Differential effects of clozapine and haloperidol on dopamine receptor mRNA expression in rat striatum and cortex.

The regulation of the dopamine (DA) receptors is of considerable interest, in part because treatment with antipsychotic drugs is known to upregulate striatal D2-like receptors. While previous studies have focused on the regulation of striatal DA receptors, less is known about the pharmacological regulation of cortical DA receptors. The purpose of this study was to examine the regulation of DA mRNA receptor expression in the cortex compared to the striatum following treatment with antipsychotic agents. Adult male Sprague-Dawley rats were injected daily with haloperidol (2 mg/kg/day), clozapine (20 mg/kg/day) or a control vehicle for a period of 14 days. Following treatment, brains were subjected to in situ hybridization for the mRNAs encoding the five dopamine receptors; only D1, D2, and D3 receptor mRNAs were detected in these regions. Haloperidol tended to either modestly upregulate or have no effect on dopamine receptor mRNAs detected in striatal structures, while clozapine generally downregulated these mRNAs. On the other hand, in the cortex, both drugs had striking effects on D1 and D2 mRNA levels. Cortical D1 mRNA was upregulated by haloperidol, but this effect was primarily restricted to cingulate cortex; clozapine also upregulated D1 mRNA, but primarily in parietal regions. Haloperidol downregulated D2 mRNA in the majority of cortical regions, but most dramatically in frontal and cingulate regions; clozapine typically upregulated this mRNA, but primarily in regions other than frontal and cingulate cortex. These results indicate that clozapine and haloperidol each have regionally-specific effects, and differentially regulate dopamine receptor mRNA expression in striatal and cortical regions of the rat brain.

Dopamine receptor mRNA expression in human striatum and neocortex.

The distributions of the transcripts encoding the five dopamine receptors have been determined in the human striatum and selected regions of the neocortex. In the striatum significant levels of dopamine receptor expression are restricted to the D1, D2, and D3 receptors. D1 and D2 receptor messenger ribonucleic acids (mRNAs) are homogeneously distributed throughout the caudate, putamen, and nucleus accumbens. D3 receptor mRNA is particularly enriched in the nucleus accumbens, with moderate levels in the ventral putamen. In the prefrontal cortex D1 and D4 receptor mRNAs are the most abundant, although the other three transcripts are seen at lower levels. A similar pattern is seen in the temporal neocortex. In the occipital cortex, D1 receptor mRNA is the most abundant, D3 the rarest, while the other three transcripts are present at modest levels of expression. These data add to a growing understanding of the neuroanatomical distribution of these transcripts in the human brain. They are essential to understand in the context of the limbic circuitry of the brain, as new hypotheses of dysfunction of dopaminergic neurotransmission are advanced in psychiatry and as these receptor subtypes are targeted for development of novel pharmacological treatments.

Differential regulation of hippocampal AMPA and kainate receptor subunit expression by haloperidol and clozapine.

Dopamine-glutamate interactions within discrete neural circuits are increasingly recognized as potential substrates for dysregulation in schizophrenia, and as a result, potential targets for pharmacological intervention in this illness. We examined the regulation, by haloperidol (2 mg kg-1 day-1) and clozapine (20 mg kg-1 day-1), of the mRNAs encoding the four AMPA receptor subunits (gluR1-gluR4), three low-affinity kainate receptor subunits (gluR5-gluR7), and two high-affinity kainate subunits (KA1 and KA2) in the rat hippocampal formation and associated entorhinal cortex. A complex and differential pattern of AMPA and kainate subunit mRNA regulation by clozapine and haloperidol was observed in this study. Both drugs caused significant alterations of most of these mRNAs, but in a heterogeneous and region-specific fashion. These data suggest that these antipsychotic drugs alter the expression of the genes encoding the subunits that express ionotropic glutamate receptors. Given the importance of glutamatergic mechanisms and the hippocampal formation in schizophrenia, these data suggest a potential substrate for neurotransmitter dysregulation in this illness, as well as a potential target for therapeutic intervention.

The human dopamine D5 receptor gene: cloning and characterization of the 5'-flanking and promoter region.

