Effects of subchronic clozapine and haloperidol on striatal glutamatergic synapses

Chronic Oral Olanzapine Treatment but not Haloperidol Decreases [3H] MK-801 Binding in the Rat Brain Independent of Dietary Conditions

Haloperidol and olanzapine are first and second-generation antipsychotic (neuroleptic) medications approved to treat schizophrenia. Glutamate signaling is known to play an important role in the manifestation of schizophrenia symptoms, as phencyclidine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, replicates and exasperates these symptoms. While initial reports show that neuroleptic treatments can impact aspects of NMDAR expression, there is little attention on the interaction between neuroleptics and dietary conditions. Thus, we examined the impact of chronic haloperidol and olanzapine treatment under both normal and high-fat dietary conditions on NMDAR expression. Adult male rats were treated for 28-days with either oral vehicle, haloperidol (1.5mg/kg), or olanzapine (10mg/kg), and fed either a standard control diet or a high-fat diet. In-vitro receptor autoradiography binding was performed using [³H] MK-801 as a measure of NMDAR expression. Results showed that olanzapine, irrespective of the diet, significantly decreased [³H] MK-801 binding within the cingulate cortex, substantia nigra, insular cortex, piriform cortex, ectorhinal cortex and perirhinal cortex, the forelimb region of the somatosensory cortex, and all quadrants of the caudate-putamen. In contrast, haloperidol treatment did not impact [³H] MK-801 binding, and we also report no effect of diet on [³H] MK-801 binding. These data suggest that the effects seen in olanzapine treatment are not mediated by diet, nor does a 28-day chronic high-fat diet alter [³H] MK-801 binding. Furthermore, these data also importantly support that combined consumption of a high-fat diet and pharmacological treatments are not immediately detrimental to NMDARs and contribute to the expansive literature of precision medicine.

Lmtk3-KO Mice Display a Range of Behavioral Abnormalities and Have an Impairment in GluA1 Trafficking

Accumulating evidence suggests that glutamatergic signaling and synaptic plasticity underlie one of a number of ways psychiatric disorders appear. The present study reveals a possible mechanism by which this occurs, through highlighting the importance of LMTK3, in the brain. Behavioral analysis of Lmtk3-KO mice revealed a number of abnormalities that have been linked to psychiatric disease such as hyper-sociability, PPI deficits and cognitive dysfunction. Treatment with clozapine suppressed these behavioral changes in Lmtk3-KO mice. As synaptic dysfunction is implicated in human psychiatric disease, we analyzed the LTP of Lmtk3-KO mice and found that induction is severely impaired. Further investigation revealed abnormalities in GluA1 trafficking after AMPA stimulation in Lmtk3-KO neurons, along with a reduction in GluA1 expression in the post-synaptic density. Therefore, we hypothesize that LMTK3 is an important factor involved in the trafficking of GluA1 during LTP, and that disruption of this pathway contributes to the appearance of behavior associated with human psychiatric disease in mice.

Treatment-resistant schizophrenia: current insights on the pharmacogenomics of antipsychotics

Up to 30% of people with schizophrenia do not respond to two (or more) trials of dopaminergic antipsychotics. They are said to have treatment-resistant schizophrenia (TRS). Clozapine is still the only effective treatment for TRS, although it is underused in clinical practice. Initial use is delayed, it can be hard for patients to tolerate, and clinicians can be uncertain as to when to use it. What if, at the start of treatment, we could identify those patients likely to respond to clozapine - and those likely to suffer adverse effects? It is likely that clinicians would feel less inhibited about using it, allowing clozapine to be used earlier and more appropriately. Genetic testing holds out the tantalizing possibility of being able to do just this, and hence the vital importance of pharmacogenomic studies. These can potentially identify genetic markers for both tolerance of and vulnerability to clozapine. We aim to summarize progress so far, possible clinical applications, limitations to the evidence, and problems in applying these findings to the management of TRS. Pharmacogenomic studies of clozapine response and tolerability have produced conflicting results. These are due, at least in part, to significant differences in the patient groups studied. The use of clinical pharmacogenomic testing - to personalize clozapine treatment and identify patients at high risk of treatment failure or of adverse events - has moved closer over the last 20 years. However, to develop such testing that could be used clinically will require larger, multicenter, prospective studies.

Genetic association analysis of N-methyl-D-aspartate receptor subunit gene GRIN2B and clinical response to clozapine

OBJECTIVE

Approximately 30% of patients with schizophrenia fail to respond to antipsychotic therapy and are classified as having treatment-resistant schizophrenia. Clozapine is the most efficacious drug for treatment-resistant schizophrenia and may deliver superior therapeutic effects partly by modulating glutamate neurotransmission. Response to clozapine is highly variable and may depend on genetic factors as indicated by twin studies. We investigated eight polymorphisms in the N-methyl-D-aspartate glutamate receptor subunit gene GRIN2B with response to clozapine.

METHODS

GRIN2B variants were genotyped using standard TaqMan procedures in 175 European patients with schizophrenia deemed resistant or intolerant to treatment. Response was assessed using change in Brief Psychiatric Rating Scale scores following six months of clozapine therapy. Categorical and continuous response was assessed using chi-squared test and analysis of covariance, respectively.

RESULTS

No associations were observed between the variants and response to clozapine. A-allele carriers of rs1072388 responded marginally better to clozapine therapy than GG-homozygotes; however, the difference was not statistically significant (p = 0.067, uncorrected).

CONCLUSIONS

Our findings do not support a role for these GRIN2B variants in altering response to clozapine in our sample. Investigation of additional glutamate variants in clozapine response is warranted.

Chronic clozapine reduces rat brain arachidonic acid metabolism by reducing plasma arachidonic acid availability

Chronic administration of mood stabilizers to rats down-regulates the brain arachidonic acid (AA) cascade. This down-regulation may explain their efficacy against bipolar disorder (BD), in which brain AA cascade markers are elevated. The atypical antipsychotics, olanzapine (OLZ) and clozapine (CLZ), also act against BD. When given to rats, both reduce brain cyclooxygenase activity and prostaglandin E(2) concentration; OLZ also reduces rat plasma unesterified and esterified AA concentrations, and AA incorporation and turnover in brain phospholipid. To test whether CLZ produces similar changes, we used our in vivo fatty acid method in rats given 10 mg/kg/day i.p. CLZ, or vehicle, for 30 days; or 1 day after CLZ washout. [1-(14) C]AA was infused intravenously for 5 min, arterial plasma was collected and high-energy microwaved brain was analyzed. CLZ increased incorporation coefficients ki * and decreased [corrected] rates J(in,i) of plasma unesterified AA into brain phospholipids. [corrected]. These effects disappeared after washout. Thus, CLZ and OLZ similarly down-regulated kinetics and cyclooxygenase expression of the brain AA cascade, likely by reducing plasma unesterified AA availability. Atypical antipsychotics and mood stabilizers may be therapeutic in BD by down-regulating, indirectly or directly respectively, the elevated brain AA cascade of that disease.

Effect of chronic antipsychotic treatment on brain structure: a serial magnetic resonance imaging study with ex vivo and postmortem confirmation

BACKGROUND

There is increasing evidence that antipsychotic (APD) may affect brain structure directly. To examine this, we developed a rodent model that uses clinically relevant doses and serial magnetic resonance imaging (MRI), followed by postmortem histopathological analysis to study the effects of APD on brain structures.

METHODS

Antipsychotic , haloperidol, and olanzapine were continuously administered to rats via osmotic minipumps to maintain clinic-like steady state levels for 8 weeks. Longitudinal in vivo MRI scanning (T₂-weighted) was carried out at baseline, 4 weeks, and 8 weeks, after which animals were perfused and their brains preserved for ex vivo MRI scanning. Region of interest analyses were performed on magnetic resonance images (both in vivo as well as ex vivo) along with postmortem stereology using the Cavalieri estimator probe.

