Cell-specific abnormalities of glutamate transporters in schizophrenia: sick astrocytes and compensating relay neurons?

Excitatory amino-acid transporters (EAATs) bind and transport glutamate, limiting spillover from synapses due to their dense perisynaptic expression primarily on astroglia. Converging evidence suggests that abnormalities in the astroglial glutamate transporter localization and function may underlie a disease mechanism with pathological glutamate spillover as well as alterations in the kinetics of perisynaptic glutamate buffering and uptake contributing to dysfunction of thalamo-cortical circuits in schizophrenia. We explored this hypothesis by performing cell- and region-level studies of EAAT1 and EAAT2 expression in the mediodorsal nucleus of the thalamus in an elderly cohort of subjects with schizophrenia. We found decreased protein expression for the typically astroglial-localized glutamate transporters in the mediodorsal and ventral tier nuclei. We next used laser-capture microdissection and quantitative polymerase chain reaction to assess cell-level expression of the transporters and their splice variants. In the mediodorsal nucleus, we found lower expression of transporter transcripts in a population of cells enriched for astrocytes, and higher expression of transporter transcripts in a population of cells enriched for relay neurons. We confirmed expression of transporter protein in neurons in schizophrenia using dual-label immunofluorescence. Finally, the pattern of transporter mRNA and protein expression in rodents treated for 9 months with antipsychotic medication suggests that our findings are not due to the effects of antipsychotic treatment. We found a compensatory increase in transporter expression in neurons that might be secondary to a loss of transporter expression in astrocytes. These changes suggest a profound abnormality in astrocyte functions that support, nourish and maintain neuronal fidelity and synaptic activity.

Abnormal subcellular localization of GABAA receptor subunits in schizophrenia brain.

Inhibitory neurotransmission is primarily mediated by γ-aminobutyric acid (GABA) activating synaptic GABA type A receptors (GABA(A)R). In schizophrenia, presynaptic GABAergic signaling deficits are among the most replicated findings; however, postsynaptic GABAergic deficits are less well characterized. Our lab has previously demonstrated that although there is no difference in total protein expression of the α1-6, ß1-3 or γ2 GABA(A)R subunits in the superior temporal gyrus (STG) in schizophrenia, the α1, ß1 and ß2 GABA(A)R subunits are abnormally N-glycosylated. N-glycosylation is a posttranslational modification that has important functional roles in protein folding, multimer assembly and forward trafficking. To investigate the impact that altered N-glycosylation has on the assembly and trafficking of GABA(A)Rs in schizophrenia, this study used western blot analysis to measure the expression of α1, α2, ß1, ß2 and γ2 GABA(A)R subunits in subcellular fractions enriched for endoplasmic reticulum (ER) and synapses (SYN) from STG of schizophrenia (N = 16) and comparison (N = 14) subjects and found evidence of abnormal localization of the ß1 and ß2 GABA(A)R subunits and subunit isoforms in schizophrenia. The ß2 subunit is expressed as three isoforms at 52 kDa (ß2(52 kDa)), 50 kDa (ß2(50 kDa)) and 48 kDa (ß2(48 kDa)). In the ER, we found increased total ß2 GABA(A)R subunit (ß2(ALL)) expression driven by increased ß2(50 kDa), a decreased ratio of ß(248 kDa):ß2(ALL) and an increased ratio of ß2(50 kDa):ß2(48 kDa). Decreased ratios of ß1:ß2(ALL) and ß1:ß2(50 kDa) in both the ER and SYN fractions and an increased ratio of ß2(52 kDa):ß(248 kDa) at the synapse were also identified in schizophrenia. Taken together, these findings provide evidence that alterations of N-glycosylation may contribute to GABAergic signaling deficits in schizophrenia by disrupting the assembly and trafficking of GABA(A)Rs.

Glutamate transporter splice variant expression in an enriched pyramidal cell population in schizophrenia.

