Inactivation of endopeduncular nucleus impaired fear conditioning and hippocampal synaptic plasticity in rats

The internal globus pallidus (GPi) is one part of basal ganglion nucleuses which play fundamental role in motor function. Recent studies indicated that GPi could modulate emotional processing and learning, but the possible mechanism remains still unknown. In this study, the effects of endopeduncular nucleus (EP, a rodent homolog of GPi) on fear conditioning were tested in rats. GABA_A receptor agonist muscimol was bilaterally delivered into the EP 15 min before or immediately after fear conditioning in rats. We found that EP inactivation impaired the acquisition but not consolidation of fear memory in rats. Furthermore, the long-term potentiation (LTP) in hippocampal CA1 area was impaired, and the learning related phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit 1 (GluA1) at the Ser845 site in hippocampus was decreased in muscimol treated group. These results demonstrated that dysfunction of EP impaired hippocampal dependent learning and memory in rats.

Botulinum toxin A injection into the entopeduncular nucleus improves dynamic locomotory parameters in hemiparkinsonian rats

Parkinson’s disease is associated with hyperactivity of the subthalamic nucleus (STN), contributing to motor and gait disturbances. Although deep brain stimulation of the STN alleviates certain motor dysfunction, its specific effect on gait abnormalities remains controversial. This study investigated the long-term changes in locomotion following direct infusions of botulinum toxin-A into the globus pallidus internal segment (GPi) to suppress the flow of information from the STN to the GPi in a hemiparkinsonian rat model. Static and dynamic gait parameters were quantified using a CatWalk apparatus. Interestingly, botulinum toxin-A at 0.5 ng significantly reduced only the dynamic gait parameters of hemiparkinsonian rats at 1 week and 1 month post-infusion, while static gait parameters did not change. This study offers new insights into the complexity of basal ganglia in locomotor control and shows the potential of central infusion of botulinum toxin-A as a novel intervention in the study of experimental hemiparkinson’s disease.

Mood regulation. GABA/glutamate co-release controls habenula output and is modified by antidepressant treatment

The lateral habenula (LHb), a key regulator of monoaminergic brain regions, is activated by negatively valenced events. Its hyperactivity is associated with depression. Although enhanced excitatory input to the LHb has been linked to depression, little is known about inhibitory transmission. We discovered that γ-aminobutyric acid (GABA) is co-released with its functional opponent, glutamate, from long-range basal ganglia inputs (which signal negative events) to limit LHb activity in rodents. At this synapse, the balance of GABA/glutamate signaling is shifted toward reduced GABA in a model of depression and increased GABA by antidepressant treatment. GABA and glutamate co-release therefore controls LHb activity, and regulation of this form of transmission may be important for determining the effect of negative life events on mood and behavior.

Modulation of torsinA expression in the globus pallidus internus is associated with levodopa-induced dyskinesia in hemiparkinsonian rats

TorsinA is the causative protein of DYT1 dystonia, a major representative of hyperkinetic movement disorders. In this study, the distribution of torsinA was investigated in the basal ganglia of hemiparkinsonian rats with or without levodopa-induced dyskinesia (LID). Two months after 6-hydroxydopamine (OHDA) treatment, Wistar-albino rats were subjected to intermittent intraperitoneal injection of levodopa/benserazid (LID-group, n=5) or vehicle (control, n=5) for 21 days. Immunohistochemical analysis disclosed that in the caudal portion of the entopeduncular nucleus (EP), homologous to the globus pallidus internus (GPi) in primates, on the parkinsonian side, there was a significant decrease of torsinA-immunopositive neurons in rats with LID, but not in those without LID. However, Nissl-staining showed no loss of GPi neurons in rats with LID. In both groups, there was no significant difference between ipsi- and contralateral sides with respect to the density of torsinA-positive neuronal cells in the striatum, globus pallidus externus, and subthalamic nucleus. Ours are the first data to demonstrate the specific modulation of torsinA expression in the basal ganglia of the hyperkinesia model, suggesting that GPi neurons containing torsinA possess pathologic plasticity for LID.