Genomic and overlapping cDNA clones encompassing the entire 5'-untranslated region of the human D5 receptor gene were cloned and sequenced. Comparison of these human D5 receptor genomic and cDNA clones revealed the presence of two exons separated by a small and variably sized intron (of either 179 or 155 bp). We have determined that the major site of transcription initiation of the D5 gene is 2125 bp upstream from the translational initiation start site. The region 5' to the transcription initiation site lacked conventional TATA and CAAT sequences, but contained several putative binding sites for transcription factors, such as Sp1 and Ap1. Luciferase reporter gene constructs containing D5 gene sequence information up to 500 bp 5' of the transcription initiation site were able to stimulate transcription only in SK-N-SH cells but not in COS-7, CHO, P19EC, NB41A3, and SK-N-MC cell lines. Promoter deletion analysis indicated that the D5 gene promoter contained a positive modulator at 119-182 and a negative modulator 251-500 bases upstream from the site of transcription initiation. In addition, in order to detect the expression of functional D5 receptor mRNAs and not those of its expressed pseudogenes, in situ hybridization analysis of monkey and human brain using a 5' D5-specific riboprobe revealed that D5 receptor mRNA was most abundant in discrete cortical areas (layers II, IV, and VI), the dentate gyrus, and hippocampal subfields with very little message detected in the striatum. Unexpectedly, D5 mRNA antisense riboprobes labeled discrete cell bodies in the pars compacta of the substantia nigra. The characterization of the genomic organization of the D5 receptor gene and of those factors involved in its transcriptional regulation may aid in our understanding of the role this gene product plays in the generation and maintenance of dopamine D1-like receptor-mediated events.

Differential expression of autoreceptors in the ascending dopamine systems of the human brain.

The tone and regulation of the brain dopaminergic projections are, in part, determined by the presence or absence of dopamine (DA) autoreceptors: rate of DA synthesis and turnover, as well as both pattern and rate of neuronal firing, are modulated by the expression and activity of these autoreceptors. The expression of dopaminergic receptors in the midbrain DA cell groups, presumably reflecting DA autoreceptors, was determined in the brains of the rat, Old World monkey, and human. In the rat, both the substantia nigra (A9) and the ventral tegmental area (A10) appear to express DA autoreceptors. In the monkey and human, however, only the projections arising from the substantia nigra express these receptors; the limbic projections originating in the ventral tegmental area lack this substrate for DA autoregulation. These results indicate that in the human, the nigrostriatal and mesocorticolimbic dopamine systems may be differentially autoregulated.

Dopamine receptor gene expression in the human medial temporal lobe.

The distributions of the messenger RNA molecules encoding the five known dopamine receptors have been determined in the medial temporal lobe of postmortem human brain. All five receptor mRNAs are present in temporal lobe structures, although their distributions are heterogeneous. The D1-like receptors, D1 and D5, have strikingly dissimilar distributions. D1 receptor mRNA is abundant in temporal neocortex but is rare elsewhere. D5 receptor message, however, is seen in the hippocampus, subicular complex, and in temporal cortex. The D2-like receptors have similar distributions: D2, D3, and D4 receptor mRNAs are all identifiable in the hippocampal formation and in the cortical regions of the medial temporal lobe. Distinct patterns of relative regional concentrations for each message are observed, however, suggesting a neuroanatomical substrate for potential differences in dopaminergic regulation within discrete regions of the medial temporal lobe. These results provide a description of the distribution of these receptor mRNAs in normal humans and suggest multiple levels of complexity as well as regulation of the medial temporal lobe dopamine projection.

Effects of cocaine on dopamine receptor gene expression: a study in the postmortem human brain.

The effects of chronic cocaine exposure on dopamine D1 and D2 receptor gene expression in the human brain were studied in postmortem samples from chronic cocaine abusing and matched control subjects. Using in situ hybridization of receptor autoradiography to examine messenger ribonucleic acid (RNA) and binding sites, respectively, neither D1 nor D2 receptor expression was found to be changed in the nucleus accumbens, caudate, putamen, or substantia nigra of the cocaine-exposed subjects. Although chronic cocaine exposure can produce alterations in dopaminergic neurotransmission, sustained compensatory changes in dopamine receptor expression do not appear to occur in the human.

Distribution of D5 dopamine receptor mRNA in rat brain.

The distribution of the messenger RNA encoding the dopamine D5 receptor was determined in the rat brain by in situ hybridization. Using [35S]-labelled riboprobes to either the rat or human D5 receptor, this mRNA was localized to the hippocampus and the parafascicular nucleus of the thalamus. This mRNA could not be visualized in the more traditional brain regions associated with dopaminergic cell bodies or projection fields. This unusual distribution suggests a novel function in the brain for this subtype of the dopamine receptor.