ABBREVIATiONS:SHR, spontaneously hypertensive rat; WKY,Wistar Kyoto rat; NPA, N-propylnorapomorphine; 8-OH-DPAT, 8-hydroxydipropylaminotetralin;7-OH-DPAT,7-hydroxy-dipropylaminotetralin;NTS,nucleustractussolitarlus;PO-AVP,Pmp1,O-Me-Tyr@-[kg@J-vasopressin;ANF,atnalnatriureticfactor MAP,meanarterialpressure;SED,standarderrorofthe difference;ANOVA,analysisof variance;AUC,areaunderthecurve;HA,heartrate.

0022-3565/96/2771-0473$03.OWOTHEJowuw@ or PHARMAcOLOGYANDEXPERIMENTALTrnaApnmcsCopyright 0 1996 by The American Society for Pharmacology and Experimental TherapeuticsJpE,r 277:473—483,1996

ProlongedCentralEffectsof Quinpiroleon CardiovascularRegulation

MMRTEN VAN DEN BUUSE, SARAH J. MORTON, JENNIFER L CORNISH and GEOFFREY A. HEADBaker Medical Research Instftute, Commercial Road, P@ahran,Wctoria, Aust,@Jia

Accepted for publication December 18, 1995

Vol.277, No. 1Printed in U.S.A.

ABSTRACTCentral cardiovascular effects of the dopamine D2 receptoragonist quinpirolewere studied in consciousrats. The i.v. injection of 0.3 mg/kg of quinpirole in spontaneously hypertensive rats (SHR) caused a rapid but short-lastingincrease inblood pressure. Heart rate showed little change. Pretreatmentwith the centrally acting selective dopamine D2 receptor antagonist raclopnde, but not the D1 antagonistSCH23390, completelypreventedthe rise in bloodpressure.A second injectionof quinpirole, 30 mm after the first injection, induced littlechange in blood pressure, although at 4 or 24 hr after quinpiroletreatment, we observed partial and complete recovery of thepressor response, respectively. This pattern of desensitizationwas similar to that seen after administration of the dopamine D2receptor agonists N-propylnorapomorphine (0.3 mg/kg) orquinelorane (0.1 mg/kg), and was similar in spontaneously hypertensive rats, Wistar Kyoto and Sprague-Dawley rats. At 30mmaftertreatmentwithquinpirole,the hypotensioninducedbyi.v. injectionof clonidine(0.01 mg/kg) or of 8-hydroxy-dipropylaminotetralin(0.1 mg/kg)was markedlyreducedwhen corn

pared to that in saline-pretreated spontaneously hypertensiverats, suggesting a prolonged effect of quinpirole at the level ofsympathetic regulation. The rapid fall in blood pressure causedby i.v. injectionof the ganglionblockerpentolinium(10 mg/kg)was slightly,but significantlyenhancedby treatmentwith quinpirole,which suggestsan overallprolongedincreasein restingsympathetic vasomotor tone. This would be difficult to reconcile with an inhibitionof the action of sympatholytic drugs,unless it is hypothesized that the increase in sympathetic Vasomotor tone was differential between different sympatheticbeds or different neuronal populations in the brain. This mayprohibitany additionalpressorresponsesand, througha central feedback mechanism,may inhibitthe action of sympatholytic drugs. No evidence was found for lasting changes incirculatinglevelsof vasopressin,angiotensinor atnalnatriureticfactor, nor were there changes in hematocrit.Cardiac sympathetic tone appearedto be enhanced,althoughvagal tone wasnormal and no major changes in baroreflex sensitivity wereobserved.

Several studies have suggested an important role of central dopamine systems in cardiovascular control and thedevelopment of hypertension (Van den Buuse and De Jong,1992). Spontaneously hypertensive rats, which develop anage-dependent and genetically determined hypertension, display a range of changes in behavioral, endocrine and cardiovascular responses to administration of dopaminergic compounds (Fuller et at., 1983; Van den Buuse and De Jong,1989; Van den Buuse and De Jong, 1992; Van den Buuse etat. , 1992). Stimulation of endogenous release of dopamine inthe brain of consciousrats causeda pressorresponse,whichwas mediated by the release of vasopressin into the circulation (Cornish and Van den Buuse, 1994; Cornish and Van denBuuse, 1995).

Quinpirole [LY 171555 (Tsuruta et at. , 1981)] is an ergoline

Received for publication July 24, 1995.

analogue that is widely used in studies on the role of centraldopamine systems in behavior (Fuller et a!., 1983; Koller etat., 1987; Eilam and Szechtman, 1989; Wan and Swerdlow,1993; Szechtman et at., 1994). Quinpirole was shown to havehigh selectivity for the dopamine D2 receptor group (Ruffoloand Shaar, 1983; Wong et a!., 1983; Cohen et at., 1984;Seeman and Schaus, 1991). The administration of quinpiroleto anesthetized rats and dogs induced a decrease in bloodpressure (Hahn and MacDonald, 1984; Nagahama et at.,1986a), which was most likely mediated by a reduction ofsympathetic vasomotor tone via dopamine D2 receptors located on sympathetic nerve endings (Clark, 1990). In contrast, i.v. injection of quinpirole in conscious rats caused animmediate pressor response (Nagahama et a!., 1986b; Vanden Buuse, 1992), which was mediated partly by an increasein sympathetic vasomotortone and partly by a rise in plasmavasopressin concentrations (Nagahama et at. , 1986b; Naga

473

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

474 vanden Buuseet al. Vol. 277

hama et at. , 1987). The pressor response to quinpirole couldbe prevented by pretreatment with the centrally acting dopamine D2 receptor antagonists, but was enhanced by theperipherally acting D2 antagonist domperidone (Nagahamaet at. , 1986b; Van den Buuse, 1992). The central action ofquinpirole on blood pressure was most likely mediated by aneffect in the NTS in the medulla oblongata (Yang et at. , 1990),and was mimicked by other centrally acting dopamine D2receptor agonists such as apomorphine, (+)-3-PPP, NPA,7-OH-DPAT, quinelorane and pergolide (Van den Buuse,1992; Van den Buuse, 1993; Van den Buuse, 1995). Such apressor response could functionally antagonize any potentialantthypertensive actions of these dopaminergic compounds(Clark, 1990) and could pose a serious side-effect during theirclinical use for central disorders such as Parkinson's disease(McNay et at. , 1987). However, upon continuous systemicadministration of quinpirole in conscious rats, the pressorresponse was short-lived and reversed into a depressor response (Igarashi et al. , 1987). Moreover, 30 miii after a bolusinjection of quinpirole, when the initial pressor response haddissipated, a further injection of the compound had no effecton blood pressure (Van den Buuse, 1992). Partial recovery ofthe pressor response was observed at 2 and 6 hr after theinitial treatment, whereas only after 24 hr the pressor response was completely restored (Van den Buuse, 1992).These observations suggested that the central pressor actionof quinpirole was short-lived and subject to rapid and longlasting desensitization, an effect that might involve at leastthree possible mechanisms. First, it is possible that quinpirole was rapidly metabolized or cleared from the circulation.However, pharmacokinetic studies on quinelorane, a dopamine agonist closely related to quinpirole, have shown thatthis drug remained present in the circulation for severalhours after administration (Franklin et at. , 1994). Drug metabolism studies on pergolide, another dopamine agonistclosely related to quinpirole, showed that several of the primary metabolites of this compound had dopamine receptoragonist properties similar to those of the parent compound(Clemens et al. , 1993), making it unlikely that catabolismand inactivation of quinpirole could explain the short-lastingchanges in blood pressure caused by quinpirole adininistration. Moreover, such mechanisms do not explain the apparent desensitization.Another possibilityis that the dopamineD2 receptors involved had desensitized. This seems unlikely,because such rapid and complete receptor desensitizationhas not been described before for dopamine D2 receptors(Barton et at. , 1991) and the behavioral actionsof quinpiroleand related compounds are maintained for several hours(Van den Buuse, 1992; Van den Buuse, 1995). A third possibility to explain the apparent desensitization to quinpirole isthat this effect was specific for the central cardiovascularaction of this compound. Thus, it could be that the pressorresponse dissipated due to compensation by other cardiovascular mechanisms. In this case, although blood pressurewould return to base line, quinpirole was still present in thecirculation and activating its physiological effector mechanisms, which, on additional administration of quinpirolewere unable to increase their response any further. However,the identity of the neural or hormonal mechanism that compensated for the initial quinpirole-induced pressorresponseis as yet unknown.

