Dopamine receptor antagonists reverse amphetamine-induced behavioral alteration on a differential reinforcement for low-rate (DRL) operant task in the rat

Previous studies have shown that amphetamine significantly alters operant responding on the behavior maintained on a schedule of differential reinforcement of low-rate (DRL). As such, behavioral deficiency of DRL responding has been observed by the drug-induced increase of non-reinforced responses and a leftward shift of inter-response time (IRT) curve on DRL responding in the rat. However, the neurochemical basis for amphetamine-induced DRL behavioral alternations remain to be elucidated. The present study was then designed to examine whether the effects of amphetamine were dependent on dopamine-subtyped receptors, this was carried out by the co-administration of the selective D1 and D2 receptor antagonists, SCH23390 and raclopride respectively. Rats were first trained to perform on DRL 10-sec task and then divided into four groups, which received separate types of double injections before the behavioral session. The four groups were the saline control group, the amphetamine alone group, the dopamine antagonist alone group, and the combination of [corrected] amphetamine and dopamine antagonist group. The saline control group performed DRL responding in an efficient manner with a major index for the peak time of the IRT curve, which was fairly localized within the 10-sec bin throughout the test phase. The subjects injected with amphetamine (1 mg/kg) significantly shortened IRT that led to a leftward shift of IRT curve, which was further revealed by a decreased peak time without significant effectiveness on the peak rate and burst response. Even though the group given SCH23390 or raclopride alone showed profound disruption on DRL behavior by flattening the IRT curve, the co-administration of amphetamine with SCH23390 or raclopride reversed the aforementioned amphetamine-induced behavioral deficiency on DRL task. Together, these results suggest that the dopamine D1 and D2 receptors are involved and important to the temporal regulation of DRL response under psychostimulant drug treatment. Furthermore, this highlights the involvement of the brain dopamine systems in the temporal regulation of DRL behavior performance.     

Effects of SCH23390 and raclopride on a run-climb-run behavioral task in rats

The present study was designed to compare the putative differential behavioral consequences of treatment with SCH23390 (a selective dopamine D1 receptor blocker) and raclopride (a selective dopamine D2 receptor blocker) by employing a run-climb-run (RCR) behavioral task of different lengths. Rats were trained to traverse an uncovered floor alleyway (150 cm), climb a vertical rope (70 or 130 cm), and run across an upper board (100 cm) to access water for the reinforcement. At doses of 0.05, 0.10 and 0.15 mg/kg administered intraperitoneally 60 min before the behavioral session, both SCH23390 and raclopride significantly increased the total time to complete the tasks in a dose-related fashion. Microstructural analysis on the RCR behavioral performance revealed that the most apparent impairment induced by either drug was observed as the subject shifted motion from the end of the floor alleyway to the rope when hopping or to initiate climbing. However, the motion shift from climbing to running on the upper board was significantly impaired by raclopride, but not by SCH23390. Surprisingly, neither SCH23390 nor raclopride affected the climbing response itself. Running responses on the floor alleyway board were significantly disrupted by raclopride, whereas those on the upper board were significantly disrupted by SCH23390. Deficits induced by both drugs were more profound for the longer compared to the shorter rope, and were most notably shown at the transition area from running to climbing. These data indicate that both dopamine D1 and D2 receptors are involved in the RCR behavior performance. The results also suggest that the cost of motoric demand for behavioral performance is important for evaluating of the effects of drugs blocking dopamine receptors.     

The influence of systemically administered selective dopamine antagonists of D1 and D2/D3 types on alimentary and escape conditioned placing reactions in cats

Systemic administration of the specific antagonists of D1 (SCH23390, 0.005-0.1 mg/kg) and D2/D3 (raclopride, 0.1-0.25 mg/kg) dopamine receptors leeds to dose-dependent increase of the reaction time and decrease of conditined reflex probability up to full blocking (in the case of SCH23390) of alimentary and escape conditioned placing reaction in cats. The action of both antagonists was far more suppressive regarding conditioned escape reflex. The action of SCH23390 was far more effective than that of raclopride concerning both types of conditioned reflexes.     

