Alternative splicing directs the expression of two D2 dopamine receptor isoforms

D2 dopamine receptor gene expression in the rat striatum during ontogeny: an in situ hybridization study

D2 dopamine receptor (D2R) gene expression in the rat striatum was studied by in situ hybridization throughout the pre- and the postnatal period from gestational day 12 to postnatal day 8. D2R mRNA was detected with 35S-labelled oligonucleotide probes, one that hybridized equally to the two isoforms of the D2R mRNA (D2(415) and D2(444)) and the other that hybridized specifically to the large isoform (D2(444)). D2R mRNA was first detected in the striatal primordium at day 14 of gestation with the probe that recognizes indifferently the two isoforms and with the probe specific for the D2(444) mRNA. At day 16, D2R mRNA was present in the lateral part of the striatum and in the germinal ventricular zone lining the lateral ventricle. At day 18, D2R mRNA was found in neurons of the caudate-putamen, the nucleus accumbens, the olfactory tubercle and the subependymal zone lining the lateral ventricle. The microautoradiographic analysis demonstrated that the labelled cells have a neuroblastic and immature aspect before birth. After birth the topography and aspect of labelled cells was similar to the one observed in the adult animals. D2R mRNA was present in neurons of the caudate-putamen, the nucleus accumbens and the olfactory tubercle. In the caudate-putamen there was a latero-medial gradient of labelling. From postnatal day 2 onward the D2R gene was expressed in two striatal cell types, small neurons probably enkephalinergic, and large-sized neurons with prominent cytoplasm, most probably cholinergic.(ABSTRACT TRUNCATED AT 250 WORDS)

Shorter variants of the D3 dopamine receptor produced through various patterns of alternative splicing

Using Polymerase Chain Reaction amplification of mRNAs from several areas of rat brain we have shown the occurrence of two shorter transcripts of the dopamine D3 receptor gene, in addition to that corresponding to the D3 receptor. Cloning and sequencing of these transcripts, together with the establishment of the exon-intron organization of the D3 receptor gene, shown these transcripts to result from different processes of alternative splicing. The first transcript encodes a 100 amino acid protein, being produced by splicing of an exon whose absence deletes the third transmembrane domain and gives rise downstream to a frameshift in the open reading frame. In the second transcript, an in frame 54 bp deletion is produced by splicing occurring at an internal acceptor site, suppressing half of the second extracellular loop and a small sequence in the fifth transmembrane domain. This transcript was stably expressed in CHO cells which, however, failed to reveal any dopaminergic ligand binding activity. The functional significance and possible role of these shorter variants of the dopamine D3 receptor in cell signalling remain to be established.

Visualization of a dopamine D1 receptor mRNA in human and rat brain

Using 32P-labeled oligonucleotides derived from the coding region of human dopamine D1 receptor mRNA we have localized in the human and rat brain the cells containing the mRNAs coding for this receptor. Dopamine D1 receptor mRNA in human brain was found to be contained in the neurons of the caudate and putamen nuclei as well as in the nucleus accumbens, some cortical regions and some nuclei of the amygdala. In the rat brain, cells containing D1 receptor mRNA were enriched in caudate-putamen and accumbens nuclei, olfactory tubercle, islands of Calleja, some cortical areas and in several thalamic nuclei. Moreover, in both species, it was absent from the neurons of the substantia nigra both pars compacta and pars reticulata and ventral tegmental area as well as from the globus pallidus pars lateralis and medialis in human and globus pallidus and entopeduncular nucleus in rat. In general, a good agreement was found with the distribution of binding sites labeled with the D1 antagonist SCH 23390. The main exception was the absence of D1 receptor mRNA in globus pallidus and substantia nigra, regions where high densities of receptor sites are found. These data support the notion that sites in these two regions are localized to projections from striatal neurons and that dopaminergic neurons do not express this receptor.