Normalization of glutamate decarboxylase gene expression in the entopeduncular nucleus of rats with a unilateral 6-hydroxydopamine lesion correlates with increased GABAergic input following intermittent but not continuous levodopa

The expression of mRNA encoding for the 67 kilodalton isoform of glutamate decarboxylase (GAD67) was examined by in situ hybridization histochemistry in the entopeduncular nucleus (EP) of adult rats with a 6-hydroxydopamine unilaterally lesion of dopamine neurons. Our results provide original evidence that continuous or intermittent levodopa administration is equally effective at reversing the lesion-induced increase in GAD67 mRNA expression in the EP when compared with vehicle controls. To characterize the GABAergic interactions that may mediate levodopa-induced alterations in the EP, double-labeling in situ hybridization was conducted with a combination of GAD67 radioactive and preproenkephalin or preprotachykinin digoxigenin-labeled complementary RNA probes in the striatum. Levels of GAD67 mRNA labeling were significantly increased by intermittent, but not continuous levodopa. Analysis at the cellular level in a dorsal sector of the striatum revealed that GAD67 mRNA levels increased predominantly in preproenkephalin-unlabeled neuronal profiles, presumably striatal/EP neurons (+99.3%). Saturation analyses of (3)H-flunitrazepam binding to GABA(A) receptors in the EP showed that the increase in GAD67 mRNA in preproenkephalin-unlabeled neurons by intermittent levodopa paralleled a significant decrease in number of GABA(A) receptors (Bmax) in the EP ipsilateral to the lesion. Continuous levodopa failed to alter striatal GAD67 mRNA levels, or the number or affinity of GABA(A) receptors when compared with vehicle-treated controls. These results suggest the normalization of GAD gene expression in the EP by intermittent levodopa involves an increase in GABAergic inhibition by striatonigral/EP neurons of the direct pathway. Conversely, the effects of continuous levodopa on GAD mRNA levels in the EP do not appear to be mediated by GABA.

High frequency stimulation of the entopeduncular nucleus has no effect on striatal dopaminergic transmission

High frequency stimulation (HFS) of the subthalamic nucleus (STN) is thought to be superior to stimulation of the internal pallidum (GPi) in alleviating symptoms of Parkinson's disease (PD). However, preliminary controlled studies comparing the effectiveness of both targets have not found significant differences in the improvement of parkinsonian symptoms, but have shown that STN stimulation allows a dramatic decrease in dopaminergic medication. We have previously shown that STN-HFS increases striatal extracellular dopamine (DA) metabolites, but not DA, in both naive and 6-hydroxydopamine (6-OHDA)-lesioned rats, whereas stimulation of the entopeduncular nucleus (EP), the rodent equivalent of the internal pallidum, does not affect DA or metabolite levels. Intriguingly, STN-HFS increases striatal DA release after inhibition of DA reuptake or metabolism, suggesting that this observation may have been obscured in non-drug treated animals by rapid and effective DA reuptake. Since STN-HFS further enhances DA metabolism after DA reuptake inhibition or depletion it has been proposed that STN-HFS increases both, striatal DA release and metabolism, independently. Therefore, the present study assesses the impact of EP-HFS on striatal DA release and metabolism in normal rats after inhibition of DA reuptake or metabolism, using microdialysis. In summary, our data demonstrate that, contrary to STN stimulation, EP-HFS has no effect on striatal DA release and metabolism. Thus, the present study provides a partial explanation for the reported clinical differences, and experimental evidence for differential mechanisms of action between HFS of the internal pallidum and the STN, that are most likely related to differences in functional anatomy.

