Effect of Guanine Nucleotides on Dopaminergic Agonist and Antagonist Affinity for [3H]Sulpiride Binding Sites in Rat Striatal Membrane Preparations

Abstract: [³H]Sulpiride bound to rat striatal membrane preparations with a saturable, high affinity component. This binding was displaced potently by dopamine antagonists (both classic neuroleptics and the benzamide, sulpiride) and less potently by dopamine agonists. GTP and its stable analogue Gpp(NH)p did not affect [³H]sulpiride binding to the membranes but altered the affinity for dopaminergic agonists. This effect was specific in that antagonist binding was not affected and only GTP, GDP, and Gpp(NH)p produced the effect. Similar alterations in ligand binding affinity caused by guanine nucleotides have been observed for binding sites linked to an adenylate cyclase. Such an interpretation for the case of [³H]sulpiride is contrary to suggestions that sulpiride labels only those dopamine receptors that are not cyclase linked.     

Serotonin sensitive adenylate cyclase activity in monkey anterior limbic cortex: antagonism by molindone and other antipsychotic drugs.

Serotonin (5-HT) sensitive adenylate cyclase in monkey anterior limbic cortex homogenates was further characterized and the effects of antipsychotic drugs and 5-HT anatagonists investigated. Differences in time course for stimulation by agonists and in responsiveness to receptor anatagonists of 5-HT-and dopamine (DA)-stimulated activities, were observed. Also there was an additivity of 5-HT and DA at maximally effective concentrations. Classical 5-HT antagonists blocked the 5-HT sensitive adenylate cyclase with a rank order of potency: methiothepin > cyproheptadine > methysergide. These 5-HT antagonists also effectively inhibited DA sensitive adenylate cyclase. Most antipsychotic drugs tested antagonized 5-HT stimulated activity although these drugs exhibited greater efficacies in blocking DA stimulated activity. Exceptions were molindone which failed to antagonize DA sensitive adenylate cyclase but effectively blocked 5-HT sensitive cyclase and pipamperone which was inactive in both cyclase systems. Haloperidol was a more selective antagonist of the DA sensitive adenylate cyclase than were the other antipsychotic drugs tested.     

Study by the intracerebral microdialysis method of the effects of atypical neuroleptics and anxiolytics on striatal release and metabolism of dopamine in awake rats]

Using brain microdialysis in awake rats effects of risperidone, ritanserin, buspirone, sulpiride and 5-methoxy-N,N-dimethyltryptamine (MeODMT) on striatal dopamine (DA) release and metabolism were studied. Risperidone, sulpiride and buspirone increased levels of DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Ritanserin failed to affect DA release, while increased DOPAC and HVA levels. MeODMT had no effect on striatal DA release and metabolism. Possible interaction between DA and serotonin systems is discussed.     

Dopaminergic Mechanisms in the Teleost Retina

A specific dopamine-sensitive adenylate cyclase has been identified in homogenates of the teleost (carp) retina. Maximal stimulation by 100 microM-dopamine resulted in a 5--10-fold increase in adenylate cyclase activity with half-maximal stimulation occurring at a concentration of 1 microM. l-Noradrenaline and l-adrenaline were some 10 times less potent than dopamine whilst the alpha- and beta-adrenoreceptor agonists, l-phenylephrine and dl-isoprenaline were inactive. Apomorphine elicited a partial stimulation of adenylate cyclase activity whilst various ergot alkaloids produced mixed agonist/antagonist responses. Dopamine-stimulated adenylate cyclase activity was potently antagonised by various neuroleptic drugs including fluphenazine, alpha-flupenthixol and alpha-piflutixol, and to a lesser extent by the butyrophenone derivatives haloperidol and spiperone. The benzamide derivatives, metoclopramide and sulpiride, together with the alpha- and beta-adrenoreceptor blocking agents, phentolamine and propranolol respectively were essentially inactive at blocking dopamine-stimulated adenylate cyclase activity. These data suggest the presence of a highly specific dopamine-sensitive adenylate cyclase in homogenates of teleost retina possessing similar pharmacological properties to the dopamine-sensitive adenylate cyclase observed in the mammalian central nervous system.     

