menthol review

8/13/2019 MenThol review

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J Pharm. Pharmacol. 1994 ,46: 618-630Received February 23, 1994Accepted April 22, 1994Review

0 994 J Pharm. Pharmacol.

Menthol and Related Cooling CompoundsR. E C C L E S

Common Cold and Nasal Research, Department o Physiology, University o Wales College o Cardiff, Cardiff CF1 ISS K

Production and Chemistry of MentholMetabolismStructure-activity RelationshipsMechanism of ActionPharmacologyRespiratory systemNasal decongestant activityRespiratory reflexesAntitussive propertiesMucus production and mucociliary clearancePulmonary functionSkinOral cavityGastrointestinal tractEffects on olfactionSensory impact of mentholMiscellaneousToxicology

Summary-Menthol and related cooling compounds such as coolant agent lo, are widely used inproducts ranging from common cold medications to toothpastes, confectionery, cosmetics and pesticides.The review brings together a range of information on production and chemistry of menthol, and itsmetabolism, mechanism of action, structure-activity relationships, pharmacology and toxicology. Inparticular, the coolant action and carminative actions of menthol are discussed in terms of actions oncalcium conductance in sensory nerves and smooth muscle. The actions of menthol on the nose,respiratory reflexes, oral cavity, skin and gastrointestinal tract are reviewed.

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Menthol and related cooling compounds are widely used in awide range of products ranging from common cold medica-tions to toothpastes, confectionery and pesticides. Mentholwas at first considered as a flavouring agent or a fragrancewhich enhanced the palatability of medications and confec-tionery, but pharmacological studies have uncovered a rangeof biological activity on sensory nerves and smooth muscle.Menthol has been used for medicinal purposes for over onehundred years in the West, but our knowledge of thisfascinating compound is still very limited despite its verywidespread use. Research on menthol is a fertile area forthose interested in developing new drugs for the treatment ofrespiratory, dermatological and gastrointestinal disturbances.

Production and Chemistry of MentholMenthol is a naturally occurring compound of plant originwhich gives plants of the Men f h a species the typical minty

smell and flavour. The plant oil, often referred to aspeppermint oil (from Mentha pipev i ta or cornmint oil(from Mentha arvensis , is readily extracted from the plantby steam distillation. Before World War I1 menthol wasobtained primarily from plant sources, with China andJapan being the main producers. During the disruption oftrade routes during World War 11, Brazil took over as themain producer of raw plant oil and became the largestproducer and supplier of menthol (Anon 1984).Cornmint oil obtained by steam distillation from theflowering shrub Men f h a arvensis contains 70-80% of (-)-menthol, which can be crystallized out from the cornmint oilmixture. Since the crystalline product contains traces ofcornmint oil, menthol obtained from this plant source has aslightly herbal minty smell. Mentha arvensis is grown com-mercially in Brazil, Japan, Paraguay and China as a source ofcornmint oil. Pure (-)-menthol can be obtained from corn-mint oil by recrystallization from low-boiling point solvents.


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MENTHOL AND RELATED COOLING COMPOUNDS 619Peppermint oil made from Mentha piperita contains up to50% menthol, but due to its high price, peppermint oil is notused for the production of menthol. Peppermint oil is mostlyproduced in the USA and is used mainly as a flavouring fortoothpastes, other oral hygiene products and chewing-gum.Menthol can also be extracted or synthesized from otheressential oils such as cirtonella oil, eucalyptus oil and Indianturpentine oil, and synthesis of menthol from thymol com-petes with isolation from natural mint oils.Menthol (C,,H,,O, mol. wt 156.27) is a cyclic terpenealcohol, and although this type of alcohol occurs widely innature very few of these alcohols have the chemical proper-ties that make them important fragrance or flavour com-pounds. Menthol has three asymmetric carbon atoms in itscyclohexane ring, and therefore occurs as four pairs ofoptical isomers; (-)- and (+)-menthol, (-)- and (+)-neomenthol, (-)- and (+)-isomentho1 and (-)- and (+)-neoisomenthol, as illustrated in Fig. 1 (Bauer et a1 1990).(-)-Menthol is the isomer that occurs most widely in nature

(+)-menthol (+)+eo (+)-is0 (+)-isoneo

(-)-menthol (-)-neo (-)-is0 (-)-isone0FIG.1. The four pairs of optical isomers of menthol. (-)-Menthol isthe most common naturally occurring isomer and also the isomerwith the subjective cooling action.and is the one assumed by the name menthol. It has thecharacteristic peppermint odour and exerts a cooling sensa-tion when applied to skin and mucosal surfaces (Watson et a11978). The other isomers of menthol have a similar, but notidentical, odour and do not have the same cooling action as(-)-menthol (Eccles 1990). The (-)- and (+)-isomers ofmenthol each have identical physical properties apart fromtheir specific optical effect on rotation of light. Neomenthol,neoisomenthol, menthol and isomenthol differ slightly intheir boiling points with a range of 211.7-218.6"C. Theisomers also differ in their physical characteristics, as atroom temperature (+)-neomenthol is a colourless liquid andisomenthol and menthol are white crystals.

MetabolismMenthol is a highly lipid-soluble substance and like manysimilar organic substances metabolism of menthol involvesthe formation of glucuronide compounds, which are muchmore water soluble and more readily excreted in the urine.Menthol administered orally or absorbed through the skin orrespiratory epithelium is transported to the liver by thecirculation. There may be some phase metabolism ofmenthol in the skin and gut on absorption but most occurs

in the liver. In the liver, menthol is hydroxylated by micro-soma1 enzymes to form mainly p-menthane-3,8 diol, which ismetabolite I1 as illustrated in Fig. 2. The hydroxylatedmenthol compound is then conjugated with glucuronideand then circulated to the kidneys for excretion in theurine. Madyastha & Srivatsan (1988) in studies on themetabolism of (-)-menthol in rats demonstrated that therewere four hydroxylated menthol compounds in rat urinefollowing oral administration of 800 mg menthol kg-' perday for 20 days. The major metabolites werep-menthane-3,8diol(I1) and 3,8-dihydroxy-p-menthane-7-carboxyliccid V)as shown in Fig. 2. Minor metabolites 3,8-oxy-p-menthane-7-carboxylic acid IV) and p-menthane-3-9-diol 111) were alsofound. Oral administration of 800 mg menthol kg-' per day,for up to seven days was also shownmicrosomal enzymes cytochrome P450chrome c (P450) reductase.

to induce the liverand NADPH-cyto-

FIG. . Metabolism of menthol. The major metabolites arep-menthane-3,8 diol (11) and 3,8-dihydroxy-p-menthane-7-car-boxylic acid V). See text for further details. After Madyastha &Srivatsan (1988).The glucuronidation of menthol involves UDP-glucuro-nyltransferase which is responsible for the glucuronidationof monoterpenoid alcohols in general (Nakaoka 1990).Menthol glucuronide compounds in urine can be readilydeconjugated by treating raw urine with P-glucuronidaseand the menthol content of urine can be specifically assayed

by gas chromatography (Bell et a1 1981; Kaffenberger &Doyle 1990).Structure-activity Relationships

In discussing structure-activity relationships for menthol itis the cooling action of menthol which has been moststudied. The cooling action can be investigated either byusing panels of volunteers with subjective scores for coolingeffects (Watson et a1 1978), or by electrophysiologicalinvestigations on nerve cell activity (Swandulla et al 1987).Studies on the electrophysiological activity of culturedvertebrate neurons and snail neurons have shown that (-)-menthol has a very specific dose-dependent effect on calciumflux across the cell membrane, whereas the closely relatedcompound, cyclohexanol, was without activity and +)-


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620 R. ECCLESmenthol was only half as active as its stereoisomer (Swan-dulla et a1 1986, 1987).A systematic investigation of structure-activity relation-ships was undertaken by Watson et al (1978), who investi-gated the cooling activity of over 1200 synthetic compoundsin order to develop a non-volatile coolant compound for usein shavin g foams. The su bjective tests used by W atson e t a1(1978) to determine the coolant activity of com pounds werebased on the application of coolant compounds onto thesurface of the tongue to determine oral threshold for cool-ing, or directly onto the skin of the inside of th i forearm todetermine the degree of cooling on an eight point subjectivescale. The authors established that four important criterianeeded to be satisfied for a c ompou nd to possess effectivecooling activity: a hydrogen bonding group, a compacthydrocarbon skeleton, a correct hydrophilic/hydrophobicbalance, and, a mol. wt in the range 150-350.The hydrogen-bonding group of menthol is the hydroxylgroup, bu t this can be substituted w ith a variety of hydro-gen-bonding groups an d still retain coo lant activity. Watsonet a1 (1978) studied a series of N-alkyl-carbox amide gro upswhich had the advantage over menthol of low volatility.Two ex amples of these compounds are illustrated in F ig. 3,with the average oral cooling threshold recorded (pg) givenin brackets.Studies by Am ano (1986) on cooling and pungent agentsfor use as flavouring compoun ds led to the discovery of a veryactive cooling compound, 3-1 menthoxy propane- 1 2-diol,which is often referred to as coolan t agent 10 or MPD, andwhose structure is illustrated in Fig. 3. The compound iswidely used in cosmetics, soaps, dentifrices, mouthwashes,chewing-gums, tobaccos and medical plasters. Coolant agent10 has an oral cooling threshold in man as low as 1ppm inaqueo us solution, which is only one-fifth that of (-)-mentho l,and its duration of cooling action is 20-25min at IO ppm,which is twice as long as (-)-menthol (Ama no 1986). Coolan tagent 10 resembles menth ol in its ability to cause a subjectivesensation of coolness, but it d iffers in other respects being acolourless liquid with only a faint minty odour.Cooling agent AG-3-5 1- 2-hydroxyphenyI)-4- 3 nitro-phenyl -l,2,3,6-tetrahydropyrimidine-2-one , as shown inFig. 3, was discovered by a chance finding when Wei(1983) noted that accidental self-contamination of thenostrils eyelids and lips with the compo und caused a verystrong sensation of coolness. Cooling agent AG-3-5 wasfound t o be a po tent cooling agent an d Wei (1983) proposedthat the compound could be very useful as it lacked theflavour and od our of menthol an d was not readily absorbedthrough the skin.Apart from the cooling effect, and a degree of flavourpotentiation and odour modification, there is no commonproperty of cooling compounds. For instance, there is noassociation between minty smell and cooling. (-)-Mentholhas the greatest cooling activity of the eight stereoisomers ofmenthol, with (+)-menthol 45 times less active than (-)-menthol, as determined by oral threshold for coolingactivity (Watson et a1 1978). (+)-Menthol is also relativelyinactive compared with (-)-menthol when nasal coolingsensation is used as a measure of activity Eccles et a1 1988a).Menthol is an alcohol derivative and could act by indu-cing general cell mem brane labilization o r mo difying mem-

