Extracerebellar role for Cerebellin1: modulation of dendritic spine density and synapses in striatal medium spiny neurons.

Cerebellin1 (Cbln1) is a secreted glycoprotein that was originally isolated from the cerebellum and subsequently found to regulate synaptic development and stability. Cbln1 has a heterogeneous distribution in brain, but the only site in which it has been shown to have central effects is the cerebellar cortex, where loss of Cbln1 causes a reduction in granule cell-Purkinje cell synapses. Neurons of the thalamic parafascicular nucleus (PF), which provide glutamatergic projections to the striatum, also express high levels of Cbln1. We first examined Cbln1 in thalamostriatal neurons and then determined if cbln1 knockout mice exhibit structural deficits in striatal neurons. Virtually all PF neurons express Cbln1-immunoreactivity (-ir). In contrast, only rare Cbln1-ir neurons are present in the central medial complex, the other thalamic region that projects heavily to the dorsal striatum. In the striatum Cbln1-ir processes are apposed to medium spiny neuron (MSN) dendrites; ultrastructural studies revealed that Cbln1-ir axon terminals form axodendritic synapses with MSNs. Tract-tracing studies found that all PF cells retrogradely labeled from the striatum express Cbln1-ir. We then examined the dendritic structure of Golgi-impregnated MSNs in adult cbln1 knockout mice. MSN dendritic spine density was markedly increased in cbln1(-/-) mice relative to wildtype littermates, but total dendritic length was unchanged. Ultrastructural examination revealed an increase in the density of MSN axospinous synapses in cbln1(-/-) mice, with no change in postsynaptic density length. Thus, Cbln1 determines the dendritic structure of striatal MSNs, with effects distinct from those seen in the cerebellum.

Glucocorticoid modulation of tryptophan hydroxylase-2 protein in raphe nuclei and 5-hydroxytryptophan concentrations in frontal cortex of C57/Bl6 mice.

Considerable attention has focused on regulation of central tryptophan hydroxylase (TPH) activity and protein expression. At the time of these earlier studies, it was thought that there was a single central TPH isoform. However, with the recent identification of TPH2, it becomes important to distinguish between regulatory effects on the protein expression and activity of the two isoforms. We have generated a TPH2-specific polyclonal antiserum (TPH2-6361) to study regulation of TPH2 at the protein level and to examine the distribution of TPH2 expression in rodent and human brain. TPH2 immunoreactivity (IR) was detected throughout the raphe nuclei, in lateral hypothalamic nuclei and in the pineal body of rodent and human brain. In addition, a prominent TPH2-IR fiber network was found in the human median eminence. We recently reported that glucocorticoid treatment of C57/Bl6 mice for 4 days markedly decreased TPH2 messenger RNA levels in the raphe nuclei, whereas TPH1 mRNA was unaffected. The glucocorticoid-elicited inhibition of TPH2 gene expression was blocked by co-administration of the glucocorticoid receptor antagonist mifepristone (RU-486). Using TPH2-6361, we have extended these findings to show a dose-dependent decrease in raphe TPH2 protein levels in response to 4 days of treatment with dexamethasone; this effect was blocked by co-administration of mifepristone. Moreover, the glucocorticoid-elicited inhibition of TPH2 was functionally significant: serotonin synthesis was significantly reduced in the frontal cortex of glucocorticoid-treated mice, an effect that was blocked by mifepristone co-administration. This study provides further evidence for the glucocorticoid regulation of serotonin biosynthesis via inhibition of TPH2 expression, and suggest that elevated glucocorticoid levels may be relevant to the etiology of psychiatric diseases, such as depression, where hypothalamic-pituitary-adrenal axis dysregulation has been documented.

Cortical regulation of dopamine depletion-induced dendritic spine loss in striatal medium spiny neurons.