Stereospecific effect of flupenthixol on neuroreceptor gene expression

An experimental method to test the hypothesis that antipsychotic (neuroleptic) agents influence gene expression in the mouse brain has been developed using the cis and trans stereoisomers of flupenthixol. The cis form of the drug is known to be clinically effective against some of the psychotic symptoms of schizophrenia as opposed to the trans isomer which is relatively inactive. A 2- to 3-fold increase in the abundance of dopamine 2 receptor mRNA was observed in the cis treated mice after a period of ten weeks. No change was observed in the expression of the dopamine D2 receptor gene upon treatment with the trans isomer. No change in the amount of 5-HT1A, 5-HT1C, alpha 1 adrenergic, beta 1 and beta 2 adrenergic neuroreceptor mRNA was found in the mice treated with active drug. The results show a long-term adaptation to D2 antagonism at the level of gene expression which occurs over a similar time scale to that of the clinical response to neuroleptic treatment of schizophrenia.

Diversity in receptor signalling: cellular individuality and the search for selective drugs

Recent developments in the molecular pharmacology of receptor signalling are reviewed. Receptors and other signalling molecules belong to superfamilies characterized by variations on given themes. A remarkable degree of diversity is generated by unique combinations of molecular entities, and there are also interactions between parallel signal transduction pathways. Molecular biology techniques have revealed new targets for drug development, as well as new methods of drug testing. However, it is not yet clear whether molecular and biochemical specificity always correspond to clinical selectivity.

Differential expression of dopaminergic D2 receptor messenger RNAs during development

To define the possible roles of multiple types of dopamine D2 receptors, mRNA levels for two forms of D2 receptor were determined during a variety of developmental stages. Transcripts encoding the 444 amino acid form appear as early as embryonic day 14. In contrast, the mRNA encoding the 415 amino acid form of the receptor does not appear until embryonic day 17, and remains a minor form throughout prenatal development. The adult levels of the mRNAs of these two D2 receptor forms are not attained until long after birth.

Cloning of the gene for a human dopamine D4 receptor with high affinity for the antipsychotic clozapine

Dopamine receptors belong to the family of G protein-coupled receptors. On the basis of the homology between these receptors, three different dopamine receptors (D1, D2, D3) have been cloned. Dopamine receptors are primary targets for drugs used in the treatment of psychomotor disorders such as Parkinson's disease and schizophrenia. In the management of socially withdrawn and treatment-resistant schizophrenics, clozapine is one of the most favoured antipsychotics because it does not cause tardive dyskinesia. Clozapine, however, has dissociation constants for binding to D2 and D3 that are 4 to 30 times the therapeutic free concentration of clozapine in plasma water. This observation suggests the existence of other types of dopamine receptors which are more sensitive to clozapine. Here we report the cloning of a gene that encodes such a receptor (D4). The D4 receptor gene has high homology to the human dopamine D2 and D3 receptor genes. The pharmacological characteristics of this receptor resembles that of the D2 and D3 receptors, but its affinity for clozapine is one order of magnitude higher. Recognition and characterization of this clozapine neuroleptic site may prove useful in the design of new types of drugs.

Characterization of a dopamine receptor (DA2K) in the kidney inner medulla

Dopamine (DA) produces a natriuretic/diuretic response in the kidney by mechanisms that are still not well understood. There is some indication that DA2 receptors may be involved in mediating the effects of DA, but little is known regarding the nature of this receptor in the kidney. Autoradiographic localization of [3H]spiperone, a DA2 antagonist, indicated that high-density binding was restricted to inner medullary collecting ducts (IMCDs). [3H]Spiperone binding was saturable, high affinity (Kd, 17.2 +/- 1.65 nM), and high density (Bmax, 935 +/- 83 fmol per mg of protein). The photosensitive spiperone analogue N-(p-azido-m-[125I]iodophenethyl)spiperone labeled similar sized proteins of Mr = 120,000 in membranes prepared from the kidney inner medulla, striatum, and pituitary. However, the rank-order competition profile for the [3H]spiperone binding in the kidney inner medulla differed from the DA2 receptor in striatum and pituitary and, furthermore, RNA (Northern) blot analyses of kidney inner medullary RNA with brain DA2 receptor oligonucleotide probes were negative. Functionally, DA stimulated prostaglandin E2 production by IMCD cells, an effect that could be blocked by the DA2 antagonist domperidone. These results indicate that the kidney inner medulla expresses a functional DA receptor that may represent a newly identified DA receptor subtype (here designated DA2K). Moreover, these results suggest that the kidney inner medulla may be a significant site at which DA, either directly or indirectly, influences water and electrolyte excretion.

