The major dopamine D2 receptor: molecular analysis of the human D2A subtype

Identification of an additional gene belonging to the alpha 2 adrenergic receptor family in the human genome by PCR

We here describe the cloning of an additional gene, called alpha 2-1.8, which is similar to the previously cloned human alpha 2-adrenergic receptor located on chromosome 4. The alpha 2-1.8 gene was identified by using the polymerase chain reaction with primers specific for sequences in transmembrane regions 2 and 5 of the previously isolated human alpha 2-C4 and alpha 2-C10 adrenoceptor genes, which are localized on chromosomes 4 and 10, respectively. The new gene was identified by amplifying the 1.8 kb size fractionated region of PstI restriction cut human genomic DNA. The previously cloned alpha 2-C10 and alpha 2-C4 genes were recovered at their expected locations, 0.96 and 5.9 kb, respectively. We have identified 387 bases of the new alpha 2-1.8 gene, and its sequence is identical to the previously described alpha 2-C4 gene, but it is distinct from the alpha 2-C10 and alpha 2-C2 genes. Our results demonstrate that the alpha 2-C4 adrenergic receptor exists in more than one copy in the human genome.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Analysis of the structure and expression of the human dopamine D2A receptor gene

In order to study the possible involvement of dopamine receptors in the pathophysiology of various neurological and psychiatric disorders, we have isolated the human D2A gene. Like the rat D2A gene, the human gene contains at least eight exons and spans at least 52 kb. Exons 2-8 are clustered within 14 kb of genome. Exon 1 is separated from exon 2 by at least 38 kb. We and others have shown that alternative utilization of exon 6 gives rise to alternative D2A transcripts. Despite the extreme size of intron 1, no alternative transcription between exons 1 and 2 can be detected in basal ganglia and pituitary using polymerase chain reaction analysis. The relative abundance and tissue distribution of the alternative D2A transcripts were examined in 18 human brain regions. The relative expression of the transcripts varied by at least 70-fold across the brain regions surveyed. As expected, high levels of transcripts were detected in caudate, putamen, and pituitary. Moderate levels were detected in regions of catecholamine-containing cell bodies and in the amygdala. In contrast to the rat brain, very low levels of transcripts were detected in cortical regions.

Differential expression of the mouse D2 dopamine receptor isoforms

We have identified and characterized the cDNAs corresponding to the mouse D2 dopamine receptors. We show that in the mouse the D2 dopamine receptor is found in two forms, generated by alternative splicing of the same gene, mRNA distribution analysis of areas expressing the D2 receptors shows that the larger form is the most abundant, except in the brain stem where the shorter form is predominant. Membranes of mammalian cells transiently transfected with both forms of D2 receptor bind [3H]spiperone with a high affinity.

Distribution of dopamine D2 receptor mRNA splice variants in the rat by solution hybridization/protection assay

We investigated the distribution of the two dopamine D2 receptor mRNA splice variants in the rat using a sensitive and quantitative solution hybridization/nuclease protection assay. In all brain and endocrine regions studied, both splice variants were detected and the mRNA of the longer form (D2L) was more abundant than that of the shorter form (D2S). The lowest percentages of D2S were found in the pituitary and adrenal glands.

Distribution of D2 dopamine receptor mRNA in the primate brain

1. The distribution of the messenger RNA (mRNA) encoding the D2 dopamine receptor has been mapped in the monkey brain by in situ hybridization. 2. Using [35S]-labelled riboprobes corresponding to the region of the D2 dopamine receptor spanning the third cytosolic loop and the sixth and seventh transmembrane domains, specific hybridization was observed in a number of neural structures. 3. High levels of mRNA expression were observed in the caudate, putamen, and claustrum. Significant amounts were also identified in the hippocampus, lateral geniculate nucleus, much of the cortex, amygdala, pons, and thalamus. High levels of this mRNA were also visualized in the substantia nigra, likely reflecting autoreceptor synthesis. 4. While the distribution of D2 dopamine receptor mRNA was similar between the monkey and previously published maps in the rat, several differences were noted. 5. These results demonstrate the feasibility of visualizing this mRNA in the primate brain, and suggest that a similar analysis of human postmortem brain material may be possible.