The proximate cause of Parkinson's disease is striatal dopamine depletion. Although no overt toxicity to striatal neurons has been reported in Parkinson's disease, one of the consequences of striatal dopamine loss is a decrease in the number of dendritic spines on striatal medium spiny neurons (MSNs). Dendrites of these neurons receive cortical glutamatergic inputs onto the dendritic spine head and dopaminergic inputs from the substantia nigra onto the spine neck. This synaptic arrangement suggests that dopamine gates corticostriatal glutamatergic drive onto spines. Using triple organotypic slice cultures composed of ventral mesencephalon, striatum, and cortex of the neonatal rat, we examined the role of the cortex in dopamine depletion-induced dendritic spine loss in MSNs. The striatal dopamine innervation was lesioned by treatment of the cultures with the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) or by removing the mesencephalon. Both MPP+ and mesencephalic ablation decreased MSN dendritic spine density. Analysis of spine morphology revealed that thin spines were preferentially lost after dopamine depletion. Removal of the cortex completely prevented dopamine depletion-induced spine loss. These data indicate that the dendritic remodeling of MSNs seen in parkinsonism occurs secondary to increases in corticostriatal glutamatergic drive, and suggest that modulation of cortical activity may be a useful therapeutic strategy in Parkinson's disease.

Striatal plasticity in parkinsonism: dystrophic changes in medium spiny neurons and progression in Parkinson's disease.

Striatal dopamine loss in Parkinson's Disease (PD) sets into play a variety of compensatory responses to help counter dopamine depletion. Most of these changes involve surviving dopamine neurons, but there are also changes in striatal medium spiny neurons (MSNs), which are the major target of dopamine axons. Among these changes are decreases in MSN dendritic length and spine density, which may dampen excessive corticostriatal glutamatergic drive onto MSNs that occurs secondary to dopamine loss. An increasing knowledge of dendritic changes in PD suggests strategies for tracking progressive worsening of symptoms and is opening new ideas on novel therapeutic strategies for PD.

Dendritic degeneration in neostriatal medium spiny neurons in Parkinson disease.

Dysfunction of neostriatal medium spiny neurons (MSNs) is hypothesized to underlie late-stage motor complications of Parkinson disease (PD). The authors demonstrate shortened dendrite length of MSNs that was similar in four regions of neostriatum in late-stage PD. In contrast, MSN dendrite spine degeneration was unevenly distributed with the greatest loss in caudal putamen. The authors propose that these structural changes in MSN may contribute to late-stage motor complications of PD.

Anatomical substrates of orexin-dopamine interactions: lateral hypothalamic projections to the ventral tegmental area.

Dopaminergic projections to the forebrain arising from the mesencephalic ventral tegmentum modulate information processing in cortical and limbic sites. The lateral hypothalamus is crucial for the coordination of behavioral responses to interoceptive cues. The presence of a hypothalamic input to the ventral tegmental area has been known for some time, but the organization of this pathway has received little attention. Among the neuropeptides found in the hypothalamus are the orexins, which are selectively expressed in the lateral hypothalamus and adjacent perifornical area and are critically involved in homeostatic regulatory processes, including arousal and feeding. We examined the anatomical relationships between orexin and dopamine neurons in rats, with particular attention to characterizing the lateral hypothalamic projection to midbrain dopamine neurons. Iontophoretic deposits of the retrograde tracer FluoroGold into the ventral tegmental area revealed a large number of retrogradely-labeled cells that formed a band extending from the medial perifornical area arching dorsally over the fornix and then ventrolaterally into the lateral hypothalamus; approximately 20% of these cells expressed orexin A-like immunoreactivity. Moreover, axons that were anterogradely labeled from the lateral hypothalamus were seen throughout the ventral tegmental area, and were often in close proximity to the dendrites and somata of dopamine neurons. Dopamine and orexin fibers were found to codistribute in the medial prefrontal cortex; orexin fibers were present in lower density in the medial shell of the nucleus accumbens, and the central and posterior basolateral nuclei of the amygdala. We conclude that the lateral hypothalamic/perifornical projection represents an anatomical substrate by which interoceptive-related signals may influence forebrain dopamine function.

Neurotensin-deficient mice show altered responses to antipsychotic drugs.

The peptide transmitter neurotensin (NT) exerts diverse neurochemical effects that resemble those seen after acute administration of antipsychotic drugs (APDs). These drugs also induce NT expression in the striatum; this and other convergent findings have led to the suggestion that NT may mediate some APD effects. Here, we demonstrate that the ability of the typical APD haloperidol to induce Fos expression in the dorsolateral striatum is markedly attenuated in NT-null mutant mice. The induction of Fos and NT in the dorsolateral striatum in response to typical, but not atypical, APDs has led to the hypothesis that the increased expression of these proteins is mechanistically related to the production of extrapyramidal side effects (EPS). However, we found that catalepsy, which is thought to reflect the EPS of typical APDs, is unaffected in NT-null mutant mice, suggesting that NT does not contribute to the generation of EPS. We conclude that NT is required for haloperidol-elicited activation of a specific population of striatal neurons but not haloperidol-induced catalepsy. These results are consistent with the hypothesis that endogenous NT mediates a specific subset of APD actions.