Effect of subthalamic nucleus or entopeduncular nucleus lesion on levodopa-induced neurochemical changes within the basal ganglia and on levodopa-induced motor alterations in 6-hydroxydopamine-lesioned rats

Inactivation of the subthalamic nucleus (STN) or the internal segment of the pallidum (GPi)/entopeduncular nucleus (EP) by deep brain stimulation or lesioning alleviates clinical manifestations of Parkinson's disease (PD) as well as reducing the side-effects of levodopa treatment. However, the effects of STN or entopeduncular nucleus (EP) lesion on levodopa-related motor fluctuations and on neurochemical changes induced by levodopa remain largely unknown. The effects of such lesions on levodopa-induced motor alterations were studied in 6-hydroxydopamine (6-OHDA)-lesioned rats and were assessed neurochemically by analyzing the functional activity of the basal ganglia nuclei, using the expression levels of the mRNAs coding for glutamic acid decarboxylase and cytochrome oxidase as molecular markers of neuronal activity. At the striatal level, preproenkephalin (PPE) mRNA levels were analyzed. We found in 6-OHDA-lesioned rats that a unilateral STN or EP lesion ipsilateral to the 6-OHDA lesion had no effect on either the shortening in the duration of the levodopa-induced rotational response or the levodopa-induced biochemical changes in the basal ganglia nuclei. In contrast, overexpression of PPE mRNA due to levodopa treatment was reversed by the STN or EP lesion. Our study thus shows that lesion of the EP or STN may counteract some of the neurochemical changes induced by levodopa treatment within the striatum.

Behavioral/neurophysiological investigation of effects of combining a quinolinic acid entopeduncular lesion with a fetal mesencephalic tissue transplant in striatum of the 6-OHDA hemilesioned rat

Behavioral and electrophysiological methods were used to investigate the effects of combining a unilateral quinolinic acid lesion of the entopeduncular nucleus (QA/EP) with a striatal transplant of fetal ventral mesencephalic tissue in the 6-hydroxydopamine (6-OHDA) hemilesioned rat model for Parkinson's disease. The subjects were 6-OHDA-lesioned rats selected because their response to amphetamine treatment was a strongly biased rotation toward the side of the 6-OHDA lesion in the substantia nigra, at the expense of other evoked behaviors associated with amphetamine. Two experiments were performed. In the first, the motor effects of the QA/EP lesion alone and of the combination of QA/EP lesion with striatal transplant were determined by measuring six aspects of the motor response. In the second the electrophysiological effects of the two interventions on the responses of neurons in the subthalamic nucleus to amphetamine and apomorphine were determined in the 6-OHDA-lesioned rats. The results of the first experiment show that the QA/EP lesion by itself produced an attenuation of the rotation response and, simultaneously, an increase of oral stereotypy. They also show that the combination of QA/EP lesion with striatal transplant was more effective than the single intervention, inducing more attenuation of rotation and more oral stereotypy. The results of the second experiment show that the responses of subthalamic neurons to amphetamine in the behaving hemiparkinsonian rat with combined QA/EP lesion and transplant were larger than the responses in the hemiparkinsonian rat with the QA/EP lesion alone. However, even these larger responses in the rats with combined intervention were not as large as those recorded at the same time in the subthalamus in the opposite, intact, hemisphere. The results of the two experiments, both of which show enhanced motor and neuronal sensitivity to amphetamine after the combined intervention, suggest that such a multiple approach might prove more beneficial than a one-site intervention targeting either the EP or the striatum.

Behavioral and subthalamic effects of combining a fetal ventral mesencephalic transplant in striatum with an electrolytic lesion of the entopeduncular nucleus in the rat with a unilateral 6-OHDA lesion of substantia nigra