RESULTS

Chronic (8 weeks) exposure to both haloperidol and olanzapine resulted in significant decreases in whole-brain volume (6% to 8%) compared with vehicle-treated control subjects, driven mainly by a decrease in frontal cerebral cortex volume (8% to 12%). Hippocampal, corpus striatum, lateral ventricles, and corpus callosum volumes were not significantly different from control subjects, suggesting a differential effect of APD on the cortex. These results were corroborated by ex vivo MRI scans and decreased cortical volume was confirmed postmortem by stereology.

CONCLUSIONS

This is the first systematic whole-brain MRI study of the effects of APD, which highlights significant effects on the cortex. Although caution needs to be exerted when extrapolating results from animals to patients, the approach provides a tractable method for linking in vivo MRI findings to their histopathological origins.

Alterations in dopamine and glutamate neurotransmission in tetrahydrobiopterin deficient spr-/- mice: relevance to schizophrenia

Tetrahydrobiopterin (BH4) is a pivotal cofactor for enzymes responsible for the synthesis and release of monoamine neurotransmitters including dopamine (DA) and serotonin (5-HT) as well as the release of glutamate (Glu). Deficiencies in BH4 levels and reduced activities of BH(4)-associated enzymes have been recently reported in patients with schizophrenia. Accordingly, it is possible that abnormalities in the biochemical cascades regulated by BH(4) may alter DA, 5-HT and Glu neurotransmission, and consequently contribute to the pathophysiology of different neuropsychiatric diseases including schizophrenia. The development of a novel strain of mutant mice that is deficient in BH(4) by knocking out the expression of a functional sepiapterin reductase gene (spr -/-) has added new insights into the potential role of BH(4) in the pathophysiology and improved treatment of schizophrenia.

A novel mechanism and treatment target for presynaptic abnormalities in specific striatal regions in schizophrenia

Abnormalities of amount and function of presynaptic terminals may have an important role in the mechanism of illness in schizophrenia. The SNARE proteins (SNAP-25, syntaxin, and VAMP) are enriched in presynaptic terminals, where they interact to form a functional complex to facilitate vesicle fusion. SNARE protein amounts are altered in the cortical regions in schizophrenia, but studies of protein-protein interactions are limited. We extended these investigations to the striatal regions (such as the nucleus accumbens, ventromedial caudate (VMC), and dorsal caudate) relevant to disease symptoms. In addition to measuring SNARE protein levels, we studied SNARE protein-protein interactions using a novel ELISA method. The possible effect of antipsychotic treatment was investigated in parallel in the striatum of rodents that were administered haloperidol and clozapine. In schizophrenia samples, compared with controls, SNAP-25 was 32% lower (P=0.015) and syntaxin was 26% lower (P=0.006) in the VMC. In contrast, in the same region, SNARE protein-protein interactions were higher in schizophrenia (P=0.008). Confocal microscopy of schizophrenia and control VMC showed qualitatively similar SNARE protein immunostaining. Haloperidol treatment of rats increased levels of SNAP-25 (mean 24%, P=0.003), syntaxin (mean 18%, P=0.010), and VAMP (mean 16%, P=0.001), whereas clozapine increased only the VAMP level (mean 13%, P=0.004). Neither drug altered SNARE protein-protein interactions. These results indicate abnormalities of amount and interactions of proteins directly related to presynaptic function in the VMC in schizophrenia. SNARE proteins and their interactions may be a novel target for the development of therapeutics.

Ethanol self-administration modulation of NMDA receptor subunit and related synaptic protein mRNA expression in prefrontal cortical fields in cynomolgus monkeys

BACKGROUND

Functional impairment of the orbital and medial prefrontal cortex underlies deficits in executive control that characterize addictive disorders, including alcohol addiction. Previous studies indicate that alcohol alters glutamate neurotransmission and one substrate of these effects may be through the reconfiguration of the subunits constituting ionotropic glutamate receptor (iGluR) complexes. Glutamatergic transmission is integral to cortico-cortical and cortico-subcortical communication, and alcohol-induced changes in the abundance of the receptor subunits and/or their splice variants may result in critical functional impairments of prefrontal cortex in the alcohol-addicted state.

METHODS AND RESULTS

The effects of chronic ethanol self-administration on glutamate receptor ionotropic NMDA (GRIN), as well as GRIN1 splice variant mRNA expression was studied in the orbitofrontal cortex (OFC; Area 13), dorsolateral prefrontal cortex (DLPFC; Area 46) and anterior cingulate cortex (ACC; Area 24) of male cynomolgus monkeys. Chronic ethanol self-administration resulted in significant changes in the expression of NMDA subunit mRNA expression in the DLPFC and OFC, but not the ACC. In DLPFC, the overall expression of NMDA subunits was significantly decreased in ethanol treated monkeys. Slight but significant changes were observed for synaptic associated protein 102 kD (SAP102) and neuronal nitric oxide synthase (nNOS) mRNAs. In OFC, the NMDAR1 variant GRIN1-1 was reduced while GRIN1-2 was increased. Furthermore, no significant changes in GFAP protein levels were observed in either the DLPFC or OFC.

CONCLUSION

Results from these studies provide the first demonstration of posttranscriptional regulation of iGluR subunits in the primate brain following long-term ethanol self-administration. Furthermore, changes in these transcripts do not appear to reflect changes in glial activation or loss. Further studies examining the expression and cellular localization of subunit proteins and receptor pharmacology would shed more light on the findings reported here.

Effects of risperidone on glutamate receptor subtypes in developing rat brain

Levels of ionotropic glutamate (Glu) N-methyl-d-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA), and kainic acid (KA) receptors in forebrain regions of juvenile rats (age 42 days) were quantified after 3 weeks of treatment with three different doses of risperidone (0.3, 1.0 and 3.0 mg/kg) and compared findings to those in adult rats treated with risperidone (3.0 mg/kg/day) previously. Risperidone (at 0.3 mg/kg/day) did not alter levels of three ionotropic Glu receptors in all brain regions examined. Risperidone (at 1.0 and 3.0 mg/kg/day) significantly decreased NMDA binding in caudate-putamen of juvenile and adult animals. In contrast, the same two doses of risperidone decreased NMDA receptors in nucleus accumbens of juveniles and not adults. Risperidone (at 1.0 and 3.0 mg/kg/day) increased AMPA receptors in medial prefrontal cortex and caudate-putamen of juvenile animals, whereas risperidone (at 3.0 mg/kg) increased AMPA receptors in caudate-putamen and hippocampus of adults. Kainate receptors were not altered by any dose of risperidone in any brain region examined in developing and mature animals. The findings indicate that risperidone exerts dose-dependent effects on Glu receptor subtypes in developing animals, and that Glu receptor responses to repeated administration of risperidone are different in juvenile animals than adults.

Substances that can change alternative splice-site selection

Alternative pre-mRNA splicing is an important element in eukaryotic gene expression, as most of the protein-coding genes use this process to generate multiple protein isoforms from a single gene. An increasing number of human diseases are now recognized to be caused by the selection of 'wrong' alternative exons. Research during the last few years identified a number of low-molecular-mass chemical substances that can change alternative exon usage. Most of these substances act by either blocking histone deacetylases or by interfering with the phosphorylation of splicing factors. How the remaining large number of these substances affect splicing is not yet fully understood. The emergence of these low-molecular-mass substances provides not only probes for studying alternative pre-mRNA splicing, but also opens the door to the possible harnessing of these compounds as drugs to control diseases caused by the selection of 'wrong' splice sites.

Stage II follow-up on a linkage scan for bipolar disorder in the Ashkenazim provides suggestive evidence for chromosome 12p and the GRIN2B gene

PURPOSE

We had previously performed a genome-wide linkage scan for bipolar affective disorder in an Ashkenazi Jewish sample, a population likely to have reduced genetic heterogeneity. This study is a second stage follow-up focusing on regions that showed positive linkage scores in our previous scan but were not fine-mapped at that time.