The neurotensin antagonist SR 48692 attenuates haloperidol-induced striatal Fos expression in the rat.

Neurotensin interacts with central dopamine systems and has been suggested to exert antipsychotic drug-like actions. Antipsychotic drugs such as haloperidol induce striatal immediate-early gene expression. In order to study neurotensin's role in antipsychotic drug actions, rats were pretreated with the neurotensin antagonist SR 48692 and then injected with haloperidol. SR 48692 dose-dependently decreased haloperidol-elicited immediate-early gene expression in the dorsolateral and central striatum but not other striatal areas. SR 48692 reduced Fos expression in the striatal patch (striosome) and matrix compartments, with a significantly greater effect in the patch. These data suggest that neurotensin may play a role in the actions of haloperidol. In view of proposed functional roles of the striatal patch and matrix, we suggest that neurotensin may be important in the therapeutic rather than side effects of antipsychotic drugs.

Distribution of serotonin 5-HT(2A) receptors in afferents of the rat striatum.

Treatment with conventional antipsychotic drugs (APDs) is accompanied by extrapyramidal side effects (EPS), which are thought to be due to striatal dopamine D(2) receptor blockade. In contrast, treatment with atypical APDs is marked by a low incidence or absence of EPS. The reduced motor side effect liability of atypical APDs has been attributed to a high serotonin 5-HT(2A) receptor affinity coupled with a relatively low D(2) affinity. Despite the high density of 5-HT(2A) binding sites in the striatum, there are few detectable 5-HT(2A) mRNA-expressing neurons in the striatum. This suggests that most striatal 5-HT(2A) receptors are heteroceptors located on afferent axons. A combined retrograde tracer-immunohistochemistry method was used to determine the sites of origin of striatal 5-HT(2A)-like immunoreactive axons. 5-HT(2A)-like immunoreactive neurons in both the cortex and globus pallidus were retrogradely labeled from the striatum; very few nigrostriatal or thalamostriatal neurons expressed 5-HT(2A)-like immunoreactivity. Within the striatum, parvalbumin-containing interneurons displayed 5-HT(2A) immunolabeling; these neurons are the targets of cortical and pallidal projections. Our data indicate that cortico- and pallido-striatal neurons are the major source of 5-HT(2A) receptor binding in the striatum, and suggest that cortico- and pallido-striatal neurons are strategically positioned to reduce the motor side effects that accompany striatal D(2) receptor blockade or are seen in parkinsonism.

Stoichiometry of tyrosine hydroxylase phosphorylation in the nigrostriatal and mesolimbic systems in vivo: effects of acute haloperidol and related compounds.

Electrical stimulation of the medial forebrain bundle increases (32)P incorporation into striatal tyrosine hydroxylase (TH) at Ser (19), Ser(31), and Ser(40). In the present studies, the effects of acute haloperidol and related drugs on sitespecific TH phosphorylation stoichiometry (PS) in the nigrostriatal and mesolimbic systems were determined by quantitative blot immunolabeling using phosphorylation statespecific antibodies. The striatum (Str), substantia nigra (SN), nucleus accumbens (NAc), and ventral tegmental area (VTA) from Sprague-Dawley rats were harvested 30-40 min after a single injection of either vehicle, haloperidol (2 mg/kg), raclopride (2 mg/kg), clozapine (30 mg/kg), or SCH23390 (0.5 mg/kg). In vehicle-injected control rats, Ser(19) PS was 1.5- to 2. 5-fold lower in Str and NAc than in SN and VTA, Ser(31) PS was two-to fourfold higher in Str and NAc than in SN and VTA, and Ser(40) PS was similar between the terminal field and cell body regions. After haloperidol, Ser(40) PS increased twofold in Str and NAc, whereas a smaller increase in SN and VTA was observed. The effects of haloperidol on Ser(19) PS were similar to those on Ser(40) in each region; however, haloperidol treatment increased Ser(31) PS at least 1.6-fold in all regions. The effects of raclopride on TH PS were comparable to those of haloperidol, whereas clozapine treatment increased TH PS at all sites in all regions. By contrast, the effects of SCH23390 on TH PS were relatively small and restricted to the NAc. The stoichiometries of site-specific TH phosphorylation in vivo are presented for the first time. The nigrostriatal and mesolimbic systems have common features of TH PS, distinguished by differences in TH PS between the terminal field and cell body regions and by dissimilar increases in TH PS in the terminal field and cell body regions after acute haloperidol.