The aim ofour study was to further investigate the central

pressoraction of quinpirole. Becausethe apparent desensitization to quinpirole in SHR could have been unique to thiscompound or unique to this rat strain, we compared thisphenomenon with desensitization to quinelorane and NPA inSHR and with desensitization to quinpirole in WKY andSprague-Dawley rats. To determine the functional significance ofthe apparent desensitization and whether functional“¿�cross-desensitization―had occurred, the influence of pretreatment with quinpirole on the antthypertensive action ofthe sympatholyticdrugsclonidineand 8-OH-DPAT was studied. Possible effector mechanisms involved in the effect ofquinpirole and its desensitization, including sympathetictone, were studied at 30 miii after administration, whenblood pressure had returned to base line and desensitizationwas complete.

MethodsRats, surgery and general protocoi. Male SHR, WKY and

Sprague-Dawley rats were obtained from the Baker Medical Research Institute breeding stock. Except where indicated otherwise,all experiments were performed in conscious SHR, because rats fromthis strain were found to display less locomotor hyperactivity inresponse to treatment with quinpirole (Van den Buuse, 1992). Therats were between 250 and 300 g body weight and were kept three tofive per cage with free access to pellet food and tap water.

Surgery was performed at least 1 wk before the experiments. Therats were anesthetized with an intraperitoneal mixture of pentobarbital (Nembutal, 30 mg/kg), methohexital (Brietal, 40 mg/kg) andatropine sulfate (0.5 mg/kg). A midline incision was made in theabdominal skin and muscle wall and the abdominal aorta was exposed. The aortic cannula consisted ofa 60-cm length ofvinyl tubing(SV-40, Dural Plastics, Australia) with a 2.5 cm length of teflontubing (S@[1@30, Small Parts, Miami, FL) inserted. The teflon tip ofthis cannula was cut back to approximately 3 to 4 mm and insertedinto the aorta while it was clamped off with nontraumatic vascularclamps. The cannula was fixed to the vessel with a drop oftissue glue(Loctite, Caringbah, NSW, Australia). The other end of the cannulawas looped two to three times in the abdominal cavity before beingtunnelled under the skin, exteriorized in the neck of the rat andclosed with a pin. The vena cava was catheterized via the jugularvein with a 10-cm length of vinyl tubing (SV-40, Dural Plastics,Silverwater, NSW, Australia) through an incision lateral to thelarynx. This cannula was also tunneled under the skin, exteriorizedat the back of the neck, and ifiled with heparinized saline. Each ratwas given 4 to 5 ml of Hartman's solution i.p., after which allincisions were sutured, and it was allowed to recover in a warmrecovery box. Each rat was singly housed after surgery.

On the day of the experiments, the rats were taken to the experiment room,weighed and allowed to acclimatize for at least 30 mm.The experiments were performed while the rats were in their homecages. The arterial catheters were connected to Statham P23XLtransducers and an 8-channel Neomedix Systems Neotrace recorderor Grass model 7 polygraph. HR was derived off the blood pressurepulse by Baker Medical Research Institute tachographs. Both bloodpressure and HR were thgitized and recorded with a MetrabyteDAS-8 analog-digital card and a data-acquisition program (NEWAD,Baker Medical Research Institute).

Before the experiments, patency of the i.v. catheters was assessedby injection of a 0.1 mg/mI nitroprusside solution. During the experiments, a standard protocol was used (Van den Buuse, 1992), whichconsisted of 1) a 5-mm base-line period; 2) i.v. injection of saline (1mi/kg) or other pretreatments and a 10-mm reading of blood pressure and HR; 3) i.v. injection ofthe peripherally acting dopamine D2receptor antagonist domperidone (1 mg/kg) and a 10-mm reading ofblood pressure and HR; 4) i.v. injection of saline (1 mI/kg) or quin

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

1996 Quinpirole and Blood Pressure Control 475

pirole (0.3 mg/kg) followedby a 30-mm reading ofblood pressure andHR and 5) in a number of experiments: additional injections ofquinpirole or other compounds, induction of pressure ramps, i.v.injection of blocking agents or blood sampling (see below). Aftercompletion ofthe experiment, some rats were returned to the animalhouse and used for additional experiments at least 48 hr later. Thedose of quinpirole was chosen on the basis of previously performeddose-response experiments (Van den Buuse, 1992) while the dose ofother compoundswas based on previousexperimental and literaturedata as indicated where appropriate.

Effects of raclopride and SCH23390 on the pressor response to quinpirole. To determine dopamine receptor specificityin the effects of quinpirole on bloodpressure and the possible interactionofdopamineD1andD2receptors,wepretreatedSHRwith0.5mg/kg ofthe dopamine D2 receptor antagonist raclopride (Hall et al.,1988) or 0.1 mg/kgofthe D1antagonist SCH23390 (Iorioet al., 1983;Hyttel and Arnt, 1987) 20 mm before injection of quinpirole. Bloodpressure and HR were measured at 1, 2, 3, 4, 5, 10, 15, 20, 25 and 30mm after quinpirole treatment.

Comparison of quinpirole with quinelorane and NPA. Toverify whether the apparent desensitization to the presser action ofquinpirole (Van den Buuse, 1992) could be observed with otherdopaminergic compounds acting on D2 receptors, we tested the effects of administering quinelorane or NPA. Intravenous injection ofquinelorane, a compound structurally related to quinpirole (Foreman et al. , 1989), has been shown to induce a similar pressor response to that of quinpirole (Van den Buuse, 1995). Similarly, i.v.injection of NPA has been shown to induce a pressor response (Vanden Buuse, 1992). Quinpirole(0.3 mg/kg),quinelorane(0.1 mg/kg)orNPA (0.3 mg/kg) were injected i.v. at step 4 of the general protocol.The maximal increase in bloodpressure, usually at 1 to 2 mm afteradministration, was recorded. Each compound was administeredagain in the same rats 30 ruin, 4 or 24 hr after the first injection andthe maximal change in bloodpressure at 1 to 2 mm after treatmentwas recorded. Rats that were treated 4 or 24 hr after the initialagonist treatments, were given additional domperidone as in thestandard protocol.

Comparison of SHR with WKY and Sprague-Dawley rate.Quinpirole (0.3 mg/kg) was injected intravenously accordingto thestandard protocol, followed by an additional injection 30 miii, 4 or 24

hr later. MaXimal changes in blood pressure were taken at 1 to 2mizi after administration and compared between SHR, WKY andSprague-Dawley rats.

Cross-desensitization: the effect of quinpirole on the antihypertensive action of clonidine and 8-OH-DPAT. SHR wereadministered saline or quinpirole according to the standard protocol.At 30 nun after these treatments, the rats received injections i.v.with 0.01 mg/kgofthe a@adrenoceptoragonist clomdineor0.1 mg/kgof the 5-HT1Areceptoragonist 8-OH-DPAT.Blood pressure and HRwere monitored for 60 miii after these treatments.

Effector mechanisms involved in the desensitization toquinpirole. The activity ofa number ofneural and hormonal mechanisms in the apparent desensitization to quinpirole was determined30 mm after its administration. This time-point was chosen becauseblood pressure had returned to normal and desensitization wasfound to be complete (Van den Buuse, 1992).

We assessed sympathetic vasomotortone indirectly (Okunoet al.,1983; Jablonskis and Howe, 1993) by measuring the acute fall inblood pressure caused by i.v. injection of the ganglion blocker, pentolinium (10 mg/kg).

Vasopressinergic activity was assessed by measuring the acutechanges in blood pressure to i.v. injection of the vasopressin V1receptor antagonist PO-AVP (30 gig/kg). This dose ofthis antagonisthas been shown to block the 20 mm Hg pressor response to i.v.injection of 10 ng/kg of vasopreasin (Cornish and Van den Buuse,1995). It was anticipated that a raised level of plasma vasopressinconcentrations could be detected by an acute fall in blood pressure toadministration ofthe antagonist. In a limited number ofanimals, we

directly measured plasma concentrations of vasopressin by radioim

munoassay.We determined whether there were changes in the activity of the

remn-angiotensin system by measuring the acute fall in blood pressure to i.v. administrationofthe angiotensin AT1receptorantagonistlosartan (30 mg/kg) (Wong et al., 1990).