METHODS

We genotyped an additional 145 highly polymorphic microsatellites and conducted linkage analyses using standard laboratory and analytical methods.

RESULTS

We saw an improvement of the evidence for linkage in most regions, with the most notable change on chromosome 12p13.1-p12.3, where the evidence of linkage is now suggestive. This region harbors the gene encoding the ionotropic glutamate receptor subunit 2B (GRIN2B), a gene that previously yielded evidence for association in a candidate gene study on 323 Ashkenazi Jewish bipolar case-parent trios. We find that the evidence for linkage is significantly correlated with the presence of the putative high-risk allele identified in our candidate gene study.

CONCLUSIONS

Following up weaker signals can significantly improve linkage signals even after relatively small increases in information content. Our results on chromosome 12p support GRIN2B as a candidate gene for bipolar disorder that needs further investigation.

Long-term treatment of rats with haloperidol: lack of an effect on brain N-acetyl aspartate levels

Proton magnetic resonance spectroscopy (1H-MRS) studies of schizophrenia suggest an effect of the disease or of antipsychotic medications on brain N-acetyl aspartate (NAA), a marker of neuronal viability. We studied in rat the effect of haloperidol on NAA, glutamate, and glutamine in several brain regions where metabolite reductions have been reported in chronically medicated patients with schizophrenia. Two groups of 16 rats each were treated with haloperidol depo (38 mg/kg/month) and vehicle for 6 months and were killed. Concentrations of metabolites were determined by high-resolution magic angle proton magnetic resonance spectroscopy (HR-MAS 1H-MRS) at 11.7 T in ex-vivo punch biopsies from the following brain regions: medial frontal and cingulate cortex, striatum, nucleus accumbens, dorsal and ventral hippocampus, amygdala, and temporal cortex. Factorial ANOVA of NAA concentrations revealed no significant effect of drug group (F(1,212) = 1.5; p = 0.22) or a group by brain region interaction (F(7,212) = 1.0; p = 0.43). There was a significant main effect of region (F(7,212) = 17.8; p < 0.001) with lower NAA in the striatum. A prolonged exposure to the dopamine D2 receptor blockade effects of haloperidol does not result in changes in NAA, glutamate, glutamine, and other metabolites in the proton spectrum. These results are consistent with the only other two studies of the effect of antipsychotic drugs on NAA in the rat brain. The documented lower NAA in chronically treated schizophrenia patients is most likely not a simple effect of antipsychotic medications.

Chronic neuroleptic treatment reduces endogenous kynurenic acid levels in rat brain

The brain and cerebrospinal fluid levels of kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation and antagonist of the glycine(B) receptor and the alpha7 nicotinic acetylcholine receptor, are elevated in persons with schizophrenia. To evaluate whether this increase is related to antipsychotic medication, we examined the effects of haloperidol (HAL), clozapine (CLOZ) or raclopride (RAC) on brain KYNA levels in rats. Animals received either acute drug injections or ingested the drugs chronically with the drinking water. Acute application or one-week drug exposure had no effect on brain KYNA levels. After one month, HAL, CLOZ and RAC all caused significant reductions in KYNA levels in striatum, hippocampus and frontal cortex. Quantitatively similar reductions in the brain tissue content of KYNA were observed after one year of HAL administration. All these effects were accompanied by equivalent decreases in the extracellular concentration of KYNA, measured by striatal microdialysis. Separate animals received an intrastriatal infusion of (3)H-kynurenine to probe the entire kynurenine pathway acutely in rats treated with HAL for one year. These animals showed reduced (3)H-KYNA production, but no changes in the formation of other kynurenine pathway metabolites. By enhancing glutamatergic and cholinergic neurotransmission, reduced brain KYNA levels may play a role in the clinical effects of prolonged antipsychotic medication.

SR Proteins as Potential Targets for Therapy

Serine- and arginine-rich (SR) proteins constitute a highly conserved family of pre-mRNA splicing factors that play key roles in the regulation of splice site selection, and thereby in the control of alternative splicing processes. In addition to conserved sequences at the splice junctions, splice site selection also depends upon different sets of auxiliary cis regulatory elements known as exonic and intronic splicing enhancers (ESEs and ISEs) or exonic and intronic silencers (ESSs and ISSs). Specific binding of SR proteins to their cognate splicing enhancers as well as binding of splicing repressor to silencer sequences serve to enhance or inhibit recognition of weak splice sites by the splicing machinery. Given that the vast majority of human genes contain introns and that most pre-mRNAs containing multiple exons undergo alternative splicing, mutations disrupting or creating such auxiliary elements can result in aberrant splicing events at the origin of various human diseases. In the past few years, numerous studies have reported several approaches allowing correction of such aberrant splicing events by targeting either the mutated sequences or the splicing regulators whose binding is affected by the mutation. The aim of the present review is to highlight the different means by which it is possible to modulate the activity of SR splicing factors and to bring out those holding the greatest promises for the development of therapeutic treatments.

Impact of schizophrenia and chronic antipsychotic treatment on [123I]CNS-1261 binding to N-methyl-D-aspartate receptors in vivo

BACKGROUND

Antipsychotic drugs modulate N-methyl-D-aspartate (NMDA) receptor function in animals. The novel single photon emission tomography (SPET) radiotracer [123I]CNS-1261 binds to the PCP/MK-801 intrachannel site of the NMDA receptor, allowing the noninvasive estimation of NMDA receptor activity in living humans. We used [123I]CNS-1261 to determine whether binding to the NMDA receptor intrachannel PCP/MK-801 site is affected by schizophrenia or by treatment with typical antipsychotics and clozapine in vivo.

METHODS

Three groups of schizophrenia patients were recruited-drug free (n = 5), typical antipsychotic treated (n = 7), and clozapine treated (n = 9)-as well as a control group of healthy normal volunteers (n = 13). All underwent [123I]CNS-1261 SPET scanning. Total volume of distribution of [123I]CNS-1261 was determined within predefined user-independent regions of interest after alignment of all images to a common template.

RESULTS

There was no apparent difference in total volume of distribution of [123I]CNS-1261 in drug-free patients relative to healthy control subjects. A nonsignificant reduction in total volume of distribution was observed in typical antipsychotic treated patients. A significant decline in total volume of distribution of [123I]CNS-1261 was observed in all examined brain regions in the clozapine-treated patient group relative to healthy control subjects (p < .005).

CONCLUSIONS

Clozapine treatment resulted in a global reduction in [123I]CNS-1261 binding to the NMDA receptor intrachannel PCP/MK-801 site in vivo. This supports an effect of the drug on glutamatergic systems that could be exploited for future antipsychotic drug discovery.

Increased expression of glutaminase and glutamine synthetase mRNA in the thalamus in schizophrenia

Numerous molecules enable the handling of glutamate that is destined for neurotransmitter release, including transporters, receptors and glutamatergic enzymes. Previous work in our lab has shown altered levels of transcript expression of excitatory amino acid transporters and a vesicular glutamate transporter in the thalamus in schizophrenia. These changes suggest that molecules that facilitate the release and reuptake of glutamate may be abnormal in schizophrenia. In this study we determined the levels of expression of phosphate activated glutaminase (PAG), which converts glutamine to glutamate, and glutamine synthetase (GS), which converts glutamate to glutamine, with the hypothesis that thalamic PAG and GS transcript expression is altered in schizophrenia. We investigated expression of PAG and GS mRNA using in situ hybridization in six different thalamic nuclei (anterior, dorsomedial, centromedial, ventral anterior, ventral and reticular) from 13 persons with schizophrenia and 8 comparison subjects and found that transcripts for PAG and GS were significantly increased in schizophrenia. Increased PAG and GS transcripts suggest enhanced glutamatergic neurotransmission in the thalamus and its efferent targets in schizophrenia.