Differential effects of dopamine D1 and D2 receptor antagonists on conditioned orienting behavior in the rat

Brain dopamine (DA) systems are known to be important in regulation of behavior conditioned to appetitive stimuli. Nevertheless, despite a large body of evidence showing behavioral deficits in the operant conditioning paradigm produced by DA receptor blockade, there have been relatively few studies directly assessing behavioral changes in classical conditioning paradigm under this drug treatment. By employing an appetitive Pavlovian conditioning task, the present work investigated the effects of selective D1 and D2 receptor antagonists on the expression and acquisition of the conditioned orienting response (COR) and food-cup approach. SCH23390 (0, 0.05, and 0.10 mg/kg) and raclopride (0, 0.1, and 0.2 mg/kg) were administered via an intra-peritoneal route in a between-group design. Data from Experiment 1 showed that both SCH23390 and raclopride suppressed expression of the COR and food-cup approach, but only the impairment produced by raclopride reached a significant level. In Experiment 2, with SCH23390 being administered during the acquisition phase, the suppressed COR was completely restored in a subsequent (24 h later) drug-free session. In contrast, the suppressed COR in raclopride-pretreated groups was only partially restored. These findings support the view that the DA system plays a role in the neural substrates underlying this appetitive conditioning. In addition, D2 receptors are more likely involved in the modulation of learning process of the COR than D1 receptors.     

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.     

Obese OLETF rats exhibit increased operant performance for palatable sucrose solutions and differential sensitivity to D2 receptor antagonism

CCK-1-receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and exhibit a greater preference for sucrose compared with lean controls [Long-Evans Tokushima Otsuka (LETO)]. To directly assess motivation to work for sucrose reward in this model of obesity and type 2 diabetes, we examined the operant performance of OLETF rats at nondiabetic and prediabetic stages (14 and 24 wk of age, respectively) on fixed-ratio (FR) and progressive-ratio (PR) schedules of reinforcement. To evaluate the involvement of dopamine systems, the effects of the D1 receptor antagonist SCH23390 (100 and 200 nmol/kg ip) and the D2 receptor antagonist raclopride (200 and 400 nmol/kg ip), were also tested on PR responding for sucrose. Compared with age-matched LETO rats, 14-wk-old OLETF rats emitted more licks on the "active" empty spout operant on the FR-10 schedule of reinforcement to obtain 0.01 M and 0.3 M sucrose and completed higher ratio requirements on the PR schedule to gain access to 0.3 M and 1.0 M sucrose. At 24 wk, this effect was limited to 1.0 M sucrose. Both antagonists were potent in reducing operant responding to 0.3 M sucrose in both strains at both ages, and there was no strain effect to SCH23390 at either age. OLETF rats, on the other hand, showed an increased sensitivity to the higher dose of raclopride, resulting in reduced responding to sucrose reinforcement at 24 wk. Taken together, these findings provide the first direct evidence for an increased motivation for sucrose reward in the OLETF rats and suggest altered D2 receptor regulation with the progression of obesity and prediabetes.     

The effect of SCH 23390 against toxic doses of cocaine, d-amphetamine and methamphetamine

The effect of SCH 23390, a dopamine-one (D1) antagonist, in preventing acute toxicity induced by lethal doses of cocaine, d-amphetamine, and methamphetamine was studied in the rat. Animals were first pretreated with SCH 23390 (0.0, 0.5, 1.0, and 2.5 mg/kg, i.p.) and then were challenged with cocaine (70 mg/kg, i.p., an LD85), d-amphetamine (75 mg/kg, i.p., an LD95), and methamphetamine (100 mg/kg, i.p., an LD90). SCH 23390 did not alter the incidence of stimulant-induced seizures compared to the vehicle controls. Significant protection against cocaine-induced death was afforded only by the lowest dose of SCH 23390 tested. Significant protection against d-amphetamine-induced death was provided by all doses, with a dose dependent effect noted so that the incidence decreased from 95% for vehicle to 30% (p less than or equal to 0.01) with 2.5 mg/kg SCH 23390 pretreatment. No statistically significant reduction in the incidence of methamphetamine-induced death was seen with SCH 23390 pretreatment. The ability of SCH 23390 to protect against d-amphetamine, but apparently not against methamphetamine-induced death, suggests that different mechanisms of toxicity may exist between these drugs at high doses.     