Dysregulation of the glutamate transporters EAAT1 and EAAT2 and their isoforms have been implicated in schizophrenia. EAAT1 and EAAT2 expression has been studied in different brain regions but the prevalence of astrocytic glutamate transporter expression masks the more subtle changes in excitatory amino acid transporters (EAATs) isoforms in neurons in the cortex. Using laser capture microdissection, pyramidal neurons were cut from the anterior cingulate cortex of postmortem schizophrenia (n = 20) and control (n = 20) subjects. The messenger RNA (mRNA) levels of EAAT1, EAAT2 and the splice variants EAAT1 exon9skipping, EAAT2 exon9skipping and EAAT2b were analyzed by real time PCR (RT-PCR) in an enriched population of neurons. Region-level expression of these transcripts was measured in postmortem schizophrenia (n = 25) and controls (n = 25). The relationship between selected EAAT polymorphisms and EAAT splice variant expression was also explored. Anterior cingulate cortex pyramidal cell expression of EAAT2b mRNA was increased (P < 0.001; 67%) in schizophrenia subjects compared with controls. There was no significant change in other EAAT variants. EAAT2 exon9skipping mRNA was increased (P < 0.05; 38%) at region level in the anterior cingulate cortex with no significant change in other EAAT variants at region level. EAAT2 single-nucleotide polymorphisms were significantly associated with changes in EAAT2 isoform expression. Haloperidol decanoate-treated animals, acting as controls for possible antipsychotic effects, did not have significantly altered neuronal EAAT2b mRNA levels. The novel finding that EAAT2b levels are increased in populations of anterior cingulate cortex pyramidal cells further demonstrates a role for neuronal glutamate transporter splice variant expression in schizophrenia.

Expression of the NR2B-NMDA receptor trafficking complex in prefrontal cortex from a group of elderly patients with schizophrenia.

Dysregulated glutamate neurotransmission has been implicated in the pathophysiology of schizophrenia. In particular, hypofunction of the NMDA glutamate receptor has been proposed to play an important role in mediating cognitive deficits in patients. The two NMDA receptor subunits, NR2A and NR2B, are distinctly regulated during development and are associated with different intracellular pathways and functions, which suggest that these receptors play separate roles in the control of higher cognitive functions such as learning and memory. Trafficking of the NR2B subunit-containing receptor is regulated by a microtubule-associated trafficking complex consisting of the KIF17, APBA1, CASK, and mLin7 proteins. Several studies have demonstrated an integrated functional regulation of this trafficking complex with NR2B receptor subunit expression, which in turn has been linked to higher cognitive functions. In the present work, we investigated whether expression of this NR2B-associated trafficking complex might be abnormal in schizophrenia. We analyzed the expression of KIF17, APBA1, CASK, mLin7A and mLin7C in postmortem brain from patients with schizophrenia a comparison group. Analysis of transcripts for all of these proteins revealed particularly prominent expression in cortical layer III and layer IV, which overlapped with NR2B but not NR2A transcripts. We found altered expression of transcripts for the CASK, ABPA1, and mLin7 molecules and the CASK, mLin7 proteins, suggesting that NR2B-containing NMDA receptor transport could be selectively compromised in schizophrenia, and that these changes likely involve altered NR2B function in a subset of cortical neurons.

Changes in NMDA receptor subunits and interacting PSD proteins in dorsolateral prefrontal and anterior cingulate cortex indicate abnormal regional expression in schizophrenia.