Transcription mediated by a cAMP-responsive promoter element is reduced upon activation of dopamine D2 receptors

Dopaminergic D2 receptors mediate the effect of dopamine on cellular effector systems by means of guanine nucleotide-binding proteins (G proteins). The major biochemical effect evoked by these receptors is the inhibition of adenylyl cyclase. As a consequence, the activation of D2 receptors lowers the intracellular cAMP level. Two cDNAs, originated by alternative splicing of the same gene, have been isolated: D2A and D2B. They code for two proteins of 444 and 415 amino acids. These proteins display high affinity for selective D2 dopamine ligands. D2A differs from D2B by an insertion of 87 nucleotides in its cDNA, which is located in a region of the protein considered important for the coupling to G proteins. To investigate functional differences between the two dopamine D2 receptor isoforms, we transiently expressed them in cultured cells. To do so we developed an assay to study membrane receptors that are coupled to the adenylyl cyclase. Using this assay, we were able to show that the stimulation of the adenylyl cyclase induced by the activation of the beta 2-adrenergic receptor is inhibited more efficiently by D2B than D2A. The effects elicited by the D2 receptors are mediated by pertussis toxin-sensitive G proteins. Treatment of transfected cells with pertussis toxin abolishes the inhibitory effects in a dose- and receptor isoform-dependent manner. Our results suggest that the two dopamine receptor isoforms are differentially coupled to G proteins.

Cloning and sequence analysis of brain cDNA encoding a Xenopus D2 dopamine receptor

A D2 dopamine receptor pharmacologically different from the mammalian D2 receptor has previously been characterized in the amphibian Xenopus laevis. Here we report the cloning of a Xenopus D2 receptor which revealed about 75% amino acid sequence identity with its mammalian counterpart and the presence of an additional 33 amino acid sequence in the 3rd cytoplasmic loop instead of the additional 29 residues in the large form of the mammalian D2 receptor. All 7 predicted transmembrane domains are highly conserved between the Xenopus and mammalian D2 receptors, as are the 1st and 2nd intracellular loop, the 1st and 3rd extracellular loop and the carboxy-terminal portion of the receptors. The amino-terminal portion, the 2nd extracellular loop and the middle portion of the 3rd intracellular loop of these receptors, however, differ considerably. Knowledge of the locations of these regions of conservation and divergence within the D2 receptors of Xenopus and mammals will help to delineate portions of the receptor molecule that are functionally important. Interestingly, the 5'-untranslated region of the Xenopus D2 receptor mRNA contains 4 small open reading frames which may affect translational efficiency.

Increased abundance of alternatively spliced forms of D2 dopamine receptor mRNA after denervation

The existence of two molecular forms of D2 dopamine receptors suggests that differences in the distribution or regulation of the two forms could be exploited for the pharmacological treatment of disease. Using probes selective for each alternatively spliced variant of D2 receptor mRNA, we determined that both variants were widely distributed in rat brain and pituitary but that the ratio of the forms varied among regions. mRNA for the 444-amino acid-long variant, D2(444), was the most abundant form in pituitary and neostriatum. Intermediate levels of both D2(444) mRNA and the short form, D2(415), were detected in midbrain, and low levels of D2(444) and D2(415) mRNAs were detected in all other regions examined, including hippocampus, cerebellum, and cortex. The D2(444)/D2(415) ratio was generally lower in the regions of low expression than in pituitary and neostriatum. Dopamine-depleting lesions increased the density of D2 receptors in the denervated neostriatum by 29% without altering the affinity of the receptors for [3H]spiperone. The proliferation of receptors appeared to be due to a lesion-induced increase of up to 120% in the abundance of both variants of mRNA in the neostriatum.

