Pharmacologic evidence for direct dopaminergic regulation of striatal acetylcholine release

This study was done to provide pharmacologic evidence for the location of those striatal dopamine D-1 and D-2 receptors that participate in the regulation of local acetylcholine (ACh) release. Striatal tissue slices from adult male Sprague-Dawley rats were preloaded with [3H]choline and superfused in separate experiments with buffer containing either: a D-2-specific agonist (LY141865 or LY171555), a D-2 specific antagonist (L-sulpiride), a D-1 specific agonist (SKF38393), or a D-1 antagonist (SCH23390), in the presence or absence of tetrodotoxin (TTX), used to block interneuronal activity. With either D-2 agonist there was a dose-dependent decrease in K+-stimulated [3H]ACh release, maximally at 5 X 10(-7)-10(-6) M [agonist] and to the same extent with each drug. Both SKF38393 and SCH23390 increased [3H]ACh release at tested concentrations of these agents. Results were unchanged when any of the drugs used was superfused in the presence of TTX, 5 X 10(-7) M. These data are consistent with the hypothesis that populations of striatal D-1 and D-2 receptors exist on local cholinergic neurons, where they regulate ACh release. Alternative interpretations are discussed.     

Binding of a novel dopaminergic agonist radioligand [3H]-fenoldopam (SKF 82526) to D-1 receptors in rat striatum

Fenoldopam (SKF 82526), a dopamine agonist which exhibits D-1 receptor subtype selectivity, was evaluated as a radioligand for this receptor subtype. In saturation studies in rat striatal membrane preparations, [3H]-fenoldopam appeared to label a single binding site with a KD of 2.3 +/- 0.1 nM and a Bmax of 590 +/- 40 fmoles/mg protein. In competition binding experiments, binding was shown to be stereoselective, and rank ordering of affinities of dopaminergic and non-dopaminergic compounds closely correlated with potencies of these compounds in stimulating or inhibiting dopamine-sensitive adenylate cyclase (D-1) and in binding to D-1 sites labelled with the antagonist [3H]-cis-flupenthixol. The most potent competitors were the recently identified D-1 selective antagonists, SCH 23390 and SKF R-83566. [3H]-Fenoldopam was also used to assess agonist/D-1 receptor interactions. The results suggest that [3H]-fenoldopam is a useful and selective agonist radioligand for the D-1 receptor.     

Behavioural expression of D-1 receptor supersensitivity depends on previous stimulation of D-2 receptors

SKF 38393 (2 mg/kg s.c.), a reportedly selective D-1 agonist, failed to induce contralateral turning behaviour in naive rats bearing 12 days old unilateral 6-hydroxydopamine lesions. On the other hand strong contraversive turning in response to SKF 38393 was obtained if rats had been tested 2 or 7 days before with apomorphine (0.1 mg/kg s.c.) or with LY 171555 (0.2 mg/kg s.c.), a selective D-2 receptor agonist. Contraversive turning in response to SKF 38393 was blocked by a low dose (0.05 mg/kg s.c.) of the specific D-1 antagonist SCH 23390. The results indicate that the behavioural expression of D-1 receptor supersensitivity following lesion of dopaminergic neurons depends on previous exposure to a stimulation of D-2 receptors.     

Forskolin Potentiates the Stimulation of Rat Striatal Adenylate Cyclase Mediated by D-1 Dopamine Receptors, Guanine Nucleotides, and Sodium Fluoride

We report here that forskolin acts in a synergistic manner with dopaminergic agonists, guanine nucleotides, or sodium fluoride to potentiate the stimulation of rat striatal adenylate cyclase mediated by these reagents. In the presence of 100 microM GTP, 100 microM guanyl-5'-yl imidodiphosphate [Gpp(NH)p], or 10 mM NaF, there is a greater than additive increase in forskolin-stimulated enzyme activity as well as a concomitant decrease (two- to fourfold) in the EC50 value for forskolin stimulation of striatal enzyme activity. In the presence of various concentrations of forskolin (10 nM-100 microM), the stimulation of adenylate cyclase elicited by GTP, Gpp(NH)p, and NaF is potentiated 194-1,825%, 122-1,141%, and 208-938%, respectively, compared with the stimulation by these agents above basal activity in the absence of forskolin. With respect to 3,4-dihydroxyphenylethylamine (dopamine) receptor-mediated stimulation of striatal enzyme activity, the stimulation of enzyme activity by dopaminergic agonists, in the absence or presence of forskolin, was GTP-dependent and could be antagonized by the selective D-1 antagonist SCH23390 (100 nM), indicating that these effects are mediated by D-1 dopamine receptors. In the presence of 100 microM GTP, forskolin at various concentrations markedly potentiates the stimulation elicited by submaximal as well as a maximally effective concentrations of dopamine (100 microM) and SKF38393 (1 microM). At higher concentrations of forskolin (10-100 microM) the stimulation elicited by the partial agonist SKF38393 is comparable to that of the full agonist dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of D-1 receptor system on the D-2 receptor-mediated hypothermic response in mice