Demonstration and Characterization of Opiate Inhibition of the Striatal Adenylate Cyclase

Abstract: The conditions in which Leu⁵-enkephalin inhibition of striatal adenylate cyclase was observed were defined. It was determined that enkephalin inhibition was dependent on GTP. The apparent K_m for GTP in opiate inhibition was determined to be 0.5 and 2 μM when 0.1 mM- and 0.5 mM-ATP were used as substrate. ITP, but not CTP or UTP, could substitute for GTP in the reaction. Though the addition of monovalent cations—Na⁺,K⁺, Li⁺, Cs⁺, and choline⁺—stimulated striatal adenylate cyclase activity, enkephalin inhibition of striatal adenylate cyclase did not require Na⁺ when theophylline was used as the phosphodiesterase inhibitor. Under optimal conditions, i.e., 20 μM-GTP and 100 mM-Na⁺, Leu⁵-enkephalin inhibited the striatal adenylate cyclase activity by 23–27%. When the enkephalin regulation of the cyclase activity was further characterized, it was observed that Leu⁵-enkephalin inhibited the rate of the enzymatic reaction. Kinetic analysis revealed that the opioid peptide decreases V_max values but not the K_m values for the substrates Mg²⁺ and Mg-ATP. Agents such as MnCl₂, NaF, and guanyl-5′-ylimido-diphosphate, which directly activated the adenylate cyclase, antagonized the opiate inhibition. Levorphanol and (–)naloxone were more potent than dextrorphan and (+)naloxone in inhibiting adenylate cyclase and in reversing the enkephalin inhibition, respectively. There were differences in the potencies of various opiate peptides in their inhibition of striatal adenylate cyclase activity, with Met⁵- > Leu⁵-enkephalin > β-endorphin. The opiate receptor through which the enkephalin inhibition was observed is most likely δ in nature, since in the presence of either Na⁺ or K⁺, the magnitude of the alkaloid inhibition was reduced, whereas the peptide inhibition was either potentiated or not affected.     

Inhibition of a Low Km GTPase Activity in Rat Striatum by Calmodulin

In rat striatum, the activation of adenylate cyclase by the endogenous Ca2+-binding protein, calmodulin, is additive with that of GTP but is not additive with that of the nonhydrolyzable GTP analog, guanosine-5'-(beta, gamma-imido)triphosphate (GppNHp). One possible mechanism for this difference could be an effect of calmodulin on GTPase activity which has been demonstrated to "turn-off" adenylate cyclase activity. We examined the effects of Ca2+ and calmodulin on GTPase activity in EGTA-washed rat striatal particulate fractions depleted of Ca2+ and calmodulin. Calmodulin inhibited GTP hydrolysis at concentrations of 10(-9)-10(-6) M but had no effect on the hydrolysis of 10(-5) and 10(-6) M GTP, suggesting that calmodulin inhibited a low Km GTPase activity. The inhibition of GTPase activity by calmodulin was Ca2+-dependent and was maximal at 0.12 microM free Ca2+. Maximal inhibition by calmodulin was 40% in the presence of 10(-7) M GTP. The IC50 for calmodulin was 100 nM. In five tissues tested, calmodulin inhibited GTP hydrolysis only in those tissues where it could also activate adenylate cyclase. Calmodulin could affect the activation of adenylate cyclase by GTP in the presence of 3,4-dihydroxyphenylethylamine (DA, dopamine). Calmodulin decreased by nearly 10-fold the concentration of GTP required to provide maximal stimulation of adenylate cyclase activity by DA in the striatal membranes. The characteristics of the effect of calmodulin on GTPase activity with respect to Ca2+ and calmodulin dependence and tissue specificity parallel those of the activation of adenylate cyclase by calmodulin, suggesting that the two activities are closely related.(ABSTRACT TRUNCATED AT 250 WORDS)     

Different effects of typical and atypical neuroleptics on K+-stimulated dopamine release from isolated rat striatum]

Effects of haloperidol (10(-7)-alpha 10(-5) M), trifluoperazine, metoclopramide, tiapride, sulpiride, thioridazine, clozapine remoxipride, raclopride, cis- and trans-isomers of carbidine, SCH 23390 (all at the 10(-6) M) on the K(+)-stimulated (28 mM) dopamine (DA) release from isolated rat striatum were studied. Haloperidol at the concentration of 10(-7) and 10(-6) M failed to affect, while at 10(-5) M the drug decreased the stimulated striatal DA release. Trifluoperazine, metoclopramide and tiapride were shown not to modify this process. Sulpiride, thioridazine, clozapine, remoxipride, raclopride, isomers of carbidine were found to increase significantly the stimulated striatal DA release. SCH 23390 failed to affect K(+)-stimulated release of DA in the striatum and also did not change K(+)-stimulated release enhancement produced by raclopride. It is suggested that the mechanism underlying observed effects of the drugs may contribute to pharmacological profile of atypical neuroleptics.     