Involvement of Hippocampal D1-Like Dopamine Receptors in the Inhibitory Effect of Cannabidiol on Acquisition and Expression of Methamphetamine-Induced Conditioned Place Preference

Cannabidiol (CBD) is a non-psychotomimetic compound with strong potential to decrease the psychostimulant’s rewarding effect with unclear receptors. Furthermore, as a part of the reward circuit, the hippocampus plays a crucial role in regulating the reward properties of drugs as determined by conditioned place preference (CPP). In the current research, CPP was used to evaluate the role of intra-CA1 microinjection of D1-like dopamine receptor antagonists in CBD's inhibitory effect on the acquisition and expression phases of methamphetamine (METH). Animals were treated by METH (1 mg/kg; sc) in a five-day schedule to induce CPP. To find out the impact of D1-like dopamine receptor antagonist, SCH23390, in the CA1 on the inhibitory influence of CBD on the acquisition of METH, the rats received intra-CA1 administration of SCH23390 (0.25, 1, and 4 µg/0.5 µl) following ICV treatment of CBD (10 µg/5 µl) over conditioning phase of METH. Furthermore, animals were given SCH23390 in the CA1 ensuing ICV microinjection of CBD (50 µg/5 µl) in the expression phase of METH to rule out the influence of SCH23390 on the suppressive effect of CBD on the expression of METH CPP. Intra-CA1 microinjection of SCH23390 abolished CBD's suppressive impact on both METH-induced CPP phases without any side effect on the locomotion. The current research disclosed that CBD inhibited the rewarding characteristic of METH via D1-like dopamine receptors in the CA1 region of the hippocampus.

     

Involvement of dopamine receptors in binge methamphetamine-induced activation of endoplasmic reticulum and mitochondrial stress pathways

Single large doses of methamphetamine (METH) cause endoplasmic reticulum (ER) stress and mitochondrial dysfunctions in rodent striata. The dopamine D(1) receptor appears to be involved in these METH-mediated stresses. The purpose of this study was to investigate if dopamine D(1) and D(2) receptors are involved in ER and mitochondrial stresses caused by single-day METH binges in the rat striatum. Male Sprague-Dawley rats received 4 injections of 10 mg/kg of METH alone or in combination with a putative D(1) or D(2) receptor antagonist, SCH23390 or raclopride, respectively, given 30 min prior to each METH injection. Rats were euthanized at various timepoints afterwards. Striatal tissues were used in quantitative RT-PCR and western blot analyses. We found that binge METH injections caused increased expression of the pro-survival genes, BiP/GRP-78 and P58(IPK), in a SCH23390-sensitive manner. METH also caused up-regulation of ER-stress genes, Atf2, Atf3, Atf4, CHOP/Gadd153 and Gadd34. The expression of heat shock proteins (HSPs) was increased after METH injections. SCH23390 completely blocked induction in all analyzed ER stress-related proteins that included ATF3, ATF4, CHOP/Gadd153, HSPs and caspase-12. The dopamine D(2)-like antagonist, raclopride, exerted small to moderate inhibitory influence on some METH-induced changes in ER stress proteins. Importantly, METH caused decreases in the mitochondrial anti-apoptotic protein, Bcl-2, but increases in the pro-apoptotic proteins, Bax, Bad and cytochrome c, in a SCH23390-sensitive fashion. In contrast, raclopride provided only small inhibition of METH-induced changes in mitochondrial proteins. These findings indicate that METH-induced activation of striatal ER and mitochondrial stress pathways might be more related to activation of SCH23390-sensitive receptors.     