To measure plasma levels of ANF, we rapidly anesthetized twogroups of SHR by i.v. injection of pentobarbital and collected blood

from the abdominal aorta through a midline incision. The blood wascentrifuged for 30 mm at 2500 rpm and plasma was separated andfrozen for radioimmunoassay (Woods, 1988).

Blood hematocrit was determined in SHR, which were treatedwith saline or quinpirole i.v. (0.3 mg/kg). Small blood samples weretaken fromthe arterial catheter twice during each experiment, oncejust before the onset of blood pressure recordings, and again at 30

ruin after injection of saline or quinpirole.Quinpirole and HR controL To assess the effect ofquinpirole on

autonomicactivity to the heart, the SHR were injected i.v. at 30 mmafter saline- or quinpirole-treatment with the@ adrenoceptor an

tagonist atenolol (1 mg/kg) and, 5 mm later, with methylatropine (1mg/kg). The acute changes in HR were taken as an index of cardiac

sympathetic and cardiac vagal tone, respectively (Head and Adams,1988).

Reflex control of HR was investigated 30 mm after administration of either saline or qumnpirole.The baroreceptor-HRreflex wasanalyzed by the ramp technique, which included moderately fast(30 sec to 1 mm) ramp increases or decreases in blood pressureinduced by i.v. infusions of methoxamine (0.1 mg/mI) or sodiumnitroprusside (0.1 mg/mi), respectively, until a change in bloodpressure of 50 to 75 mm Hg was achieved and HR changes hadreached a plateau. MAP and HR data were averaged over 2-secperiods and fitted to a sigmoid logistic function by an iterativeleast squares method using a nonlinear regression equation: HR =P1 + [P2/i + ePSU@@@ P4)](Head and McCarty, 1987). In thisequation, P1 is the calculated lower HR plateau, P2 the HR range,P3 a range-independent curvature coefficient, and P4 the BP5O,i.e. , the MAP at half the HR range. The upper HR plateau can becalculated as P1 + P2 and the average baroreflex gain betweenthe two infliction points by G = -P2 x P3/4.56. The curve wasforced through the resting MAP and HR (Head and McCarty,1987). The curve fitting procedure was performed with a PC-basedcomputer program (RAMP, version 3.0, Baker Medical ResearchInstitute), which calculated baroreflex curve parameters for eachrat. The parameters were then group-averaged and used to plotgroup baroreflex curves.

Experimental drugs. The following compounds were purchasedfrom Research Biochemicals (Natick, MA): quinpirole HC1, domperidone, (R)-(-)-NPAHC1,SCH23390 HC1,8-OH-DPATHBr. PO-AVPwas purchased from Peninsula Laboratories (Belmont, CA). Raclopride tartrate was a gift from Astra, Sodertaije, Sweden. Quinelorane HC1was a gift fromLilly (Indianapolis, IN), and losartan was agift from Du Pont (Wilmington, DE). Other drugs were obtained fromSigma Chemical Co. (St. Louis, MO). The doses of the drugs were

expressed as their salts. All drugs were dissolved in saline beforeuse, except for SCH23390, which was dissolved in distilled water,and domperidone, which was first dissolved in a minimal amount ofHCIand diluted to the appropriateconcentration.

Data analysis. All data are expressed as mean values ±S.E.M. orSED. AUC was calculated using Prism, version 1.02 (GraphpadSoftware), and expressed are arbitraryunits (miii Hg x time units).Differences between groups were analyzed with ANOVA with re

peated measures where appropriate. Further between-group cornparisons were performed with Student-Newman Keuls test or pairedand unpaired Student's t tests. Differences were considered significant when P < .05.

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

476 vandenBuuseatal. Vol. 277

hance it (fig. 1). AUC for the three pretreatment groups was266 ±46, 25 ±11 and 325 ±43 U, respectively. Although theAUC for the raclopride-treated SHR was significantly lessthan that of saline-treated SHR, there was no significantdifference between saline- and SCH23390-pretreated SHR.

HR did not show significant changes in response to i.v.injection of quinpirole in saline-pretreated or raclopride-pretreated SHR. By contrast, in SCH23390-pretreated SHR, amoderate, but significant bradycardia developed after 5 mmafter injection of quinpirole, which persisted while bloodpressure was returning to base line (fig. 1).

Comparison of quinpirole with quinelorane andNPA. The i.v. injection of 0.3 mg/kg of quinpirole, 0.1 mg/kgof quinelorane or 0.3 mg/kg of NPA caused acute pressorresponses with a similar time-course, with the greatest effectproduced by quinelorane. Thus, blood pressure reached itsmaximum 1 to 2 mm after injection and returned to base linewithin 30 ruin after treatment. With all three compounds,asecond injection 30 mm after the first injection failed toproduceany prominent change in blood pressure (see fig. 2for maximum changes in blood pressure). Increasing thetime-interval between the two injections to 4 hr allowed somerecovery of the pressor response, although the increases inblood pressure were still significantly less than those afterthe initial treatments (fig. 2). When the interval betweeninjections was increased to 24 hr, the pressor responses toinjection of quinpirole or NPA were fully restored. However,the pressor response to injection of quinelorane was stillsignificantly reduced (fig. 2).

Comparison of SHR with WKY and Sprague-Dawleyrats. The i.v. injection of 0.3 mg/kg of quinpirole causedsimilar pressor responses in WKY and Sprague-Dawley ratsas in SHR, although the extent of the pressor responsetended to be smaller in WKY. In rats from all three strainsblood pressure reached its maximum 1 to 2 mm after injection and returned to base line within 30 mm after treatment.Similar to SHR (Van den Buuse, 1992), in WKY and SpragueDawley rats a secondinjection 30 mm after the first injectionof quinpirole failed to produce a pressor response (see fig. 3for maximum changes in blood pressure). Increasing thetime-interval between the two injections to 4 hr allowed somerecovery of the pressor response, although the quinpiroleinduced increases in blood pressure were still significantlysmaller than those after the initial treatments in SHR andSprague-Dawley rats (fig. 3). When the interval betweeninjections was increased to 24 hr, the pressor responses toinjection of quinpirole was fully restored in all three strains(fig. 3).

Cross-desensitization: the effect of quinpirole on theantihypertensive action of clonidine and 8-OH-DPAT.As the above experiments suggestedthat quinpirole treatment might cause a lasting increase in sympathetic vasomotor and cardiac tone despite a return to normal bloodpressure and HR, we examined the influence of pretreatmentwith quinpirole on the antthypertensive action of two sympatholytic drugs: 8-OH-DPAT and clonidine. In these experiments, pretreatment with quinpirole, but not saline, causeda pressor response of approximately 20 mm Hg (fig. 4) afterwhich blood pressure returned to base line. Subsequent injection of 8-OH-DPAT or clonidine caused a marked andsignificant fall in blood pressure and HR in saline-treatedSHR. The effect of 8-OH-DPAT on blood pressure and HR

Bloodpressure

Q

Heartrate

0 IÔ @o

Timeafterinjection(mm)

ResultsEffects of raclopride and SCH23390 on the pressor

response to quinpirole. The effect of i.v. injection of 0.3mg/kg of quinpirole on blood pressure and HR is shown infigure 1. Quinpirole treatment caused an immediate pressorresponse, with a maximum increase of nearly 30 mm Hg atapproximately 1 mm after injection, after which blood pressure gradually returned to base line. The i.v. pretreatmentwith 0.5 mg/kg of the dopamine D2 receptor antagonist raclopride caused a virtually complete abolition of the quinpiroleinduced pressor response (fig. 1). By contrast, pretreatmentwith 0.1 mg/kg of the D1 receptor antagonist SCH23390 didnot reduce the pressor response, but rather tended to en

40

30

IE 20E

@ 10

0

25

0

@-25

Pretreatment:—¿�.-- Saline

—¿�o.-.Raciopride—¿�@0--SCH23390

Fig. 1. The effect of pretreatmentwith saline(1 mVkg,n = 15),thedopamineD2receptorantagonistraclopnde(0.5mg/kg,n = 10),or theDl receptor antagonist SCH23390 (0.1 mg/kg, n = 16) on mean arterialpressure(MAP)andheartrate(HA)changesinducedby i.v. injectionofquinpirole (0.3 mg/kg). Base-line blood pressure values were 157 ±4,157 ±7 and 160 ±3 mm Hg, respectively. In saline-pretreated SHA,quinpiroleinduceda markedpressorresponse,withbloodpressuresignificantly increased compared to base line at 1 , 2, 3, 4, 5 and 10 mmafter injection(P < .05, One-wayrepeatedmeasuresANOVAandBonferroni corrected t test). In SHA pretreated with raclopnde, bloodpressure was slightly, but significantly elevated over base line only at 1mm after injection of quinpirole. In SHR pretreated with SCH23390,blood pressure was significantlyincrease over base lineat 1, 2. 3, 4, 5and10 mmafterinjectionofquinpirole.Base-lineHRvalueswere348±9, 320 ±8 and 356 ±9 bpm, respectively. There were no significantchangesin HRinducedby injectionof quinpirolein the SHApretreatedwith saline or raclopnde. In SHApretreated with SCH23390, quinpiroleinduced a significant bradycardia, with HR values significantly lowerthan baseline at 10, 15, 20, 25 and 30 mm after quinpirole treatment (P< .05, One-way ANOVA and Bonferroni corrected t test).