Acute reserpine and subchronic haloperidol treatments change synaptosomal brain glutamate uptake and elicit orofacial dyskinesia in rats

Reserpine- and haloperidol-induced orofacial dyskinesia are putative animal models of tardive dyskinesia (TD) whose pathophysiology has been related to free radical generation and oxidative stress. In the present study, the authors induced orofacial dyskinesia by acute reserpine and subchronic haloperidol administration to rats. Reserpine injection (one dose of 1 mg/kg s.c.) every other day for 3 days caused a significant increase in vacuous chewing, tongue protrusion and duration of facial twitching, compared to the control. Haloperidol administration (one dose of 12 mg/kg once a week s.c.) for 4 weeks caused an increase in vacuous chewing, tongue protrusion and duration of facial twitching observed in four weekly evaluations. After the treatments and behavioral observation, glutamate uptake by segments of the brain was analyzed. A decreased glutamate uptake was observed in the subcortical parts of animals treated with reserpine and haloperidol, compared to the control. Importantly, a decrease in glutamate uptake correlates negatively with an increase in the incidence of orofacial diskinesia. These results indicate that early changes in glutamate transport may be related to the development of vacuous chewing movements in rats.

Nerve terminal glutamate immunoreactivity in the rat nucleus accumbens and ventral tegmental area after a short withdrawal from cocaine

Cocaine administration has been shown to alter glutamate transmission in numerous studies. Using quantitative electron microscopic immunogold labeling, our laboratory has previously reported that nerve terminal glutamate immunoreactivity is transiently altered following cocaine administration. The present study was undertaken to examine presynaptic nerve terminal glutamate immunoreactivity at shorter time points after withdrawal from cocaine. Animals received saline or cocaine for 7 days followed 3 days later by a cocaine or saline challenge. Most (>75%) cocaine-challenged animals had a heightened locomotor response to cocaine compared to the first day of cocaine and were considered behaviorally sensitized. One day after the challenge, glutamate immunogold-labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens and ventral tegmental area. A single dose of cocaine did not alter the density of presynaptic nerve terminal glutamate immunoreactivity in the nucleus accumbens (NAc) or ventral tegmental area (VTA). The density of nerve terminal glutamate immunoreactivity in the shell, but not the core, was significantly increased in the animals receiving repeated cocaine. In the VTA the density of nerve terminal glutamate immunoreactivity did not change in the cocaine-sensitized group, but was significantly increased in the nonsensitized group. The finding that repeated cocaine treatment increased glutamate nerve terminal immunolabeling within the nucleus accumbens shell, but not the core, supports the hypothesis that glutamate synapses in the core and shell are differentially sensitive to repeated cocaine administration. Overall, our study does not support a role for changes in presynaptic glutamate in the development of behavioral sensitization.

Treatment of rats with antipsychotic drugs: lack of an effect on brain N-acetyl aspartate levels

BACKGROUND

Proton magnetic resonance spectroscopy (1H-MRS) studies of schizophrenia suggest an effect of the disease or of antipsychotic medications on brain N-acetyl aspartate (NAA), a marker of neuronal viability. We studied in the rat the effect of antipsychotic drugs on NAA in several brain regions where NAA reductions have been reported in chronically medicated patients with schizophrenia.

METHODS

Three groups of nine rats each were treated with haloperidol (6 mg/kg/day), clozapine (70 mg/kg/day) and vehicle for 6 weeks and were sacrificed. Concentrations of NAA were determined by high-performance liquid chromatography (HPLC) from the following brain regions: cortex, striatum, thalamus, hippocampus and cerebellum.

RESULTS

Mixed-factorial ANOVA of NAA concentrations revealed no significant effect of drug group [F(2, 24) = 0.034; p = 0.966] or a group by brain region interaction [F(8, 44) = 0.841; p = 0.572]. There was a significant main effect of region [F(4, 21) = 6.104; p = 0.002] with higher NAA in the cortex.

CONCLUSIONS

These results are consistent with the only other study of the effect of typical and atypical antipsychotic drugs on NAA in the rat brain. The well-documented lower NAA in chronically treated schizophrenia patients is probably not a simple effect of antipsychotic medications.

The impact of splice isoforms on voltage-gated calcium channel alpha1 subunits

Semi-conserved exon boundaries in members of the CACNA1 gene family result in recurring pre-mRNA splicing patterns. The resulting variations in the encoded pore-forming subunit of the voltage-gated calcium channel affect functionally significant regions, such as the vicinity of the voltage-sensing S4 segments or the intracellular loops that are important for protein interaction. In addition to generating functional diversity, RNA splicing regulates the quantitative expression of other splice isoforms of the same gene by producing transcripts with premature stop codons which encode two-domain or three-domain channels. An overview of some of the known splice isoforms of the alpha(1) calcium channel subunits and their significance is given.

The relevance of alternative RNA splicing to pharmacogenomics

The importance of alternative RNA splicing in the generation of genetic diversity is now widely accepted. This article highlights how alternative RNA splicing can have an impact on drug efficacy and safety, and demonstrates its potential pharmacogenomic value. The analysis of the repertoire of alternative RNA splicing events could potentially identify markers of pharmacogenomic relevance with high sensitivity and specificity and also provides a route through which genes can be selected for single nucleotide polymorphism (SNP) genotyping. Recent methodological advances, including microarray and splice-dedicated expression profiling, have made it possible to perform high-throughput alternative splicing analyses.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Molecular aspects of glutamate dysregulation: implications for schizophrenia and its treatment

The glutamate system is involved in many aspects of neuronal synaptic strength and function during development and throughout life. Synapse formation in early brain development, synapse maintenance, and synaptic plasticity are all influenced by the glutamate system. The number of neurons and the number of their connections are determined by the activity of the glutamate system and its receptors. Malfunctions of the glutamate system affect neuroplasticity and can cause neuronal toxicity. In schizophrenia, many glutamate-regulated processes seem to be perturbed. Abnormal neuronal development, abnormal synaptic plasticity, and neurodegeneration have been proposed to be causal or contributing factors in schizophrenia. Interestingly, it seems that the glutamate system is dysregulated and that N-methyl-D-aspartate receptors operate at reduced activity. Here we discuss how the molecular aspects of glutamate malfunction can explain some of the neuropathology observed in schizophrenia, and how the available treatment intervenes through the glutamate system.

The effects of acute or repeated cocaine administration on nerve terminal glutamate within the rat mesolimbic system

Cocaine administration alters glutamate function within several brain regions. Using quantitative electron microscopic immunocytochemistry, the present study investigated the effect of repeated intermittent cocaine (resulting in behavioral sensitization) or acute cocaine administration on the density of glutamate immunogold labeling within nerve terminals. Rats were treated daily with saline or cocaine for 7 days. Following a 14-day withdrawal animals were challenged with saline or cocaine. On the challenge day, most (75%) animals that received cocaine repeatedly showed a heightened locomotor response to cocaine compared to the first day of cocaine administration, and were considered behaviorally sensitized.Three days after the challenge, glutamate immunogold labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens, ventral tegmental area and medial prefrontal cortex. There was a decrease in such labeling in the nucleus accumbens in the group receiving acute cocaine. Locomotor activity was positively correlated with glutamate immunolabeling within nerve terminals in the nucleus accumbens core only for the cocaine-sensitized group. Nerve terminal glutamate immunolabeling in the nucleus accumbens core, but not the shell, was increased in the non-sensitized compared to the cocaine-sensitized group. In the ventral tegmental area, glutamate immunolabeling was significantly higher in the cocaine-sensitized compared to the acute cocaine group. In the prefrontal cortex, there were no significant differences in glutamate immunogold labeling between treatment groups. This study indicates that acute cocaine administration significantly decreases nerve terminal glutamate immunoreactivity in the nucleus accumbens. We suggest that sensitization results in differential changes in the nucleus accumbens core versus the shell, and may alter presynaptic mechanisms regulating glutamate release or re-uptake in the core.