cooling agent 10 cooling agentAG-3-5 7FIG. . Synthetic coolant compounds. The structure of menthol isshown for comparison. The figures in parentheses are the oralthresholds (pg) for cooling sensation for man. Compounds I1 andIX were synthesized by W atson e t a1 (1978). Coo lant agent 10 wassynthesized by Amano (1986), and cooling agent AG-3-5 wasdiscovered by Wei (1983).

NO

brane fluidity due to its lipophilic properties. However,similar lipophilic compounds such as cyclohexanol arewithout cooling activity. Studies on the effects of mentholon r at lingual nerve response and com parisons with a rangeof related alcohols demonstrated that menthol behavedquite differently from the other substances tested, and theresults were consistent with menthol interacting with aspecific receptor (Simon & Sostman 1991). This informa-tion, together with the observations that the effects ofmenthol are fully reversible, stereochemically selective andcannot be evoked by intracellular application, indicate thatmenthol exerts its cooling effects by interaction with aspecific pharmacological receptor (Watson et a1 1978;Swandulla et a1 1986; Eccles et a1 1988a).

Mechanism of ActionOne of the major effects of menthol when applied t o the skinor a mucosal surface is to cause a sensation of coolness orwarmth and this was attributed to stimulation of thermo-receptors by Goldsheider as early as 1886. The perceivedtemperature effect is not caused by evap oration of m entholor due to vasodilation but is due to a specific action ofmenthol on sensory nerve endings. The sensation of cold orwarmth is determined by the activity of thermoreceptors inthe skin and mucosal surfaces. These cold and warmreceptors are considered to be free nerve endings withoutany specialized end organ (Hensel 1982).It has been known for some time that changes in thecalcium concentration around thermoreceptors causechanges in the sensation of temperature. In m an, intraven-ous injection of calcium solutions has been reported to causea diffuse sensation of warmth (Hirschsohn & Maendl 1922).The effects of calcium solutions on human sensations ofwarmth are supported by electrophysiological studies onwarm and cold receptors in the nasal area of the cat.Intravenous administration of calcium solution in theanaesthetized cat caused a marked increase in the frequencyof warm-receptor discharge and a depression in the dis-charge of cold receptors (Hensel & Schafer 1974). Subse-quent studies on the cat have demonstrated tha t a decrease


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MENTHOL AND RELATED COOLING COMPOUNDS 621in external calcium concentration caused by intravenousadministration of the calcium-chelating agent EDTA,caused an increase in cold-receptor discharge from nasalskin (Schafer et al 1982). Similarly, a decrease in the calciumconcentration in the perfusion fluid of the isolated tongue ofthe cat was shown to enhance cold-fibre activity in the sameway as mentholThe above studies indicate that calcium ions play a majorrole in determining the activity of cold receptors and thatmovement of calcium across the sensory nerve-cell mem-brane controls the membrane potential and the electricalactivity of the cold receptor. Our understanding of thetransducer activity of cold receptors is still developing, butit is generally accepted that the cold receptor exhibitsoscillations in the membrane potential and that wheneverthe membrane potential depolarizes above threshold abursting discharge of action potentials is fired (Braun et al1980). The oscillations in membrane potential are caused bya regenerative influx and efflux of calcium ions. The efflux ofcalcium from the cold receptor causes hyperpolarization ofthe receptor and inhibits the discharge of action potentials.It is this efflux of calcium which is believed to be sensitive tochanges in external calcium concentration as the influx ofcalcium stimulates the efflux mechanism (Eckert & Lux1976). Therefore, a decrease in external calcium concentra-tions as caused by EDTA slows the efflux of calcium fromthe cold receptor and the resultant depolarization of the cellmembrane causes an increased electrical discharge from thecold receptor.Menthol exerts its effects on cold receptors by interferingwith the movement of calcium across the cell membrane andthere is evidence that menthol acts in the same way asEDTA. Studies on the discharge activity of cat nasal andlingual cold receptors have demonstrated that intravenousinfusion of an aqueous solution of menthol in micromolarconcentrations caused an increase in the electrical dischargefrom cold receptors (Schafer et al 1986). The effects ofmenthol on cold-receptor discharge were abolished byintravenous infusion of a calcium solution. The authorsconcluded that these findings indicated that mentholcaused receptor depolarization and increased nervous dis-charge by inhibiting the efflux of calcium from the coldreceptor (Schafer et a1 1986).The inhibitory action of menthol on calcium efflux fromcold receptors is not due to blocking of open calciumchannels. Studies on snail neurons and vertebrate neuronsin culture have shown that menthol acts on a cell membranemechanism which regulates calcium efflux (Swandulla et al1986,1987; Dorshenko et a1 1989). This mechanism does notrequire menthol to penetrate the nerve cell, as intracellularinjection of saturated menthol solutions did not affectcalcium efflux (Swandulla et a1 1987).The actions of peppermint oil and menthol on calciumchannel-dependent processes in intestinal, neuronal andcardiac preparations were studied in detail by Hawthornet al (1988). These investigators demonstrated that mentholwas almost twice as potent as peppermint oil as an inhibitorof potassium depolarization-induced and electricallyinduced contractions of guinea-pig ileum and cardiacmuscle. Both menthol and peppermint oil inhibited radio-labelled nitrendipine binding to smooth and cardiac muscle

M ) (Schafer & Braun 1992).

and the authors concluded that menthol exhibited calcium-channel blocking properties.The early observations that administration of calciumsolutions affected the sensations of warmth and cold(Hirchsohn & Maendl 1922) clearly linked temperaturesensation with calcium. Later studies on menthol havenow demonstrated that menthol exerts its actions on theperception of cold and warmth by influencing the movementof calcium in thennoreceptors (Schafer et a1 1991). Furtherclarification of the mechanism of action of menthol isdependent on a better understanding of the role of calciumin modulating nerve-cell activity. Pharmacological studiescomparing the effects of menthol with calcium antagonistssuch as verapamil and dihydropyridine (Side11 et al 1990) arestarting to untangle this complex interaction of menthol oncalcium flux in the nerve cell, and we can expect rapiddevelopments in this field of research.The cool sensation caused by application of menthol tothe skin or mucosal surfaces is apparent when low orthreshold concentrations of menthol are used and is relatedto stimulation of cold receptors, but menthol has bothirritant and local anaesthetic actions when used in higherconcentrations. Menthol in 5-10% concentration inmineral oil causes a burning sensation when applied to theskin and this sensation has been attributed to stimulation ofnociceptors in the skin (Green 1986). The local anaestheticaction of menthol was studied by Macht (1939) who used a1 :500 solution of menthol in 9.5% alcohol. Macht (1939)demonstrated local anaesthetic activity on frog skin, rabbiteye and guinea-pig skin and he also attributed the centralnervous system effects of menthol to its local anaestheticactivity. A local anaesthetic action of menthol on sensoryreceptors of the human tongue was reported by Hensel &Zotterman (1951). Similarly, Hellekant (1969), reported thathigh concentrations of menthol (0.2 g L-' in water) orprolonged exposure to menthol caused anaesthesia ofsensory receptors in the cat tongue. Menthol in aqueoussolution is used as an anaesthetic for experiments on snailswhere it causes a fully reversible depression of activity incentral neurons (Haydon et a1 1982).The local anaesthetic activity and irritant activity ofmenthol may be caused by effects on calcium conductancein sensory nerves and have similarities with menthol's effectson thermoreceptors. This is quite likely for the localanaesthetic properties, but the irritant activity may be anonspecific action which is also found in other compoundssuch as camphor, which have quite a different pharmacolo-gical profile from menthol.