Abnormal expression of the N-methyl-D-Aspartate (NMDA) receptor and its interacting molecules of the postsynaptic density (PSD) are thought to be involved in the pathophysiology of schizophrenia. Frontal regions of neocortex including dorsolateral prefrontal (DLPFC) and anterior cingulate cortex (ACC) are essential for cognitive and behavioral functions that are affected in schizophrenia. In this study, we have measured protein expression of two alternatively spliced isoforms of the NR1 subunit (NR1C2 and NR1C2') as well as expression of the NR2A-D subunits of the NMDA receptor in DLPFC and ACC in post-mortem samples from elderly schizophrenic patients and a comparison group. We found significantly increased expression of NR1C2' but not of NR1C2 in ACC, suggesting altered NMDA receptor cell membrane expression in this cortical area. We did not find significant changes in the expression of either of the NR1 isoforms in DLPFC. We did not detect changes of any of the NR2 subunits studied in either cortical area. In addition, we studied expression of the NMDA-interacting PSD molecules NF-L, SAP102, PSD-95 and PSD-93 in ACC and DLPFC at both transcriptional and translational levels. We found significant changes in the expression of NF-L in DLPFC, and PSD-95 and PSD-93 in ACC; increased transcript expression was associated with decreased protein expression, suggesting abnormal translation and/or accelerated protein degradation of these molecules in schizophrenia. Our findings suggest abnormal regional processing of the NMDA receptor and its associated PSD molecules, possibly involving transcription, translation, trafficking and protein stability in cortical areas in schizophrenia.

DNA microarray analysis of functionally discrete human brain regions reveals divergent transcriptional profiles.

Transcriptional profiles within discrete human brain regions are likely to reflect structural and functional specialization. Using DNA microarray technology, this study investigates differences in transcriptional profiles of highly divergent brain regions (the cerebellar cortex and the cerebral cortex) as well as differences between two closely related brain structures (the anterior cingulate cortex and the dorsolateral prefrontal cortex). Replication of this study across three independent laboratories, to address false-positive and false-negative results using microarray technology, is also discussed. We find greater than a thousand transcripts to be differentially expressed between cerebellum and cerebral cortex and very few transcripts to be differentially expressed between the two neocortical regions. We further characterized transcripts that were found to be specifically expressed within brain regions being compared and found that ontological classes representing signal transduction machinery, neurogenesis, synaptic transmission, and transcription factors were most highly represented.

Vesicular glutamate transporter transcript expression in the thalamus in schizophrenia.

Previously, we have reported alterations in thalamic NMDA receptor subunit and excitatory amino acid transporter expression in schizophrenia, consistent with the hypothesis that thalamic glutamatergic dysfunction may contribute to the pathophysiology of this illness. We have generalized this hypothesis to include other molecules of the glutamate synapse. Using riboprobes specific for human brain-specific Na+-dependent inorganic phosphate transporter (BNPi) and differentiation-associated Na+/Pi co-transporter (DNPi), both vesicular glutamate transporters, in situ hybridization was performed in the thalami of persons with schizophrenia and comparison subjects. We detected increased expression of DNPi mRNA in the thalamus in schizophrenia, while BNPi mRNA was not expressed in the thalamus in any subjects. These findings support the hypothesis of glutamatergic dysfunction in the thalamus in schizophrenia.

Expression of excitatory amino acid transporter transcripts in the thalamus of subjects with schizophrenia.

OBJECTIVE: Recent investigations of schizophrenia have targeted glutamatergic neurotransmission, since phencyclidine, an N-methyl-D-aspartate (NMDA) receptor antagonist, can induce schizophreniform psychosis. The authors previously reported alterations in thalamic NMDA receptor subunit expression in schizophrenia, consistent with the hypothesis that thalamic glutamatergic hypofunction may contribute to the pathophysiology of this illness. In this study they generalized this hypothesis to include other molecules of the glutamate synapse, specifically excitatory amino acid transporters (EAATs), whose normal expression and regulation in the thalamus may also be disrupted in subjects with schizophrenia. METHOD: In situ hybridization with riboprobes specific for the human excitatory amino acid transporter transcripts EAAT1, EAAT2, and EAAT3 was performed in discrete thalamic nuclei in persons with schizophrenia and comparison subjects. RESULTS: Higher expressions of transcripts encoding EAAT1 and EAAT2, but not EAAT3, were detected in the thalamus of subjects with schizophrenia. CONCLUSIONS: These findings support the hypothesis of glutamatergic dysfunction in schizophrenia and suggest that molecules other than glutamate receptors are abnormally expressed in glutamatergic synapses in this illness.