ON THE RELEVANCE OF PREFERENTIAL INCREASES OF MESOLIMBIC VERSUS STRIATAL DOPAMINE TURNOVER FOR THE PREDICTION OF ANTIPSYCHOTIC ACTIVITY OF PSYCHOTROPIC DRUGS

Abstract— Drugs possessing (chlorpromazine, haloperidol, clozapine, thioridazine and sulpiride) or lacking (benzoctamine and perlapine) antipsychotic activity were compared with respect to their ability to enhance x-methyl-p-tyrosine-induced dopamine disappearance from the mesolimbic area and corpus striutum of rat brain. In addition, their effects on the endogenous concentrations of homovanillic (HVA) and 3.4-dihydroxyphenylacetic (DOPAC) acids in these two brain areas were determined. Some of the drugs enhanced dopamine disappearance in the mesolimbic area more than in the striatum. The most active in this respect were sulpiride. perlapine and chlorpromazine. By contrast, haloperidol was slightly more active in the striatum than in the mesolimbic area. None of the drugs was more efficient in elevating HVA levels in the mesolimbic area than in the striatum. However, there were large differences in the relative extent of the HVA increases in the two regions. Benzoctamine, perlapine and chlorpromazine increased HVA concentrations in the mesolimbic area nearly as much as in the striatum. Thioridazine and haloperidol, however, elevated striatal HVA much more effectively. Haloperidol and clozapine increased the DOPAC concentration in both areas to about the same extent. The other drugs were more active in the striatum. The largest difference between both regions was shown by chlorpromazine. Perlapine and benzoctamine, both lacking antipsychotic activity, produced much larger increases of HVA than of DOPAC. This is in contrast to the results obtained with true neuroleptics and may reflect an involvement of release phenomena in the action of these two drugs on dopamine metabolism. These results suggest that a preferential increase of dopamine turnover in the mesolimbic area is not necessarily linked to a better ratio of antipsychotic activity vs. extrapyramidal side effects. Moreover, an antiacetylcholine component of dopamine receptor blocking drugs does not seem to be a prerequisite for preferential activity on dopamine turnover in the mesolimbic system.     

Dopamine-sensitive adenylate cyclase of the caudate nucleus of rats treated with morphine.

The addition of narcotic analgesics $\text{in vitro}$ to nerve ending preparations from rat caudate nucleus in an assay of adenylate cyclase activity (AC) resulted in an inhibition of basal AC only at drug concentrations of 10−⁴M or higher, and no inhibition of dopamine-stimulated (DA) AC at these drug concentrations. The acute administration of morphine at a moderately high dose (60 mg/kg) produced an increase in striatal cAMP levels, and increases in basal and DA-AC in caudate nerve-endings. In morphine-tolerant rats, striatal cAMP levels and basal AC were similar to control values, while DA-AC was elevated. These results suggest: (1) that opiates do not act directly on DA-AC, the ‘dopamine receptor’, and (2) that the observed behavioural DA sensitivity in tolerant animals may be produced by the DA-AC supersensitivity.     

Role of the Dopaminergic System in the Acquisition, Expression and Reinstatement of MDMA-Induced Conditioned Place Preference in Adolescent Mice

Background

The rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in conditioned place preference (CPP) procedures, but the involvement of the dopaminergic system in MDMA-induced CPP and reinstatement is poorly understood.