PharmacologyRespiratory systemMenthol is widely used in medications for the relief ofcommon cold symptoms such as nasal congestion andcough, but there is little hard scientific evidence to supportany nasal decongestant or antitussive activity.Menthol was introduced into Europe and the UnitedStates at the end of the nineteenth century and in 1890,Potter, in a review of 'The use of menthol in diseases of theupper air passages' stated that "while menthol seems to be adrug with a future of great usefulness, it needs to be carefully


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622 R. ECCLESstudied and investigated, so that we may become familiarwith its action and know its limitations. It is now over onehundred years since Potter wrote on menthol, yet ourunderstanding of the pharmacology of this compound isstill very limited and often confused.Nasal decongestant activity Menthol-containing inhala-tions, rubs, and lozenges are often described as nasaldecongestants yet there are several studies which haveclearly shown the opposite effect, i.e. a tendency toincreased nasal congestion. As early as 1927, Fox, in aseries of experiments on dogs, demonstrated that topicalapplication of 1 and 5 solutions of menthol to the nasalmucosa caused nasal congestion whereas topical adrenalinesolution (1 :1000) had a marked nasal decongestant action.

Administration of menthol via a nasal inhaler has alsobeen shown to cause nasal congestion (Butler & Ivy 1943).In these experiments on volunteers, inhalation of mentholproduced a marked increase in nasal resistance within 1minafter administration and this congestion was sustained over210min. Despite the nasal congestion, the subjects did notfeel congested but actually believed that their nasal pas-sages felt clear (Butler & Ivy 1943). The nasal congestioncaused by menthol may be due to an irritant action asmenthol, when applied topically to both the skin andnasal mucosa, has been shown to cause hyperaemia and aburning or smarting sensation (Glass & Bliss 1939). Theirritant action of menthol is dependent on the concentrationof menthol, as a 0.5 solution of menthol in liquidpetrolatum was irritating to the nasal mucosa in all subjectstested, whereas a 0.1 solution in physiological saline wasnon-irritant in most subjects (Glass & Bliss 1939).

Despite the results of the studies by Fox (1927) and Butler& Ivy (1943), menthol and similar aromatics were stillbelieved to have a nasal decongestant action and theywere recommended as nasal decongestants in professionalliterature (British National Formulary 1974). Studies onhealthy volunteers by Burrow et al (1983) clearly demon-strated that inhalation of camphor, eucalyptus and mentholvapour had no effect on nasal resistance to airflow asmeasured by rhinomanometry, but exercise caused amarked decrease in nasal resistance in the same group ofsubjects. This was the first investigation to use rhinomano-metry as a means of investigating the effects of menthol onthe nose. In the study by Burrow et a1 (1983), the volunteerswere asked if their sensation of nasal airflow was improvedafter breathing the aromatics and after exercise. One hun-dred percent of subjects who breathed menthol reported anincreased sensation of nasal airflow whereas only 20 ofsubjects reported improved airflow after exercise, despite a70 decrease in nasal resistance to airflow associated withexercise and no change in nasal resistance after menthol.This study clearly demonstrated that menthol inhalationcaused a subjective nasal decongestant effect without anyobjective decongestant action.

The method of measuring the subjective decongestantaction of menthol was refined in subsequent studies whichused scoring from -6 (completely obstructed nose) to f 6(completely clear nose) and a 100-mm visual analogue scalewith the extremes labelled nose feels extremely blocked andnose feels extremely clear (Eccles et al 1987a,b, 1988a,b).

These studies confirmed that menthol had no effect on nasalairway resistance but caused a marked increase in nasalsensation of airflow, which could now be quantified for thefirst time as a subjective score.

Menthol lozenges are widely marketed for the relief ofcommon cold symptoms and although they may have asoothing effect on sore throat, their main action appears tobe a subjective sensation of improved nasal airflow. In astudy on subjects suffering from nasal congestion associatedwith the common cold, Eccles et a1 (1990), demonstratedthat oral administration of an 11-mg menthol lozengecaused a subjective sensation of improved airflow withoutany change in nasal airway resistance. The effects of inges-tion of a menthol lozenge on the subjective sensation ofnasal airflow and an objective measure of nasal airwayresistance are illustrated in Fig. 4.

The cool sensation of increased nasal airflow caused byinhalation of menthol is believed to be due to stimulation ofcold receptors served by the trigeminal nerve supply to thenose (Jones et a1 1987; Eccles 1990). Jones et al (1989)studied the distribution of thermoreceptors within thenasal vestibule and compared the sensitivity of this areawith the nasal mucosa inside the nasal cavity and the malarskin lining the outside of the nose. They found evidence forthermoreceptors within the nasal vestibule and on thesurface of the nose, but thermoreceptors were not foundon the nasal mucosa.

The location of the cold receptors in the nose is believed tobe mainly in the nasal vestibule, which is lined by asquamous epithelium similar to facial skin (Jones et a11987; Aldren & Tolley 1991; Clarke et al 1992), but there

150.ip 100-m 5mCum.2 02-mv

5 Ibasal lb 40 8Time (min)

FIG. . The effects of ingestion of an 1I-mg (-)-menthol lozenge onsubjective sensation of nasal congestion 0 ,m) and nasal resistanceto airflow 0, n volunteers with common cold. The subjectivesensation of nasal congestion was significantly reduced after 10minafter ingestion of the lozenge but nasal airway resistance as measuredby rhinomanometry was unaffected. Shaded symbols m, 0 epre-sent the values for the menthol-treated group (n = 30) and the opensymbols 0 ,0)epresent the mean values for the placebo-treatedgroup (n = 32). Results taken from Eccles et a1 (1990).


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MENTHOL AND RELATED COOLING COMPOUNDS 623is also evidence supporting the involvement of the nasalmucosa in the sensation of nasal airflow based on a findingthat local anaesthesia of the nasal mucosa reduces thesensation of nasal airflow (Eccles et al 1988~).

When menthol is taken orally in lozenge form, theincreased sensation of nasal airflow is believed to be dueto menthol vapour reaching the nasal cavity via the nostrilswhen the mouth is opened and via the nasopharynx duringswallowing. However, there is evidence that menthol mayinfluence the sensation of nasal airflow by a purely oral formof stimulation as it has been proposed that the majorpalatine nerve serving the hard palate has a role in nasalsensation of airflow (Naito et a1 1991).Respiratory rejexes. Inspiratory airflow cools the nose andlarynx and stimulates cold receptors to cause a sensation ofairway cooling which is associated with airflow. As well asproviding a sensation of cool airflow, the stimulation ofupper airway cold receptors causes a reflex inhibition ofrespiration and inhibition of upper airway accessory respira-tory muscle activity in conscious man (McBride& Whitelaw1981; Eccles & Tolley 1987) and in the anaesthetized dog(Mathew et al 1990). Inhalation of menthol causes the samereflex inhibition of respiration as a cold air stimulus butwithout any change in airway temperature in the anaesthe-tized guinea-pig (Orani et al 1991), the anaesthetized dog(Sant Ambrogio et al 1992), and in conscious man (Eccles etal 1989; De Cort et al 1993; Sloan et al 1993). The effects ofnasal airflow and menthol are not always inhibitory torespiratory activity as in the anaesthetized cat, nasal airflowenhances the activity of nasal dilator muscles and mentholpotentiates this response (Davies & Eccles 1985, 1987).

The inhibition of respiration caused by cold-air stimula-tion of the nostrils in man may cause a brief period ofapnoea in newborn infants. Similarly, administration ofmenthol vapour directly to the nose of infants has beenshown to cause a brief period of apnoea (Javorka et a1 1980).Those authors demonstrated that menthol vapour adminis-tered from an open container of menthol crystals placed1cm from the nostrilscaused a brief period of apnoea (mean4.7s in 43 of cases) in a study on 44 premature infants(gestation age 19-37 weeks). It is not clear from this study ifthe reflex apnoea was induced solely by cold receptorstimulation; menthol crystals placed so close to the nosewould deliver a high concentration of menthol vapourwhich could be irritant. Javorka et al (1980) proposed thatthe apnoea was the result of a protective reflex to preventinhalation of irritant substances, and this is supported bythe increase in heart rate observed after menthol exposure,which may represent a general arousal reaction.