Involvement of D1 dopamine receptors in the nicotine-induced noradrenaline release from hypothalamic and preoptic noradrenaline nerve terminal systems

Nicotine (4 × 2 mg/kg, i.p.) was given every 30 min for 2 h to male rats. Some rats were pretreated with the D1 dopamine (DA) receptor antagonist SCH 23390 (1 mg/kg, i.p.) or with the D2 DA receptor antagonist raclopride (1 mg/kg, i.p.), 5 min before nicotine treatment. Hypothalamic and preoptic catecholamine levels were measured by quantitative histofluorimetry in discrete DA and noradrenaline nerve terminal systems.Nicotine treatment produced a depletion of catecholamine stores in noradrenaline and DA nerve terminals of the hypothalamus, the preoptic area and the median eminence, an action which was counteracted by SCH 23390 but not by raclopride.The results indicate that hypothalamic D1 DA receptors may regulate the sensitivity of the nicotinic cholinoceptors and increase their ability to release hypothalamic noradrenaline. A possible role of D1 DA receptor antagonists to reduce the ability of nicotine treatment to produce rapid increases in LH, prolactin and corticosterone secretion and tonic arousal is implicated.     

Endogenous Dopamine Facilitates Striatalln Vivo Acetylcholine Release by Acting on D1 Receptors Localized in the Striatum

Intrastriatal application of the D1 antagonist SCH 23390 by two procedures, reverse dialysis (20 microM) and local injection (0.45 nmol per striatum), elicited a reduction in acetylcholine (ACh) release superimposable on that induced by systemic administration. The novel selective D1 antagonist SCH 39166 produced a similar decreasing effect on striatal ACh release on local injection (0.45 nmol per striatum). On the other hand, local application of SCH 23390 into the frontal cortices (0.45 nmol per side) failed to alter striatal ACh overflow, indicating that the drug does not diffuse out of its injection site to any significant extent. The dopamine release inducer d-amphetamine (2 mg/kg s.c.) and the dopamine uptake inhibitor cocaine raised ACh release like the D1 agonists. These effects were completely blocked by 10 microM SCH 23390 applied by reverse dialysis. The results suggest that D1 receptors regulating ACh release are located in the striatum.     

Operant performance following tail-pinch in the rat: effects of d-amphetamine

To extend the investigation of tail-pinch induced behavioral changes, rats performing on a differential reinforcement of low rates of 10 sec (DRL10), a fixed-interval of 60 sec (F160), and a fixed-ratio of 20 (FR20) schedules were exposed to a paper clip applied to the tail. While a 10 min tail-pinch conducted 1 hr before operant sessions significantly altered the DRL10 behavior, this stressor had little effect on either F160 or FR20 responding. Marked DRL10 behavior performance changes following tail-pinch included increases in the number of lever presses, decreases in the number of the reinforcers, and disruption in the frequency distribution of inter-response times (IRT). These DRL10 operant deficits were diminished when the subject received a tail-pinch pretreatment followed by d-amphetamine treatment (0.2 and 2.0 mg/kg). In combination with biochemical data from others, the present results suggest that catecholamine systems are involved in modulation of DRL10 behavior following tail-pinch.     