Current developments in G-protein-coupled receptors

The rate at which receptors have been cloned has recently increased dramatically--existing families have been extended and new families created. The rapid cloning by homology of 'orphan receptors' has also stimulated the development of a new reverse pharmacology.

Expression of three human beta-adrenergic-receptor subtypes in transfected Chinese hamster ovary cells

The genes coding for three pharmacologically distinct subtypes of human beta-adrenergic receptors (beta 1 AR, beta 2 AR and beta 3 AR) were transfected for expression in Chinese hamster ovary (CHO) cells. Stable cell lines expressing each receptor were analyzed by ligand binding, adenylate cyclase activation and photoaffinity labeling, and compared to beta AR subtypes observed in previously described tissues, primary cultures and transfected cell lines. Each of the three receptor subtypes displayed saturable [125I]iodocyanopindolol-binding activity. They showed the characteristic rank order of potencies for five agonists, determined by measuring the accumulation of intracellular cAMP. These recombinant cell lines express a homogeneous population of receptors and display the known pharmacological properties of beta 1 AR and beta 2 AR, in human tissues. It is therefore likely that the pattern of ligand binding and adenylate cyclase activation, mediated by the new beta 3 AR in CHO cells, also reflects the yet-undetermined pharmacological profile in humans.

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.

Investigation of striatal dopamine D2 receptor acquisition following prenatal neuroleptic exposure

To test the hypothesis that neuroleptic blockade impairs the development of striatal dopamine D2 receptors, pregnant rats were given haloperidol, thiothixene, or trifluoperazine for gestational days 15-18 (short-term exposure) or days 5-20 (long-term exposure). All of the drugs were demonstrated to cross the placenta and enter the fetal brains equally well. Striatal dopamine D2 receptors of the pups were assayed on postnatal day 14. Neither receptor density nor receptor affinity was altered significantly by the short- or long-term prenatal neuroleptic treatment.

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.

Structure/function relationship of proteins belonging to the family of receptors coupled to GTP-binding proteins

The structural properties of a number of proteins belonging to the family of receptors coupled to GTP binding proteins are discussed in relation to their function. The structure of the ligand binding site and of the regions involved in coupling to the G proteins are analyzed mainly for the adrenergic and muscarinic cholinergic receptors, for which site-directed mutagenesis and chimaeric constructions have been studied. The structure of the genes are compared and the presence of various regulatory elements is discussed in relation to control of expression. Mechanism of desensitization and internalization, while mostly studied for the beta 2-adrenergic receptor, are proposed to be generally applicable to all G-protein-coupled receptors.

Cocaine induces striatal c-fos-immunoreactive proteins via dopaminergic D1 receptors

The protooncogene c-fos produces a phosphoprotein, Fos, which regulates gene transcription processes. In neuronal systems, Fos has been proposed to couple synaptic transmission to changes in gene expression by acting in the cell nucleus in concert with other proteins to form complexes in the promoter regions of target genes. We report here that the acute administration of a single dose of the indirect-acting dopaminergic agonist cocaine increases multiple Fos proteins in rat caudate nucleus. The increase is dose-dependent and is apparent immunocytochemically at 1 hr, maximal at 2 hr, and absent 48 hr after treatment. The increase seen immunocytochemically is composed of several molecular weight species as assessed by Western blotting of proteins from isolated striatal cell nuclei. Administration of the specific dopaminergic receptor antagonists sulpiride and SCH-23390 prior to cocaine support a significant role for D1 but not for D2 receptors in mediating this effect. These data indicate that D1 dopamine receptors are linked to a cellular immediate-early gene system(s) and suggest an action of cocaine at one or more levels of gene expression via modulation of transcriptional processes in activated cells.