Since menthol-containing medications are widely used inthe treatment of common cold in infants, any tendency ofmenthol to cause apnoea is viewed with great concern.Following several reports in the German medical literatureof adverse events in infants after treatment with menthol-containing ointments (Bettecken 1964; Moll 1964), theGerman medical authorities issued a statement warningagainst the use of menthol medications in infants (Arznei-mittelkommission der Deutschen Arzteschaft 1964). Thepublic warning by the German medical authorities stimu-lated interest in this area and an international symposium

was held in Paris in April 1966 to debate the safety ofmenthol medications. The symposium proceedings bringtogether a large amount of clinical data on the use ofmenthol medication in thousands of cases in infants suffer-ing from acute respiratory tract infection. The generalconclusions of the symposium were that commonly usedvaporub remedies were safe to use in infants but that theyshould not be applied directly to the nostrils (Dost & Leiber1967).Antitussive propertie s. The use of menthol as an antitussivegoes back over one hundred years, as it was in 1890 t h a tLunsford Richardson developed a topical rub containingmenthol for the treatment of whooping cough (Poetsch1967). The vaporub developed by Richardson is stillmarketed for the treatment of cough together with numer-ous other menthol-containing syrups, lozenges and topicalrubs. Despite the very widespread use of menthol productsas antitussives, there is very little literature available in thepublic domain to support antitussive efficacy.Inhalation of menthol has been shown to inhibit respira-tion via stimulation of upper airway cold receptors (Eccles etal 1989; Orani et al 1991; Sant Ambrogio et a1 1992), andmenthol may act to inhibit cough via this mechanism.Respiratory reflexes such as cough are closely linked to thebrainstem centres regulating respiration and the generalinhibition of respiratory activity caused by menthol couldalso influence the frequency and intensity of cough. Mentholmay also act as an antitussive by influencing the activity ofsensory nerves in the upper airway which initiate cough. It isnot clear at present which sensory nerves are involved inproducing cough, but it is generally agreed that the larynx isthe most sensitive site for the initiation of cough (Karlssonet a1 1988; Fuller & Jackson 1990). Menthol has been shownto influence the activity of cold receptors in the larynx (SantAmbrogio et al 1991) and may also influence the activity oflaryngeal sensory receptors involved in the cough reflex. Themechanisms by which menthol may act as an antitussive areat present speculative, but they are based on establishedknowledge that menthol has already been shown to influ-ence the activity of upper-airway sensory receptors and tomodulate respiratory reflexes.Mucus production and mucociliary clearance. The secretionof mucus and the continuous mucociliary clearance from theupper airway are important defence mechanisms againstinfection. There are conflicting reports in the literature thatmenthol and other aromatics may either enhance or depressthese defence mechanisms. Menthol is a popular ingredientfor steam inhalations taken for the treatment of cough andas an expectorant to promote clearance of mucus from therespiratory tract. Oswald (1959) concluded that the mostdesirable property of menthol and other aromatics taken insteam inhalation was their pleasant smell because the mainvirtue of steam inhalation is the expectorant effect of hotmoist air. However, studies by Boyd& Sheppard (1969) onanaesthetized rabbits demonstrated that administration ofmenthol by steam inhalation caused an increase in mucusproduction and a decrease in the specific gravity of respira-tory tract fluid. This effect of menthol was produced byamounts of menthol which added no detectable odour of


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624 R. ECCLESmenthol to the inspired air and which corresponded to asystemic absorption of less than 20 pg kg- body weight.The authors concluded that the bronchomucotropic effectsof menthol were due to direct local stimulation of mucus-secreting cells in the respiratory tract. The effect of mentholon mucus production was dose dependent, as inhalation ofhigh concentrations of menthol depressed both the volumeand mucus content of respiratory-tract fluid.

The stimulant action of menthol on mucus productionmay be beneficial, as bacteria adhere avidly to respiratory-tract mucus (Plotkowski et a1 1993). Ciliary clearance ofmucus is essential in order to prevent infection and thereforeany effect of menthol on ciliary activity is of interest. Usingan in-vitro model of the frog oesophagus, Das et al (1970)demonstrated that 0.1YOmenthol in Ringer solution stimu-lated mucociliary clearance, whereas Su et a1 (1993) using arat tracheal preparation demonstrated that menthol at0.01 concentration decreased ciliary beat frequency.Although Su et al (1993) claim that menthol is ciliotoxic ina dose-dependent manner, they also suggest that in-vivostudies need to be made on human subjects to determine anyeffects on mucociliary clearance and respiratory defence.

There does not appear to be any report of in-vivo studiesin man on the effects of menthol on respiratory defence.However, Huber et a1 (1973) reported in abstract form thatexposure to a vapour rub containing menthol and otheraromatics resulted in a depression of respiratory defence, asmeasured by bacterial aerosol challenge in mice. Thispublication aroused great clinical interest as the vaporubpreparation was widely used in children for the symptomatictreatment of common cold. In a fuller paper, Jakab& Green(1 975) contradicted the earlier report by Huber et a1 (1973)and demonstrated that exposure to vaporub vapours did notcause any depression of pulmonary antibacterial activity.The findings of Jakab & Green (1975) were later confirmedby Goldstein et al (1976), who demonstrated that rates ofpulmonary bacterial transport and inactivation in the ratand mouse were unaffected by exposure to vapours ofcamphor, menthol, eucalyptol and turpentine. Jakab &Green (1975) took the trouble to measure the concentra-tions of the vapours and they carefully controlled theconditions of exposure and assessments of respiratorydefence, and their work throws considerable doubt on theearly report by Huber et al(l973) that inhalation of mentholand other aromatics may compromise respiratory defence.

The studies on respiratory defence mechanisms investi-gated the effects of a mixture of aromatic vapours and,therefore, it is difficult to separate out any specific effect ofmenthol. However, the substantial studies by Jakab &Green (1975) and Goldstein et a1 (1976) showed that themixture of aromatics was without effect on respiratorydefence and one can conclude that menthol vapour alonewould also be innocuous unless there was some complexinteraction between the components of the vapour mixture.Pulmonary function. Because of the commercial pressure toshow efficacy of action for a registered product with amixture of aromatics (including menthol) there are rela-tively few studies on the single ingredients. Menthol mayhave effects on the airways in subjects with common cold, asCohen & Dressler (1982) reported that a mixture of aro-

matics (including menthol) improved a number of pulmon-ary function indices. However, that report which indicates abeneficial effect of aromatics inhalation has not been con-firmed and it is difficult to attribute any effect of the mixtureof vapours to an action of menthol.

Menthol vapour may exert some effect on lung functionby influencing lung surface tension, as in-vitro studies onsynthetic surfactant films have shown that menthol doeslower surface tension and in-vivo studies on anaesthetizedrabbits have shown that eucalyptol exhibits surfactantproperties and increases lung compliance (Zanker et a11980).SkinThe main effect of menthol when applied to hairy skin is tocreate a cooling sensation similar to the nasal coolingsensation described above. The coolant action of mentholon skin made it a popular ingredient in shaving creams(Watson et a1 1978) and in order to avoid the eye irritationcaused by menthol vapour, relatively non-volatile menthol-like coolant compounds were developed. However, thesensory effects of menthol on skin are more complex thana straightforward stimulation of cold receptors, as mentholalso modulates the sensations of warmth and skin irritation.Menthol (0.2 and 2 in mineral oil) when applied to thevermillion border of the human lip causes an attenuation ofthe sensation of warmth, which appears to be due to effectson warm receptors rather than a masking of the sensation ofwarmth due to stimulation of cold receptors (Green 1986).Higher concentrations of menthol (5 and 10 in mineraloil) caused a strong burning sensation which may be due tostimulation of nociceptors rather than warm receptors(Green 1986). Green (1992) also made psychophysicalmeasurements on the sensory effects of menthol appliedtopically to the human forearm under controlled thermalconditions. Menthol as a 5 solution in ethanol heightenedthe perception of skin cooling and attenuated the perceptionof moderate skin warming. Menthol was also shown to havepungent or irritant properties which were enhanced bycooling of the skin, and Green (1992) suggested thatmenthol may stimulate cold-sensitive nociceptors as wellas cold receptors.

Menthols property to stimulate skin nociceptors may beresponsible for the counter-irritant action of menthol, as thenociceptors initiate an axon reflex with subsequent releaseofvasodilator peptides. An increase in skin temperature andunderlying muscle temperature has been shown after topicalapplication of a mixture of eucalyptus oil and menthol, andthis may be beneficial for pain relief and useful to athletes asa passive form of warm-up (Hong & Shellock 1991). Theeffects of menthol on cold receptors and nociceptors mayalso be beneficial in the treatment of pruritus, as itching ismediated via unmyelinated c fibres which also transmit thesensation of pain. Topical menthol (0.25 ) has been widelyused for many years to treat pruritus but there are nocontrolled studies to verify the efficacy of menthol as atopical treatment for this condition (Greco & Ende 1992).

The irritant effect of menthol causes local vasodilationand this together with the lipophilic nature of menthol mayaid topical drug penetration. Studies on hairless mouse skinhave shown that menthol (1-5 w/v) can aid the penetra-


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M E NT HOL ND RELATED COOLING COMPOUNDS 625tion of compounds such as indomethacin and cortisone(Katayama et al 1992), morphine hydrochloride (Mori-mot0 et al 1993) and methyl salicylate (Yano et a1 1991).Irritancy at high concentrations is not a general propertyof coolant compounds as coolant agent 10 has no irritantactivity on human skin and rabbit eye at concentrations upto l o , despite being a more potent coolant compoundthan menthol (Amano 1986; Takasago Co. 1993).Oral cavityCornmint oil and peppermint oil are widely used forflavouring toothpastes, chewing-gum and confectionery.These oils contain around 70% (-)-menthol which isresponsible for the pleasant cooling and refreshing taste ofthe oils.Menthol has a complex sensory effect in the oral cavity asit influences the activity of both gustatory and temperaturereceptors. In studies on the electrical activity of the chordatympani nerve of the anaesthetized cat, Hellekant (1969)demonstrated that rinsing the tongue with an aqueousmenthol solution (0.1 g L-I) caused a slowly increasingactivity in all gustatory fibres. Menthol also altered theresponse of the taste receptors to other sapid solutions,and prolonged exposure to menthol or higher concentra-tions of menthol (0.2 g L- water) anaesthetized the tastereceptors. Hensel & Zotterman (1951) studied the effects ofan aqueous solution of menthol on temperature-inducedresponses from the tongue of the cat and reported thatmenthol caused chemical sensitization of the thermorecep-tors to thermal stimuli. Menthol in an aqueous solution(1 :10000) was shown to enhance cold-receptor activitywithout any action on warm receptors. Hensel & Zotter-man (1951) also commented that the local anaesthetic actionof high concentrations on menthol in oil was easily demon-strated in man, as 1 or 2 min after application of menthol tothe tip of the tongue there was an obvious decrease in bothcold and tactile sensibility. Dodt et al (1953), in a similarstudy on the thermoreceptors of the tongue of the cat, foundthat acetylcholine and menthol behaved in a similar way asthey both sensitized cold receptors. The authors alsoreported that menthol influenced the activity of warmreceptors and caused a shift of the range of steady dischargeof the warm receptors towards the cold side, i.e. sensitizedwarm receptors. Dodt et al (1953) claimed that the earlierstudy by Hensel& Zotterman (1951) did not find any effectof menthol on warm-receptor discharge because the studywas limited to very few warm-fibre preparations and toonset of heat paralysis in the warm-fibre response above40C.The complex oral sensations induced by menthol werecommented on by Green (1985), who found that mentholenhanced cold sensations in the mouth but could enhance orattenuate warm sensations depending on the time period ofpretreatment with menthol.The complexity of oral sensations caused by menthol maybe explained by a sensitization of both warm and coldreceptors together with modulation of taste-receptor activ-ity. In high concentrations menthol has both an irritantaction and a local anaesthetic effect and gradations of theseeffects complicate the sensory actions of menthol. Studies onthe chorda tympani nerve response to menthol in the

