Metabotropic glutamate receptors as therapeutic targets for cognitive disorders.

Since more than one decade, metabotropic glutamate receptors have been under investigation as targets for various CNS disorders such as anxiety, pain, depression, schizpohrenia, Alzheimer's disease and Parkinson's disease. It has been shown that some mGluRs play a crucial role in cognitive processes such as learning and memory, which was initially, demonstrated using knockout mice for each receptor subtype. Later, selective pharmacological tools were developed allowing more specific examinations of the involvement of mGluR1-8 in various forms of learning and memory. Ligands for group I and II mGluRs have been proposed as promising candidates for the treatment of cognitive disorders such as schizophrenia, Fragile X syndrome, Alzheimer's and Parkinson's disease and post traumatic stress disorder, of which some have made it to clinical testing. The present paper reviews relevant data on the role of mGluRs in learning and cognition processes focusing on their utility as targets for cognition enhancement in several CNS diseases.

The role of group I metabotropic glutamate receptors in schizophrenia.

It has been proposed that glutamatergic transmission, in particular NMDA receptor function, might be altered in schizophrenia. This hypothesis is mainly based on the observation that uncompetitive NMDA receptor antagonists, e.g. phencyclidine, evoke psychotic symptoms in healthy subjects, whereas agonists interacting at the glycine site of the NMDA receptor complex, e.g. glycine or D-serine, administered jointly with typical neuroleptics, can alleviate schizophrenic symptoms. The function of NMDA receptors may be modulated by group I mGluRs (mGluR1 and mGluR5), which have also been shown to be altered in schizophrenia. In rodents, mGluR5 antagonists, but not mGluR1 ones, potentiate the locomotor activity and the deficit of prepulse inhibition (PPI) induced by uncompetitive NMDA receptor antagonists. These antagonists (of either type) administered alone are not active in the above tests. Hence, antagonists of mGluR1 and mGluR5 may evoke different effects on the NMDA receptor antagonists-induced behavior and, possibly, on schizophrenic symptoms.

An influence of ligands of metabotropic glutamate receptor subtypes on parkinsonian-like symptoms and the striatopallidal pathway in rats.

Several data indicate that inhibition of glutamatergic transmission may be important to alleviate of parkinsonian symptoms. Therefore, the aim of the present paper is to review recent studies on the search for putative antiparkinsonian-like effects of mGluR ligands and their brain targets. In order to inhibit glutamatergic transmission, the group I mGluRs (mGluR1 and mGluR5) were blocked, and group II (mGluR2/3) or III (mGluR4/7/8) mGluRs were activated. Systemic or intrastriatal administration of group I mGluR antagonists (mGluR5 - MPEP, MTEP; mGluR1 - AIDA) was found to inhibit parkinsonian-like symptoms (catalepsy, muscle rigidity) in rats. MPEP administered systemically and mGluR1 antagonists (AIDA, CPCCOEt, LY367385) injected intrastriatally reversed also the haloperidol-increased proenkephalin (PENK) mRNA expression in the striatopallidal pathway. Similarly, ACPT-1, a group III mGluR agonist, administered into the striatum, globus pallidus or substantia nigra inhibited the catalepsy. Intrastriatal injection of this compound reduced the striatal PENK expression induced by haloperidol. In contrast, a group II mGluR agonist (2R,4R-APDC) administered intrastriatally reduced neither PENK expression nor the above-mentioned parkinsonian-like symptoms. Moreover, a mixed mGluR8 agonist/AMPA antagonist, (R,S)-3,4-DCPG, administered systemically evoked catalepsy and enhanced both the catalepsy and PENK expression induced by haloperidol. The results reviewed in this article seem to indicate that group I mGluR antagonists or some agonists of group III may possess antiparkinsonian properties, and point at the striatopallidal pathway as a potential target of therapeutic intervention.

Functional interaction of NMDA and group I metabotropic glutamate receptors in negatively reinforced learning in rats.