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

IQulnpirole 0.3 mglkg40@

@30] 1@

E2O@@@I I

@10 __

@ 01 •¿�;@;1

I@ 0.1màiii@I40,

@30jI@ 1@E 204 1@ L@E1

@:I@ioI

@ I

01

40i NPAO.3mgIkg@

@3gI i __Ii

< IT

•¿�* “¿�/A@20j _@:I@1o1 *0@

30mm 4hrs 24hrs

Interval between Injections

I—¿� letlnje@ion@ 2ndIr@ectIon@Fig. 3. Apparent desensftization to the presser effect of quinpirole inSHR,WKVand Sprague-Dawley(SD)rats.Data are meanmaximalchangeinbloodpressure±SEMinresponseto an Initialinjectionofeachdrug (filledbars)and to a secondinjectionof the samedrug 30mm, 4 or 24 hr later (hatched bars). Base-line blood pressure values inSHRwerel57 ±6and 154 ±7mmHg(n = 8), 175 ±6and 163 ±6mm Hg (ii = 7) and 161 ±6 and 157 ±4 mm Hg (n = 10), respectively.Base-linebloodpressurevaluesinWKYwere133±3 and126±4mmHg(n=8),131 ±Sandl3l ±4mmHg(n=8)and134±7and125±5 mm Hg (n = 8), respectively. Base-line blood pressure values InSp@gue-DawIeyratswere104±3andll2 ±4mmHg(n = 6),101±4andlO2 ±9mmHg(n = 6)and95 ±6and100±6mmHg(n = 5),respectively.@ < .05 for differencewith the correspondingcontrolpressor response (two-way ANOVA and Student's Newman-Keulstest).

dycardic action of 8-OH-DPAT, with no significant differences between the pretreatment groups for the percentagefall in HR or the AUC. After pretreatment with quinpirole,the hypotensive action of clonidine was virtually abolished(fig. 4). Thus, the maximal percentage fall in blood pressurein response to treatment with clonidine was reduced from-16.6 ±1.9% in SHR pretreated with saline to 0.6 ±1.2% inSHR pretreated with quinpirole and the AUC for theclonidine-induced hypotension was negligible in SHR pretreated with quinpirole. The bradycardia caused by treatment with clomdine was partially prevented, particularly inthe early stages after injection (fig. 4). Thus, at 15 mm, whenthe effect of clomdine was maximal in controls, the percent

I—¿� letir@on@ 2ndmnjecdonFig. 2. Apparent desensitization to the pressor effect of qumnpirole,quinelorane or NPA In SHR. Data are mean maximal change in meanarterial pressure (MAP) ±S.E.M. in response to an initial injection ofeachdrug (filledbars)and to a secondinjectionof the samedrug 30mm,4 or 24 hr later(hatchedbars).Base-linebloodpressurevaluesforthequinpiroleexperimentwere157 ±6 and154 ±7 mmHg(n = 8),175 ±6and 163 ±6mm Hgfri = 7)and 161 ±6and 157 ±4mm Hg(n = 10), respectively. Base-line blood pressure values for the quineloraneexperimentwere155±5and156±4 mmHg(n= 9),162±7 and167±7mmHg(n=8)and154±6and166±8mmHg(n =8),respectively.Base-lineblood pressurevaluesfor the NPAexperimentwerel57 ±4and144 ±3mmHg(n = 8), 166 ±5and164 ±5mmHg(n=8)andl4O±5and146±8mmHg(n=7),respectively.*P< .05 for difference wfth the corresponding control pressor response(two-wayANOVAandStudent'sNewman-Keulstest).

was maximal at 15 and 50 mm after administration, respectively. The effect of clonidine on blood pressure and HR wasmaximal at 15 mm after administration. After pretreatmentwith quinpirole the hypotensive action of 8-OH-DPAT wasmarkedly inhibited (fig. 4). Thus, the maximal percentagefall in blood pressure in responseto treatment with 8-OHDPAT was significantly reduced in SHR pretreated withquinpirole (-7.3 ±1.7%) when compared to SHR pretreatedwith saline (-17.3 ±2.6%). Similarly, the AUC for the 8-OHDPAT-induced hypotension was significantly smaller in SHRpretreated with quinpirole (377 ±77 U) when compared toSHR pretreated with saline (1085 ±202 U). By contrast,there was little effect of quinpirole pretreatment on the bra

1996 Quinpirole and Blood Pressure Control 477

SHRI40-i

@30J

@l _

I I

@20J __

@,10J *.@1@ •¿�@4 @1

‘¿�@ I

30,

I 20-1@

ITE IE I __@‘¿�io1 *

@0I•¿�@

40 ISDMteI

@3@1 __Ii __

< I ____

@ 20-i

@101 *01 /A

30mm 4hrs 24hrs

Interval between Injections

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

0 20 40 60Pretreatment—¿�0-—Saline—¿�-—Quinpirole

0 20 40 60

478 vandenBuuseetal. Vol.277

8-OH-DPAT Clonidine*

20

10

0

20

10

IE°

0

Bloodpressure

-20

-30

0

Heart rate

-7@

0 20 40 60Time(mm) Time(mm)

Fig. 4. Theeffectofpretreatmentwithsaline(whitebars,opencircles)orquinpirole(0.3mg/kg,blackbars,filledcircles)onthedepressoractionof i.v. administrationof 8-OH-DPAT (0.1 mg/kg, left panels)or clonidine(0.01 mg/kg, rightpanels)in consciousSHR. Top panelsshow bloodpressureresponses,bottompanelsshowHRresponses.Dataaremeanchangein MAP(L@MAP)or HR±S.E.M.Barsin eachpanelindicatethemaximumchangein bloodpressureor HR at 1 to 2 mmafter i.v. injectionof salineor quinpirole.Subsequenttime-coursesshowthe effectsof8-OH-DPATorclonidine,injectedat30mmafterquinpirole,whenbloodpressurehadreturnedtobaseline.Baselinebloodpressurevaluesbeforeeachtreatmentweresaline/8-OH-DPAT153 ±6 and153 ±7 mmHg,respectively(n= 7);quinpirole/8-OH-DPAT161±4 and160±5 mmHg(n = 8); saline/clonidine 161 ± 4 and 162 ± 3 mm Hg (n = 9); quinpirole/clonidine 161 ± 5 and 151 ± 5 mm Hg (n = 9). Base-line HR values were

saline/8-OH-DPAT328 ±13and339 ±19B/mm;quinpirole/8-OH-DPAT326 ±8 and315 ±10 B/mm;saline/clonidine330 ±8 and360 ±IiB/mm;quinpirole/clonidine335 ±12 and325 ±14 B/mm.@ < .05for differencebetweensaline-pretreatedSHRandquinpirole-pretreatedSHR.

age fall in HR was -4.1 ±2.2% in SHR pretreated withquinpirole compared to -13.9 ±2.7% in SHR pretreated withsaline (P < .05). Overall, however, the AUC for the clonidineinduced bradycardia was not significantly different in SHRpretreated with quinpirole (2401 ±532 U) when compared toSHR pretreated with quinpirole (1564 ±348 U).

Effector mechanisms involved in the desensitizationto quinpirole. Figure 5 and table 1 show the results ofexperiments that were aimed at measuring the activity of anumber of cardiovascularregulatory mechanismsin SHR at30 mm after administration of quinpirole, when blood pressure had returned to base line but desensitization to a subsequent administration was complete. In all these experiments, administration of quinpirole caused a significantpressor response (fig. 5 and table 1).