Striatal ionotropic glutamate receptor expression in schizophrenia, bipolar disorder, and major depressive disorder.

Abnormalities of the ionotropic glutamate receptors (N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid [AMPA], and kainate) have been reported in the brain in schizophrenia, although in complex, region-specific patterns. While limbic cortex and medial temporal lobe structures have been most often studied in psychiatric illnesses, glutamate receptors are expressed in other brain regions associated with limbic circuitry, especially the striatum. In this study, we have determined striatal ionotropic glutamate receptor expression in brains from persons with schizophrenia, bipolar disorder, major depression, and a comparison group, using samples from the Stanley Foundation Neuropathology Consortium. We have determined the expression of these receptors at multiple levels of gene expression by using both in situ hybridization and receptor autoradiography. The expression of nearly all of these molecules was not different in these psychiatric conditions. The only significant changes noted were NR2D and gluR1 transcripts, and [(3)H]AMPA binding. This is the first comprehensive study of striatal ionotropic glutamate receptor expression in schizophrenia and affective disorders, and suggests that there are minimal changes in these receptors in this region of the brain in these illnesses.

Ontogeny of ionotropic glutamate receptor expression in human fetal brain.

Glutamate receptors have multiple roles in the central nervous system. Recent evidence suggests that the iontropic glutamate receptors are critical during brain development, particularly for corticogenesis, neuronal migration, and synaptogenesis. In this study, we examined subunit mRNA expression and binding sites of the NMDA, AMPA, and kainate receptors from gestational weeks 8-20 in human fetal brain. Expression of glutamate receptors was high during several periods in these brains. Different levels of expression of each NMDA, AMPA, and kainate receptor subunit transcripts were present during development, with a greater abundance of NR1, NR2B, NR2D, GluR7, and KA1 mRNA at most gestational ages. Binding sites for NMDA, AMPA, and kainate receptors were all detected, but each had a unique pattern of expression. These results demonstrate that glutamate receptors are expressed early in human brain development, and undergo complex changes over time consistent with their role in normal development.

Abnormal kainate receptor expression in prefrontal cortex in schizophrenia.

Abnormalities of molecules associated with the glutamate synapse have been implicated in the pathophysiology of schizophrenia. Of the many glutamate receptors, those most commonly suggested to be involved in schizophrenia are the ionotropic subtypes, the NMDA, AMPA, and kainate receptors. Both the NMDA and AMPA subtypes have been extensively studied in postmortem brains of individuals with schizophrenia, but relatively little is known about the expression of the kainate subtype of glutamate receptor. In this study, we have determined cortical and striatal kainate receptor expression in brains from persons with schizophrenia and a comparison group, using both in situ hybridization and receptor autoradiography. At the level of subunit mRNA expression, a shift in subunit stoichiometry was evident in multiple regions of the prefrontal cortex, with increased expression of gluR7 mRNA and decreased expression of KA2 mRNA. Decreased kainate receptor binding was also observed in the subjects with schizophrenia, but was restricted to infragranular laminae of the prefrontal cortex. No differences in kainate receptor binding or subunit mRNA levels were found in striatum or occipital cortex, suggesting that these findings may be restricted to association cortex. These data add to the growing literature implicating ionotropic glutamate receptor disturbances in schizophrenia, and indicate that in addition to AMPA and NMDA receptors, the kainate receptors are also abnormally expressed in this illness.

Ionotropic glutamate receptor binding and subunit mRNA expression in thalamic nuclei in schizophrenia.