Methodology/Principal Findings

In this study, the effects of the DA D1 antagonist SCH 23390 (0.125 and 0.250 mg/kg), the DA D2 antagonist Haloperidol (0.1 and 0.2 mg/kg), the D2 antagonist Raclopride (0.3 and 0.6 mg/kg) and the dopamine release inhibitor CGS 10746B (3 and 10 mg/kg) on the acquisition, expression and reinstatement of a CPP induced by 10 mg/kg of MDMA were evaluated in adolescent mice. As expected, MDMA significantly increased the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, and a priming dose of 5 mg/kg reinstated the extinguished preference. The higher doses of Haloperidol, Raclopride and CGS 10746B and both doses of SCH 23390 blocked acquisition of the MDMA-induced CPP. However, only Haloperidol blocked expression of the CPP. Reinstatement of the extinguished preference was not affected by any of the drugs studied. Analysis of brain monoamines revealed that the blockade of CPP acquisition was accompanied by an increase in DA concentration in the striatum, with a concomitant decrease in DOPAC and HVA levels. Administration of haloperidol during the Post-C test produced increases in striatal serotonin, DOPAC and HVA concentrations. In mice treated with the higher doses of haloperidol and CGS an increase in SERT concentration in the striatum was detected during acquisition of the CPP, but no changes in DAT were observed.

Conclusions/Significance

These results demonstrate that, in adolescent mice, the dopaminergic system is involved in the acquisition and expression of MDMA-induced CPP, but not in its reinstatement.     

Modulation of adenylate cyclase activity by Ca2+, phospholipid-dependent protein kinase in rat brain striatum

Summary The effects of purified Ca²⁺, phospholipid-dependent protein kinase (C-kinase) were studied on adenylate cyclase activity from rat brain striatum. C-kinase treatment of the membranes stimulated adenylate cyclase activity, the maximal stimulation between 50–80% was observed at 3.5 U/ml, whereas the catalytic subunit of cAMP dependent protein kinase did not show any effect on enzyme activity. The inclusion of Ca²⁺ and phosphatidyl serine did not augment the percent stimulation of adenylate cyclase by C-kinase, however EGTA inhibited the stimulatory effect of C-kinase on enzyme activity. Furthermore, the known inhibitors of C-kinase such as polymyxin-B and 1-(5-Isoquinoline sulfonyl)-2-methylpiperazine dihydrochloride (H-7) also inhibited the stimulatory effect of C-kinase on adenylate cyclase activity. In addition, in the presence of GTP the stimulatory effects of C-kinase on basal and N-Ethylcarboxamide adenosine- (NECA-), dopamine-(DA) and forskolin- (FSK) sensitive adenylate cyclase activities were augmented. On the other hand, the inhibitory effect of high concentrations of GTP on enzyme activity was attenuated by C-kinase treatment. In addition, oxotremorine inhibited adenylate cyclase activity in a concentration dependent manner, with an apparent Ki of about 10 µM and C-kinase treatment almost completely abolished this inhibition. These data suggest that C-kinase may play an important role in the regulation of adenylate cyclase activity possibly by interacting with a guanine nucleotide regulatory protein.Abbreviations C-kinase Ca^2– phospholipid-dependent protein kinase - NECA N-Ethylcarboxamide adenosine - DA Dopamine - FSK Forskolin - PMA Phorbol 12-(-Myristate), 13-Acetate, H-7, 1-(5-isoquinoline sulfonyl)-2-methylpiperazine dihydrochloride Presented in part at the VIth International Conference on Cyclic nucleotides, calcium and protein phosphorylation signal transduction in biological systems. September 2-6, 1986, Bethesda, MD (USA).M.B.A.-S. was Canadian Heart Foundation Scholar during the course of these studies.     

Muscarinic Receptors Modulate Dopamine-Activated Adenylate Cyclase of Rat Striatum

We investigated the effect of acetylcholine (ACh) on the activation of adenylate cyclase by dopamine (DA) in a lysed synaptosomal preparation from rat striatum. ACh reduced both basal and the DA-activated adenylate cyclase with an apparent IC50 of approximately 1 microM. From a kinetic analysis it appeared that ACh reduced the Vmax for activation by DA but not the activation constant for DA. For most preparations the Vmax was reduced by 30-40%. The presence of atropine did not affect the activation of the enzyme by DA but it blocked the inhibition by ACh. Following 6-hydroxydopamine lesion of the nigrostriatal pathway, the enzyme became supersensitive to activation by DA and also more sensitive to inhibition by ACh. Inhibition of adenylate cyclase by ACh appeared to be rather specific for activation by DA, as ACh had no effect on activation of adenylate cyclase by the adenosine analogue N6-(L-2-phenylisopropyl)adenosine. These results indicate that some striatal muscarinic and dopaminergic receptors are probably coupled to the same adenylate cyclase domain. Moreover, they suggest a biochemical model for the dynamic balance of cholinergic and dopaminergic neurons that innervate the striatum.     