anaesthetized rat have shown that the nerve responseelicited by menthol (064mM in aqueous solution) onlylasts around 2.5s, but that the tongue receptors remaininsensitive to menthol for up to 10min after application ofthe solution (Lundy & Contreras 1993). Although stimula-tion with menthol prevented taste receptors from respond-ing to subsequent presentations of menthol, the preparationresponded normally to sodium and potassium solutions andsolutions of quinine and citric acid, and prior stimulationwith one of these solutions resulted in the recovery of thementhol response. Lundy & Contreras (1993) explained thiseffect of menthol as being due to cell depolarization causedby menthol binding with receptor proteins on the plasmamembrane of the taste receptor.The oral sensations of warmth and cold are accompaniedby a sensation of nasal cooling and a peppermint smell, asmenthol vapour from the oral cavity reaches the nose. Oraladministration of menthol may also directly influence nasalsensation of airflow by effects on the major palative nervewhich innervates the palatal mucosa (Naito et a1 1991).Gastrointestinal tractPeppermint oil has been used for many years in herbalremedies for the treatment of digestive disorders and it hasbeen shown to be effective in the symptomatic treatment ofirritable bowel syndrome (Rees et a1 1979), and to reducecolonic spasm when injected directly into the large intestineduring colonoscopy (Leicester& Hunt 1982). When used forthe treatment of irritable bowel syndrome, peppermint oil isrecommended to be taken in enteric-coated capsules asrelease of the oil in the stomach causes relaxation of thelower oesophageal sphincter and heart burn (Somerville eta1 1984). Peppermint oil is a carminative with potentantispasmodic properties and it is likely that these actionsare mainly due to the high (-)-menthol content of pepper-mint oil, although Evans et a1 (1975) suggest that thecarminative action of menthol and similar compoundssuch as camphor, thymol, eugenol, and carvone, is non-specific and related to the aqueous solubility.Menthol has been shown to inhibit the histamine- andacetylcholine-induced contractions of the guinea-pig iso-lated taenia coli (Reis & Bertini 1984) and to have smoothmuscle relaxant activity on the isolated vas deferens, uterusand intestine (Macht 1939). The fact that menthol has beenshown to be a smooth-muscle relaxant supports the ideathat the activity of peppermint oil is mainly related to itshigh menthol content. However, since peppermint oil is amixture of biologically-active substances, some cautionneeds to be taken in attributing all the properties ofpeppermint oil to menthol.Menthol has been shown to inhibit the contractions of theguinea-pig ileum by an effect on calcium conductance insmooth muscle (Hawthorn et al 1988). This inhibitoryaction of menthol on gut smooth-muscle calcium conduct-ance may be the basis for the efficacy of peppermint oil intreating irritable bowel syndrome, as peppermint oil containsa high percentage of menthol. In-vitro studies on guinea-pigand human gut smooth muscle by Taylor et a1 (1984, 1985)indicate that menthol exerts an inhibitory effect on gutsmooth muscle by decreasing the influx of extracellularcalcium through potential-dependent channels, whilst


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626 R. ECCLEShaving no effect on the intracellular mobilization of calcium.Thus, the actions of menthol on both nervous tissue andsmooth muscle are mediated by effects on calcium conduc-tance across the cell membrane, and this basic action linksmenthols cooling and carminative properties.Effects on olfactionThe cooling sensation caused by inhalation of menthol isbelieved to be primarily due to stimulation of trigeminalnerve fibres supplying the nasal vestibule and nasal mucosa,but menthol also has a distinctive minty smell which ispresumably due to stimulation of olfactory nerves. Theintegrity of the olfactory system is not essential for thedetection of menthol as anosmics can detect menthol viapungency (Commetto-Muniz & Cain 1990). Chronic expo-sure to menthol has been shown to cause symptoms ofoccupational hyposmia (Naus 1983), whereas an acuteexposure to an aqueous solution of menthol applied to thehuman olfactory area by means of a drop pipette has beenshown to decrease the smell threshold to coffee, which isbelieved to be a pure olfactory stimulus (Skouby & Zilstorff-Pedersen 1954). The ability to detect menthol declines withage (Murphy 1983), and this may represent a general declinein nasal trigeminal/olfactory sensitivity due to accumulateddamage of nasal infection and it indicates that caution mustbe taken when applying results of research on healthy youngadults to the general population.The trigeminal nerve fibres supplying the nasal mucosahave been shown to respond to a wide variety of chemicalstimuli and are believed to provide a sense of irritation oftenreferred to as a common chemical sense, whose primefunction is to protect the animal from potentially harmfulchemicals (Silver 1990). The nerve fibres involved in thesense of nasal irritation appear to be free trigeminal nervefibres which penetrate between the epithelial cells of thenasal mucosa (Silver 1992).Sensory impact of mentholThe sensory impact of menthol when applied to skin or amucosal surface depends on the concentration of menthol.Low concentrations give a cool sensation whereas highconcentrations of 2-5% menthol cause irritation and localanaesthesia. There is little information on the dose-responsecharacteristicsof menthol and it is often difficult to comparestudies when different aqueous and organic vehicles are usedfor application of menthol, as the solvent influences thebiological activity of the menthol. There is also evidencethat there is a marked difference in sensitivity betweendifferent body surfaces for the cooling action of menthol.Watson et al( l978) stated that the eye was the most sensitiveregion of the body, with thresholds measured in nanogramsand a simple test for cooling activity in volatile compoundsbeing to hold an open bottle near to the eye. The sensitivity ofbody surfaces was ordered as eye > tongue > buccalregion >ano-genital area > axilla > inside forearm-breast>other arm areas, thigh, back >hands, feet palms, soles(Watson et a1 1978). This gradation of sensitivity is probablyrelated to the density of thermoreceptors in these surfaces.The nasal-cooling response was not listed by Watson et al(1978) but from our own experience it probably comes a closesecond to the eye as a very sensitive area.

The sensory impact of menthol in the nose when inspiredas a vapour or administered intranasally in solution is acomplex mix of sensations. The cool sensation dominates atlow concentrations of menthol with stimulation of trigem-inal cold receptors and this is supplemented by the mintysmell caused by stimulation of olfactory nerves. In higherconcentrations menthol stimulates trigeminal nerves to givea chemical sense related to the detection of irritantcompounds, and this may be related to stimulation ofnociceptors. Depending on the concentration of menthol,this irritant sensation may be just a pungent sensation, or inextreme, a sensation of burning and pain. The cool sensationis generally perceived as a pleasant sensation, especiallywhen providing relief from nasal congestion, but highconcentrations of menthol are irritant and may elicit pro-tective respiratory reflexes and cause nasal congestion.The sensory impact in the mouth is complex, as mentholvapour will also reach the nasal cavity and stimulateolfactory and trigeminal sensory receptors. Similarly in theoral cavity, menthol will stimulate taste receptors andthermoreceptors and, as discussed above, the effects ofmenthol on these sensory receptors are dependent on boththe concentration of menthol and the duration of exposure.The well-known pleasant minty taste of menthol andpeppermint oil is very familiar, but when it comes todissecting out the various sensations and ascribing them tonasal and oral receptors the picture is extremely complex.MiscellaneousMenthol is commonly used in oral and nasal medicationsand comes into contact with mucosal epithelia which arefrequently subject to bacterial infection and allergic reac-tions. Therefore, it is not surprising that investigators havestudied the antibacterial and anti-allergic properties ofmenthol and other essential oil components.Moleyar & Narasimham (1 992) studied the antibacterialactivity of fifteen essential oil components towards food-borne bacteria and demonstrated that cinnamic aldehydewas the most active compound but that menthol also hadsignificant antibacterial activity.Studies on the anti-allergic effects of constituents ofchewing-gum by Arakawa et al (1992) have shown thatintraperitoneal administration of menthol in the guinea-pig inhibits homologous passive cutaneous anaphylaxismediated by IgE antibody. Similarly, menthol was shownto suppress antigen-induced histamine release from ratperitoneal mast cells and the authors concluded thatmenthol in chewing-gum would have an anti-allergic effect.Cooling of the urinary bladder in man and other animalshas been shown to cause a reflex contraction of the detrusormuscle and micturition. This reflex is believed to be mediatedby cold receptors in the bladder wall. Lindstrom & Mazieres(1991) studied the effect of menthol on the urinary bladdercooling reflex in anaesthetized cats and demonstrated that thebladder cooling reflex was greatly exaggerated by intralum-inal exposure of the bladder or urethra to a 0.6 mM solution ofmenthol. This action of menthol appeared to be a specificsensitizing action on bladder cold receptors as the activity ofbladder mechanoreceptors was unaffected. Thus, mentholseems to have similar actions on respiratory cutaneous andbladder cold receptors.