Behavioral and electrophysiological methods were used to determine whether a transplant of dopamine-rich fetal tissue in striatum combined with an electrolytic lesion of the entopeduncular nucleus have additive effects in the unilaterally lesioned rat model for Parkinson's disease. The subjects were rats with the left substantia nigra lesioned with 6-hydroxydopamine (6-OHDA) and responding to systemic amphetamine with rotation toward the side of the lesion (ipsilateral rotation). The motor response to amphetamine was fractionated into six aspects, half reflecting the unilateral deafferentation in striatum and half representing those aspects of the response evoked in normal rats. After collection of baseline values, 25 rotators received a transplant of fetal ventral mesencephalic tissue in the left striatum and 20 received a transplant and, at the same time, an electrolytic lesion of the left entopeduncular nucleus. Testing for the motor response to amphetamine resumed after 4 weeks of recovery and continued at weekly intervals for 5 weeks. Upon completion of these tests, each rotator was implanted with multiple electrodes in the subthalamic nucleus. After recovery, multiunit responses to amphetamine and apomorphine were recorded from several electrodes in parallel during the motor response to the drugs. In rotators with transplant only, treatment with amphetamine evoked oral stereotypy and an attenuated ipsilateral rotation response. In rotators with combined transplant and entopeduncular lesion, ipsilateral rotation did not change or increased. Subthalamic responses to amphetamine and apomorphine were larger in rotators with combined transplant and entopeduncular lesion than in rotators with transplant alone. These findings indicate that the combination of transplant and pallidotomy in the 6-OHDA rat model for parkinsonism does not lead to additive benefits, an effect that may have been due to the nonselectivity of the electrolytic damage and/or of the lesion extending beyond the entopeduncular nucleus into the lateral hypothalamus.

Effects of dynorphin on rat entopeduncular nucleus neurons in vitro

The entopeduncular nucleus (EP) receives dense neostriatal afferent axons that contain dynorphin (DYN, an endogenous kappa-receptor agonist), in addition to GABA and substance P. To examine the role of DYN in the EP, whole-cell recordings were performed in rat brain slice preparations. Based on the physiological and morphological characteristics, all the neurons recorded were similar to the Type-I EP neuron described in a previous study. The kappa-receptor agonist dynorphin A (1-13) (DYN13) hyperpolarized and decreased the input resistance of approximately one-quarter of the EP neurons examined. The hyperpolarization was due to an increase in potassium conductance since current-voltage relationship curves obtained before and after DYN13 application crossed at the potassium equilibrium potential. In the presence of the glutamate blocker 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide and 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid in artificial cerebrospinal fluid, stimulation of the globus pallidus evoked bicuculline-sensitive multi-component GABAergic responses in EP neurons. Application of DYN13 equally reduced the amplitudes of the short-latency response, conceivably evoked by pallido-EP axons, and the medium-latency response, conceivably evoked by striato-EP axons. These effects were reversed by bath application of a non-selective opioid antagonist naloxone or by a kappa-opioid receptor-selective antagonist nor-binaltorphimine dihydrochloride (nor-BNI), but not by the partial differential -antagonist naltrindole or the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2). DYN13 also reduced the frequency of tetrodotoxin-insensitive miniature-inhibitory postsynaptic potential (mIPSPs) without changing their amplitude distributions. The decrease of the frequency of mIPSPs was reversible upon washing and was also completely blocked by nor-BNI. The results of the present study on the EP indicated that DYN released from striatal axons might exert at least three different effects on these target nuclei. Firstly, DYN might provide negative feedback regulation of striatal GABAergic outputs at their termination sites. Secondly, DYN released from the striatal terminals might diffuse to the pallidal terminals, regulating their GABA release. Thirdly, DYN might exert a direct inhibition of EP neurons. Thus, DYN released from striatal axons might control the activity of EP neurons by reducing the GABAergic transmission and also by hyperpolarizing postsynaptic membrane.