D1 agonist SKF 38393 antagonizes pentobarbital-induced narcosis and depression of hippocampal and cortical cholinergic activity in rats

SKF 38393 (5 mg/kg), but not quinpirole, shortened the duration of loss of righting reflex produced in pentobarbital-narcotized rats. This effect was blocked by atropine (2 mg/kg), but not by atropine methylbromide, suggesting involvement of central cholinergic mechanisms. The analeptic effect was also blocked by SCH 23390 (0.2 mg/kg) or raclopride (2 mg/kg). SKF 38393 also increased sodium dependent high affinity choline uptake (HACU) in cortical and hippocampal synaptosomes that had been depressed by pentobarbital. SCH 23390 or raclopride prevented the SKF 38393 reversal of the depressed HACU, indicating that both D1 and D2 mechanisms were involved mediating the analeptic effect. These results provide neurochemical evidence that cortical and hippocampal D1-mediated cholinergic activation results in a behavioral arousal (analeptic) response. They also suggest that DA mechanisms may be involved in regulation of cortical and hippocampal cholinergic neurons.     

beta-Lactotensin, a neurotensin agonist peptide derived from bovine beta-lactoglobulin, enhances memory consolidation in mice

beta-Lactotensin (His-Ile-Arg-Leu) is an ileum-contracting tetrapeptide isolated from bovine beta-lactoglobulin. We previously reported that a neurotensin agonist beta-lactotensin shows antinociceptive effect through neurotensin NT(2) receptor. We found that centrally or orally administered beta-lactotensin at a dose of 60nmol/mouse or 300-500mg/kg, respectively, increased memory consolidation in the step-through-type inhibitory avoidance test in mice. The memory-enhancing activity of beta-lactotensin was inhibited by the dopamine D(2) receptor antagonist raclopride but not the D(1) receptor antagonist SCH23390. Taken together, beta-lactotensin might improve memory consolidation through activating the dopamine D(2) receptor.     

Functional alteration by NMDA antagonist: effects of L-Dopa,neuroleptics drug and postnatal administration

Summary.  Antiakinsic effects of the uncompetitive NMDA antagonists, memantine, amantadine and MK-801, and competitive antagonists, CGP 40116, alone or in co-administration with acute subthreshold dose of L-Dopa (5 mg/kg) in MPTP-treated mice, functional alterations induced by acute MK-801 in combinations with neuroleptic compounds or behavioural deficits following postnatal administration of MK-801 were investigated. Memantine and amantadine injected 60 min before the subthreshold dose of L-Dopa (5 mg/kg), induced antiakinesic actions in hypokinesic MPTP-treated mice. Concurrently, higher doses of memantine and MK-801 caused dyskinesic changes, reducing further rearing (10 and 30 mg/kg) and locomotor (30 mg/kg) behaviour of the MPTP mice; MK-801 elevated locomotion (0.1 mg/kg) but reduced rearing (0.3 mg/kg). In control, saline-treated mice, memantine (3, 10 and 30 mg/kg) and MK-801 (0.1 and 0.3 mg/kg) increased locomotor behaviour but decreased rearing behaviour. In rats, MK-801 induced marked increases in locomotor activity and disruptions of circular swim maze acquisition that were to greater or lesser extents blocked or potentiated by neuroleptic compounds: SCH 23390 (0.005 and 0.05 mg/kg) and clozapine (5.0 and 10.0 mg/kg) dose-dependently antagonised MK-801 (0.3 mg/kg) induced locomotor activity whereas raclopride (0.1 mg/kg) and haloperidol (0.1 mg/kg) attenuated it dose-specifically. Amperozide (0.5 mg/kg) attenuated the MK-801 effect but potentiated it at the 2.0 mg/kg dose. In the circular swim maze, raclopride (0.01 mg/kg) and SCH 23390 (0.05 mg/kg) improved the acquisitive performance of rats administered MK-801 (0.03 mg/kg) acutely whereas clozapine (10.0 mg/kg) and amperozide (2.0 mg/kg) deteriorated the performance of MK-801-treated rats. Postnatal administration of MK-801 (0.05 mg/kg, day 11 after birth) induced severe functional alterations in adult mice. At 70 days of age, MK-801 mice showed an initial hypoactivity followed by marked hyperactivity in the motor activity test chambers. These mice showed deficits in habituation, a nonassociative form of learning. Their hyperactivity in the test chambers was reversed by a low dose of d-amphetamine (0.25 mg/kg). Taken together, these findings display a wide range of acute/long-term functional alterations induced by NMDA antagonists, particularly MK-801, associated with animal models of brain disorders. Received July 9, 2001 Accepted August 6, 2001 Published online June 17, 2002     