OBJECTIVE: Both thalamic and glutamatergic dysfunction have been implicated in the pathophysiology of schizophrenia. The authors examined ionotropic glutamate receptor expression in postmortem samples from patients with schizophrenia and comparison subjects, using the hypothesis that glutamate receptor expression differs in limbic nuclei of the thalamus in schizophrenia. METHOD: N-Methyl-D-aspartate (NMDA), AMPA, and kainate receptor expression was determined in six thalamic nuclei from 12 subjects with DSM-III-R diagnoses of schizophrenia and eight psychiatrically normal individuals. The authors used in situ hybridization to determine NMDAR1, NMDAR2A-NMDAR2D, gluR1-gluR7, KA1, and KA2 subunit mRNA levels and receptor autoradiography to determine binding to glutamate binding sites of the three receptor subtypes and to the glycine, polyamine, and ion channel binding sites of the NMDA receptor. RESULTS: Glutamate receptor expression was lower at both transcriptional (NMDAR1, NMDAR2B, NMDAR2C, gluR1, gluR3, and KA2 subunit mRNAs) and posttranscriptional ([(3)H]ifenprodil and [(3)H]MDL105,519 binding to polyamine and glycine sites of the NMDA receptor) levels in the thalamus in patients with schizophrenia than in comparison subjects, but differences were most prominent in nuclei with reciprocal projections to limbic regions. CONCLUSIONS: Abnormalities in NMDA, AMPA, and kainate receptor expression in limbic thalamus are suggestive of the NMDA receptor hypoactivity hypothesis of schizophrenia and are consistent with diminished glutamatergic activity in the thalamus in schizophrenia. Alternatively, these results could suggest abnormal glutamatergic innervation in afferent and/or efferent regions, which are limbic structures that have been implicated in this illness. These results may provide a neurochemical anatomical substrate for antipsychotic therapies targeting ionotropic glutamate receptors.

Ionotropic glutamate receptor modulation preferentially affects NMDA receptor expression in rat hippocampus.

Electrophysiological data suggest that alterations in the function of one glutamate receptor subtype may affect the function of other subtypes. Further, previous studies have demonstrated that NMDA receptor antagonists affect NMDA and kainate receptor expression in rat hippocampus. In order to address the mutual regulation of NMDA, AMPA, and kainate receptor expression in rat hippocampus, we conducted two experiments examining the effects of NMDA and non-NMDA glutamate receptor modulators on NMDA, AMPA, and kainate receptor expression using in situ hybridization and receptor autoradiography. NMDA receptor expression was preferentially affected by systemic treatments, as all drugs significantly altered [(3)H]MK-801 binding, and several drugs increased [(3)H]ifenprodil binding. GYKI52466 and aniracetam treatments resulted in changes in both [(3)H]ifenprodil binding and NR2B mRNA levels, consistent with the association of this subunit and binding site in vitro. There were more modest effects on AMPA and kainate receptor expression, even by direct antagonists. Together, these data suggest that ionotropic glutamate receptors interact at the level of expression. These data also suggest that drug regimens targeting one ionotropic glutamate receptor subtype may indirectly affect other subtypes, potentially producing unwanted side effects.

Nucleus-specific expression of ionotropic glutamate receptor subunit mRNAs and binding sites in primate thalamus.