RATIONALE: The role of glutamatergic system in learning and memory has been extensively studied, and especially N-methyl-D: -aspartate (NMDA) receptors have been implicated in different learning and memory processes. Less is known, however, about group I metabotropic glutamate (mGlu) receptors in this field. Recent studies indicated that the coactivation of both NMDA and group I mGlu receptors is required for the induction of long-term potentiation (LTP) and learning. OBJECTIVE: The purpose of the study is to evaluate if there is a functional interaction between NMDA and group I mGlu receptors in two different models of aversive learning. METHODS: Effects of NMDA, mGlu1, and mGlu5 receptor antagonists on acquisition were tested after systemic coadministration of selected ineffective doses in passive avoidance (PA) and fear-potentiated startle (FPS). RESULTS: Interaction in aversive learning was investigated using selective antagonists: (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM) for mGlu1, [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) for mGlu5, and (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate [(+)MK-801] for NMDA receptors. In PA, the coapplication of MTEP at a dose of 5 mg/kg and (+)MK-801 at a dose of 0.1 mg/kg 30 min before training impaired the acquisition tested 24 h later. Similarly, EMQMCM (2.5 mg/kg) plus (+)MK-801 (0.1 mg/kg), given during the acquisition phase, blocked the acquisition of the PA response. In contrast, neither the combination of MTEP (1.25 mg/kg) nor EMQMCM (5 mg/kg) plus (+)MK-801 (0.05 mg/kg) was effective on the acquisition assessed in the FPS paradigm. CONCLUSION: The findings suggest differences in the interaction of the NMDA and mGlu group I receptor types in aversive instrumental conditioning vs conditioning to a discrete light cue.

Effect of 5-HT3 receptor antagonist MDL 72222 on behaviors induced by ketamine in rats and mice.

Phencyclidine and ketamine (but not other NMDA channel blockers, such as memantine) produce psychotomimetic effects. Since unlike memantine, phencyclidine-like compounds show no significant affinity at 5-HT(3) receptors, we investigated if behavioral effects of ketamine could be reduced by 5HT(3) receptor blockade. Ketamine (3-40 mg/kg) produced ataxia, stereotypes and diminished exploratory activity in mice, and reduced prepulse inhibition of acoustic startle response, lowered accuracy in fixed consecutive number and in delayed non-matching-to-sample tasks in rats. The 5HT(3) receptor antagonist MDL 72222 (0.3-3 mg/kg) administration did not reverse any of these deficits and exerted no effects on discriminative stimulus properties of ketamine. In the tail suspension test, both ketamine and MDL 72222 produced anti-immobility effects when given alone (50-66 and 3 mg/kg, respectively) and together (12.5-25 and 1 mg/kg). The present data suggest that 5-HT(3) receptor blockade does not reverse the behavioral deficits of ketamine and may even enhance its certain effects, such as the antidepressant-like action.

mGluR5, but not mGluR1, antagonist modifies MK-801-induced locomotor activity and deficit of prepulse inhibition.

Hypoglutamatergic theory of schizophrenia is substantiated by observation that high affinity uncompetitive antagonists of NMDA receptors such as PCP can induce psychotic symptoms in humans. Recently, metabotropic glutamate receptors of the mGluR5 type have also been discussed as possible players in this disease. However, less is known about the potential contribution of mGluR1 in schizophrenia. Therefore, the aim of the present study was to compare the effect of selective mGluR1 antagonist EMQMCM, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist (MTEP ([(2-methyl-1, 3-thiazol-4-yl) ethynyl] pyridine) either alone or in combination with (+)MK-801 in a prepulse inhibition (PPI) model and locomotor activity tests. Additionally, the effect of both mGluR1 and mGluR5 antagonists on (+)MK-801-evoked ataxia was tested. In contrast to (+)MK-801, which induced disruption of PPI, neither MTEP (1.25-5 mg/kg) nor EMQMCM (0.5-4 mg/kg) altered the PPI. However, MTEP, but not EMQMCM, enhanced disruption of PPI induced by (+)MK-801. Although neither mGluR1 nor mGluR5 antagonists given alone changed locomotor activity of rats, MTEP at 5 mg/kg potentiated the effect of (+)MK-801 while EMQMCM (up to 4 mg/kg) turned out to be ineffective. On the other hand, EMQMCM, but not MTEP, enhanced ataxia evoked by MK-801. The present results demonstrate that blockade of mGluR1 and mGluR5 evokes different effects on behavior induced by NMDA receptor antagonists.