Association analysis for NMDA receptor subunit 2B (GRIN2B) genetic variants and psychopathology and clozapine response in schizophrenia

It is known that a syndrome resembling schizophrenia is produced by the N-methyl-d-aspartate receptor antagonists. It has also been demonstrated that the level of an ionotropic N-methyl-d-aspartate 2B subunit (GRIN2B) of the glutamate receptor tends to increase after subchronic administration of clozapine, suggesting that GRIN2B may play an active role in the pathogenesis of schizophrenia and the function of clozapine medication. We studied 100 schizophrenic patients, investigating the associations for the GRIN2B genetic variants, and psychiatric symptoms and clozapine response. No significant differences were demonstrated comparing these three groups in terms of the baseline Brief Psychiatric Rating Scale (BPRS) score (P = 0.441). The percentage of patients scoring within 20% of baseline BPRS after clozapine treatment was similar for the three genotype groups (P = 0.132). A marginally higher mean clozapine dosage was revealed, however, for patients bearing the 2664C/C genotype (P = 0.013). Although replication of this research is required to confirm the results, an association for the GRIN2B C2664T polymorphism and clozapine treatment is suggested from our findings, which may assist in the prediction of optimal dosage for schizophrenic patients.

Blockade of NMDA receptors by MK-801 reverses the changes in striatal glutamate immunolabeling in 6-OHDA-lesioned rats

A lesion of the dopamine (DA)-containing nigrostriatal pathway with 6-hydroxydopamine (6-OHDA) results in an increase in the density of nerve terminal glutamate immunolabeling and in the mean percentage of asymmetrical synapses containing a discontinuous postsynaptic density [Meshul et al. (1999) Neuroscience 88:1-16]. Similar alterations in striatal glutamate synapses have been reported following blockade of striatal DA D-2 receptors with subchronic haloperidol treatment [Meshul et al. (1994) Brain Res 648:181-195]. The haloperidol-induced change in glutamate synapses was blocked by coadministration of the N-methyl-D-aspartate (NMDA) noncompetitive receptor antagonist MK-801. In order to determine if blockade of NMDA receptors could alter the density of nerve terminal glutamate immunolabeling following a 6-OHDA lesion of the nigrostriatal pathway, MK-801 was administered to lesioned animals for 14 days. In addition, the number of apomorphine-induced contralateral rotations was determined prior to and following the administration of MK-801. MK-801 administration reversed the increase in the density of nerve terminal glutamate immunolabeling due to a 6-OHDA lesion. There was a small but significant decrease in the number of apomorphine-induced contralateral rotations following administration of MK-801 compared to the number of rotations prior to treatment with the NMDA antagonist. These results demonstrate that blockade of postsynaptic NMDA receptors affects the density of presynaptic glutamate immunolabeling and that this change in nerve terminal glutamate density is associated with a decreased behavioral response to direct DA receptor stimulation. Whether the effect of MK-801 is directly on the striatum or acts through other excitatory pathways of the basal ganglia remains unclear.

Neurotoxicity of NMDA antagonists: a glutamatergic theory of schizophrenia based on selective impairment of local inhibitory feedback circuits

Modulation of recurrent inhibition is critical not only for the normal function of highly excitable regions of the brain, especially the limbic system, but may also be a primary determining factor for the viability of neurons in these regions. Standard extracellular and intracellular recordings from in vitro brain slices of rat hippocampi were employed to show that recurrent inhibition onto CA1 neurons can be modulated by N-methyl-D-aspartate (NMDA) antagonists. Besides reducing the amplitude of inhibitory postsynaptic potentials (IPSPs) at resting membrane potential conditions, different NMDA antagonists, including the endogenous substance N-acetyl-L-aspartyl-L-glutamic acid (NAAG), are able to block long-term potentiation (LIP) of recurrent inhibition completely at concentrations that are not sufficient to block LTP of the excitatory drive onto pyramidal neurons. This LTP of recurrent inhibition may play a significant role in stimulus discrimination and learning, as simulated in a biophysical computer model of a basic neuronal circuit. Both the amplitude of the IPSP and LTP of the recurrent inhibitory circuit also undergo developmental changes showing their highest expression and vulnerability to chronic NMDA antagonist injections in juvenile rats. Finally, blocking NMDA receptor-dependent transmission in the recurrent inhibition loop may lead to an overall increased excitability of the neuronal network. This may resemble the positive schizophrenic symptoms observed in man, presumably caused by elevated levels of the endogenous NMDA antagonist NAAG.

Expression of complexin I and II mRNAs and their regulation by antipsychotic drugs in the rat forebrain

Complexin (cx) I and II are homologous synaptic protein genes which are differentially expressed in mouse and human brain and differentially affected in schizophrenia. We characterized the distribution of cx I and II mRNAs in rat forebrain and examined whether their abundance, or the transcript of the synaptic marker synaptophysin, is affected by 14 days' administration of antipsychotic drugs (haloperidol, chlorpromazine, risperidone, olanzapine, or clozapine). Cx I mRNA predominated in medial habenula, medial septum-diagonal band complex, and thalamus, whereas cx II mRNA was more abundant in most other regions, including isocortex and hippocampus. Within the hippocampus, cx I mRNA was primarily expressed by interneurons and cx II mRNA by granule cells and pyramidal neurons. Localized cx II mRNA signal was seen in the dentate gyrus molecular layer, suggestive of its transport into granule cell dendrites. Antipsychotic treatment produced selective, modest effects on cx mRNA expression. Cx I mRNA was elevated by olanzapine in dorsolateral striatum and frontoparietal cortex, while the abundance of cx II mRNA relative to cx I mRNA was decreased in both areas by olanzapine and haloperidol. Chlorpromazine increased cx II mRNA in frontoparietal cortex and synaptophysin mRNA in dorsolateral striatum. In summary, the data have implications both for understanding the effects of antipsychotic medication on synaptic organization, and for synaptic protein expression studies in patients treated with the drugs.

Haloperidol reverses the changes in striatal glutamatergic immunolabeling following a 6-OHDA lesion

We reported previously that 3 months following a unilateral lesion of the nigrostriatal pathway with 6-hydroxydopamine (6-OHDA), there was a decrease in the extracellular level of striatal glutamate as determined by in vivo microdialysis. This resulted in an accumulation or increase in the density of nerve terminal glutamate immunolabeling (Meshul et al., 1999). We also reported on blockade of dopamine D-2 receptors with haloperidol resulting in ultrastructural changes within the striatum consistent with increased functioning of the glutamatergic corticostriatal pathway (Meshul and Tan 1994). We hypothesized that administration of haloperidol to 6-OHDA-lesioned rats may be capable of activating the corticostriatal pathway and thereby counteracting the effects of the unilateral nigrostriatal lesion. Striatal glutamatergic function was evaluated using electron microscopy and quantitative glutamate immunocytochemistry. Starting 1 month after a unilateral lesion of the nigrostriatal pathway with 6-OHDA, haloperidol (0.5 mg/kg/d) was administered for the next 2 months. Within the dorsolateral caudate nucleus, the main area of innervation from the motor cortex, haloperidol blocked the 6-OHDA-induced increase in the density of nerve terminal glutamate immunolabeling. Within all three experimental groups (6-OHDA, haloperidol, 6-OHDA/haloperidol) there was an increase in the mean percentage of striatal asymmetrical synapses containing a perforated postsynaptic density. In addition, haloperidol treatment resulted in a reduction in the number of apomorphine-induced contralateral rotations in unilaterally 6-OHDA lesioned rats. The data suggests that the decrease in striatal glutamatergic function 3 months following a unilateral 6-OHDA lesion can be reversed by daily haloperidol treatment. This finding is discussed in terms of current therapy for Parkinson's disease. Synapse 36:129-142, 2000. Published 2000 Wiley-Liss, Inc.

