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Pharmacological Evidence for the Subclassification of Central Dopamine Receptors in the Rat Alma J

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Pharmacological Evidence for the Subclassification of Central Dopamine Receptors in the Rat Alma J Br. J. Pharmac. (1982), 77, 185-194 PHARMACOLOGICAL EVIDENCE FOR THE SUBCLASSIFICATION OF CENTRAL DOPAMINE RECEPTORS IN THE RAT ALMA J. GOWER & A.S. MARRIOTT Department of Pharmacology, Glaxo Group Research Limited, Ware, Hertfordshire SG12 ODJ 1 The relative potencies of dopamine receptor agonists in causing stereotypy iireats when injected into the olfactory tubercles, and contralateral rotation when injected unilaterally into the caudate nucleus of rats with lesions of the nigro-striatal dopamine pathway, were determined. The actions of some agonists in eliciting these responses following peripheral injection, and the relative potencies of dopamine receptor antagonists in inhibiting them were also determined. 2 Dopamine, apomorphine and 2-amino-5,6 and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahy- dronaphthalene (A-5, 6 DTN, A-6, 7 DTN) and N,N dipropyl A-5, 6DTN induced both responses. In contrast, 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-lH-3-benzazepine HCl (SK & F 38393) whether injected intracerebrally or peripherally, induced contralateral rotation but not stereotypy. 3 Contralateral rotation and stereotypy induced by apomorphine or N,N dipropyl A-5,6 DTN were inhibited by haloperidol, pimozide and fluphenazine but these drugs failed to inhibit rotation induced by SK & F 38393. Clozapine inhibited rotation induced by SK & F 38393, apomorphine or N,N dipropyl A-5,6 DTN but failed to inhibit stereotypy. Loxapine was more potent in inhibiting stereotypy than rotation, whereas clothiapine inhibited rotation and stereotypy at similar doses irrespective of the agonist used to elicit the response. 4 Contralateral rotation induced by SK & F 38393 was not inhibited by yohimbine, prazosin, atropine, methysergide, mepyramine or propranolol. 5 The results provide evidence that contralateral rotation induced by dopamine receptor agonists is mediated by two different classes of dopamine receptors and that these receptors differ from those mediating the stereotypy response. 6 The receptors mediating these responses appear classifiable in terms of their sensitivity to the agonist actions of SK & F 38393 or apomorphine respectively. SK & F 38393-sensitive receptors are susceptible to blockade by clozapine but are not blocked by haloperidol, pimozide or fluphenazine. Apomorphine-sensitive receptors are susceptible to blockade by haloperidol, pimozide and fluphenazine but appear divisible into two sub-classes depending on whether or not they are blocked by clozapine and on their sensitivity to blockade by loxapine. Introduction A number of dopamine receptor agonists and an- tagonists could provide useful information for a func- tagonists have selective actions which suggest that tionally related subclassification of central dopamine they may differentiate between central dopamine receptors. The agonists were injected directly into receptors. For example, the neuroleptic drug, dopamine-rich areas of the brain which have been clozapine, differs from other dopamine receptor an- associated with defined dopaminergically mediated tagonists in that it fails to inhibit apomorphine- responses. Relative potencies were determined in induced stereotypy or to block autoreceptors on causing stereotypy in rats when injected bilaterally dopaminergic nigro-striatal nerve terminals (Stille, into the olfactory tubercles (see Costall, Naylor & Lauener & Eichenberger, 1971; Roth, 1980). The Neumeyer, 1975; Krieger, 1980) and contralateral new dopamine receptor agonist, SK & F 38393, has rotation when injected unilaterally into the caudate been shown to cause contralateral rotation in rats nucleus of rats with unilateral lesions of the nigro- prepared with unilateral nigro-striatal lesions but, striatal dopamine pathway (see Ungerstedt, 1971). unlike other dopamine receptor agonists it does not The actions of some dopamine agonists known to induce stereotypy in this species or vomiting in dogs penetrate the blood brain barrier were determined (Setler, Sarau, Zirkle & Saunders, 1978). In the following peripheral injection. The relative potencies present study we have examined the possibility that of dopamine receptor antagonists in inhibiting the these and other dopamine receptor agonists and an- agonist-induced responses were also determined. 