The hypothermia induced by apomorphine, a mixed dopamine (DA) agonist in male Swiss-Webster mice, was not blocked by the selective D-1 antagonist SCH 23390 but was completely blocked by the selective D-2 antagonists haloperidol, sulpiride and YM-09151-2. The selective D-1 agonist SKF 38393 did not elicit hypothermic response but the selective D-2 agonist quinpirole caused a marked lowering of rectal temperature. D-2 antagonists blocked this response to quinpirole. SCH 23390 enhanced and SKF 38393 attenuated the hypothermia induced by quinpirole. Ineffective doses of haloperidol and SKF 38393, when given together, completely blocked the effect of quinpirole. It was concluded that hypothermia is a D-2 receptor mediated response but modulated by the D-1 receptor system. In another series of experiments the influence of neuroleptics and antidepressants on the hypothermic effect of apomorphine and quinpirole was investigated. The hypothermic effect of a low dose (1 mg/kg) of apomorphine was blocked by the D-2 receptor antagonists, but not by classical antidepressants. However, the response to a high dose (10 mg/kg) of apomorphine was blocked by both classical antidepressants and D-2 antagonists (except haloperidol). These drugs did not show similar effect on quinpirole-induced hypothermia. It is clear that the hypothermic response, especially that of quinpirole, is not a suitable model for testing either neuroleptics or antidepressants.     

Binding of [3H]SKF 38393 to Dopamine D-l Receptors in Rat Striatum In Vitro; Estimation of Receptor Molecular Size by Radiation Inactivation

[3H]SKF 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine) binds with high affinity to 3,4-dihydroxyphenylethylamine (dopamine) D-1 receptors in rat striatum in vitro (KD = 7 and 14 nM in nonfrozen and frozen striatum, respectively). The number of binding sites (Bmax) was approximately 80.0 pmol/g of original tissue, a value similar to the Bmax for the dopamine D-1 antagonist SCH 23390. Nondisplaceable [3H]SKF 38393 binding was approximately 45% of total binding. Irradiation (0-4 Mrad) of frozen whole striata decreased the number of [3H]SKF 38393 binding sites monoexponentially without changing the binding affinity. The functional molecular mass for the agonist dopamine D-1 binding site was 132,800 daltons, which is higher than the functional molecular mass of the antagonist dopamine D-1 binding site (approximately 80,000 daltons).     

Coexistence of inhibitory dopamine D-1 and excitatory D-2 receptors on the same caudate nucleus neurons

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to determine whether or not dopamine D-1 and D-2 receptors co-exist in the same caudate nucleus (CN) neurons that receive inputs from the substantia nigra (SN), and in which spikes elicited by SN stimulation were blocked by domperidone, a selective D-2 antagonist. Iontophoretic application of dopamine produced a dose-dependent inhibition of spontaneous firing in 2 of 4 spontaneously active CN neurons and an increase in firing in the remaining 2 neurons. However, dopamine inhibited the glutamate-induced firing in 31 of 32 CN neurons that were not spontaneously active. Similar inhibition with iontophoretically applied SKF 38393, a selective D-1 agonist, was observed in 33 of 34 spontaneously inactive neurons tested. When the effects of dopamine, SKF 38393 and bromocriptine (D-2 agonist) were examined on the same CN neurons, the inhibitory effects of both dopamine and SKF 38393 were seen in 14 of 15 neurons, and both an inhibition by SKF 38393 and an excitation by bromocriptine were observed in 15 of 17 neurons. The inhibitory effects of dopamine and SKF 38393 were antagonized by haloperidol and SCH 23390 (D-1 antagonist) without being affected by domperidone. Furthermore, the dopamine-induced inhibition was converted to an excitation during simultaneous application of SCH 23390 in 6 of 10 CN neurons, and this excitation was antagonized by domperidone. These results strongly suggest that the inhibitory D-1 and excitatory D-2 receptors co-exist on the same CN neurons receiving inputs from the SN.     