As an index of sympathetic vasomotortone, we measuredthe change in blood pressure caused by i.v. administration ofthe ganglion blocking agent, pentolinium. Intravenous injection of pentolinium (10 mg/kg) caused a rapid and profoundfall in blood pressure at 2 to 4 mm after administration. Thepentolinium-induced hypotension was greater in SHR thathad been previously treated with quinpirole compared toSHR that had been treated with saline (fig. 5). Thus, bloodpressure was significantly lower in SHR that were treatedwith quinpirole and pentolirnum when compared to SHR thatreceived saline and pentolinium (fig. 5). Similarly, the percentage decrease in blood pressure caused by injection of

pentoliium was significantly greater in quinpirole-treatedSHR compared to saline-treated SHR (fig. 5).

As an index of the contribution of circulating vasopressinand angiotensin to blood pressure in quinpirole-treated rats,we measured the changes in blood pressure caused by i.v.administration ofthe vasopressin V1 receptor antagonist P0-AYE'(30 p.g/kg i.v.) or the angiotensin AT1 antagonist losartan (30 mg/kg i.v.), respectively. In both experiments, quinpirole administration causedthe expectedpressor responsewith return to base-line blood pressure within 30 mm. Administration of PO-AVP at this time-point caused a smalldecrease in blood pressure in saline-treated, but not quinpirole-treated rats, resulting in a small, but significantly different percentage change in blood pressure caused by P0-AVP injection. However, absolute blood pressure values werenot significantly different between the groups (table 1). Administration oflosartan causeda moderate decreasein bloodpressure in both saline-treated and quinpirole-treated rats.There was no significant difference between the groups inblood pressure after losartan administration or in the percentage change in blood pressure caused by this treatment(table 1).

Blood was obtained from SHR that had been treated withquinpirole or saline 30 miii before. Although administration

of quinpirole caused the expected pressor response with return to base line within the 30-min period, plasma levels of

¶CE

@.-25

0 20 40 60

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

Effect of admInIstration of saline or qumnpirole(0.3 mg/kg I.v.) on changes In blood pressure Induced by administration of PO-AVP (30gig/kg l.v. or losartan (30 mg/kg I.v.) or on plasma ANF concentrations or hematocrft values In conscious SHR

-60*

1996

ANF were not significantly different between the groups(table 1).

Blood hematocrit was measured before and after the administration of saline or quinpirole. As in the other experiments, injection ofquinpirole caused a pressor response afterwhich blood pressure returned to baseline. Hematocrit val

TABLE1

Qulnpirole and Blood Pressure Control 479

ues were not different between the groups either before orafter treatment with saline or quinpirole (table 1).

Quinpirole and HR control. To further assess the effects0 of quinpirole treatment on autonomic function, we measured

the acute changesin HR following i.v. injection of atenolol (120 mg/kg) or methylatropine (1 mg/kg) as an index of cardiac

sympathetic and vagal tone, respectively.As before, in theseSHR quinpirole administration induced a significant pressor

-40 response of29 ±2 mm Hg, with little change in HR. Injectionof saline, atenolol or methylatropine did not cause any significant change in blood pressure (not shown). By contrast,injection of atenolol caused a marked and significant fall inHR, and this effect was greater in SHR pretreated withquinpirole than in those pretreated with saline. Thus, HRwas significantly lower in quinpirole-treated SHR than insaline-treated SHR after, but not before, injection of atenolol.Similarly, the percentage fall in HR caused by atenolol treatment was significantly greater in quinpirole-treated SHRthan in saline-treated SHR (fig. 6). Injection of methylatropine, subsequentto atenolol, causedan increase in HR thatwas not significantly different between quinpirole-pretreatedand saline-pretreated SHR (fig. 6).

Baroreceptor-HR reflex control was assessed by measuringHR changes in response to induced ramp increases and decreases in blood pressure 30 mm after treatment with salineor quinpirole (see “¿�Methods―).Quinpirole administrationcaused a significant pressor response of 31 ±3 mm Hghowever, base-line blood pressure before or 30 mm aftertreatments were not significantly different between thegroups (table 2). Similarly, there were no significant differences in HR at any time-point during the experiment (table2). Figure 7 shows the group baroreflex curves generatedfrom jndividual barorefiex curves from saline-treated andquinpirole-treated SHR. The upper HR plateau, i.e. , maximum HR increases caused by decreases in blood pressure,was slightly but significantly reduced in SHR treated withquinpirole as compared to controls. However, none of theother barorefiex parameters, including HR range and baroreflex gain, were significantly altered (fig. 7, table 2).

*Pentollnlum % thange in MAP30-

@ 0-IEE

.@

-60-

-90-

Time (mm)

-0-s@-•-a@ —¿�

FIg. 5. Left panel: The effect of treatment with pentolinium (10 mg/kgi.v.,Pent)onbloodpressureofSHRwhichwerepretreatedwithsaline(opencircles,n = 9)or wfthquinpirole(0.3mg/kgi.v.,solidcircles,n =9). RestIngMAPat the startof the experimentwas 150 ±4 and 148 ±6 mmHg,respectively.DataaremeanchangeinMAP(@MAP)±SEMat three time-points: 1 mm after pretreatment (maximumsallne/qulnpiroleresponse),30 mmafterpretreatment(returnto restingMAP),maximumfall In blood pressureaftertreatmentwith pentolinlum.@ < .05for differenceIn MAP betweensaline-treatedand quinplrole-treatedrats (two-way ANOVA). Right panel: The percent change in MAPcaused by injectionof pentollniumcomparedto base-lineMAPjustbefore treatment, i.e., 30 mm after injection of saline (open bars) orquinpirole (solid bars). P < .05 for difference between saline-treatedand quinpirole-treated SHR (paired t test)

SaUQuin

0 1 30 Pent

PO-AVP Ex@mentPretreatmentGroupSaline(n=7)Quinpirole(n = 7)Resting

MAP138±7134±5Maximal

L'@MAP3±2

36 ±3'i@MAP

30 mm—¿�1±2—¿�2±2@MAP

P0-AVP—¿�5±2

1 ±I%

changeMAP—¿�4±1

1 ±1.Losartan

&perimentPretreatment GroupSalIne(n=8)Qulnpirole(n=8)Resting

MAP146±4139±4Maximal

@MAP—¿�2±133±2@MAP

30 mm0±2

—¿�3±4@MAP

Losartan—¿�14±2—¿�11±2%

change MAP—¿�10±2

—¿�8±2ANFExperiment

PretreatmentGroupSalIne(n=7)Quinpirolefri= 7)Resting

MAP152±8145 ±7Maximal

L@MAP1±2

29 ±3i@MAP

30 mm5±5

—¿�8±3Plasma

ANF (pg/mi)48.3±2.549.7 ±5.0Hernatocrit

ExperimentPretreatment GroupSaline(n=7)Qulnpirole(n=7)Resting

MAP148±6140±3Maximal

i@MAP—¿�2±224±?L@MAP

30 mm—¿�9±4

4±2Hematocrft

1 (%)43±144±1Hematocrft

2 (%)42±143±3

Dataare mean ±SEMand expressed as absolute or change inmean arterialpressure (MAP,mm Hg)unless otherwise indicated. HematOCritvalues were measuredbefore (hematocrit1)and 30 mmafter administrationof saline or quinpirole(hematocrit2).

a p < .os for difference between values for saline-treated and quinplrole-treated SHR.

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

480 vanden Buuseat al Vol.277

AtSnOIOI 150

SallQtin

I% changeinHR

10 100C

E

I“¿�1

Controls

5O@

40

20

0@

-20

-40@

Quinplrole

0@

*

0 1 30 AtenA@O-50•

Time (mm)

-100•-O-S@ne@—¿�•—Quin@e@

FIg. 6. Leftpanel:Theeffectof treatmentwithatenolol(1 mg/kgi.v.,Aten)and methylatropine (1 mg/kg i.v.,Atro)on heart rate (HA)of SHRwhichwerepretreatedwith saline(opencircles,n = 8)or with quinpirole (0.3 mg/kg i.v., solid circles, n = 8). Resting HR at the start of theexperiment was 360 ±6 and 359 ±16 mm Hg, respectively. Data aremeanchangein mean HR ±SEM at three time-points:1 mm afterpretreatment (maximum sallne/quinpirole response), 30 mm after pretreatment(returnto restingHA),maximumchangelin HA after treatmerit with atenolol and methylatropine.@ < .05 for difference in HRbetween saline-treated and quinpirole-treated rats (two-way ANOVA).Right panel: The percentagechange in HR caused by injectionofatenololormethylatropinecomparedto HRjustbeforetreatment,i.e.,30 mm after Injectionof saline (open bars) or quinpirole(solidbars) or 5mm after injectionof atenolol.*@)< @o5for differencebetweensalinetreatedandquinpirole-treatedSHR(pairedt test)

DiscussionThis study provided a number of new insights into the

mechanisms behind the central pressor action of the dopamine D2 receptor agonist quinpirole. The main findings werethat the pressor response was subject to rapid and longlasting desensitization similarly in SHR, WKY and SpragueDawley rats and that this effect was also observed with thedopamine agonists quinelorane and NPA. Importantly, thehypotensive action of the sympatholytic drugs 8-OH-DPATand clonidine were found to be markedly inhibited afterpretreatment of the SHR with quinpirole, suggestinga prolonged action of quinpirole treatment on sympathetic regulation despite the return of blood pressure to baseline. Theexperiments using pentolinium and atenolol suggestedthat,30 mm after treatment with quinpirole, there was an overallelevated sympathetic vasomotor and cardiac sympatheticoutflow, although other cardiovascular regulatory mechanisms appeared to be normal.