Effects of mGlu1 and mGlu5 receptor antagonists on negatively reinforced learning.

Effects on aversive learning of the novel highly selective mGlu5 receptor antagonist [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and mGlu1 receptor antagonist (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM) were tested, after systemic administration, in the passive avoidance (PA) and fear potentiated startle (FPS) paradigms. Both MTEP at 10 mg/kg and EMQMCM at 5 and 10 mg/kg, given 30 min before training, impaired acquisition of the passive avoidance response (PAR). Co-administration of MTEP and EMQMCM at doses ineffective when administered alone, produced anterograde amnesia when given 30 min before the acquisition phase. Neither EMQMCM (5 mg/kg) nor MTEP (10 mg/kg) impaired retention of the PAR after direct post-training injections. EMQMCM (5 mg/kg), but not MTEP (10 mg/kg) blocked the PAR when given 30 min before testing. Pre-training administration of MTEP at doses of 2.5 and 5 mg/kg inhibited fear conditioning in the FPS when tested 24 h later. In contrast, EMQMCM was ineffective. Our findings suggest diverse involvement of mGlu1 and mGlu5 receptors in negatively reinforced learning.

Opposite influence of MPEP, an mGluR5 antagonist, on the locomotor hyperactivity induced by PCP and amphetamine.

Potential antipsychotic effects of a selective non-competitive antagonist of metabotropic glutamate receptor 5 (mGluR5), 2-methyl-6-phenylethynylpyridine (MPEP), was examined in two commonly used screening tests: (1) the hyperactivity induced by an NMDA receptor antagonist phencyclidine (PCP), and (2) the hyperactivity induced by an indirect dopamine agonist, D-amphetamine. PCP was administered at a dose of 2.5 mg/kg s.c. and D-amphetamine was given at a dose of 1 mg/kg s.c. MPEP (5 mg/kg i.p.) significantly enhanced the locomotor activity increased by PCP, but inhibited amphetamine-induced hyperactivity. The opposite effect of MPEP in the two above-mentioned models questions significance of the blockade of mGluR5 receptors to antipsychotic effects.

The role of metabotropic glutamate receptors in regulation of striatal proenkephalin expression: implications for the therapy of Parkinson's disease.

Overactivity of the striatopallidal pathway, associated with an enhancement of enkephalin expression, has been suggested to contribute to the development of parkinsonian symptoms. The aim of the present study was to examine whether the blockade of group I metabotropic glutamate receptors: subtypes 1 and 5 (mGluR1/5), or stimulation of group II: subtypes 2 and 3 (mGluR2/3) may normalize enkephalin expression in the striatopallidal pathway in an animal model of parkinsonism. The proenkephalin mRNA level measured by in situ hybridization in the striatum was increased by pretreatments with haloperidol (1.5 mg/kg s.c., three times, 3 h apart). Triple (3 h apart), bilateral, intrastriatal administration of selective antagonists of mGluR1: (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (3 x 5 microg/0.5 microl) or 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (3 x 2.5 microg/0.5 microl), reversed the haloperidol-induced increases in proenkephalin mRNA levels in the rostral and central regions of the striatum. Similarly, repeated (6 times, 1.5 h apart), systemic injections of an antagonist of mGluR5, 2-methyl-6-(phenylethynyl)pyridine (6 x 10 mg/kg i.p.) counteracted an increase in the striatal proenkephalin mRNA expression elicited by haloperidol. None of the abovementioned antagonists of mGluR1 and mGluR5 per se influenced the proenkephalin expression. Differential effects were induced by agonists of the group II mGluRs, viz. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine administered intraventricularly (3 times at 0.1-0.2 microg/4 microl, 3 h apart) increased both the normal and haloperidol-increased proenkephalin mRNA level, whereas (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate injected intrastriatally (3 times at 15 microg/0.5 microl, 3 h apart) was ineffective. The present study indicates that the blockade of striatal glutamate receptors belonging to the group I (mGluR1 and mGluR5) but not stimulation of the group II mGluRs may normalize the function of the striatopallidal pathway in an animal model of parkinsonism, which may be important for future antiparkinsonian therapy in humans.