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MENTHOL AND RELATED COOLING COMPOUNDS 627Menthol causes a cool sensation when applied to the skin

and this may be the cause of a behavioural response knownas the wet shakes, which is observed following administra-tion of menthol in rats. Wei (1983) described the wet shakesin rats treated with menthol and another coolant compoundAG-3-5 as rotational body movements similar to thosemade by a dog when wet. The ED50 for menthol for wetshakes was found to be 35 mg kg-l compared with an ED50of only 0.18 mg kg- for compound AG-3-5 when adminis-tered by intraperitoneal injection. The wet-shakes behaviourwas attributed to a cutaneous action of the drugs whichprobably caused a sensation of skin cooling.

An unusual use for menthol is as a pesticide in controllingtracheal mite infestations in honey bee colonies. A 50-gporous sachet of menthol is placed within the hive and thementhol vapour kills mites within the bee tracheae (Duff &Furgala 1991; Delaplane 1992). Menthol is the only pesti-cide approved by the US Environmental Protection Agencyfor bee mite control.

ToxicologyMenthol, peppermint oil and synthetic coolant compoundsare widely used in the food and pharmaceutical industries;peppermint oil is the worlds third most important flavour-ing exceeded in popularity only by vanilla and citrusflavours (Thorup et a1 1983a). World production ofmenthol was estimated as 3500tonnes in 1984 and isprobably considerably more today (Anon 1984). Pepper-mint oil contains 30-50 menthol and the world produc-tion of peppermint oil is about 8000 tonnes per year. Forsuch a commonly ingested substance it is surprising thatthere are relatively few studies on the toxicology of mentholand this supports the generally held opinion that menthol isa relatively nontoxic and safe substance.

The acceptable daily intake for menthol is quoted as0-0.2mgkg-I (FAO/WHO 1976), but this figure is notsupported by any toxicological data. The authors havequoted a very low value for chronic menthol intake to erron the side of safety in the absence of any hard data and theyacknowledge that much larger doses have been taken byman without any ill effect.

Martindale, The Extra Pharmacopoeia (1989) states thatthe fatal dose in man has been estimated to be about 2g.This is probably a gross underestimate as it represents a doseof only around 28 mg kg- and in metabolic studies on rats adaily dose of 800mg kg- was given for 20 days without anyreport of serious adverse effects, apart from induction ofliver enzymes (Madyastha & Srivatsan 1988). Macht (1939)reported the lethal dose in rats to be 1.5 g kg-I . In a studywhich primarily investigated the cooling action of menthol,the LD50 in rats was found to be 700mg kg-, with deathattributed to the well known anaesthetic properties of thiscompound with loss of the righting reflex and depression ofrespiration (Wei 1983). Macht (1939) commented that themost striking pharmacological property of menthol was itseffect on the central nervous system, as with high dosesanimals soon became depressed and gradually lost con-sciousness, an action which he attributed to the localanaesthetic activity of menthol. Menthol has been shownto reversibly depress the activity of snail neurons, where it is

often used as an anaesthetic agent and there is also someevidence that menthol inhibits synaptic transmission in thesnail (Haydon et a1 1982). In comparison with menthol,camphor exhibited marked convulsant activity in the rat(Macht 1939).

The synthetic coolant compounds have been found to beeven less toxic than menthol when judged by the rat LDSO.Coolant agent 10 has an acute oral LD50 of 5.7gkg-(Amano 1986), and coolant agent AG-3-5 has an acuteLD50 of 1.5gkg- (Wei 1983).

Neuropathy has been induced in rats given peppermint oil(40-100mg kg- per day), with histological changes in thewhite matter of the cerebellum (Thorup et a1 1983a). Sincepeppermint oil contains 1-3 pulegone, which is known tobe toxic, a comparison of the toxicology of menthol withpulegone was undertaken in order to determine if mentholcould also be implicated as a cause of neuropathy. Rats weregiven menthol by gavage for 28 days at 0, 200, 400 and800mgkg- and showed few toxic effects (Thorup et al1983b). For menthol, the only effects were an increase inliver weight and increased vacuolization of hepatocyteswhich was evident at all doses, and probably represents aninduction of liver enzymes. In contrast, pulegone, at doses of80 and 60 mg kg- , caused atonia and histopathologicalchanges in both liver and brain tissues. No sign of encepha-lopathy was observed in rats given menthol. Thus, theneurotoxicity of peppermint oil is probably related to thepulegone content rather than menthol.

Chronic exposure to high concentrations of mentholvapour has been shown to have no gross toxic effects inrats (Rakieten et al 1954). Those investigators exposedrats to (-)-menthol vapour at several concentrationsbetween 0.087 and 0.166ppm for over 6 h a day, for up t o79 days, and found only slight changes indicative of irrita-tion of the eyes and lungs at the highest concentration.The authors also report that after the rats had been killed,tracheal ciliary activity was normal in all animals,although no objective measurement of ciliary beat fre-quency or mucociliary clearance was used by the authorsto support this claim.

In-vitro studies on a variety of animal tissues includinghamster brown adipocytes have shown that the major effectof menthol in-vitro at a concentration of 0.32-0.76 mM is tocause deterioration of biological membranes (Bernson &Pettersson 1983). The authors also demonstrated thatnoradrenaline-induced respiratory activity in the adipo-cytes was inhibited by menthol.

Menthol is used in many brands of cigarettes and thesmoking of mentholated cigarettes represents a chronicform of menthol intake. The percentage of mentholated-cigarette users has been shown to be much higher inAmerican Blacks and Asians than in Whites, especially inthe younger age groups (Sidney et al 1989). Since theincidence of oesophageal cancer mortality among Ameri-can Blacks is over three times the rate for Whites and theincrease in oesophageal cancer roughly parallels the increasein menthol cigarette sales, it has been proposed that there isa link between smoking mentholated cigarettes and oeso-phageal cancer (Herbert & Kabat 1989). However, studieslooking at the consumption of mentholated cigarettes andoesophageal cancer have not shown any clear link, and the


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628 R. ECCLESissue of menthol-cigarette smoking and oesophageal canceris unresolved (Herbert & K aba t 1989). In-vitro studies ongenotoxicity on animal and huma n tissues have not foundany chromosomal-damaging effects of menthol and atpresent there is no evidence to implicate menthol as acarcinogen (Murthy et al 1991).In general, mentho l appears t o be a substance of very lowtoxicity in acute studies, but more information is required todefine a safe daily intake for chronic intake.

ReferencesAldren, C., Tolley, N. S. (1991) Further studies on nasal sensationof airflow. Rhinology 29: 49-55Amano, A. (1986) Cooling and pungent agents. J. Jap. SOC.Cut.Health 17: 77-86Anon (1984) The situation of the world menthol market. Tabak. J.Int. 5: 404-405Arakawa, T., Shibata, M., Hosomi, K., Watanabe, T., Honma, Y.,Kawasumi, K., Takeuchi, Y. (1992) Anti-allergic effects of pepper-mint oil, chicle and jelutong. J . Food Hyg. SOC. ap. 33: 569-575Arzneimittekommission der Deutschen Arzteschaft (1964) War-nung vor. der Anwendung mentholhaltiger Praparate bei Sauglin-gen und Kleinkindern. Deutsches Arzteblatt 1763Bauer, K., Garbe, D., Surburg, H. (1990) Common Fragrance andFlavor Materials: Preparation, Properties and Uses. VCH,Weinheim, GermanyBell, G. D., Henry, D. A., Richmond, C. R. (1981) A specific g.1.c.assay for menthol in urine. Br. J. Clin. Pharmacol. 12: 281PBernson, V. S. M., Pettersson, B. (1983) The toxicity of menthol inshort term assays. Chem. Biol. Interactions 46: 233-246Bettecken, F. (1964) Nil nocere Vergiftungsssymptome bei Sauglin-gen nach Gebrauch mentholhaltiger Salben. Munch. Med.Wschr. 106: 1218-1219Boyd, E. M., Sheppard, E. P. (1969) A bronchomucotropic action inrabbits from inhaled menthol and thymol. Arch. Int. Pharmaco-dyn. Ther. 182: 206-214Braun, H. A., Bade, H., Hensel, H. (1980) Static and dynamicdischarge patterns of bursting cold fibres related to hypotheticalreceptor mechanisms. Mugers Arch. 386: 1-9British National Formulary (1974-1976) British Medical Associa-tion, London, p. 159Burrow, A., Eccles, R., Jones, A. S . (1983) The effects of camphor,eucalyptus and menthol vapour on nasal resistance to airflow andnasal sensation. Acta Otolaryngol. 96: 157- 161Butler, D. B., Ivy, A. C. (1943) Effects ofnasal inhalers on erectile issuesof the nose. Archiv. Otolaryngol. Head Neck Surg. 38: 309-317Clarke, R. W., Jones, A. S., Charters, P., Sherman, I. (1992) Therole of mucosal receptors in the nasal sensation of airflow. Clin.Otolaryngol. 17: 383-387Cohen, B. C., Dressler, W. (1982) Acute aromatics inhalationmodifies the airways. Effects of the common cold. RespirationCommetto-Muniz, J. E., Cain, W. S. (1990) Thresholds for odorand nasal pungency. Physiol. Behav. 48: 719-726Das, P. K., Rathor, R. S., Sinha, P. S., San Yal, A. K. (1970) Effecton ciliary movements of some agents which come in contact withthe respiratory tract. Ind. J. Physiol. Pharmacol. 1 4 297-303Davies, A. M., Eccles, R. (1985) The effects of nasal airflow andmenthol on the electromyographic activity of the anaesthetisedcat. Br. J. Pharmacol. 85: 254PDavies, A. M., Eccles, R. (1987) Electromyographic responses of anasal muscle to stimulation of the nasal vestibule in the cat. J.Physiol. 391: 25-38De Cort, S. C., Rowe, J. S., Eccles, R. (1993) Cardiorespiratory effectsof inhalation of L-menthol in healthy humans. J. Physiol. 473: 47PDelaplane, K. S. (1992) Controlling tracheal mites (Acari: Tarso-nemidae) in colonies of honey bees (Hymenoptera: Apidae) withvegetable oil and menthol. J. Econ. Entom. 85: 2118-2124Dodt, E., Skouby, A. P., Zotterman, Y. (1953) The effect ofcholinergic substances on the discharge from thermal receptors.Acta Physiol. Scand. 28: 101-1 14