Effects of raclopride and SC1123390 on the pressorresponse to quinpirole. The pressor response to quinpirolecould be blocked by pretreatment with the dopamine D2receptor antagonist raclopride, but not the D1 antagonistSCH23390. Although this confirms that quinpirole mostlikely activated D2 receptors,it alsoindicates that there wasno functional interaction between D2 receptors and D1 receptors in the effectofthis compoundon bloodpressure.Suchaninteraction has been shown to be important for many of the

-ioo -so 0 50 ioo

Change InMAP (mm Hg)

Fig. 7. Theeffectof administrationof saline(controls,n = 7)or quinpirole (0.3 mg/kg, n = 8) on baroreceptor-HR reflex of conscious SHR30 mm afteradmInistration.Data are normalizedfor differencesinbase-line values and depicted as the change in MAP or HR ±S.E.M.from the initial base-line values before treatments. For further details ofbarorefiexparameters,seetable2. *@< .05fordifferenceinupperHAplateaubetweenthegroups.

behavioral effects of quinpirole and related drugs, whereactivation of dopamine D1 receptors “¿�enabled―the activationof D2 receptors to produce its full effects. Such a functionallink between D1and D2receptors was lost in rats chronicallydenervated by treatment with reserpine or central 6-hydroxydopamine (Arnt, 1985; Waddington, 1989).

Comparison of quinpirole with quinelorane andNPA. Several dopaminergic agonists cause a centrally mediated pressor response, including apomorphine, (+)-3-PPP,NPA, 7-OH-DPAT, quinelorane and pergolide (Van denBuuse, 1992; Van den Buuse, 1993; Van den Buuse, 1995).Our results show that the pressor response to administrationof quinelorane or NPA caused apparent desensitization similar to that producedby quinpirole, suggestingthat this phenomenon is not unique to this compound but appears to be ageneral phenomenon with dopamine D2 receptor agonists.

Comparison of SRR with WKY and Sprague-Dawleyrats. The acute effect ofquinpirole on blood pressure in SHRwas very similar to that in WKY or Sprague-Dawley rats, asalso suggested in earlier studies (Nagahama et al., 1986b;Van den Buuse, 1992). We now observed that, in addition, theextent of desensitization to the pressor action of quinpirolewas virtually identical in all three strains, suggesting thisphenomenon was not unique to SHR. Previously it has beenshown that, unlike in normotensive rats, quinpirole producedlittle locomotorhyperactivity in SHR (Fuller et al. , 1983;Vanden Buuse, 1992), suggesting forebrain postsynaptic D2 receptor mechanisms were altered in this strain. Our resultsconfirm earlier findings that such changes were not generaland that D2-mediated effectsmay in fact be greater, similaror smaller in SHR, dependingon the central dopamine sys

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

Sallne4reatedSHR(n = 7)Quinplrole-Treated SHR(n =8)RestIngMAP(mmHg)147±5.5140±5Resting

heart rate (B/mm)354 ±15341 ±11MAPat maximalchange(mmHg)152±7171 ±5@Heartrateat madmalchange(B/mm)362±14333±9MAP3omlnaftertreatment(mmHg)149±6140±4Heart

rate30 mmaftertreatment(B/mm)355 ±15322 ±17BarorefiexparametersLower

heartrateplateau(P1,B/mm)270 ±31242 ±18Heartrate range(P2,B/mm)

Curvaturecoefficient(P3,mm Hg1@199±24

—¿�0.064±0.014178±13

—¿�0.079±0.011BP5O(P4,mmHg)146±10135±8Upper

heartrate plateau(P1+ P2,B/mm)469 ±10420 ±18@Averagegain (B/mm/mm Hg)-2.36 ±0.44-2.89±0.35

1996 Quinpirols and Blood Pressure Control 481

TABLE2Effect of administration of sails or qulnpirole (0.3mglkg) on mean arterial pressure (MAP)and heart rats and on barorefiexparameters at 30 mm after Injection In conscious SHR

Dataaremean±SEMof7 and8 SHR,respectively.a P < .05 for difference between SHR treated with saline and SHR treated with quinplrole.

tern involved and the test used (Van den Buuse and De Jong,1992; Van den Buuse et aL, 1992).

Effector mechanism. involved in the desensitizationto quinpirole. Studies by Nagahama et cii. (1986b) haveshown that the pressor action ofquinpirole was accompaniedby a significant rise in plasma levels of noradrenaline andadrenaline and could be partly inhibited by pretreatmentwith phenoxybenzamine. Similarly, the preesor response wasaccompanied by a significant rise in plasma levels of vasopreasin and could be inhibited by pretreatment with a vasopressm antagonist (Nagahama et a!., 1986b). Pretreatmentwith both phenoxybenzamine and a vasopressin antagonistcompletely blocked the presser effect of quinpirole, suggestiug that it was mediated by an increase in both sympatheticactivity and vasopressin release. We were interested to seewhether at 30 mm after administration of quinpirole, whenblood pressure had gone back to base-line values but theapparent desensitization was complete, the activity ofa number ofcardiovascular regulatory mechanisms, including sympathetic tone and vasopressinergic tone, was altered. In SHRtreated with quinpirole, the hypotensive action of clonidineand 8-OH-DPAT were significantly reduced, indicating that“¿�cro8s-densensitization―had occurred between the effect ofquinpirole and other centrally acting cardiovascular drugs,most likely at the level of sympathetic regulation. In SHRtreated with quinpirole, we observed a significantly greaterfall in blood pressure and HR caused by administration ofpentolimum or atenolol, respectively. Thus, sympathetic outflow appeared to be enhanced in these rats, despite a “¿�normar' bl@d pressure. We did not find evidence for markedlyaltered vasopressinergic or angiotensinergic activity or ofchanges in plasma levels of ANF or hematocrit. This wouldindicate that at least the reported increase in vasopressinrelease, involved in the quinpirole-induced presser response(Nagahama et al. , 1986b), is not maintained and is thereforeunlikely to play a role in the apparent desensitization. Slutilarly, changes in the activity of the renin-angiotensin systern, ANF release or plasma volume did not appear to beinvolved in the retarn of blood pressure to base line and inthe apparent desensitization. It is important to note that,although the fall in blood pressure after gangliomc blockademay be taken as an indirect measure of sympathetic vasomotor tone (Okuno et al., 1983; Jablonskis and Howe, 1993),some caution should be retained in the interpretation of this

result. The level ofblood pressure after ganglionic blockade isnot the minimum, because there is compensation for thehypotension by increased release of vasopressin and renin(Jablonskis and Howe, 1993). Thus, the hypotensive action ofgangliomc blockade can only be used as an indicator of poesible changes in sympathetic vasomotor tone if hormonalresponses are similar between groups. In our experiments,the relative contribution ofthe renin-angiotensin system appeared to be unaltered. There was, however, a small butsignificant difference in the acute blood pressure response totreatment with a vasopressin antagonist. To which extentthis possible difference in vasopressinergic activity may haveinfluenced the results of the experiments with ganglionicblockade, will have to be determined in experiments usingboth treatments together (Jablonskis and Howe, 1993). Inany case, the blood pressure fall in response to treatmentwith pentoliniurn was not reduced in quinpirole-treated SHR(see below). With respect to HR, we did not observe a significant change in the tachycardia response to injection ofmethylatropine, suggesting that vagal activity was unalteredand was not compensating for the increased sympatheticcardiac tone. It is at present unclear by which mechanismblood pressure returned to base line after the initial pressoreffect of quinpirole. If it was assumed that the increasedrelease ofvasopressin was not maintained, this could explainsome of this return to base line, but the sustained sympathetic activation would have to be compensated for by someother mechanism, such as a change in cardiac output.