The differential vulnerability of striatal projection neurons in 3-nitropropionic acid-treated rats does not match that typical of adult-onset Huntington's disease

In adult-onset Huntington's disease (HD), striatal projection neurons are much more vulnerable than striatal interneurons, but even striatal projection neurons show differences in their vulnerability, with the striatal projection neurons projecting to the internal segment of the globus pallidus being the least vulnerable. Previous studies have shown that systemic chronic treatment with 3-nitropropionic acid (3NP), an inhibitor of succinate dehydrogenase, induces the preferential loss of striatal projection neurons over striatal interneurons that is characteristic of HD, which has been taken to support the hypothesis that the pathogenic defect in HD may involve impaired energy metabolism. We sought to determine whether the patterns of survival for striatal projection neurons in 4-month-old rats after chronic systemic 3NP treatment also resemble those in adult-onset HD. We assessed the projection neuron survival using neuropeptide immunolabeling of striatal efferent fibers in striatal target areas and quantified the degree of fiber loss in the striatal target areas using computer-assisted image analysis. We found that 3NP produced relatively equal loss of striatal fibers and terminals in the globus pallidus, substantia nigra, and entopeduncular nucleus, indicating a nondifferential vulnerability of striatal projection neurons to 3NP-induced impairment in energy metabolism. The results suggest that the 3NP rat model does not fully mimic adult-onset HD pathogenesis.

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.

Dopaminergic and cholinergic stimulation of the ventrolateral striatum elicit rat jaw movements that are funnelled via distinct efferents

It has been reported that two distinct types of jaw movements can be elicited by bilateral injections of drugs into the ventrolateral striatum: (1) dopamine receptor-mediated jaw movements that are elicited by a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), and (2) acetylcholine receptor-mediated jaw movements that are elicited by carbachol (2.5 microg). In the present study, electromyographic analysis was used to characterise these movements: the dopamine receptor-mediated jaw movements were marked by a dominant digastric activity during jaw opening and a dominant masseter activity during jaw closing (digastric/masseter type), whereas the acetylcholine receptor-mediated jaw movements were marked by a dominant digastric activity during jaw opening without any significant change in masseter activity during jaw closing (digastric type). The main goal was to (in)validate the hypothesis that these two types of jaw movements are funnelled via distinct gamma-aminobutyric acid (GABA)ergic output channels. Bilateral injections of both muscimol (25 and 50 ng/0.2 microl per side) and bicuculline (50 and 150 ng/0.2 microl per side) into the ventral pallidum, entopeduncular nucleus or dorsolateral part of the substantia nigra pars reticulata essentially inhibited dopamine receptor-mediated jaw movements to various degrees. In contrast, acetylcholine receptor-mediated jaw movements were inhibited by muscimol given into the entopeduncular nucleus and dorsolateral part of the substantia nigra pars reticulata, whereas these movements were enhanced by bicuculline. The acetylcholine receptor-mediated jaw movements were not affected by muscimol injections into the ventral pallidum, but were inhibited by bicuculline injections. Studies on such injections into the ventral pallidum, entopeduncular nucleus or dorsolateral part of the substantia nigra pars reticulata of naive rats revealed that jaw movements of the digastric/masseter type were elicited either by muscimol injections into the dorsolateral part of the substantia nigra pars reticulata or by combined injections of muscimol and bicuculline into the entopeduncular nucleus, and that jaw movements of the digastric type were elicited only by combined injections of muscimol and bicuculline into the entopeduncular nucleus. Together, the data allow the conclusion that dopamine receptor-mediated and acetylcholine receptor-mediated jaw movements are two distinct types of jaw movements that are funnelled via separate GABAergic output channels. It is suggested that the three different profiles of responses to GABAergic drugs in animals showing either dopamine receptor-mediated or acetylcholine receptor-mediated jaw movements reflect the involvement of three distinct types of output neurons of the striatum, namely: type I neurons with collateralised axons to the ventral pallidum, entopeduncular nucleus and dorsolateral part of the substantia nigra pars reticulata, mediating the dopamine receptor-mediated jaw movements; type II neurons with collateralised axons to the globus pallidus that, in turn, project to the entopeduncular nucleus and the dorsolateral part of the substantia nigra pars reticulata, mediating directly the acetylcholine receptor-mediated jaw movements; and type III neurons with a single axon to the ventral pallidum, mediating indirectly the acetylcholine receptor-mediated movements. It is evident that future studies are required to provide direct evidence in favour of the latter hypothesis.

