Neuropharmacology Vol. 26, No. 213, pp. 11 l-l 13, 1987 Printed in Great Britain. All rights reserved

W28-39-08/87 $3.00 + 0.00 Copyright 0 1987 Pergamon Journals Ltd

EFFECT OF PROLACTIN ON TOLERANCE AND DEPENDENCE TO ACUTE ADMINISTRATION

OF MORPHINE

S. RAMASWAMY and J. S. BAPNA

Department of Pharmacology, Jawaharlal Institute of Postgraduate Medical Education and Research, Pondicherry-605 006, India

(Accepted’20 March 1986)

Summary-The effect of exogenous prolactin on the development of acute tolerance to and dependence on morphine was studied. As described elsewhere, analgesia induced by prolactin was subject to tolerance. Pretreatment with prolactin enhanced the development of tolerance to analgesia induced by morphine while it effectively suppressed the withdrawal symptoms. These results indicate that prolactin behaves like an opioid agonist and suggest that it may have therapeutic implications in the management of opioid dependence.

Key words: prolactin, morphine, analgesia, tolerance, dependence.

Hyperprolactinaemia in rats was found to induce an opioid mediated reduction in responsiveness to elec- trically induced foot-shock (Drago, Gispen and Bohus, 1981). Recently, Ramaswamy, Pillai and

Bapna (1983) reported that the exogenous adminis- tration of prolactin in mice produced a dose- dependent, analgesic effect in chemical assay. This effect was attenuated by naloxone; acute tolerance developed to this action of prolactin and there was a cross tolerance between prolactin and morphine (Ramaswamy, Viswanathan and Bapna, 1985). These findings suggest that prolactin essentially involves opioid mechanisms for inducing analgesia, resem- bling that of morphine. However, Drago, Kovacs and Scapagnini (1984), in their study on hyper- prolactinaemic rats, reported that tolerance to morphine-induced analgesia did not develop in these rats. They explained this phenomenon on the poten- tiating effect of prolactin on the action of the mor- phine. The present study investigated the influence of exogenous prolactin on analgesia induced by acute administration on morphine and on the development of acute tolerance to and dependence on morphine.

METHODS

Swiss male albino mice (20-25 g) were used. Saline- treated animals served as controls.

Analgesic assay

The analgesia was studied by injecting acetic acid (0.6%, lOml/kg, i.p.) and observing the number of writhings for a period of 10 min. The reduction in the number of writhings was considered to be an analge- sic effect.

The acute pattern of tolerance to the analgesic

effect of morphine (1 mg/kg, s.c.) or prolactin (0.04 mg/kg, i.p.) was studied by exposing different groups of animals to the agonists at 9, 13 and 17 hr on the same day. The analgesia was tested as de- scribed above 15 min after each injection. Each ani-

mal was subjected to an analgesic test once only. A significant reduction in the degree of analgesia, as compared to that observed after the first injection, was considered as development of tolerance.

The influence of prolactin on acute tolerance to

morphine was studied by pretreating another group of animals with prolactin (O.O4mg/kg, i.p.) just be- fore every injection of morphine as described in the above schedule for acute tolerance. The analgesia was determined 15 min after each injection of morphine.

Acute dependence on morphine was induced as

described by Pillai, Ramaswamy, Gopalakrishnan and Ghosh (1982). The animals were treated with a single injection of morphine sulphate (32 mg/kg s.c.; Govt. Opium and Alkaloid Works, Ghazipur). The withdrawal syndrome was precipitated by the admin- istration of naloxone hydrochloride (16 mg/kg, i.p. Endo Labs) at 30 min after an injection of morphine to record wet shakes and, at 120min, to record jumping. These optimum time intervals were chosen following the report of Pillai er al. (1982). After the injection of naloxone the mouse was placed on a raised circular platform (30 cm high, 12 cm dia) and observed for a period of 20 min. As soon as the animal jumped, it was placed back on the platform for further observation. The number of jumps or wet shakes for each animal was recorded.