Propranolol modifies platelet serotonergic mechanisms in rats.

Though the mechanisms for the vascular actions of vasodilatory beta-blockers are mostly determined, some of their interactions with monoaminergic systems are not elucidated. Because there are evidences supporting a possible involvement of serotonin (5-HT) in the actions of beta-blockers, we studied the effect of propranolol on peripheral serotonergic mechanisms in normotensive and Goldblatt two-kidney - one clip (2K1C) hypertensive rats. In both groups of animals propranolol decreased systolic blood pressure, significantly increased whole blood serotonin concentration and at the same time it decreased platelet serotonin level. The uptake of the amine by platelets from hypertensive animals was lower than that of normotensive animals and it was decreased by propranolol only in the latter. In both groups propranolol inhibited potentiation of ADP-induced platelet aggregation by serotonin. In conclusion, this study provides evidence that propranolol modifies platelet serotonergic mechanisms in normotensive and renal hypertensive rats.

Decline in motor functions in aging is related to the loss of NMDA receptors.

The aim of the study was to assess the contribution of central dopaminergic and glutamatergic systems to the age-dependent loss of motor functions in rats. Rats of three age groups were compared: young (3-5-month-old), middle-aged (20-21-month-old) and old (29-31-month-old). The obtained results showed an age-dependent decline in the electromyographic (EMG) resting and reflex activities in the gastrocnemius and tibialis anterior muscles, as well as in the T-maze performance. Although these disturbances were accompanied with significant age-dependent decreases in the binding to NMDA, AMPA and dopamine D2 receptors, and a decline in the number of nigral dopamine neurons, they were significantly correlated with the loss of the binding to NMDA receptors only. The reduction in T-maze performance with aging was additionally correlated with a decrease in motor functions (EMG activity). The study suggests a crucial role of the loss of NMDA receptors in age-dependent motor disabilities, as well as in disturbances measured in the T-maze.

The role of glutamate receptors in antipsychotic drug action.

It has recently been postulated that disturbances in glutamatergic neurotransmission may contribute to the pathophysiology of schizophrenia. Therefore the aim of the present study was to evaluate the role of glutamate NMDA and group II metabotropic receptors in the antipsychotic drug action. To this aim the influence of some well-known neuroleptics on cortical NMDA receptors was examined. Furthermore, their behavioral effects were compared with those of the novel agonist of group II glutamate metabotropic receptors, LY 354740, in some animal models of schizophrenic deficits. We found that long-term administration of the typical neuroleptic haloperidol and the atypical one clozapine increased the number of NMDA receptors labelled with [3H]CGP 39653 in different cortical areas. Long-, but not short-term, treatment with haloperidol and raclopride diminished the deficit of prepulse inhibition produced by phencyclidine, which is a model of sensorimotor gating deficit in schizophrenia. In contrast, neither short- nor long-term treatment with clozapine influenced the phencyclidine effect in that model. Acute treatment with LY 354740 reversed neither (1) the deficit of prepulse inhibition produced by phencyclidine or apomorphine, nor (2) the impairment in a delayed alternation task induced by MK-801, which is commonly used to model the frontal lobe deficits associated with schizophrenia. The present study suggests that an increase in the density of cortical NMDA receptors may be important to a longterm neuroleptic therapy. Conversely, the results do not support the role of group II metabotropic glutamate receptors in the antipsychotic drug action.

Influence of long-lasting administration of neuroleptics on cortical NMDA receptors and phencyclidine-induced deficit in the sensorimotor gating in rats.