The distribution and origin of the calretinin-containing innervation of the nucleus accumbens of the rat.

The nucleus accumbens of the rat consists of several subregions that can be distinguished on the basis of histochemical markers. For example, the calcium-binding protein calbindin D28k is a useful marker of the core compartment of the nucleus accumbens. Calretinin, another calcium-binding protein, is found in a dense fibre plexus in the accumbal shell and septal pole regions. The source of the accumbal calretinin innervation is not known. We examined the distribution of calretinin in the nucleus accumbens and used tract-tracing and lesion methods to determine the source of this calretinin innervation. Intense calretinin immunoreactivity was present in the medial shell, but the density of calretinin axons diminished sharply in the ventrolateral shell. Regions of dense calretinin immunostaining and those areas with calbindin-like immunoreactive cell bodies were generally segregated in the nucleus accumbens, although some overlap in the transition region between the core and shell was seen. Small clusters of calretinin-immunoreactive fibres were seen in the core, where they were restricted to calbindin-negative patches. Injections of the anterograde tracer biotinylated dextran amine into the paraventricular thalamic nucleus labelled fibres in calretinin-rich regions of the accumbens. Conversely, injections of Fluoro-gold into the accumbal shell retrogradely labelled numerous cells in the paraventricular thalamic nucleus that were calretinin-immunoreactive. Electrolytic lesions of the paraventricular thalamic nucleus reduced calretinin levels in the shell by approximately 80%. These data indicate that the calretinin innervation of the nucleus accumbens is derived primarily from the thalamic paraventricular nucleus, and marks accumbal territories that are largely complementary to those defined by calbindin.

DOI-Induced activation of the cortex: dependence on 5-HT2A heteroceptors on thalamocortical glutamatergic neurons.

Administration of the hallucinogenic 5-HT(2A/2C) agonist 1-[2, 5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) induces expression of Fos protein in the cerebral cortex. To understand the mechanisms subserving this action of DOI, we examined the consequences of pharmacological and surgical manipulations on DOI-elicited Fos expression in the somatosensory cortex of the rat. DOI dose-dependently increased cortical Fos expression. Pretreatment with the selective 5-HT(2A) antagonist MDL 100,907 completely blocked DOI-elicited Fos expression, but pretreatment with the 5-HT(2C) antagonist SB 206,553 did not modify DOI-elicited Fos expression. These data suggest that DOI acts through 5-HT(2A) receptors to increase cortical Fos expression. However, we found that DOI did not induce Fos in cortical 5-HT(2A) immunoreactive neurons but did increase expression in a band of neurons spanning superficial layer V to deep III, within the apical dendritic fields of layer V 5-HT(2A)-immunoreactive cells. This band of Fos immunoreactive neurons was in register with anterogradely labeled axons from the ventrobasal thalamus, which have previously been shown to be glutamatergic and express the 5-HT(2A) transcript. The effects of DOI were markedly reduced in animals pretreated with the AMPA/KA antagonist GYKI 52466, and lesions of the ventrobasal thalamus attenuated DOI-elicited Fos expression in the cortex. These data suggest that DOI activates 5-HT(2A) receptors on thalamocortical neurons and thereby increases glutamate release, which in turn drives Fos expression in cortical neurons through an AMPA receptor-dependent mechanism. These data cast new light on the mechanisms of action of hallucinogens.

Clozapine pretreatment modifies haloperidol-elicited forebrain Fos induction: a regionally-specific double dissociation.