43: 285-293

Dorshenko, P. A., Kostyuk, P. G., Lukyanets, E. A. (1989)Blockade of calcium channels by menthol. Biol. Memb. 61: 42-50Dost, F . H. , Leiber, B. (eds) (1967) Menthol and Menthol-contain-ing External Remedies. Use, Mode of Effect and Tolerance inChildren. George Thieme Verlag, StuttgartDuff, S. R., Furgala, B. (1991) Some effects of menthol on honey-bee tracheal mite infestations in non migratory honey-bee colon-ies in Minnesota. Am. Bee J. 131: 315-317Eccles, R. (1990) Effects of menthol on nasal sensation of airflow.In: Green, B. G., Mason, J. R., Kare, M. R. (eds) ChemicalSenses, Volume 2, Irritation. Marcell Dekker Inc., New York, ppEccles, R., Tolley, N. S. (1987) The effect of a nasal airflow stimulusupon human alae nasi e.m.g. activity. J. Physiol. 394: 78PEccles, R., Lancashire, B., Tolley, N. S. (1987a) Experimentalstudies on nasal sensation of airflow. Acta Otolaryngol. 103:Eccles, R., Lancashire, B., Tolley, N. S. (1987b) The effects ofaromatics on inspiratory and expiratory nasal resistance toairflow. Clin. Otolaryngol. 12: 11-14Eccles, R., Griffiths, D. H., Newton, C. G., Tolley, N. S. (1988a)The effects of D and L isomers of menthol upon nasal sensation ofairflow. J. Laryngol. Otol. 102: 506-508Eccles, R., Griffiths, D. H., Newton, C. G., Tolley, N. S. (1988b)The effects of menthol isomers on nasal sensation of airflow. Clin.Otolaryngol. 13: 25-29Eccles, R., Morris, S. , Tolley, N . S. (19%) The effects of nasalanaesthesia upon nasal sensation of airflow. Acta Otolaryngol.Eccles, R., Lancashire, B., Tolley, N. S. (1989) The effect of L-menthol on electromyographic activity of the alae nasi muscle inman. J. Physiol. 412: 34PEccles, R., Jawad, M. S., Morris, S. (1990) The effects of oraladministration of (-)-menthol on nasal resistance to airflow andnasal sensation of airflow in subjects suffering from nasalcongestion associated with the common cold. J. Pharm. Pharma-col. 42: 652-654Eckert, R., Lux, H. D. (1976) A voltage sensitive persistent calciumconductance in neuronal somata of Helix. J . Physiol. 254: 129-152Evans, B. K., Heatley, R. V., James, K. C., Luscombe, D. K. (1975)Further studies on the correlation between biological activity andsolubility of some carminatives. J. Pharm. Pharmacol. 27(Suppl.): 66PFAOjWHO (1976) Evaluation of certain food additives. TwentiethReport of the Joint FAOjWHO Expert Committee on FoodAdditives, Tech. Rep. Ser. Wld. Hlth. Org. no. 599Fox, N. (1927) Effect of camphor, eucalypt01 and menthol on thevascular state of the mucous membrane. Archiv. Otolaryngol.Head Neck Surg. 6: 112-122Fuller, R. W. , Jackson, D. M. (1990) Physiology and treatment ofcough. Thorax 45: 425-430Glass, H. B., Bliss, R. (1939) The menthols. Drug Cosmet. Ind. 44:Goldsheider, A. (1886) Ueber specische Wirking des Menthols.Arch. Anat . Physiol Abt. Leipzig 555-558Goldstein, E., Cooper, A. D., Takington, B. (1976) Effect ofinhaling medication vapors from a colds preparation on murinepulmonary bacterial defense systems. J. Toxicol. Environ. Health2: 371-388Greco, P. J., Ende, J. (1992) Pruritus: a practical approach. J. Gen.Intern. Med. 7: 340-349Green, B. G. (1985) Menthol modulates oral sensations of warmthand cold. Physiol. Behav. 35: 427-434Green, B. G. (1986) Menthol inhibits the perception of warmth.Physiol. Behav. 38: 833-838Green, B. G. (1992) The sensory effects of I-menthol on human skin.Somatosens. Mot. Res. 9: 235-244Hawthorn, M. , Ferrante, J., Luchowski, E., Rutledge, A, , Wei, X .Y., Triggle, D. J (1988) The actions of peppermint oil andmenthol on calcium channel dependent processes in intestinal,neuronal and cardiac preparations. Aliment. Pharmacol. Ther. 2:

275-291

303-306

106: 152-155

289-291

101-118


12/13

629ENTHOL AND RELATED COOLING COMPOUNDSHaydon, P. G., Winlow, W., Holden, A. V. 1982) The effects ofmenthol on central neurons of the pond snail, Lymnaea stagnalis(L.) Comp. Biochem. Physiol. 73C: 95-100Hellekant, G. 1969) The effect of menthol o n taste receptors. ActaPhysiol. Scand. 76: 361-368Hensel, H. 1982) Thermal Sensations and Thermoreceptors inMan. Charles C. Thomas, Springfield, IllinoisHensel, H., Schafer, K. 1974) Effects of calcium on warm and co ldreceptors. Pflug ers Arch. 352: 87-90Hensel, H., Zotterman, Y. 1951) The effect of menthol on thethermoreceptors. Acta Physiol. Scand. 24: 27-34Herbert, J. R., Kabat, G . C. 1989) Menthol cigarette smoking andoesophageal cancer. Int. J . Epidemiol. 18: 37-44Hirschsohn, J., Maendl, H. 1922) Studien zur Dynamik derendovenosen Injektion bei Anwendung von Calcium. Wien.Arch. Inn. Med. 4: 379-414Hong, C.-Z., Shellock, F. G. 1991) Effects of a topically appliedcounterirritant (Eucalypromint) on cutaneous blood flow and o nskin and muscle temperatures. Am. J. Phys. Med. Rehabil. 70:Huber, G. , Broderick, A., Finder, E., Simmons, G., OConnell, D.

1 973) Impairment of intrapulmonary bacterial inactivationfollowing administration of a commonly used cold remedy.Chest 64: 397Jakab, G. J., Green, G. M. 1975) The effect of the vapors of acommonly used remedy for colds on pulmonary antibacterialdefenses. Chest 68: 389-390Javorka, K., Tomori, Z., Zavarska, L. 1980) Protective anddefensive airway reflexes in premature infants. Physiol. Boemo-