Structural Studies on D2 Dopamine Receptors: Mutation of a Histidine Residue Specifically Affects the Binding of a Subgroup of Substituted Benzamide Drugs

Abstract: A histidine residue (His³⁹⁴) that is likely to be located in the ligand-binding region of the D₂ dopamine receptor has been mutated to a leucine (Leu³⁹⁴), and the properties of the mutant receptor have been determined. For a range of antagonists the mutation has only a minor effect on the affinity of the receptor for the antagonist. The mutation does, however, elicit a structurally specific effect on the affinity with which certain members of the substituted benzamide class of antagonist bind to the receptor. Some of these drugs, e.g., sulpiride, sultopride, and tiapride, bind with reduced affinity to the mutated receptor, whereas others, e.g., clebopride and metoclopramide, bind with increased affinity. However, the Na⁺/H⁺ sensitivity of the binding of sulpiride to the receptor is not reduced by the mutation. These findings have been interpreted in terms of the productive or unfavourable interaction of the His³⁹⁴ residue with these compounds.     

Alterations in cerebral β-adrenergic receptor-adenylate cyclase system induced by halothane,ketamine and ethanol

The effect of halothane, ketamine and ethanol on β-adrenergic receptor adenylate cyclase system was studied in the brain of rats. An anesthetic concentration of halothane and ketamine added in vitro decreased the stimulatory effect of norepinephrine on cyclic AMP formation in slices from the cerebral cortex. On the other hand, ethanol increased the basal activity of cerebral adenylate cyclase without affecting on the norepinephrine-stimulated activity. The increase of the basal activity induced by ethanol was not antagonized by propranolol, a β-adrenergic antagonist. In the crude synaptosomal (P₂) fraction, these drugs had no significant effect on the basal adenylate cyclase activity, binding of [³H]dihydroalprenolol to β-receptor, and binding of [³H]guanylylimido diphosphate ([³H]Gpp(NH)p) to guanyl nucleotide binding site. In contrast, the adenylate cyclase activity stimulated by Gpp(NH)p or NaF was significantly inhibited by an anesthetic concentration of these drugs. An anesthetic concentration of these drugs increased the membrane fluidity of P₂ fraction monitored by the fluorescence polarization technique. The addition of linoleic acid (more than 500 μM) also induced not only the increase of fluidity, but also the decrease of Gpp(NH)p- or NaF-stimulated adenylate cyclase activity in the cerebral P₂ fraction. The present results suggest that general anesthetics may interfere with the guanyl nucleotide binding regulatory protein-mediated activation of cerebral adenylate cyclase by disturbing the lipid region of synaptic membrane.     

Striatal adenylate cyclase activity following reserpine and chronic chlorpromazine administration in rats.

Adenylate cyclase activity was assayed in rat striatal homogenates. Dopamine and, to a lesser extent, 1-norepinephrine added $\text{in vitro}$ produced a dose-dependent enhancement of adenylate cyclase activity. Fluphenazine did not alter basal enzyme activity, but prevented both dopamine- and 1-norepinephrine-elicited increases. No significant changes in basal- or dopamine-stimulated adenylate cyclase activity were found in homogenates from rats pretreated with chlorpromazine for 21 days or reserpine for 2 days. It is concluded that the behavioral and neurophysiologic postsynaptic supersensitivities that follow similar pretreatments are not mediated by alterations in the sensitivity of striatal adenylate cyclase to dopamine.     

3H-SCH 23390 binding sites in the rat substantia nigra: evidence for a presynaptic localization and innervation by dopamine

Chronic treatment with SCH 23390, a selective D-1 dopamine receptor antagonist, elicited a 32% increase in the density of 3H-SCH 23390 binding sites in nigral membrane preparations but failed to change the apparent KD of the ligand for its binding sites. Haloperidol, a D-2 dopamine receptor antagonist which blocks the dopamine-sensitive adenylate cyclase and (-) sulpiride, a selective D-2 dopamine receptor blocker, which does not block the dopamine-sensitive adenylate cyclase, failed to change both the Bmax and KD of 3H-SCH 23390 binding. Finally, the intrastriatal injection of kainic acid produced a marked decrease of both GAD activity and GABA content and 3H-SCH 23390 binding sites (65%) in the homolateral substantia nigra. The results show that in the rat substantia nigra most of the 3H-SCH 23390 binding sites have a presynaptic localization on the striato-nigral GABAergic afferent terminals and suggest that dopamine released from nigral dendrites exerts a tonic influence on these presynaptic D-1 dopamine receptors.     