Differential effects of haloperidol and clozapine on ionotropic glutamate receptors in rats

Despite multiple lines of investigation the effect of neuroleptics on glutamate-mediated neurotransmission remains controversial. To study the effects of typical and atypical neuroleptics on selected parameters of glutamate-mediated neurotransmission, male Sprague-Dawley rats were randomly assigned to a 21-day oral treatment course with vehicle, haloperidol (HDL), or clozapine (CLZ). Coronal slices of rat brain were then incubated with tritiated ligands to measure NMDA, AMPA, and kainate receptor, and glutamate reuptake site density. Regions of interest included the frontal cortex, anterior cingulate cortex, dorsal striatum, ventral striatum, and the nucleus accumbens. CLZ increased the density of AMPA receptors significantly in the frontal and anterior cingulate cortices compared with normal controls. In the dorsal and ventral striatum, and nucleus accumbens as a whole, CLZ-treated rats had a higher AMPA receptor density compared with both the HDL- and vehicle-treated controls. Additionally, within the nucleus accumbens, CLZ-treated rats had a higher density of AMPA receptors compared with the HDL group in the core, and at trend level in the shell. There was a group by region interaction for NMDA receptor density, primarily reflecting the tendency of HDL treated rats to have high receptor densities in the frontal and anterior cingulate cortices. Kainate receptors and glutamate reuptake site densities did not differ significantly across groups. These results suggest a critical role for glutamate in the mediation of atypical antipsychotic drug action in anatomically-specific regions, and further encourage the investigation of glutamate neurotransmitter systems in schizophrenia.

The neuropathological effects of antipsychotic drugs

In addition to their neurochemical effects, antipsychotic (neuroleptic) drugs produce structural brain changes. This property is relevant not only for understanding the drugs' mode of action, but because it complicates morphological studies of schizophrenia. Here the histological neuropathological effects of antipsychotics are reviewed, together with brief mention of those produced by other treatments sometimes used in schizophrenia (electroconvulsive shock, lithium and antidepressants). Most data come from drug-treated rats, though there are also some human post-mortem studies with broadly congruent findings. The main alteration associated with antipsychotic medication concerns the ultrastructure and proportion of synaptic subpopulations in the caudate nucleus. In rats, synapses and dendrites in lamina VI of the prefrontal cortex are also affected. The changes are indicative of a drug-induced synaptic plasticity, although the underlying mechanisms are poorly understood. Similarly, it is unclear whether the neuropathological features relate primarily to the therapeutic action of antipsychotics or, more likely, to their predisposition to cause tardive dyskinesia and other motor side-effects. Clozapine seems to cause lesser and somewhat different alterations than do typical antipsychotics, albeit based on few data. There is no good evidence that antipsychotics cause neuronal loss or gliosis, nor that they promote neurofibrillary tangle formation or other features of Alzheimer's disease.

Enhancement of N-methyl-D-aspartate (NMDA) immunoreactivity in residual dendritic spines in the caudate-putamen nucleus after chronic haloperidol administration

Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype in the caudate-putamen nucleus (CPN) have been implicated in the adverse motor effects produced by chronic administration of the typical antipsychotic drug haloperidol. To determine the functionally relevant sites, we examined the electron microscopic immunocytochemical localization of the R1 receptor subunit (NMDAR1) in the dorsolateral CPN of rats receiving 4 months of biweekly depot intramuscular injections of either haloperidol or vehicle. In all animals, NMDAR1 immunoreactivity was seen mainly in dendritic spines, but was also present in a few somata and dendrites of spiny neurons, axon terminals, and glia. In comparison with controls, the dissector stereological analysis showed a significant reduction in the numerical density of total NMDAR1-labeled and unlabeled dendritic spines in the dorsolateral CPN after haloperidol administration. When labeled spines were identified separately based exclusively on the presence of immunoreactivity within a single plane of section, there was, however, a significant increase in the numerical density of NMDAR1-containing spines in haloperidol vs. control animals. This increase was not seen using a classic dissector, suggesting that the enhancement was mainly attributed to more frequent detection of spines having higher levels of NMDA immunoreactivity. Our results are the first to identify dendritic spines in the dorsolateral CPN as preferential sites for the regulated expression of NMDA receptors following chronic administration of haloperidol.

Region-specific induction of deltaFosB by repeated administration of typical versus atypical antipsychotic drugs

Whereas acute administration of many types of stimuli induces c-Fos and related proteins in brain, recent work has shown that chronic perturbations cause the region-specific accumulation of novel Fos-like proteins of 35-37 kD. These proteins, termed chronic FRAs (Fos-related antigens), have recently been shown to be isoforms of DeltaFosB, which accumulate in brain due to their enhanced stability. In the present study, we sought to extend earlier findings that documented the effects of acute administration of antipsychotic drugs (APDs) on induction of Fos-like proteins by investigating the ability of typical and aytpical APDs, after chronic administration, to induce these DeltaFosB isoforms in several brain regions implicated in the clinical actions of these agents. By Western blotting we found that chronic administration of the typical APD, haloperidol, dramatically induces DeltaFosB in caudate-putamen (CP), a brain region associated with the extrapyramidal side effects of this drug. A smaller induction was seen in the nucleus accumbens (NAc) and prefrontal cortex (PFC), brain regions associated with the antipsychotic effects of the drug. In contrast, chronic administration of the prototype atypical APD clozapine failed to significantly increase levels of DeltaFosB in any of the three brain regions, and even tended to reduce DeltaFosB levels in the NAc. Two putative atypical APDs, risperidone and olanzapine, produced small but still significant increases in the levels of DeltaFosB in CP, but not NAc or PFC. Studies with selective receptor antagonists suggested that induction of DeltaFosB in CP and NAc is most dependent on antagonism of D2-D3 dopamine receptors, with antagonism of D1-like receptors most involved in the PFC. Immunohistochemical analysis confirmed the greater induction of DeltaFosB in CP by typical versus atypical APDs, with no significant induction seen in PFC with either class of APD. Together, these findings demonstrate that repeated administration of APDs results in the induction of long-lasting Fos-like transcription factors that could mediate some of the persistent and region-specific changes in brain function associated with chronic drug exposure. Synapse 33:118-128, 1999.

Antidepressants for the new millennium

Despite a remarkable structural diversity, most conventional antidepressants may be viewed as 'monoamine based', increasing the synaptic availability of serotonin, norepinephrine, and/or dopamine. Both preclinical and recent clinical studies indicate that compounds which reduce transmission at N-methyl-D-aspartate (NMDA) receptors are antidepressant. Moreover, chronic administration of antidepressants to mice alters both the mRNA levels encoding N-methyl-D-aspartate receptor subunits and radioligand binding to these receptors within circumscribed areas of the central nervous system. It is hypothesized that these two different treatment strategies converge to produce an identical functional endpoint: a region-specific dampening of NMDA receptor function. The pathways leading to this convergence provide a rudimentary framework for discovering novel antidepressants.

NMDA receptor blockade attenuates the haloperidol induction of Fos protein in the dorsal but not the ventral striatum

Neuroleptic blockade of dopamine receptors is known to produce an increase in the expression of Fos. This increase may be related to elevations in glutamate transmission which in turn activates N-methyl-D-aspartate (NMDA) receptors. In the present study, we examine the role of these receptors in the haloperidol-induced augmentation of Fos in the caudate-putamen and nucleus accumbens of Wistar rats. Animals were divided into four groups for each experiment and each was injected either with saline; a noncompetitive NMDA antagonist, dizocilpine maleate (MK801, 5 mg/kg); haloperidol (0.5 mg/kg); or MK801 followed by an injection of haloperidol. Fos-immunoreactive cells appear in large numbers in all parts of the striatum 3 h after the administration of haloperidol. Pretreatment with MK801 attenuates the haloperidol-induced increase in Fos in the caudate-putamen. However, antagonism of the NMDA receptor does not significantly reduce the density of Fos-immunoreactive cells in any territory of nucleus accumbens, i.e., shell, core, or rostral pole. These data suggest that haloperidol acts in an NMDA-dependent manner in the caudate-putamen, but independently in parts of nucleus accumbens traditionally considered to be targets of antipsychotic drugs.