In another group of animals, pretreatment with

prolactin (O.O4mg/kg) was given 30 min before an

112 S. RAMASWAMY and J. S. BAPNA

Table 1. Influence of prolactin (PRL) on acute tolerance to analgesia induced by morphine

Number of writhings Pretreatment Treatment .--.-

(mg/kg) (mg/kg) 1st injection 2nd injection 3rd injection

Saline Saline 22.8 * I .4 23.5 f 1.9 21.9+ 1.8 Satine Morphine 1 7.3 * 0.5** 9.6 & 1 P* 17.0 f. 1.3*t Saline PRL 0.04 11.2 f 0.9** 16.2 + 1.4*‘t 21.4+0.8? PRL 0.04 Morphine 1 2.1 + 0.7**ff _ 15.4+ 1.4*t Zl.Ok 1.7t The values represent mean rt SEM of 6 experiments. *P < 0.05 and **P < 0.01 as compared with their corresponding saline value. +P ~0.05 as compared with the value for 1st injection. $P ~0.05 as compared with value for 1st injection of morphine.

injection of naloxone and the withdrawal symptoms were recorded as described above.

The results were analysed using Dunnet’s t-test. For dependence studies Mann-Whitney’s I.J test was employed.

The following drugs were used: prolactin (Ovine, NIH, Bethseda), morphine hydroxhloride (Govt. Opium and Alkaloid Works, Ghazipur) and na- ioxone hydrochloride (Endo Labs, New York).

RESULTS

The first injection of prolactin produced a significant reduction in the number of writhings, thus indicating an analgesic effect. This effect decreased after the second injection, while the third injection did not produce any analgesia (Table 1). Similar results were observed for morphine; however, a mild but significant degree of analgesia was seen even after the third injection (Table f).

In animals pretreated with prolactin, a potentiated analgesic effect after the first injection of morphine was observed. However, in these animals, the second injection of morphine produced a reduced analgesic effect, as compared to that of morphine alone (Table 1). The third injection of morphine in these prolactin-treated animals failed to produce any anal- gesia (Table 1).

Pretreatment with prolactin suppressed the with- drawal syndrome precipitated by naloxone as shown by the reduced number of jumps and wet shakes (Table 2). The total number of animals showing these symptoms was also reduced.

Table 2. Effect of prolactin (PRL) on acute dependence on morphine 1

Mean withdrawal symptoms

Pretreatment’ Jumoine Wet shakes

Control PRL 0.04 mg/kg

6.8 (S/IO) 5.7 (7/l I) 2.9 (4/13)’ 0.2 (Z/12)”

‘Acute dependence was induced by the injection of morphine (32 mg/kg, s.c.). Wet shakes and jumping were precipitated by naloxone (16 mg/kg, i.p.) at 30 or 120 min, respectively after injection of morphine.

Ifhe pretreatment was given 30min before naloxone. The numbers in parenthesis indicate the number of

animals cxhibit~ng the s~ptoms/num~r tested. *p eO.05 and **P <O.Ol compared 10 control

(Mann-Whitney U test).

DISCUSSION

The present data confirms earlier observations that acute tolerance developed to the analgesic effect of morphine as well as prolactin (Ramaswamy, Pillai, Gopalakrishnan and Ghosh, 1983b; Ramaswamy et ai., 1985). In the acetic acid_ test, pretreatment with prolactin potentiated the acute analgesic effect of morphine on the first exposure. This finding con- forms with that of Drago et al. (1984) who used the thermal assay in hyperprolactinaemic rats. However, there was no potentiation on subsequent treatment with these drugs.