Lethal effects of cocaine are reduced by the dopamine-1 receptor antagonist SCH 23390 but not by haloperidol

The prevalence of cocaine abuse has been associated with a host of medical complications and deaths. We investigated the effects of two dopamine antagonists with different affinities for dopamine-1 and dopamine-2 receptor subtypes on cocaine-induced lethality. Male Fischer-344 rats were given cocaine HCl (i.p.) and observed for lethality at 24 hrs. Cocaine was not lethal at 50 mg/kg and produced a steep dose-effect function from 60 to 100 mg/kg. Lethality was 88.9% at 100 mg/kg and the LD 50 was 79.7 mg/kg (95% CL: 74.8-84.9). Doses as high as 180 mg/kg failed to kill all rats. Lethality was often but not invariably associated with convulsions. Haloperidol (0.3-3 mg/kg i.p.) given 30 min prior to cocaine did not alter the lethal effects of cocaine but did reduce the lethality of methamphetamine. SCH 23390 (0.1-1 mg/kg i.p., 30 min prior) shifted the cocaine dose-effect function to the right at 0.3 mg/kg. Maximum protection was conferred by 0.3 mg/kg SCH 23390 where the LD 50 was increased to 100.1 mg/kg (95% CL: 91.5-109.5). Comparable protection was not observed if SCH 23390 was given 5 min after cocaine. These results suggest that dopamine receptors may play a role in the lethal effects of cocaine and that the D1 dopamine receptor subtype appears to be more relevant to lethality than the D2 subtype.     

Lack or Inhibition of Dopaminergic Stimulation Induces a Development Increase of Striatal Tyrosine Hydroxylase-Positive Interneurons

We examined the role of endogenous dopamine (DA) in regulating the number of intrinsic tyrosine hydroxylase-positive (TH⁺) striatal neurons using mice at postnatal day (PND) 4 to 8, a period that corresponds to the developmental peak in the number of these neurons. We adopted the strategy of depleting endogenous DA by a 2-day treatment with α-methyl-p-tyrosine (αMpT, 150 mg/kg, i.p.). This treatment markedly increased the number of striatal TH⁺ neurons, assessed by stereological counting, and the increase was highly correlated to the extent of DA loss. Interestingly, TH⁺ neurons were found closer to the clusters of DA fibers after DA depletion, indicating that the concentration gradient of extracellular DA critically regulates the distribution of striatal TH⁺ neurons. A single i.p. injection of the D1 receptor antagonist, {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 (0.1 mg/kg), the D2/D3 receptor antagonist, raclopride (0.1 mg/kg), or the D4 receptor antagonist, L-745,870 (5 mg/kg) in mice at PND4 also increased the number of TH⁺ neurons after 4 days. Treatment with the D1-like receptor agonist {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393 (10 mg/kg) or with the D2-like receptor agonist, quinpirole (1 mg/kg) did not change the number of TH⁺ neurons. At least the effects of {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 were prevented by a combined treatment with {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393. Immunohistochemical analysis indicated that striatal TH⁺ neurons expressed D2 and D4 receptors, but not D1 receptors. Moreover, treatment with the α4β2 receptor antagonist dihydro-β-erythroidine (DHβE) (3.2 mg/kg) also increased the number of TH⁺ neurons. The evidence that DHβE mimicked the action of {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 in increasing the number of TH⁺ neurons supports the hypothesis that activation of D1 receptors controls the number of striatal TH⁺ neurons by enhancing the release of acetylcholine. These data demonstrate for the first time that endogenous DA negatively regulates the number of striatal TH⁺ neurons by direct and indirect mechanisms mediated by multiple DA receptor subtypes.