D2 dopamine receptor but not AMPA and kainate glutamate receptor genes show altered expression in response to long term treatment with trans- and cis-flupenthixol in the rat brain

Glutamate receptor function has been hypothesized as an important factor in both the aetiology and treatment of schizophrenia. We have used a multiprobe oligonucleotide solution hybridization (MOSH) technique to examine the regulation of gene expression of the GluR1-7, KA1, and KA2 glutamate receptor subunits in the left rat brain following treatment with the optical isomers of flupenthixol at a dose of 0.2 mg kg-1 day-1 over a period of 4, 12, 24 weeks in order to understand how specific glutamate receptor genes are involved in the treatment of schizophrenia. The GluR2/3 and GluR6/7 subunit immunoreactivity in the right brain following 4 and 24 weeks of drug treatment was also examined by Western blotting. Neither trans- nor cis-flupenthixol was found to alter the gene expression of any of the 9 non-NMDA glutamate receptor subunits. On the other hand, we found a nearly two-fold increase in gene expression of the D2 dopamine receptor in specific brain regions. These results suggest that non-NMDA types of glutamate receptor subunits, in contrast to NMDA receptors, are less likely to have a role in the action of antipsychotic drugs.

Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion

The goal of this study was to investigate changes in glutamatergic synapses in the striatum of rats at two different time-points following a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. One month following this lesion of the nigrostriatal pathway, there was an increase (70%) in the mean percentage of asymmetrical synapses within the dorsolateral striatum containing a discontinuous, or perforated, postsynaptic density, possibly suggesting an increase in glutamatergic activity. This was correlated, in the same brain region, with a decrease (44%) in the density of glutamate immunoreactivity within nerve terminals associated with all asymmetrical synapses and also with those terminals associated with a perforated postsynaptic density. These morphological changes were consistent with an increase (>two-fold) in the basal extracellular level of striatal glutamate, as measured by in vivo microdialysis. The density of GABA immunolabeling within symmetrical nerve terminals was increased (25%) at this one month time-period. Dopamine levels within the lesioned striatum were >99% depleted. However, at three months, while an increase in the mean percentage of striatal perforated synapses was maintained, a significant increase (50%) in the density of striatal nerve terminal glutamate immunolabeling within all asymmetrical synapses and those associated with a perforated postsynaptic density was observed. This was correlated with a small, but significant, decrease (32%) in the basal extracellular level of striatal glutamate. The density of GABA immunolabeling within nerve terminals associated with a symmetrical contact remained elevated at this three month time-period, while striatal dopamine levels remained depleted. While the density of nerve terminal GABA immunolabeling remained elevated at both the one and three month time-periods, there appeared to be a differential effect on glutamatergic synapses. The in vivo microdialysis data suggest that glutamate synapses were more active at a basal level at one month and become less active compared to the control group at the three month time-period. These data suggest that there are compensatory changes in glutamatergic synapses within the striatum following a 6-hydroxydopamine lesion that appear to be independent of the level of striatal dopamine or GABA. We propose that changes in the activity of the thalamo-cortico-striatal pathway may help to explain the differential time-course change in striatal glutamatergic synaptic activity.

Neuroleptics with differential affinities at dopamine D2 receptors and sigma receptors affect differently the N-methyl-D-aspartate-induced increase in intracellular calcium concentration: involvement of protein kinase

This study examined the effect of chronic antipsychotic treatment on the NMDA-elicited changes in intracellular free Ca2+ concentration ([Ca2+]i) in the primary culture of rat frontal cortical neurons. Antipsychotics used in the study were chosen for their differential affinities at dopamine D2 receptors and sigma receptors. The potential involvement of protein kinases in this action of antipsychotics were also examined. Chronic treatment of cells with antipsychotics (sulpiride, clozapine, and chlorpromazine) which are known to be potent dopamine D2 receptor ligands, whereas possessing low or no appreciable affinity for sigma receptors, caused a dose-dependent potentiation of the NMDA-induced increase in [Ca2+]i. On the contrary, haloperidol, which is as potent a sigma receptor ligand as a dopamine D2 receptor ligand, did not affect the NMDA-elicited increase in [Ca2+]i. Sulpiride increased the maximum effect afforded by different concentrations of NMDA and shifted the dose-response curve of NMDA to the left (EC50 value from 12.5 microM to 1.39 microM). Consistent with sulpiride's affinity at dopamine D2 receptors, this action of sulpiride was stereoselective: S(-)-sulpiride was active whereas R(+)-sulpiride was inactive. Treatment of cells with dopamine (3 microM) tends to decrease the NMDA-induced increase in [Ca2+]i. Sulpiride at 1 microM totally abolished this action of dopamine and restored its potentiating action on the NMDA-induced increase in [Ca2+]i. Haloperidol, a potent dopamine D2 and sigma receptor ligand, did not affect the sulpiride's potentiating action on the NMDA-induced responses. On the other hand, chronic treatment of cells with a sigma receptor agonist, DTG, at a concentration producing no effect of its own (10 nM), led to an enhancement of the potentiating effect of sulpiride on NMDA-induced increase in [Ca2+]i. This action of DTG was abolished by haloperidol. Further, chronic, but not acute, treatment of cells with either a protein kinase inhibitor H-7 or a cAMP-dependent protein kinase (PKA) inhibitor H-89 abolished this effect of sulpiride on the NMDA-induced [Ca2+]i changes. These results indicate that the action of NMDA in the primary cortical neurons are regulated differently by ligands with differential affinities at dopamine D2 and sigma receptors. The results with protein kinase inhibitors indicate that the potentiation of NMDA responses by sulpiride involves intracellular biochemical events.

Cocaine-induced changes in glutamate and GABA immunolabeling within rat habenula and nucleus accumbens

We previously reported that subchronic administration of cocaine for 5 days via slow-release pellets results in pronounced degeneration in the lateral habenula (LHB) and its primary efferent tract, the fasciculus retroflexus [Ellison (1992): Brain Res 598:353-356; Ellison and Switzer (1993): Neuroreport 5:17-20]. The lateral habenula receives both GABA and glutamate afferents. In order to test the hypothesis that the cocaine-induced degeneration of the fasciculus retroflexus may be related to changes in synaptic activity of either GABA or glutamate nerve terminals within the LHB, the density of nerve terminal immunolabeling of either neurotransmitter was quantified after 5 days of chronic drug administration followed by either 1 or 14 days off the drug. The shell of the nucleus accumbens (NACs) was also analyzed, since this area is thought to be associated with the reward aspects of addictive stimulant drug administration and was previously shown not to be associated with fiber degeneration. We found that cocaine treatment resulted in a significant decrease in the density of nerve-terminal GABA immunolabeling located within the LHB in animals taken off the drug for either 1 or 14 days, while there was no change in the density of glutamate immunolabeling. In the NACs, there was a decrease in the density of glutamate immunolabeling within nerve terminals 1 day but not 14 days after cocaine administration. There was no change in the density of GABA immunolabeling within the NACs following the 1 or 14 day-off period. These results suggest that there are long-term changes in the density of GABA immunolabeling within the LHB and that the effects seen in glutamate synapses within the NACs are transitory. The long-term decrease in GABA immunolabeling within the LHB is consistent with the hypothesis that a decrease in inhibitory synaptic activity, leading to increased excitatory influence on LHB neurons, may result in neurotoxicity and the subsequent degeneration of the fasciculus retroflexus.