Comparison of the in vitro receptor binding properties of N-[3H]methylspiperone and [3H]raclopride to rat and human brain membranes

The aim of the present investigation was to study and compare the in vitro binding properties of the two radioligands N-[3H]methylspiperone ([3H]NMSP) and [3H]raclopride. These compounds, labeled with 11C, have been extensively used in positron emission tomography studies on central dopamine D2 receptors in schizophrenic patients, although with diverging results. One study (using [11C]NMSP) showed an increased dopamine receptor density in drug-naive schizophrenic patients, whereas in another study (using [11C]raclopride) the density in schizophrenic patients was no different from that in healthy controls. In the present study, using in vitro binding techniques, the density of the binding sites was found to be similar irrespective of which of the two radioligands was used (20 fmol/mg wet weight in rat striatum and 10 fmol/mg in human putamen; the 5-hydroxytryptamine 2 receptors were blocked with 40 nM ketanserin). [3H]NMSP had a 10-fold higher affinity (KD, 0.3 nM in rat striatum and 0.2 nM in human putamen) than [3H]raclopride (KD, 2.1 nM in rat striatum and 3.9 nM in human putamen), which was consistent with the longer dissociation half-life of [3H]NMSP compared with [3H]raclopride (14.8 and 1.19 min, respectively). There was an approximate overall similarity between the inhibition constants for five dopamine antagonists, chlorpromazine, haloperidol, raclopride, remoxipride, and NMSP, when using either radioligand. The Ki values were, however, two- to four-fold higher when using [3H]NMSP as the radioligand, irrespective of inhibiting compound, except for chlorpromazine (and haloperidol in human putamen). NMSP was found to inhibit the binding of [3H]raclopride competitively, whereas raclopride inhibited the binding of [3H]NMSP both competitively and noncompetitively. This difference suggests that part of the binding site is exclusively used by NMSP and can only be allosterically interfered with by raclopride. It is proposed that [3H]NMSP binds to an additional set of accessory binding sites, presumably located more distantly from the agonist binding active site than the sites to which [3H]raclopride binds.

Molecular cloning and characterization of a novel dopamine receptor (D3) as a target for neuroleptics

A dopamine receptor has been characterized which differs in its pharmacology and signalling system from the D1 or D2 receptor and represents both an autoreceptor and a postsynaptic receptor. The D3 receptor is localized to limbic areas of the brain, which are associated with cognitive, emotional and endocrine functions. It seems to mediate some of the effects of antipsychotic drugs and drugs used against Parkinson's disease, that were previously thought to interact only with D2 receptors.

Molecular cloning and expression of a D1 dopamine receptor linked to adenylyl cyclase activation

In order to clone the D1 dopamine receptor linked to adenylyl cyclase activation, the polymerase chain reaction was used with highly degenerate primers to selectively amplify a cDNA sequence from NS20Y neuroblastoma cell mRNA. This amplification produced a cDNA fragment exhibiting considerable sequence homology to guanine nucleotide-binding (G)-protein-coupled receptors that have been cloned previously. To characterize this cDNA further, a full-length clone was isolated from a rat striatal library by using the cDNA fragment as a probe. Sequence analysis of this cDNA clone indicated that it is indeed a member of the G-protein-coupled receptor family and exhibits greatest homology with the previously cloned catecholamine receptors. Northern blot analysis of various neural tissues revealed a transcript of approximately 4 kb that was predominantly located in the striatum with lesser amounts in the cortex and retina. In contrast, no mRNA was detected in the cerebellum, hippocampus, olfactory bulb, mesencephalon, or pituitary. In situ hybridization analysis also revealed a high abundance of mRNA in the striatum as well as in the olfactory tubercle. To establish the identity of this cDNA, we performed transient expression experiments in COS-7 cells. [3H]SCH-23390, a D1-selective radioligand, exhibited specific, saturable binding only in cells that were transfected with this cDNA. Competition binding analysis with a variety of dopaminergic ligands demonstrated a D1 dopaminergic pharmacology. In addition, dopamine as well as other D1-selective agonists stimulated cAMP accumulation in transfected COS-7 cells. We conclude that we have cloned a cDNA encoding the D1 dopamine receptor linked to the activation of adenylyl cyclase activity.

Effect of haloperidol on expression of dopamine D2 receptor mRNAs in rat brain

Chronic administration of the neuroleptic drug haloperidol previously has been shown to increase the density of striatal dopamine D2 receptor, which is believed to be the underlying factor in neuroleptic-induced tardive dyskinesia. To search for the mechanism of receptor upregulation, the expression of the isoforms of dopamine D2 receptor mRNA in rat striatum was analyzed by Northern, solution, and in situ hybridizations in haloperidol-treated rats (1-35 days). Northern blot analysis of poly(A)+ RNA hybridized with a probe common for both isoforms as well as an insert-specific probe for the long isoform of the receptor revealed no significant difference in hybridization signal between the control and any of the haloperidol-treated groups of rats. The receptor density, however, was increased by 30-40% in animals receiving haloperidol for 7-35 days. Solution hybridization with an antisense riboprobe specific for a consensus sequence as well as in situ hybridization with a consensus oligonucleotide probe similarly failed to detect any increase in the expression of receptor mRNA following haloperidol treatment. The results suggest that post-transcriptional mechanisms may be responsible for regulating the haloperidol-induced increase in dopamine D2 receptors.