The development of acute tolerance to morphine- induced analgesia was gradual and partial, since a mild degree of analgesia was observed even after the third injection. However, in animals pretreated with prolactin the analgesic effect of morphine was re- duced to a greater extent from a potentiated level after the second injection, and the effect was absent after the third injection. This indicates that prolactin enhanced the development of tolerance to morphine which was complete in animals pretreated with pro- lactin. This observation is in contrast to the findings of Drago et al. (1984) that higher levels of endo- genous prolactin prevented the development of toler- ance to morphine-induced analgesia in rats. They explained this antagonism by way of the potentiating effect of prolactin on morphine-induced analgesia. In the present study though, a significant potentiation of acute morphine-induced analgesia by pretreatment with proiactin was observed, there was an enhanced development of tolerance to mo~hine-indu~d anal- gesia. The possibility that differences in species of animal and assay procedures employed may con- tribute to the varied results cannot be ruled out. However, Drago and Scapagnini (1985), in their subsequent study on hyperprolactinaemic rats, ob- served a definite facilitation of the development of tolerance to morphine which is in accordance with the results of this study.

The determination of wet shakes and jumping behaviour was considered to be a reliable parameter to quantify dependence. These symptoms can be precipitated by naloxone or by withdrawal of the drug (abrupt withdrawal). The present study reveals that pretreatment with prolactin suppressed the with- drawal symptoms.

Prolactin and morphine tolerance 113

Prolactin exhibits several opioid-like actions, such

as analgesia, development of tolerance, presence of cross tolerance and antagonism to naloxone (Rama-

swamy et al., 1985). Considering these actions, it can be stated that prolactin behaves like an opioid ago- nist. Generally, a combination of an opioid-like drug with morphine will enhance the analgesic activity and development of tolerance to the latter. Since prolactin produced opioid-like effects, the enhanced analgesia and development of tolerance to morphine by pro-

lactin can be explained as discussed above. Similarly, an opioid-like compound, administered during or prior to withdrawal from morphine, will suppress the withdrawal symptoms. Pretreatment with prolactin suppressed the withdrawal symptoms, confirming a morphine like activity for prolactin. Furthermore, it has previously been suggested that prolactin may be physiologically involved in the development of toler- ance to morphine (Wimersna Greidanus, Tjon Kon Fat-Bronstein and Ree, 1978). The present results support this view and, in addition, suggest a role for prolactin in the development of physical dependence. The suppression of withdrawal symptoms by pro- latin, recorded in this study, and the earlier observed inhibition of heroin self-administration by prolactin (Drag0 et al., 1984), favour the suggestion that prolactin may have clinical significance in the man-

agement of opioid dependence and its use is worth considering.

REFERENCES

Drago F., Gispen W. H. and Bohus B. (1981) Behavioral effects of prolactin: Involvement of opioid receptors. In: Advances in Endogenous and Exogenous Opioids. Proc. Int. Narcotic Research Conf., Japan, pp. 335-337. Kodansha, Tokyo.

Drago F., Kovacs G. L. and Scapagnini U. (1984) Prolactin induced behavioral effects and opioids. In: Opioid., Mod- ulation of Endocrine Functions (Delitala G., Cotta M. and Sbrio M.. Eds) DD. 137-145. Raven Press. New York.

Drago F. and Scapagnini U. (1985) Effects of endogenous hyperprolactinaemia on opioid induced behavioral changes in rats. Brain Res. 336: 215-221.

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Ramaswamy S., Pillai N. P. and Bapna J. S. (1983a) Analgesic effect of prolactin: Possible mechanism of action Eur. J. Pharmac. 96: 171L173.

Ramaswamy S., Pillai N. P., Gopalakrishnan V. and Ghosh M. N. (1983b) The development of acute tolerance to analgesia and the sensitivity changes in mouse vas defer- ens and ileum produced by morphine. Archs inr. Pharma- codyn. Thtr. 261: 214222.

Ramaswamy S., Viswanathan S. and Bapna J. S. (1985). Prolactin analgesia tolerance and cross-tolerance with morphine. Eur. J. Pharmac. 112: 123-125.

Van Wimersma Greidanus Tj. B., Tjon Kon Fat-Bronstein H. and van Ree J. M. (1978) Antisera to pituitary hormones modulate development of tolerance to mor- phine. In: Characteristics and Functions of Opioids (Van Rec. J. M. and Terenius L., Eds), pp. 73-74. Elsevier/ North Holland, Amsterdam.