In support ofa prolonged change ofsympathetic vasomotortone after treatment with quinpirole, was the attenuation ofthe effect of two agents that inhibit sympathetic activity,clonidine or 8-OH-DPAT. One might have expected that anelevated sympathetic activity (as exemplified by the effects ofpentolinium) would have resulted in these inhibitory agentsproducing a greater hypotensive effect. This would be true ifquinpirole had caused a global rise in vasomotor tone. However, if there was a differential effect of this compound onspecific vasomotor pathways, then an increase in the activityof these pathways would cause an increase in blood pressure(the rapid quinpirole-induced presser response) which,through baroreflex feedback, would result in an inhibition ofthe activity in other, clonidine- and 8-OH-DPAT-sensitivepathways, and a normalization of blood pressure despite thecontinued effect of quinpirole. Moreover, the “¿�silencing―of

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

482 vandenBuuseet al. VoL277

part of sympathetic vasomotor outflow by such a baroreflexfeedback inhibition would prevent the effect of clomdine and8-OH-DPAT administration to occur. This scheme is atpresent purely speculative, but there is some evidence forregional specificity of the sympatho-inhibitory effects ofclonidine and 8-OH-DPAT. Particularly renal, cardiac andsplanchnic vascular beds are affected by clonidine treatment,whereas sympathetic tone in muscular beds is either notaffected or enhanced (Shaw et al., 1971; Wallin and FriskHolmberg, 1981; Esleret al., 1992). Similar, but not identical,differential effects have been described for 8-OH-DPAT (Ramage and Wilkinson, 1989). Furthermore, it has been suggested that a2-adrenoceptor agomsts such as clonidine affectonly a proportion of the vasomotor cells in the rostral yentrolateral medulla, in particular catecholamine neuronsrather than the noncatecholamine pacemaker cells (Guyenetet al. , 1989). The above suggested effect of quinpirole onspecific vasomotor pathways could also explain the apparentdesensitization that we observed in our experiments. Due tothe continued presence of quinpirole in the circulation and inthe synapse, the “¿�quinpirole-pathway―would remain activated despite the return of blood pressure to base-line levelsand it would therefore not be possible to activate it further toany significant extent (in our experiments 30 mm after administration). Only when quinpirole was cleared from thesystem would differential sympathetic activity return to itsoriginal settings and be again sensitive to quinpirole-mediated activation, in our experiments after 24 hr.

ConclusionIn conclusion, our results have provided new insight in the

role of central dopamine systems in the regulation of bloodpressure (Van den Buuse and De Jong, 1992). We observedthat, although the pressor action of quinpirole was shortlived, there were indications of relatively prolonged changesin cardiovascular homeostasis, as exemplified by the apparent desensitization and by the experiments with clomdineand 8-OH-DPAT, and pentolinium and atenolol. Clearly, further studies are needed to provide more details about theeffect ofcentral dopamine on sympathetic function, includingdirect regional measurements of sympathetic nerve activityor blood flow. In the meantime, we suggest that treatmentwith dopaminergic agonists in disorders such as Parkinson'sdisease (Quinn et al. , 1981; Durrieu et al. , 1991) may causedifferential changes in sympathetic regulation that may gounnoticed because blood pressure levels are normal. Furthermore, these findings support the concept that central dopamine systems, through differential changes in sympatheticfunction and regional blood flow, may be involved in parallelregulation of locomotor activity and corresponding cardiovascular adjustments.

References

Aiurr, J.: Behavioural stimulabon is induced by separate dopamine D-1 andD-2 receptor sites in reserpine-pretreated but not normal rats. Eur. J.Pharmacol. 113: 79—88,1985.

BMrroN,A. C., BlAcK, L. E. @nSisi@sy,D. R.: Agonist-induced desensitizationof D2 dopamine receptors in human Y-79 retinoblastoma cells. Mol. Pharmacol. 39 650—658,1991.

ClARK, B. J.: Prejunctional dopamine receptor stimulation. In Peripheral Dopamine Pathopbysiology, ed. by F. Amenta, CRC Press, Boca Raton, FL,1990, pp. 267—306.

Cw@z@s,J. A., Oiui.iua@,T. @mS@zznG, E. B.: Dopamine agonist activities ofpergolide, its metabolites, and bromocriptine as measured by prolactin in

hibition, compulsive turning, and stereotypic behavior. Arzneim. Forschung.43:281—286,1993.

COHEN, M. L, Sa.@n, C. J. @r Coi.@airr, W. E.: cr2-agonist activity of the

dopamine DA2-agonist LY171555. J. Cardiovasc. Pharmacol. 6: 1245—1248,1984.

CORNISH, J. L. @nV@u@iDEN BUUSE, M.: Pressor responses to electrical andchemical stimulation of the rat brain AlO dopaminergic system. Neurosci.Lett. 176: 142—146,1994.

CORNISH, J. L. @rwV@u@DEN BUUSE, M.: Stimulation of the rat mesolimbicdopaminergic system produces a pressor response which is mediated bydopamine D-1 and D-2 receptor activation and the release of vasopressin.Brain Res. 701: 28—38,1995.

Duiuusu, G., SENARD,J. M., Tas@, M. A., RASCOL,A. @rwMoi@rr@smuc,J. L:Effects of levodopa and bromocriptine on blood pressure and plasma catecholamines in parkinsonians. Clin. NeuropharmacoL 14: 84-90, 1991.

Eu.@x, D. ANDSZEcHTMAN,H.: Biphasic effect of D-2 agonist quinpirole onlocomotion and movements. Eur. J. Pharmacol. 161: 151—157,1989.

Esi@zR,M., DUDLEY,F., JE@m@os,G., DEBIN8KI,H., L@smsar, G., Joi@izs,P.,CRO'I'rY, B., Cou@, J. @mWiuzrr, I.: Increased sympathetic nervous activity and the effects of its inhibition with clonidine in alcoholic cirrhosis.Ann. Intern. Med. 116: 446—455,1992.

Foaz@, M. M., FULLER,R W., HyNzs, M. D., GIDDA,J. S., NIcHoi@s,C. L.,SCHAUS, J. M., Koasrzw, E. C. h@m Ci@sMzNs, J. A.: PreCliniCal studies on

quinelorane, a potent and highly selective D2-dopaminergic agonist. J. Pharmacol. Exp. Ther. 250: 227—235,1989.

FRANKLIN, R. B., Sn'r@au'@u@x, G. S.,@ K. H. @r'mQUAY, J. F.: Disposition of

14C-quinelorane in dogs following oral or intravenous dosing and transdermal patch application. Drug. Dove!. Industr. Pharm. 20: 1439-1452, 1994.

FULLER, R. W., HEMIUcK-LuEcKE, S. K, WONG, D. T., P&@ssoN, D., Tnaswcr@w,

P. G. @nHyr@as,M. D.: Altered behavioural response to a D2 agonist,LY141865, in SHR exhibiting biochemical and endocrine responses similarto those in normotensive rats. J. Pharmacol. Exp. Ther. 227: 354—359,1983.

Guy@rr, P. G., HASELTON,J. R. 1umSun, M. K: Sympathoexcitatory neurons ofthe rostroventrolateral medulla and the origin ofthe sympathetic vasomotortone. In The central neural organization of cardiovascular control, Progressin Brain Research, VoL 81, ed. by J. Ciriello, M. M. Caverson and C. Polosa,pp. 105—116,New York, Elsevier Scientific Publications, 1989.

HAHN, R. A. AND MACDONALD, B. R.: Systemic hemodynamic and regional blood

flow effects of LY 141865, a selective dopamine receptor agonist. J. Pharmacol. Exp. Ther. 23e-@558-568, 1984.

HALL, H., KOHLER,C., GAWELL,L., FABDE, L. @r@mSEDVALL,G.: Raclopride, a newselective ligand for the dopamine-D2 receptors. Progr. Neuro-Psychopharmacel. Biol. Psych. 12: 559—568,1988.