The aim of this study was to examine the role of cortical NMDA receptors in the antipsychotic action of neuroleptics. Haloperidol (1 mg/kg/day) and clozapine (30 mg/kg/day) were administered to rats in drinking water. Autoradiographic and saturation binding analyses showed that a 3-month treatment with both haloperidol and clozapine increased the density of NMDA receptors labelled with [3H]CGP 39653 (a competitive antagonist) in the parietal and insular cortices. Haloperidol additionally increased the binding of that ligand in the frontal cortex. None of those neuroleptics influenced the binding of [3H]MK-801, an uncompetitive antagonist of NMDA receptors, in the frontal, parietal or insular cortices. A 6-week and a 3-month treatment with haloperidol antagonized the deficit of prepulse inhibition induced by phencyclidine (5 mg/kg s.c.). In contrast, short-term (4-day) administration of that neuroleptic was ineffective. The present study suggests that the increased density of cortical NMDA receptors, induced by long-term neuroleptic administration, may overcome the deficit of sensorimotor gating induced by phencyclidine. However, contribution of such an effect to the antipsychotic activity needs to be established.

Chronic haloperidol and clozapine administration increases the number of cortical NMDA receptors in rats.

The aim of the present study was to examine the influence of 3-month administration of haloperidol (1 mg/kg per day) and clozapine (30 mg/kg per day) in drinking water on cortical NMDA (N-methyl-D-aspartate) receptors in rats. On day 5 of withdrawal, the animals were killed and their brains were removed. The binding of [3H]MK-801 ([3H](5R, 10S)-(+)-5-methyl-10,1 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) and [3H]CGP 39653([3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) to NMDA receptors in different cortical areas, as well as the binding of [3H]spiperone to dopamine D2 receptors in the striatum, were analysed by quantitative autoradiography. Haloperidol increased the binding of [3H]CGP 39653 in frontal, insular and parietal cortices. Clozapine increased the binding of [3H]CGP 39653 in insular and parietal cortices. Haloperidol, but not clozapine, increased the binding of [3H]spiperone in the striatum. None of the neuroleptics influenced the binding of [3H]MK-801 to cortical NMDA receptors. An additional assay revealed an increase in the Bmax value, with no significant changes in the K(D) of [3H]CGP 39653 binding in parieto-insular cortical homo-genates as a result of haloperidol and clozapine administration. The present results suggest that long-term treatments with haloperidol and clozapine increase the number of NMDA receptors in different cortical regions.

Chronic treatment with haloperidol diminishes the phencyclidine-induced sensorimotor gating deficit in rats.

Prepulse inhibition is a model in which a weak subthreshold stimulus (prepulse), presented to an individual before a strong stimulus (pulse), inhibits a startle response to the latter. A deficit of prepulse inhibition induced by dopaminomimetics and antagonists of NMDA receptors has been suggested as an animal model of the sensorimotor deficit in schizophrenia. The aim of the present study was to examine the effect of chronic treatment with the classic neuroleptic haloperidol on the disruption of prepulse inhibition induced by the uncompetitive antagonist of NMDA receptors phencyclidine (PCP, 5 mg/kg sc). Haloperidol in a dose of 1 mg/kg/day was given to rats in drinking water for 3 months. The PCP-induced reduction in prepulse inhibition was not reversed by short-term (4-day) haloperidol administration. In contrast, long-term treatment with haloperidol (6 weeks or 3 months) diminished the PCP-induced effect. The present study suggests that the improvement in sensorimotor gating in the PCP model in rats by prolonged treatment with haloperidol may reflect its antipsychotic action.

Age-related changes in glutamate receptors: an autoradiographic analysis.

The aim of the present study was to evaluate age-related changes in NMDA and AMPA receptors in old female rats. To this end a quantitative autoradiography of [3H]-MK-801 and [3H]-AMPA binding was performed in the brain of young (3-month-old), middle-aged (12-month-old) and old (36-month-old) rats. Moreover, the binding of [3H]-spiperone to D2 dopamine receptors was also examined. No changes were observed in the binding of [3H]-MK-801 or [3H]-AMPA in middle-aged rats compared to young ones. In the caudate-putamen and shell and core of the nucleus accumbens septi of old rats, a pronounced decrease in the [3H]-MK-801 binding and a decreasing tendency in the [3H]-AMPA binding were observed. Furthermore, the binding of [3H]-MK-801 and [3H]-AMPA was reduced in the hippocampal formation and, additionally, a marked decline in the [3H]-MK-801 binding in different parts of the cerebral cortex including the frontal, parietal, cingulate, pyriform and insular cortices was found. The [3H]-spiperone binding progressively decreased with age in the dorsolateral, ventrolateral and medial caudate-putamen. The present results show that aging processes lead to changes in the binding of both [3H]-MK-801 to NMDA and [3H]-AMPA to AMPA receptors in a number of structures, a phenomenon which may reflect motor and memory disturbances found in old rats and elderly humans.