0007-1188/82/090185-09 $01.00 © The Macmillan Press Ltd 1982 186 ALMA J. GOWER & A.S. MARRIOTT Methods to each rat in the group during several weekly ses- sions, no rat being injected more frequently than Animals once in 7 days. Measurement of rotation started immediately after injection and was continued for up Male hooded rats (AH/H strain) were used for rota- to 7 h. Log dose-response curves were plotted at the tion experiments. Male albino rats (AH/A strain) time of peak rotation for each agonist. gave better stereotypy responses than hooded rats In antagonist studies each antagonist was investi- and were used for all stereotypy experiments. Rats gated in a separate group of 6-12 rats. Over the used in rotation experiments weighed 140-160 g at course of several weekly experimental sessions each the time of lesion placement; all rats weighed dose of antagonist and a control injection of vehicle 230-290 g at the time of experimental use. The rats were administered intraperitoneally to each rat in the were maintained on 41B cube diet and water ad group. Each dose of antagonist or vehicle was fol- libitum. Housing and laboratory temperatures were lowed by a fixed, intraperitoneal dose of agonist. maintained at 20 - 21°C. Measurement of rotation started immediately after agonist injection and was continued for 1 h. An- Intracerebral injections tagonist dose-test intervals, timed from the injection of the antagonist to the injection of the agonist are Intracerebral injections in conscious rats were ad- given in the text. Percentage inhibition of the agonist ministered via stereotaxically implanted stainless response by each dose of the antagonist was calcu- steel guide cannulae similar to those described by lated with reference to the control response following Costall & Naylor (1974). The cannulae were kept treatment with vehicle plus agonist. patent by stainless steel stylets which extended 0.5 mm below the tips. For injections the stylets were Stereotypy experiments replaced with stainless steel injection cannulae which extended either 1.5 mm (caudate nucleus) or 2.5 mm Experimentally naive rats were used. For stereotypy (olfactory tubercles) below the guide cannulae tips. assessment each rat was placed in a 2 litre glass Drugs were injected over a 30s period in a dose beaker containing a 1 cm layer of sawdust. volume of 1- 2 pl. Nialamide, (100 mg/kg i.p.) was Stereotyped movements were scored as follows, routinely injected 2 h before intracerebral injections using a 'blind' procedure in which the observer did of agonists but was omitted when agonists were in- not know the nature of the drug treatments: jected peripherally. Injection sites were checked by injecting 1 til Evans blue dye intracerebrally and Repetitive licking movements examining frozen brain sections microscopically. All Repetitive sniffing of sawdust Discontinuous: Score = cannulae placements were located in the areas Repetitive side to side head Continuous: Score - 2 specified in the text. movements Rotation experiments Repetitive chewing movements } Discontinuous: Score = 3 Nigro-striatal lesions in rats used for rotation experi- Active chewing of sawdust Continuous: Score =4 ments were induced under pentobarbitone anaes- thesia by infusing 6-hydroxydopamine into the sub- Agonists were injected intravenously or were in- stantia nigra on one side as described by Ungerstedt jected bilaterally into the olfactory tubercles via bila- & Arbuthnott (1970). Only rats that subsequently teral guide cannulae implanted 7 days previously. rotated contralaterally following apomorphine Coordinates for the olfactory tubercle injections (0.5 mg/kg i.p.) or ipsilaterally following (+)- were: A8.6, H -2.4, L±2.5 (De Groot, 1959). amphetamine (2.5 mg/kg i.p.) were used. The rota- Dose-groups of 6 rats were used. Control rats re- tion response was measured in rotometers similar to ceived an equivalent volume of saline. Stereotypy those described by Ungerstedt & Arbuthnott (1970). was assessed in each rat at 5 min intervals starting Agonists were injected intraperitoneally or were immediately after injection and continuing until the injected directly into the caudate nucleus via a single response terminated. The 5 min scores were summed guide cannula implanted on the same side as the for each rat over each 30 min of the observation nigro-striatal lesion. Intra-caudate injection site period and means for consecutive 30 min periods coordinates were: frontal 7.5 mm; hor. 1.0 mm; sag calculated in each group. Log dose-response curves 2.5 mm (Konig & Klippel, 1963). Each agonist was at peak effect were plotted by using the highest investigated in a separate group of 6-12 rats. The 30 min mean score obtained for each dose of agonist. various doses of agonists or a control injection of In each antagonist study, each dose of antagonist saline (0.9% w/v NaCl solution) were administered was administered intraperitoneally to a dose-group CENTRAL DOPAMINE RECEPTORS 187 of 6 rats and a control group of 6 rats injected in- acacia solution in distilled water; yohimbine was traperitoneally with an equivalent volume of an- dissolved in distilled water. Nialamide was dissolved tagonist vehicle. All rats then received a fixed in- in a minimal volume of 2 N HCl, neutralized to pH travenous
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