Pharmacological and molecular basis for dopamine D-2 receptor diversity

This review will focus on the main lines of evidence that suggest the existence of multiple types of dopamine D-2 receptors. Dopamine D-2 receptors share structural elements suggesting that they belong to a gene superfamily classified as G-protein-coupled receptors and show an archetypical topology predicted to consist of seven putative transmembrane domains. Activation of D-2 receptors results in a variety of responses, including inhibition of cyclic AMP formation, inhibition of phosphoinositol turnover, increase of K-channel activity, and inhibition of Ca influx. The G protein(s) linking the D-2 receptors to these responses have not been completely identified, nor has the possible hierarchy of these regulatory proteins in transforming the incoming signal into a change of second-messenger levels. A lot of experimental data support the hypothesis that there are multiple signal-processing pathways activated by dopamine through D-2-receptor stimulation. Recently, the identification of dopaminergic drugs that discriminate among the different transduction pathways and the isolation of distinct cDNAs encoding proteins that share binding profile indicative of D-2 receptors clearly indicate multiple forms of D-2 receptors. Pharmacologically, at least two distinct categories of dopamine D-2 receptors exist in rat pituitary. The first (D-2a) is insensitive to BHT 920 and coupled to inhibition of adenylyl cyclase activity; the second (D-2b) is activated by BHT 920 and linked to voltage-dependent K channels. The two types of dopamine D-2 receptors differ in their structure, G-protein-coupled and effector. Each of the three basic receptor units shows a certain degree of heterogeneity, which may affect the quality and the kinetic of the response. This variety may represent the molecular basis for the diversity in pharmacological and functional profiles of different dopamine D-2 receptors located in various brain areas and peripheral tissues.

Aspects of the structure of the D2 dopamine receptor

Significant new information on the D2 dopamine receptor has recently become available from a combination of protein chemical and molecular genetic analyses. Molecular genetic studies have shown the receptor to be a member of the family of receptors that are linked to G proteins and that have structures predicted to contain seven transmembrane domains. Two distinct species of D2 dopamine receptor have been found which may differ in their coupling to G proteins; their distributions have been mapped at the nucleic acid level. The D2 dopamine receptor has been purified from brain and anterior pituitary and characterized. Chemical modification of the brain receptor provides evidence for the importance of a carboxyl group that interacts with ligands at the receptor binding site. Here, Philip Strange discusses these points and proposes models of receptor-ligand interaction based on the conservation of several aspartic acid residues in receptors that bind cationic amines.

Different-sized mRNAs from GH4C1 cells induce a TRH-dependent electrical response in Xenopus laevis oocytes

Poly(A)+ RNA from the GH4C1 rat pituitary cell line elicited a thyrotropin releasing hormone response in Xenopus laevis oocytes which could be measured as a change in membrane current by the voltage-clamp method. Oocytes injected with Poly(A)+ RNA from GH12C1 cells which do not bind thyrotropin releasing hormone or with buffer solution alone did not show this response. Size fractionation of total poly(A)+ RNA by sucrose density-gradient centrifugation shows two response maximal representing various mRNA fractions larger than 18S. These results indicate the presence of thyrotropin releasing hormone receptor mRNA heterogeneity where the smallest mRNA is at least 2 kb.

A second molecular form of D2 dopamine receptor in rat and bovine caudate nucleus

Overexpression of the D2 dopamine receptor has been proposed to be part of the pathology of schizophrenia. The isolation of a D2 dopamine receptor clone has assisted the molecular characterization of D2 receptor. We have now isolated an identical rat clone along with two other clones--a second related rat clone (RD-2in) and a homologous bovine clone (BD-2in), both of which contain an insert encoding an additional 29 amino acids relative to the original rat clone (RD-2o). All three clones encode D2 receptor binding sites when expressed in COS-7 cells. The amino-acid insert encoded by D-2in lies in the domain of the receptor believed to interact with the GTP-binding proteins (G proteins) of various signal transduction pathways. By using oligonucleotide probes specific for either D-2o or D-2in RNA transcripts, we have found that the level of expression of the D-2in-encoded form of the receptor is seven times that of the D-2o form in the caudate nucleus, the richest brain source of D2 receptors.