Thalamic afferents and efferents utilize glutamate as their primary neurotransmitter. There are four families of glutamate receptors that can transduce this activity, as well as regulate glutamate release from thalamic relay neurons. The three ionotropic subtypes are of particular importance, because subunit composition confers variability in functional properties of each subtype. We have quantified the expression of NMDA, AMPA and kainate receptors in the thalamus of the macaque using receptor autoradiography and in situ hybridization. NMDA receptors are multimeric associations of NR1 and NR2A-NR2D subunits that form ligand-gated ion channels. Particular subunits are associated with modulatory binding sites that affect receptor activity. NR1 was the most abundant subunit mRNA; NR2A, NR2B, and NR2D subunit mRNAs were also present, but were expressed in nucleus-specific patterns. Very high levels of [3H]ifenprodil binding to the polyamine site of the NMDA complex were detected in a fairly homogeneous distribution. Binding of the ion channel ligand [3H]MK-801 was also abundant, and limbic nuclei expressed higher levels than motor nuclei or the reticular nucleus. [3H]CGP39653 binding to the glutamate site of the NMDA receptor was the least abundant of the NMDA receptor binding sites. There was variability in the stoichiometric relationships of binding sites across nuclei, suggesting that there is heterogeneity in the pharmacological properties of NMDA receptors expressed in the thalamus. AMPA and kainate are also multimeric associations of specific subunits that form ligand-gated ion channels. These subunits are encoded by specific genes: gluR1-gluR4 for AMPA receptors, and gluR5-gluR7 and KA1-KA2 for kainate receptors. GluR4 and gluR6 mRNAs were, respectively the most abundant of the AMPA and kainate receptor subunit transcripts. Both AMPA and kainate receptor subunit transcripts were expressed in a nucleus-specific pattern. The binding of [3H]kainate was higher than that of [3H]AMPA throughout the thalamus, but AMPA subunit mRNA levels were three to five orders of magnitude higher than those encoding the kainate receptor subunits. The mismatch between the levels of expression of kainate receptor subunit transcripts and binding sites is suggestive of a presynaptic localization of kainate receptors on thalamic afferents. These results suggest that ionotropic glutamate receptors are heterogeneously expressed in the thalamus of the primate, and that their differential expression is both subunit- and nucleus-specific.

Glutamate receptor expression in schizophrenic brain.

Glutamatergic dysfunction has been suggested as a possible substrate of the pathophysiology of schizophrenia. Of the multiple glutamate receptors, those most commonly implicated in schizophrenia are the ionotropic subtypes, the NMDA, AMPA, and kainate receptors. The expression of the glutamate receptors has been determined at multiple levels of gene expression in postmortem brain samples from schizophrenics and controls; while results have not been entirely consistent from study to study, several generalizations have emerged from this literature: (1) The AMPA receptor is abnormally decreased in expression in the schizophrenic hippocampus, involving decreased levels of subunit transcripts and protein levels, as well as binding sites, (2) similar changes are seen for kainate receptor expression in the hippocampus, and (3) the obligate NMDA receptor subunit, NMDAR1, may be abnormally expressed in some cortical regions in schizophrenia. These data support the hypothesis of abnormal glutamatergic neurotransmission involving the ionotropic glutamate receptors in schizophrenia.

Metabotropic glutamate receptor mRNA expression in the schizophrenic thalamus.

BACKGROUND: The central role that the thalamus plays in information processing and sensory integration suggests that its dysfunction may be a factor in the pathophysiology of schizophrenia. Glutamate is a key neurotransmitter in thalamic function, and although all aspects of thalamic glutamate neurotransmission have not been elucidated, transcripts encoding members of each family of the glutamate receptors have been identified in the thalamus. Recently, activation of group II metabotropic glutamate receptors (mGluRs) was demonstrated in rats to ameliorate the behavioral effects associated with exposure to phencyclidine, an uncompetitive NMDA receptor antagonist that can induce psychotic symptoms, suggesting the possibility of mGluR abnormalities in schizophrenia. We investigated whether expression of thalamic mGluR mRNA is altered in this illness. METHODS: We examined the expression of the transcripts encoding the mGluR1, 2, 3, 4, 5, 7, and 8 receptors in postmortem thalamic tissue samples from elderly schizophrenic and control subjects, using in situ hybridization. We identified six thalamic nuclei in each section (anterior, dorsomedial, lateral dorsal, central medial, reticular, and nuclei of the ventral tier). RESULTS: There were no differences between elderly schizophrenic and control subjects in the expression of mGluR1, 2, 3, 4, 5, 7, or 8 transcript levels in any of these six thalamic nuclei. CONCLUSIONS: mGluR mRNA expression is not abnormal in the thalamus of patients with schizophrenia. The modulatory roles proposed for mGluRs, and the potentially important relationship between mGluRs and NMDA receptors, suggest that mGluRs may be involved in the pathophysiology of schizophrenia, but this is not detectable at this level of gene expression.