Characterization of anti-peptide antibodies for the localization of D2 dopamine receptors in rat striatum

Seven different peptides of 14-23 residues in length based on the predicted amino acid sequence of the cloned rat D2 receptor cDNA were used as immunogens to develop antibodies in rabbits. Two of these peptides were derived from the amino terminus and four were from the third cytoplasmic loop, including one to the splice variant insertion sequence and one to the carboxyl terminus of the receptor protein. These peptides were conjugated to bovine thyroglobulin prior to rabbit immunization. Antibody production was monitored by a solid-phase ELISA. With the exception of the carboxyl-terminal peptide, all of the peptide immunogens produced antiserum of high titer ranging from 1:10(4) to 1:10(6) on ELISA. Specificity of the reaction was demonstrated by the absence of a response in the preimmune serum and by the absence of cross-reactivity between the various antisera and the nonimmunization peptides. Moreover, preincubation of the antiserum with the immunization peptide, but not other peptides, blocked the subsequent ELISA reactions. Some of the antisera were additionally characterized by immunodot assays using solubilized rat striatal membranes blotted onto nitrocellulose. Positive reactions with antiserum dilutions of 1:500 were observed that were dependent on the presence and concentration of membrane protein and were not observed using preimmune serum. Additionally, immunofluorescent staining by the D2 receptor antiserum was observed on cells that had been transfected with the D2 receptor cDNA but not on untransfected cells. Immunoprecipitation of the photoaffinity-labelled and solubilized D2 receptor also suggested that the antisera were able to directly recognize the native receptor protein. Immunohistochemical localization of the D2 receptor in slices of fresh frozen and perfusion-fixed rat brain was performed using these antisera. Within the striatum, about 50% of the medium-sized neurons were labeled as well as large, putatively cholinergic interneurons.

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.

Dopamine D2 receptors attenuate phosphatidylinositol 4,5-bisphosphate in synaptosomal membranes from rat neostriatum

The effects of dopamine on [32P]ATP-labelled phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidic acid were analyzed by TLC in synaptosomal membranes of the rat neostriatum. The incorporation of 32P into these compounds was found to be stable within 1 min and was maintained during the 30 min of incubation. Dopamine (0.1-10 microM) was found to attenuate the levels of phosphatidylinositol 4,5-bisphosphate without affecting the levels of phosphatidylinositol 4-phosphate or phosphatidic acid. The maximal decrease (-35 +/- 4%) was reached at 10 microM of dopamine after 30 min of incubation. The dopamine (0.1 microM)-induced decrease was blocked by the D2 selective antagonist raclopride (1 microM), but not by the D1 selective antagonist SCH 23390 (1 microM). These findings indicate the existence of an intramembrane coupling of dopamine D2 receptors to phosphoinositide turnover and may underlie some of the physiological effects of D2 receptor stimulation.

Reversible and irreversible labelling of H1- and H2 -receptors using novel [125I] probes

We have recently designed the first 125I-labelled probes specific for the histamine H1 and H2 receptors. These reversible and irreversible antagonists are among the most potent H1 and H2 ligands and have enabled investigations into the biochemical and pharmacological properties of these two receptors. In various brain animal species, the ligand binding peptide of the H1 and H2 receptors, as determined by photoaffinity labeling, resides within 56-59 kDa peptides. In contrast, in guinea pig heart, the ligand binding domain of the H1 receptor is characterized by a higher molecular weight (68 kDa), suggesting the presence of an isoform of this protein, clearly differentiable by this biochemical property but not by its pharmacology. The reversible 125I-probes allowed us to extend the pharmacology of these receptors in several biological preparations and in human brain, and to establish their interaction with G-proteins. A detailed mapping of H1 and, for the first time, of H2 receptors, has been achieved in guinea pig brain, establishing their presence in almost all brain areas. These experiments show that there is no correlation between the density of H2 receptor and the activity of adenylate cyclase sensitive to histamine suggesting a molecular heterogeneity of this receptor.