RATIONALE: Acute administration of typical antipsychotic drugs, such as haloperidol, results in the induction of the immediate-early gene c-fos in the dorsolateral striatum. In contrast, the atypical antipsychotic drug clozapine, which lacks significant extrapyramidal side effect liability, does not induce Fos protein in the dorsal striatum. Several studies have attempted to define the mechanisms through which typical antipsychotic drugs induce striatal Fos, often by pretreating animals with specific receptor antagonists. Despite the broad receptor profile of clozapine, there has been no study of the effect of clozapine pretreatment on haloperidol-elicited striatal Fos expression. METHODS: We examined the effects of clozapine pretreatment of rats on haloperidol-elicited forebrain Fos expression, using both immunoblot and immunohistochemical methods. The effects of clozapine pretreatment were assessed in the dorsal striatum and in the different nucleus accumbens compartments, the septum, and the prefrontal cortex. RESULTS: Clozapine pretreatment markedly decreased haloperidol-elicited striatal Fos induction and blocked haloperidol-induced catalepsy. Clozapine also attenuated haloperidol-elicited Fos expression in the nucleus accumbens, but in the prefrontal cortex and ventrolateral septum the effects of haloperidol and clozapine were additive. CONCLUSIONS: An emerging body of literature suggests a high incidence of rapid relapse in schizophrenic patients when clozapine treatment is discontinued. This psychosis is relatively resistant to haloperidol and other neuroleptics, even in patients who had previously responded well to neuroleptics. The present data may shed light on the central sites associated with and perhaps model certain aspects of the relapse associated with clozapine discontinuation.

Stress induces Fos expression in neurons of the thalamic paraventricular nucleus that innervate limbic forebrain sites.

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus that responds strongly to exposure to various stressors. Many of the projection targets of PVT neurons, including the medial prefrontal cortex, nucleus accumbens, and central/basolateral nuclei of the amygdala, are also activated by stress. We sought to determine if PVT neurons that respond to stress are those that project to one or more of these forebrain sites. Retrograde tract tracing combined with immunohistochemical detection of Fos protein-like immunoreactivity was used to assess the activation of target-specific populations of PVT projection neurons by mild footshock stress in the rat. Stress markedly increased Fos protein-like immunoreactivity in PVT neurons, but without regard to the projection target of the thalamic neurons. Thus, the percentage of PVT cells that were retrogradely labeled from either the prefrontal cortex, nucleus accumbens, or amygdala, and that expressed Fos-like immunoreactivity did not differ substantially across the three forebrain sites. These data suggest that the PVT may have a role as a generalized relay for information relating to stress, and may serve an important role in the stress-induced activation of limbic forebrain areas.

5-HT2 receptor regulation of extracellular GABA levels in the prefrontal cortex.

Monoamines, including both dopamine and serotonin, synapse onto prefrontal cortical interneurons. Dopamine has been shown to activate these GABAergic interneurons, but there are no direct data on the effects of serotonin on GABA release in the prefrontal cortex. We, therefore, examined the effects of the 5-HT2a/c agonist 1-(2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) on extracellular GABA levels in the prefrontal cortex of the rat. Local infusions of DOI dose-dependently increased cortical extracellular GABA levels. In addition, systemic DOI administration resulted in Fos protein expression in glutamic acid decarboxylase67-immunoreactive interneurons of the prefrontal cortex. These data indicate that serotonin, operating through a 5-HT2 receptor, acutely activates GABAergic interneurons in the prefrontal cortex. These data further suggest that there may be convergent regulation of interneurons by dopamine and serotonin in the prefrontal cortex.

Psychostimulant-induced Fos protein expression in the thalamic paraventricular nucleus.

Lesions of glutamatergic afferents to the nucleus accumbens have been reported to block psychostimulant-induced behavioral sensitization. However, thalamic glutamatergic projections to the nucleus accumbens have received little attention in the context of psychostimulant actions. We examined the effects of acute amphetamine and cocaine administration on expression of Fos protein in the thalamic paraventricular nucleus (PVT), which provides glutamatergic inputs to the nucleus accumbens and also receives dopaminergic afferents. Immunoblot and immunohistochemical studies revealed that both psychostimulants dose-dependently increased PVT Fos expression. PVT neurons retrogradely labeled from the nucleus accumbens were among the PVT cells that showed a Fos response to amphetamine. D2 family dopamine agonists, including low doses of the D3-preferring agonist 7-OH-DPAT, increased the numbers of Fos-like-immunoreactive neurons in the PVT. Conversely, the effects of cocaine and amphetamine on PVT Fos expression were blocked by pretreatment with the dopamine D2/3 antagonist raclopride. Because PVT neurons express D3 but not other dopamine receptor transcripts, it appears that psychostimulants induce Fos in PVT neurons through a D3 dopamine receptor. We suggest that the PVT may be an important part of an extended circuit subserving both the arousing properties and reinforcing aspects of psychostimulants.