HEAD, G. A. ANDADAMS,M. A.: Time course of changes in baroreceptor reflexcontrol ofheart rate in conscious SHR and WKY: Contribution ofthe cardiacvagus and sympathetic nerves. Clin. Exp. Pharmacol Physiol. 15: 289—292,1988.

HEAD, G. A. ANDMcC@arv, R.: Vagal and sympathetic components ofthe heartrate range and gain ofthe baroreceptor-heart rate reflex in conscious rats. J.Auton. Nerv. Syst. 21: 203—213,1987.

HYTTEL,J. ANDARNT, J.: Characterization ofbinding of3H-SCH23390 to dopamine D-1 receptors. Correlation to other D-1 and D-2 measures and effect ofselective lesions. J. Neural Transm. 68: 171—189,1987.

IGARASHI,Y., Ciizr@i,Y. F., Wyss, J. M., Lu@nsaiaaa, M. D. @snOp@sa, S.:Continuous intravenous infusion of LY171555, a potent selective D2 receptor agonist, lowers blood pressure in the conscious rat. Pharmacology 35:194—202,1987.

lomo, L. C., B@iu@m'r,A., LEITZ,F. H., Housza, V. P. @rwKORDUBA,C. A.:5CH23390, a potential benzazepine antipsychotic with unique interactionsof dopaminergic systems. J. Pharmacol. Exp. Ther. 228: 462-468, 1983.

JABL0NSKIS,L T. s%@itHOWE, P. R. C.: Vasopressin compensates for acute loss ofsympathetic pressor tone in spontaneoualy hypertensive rats. Cliii. Exp.Pharmacol. Physiol. 2@h380-383, 1993.

KOLLER,W., HERB.STER,G., ANDERSON,D., WACK, R. @mGORDON,J.: Quinpirolehydrochloride, a potential anti-parkinsonism drug. Neuropharmacology 261031—1036,1987.

MCNAY, J. L., Hzi@tay, D. P. @uwDELONG, A. F.: LY171555, a selective dopamine(DA-2) agonist, increases the blood pressure of hypertensive patients. Clin.Exp. Hypert. A 9 1098, 1987.

NAGAHAMA,S., Ciizr@i,Y. F., LINDHEIMER,M. D.@ OP@Ra, S.: Mechanism of thedepressor action of LY171555, a specific dopamine D2 receptor agonist, inthe anesthetized rat. J. Pharmacol. Exp. Ther. 239 426-432, 1986a.

NAGAHAMA,S., Cn@, Y. F., Lonumssa, M. D.@ Op@uui@,S.: Mechanism of thepressor action ofLY171555, a specific dopamine D2 receptor agonist, in theconscious rat. J. Pharmacol. Exp. Ther. 236: 735—742, 1986b.

NAGAHAMA, S., Soo, A. H., CHKN, Y. F., LINDREIMER, M. D. @nOp@an, S.: Roleof vasopressin in the cardiovascular effects of LY171555, a selective dopamine D2 receptor agonist: Studies in conscious Brattleboro and Long-Evansrats. J. Pharmacol. Exp. Ther. 242: 143—151,1987.

Oicu@o,T., NAGAHAMA,S., Li@msamssa,M. D. @nOp@uui.,S.: Attenuation of thedevelopment of spontaneous hypertension in rats by chronic central adniinistration ofcaptopril. Hypertension 5: 653-662, 1983.

QUn@N,N., IJLAs, A., Lirmxn'ra, F. Auw AGID, Y.: Bromocriptine in Parkinson'sdisease: A study of cardiovascular effects. J. Neurol. Neurosurg. Psych. 44:426—429,1981.

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from

1996 Quinpiroleand BloodPressureControl 483

RAMAGE,A. G. @t@nWzuaNsoN, S. J.: Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions ofputative 5-HTIA and a@-adrenoreceptor agonists in anesthetized cats. Br. J.Pharmacol. 98: 1157—1164,1989.

RUFFOLO, R. R. .i. @nSin@*a, C. J.: Relative potency ofLY141865 at dopamineDA2 and histamine H2 receptors. Eur. J. PharmacoL 92:285-206,1983.

SEEMAN, P. ANDSCHAUS,J. M.: Dopamine receptors labelled by [3Hlquinpirole.Eur. J. Pharmacol. 203: 105-105, 1991.

Sitaw, J., Hu@yoK,S. N. @mKom@m,P. I.: The peripheral circulatory effects ofclonidine and their role in the production of arterial hypotension. Eur. J.Pbarmacol. 14: 101—111,1971.

SzECHTMAN,H., DAu, H., Murr@, S., EINAT, H. @snSuwv@, R. M.: Effect ofdose and interdose interval on locomotor sensitization to the dopanuneagonist quinpirole. PharmacoL Biochem. Behav. 4& 921-928,1994.

TSURuTA, K, FREY, E. A., Gazws, C. W., CoTs, T. E., Esici@x,R. L @nJ. W.: Evidence that LY-141865 specifically stimulates the D-2 dopaminereceptor. Nature 292: 463—465,1981.

VAN DEN Buusz, M.: Central effects ofquinpirole on blood pressure of spontaneously hypertensive rats. J. Pharmacol. Exp. They. 262: 303—311,1992.

VAN DEN Buusa, M.: Effects of 7-hydroxy-N, N-di-n-propylaminotetralin onbehaviour and blood pressure of spontaneously hypertensive rats. Eur. J.Pharmacol. 243: 169—177,1993.

VAN DENBuusz, M.: Differential effects ofquinelorane and pergoide on behaviour, blood pressure, and body temperature of spontaneously hypertensiverats and wistar-Kyoto rats. PharmacoL Biochem. Behav. @0:389-397, 1995.

VAN DENBuusz, M. ANDDE JONO, W.: Differential effects ofdopaminergic drugson open-field behaviour of spontaneously hypertensive rats and normotensive wistar-Kyoto rats. J. Pharmacol. Exp. Ther. 248: 1189-1196, 1989.

V@ DENBuusz, M. ANDDz J0NG, W.: Pharmacology of central dopaminergicmodulation in hypertension. In Antthypertensive Drugs Today, Progress inHypertension, VoL 2, ed. by H. Saito, M. Minami and S. H. Parvez, pp.157-188, Utrecht, The Netherlands, VSP Press, 1992.

VANDENBuusz, M., JoNEs, C. R. @imWAai@izi@,J.: Brain dopamine D-2 receptormechanisms in spontaneously hypertensive rats. Brain Res. Bull. 28:289-297, 1992.

WADDINGTON,J. L: Functional interactions between D-1 and D-2 dopaminereceptor systems: Their role in the regulation of psychomotor behaviour,putative mechanisms, and clinical relevance. J. PsychopharmacoL 3: 54-63,1989.

WALUN, B. G.@ Fmsx.Hoi@unsao, M.: The antihypertensive mechanism ofclomdine in man: Evidence against a general reduction of sympatheticactivity. Hypertension 3: 340-346, 1981.

WAN, F. J. AND SWERDLOW,N. R.: Intra-accumbens infusion of quinpirole impairs senaoriznotor gating of acoustic startle in rats. Psychopharmacology113: 103—106,1993.

WONG, D. T., BYs@rrsR, F. P., Ram, L K, Fuuaa, R. W., Pmay, K W. @mKOENFELD, E. C.: Effect of a stereospecific D2.dopaxnine agonist on acety!choline concentration in corpus striatum ofrat brain. J. Neural Tranam. 58:55-67, 1983.

WONG,P. C., PRICE,W. A. j., Cmu, A. T., Du@ci*, J. V., C@an@i,D. J., Waxiza, P.P., JOHNSON,A. L @nThwssu@Ns, P. B. M. W. M.: Rypotensive action ofDuP 753, an angiotensin II antagonist, in spontaneously hypertensive rats.Hypertension 15: 459-468, 1990.

WOODS, R. L: Contribution of the kidney to the metabolic clearance of atrialnatriuretic factor. Am. J. PhysioL 256: E934-E941, 1988.

YANG,R. H., IGARA5HI,Y., Wvss, J. M. *@coCasri, Y. F.: Dopaniine D2receptors in the posterior region of the nucleus tractus solitarius mediatethe central pressor action of quinpirole (LY171555). Brain Res. Bull. 24:97—103,1990.

Send reprint requests to: Dr. M. van den Buuse, Baker Medical ResearchInstitute, Commercial Road, P.O. Box 348, Prahran, \Tictoria 3181, Australia.

at Monash U

niversity on February 14, 2013

jpet.aspetjournals.orgD

ownloaded from