Further evidence for the subsensitivity of striatal AMPA receptors, induced by chronic haloperidol administration: an autoradiographic study.

The aim of the present study was to investigate the influence of chronic treatment with haloperidol on the striatal N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxasole-propionic acid (AMPA) and dopamine D2 receptors using a quantitative autoradiography in rats. Haloperidol was given to animals in a dose of ca. 1 mg/kg/day in drinking water for 6 weeks or 3 months and was afterwards withdrawn for 5 days. Haloperidol increased by 20-50% the binding of [3H]spiperone in different regions of the caudate-putamen. Haloperidol decreased by ca. 30% the binding of [3H]AMPA in the ventrolateral region of intermediate part of the caudate-putamen, but did not influence the binding of [3H]MK-801. The present results suggest that, apart from supersensitivity to dopamine, chronic treatment with haloperidol also induces subsensitivity of striatal AMPA receptors.

Plasma serotonin and platelet aggregation during ischemia-reperfusion in dogs: effect of dipyridamole and coenzyme Q10.

Platelet aggregation and plasma serotonin were studied during ischemia-reperfusion of the small intestine in dogs. Blood was withdrawn from the superior mesenteric vein before and 1 h after ischemia, then 5, 30 and 60 min after reperfusion. Dipyridamole (5 mg/kg body weight) and coenzyme Q10 (CoQ10; 10 mg/kg body weight) were administered intravenously 5 min before reperfusion, following 1 h ischemia, in order to investigate their effects on platelet function and free serotonin. Ischemia-reperfusion resulted in an increased local free serotonin concentration together with an enhanced platelet response to ADP, collagen and arachidonic acid. Administration of dipyridamole and CoQ10 prior to reperfusion prevented, at least in part, augmented platelet activation and serotonin release. It appeared that dipyridamole was more potent than CoQ10. Our results may indicate a possible protective effect of dipyridamole on enhanced platelet activation during ischemia-reperfusion in dogs.

Effect of endothelin-1 on some hemostatic parameters in normotensive rats.

Some parameters of hemostasis and fibrinolysis were investigated in rats administered with endothelin-1 (ET-1). ET-1 (0.5, 1.0, 5.0 nmol/kg) dose-dependently shortened the bleeding time (BT). Concomitantly significant shortening of the clotting time (CT) was observed. ET-1 produced prolongation of the activated partial thromboplastin time (APTT), whereas prothrombin time (PT) remained unchanged. ET-1 did not influence in vitro platelet aggregation induced by ADP and collagen. The euglobulin clot lysis time (ECLT) was significantly shortened after ET-1 administration. Our results suggest that ET-1 modulates the process of hemostasis and fibrinolysis in the rat.

Specific involvement of striatal D1 and D2 dopamine receptors in the neuroleptic catalepsy in rats.

Since highly specific antagonists of D1 (SCH 39166) and D2 (raclopride) dopamine receptors have recently become available, we decided to investigate the role of striatal populations of these receptors in catalepsy - an animal model of neuroleptic-induced parkinsonism in humans. Injections of raclopride (2.5, 5 and 10 micrograms/0.5 microliters) into the ventro-rostral part of the striatum induced a strong, dose-dependent and long-lasting catalepsy. Intrastriatal injections of SCH 39166 (1.5 and 3.6 micrograms/ 0.5 microliters) also evoked a dose-dependent, but short-lasting catalepsy. The present results suggest, that neuroleptic side-effects are specifically dependent on the blockade of D2 and D1 dopamine receptors in the striatum.