Dopamine receptor-mediated regulation of striatal cholinergic activity: positron emission tomography studies with norchloro[18F]fluoroepibatidine

Large numbers of in vitro studies and microdialysis studies suggest that dopaminergic regulation of striatal acetylcholine (ACh) output is via inhibitory dopamine D2 receptors and stimulatory dopamine D1 receptors. Questions remain as to the relative predominance of dopamine D2 versus D1 receptor modulation of striatal ACh output under physiological conditions. Using positron emission tomography, we first demonstrate that norchloro[18F]fluoroepibatidine ([18F]NFEP), a selective nicotinic ACh receptor (nAChR) ligand, was sensitive to changes of striatal ACh concentration. We then examined the effect of quinpirole (D2 agonist), raclopride (D2 antagonist), SKF38393 (D1 agonist), and SCH23390 (D1 antagonist) on striatal binding of [18F]NFEP in the baboon. Pretreatment with quinpirole increased the striatum (ST) to cerebellum (CB) ratio by 26+/-6%, whereas pretreatment with raclopride decreased the ST/CB ratio by 22+/-2%. The ratio of the distribution volume of [18F]NFEP in striatum to that in cerebellum, which corresponds to (Bmax/K(D)) + 1 (index for nAChR availability), also showed a significant increase (29 and 20%; n = 2) and decrease (20+/-3%; n = 3) after pretreatment with quinpirole and raclopride, respectively. However, both the D1 agonist and antagonist had no significant effect. This suggests that under physiological conditions the predominant influence of endogenous dopamine on striatal ACh output is dopamine D2, not D1, receptor-mediated.     

Opposing Roles of Dopamine D1 and D2 Receptors in Nigral γ-[3H]Aminobutyric Acid Release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.     

SCH 23390 dissociated from conventional neuroleptics in apomorphine climbing and primate acute dyskinesia models

SCH 23390 induced only a negligible incidence of the acute dyskinetic syndrome, a predictor of neuroleptic-induced extrapyramidal liability, in squirrel monkeys. However, haloperidol-induced dyskinesias were potentiated by SCH 23390 and were blocked by the D-1 agonist, SKF 38393. When administered orally or intraperitoneally to mice, SCH 23390 showed a considerably wider dose separation than did conventional neuroleptics between antagonism of apomorphine climbing and antagonism of stereotyped sniffing. Clinically relevant distinctions may exist between D-1 and D-2 antagonists, with D-1 antagonists (exemplified by SCH 23390) showing lower, although possibly not negligible, potential to cause extrapyramidal side effects.     

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.     

Stimulation of dopamine D-1 receptors by SKF 38393 induces EEG desynchronization and behavioral arousal

The dopamine D-1 receptor agonist SKF 38393 dose-dependently (2.5-10 mg/kg) induced desynchronization of the electroencephalographic (EEG) activity and behavioral arousal in both rabbits and rats. Unlike apomorphine, SKF 38393 elicited no signs of stereotyped behavior in rabbits and minimal effects, such as episodes of grooming, in rats. The effects of SKF 38393 10 mg/kg on the EEG were prevented by the selective D-1 receptor antagonist SCH 23390 at a dose as low as 0.003 mg/kg, but not by the D-2 antagonist (-)-sulpiride (25-50 mg/kg). These data provide evidence of a role of D-1 receptors in the generation of EEG activity related to behavioral arousal. In addition, this model is a valuable tool to functionally evaluate the D-1 antagonistic properties of neuroleptics.     

D1 and D2 Dopaminergic Regulation of Acetylcholine Release from Striata of Freely Moving Rats

The effects of selective D1 and D2 dopaminergic agents on the extracellular acetylcholine (ACh) content in striata of freely moving rats were determined by the microdialysis technique. LY 171555, a selective D2 agonist, reduced ACh output by approximately 30% within 20 min at the dose of 0.2 mg/kg, i.p., whereas the D2 antagonists (-)-remoxipride (10 mg/kg, s.c.) and L-sulpiride (50 mg/kg, i.p.) induced maximal increases of approximately 50% within 10 and 20 min, respectively. In contrast, the D1 antagonist SCH 23390 (0.25 mg/kg, s.c.) decreased the extracellular ACh content by approximately 30% in 20 min, but lower doses--0.025 and 0.05 mg/kg--had no such effect. The stimulation of ACh release by LY 171555 was prevented by (-)-remoxipride but not by SCH 23390 (0.25 mg/kg, s.c.). In addition, the D1 agonist SKF 38393 failed to modify the ACh increasing effect of (-)-remoxipride. Thus, the D1 and D2 receptors subserve opposing functions on ACh release. The D1/D2 dopaminergic agonist R-apomorphine, at the does of 1 mg/kg, i.p., reduced ACh output by approximately 35% only when D1 receptors were blocked by SCH 23390 (0.025 mg/kg, s.c.). The results provide clear in vivo evidence of the tonic inhibition exerted by dopaminergic nigrostriatal input on the cholinergic system of the basal ganglia through D1 and D2 receptors.     