Jones, A. S., Crosher, R ., Wight, R. G., Lancer, J. M., B eckingham,E. 1987) The effect of local anaesthesia of the nasal vestibule onnasal sensation of airflow and n asal resistance. Clin. Otolaryngol.12: 46 1-464Jones, A. S., Wight, R. G., Durham, L. H . 1989) The distributionof thermoreceptors within the nasal cavity. Clin. Otolaryngol. 14:Kaffenberger, R. M ., Doyle, M. J. 1990) Determination of mentholglucuronide in human urine by gas chromatography using an

enzyme sensitive internal standard and flame ionization detec-tion. J. Chromatogr. 527: 59-66Karlsson, J.-A., SantAmbrogio, G., Widdicombe, J. 1988) Affer-ent neural pathways in cough and reflex bronchoconstriction. J.Appl. Physiol. 65: 1007-1023Katayama, K., Takahashi, O., Matsui, R., Morigaki, S., Aiba, T.,Kakemi, M., Koizumi, T. 1992) Effect of 1-menthol on thepermeation of indomethacin, mannitol and cortisone throughexcised hairless mouse skin. Chem . Pharm . Bull. 40: 3097-3099Leicester, R. J., Hun t, R. H . 1982) Peppermint oil reduces colonicspasm during endoscopy. Lancet ii: 989Lindstrom, S. , Mazieres, L. 1991) Effect of menthol on the bladdercooling reflex in the cat. A cta Physiol. Scand. 141: 1-10Lundy, F., Contreras, R. J. 1993) Taste prestimulation increasesthe chorda tympani nerve response to menthol. Physiol. Behav.Macht, D. I. 1939) Comparative pharmacology of menthol and itsisomers. Arch. Int. Pharmacodyn. 63: 43-58Madyastha, K. M., Srivatsan, V. 1988) Studies on the metabolismof 1-menthol in rats. Dru g Me tab. Dispos. 16: 765-772Martindale. The Extra Pharmacopoeia 1989) 29th edn, Reynolds,J. E. F. (ed.), Pharmaceutical Press, London, p. 1586Mathew, 0 P., Anderson, J. W., SantAmbrogio, F. B., SantAm-brogio, G. 1990) Cooling mediates the ventilatory depressionassociated with airflow through the larynx. Respir. Physiol. 82:McBride, B., Whitelaw, W: A. 1981) A physiological stimulus toupper airway receptors in humans. J. Appl. Physiol. 51: 1189-1197Moleyar, V., Narasimham, P. 1992) Antibacterial activity ofessential oil components. In t. J. F ood Microbiol. 16: 337-342Moll, H. 1964) Gefharen mentolhaltiger Praparate im Kindesalter.

Fortschr. Med. 82: 861Morimoto, Y., Sugibayashi, K., Koboyashi, D., Shoji, H., Yama-zaki, J. I., Kimura, M. 1993) A new enhancer-coenhancer system

29-33

S ~ O V .9: 29-35

235-239

5 4 65-70

359-368

to increase skin permeation of morphine hydrochloride in vitro.Int. J. Pharm. 91: 9-14Murphy, C. 1983) Age-related effects on the threshold, psycho-physical function and pleasantness of menthol. J. Gerontol. 38:Murthy, P. B. K., Ahmed, M. M., Regu, K. 1991) Lack ofgenotoxicity of menthol in chromosome aberration and sister

chromatid exchange assays using human lymphocytes in vitro.Toxicol. in Vitro 5: 337-340Naito, K., Ohaka, E., Kato, R., Kondo, Y. , Iwata, S. 1991) Theeffect of L-menthol stimulation of the major palatine nerve onnasal patency. Auris Nasus Larynx 18: 221-226Nakaoka, M. 1 990) Kinetic characteristics of UDP-glucuronyltransferases toward a dithiol metabolite of malotilate in hepaticmicrosomes of rats and rabbits. Xenobiotica 20: 619-628Naus, A. 1983) Alterations o f the smell acuity caused by mentho l.J. Laryngol. Otol. 97: 705-709Oran i, G. P., Anderson , J. W., SantAmbrogio, G., SantAm brogio,F. B. 1991) Upper airway cooling and L-menthol reduceventilation in the guinea pig. J. Appl. Physiol. 70: 2080-2086Oswald, N. C. 1959) Cough mixtures. Br. Med. J. 1: 292Plotkowski, M. C., Bajolet-Laudinat, O., Puchelle, E. 1993)Cellular and molecular mechanisms of bacterial adhesion torespiratory mucosa. Eur. Respir. J. 6: 903-916Poetsch, C. E. 1967) Brief history of topical rub th erapy. In: D ost,F. H., Leiber, B. (eds) Menthol and M enthol-containing ExternalRemedies. Thieme, StutgartPotter, F. H . 1890) The use of menthol in diseases of the upper a irpassages. J. Am. Med. Ass. 14: 147-149Rakieten, N., Rakieten, M. L., Boykin, M. 1954) Effects ofmenthol vapour on the intact animal with special reference tothe upper respiratory tract. J. Am. Pharm. Assoc. 43: 390-392Rees, W. D. W., Evans, B. K., Rhodes, J. 1979) Treating irrita-ble bowel syndrome with peppermint oil. Br. Med. J. 2: 835-836Reis, C. C. A., B ertini, E. 1984) Mentho l effects on con tractions inguinea pig isolated taenia coli elicited by histamin e and acetylcho-line. Rev. Cienc. Farm. 5: 155-166SantAmbrogio, F. B., Anderson, J. W., SantAmbrogio, G. 1991)Effect of L-menthol on laryngeal receptors. J. Appl. Physiol. 70:

SantAmbrogio, F. B., Anderson, J W., SantAmbrogio, G . 1992)Menthol in the upper airway depresses ventilation in newborndogs. Resp. Physiol. 89: 299-307Schafer, K., Braun, H. A. 1992) Modulation of cutaneous coldreceptor function by electrolytes, hormones and thermal adapta-tion. Physiol. Res. 41: 71-75Schafer, K., Braun, H. A., Hensel, H. 1982) Static and dynamicactivity of cold receptors a t various calcium levels. J Neurophy-siol. 47: 1017-1028Schafer, K., Braun, H. A., Isenberg , C. 1986) Effect of menthol oncold receptor activity. J. Gen. Physiol. 88: 757-776Schafer, K., Braun, H. A,, Rempe, L. 1991) Discharge patternanalysis suggests existence of a low-threshold calcium channel incold receptors. Experientia 47: 47-50Sidell, N., Verity, M. A., Nard, E. P. 1990) Menthol blocksdihydropyridine-insensitivecalcium channels a nd induces neur-ite outgrowth in human neuroblastomacells. J. Cell. Physiol. 142:Sidney, S., Tekawa, I., Friedman, G. D. 1989) Mentholatedcigarette use among multiphasic examinees. Am. J. Public.Health 79: 1415-1416Silver, W. L. 1990) Physiological factors in nasal trigeminalchemoreception. In: Green, B. G., Mason, J R., Kare, M. R.(eds) Chemical Senses, Volum e 2, Irritation. Marcell Dekker Inc.,New York, pp 21-41Silver, W. L. 1992) Neural and pharmacological basis for nasalirritation. Ann. NY Acad. Sci. 641: 152-163Simon, S. A., Sostman, A. L. 1991) Electrophysiological responsesto non-electrolytes in lingual nerve of rat and in lingual epitheliaof dog. Arch. Oral Biol. 36: 805-814Skouby, A. P., Zilstofl-Pedersen, K. 1954) The influence ofacetylcholine-like substances, menthol and strych nine on olfac-tory receptors in man. Acta Physiol. Scand. 32: 252-258

2 17-222

788-793

410-419


13/13

630 R. ECCLESSloan, A., De Cort, S. C., Eccles, R. (1993) Prolongation ofbreathold time following treatment with an I-menthol lozenge.J. Physiol. 473: 53PSomerville, K. W., Richmond, C. R., Bell, G. D. (1984) Delayedrelease peppermint oil capsules (Colpermin) for the spastic colonsyndrome: a pharmacokinetic study. Br. J Pharmacol. 18: 638-640Su, X ., Li Wan Po, A,, Millership, J S. (1993) Ciliotoxicity of

intranasal formulations: menthol enantiomers. Chirality 5: 58-60Swandulla, D., Schafer, K., Lux, H. D. (1986) Calcium channelcurrent inactivation is selectively modulated by menthol. Neu-rosci. Lett. 68: 23-28Swandulla, D., Carbone, E., Schafer, K., Lux, H. D. (1987) Effect ofmenthol on two types of calcium currents in cultured sensoryneurons of vertebrates. Hugers Arch. 409: 52-59Takasago Co. (1993) Material safety data sheet for coolant agent10. Takasago International Corporation, New Jersey, USATaylor, B. A., Collins, P., Luscombe, D. K., Duthie, H. L. (1984)The mechanism of the inhibitory action of menthol on gutsmooth muscle. Br. J. Surg. 71: 902Taylor, B. A,, Duthie, H. L., Luscombe, D. K. (1985) Calciumantagonist activity of menthol on gastrointestinal muscle. Br. J.Clin. Pharmacol. 20: 293-294

Thorup, I., Wurtzen, G., Carstensen, J., Olsen, P. (1983a) Shortterm toxicity study in rats dosed with peppermint oil. Toxicol.Lett. 19: 211-215Thorup, I., Wurtzen, G., Carstensen, J., Olsen, P. (1983b) Shortterm toxicity study in rats dosed with pulegone and menthol.Toxicol. Lett. 19: 207-210Watson, H. R., Hems, R., Rowsell, D. G. Spring, D. J. (1978) Newcompounds with the menthol cooling effect. J. SOC.Cosmet.Chem. 29: 185-200Wei, E. T. (1983) AG-3-4: a chemical which produces sensations ofcold. Environment, Drugs and Thermoregulation. 5th Int. Symp.Pharmacol. Thermoregulation, Saint-Paulde-Vence 1982,Karger, Basel, pp 183-186Yano, T., Kanetake, T., Saita, M., Noda, K. (1991) Effects of1-menthol and dl-camphor on the penetration and hydrolysis ofmethyl salicylate in hairless mouse skin. J. Pharmacobiodyn. 14:

Zanker, K. S., Tolle, W., Blumel, G. , Probst, J. (1980) Evaluation ofsurfactant-like effects of commonly used remedies for colds.Respiration 39: 150- 157

663-669

menthol review

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