Failure to down regulate NMDA receptors in the striatum and nucleus accumbens associated with neuroleptic-induced dyskinesia

The syndrome of vacuous chewing movements (VCMs) in rats is similar in many respects to tardive dyskinesia (TD) in humans. Both syndromes are characterized by delayed onset of persistent orofacial dyskinesias in a sub-group of subjects chronically treated with neuroleptics. Using the rat model, we examined the role of NMDA receptor-mediated corticostriatal neurotransmission in the expression of VCMs. Rats were treated for 36 weeks with haloperidol decanoate or vehicle and then withdrawn for an additional 28 weeks. Chronic persistent VCMs were induced in one subgroup of treated animals (+VCM), but not in another group (-VCM). Rats from +VCM, -VCM groups and vehicle-treated controls were selected for post mortem studies (n = 12 to 14 per group). NMDA receptor levels were assessed using [3H]-MK-801 binding in sections from the mid-striatum and nucleus accumbens. Chronic haloperidol treatment produced a marked reduction of NMDA receptor binding levels throughout the striatum and nucleus accumbens. Post hoc comparisons demonstrated that -VCM rats had lower NMDA receptor binding levels than +VCM and vehicle-treated controls. Ventromedial striatum and nucleus accumbens core were the most affected areas. These findings suggest that down-regulation of striatal NMDA receptor binding levels may protect against the expression of neuroleptic-induced dyskinesia.

Expression of messenger RNAs encoding ionotropic glutamate receptors in rat brain: regulation by haloperidol

In situ hybridization was used to study the regional distribution of messenger RNAs encoding ionotropic glutamate receptor subtypes in the rat brain's dopaminergic cell body regions and their forebrain projection areas. Short oligonucleotide probes specific for the messenger RNAs encoding the flip or flop splice forms of the GluR1 and GluR2 AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptor subunits, or for the messenger RNAs encoding the N-methyl-D-aspartate R1 subunit, were used. Significant differences were seen in the relative messenger RNA levels, and the distribution of the flip and flop splice forms, of GluR1 and GluR2. In the dopaminergic cell groups of the substantia nigra pars compacta and the ventral tegmental area, the flip form of both GluR1 and GluR2 dominated over the flop form. Similarly, in the core division of the nucleus accumbens, GluR1 and GluR2 flip forms dominated over the flop forms. In contrast, in the accumbens shell, the GluR1 and GluR2 flop forms dominated over the flip forms. As a comparison to the AMPA receptor subunits, N-methyl-D-aspartate R1 messenger RNA was relatively evenly distributed in all the regions analysed. The results demonstrate a heterogeneous distribution of the flip and flop splice forms of GluR1 and GluR2 in the brain's dopaminergic pathways, which could contribute to physiological differences in regulation of the pathways by glutamatergic neurotransmission. We also studied regulation of glutamate receptor subunit expression in these regions by antipsychotic drugs, based on previous reports of altered levels of subunit immunoreactivity after drug treatment. Chronic administration of the typical antipsychotic drug, haloperidol, caused a small but significant induction of GluR2 flip messenger RNA in the dorsolateral caudate putamen. This effect was not seen after chronic administration of the atypical antipsychotic drug, clozapine. Significant drug regulation of the other glutamate receptor subunits studied was not observed.

Chronic administration of imipramine and citalopram alters the expression of NMDA receptor subunit mRNAs in mouse brain. A quantitative in situ hybridization study

Chronic administration of antidepressants produces region-specific adaptive changes in the radioligand binding properties of N-methyl-D-aspartate (NMDA) receptors. We hypothesized that this effect of chronic antidepressant administration was owing to an alteration in NMDA receptor subunit composition. This hypothesis was examined using in situ hybridization with [35S]-labeled riboprobes to quantify the impact of chronic (16 d) injection with either imipramine (15 mg/kg) or citalopram (20 mg/kg) on the levels of transcripts encoding NMDA receptor subunits in mouse brain. These antidepressants altered the levels of mRNA encoding the zeta-subunit in a parallel fashion, with both drugs either reducing transcript levels (e.g., in the cortex, cerebellum, thalamus, and striatum) or producing no substantial effects (e.g., hippocampus). In contrast, these antidepressants often produced distinct, region-specific effects on mRNA levels encoding the epsilon family of subunits. For example, citalopram treatment produced widespread reductions in epsilon 1-subunit mRNA levels (e.g., in frontal cortex, CA2 of hippocampus, and amygdala), whereas imipramine reduced levels of this transcript only in the amygdala. Conversely, imipramine treatment produced widespread reductions in epsilon 2-subunit mRNA levels (e.g., in cortex, CA1-4 of hippocampus, and amygdala), whereas the effects of citalopram on levels of this transcript were largely restricted to amygdala. These findings indicate that long-term antidepressant treatment produces region-specific changes in expression of transcripts for NMDA receptor subunits, presumably altering NMDA receptor composition. Because subunit composition determines the physiological and pharmacological properties of NMDA receptors, these changes may play a critical role in the therapeutic actions of structurally diverse antidepressants.

Absence of basal ganglia amino acid neuron deficits in schizophrenia in three collections of brains

Amino acid (glutamatergic, GABAergic) neuron deficiency theories of schizophrenia offer plausible explanations of pathogenesis. However, reports of disease-related reductions in amino acid synthesizing enzymes in post-mortem brains are contradictory. We measured neuronal uptake sites for gamma-aminobutyric acid (GABA; [3H]nipecotic acid binding) and nerve terminal/glial uptake sites for L-glutamate (D-[3H aspartate binding) in three independent groups of post-mortem brains from patients with schizophrenia and control subjects. Measurements were also made of the phencyclidine site of the glutamate N-methyl-D-aspartate (NMDA) receptor. Samples from patients showed no reductions in the binding of [3H]nipecotic acid or D-[3H]aspartate in caudate, putamen or globus pallidus. On the contrary, some increased binding of both ligands was observed in patients in many comparisons with controls. There were no clear-cut changes in NMDA receptor binding. The most consistent change in the brain sets was increased [3H]nipecotic acid binding in caudate-putamen. This could be due to neuroleptic treatment. The findings produce no evidence that schizophrenia involves major loss of GABA neuron terminals in the basal ganglia or losses of corticostriatal glutamatergic projections.

Increased density of glutamate/N-methyl-D-aspartate receptors in putamen from schizophrenic patients

Saturable radioligand binding of [3H]L-689,560 to the glycine site of the N-methyl-D-aspartate (NMDA) glutamate receptor complex was employed to determine the density of this receptor in putamen, caudate and nucleus accumbens taken post mortem from schizophrenic patients and matched controls. Receptor density was found to be significantly increased in putamen of schizophrenics (P=0.012), although no significant change was found in the other two areas studied. Further experiments following 21 days administration of haloperidol to rats provided no evidence that the increase was related to prior drug treatment.

Antipsychotic drug effects on glutamatergic activity

Previous work from this laboratory indicated that some antipsychotic drugs possess unique action at N-methyl-D-aspartate (NMDA) receptors. A functional neurochemical assay showed that, at concentrations similar to those found in the cerebrospinal fluid (CSF) of schizophrenics, antipsychotic drugs augment NMDA activity while, at higher concentrations, NMDA activity is suppressed. Using similar analysis, the present paper reports that this pattern of response is also shown by the antipsychotic drugs thioridazine and chlorpromazine. In contrast, promazine, which is structurally similar to chlorpromazine but lacking both D2-effects and antipsychotic potency, had no influence on NMDA receptors. In addition, sulpiride and metoclopramide, drugs with high affinity for D2-dopamine receptors but with weak or no antipsychotic efficacy, also lack effects at the NMDA receptor. Thus, the drugs with clinical efficacy that were tested in the present and previous studies all share unique influence on NMDA receptors. Further work with other antipsychotic agents will be necessary to determine if influence on NMDA receptors contributes to antipsychotic effectiveness.

Effect of chronic clozapine administration on [3H]MK801-binding sites in the rat brain: a side-preference action in cortical areas

We studied modifications in [3H]MK801-binding sites in the rat brain after chronic clozapine. We found a 20-30% reduction of [3H]MK801-binding sites in the anterior cingulate, frontoparietal motor and frontoparietal somatosensory cortices on the left side but none on the right. We also demonstrated a 20% bilateral increase of N-methyl-D-aspartate (NMDA) receptors in the dentate gyrus of the hippocampus. No changes were found in the prefrontal cortex, caudate-putamen, nucleus accumbens, hippocampus or olfactory tubercle.