Chronic estradiol treatment increases ovariectomized rat striatal D-1 dopamine receptors

Striatal D-1 dopamine (DA) receptors were investigated following chronic 17 beta-estradiol (10 micrograms, b.i.d., s.c., for two weeks) to ovariectomized (OVX) female rats. This treatment initiated the day after ovariectomy has revealed that the maximal density in homogenates of striatal D-1 DA receptors (Bmax) labelled with [3H] SCH 23390 was increased (44% without and 28% with 120 mM NaCl in the assay buffer). Estradiol treatments initiated 2 or 4 weeks after ovariectomy did not induce D-1 DA receptor binding modifications. The affinity (Kd) of the ligand for the receptor remains unchanged by the steroid treatment while NaCl increased both the density and the affinity of [3H] SCH 23390 binding to striatal D-1 DA receptors. By autoradiography, the increase of striatal [3H] SCH 23390 binding to D-1 DA receptors after chronic estradiol treatment was found to be homogenously distributed in this brain region. Thus, chronic treatment with estradiol of ovariectomized rats leads to an increased density of striatal D-1 DA receptors but, this hormonal modulation of D-1 DA receptors is lost when treatment is started 2 weeks after ovariectomy or later.     

The discriminative stimulus properties of the D-1 agonist SK&F 38393 and the D-2 agonist (-)-NPA are mediated by separate mechanisms

The dopamine D-1 agonist SK&F 38393 (10 mg/kg) the D-2 agonist (-)-NPA (0.04 mg/kg) and d-amphetamine (1.0 mg/kg) were established as discriminative stimuli versus saline in rats. The stimulus induced by SK&F 38393 was stereoselective, since the R-(+)-, but not the S-(-)-enantiomer was effective. It was mimicked by two partial D-1 agonists with central effects, SK&F 75670 and Lu 24-040, but not by the peripheral agonist fenoldopam. D-2 agonists and d-amphetamine were ineffective. The effect of SK&F 38393 was antagonized by SCH 23390, but not by its inactive enantiomer SCH 23388 or by the D-2 antagonist YM 09151-2. The (-)-NPA stimulus was dependent on postsynaptic D-2 receptors: It was mimicked by quinpirole and pergolide in stimulant dosages, whereas the partial D-2 agonist (-)-3-PPP inhibited the effect of (-)-NPA. The dopamine synthesis inhibitor alpha-methyl-p-tyrosine did not antagonize the effect of (-)-NPA. Likewise, the above-mentioned D-1 agonists produced saline responding. D-amphetamine produced partial substitution to (-)-NPA. The (-)-NPA stimulus was blocked by YM 09151-2, but not by SCH 23390. In d-amphetamine-trained rats, quinpirole, (-)-NPA and pergolide produced generalization, whereas SK&F 38393 was ineffective. Both SCH 23390 and YM 09151-2 antagonized the effect of d-amphetamine. It is concluded that the cues induced by SK&F 38393 and (-)-NPA are mediated by separate D-1 and D-2 sites, whereas both sites contribute to the effect of d-amphetamine.     

Identification of D1-like dopamine receptors on human blood platelets

Dopamine is able to inhibit the epinephrine-induced aggregation of human blood platelets, but the mechanism of action has not been elucidated. In this study we report that membranes from human blood platelets possess high affinity, saturable and stereoselective binding sites for the D1 dopamine receptor antagonist (3H) SCH 23390. (3H) SCH 23390 appeared to label a single class of binding sites with a Bmax of 18.6 +/- 1.6 fmol/mg protein and a KD of 0.8 nM. The potencies of different dopaminergic antagonists and agonists in displacing (3H) SCH 23390 from blood platelet membranes were similar to those obtained for striatal membranes. Unlike the classically defined D1 receptors, e.g. those in striatum, the D1 receptor sites on platelets appeared not to be coupled to the adenylate cyclase system, hence the term "D1-like". The D1 agonist SKF 38393 was more potent than dopamine in inhibiting platelet aggregation induced by epinephrine, and the effects of dopamine and SKF 38393 were prevented by SCH 23390. These results suggest that the inhibitory action of dopamine on the epinephrine-induced platelet aggregation is mediated through these D1-like receptors.     