The effects of thalamic paraventricular nucleus lesions on cocaine-induced locomotor activity and sensitization.

The brain circuitry that subserves the augmented locomotor response to repeated psychostimulant administration has been the subject of intense scrutiny. The dopaminergic innervation of the nucleus accumbens is critically involved in psychostimulant-elicited behavioral sensitization, and recent studies suggest that lesions of structures that send glutamatergic projections to the nucleus accumbens alter the acquisition or expression of psychostimulant-elicited sensitization. Although certain thalamic nuclei provide a major glutamatergic input to the striatum, the involvement of the thalamus in psychostimulant-elicited sensitization has not been investigated. We therefore examined the effects of lesions of the thalamic paraventricular nucleus, which projects to the shell of the nucleus accumbens, on cocaine-elicited locomotor sensitization. Lesions of the paraventricular nucleus did not alter basal locomotor activity, but significantly enhanced the acute locomotor response to cocaine. In contrast, repeated cocaine administration did not progressively augment locomotor activity in lesioned rats, but did so in sham-lesioned animals. The thalamic lesions also blocked the conditioned locomotor response to the environment in which the cocaine injections took place. These data suggest that the thalamic paraventricular nucleus may be an integral part of extended circuitry that subserves both the conditioned and nonconditioned components of psychostimulant-induced behavioral sensitization.

Effects of desmethylclozapine on Fos protein expression in the forebrain: in vivo biological activity of the clozapine metabolite.

Several products of the hepatic metabolism of clozapine are found in high concentrations in the plasma of schizophrenic patients treated with this atypical antipsychotic drug. One of these metabolites, N-desmethylclozapine, has substantially different affinities for dopamine and serotonin metabolites than does the parent compound. However, it is not known if this metabolite is active in vivo. We examined the effect of acute administration of desmethylclozapine to rats on forebrain Fos protein expression. Clozapine induces expression of this immediate-early gene in a distinct regional pattern in the brain. Desmethylclozapine significantly increased Fos protein expression in the medial prefrontal cortex and nucleus accumbens, but not in the dorsolateral striatum, thus mirroring the effects of the parent compound. These data indicate that the desmethyl metabolite of clozapine has in vivo biological activity.

Dopaminergic regulation of extracellular gamma-aminobutyric acid levels in the prefrontal cortex of the rat.

Dopaminergic axons in the prefrontal cortex synapse with interneurons as well as pyramidal cells. Electrophysiological data suggest that dopamine depolarizes certain gamma-aminobutyric acid (GABA)-containing interneurons in the cortex. We investigated the dopaminergic regulation of extracellular GABA levels in the prefrontal cortex using in vivo microdialysis. Systemic administration of the mixed D1/D2 dopamine receptor agonist apomorphine increased extracellular GABA levels in the prefrontal cortex, but did not increase levels of glycine; the apomorphine-elicited increase in GABA levels was blocked by tetrodotoxin infusion into the prefrontal cortex. Local administration of the D2 agonist quinpirole into the cortex via the dialysis probe resulted in a dose-dependent increase in extracellular GABA levels. In contrast, administration of the D1 agonist SKF 38393 did not alter GABA levels. The ability of systemic apomorphine to increase extracellular GABA levels in the prefrontal cortex was blocked by local administration of the D2-like antagonist sulpiride to the cortex, but was not attenuated significantly by local perfusion of the D1 antagonist SCH 23390. Similarly, the ability of local infusion of the D2 agonist quinpirole to enhance extracellular GABA levels was blocked by sulpiride but not by SCH 23390. These data suggest that dopamine agonists increase the release of GABA in the prefrontal cortex through a D2-like receptor. In view of posited changes in prefrontal cortical dopamine and GABA systems in schizophrenia, it is possible that changes in GABAergic function in the cortex in schizophrenia are secondary to changes in cortical dopamine function.