Intrastriatal injection of D1 or D2 dopamine agonists affects glucose utilization in both the direct and indirect pathways of the rat basal ganglia

Two distinct pathways are thought to connect the striatum to the basal ganglia output nuclei: a direct pathway, originating from neurons bearing dopamine, D(1) receptors and an indirect pathway, originating from neurons expressing D(2) receptors. It has been recently suggested, however, that dopamine receptor sub-types may co-localize and co-operate in the striatum. We sought to verify the functional segregation of the two pathways by measuring cerebral glucose utilization following intrastriatal injection of selective D(1) (SKF 38393), D(2) (quinpirole), or non-selective indirect (amphetamine) and direct (apomorphine) dopamine agonists, in freely-moving rats. All drugs -- regardless of receptor selectivity -- reduced glucose utilization in nuclei of both the direct and indirect pathways, thus lending further support to the existence of a functional co-operation of striatal D(1) and D(2) receptors.

Facilitation of learning and long-term ventral pallidal-cortical cholinergic activation by proteoglycan biglycan and chondroitin sulfate C

We have shown previously in the rat that biglycan, a recently discovered chondroitin sulfate proteoglycan, has neurotrophic effects which are mediated by its chondroitin/dermatan sulfate chains. Here we report that biglycan has neurochemical effects when injected into the nucleus basalis magnocellularis of the ventral pallidum, a site of dense cholinergic cell bodies. The effects on the cholinergic output in the frontal cortex are long lasting, indicating profound neuroactive function akin to that expected of a long-acting hormone. Injected into the same area of the brain, as well as into the ventricles in behaviorally impaired old animals, we found that biglycan can improve learning and memory in several behavioral paradigms. Furthermore, we show that both the neurochemical effectiveness as well as the promotion of learning is carried not by the proteoglycan per se, but rather by its chondroitin sulfate moiety, thus, demonstrating for the first time memory-promoting and neuroactive effects of a glycosaminoglycan, namely, chondroitin sulfate C. The results suggest that biglycan and other extracellular matrix molecules can have neurobehavioral and pharmacological functions for beyond those traditionally attributed to this class of molecules.

Neurochemical changes in the entopeduncular nucleus and increased oral behavior in rats treated subchronically with clozapine or haloperidol

The purpose of the present experiment was to test the possibility that atypical antipsychotics and classical antipsychotics differentially regulate specific neurochemical processes within the entopeduncular nucleus. For these experiments, rats were administered clozapine (25 mg/kg), haloperidol (1 mg/kg), or Tween-80 (control) daily for 21 days. Dopamine D(1)-receptor binding was assessed with in vitro receptor autoradiographic methods and the mRNAs corresponding to the two forms of glutamate decarboxylase (glutamate decarboxylase-65 and glutamate decarboxylase-67) were analyzed using in situ hybridization histochemical methods. In addition, vacuous chewing movements (VCM) were measured throughout the drug administration period as a functional indicator of drug action and changes in striatal dopamine D(2)-receptor binding were measured as a positive control for D(2)-receptor antagonist properties of haloperidol and clozapine. In agreement with previous reports, haloperidol increased D(2)-receptor binding throughout the striatum while clozapine had a more limited impact on D(2)-receptors. Behavioral analysis revealed that both haloperidol and clozapine enhanced the display of vacuous chewing movements to a similar extent but with a different postinjection latency. In the entopeduncular nucleus, clozapine increased D(1)-receptor binding compared to controls while haloperidol was without effect. With respect to the regulation of GAD mRNAs, haloperidol increased glutamate decarboxylase-65 and glutamate decarboxylase-67 mRNA levels throughout the entopeduncular nucleus. The effects of clozapine were restricted to increases in glutamate decarboxylase-65 mRNA. These studies show that clozapine and haloperidol, both of which increase the occurrence of VCM, differentially modulate the neurochemistry of the entopeduncular nucleus.