D-1 dopamine agonist administration reduces the threshold for convulsions produced by pilocarpine

Focal, limbic seizures were produced by systemically administered pilocarpine (200 mg/kg, i.p.); as previously described this dose produces limbic stereotypies but neither convulsions nor seizure-related brain damage. The pretreatment, 5 minutes prior pilocarpine, with the D-1 agonist SKF 38393 (-ED50 = 1 mg/kg; i.p.) induced convulsions similar to those produced by a higher, convulsant dose of pilocarpine. On the other hand, the pretreatment with the D-2 agonist LY 171555 failed to induce convulsions. The D-1 receptor antagonist SCH 23390 prevented the convulsions induced by SKF 38393 plus pilocarpine (200 mg/kg). This study indicates that D-1, but not D-2, receptor stimulation converts subconvulsant doses of pilocarpine into convulsant ones.     

In Situ Molecular Size of Agonist Dopamine D-2 Binding Sites in Rat Striatum

Specific binding of [3H]N-propylnorapomorphine [( 3H]NPA) to 3,4-dihydroxyphenylethylamine (dopamine) D-2 receptors was investigated in rat striatum in vitro. For various dopamine receptor substances, the rank order of potency to inhibit [3H]NPA binding was spiroperidol greater than or equal to NPA greater than LY 171555 greater than SCH 23390 greater than SKF 38393. A single high-affinity binding site was found in membranes prepared in either Tris-citrate buffer or imidazole buffer; the affinity constants were 0.11 and 0.76 nM, respectively. The number of receptors (33 pmol/g wet weight) was independent of whether the membranes were prepared in Tris-citrate buffer or imidazole buffer and was similar to the number of receptors estimated by [3H]spiroperidol binding to dopamine receptors. Irradiation inactivation of frozen whole rat striata showed a monoexponential loss of [3H]NPA binding sites without a change in the binding affinity. The target size of the [3H]NPA binding site was 81,000 daltons, which shows that the functional molecular entity to bind the dopamine D-2 agonist was smaller than the molecular entity to bind the dopamine D-2 antagonist [3H]spiroperidol (target size, 137,000 daltons).     

Putative Neurotoxicity of SKF 38393 and Other D1 Dopaminergic Drugs Investigated in Rat Striatum

The recently alleged neurotoxicity of the D1 receptor agonist, SKF 38393, was investigated in rat striatum by measuring the enzymes acetylcholinesterase (AChE) and glutamate decarboxylase (GAD). First, unilateral intrastriatal microinjection of the excitotoxin kainic acid (2 micrograms in 1 microliter) was shown to evoke vigorous contraversive circling, followed 1 or 2 weeks later by profound decreases in striatal AChE (24 and 54%), GAD (51 and 75%), and protein (36 and 47%), as well as loss of GAD (45% at 2 weeks) in the ipsilateral substantia nigra. Similar striatal treatments with SKF 38393 (30 micrograms in 0.5-1 microliter), the related benzazepines SKF 82526 (D1 agonist, 30 micrograms in 1 microliter) and SCH 23390 (D1 antagonist, 5 micrograms in 1 microliter), or the phenanthridine D1 agonist CY 208-243 (5 micrograms in 1 microliter) failed to affect the rats' behaviour or their striatal levels of AChE, GAD, and protein. Intrastriatal SKF 38393 (30 micrograms in 0.5 microliter) also had no influence on these enzymes in the substantia nigra. It is concluded that none of the D1 dopaminergic compounds examined here was neurotoxic toward the many different cell groups that contain AChE and/or GAD in the striatum.     

Striatal cholinergic function reflects differences in D-2 dopaminergic receptor activation

The ergot derivatives, bromocriptine, lisuride and quinpirole (Ly-171555), activators of D-2 receptors, increased striatal acetylcholine (ACh) content by about 40% and induced a 30% inhibition of ACh evoked release from striatal slices, similar to the effects of the dopaminergic agonist apomorphine. These actions were a consequence of dopaminergic activation since they were antagonized by pretreatment with the neuroleptic agent, pimozide. In contrast, pretreatment with L-sulpiride (100 mg/kg), a specific antagonist for the D-2 dopaminergic receptor only, prevented the rise of ACh levels induced by apomorphine or quinpirole but did not interfere with the lisuride- or bromocriptine- induced ACh increases. Similarly, inhibition of the ACh evoked release produced by lisuride (3ωM) was prevented by pimozide (1mg/kg) but not by pretreatment with L-sulpiride. Addition of L-sulpiride (5ωM) to the Krebs solution had no effect on the inhibition of ACh-evoked release induced by lisuride, but a lower concentration (1ωM) antagonized the inhibition induced by quinpirole. Lisuride and bromocriptine responses were both insensitive to sulpiride. These results are discussed in terms of different interaction with the dopaminergic D-2 receptors by the drugs studied.