Nicotine withdrawal and psychiatric symptoms in cigarette smokers with schizophrenia.

The prevalence of smoking is markedly elevated in schizophrenia. Low smoking cessation rates and reports that some smokers with schizophrenia experience an acute increase in symptoms during attempts to quit smoking, suggest a self-medication model. Alternatively, smoking may modulate medication side effects. The effects of treated and untreated smoking abstinence on psychotic symptoms and medication side effects were examined in this study. Nineteen outpatients with schizophrenia or schizoaffective disorder participated in a randomized, double-blind, balanced crossover study: 1 day of ad libitum smoking followed by 3 days of acute smoking abstinence while wearing 22 mg/day active or placebo transdermal nicotine patches, with a return to 3 days of smoking between patch conditions. Daily symptom and side-effect ratings, nicotine and cotinine blood levels were collected. Twelve subjects completed the study. Neither positive symptoms nor mood symptoms changed. An increase in negative symptoms during the first abstinent day occurred in both placebo and active patch conditions, but was not sustained over subsequent abstinent days. Despite physiological signs of withdrawal, completers did not endorse increased nicotine withdrawal symptoms. Dropouts reported higher withdrawal symptoms, but also had no increase in psychiatric symptoms in either phase of the study. Of note, dyskinesias decreased during abstinence and placebo patch treatment, but increased during abstinence and the active patch conditions. Acute exacerbation of psychiatric symptoms is an unlikely explanation for any difficulty smokers with schizophrenia have in early abstinence.

Ionotropic glutamate receptor modulation of 5-HT6 and 5-HT7 mRNA expression in rat brain.

The novel serotonin receptor subtypes, 5-HT6 and 5-HT7, are located in limbic regions and have nanomolar affinities for atypical antipsychotics. These factors have led some to speculate about the involvement of 5-HT6 and 5-HT7 receptors in schizophrenia. However, relatively little is known about these receptor subtypes, including the regulation of their expression in limbic regions. In particular, the regulation of extracellular serotonin levels in the striatum and hippocampal formation by glutamate receptors led us to examine the effects of systemic ionotropic glutamate receptor modulator treatment on 5-HT6 and 5-HT7 receptor expression in these regions. MK-801 treatment induced a dose-dependent decrease in striatal 5-HT6 receptor mRNA levels; similarly, both aniracetam and NBQX treatments also led to decreases in striatal 5-HT6 receptor mRNA levels. Hippocampal 5-HT6 and 5-HT7 receptor expression were not dramatically affected by any of the treatments. To our knowledge, this is the first demonstration of the regulation of striatal 5-HT6 receptor mRNA expression, and provides neurochemical anatomical evidence for the interaction of serotonergic and glutamatergic systems. Furthermore, although these two neurotransmitter systems are separately implicated in schizophrenia, the glutamatergic regulation of the expression of a receptor subtype associated with schizophrenia suggests that alterations in serotonin receptor expression in schizophrenia may result, in part, from altered glutamatergic activity.

Effects of clozapine and haloperidol on 5-HT6 receptor mRNA levels in rat brain.

The high affinity of 5-HT6 receptors for atypical antipsychotic drugs, and their localization in limbic and cortical regions of the brain, suggest that they might play a role in the pathophysiology of schizophrenia. To determine if this receptor is regulated by antipsychotics, rats were injected with clozapine (20 mg/kg/day), haloperidol (2 mg/kg/day), or vehicle daily for 2 weeks, and 5-HT6 receptor mRNA levels were measured by in situ hybridization. Clozapine but not haloperidol significantly decreased 5-HT6 expression in all subfields of the hippocampus. No drug effects were observed in cortical or forebrain structures. These results suggest that downregulation of this receptor in the hippocampus might be a characteristic of atypical antipsychotic drugs, although this hypothesis will require testing with other atypical antipsychotics.