Modulation by Monoamines of Somatostatin-Sensitive Adenylate Cyclase on Neuronal and Glial Cells from the Mouse Brain in Primary Cultures

Primary cultures of mouse embryonic neuronal or glial cells from the cerebral cortex, striatum, and mesencephalon were used to identify and determine the cellular localization of somatostatin receptors coupled to an adenylate cyclase. Somatostatin inhibited basal adenylate cyclase activity on neuronal but not on glial crude membranes in the three structures examined. The somatostatin-inhibitory effect on neuronal crude membranes was still observed in the presence of (-)-isoproterenol, 3,4-dihydroxyphenylethylamine (dopamine, DA), or 5-hydroxytryptamine (5-HT, serotonin) used at a concentration (10(-5) M) inducing maximal adenylate cyclase activation. In addition, in most cases biogenic amines modified the pattern of the somatostatin-inhibitory effect, triggering either an increase in the peptide apparent affinity for its receptors or an increase in the maximal reduction of adenylate cyclase activity or both. However, 5-HT did not modify the somatostatin-inhibitory response on striatal and cortical neuronal crude membranes. The changes in somatostatin-inhibitory responses were interpreted as a colocalization of the amine and the peptide receptors on subtypes of neuronal cell populations. Finally, somatostatin was shown to inhibit adenylate cyclase activity following its activation by (-)-isoproterenol on glial crude membranes of the striatum and the mesencephalon but not on those of the cerebral cortex.     

Pertussis Toxin Attenuates D2 Inhibition and Enhances D1 Stimulation of Adenylate Cyclase by Dopamine in Rat Striatum

The response of adenylate cyclase to GTP and to dopamine (DA) was investigated in synaptic plasma membranes isolated from rat striatum injected with pertussis toxin, which inactivates the inhibitory guanine nucleotide-binding regulatory protein (Ni) of adenylate cyclase. Pertussis toxin treatment reverted the inhibitory effects on the enzyme activity elicited by micromolar concentrations of GTP and reduced by 50% the DA inhibition of cyclase activity via D2 receptors. The toxin treatment enhanced the net stimulation of enzyme activity by DA in the presence of micromolar concentrations of GTP. However, the stimulatory effect of the selective D1 receptor agonist SKF 38393 was not significantly affected. The data indicate that Ni mediates D2 inhibition of striatal adenylate cyclase and participates in the modulation of D1 stimulation of the enzyme activity by DA.     

Effect of reserpine on adenylate cyclase system of rat striatum

We have investigated the properties of dopamine-dependent adenylate cyclase in rat striatal homogenates, 20 h after reserpine treatment. In this experimental condition, we have found that the affinity of the enzyme for dopamine is greatly enhanced. On the other hand, the concentration of apomorphine required to produced half-maximal activation of the enzyme in striatal homogenates of controls and reserpine-treated rats is not changed. The unchanged affinity of adenylate cyclase for the substrate (ATP:Mg⁺⁺) indicates that reserpine probably affects the receptor component of the enzyme.     

Chronic treatment with metoclopramide induces behavioral supersensitivity to apomorphine and enhances specific binding of 3H-spiroperidol to rat striata

Metoclopramide is a dopamine (DA) antagonist with a potency and pharmacologic profile similar to chlorpromazine, but is paradoxical insofar as it is virtually inactive as an antagonist of DA-stimulated adenylate cyclase and has an extremely low affinity for DA/neuroleptic binding sites in membranes prepared from DA-rich brain regions. Chronic administration of metoclopramide to rats induced behavioral supersensitivity to apomorphine which was associated with enhanced specific binding of ³H-spiroperidol to striatal membranes. These changes were seen when metoclopramide was administered to rats in their drinking water for 39 days, but not when it was given i.p. once daily for 14 days. These findings are discussed in terms of their implications regarding the validity of currently used receptor models as predictors of DA antagonism as well as the implicit assumption that DA/neuroleptic binding sites truly represent functionally relevant DA receptors.