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.

R(-)2-fluoro-N-n-propylnorapomorphine: a very potent and D2-selective dopamine agonist

A series of 2-substituted N-n-propylnorapomorphine (NPA) derivatives were synthesized and compared with other DA agonists for affinity to D1 and D2 dopamine (DA) receptors in rat brain corpus striatum tissue. The 2-substituents tested reduced D1 affinity similarly, but enhanced D2 affinity in the rank order: F greater than OH greater than Br greater than OCH3 greater than H greater than or equal to NH2. The extraordinarily high D2 affinity (Ki = 12 pM) and D2 vs. D1 selectivity (57,500) of 2-F-NPA far-exceeded that of all other DA agonists tested, and it was about 10-times more potent than NPA in vivo.

Striatal neurons express increased level of dopamine D2 receptor mRNA in response to haloperidol treatment: a quantitative in situ hybridization study

In the present study, quantitative in situ hybridization was used to analyse the effect of haloperidol treatment on D2 dopamine receptor gene expression in the rat caudate-putamen nucleus. Variations of D2 receptor mRNA level were studied and measured at the macroscopic level of densitometric analysis of X-ray film and at the microscopic level by counting of autoradiographic silver grains in striatal cells. Macroscopic analysis demonstrated that haloperidol treatment two times 1 mg/kg per day during seven, 14 and 21 days increased D2 receptor mRNA level in the caudate-putamen. Detailed microscopic analysis demonstrated a significant increase in D2 receptor mRNA in the two neuronal populations known to express the D2 receptor gene: medium-sized neurons previously identified as enkephalinergic neurons, and large-sized neurons previously identified as cholinergic neurons. The increase was more important in cholinergic neurons (+119%) than in enkephalinergic neurons (+54%). Haloperidol treatment did not modify the number of medium-sized enkephalinergic neurons expressing the D2 receptor mRNA. In contrast, it significantly increased the percentage of large-sized neurons containing D2 receptor mRNA (from 80 to 94%). These results demonstrate that haloperidol treatment acts at the gene level to modulate D2 receptor content in striatal dopaminoceptive neurons, and that the D2 receptor mRNA increase in postsynaptic neurons contributes to dopamine supersensitivity induced by neuroleptics in the rat. This suggests that dopamine acts trans-synaptically to control D2 receptor gene expression in target striatal neurons. These results suggest that modifications of D2 receptor gene expression may be part of the biological events that lead to the movement disorders induced by neuroleptic drugs or Parkinson's disease.

Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins, guanosine-5'-triphosphate and adenosine-3':5'-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 microgram/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 microM) in 77% of the cells studied. Addition of guanosine-5'-O-(2-thiodiphosphate) (500 microM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5'-O-(3-thiotriphosphate) (100 microM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition to guanosine-5'-O-(3-thiotriphosphate), a high concentration of adenosine-3':5'-cyclic-monophosphate (100 microM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100 microM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.

Another mechanism for creating diversity in gamma-aminobutyrate type A receptors: RNA splicing directs expression of two forms of gamma 2 phosphorylation site

Diversity of gamma-aminobutyrate type A (GABAA) receptors has recently been proposed to be achieved by assembly of receptor subtypes from a multitude of subunits (alpha 1-6, beta 1-3, gamma 1-2, and delta) encoded by different genes. Here we report a further mechanism for creating GABAA receptor diversity: alternative RNA splicing. Two forms of bovine gamma 2 subunit cDNA were isolated (gamma 2S and gamma 2L) that differed by the presence or absence of a 24-base-pair (8-amino acid) insertion in the cytoplasmic domain between the third and fourth putative membrane-spanning regions. Polymerase chain reaction from RNA demonstrated that the two forms of gamma 2 subunit are expressed in bovine, human, and rat brain. Sequencing of genomic DNA clones encoding the gamma 2 subunit demonstrated that the 24-base-pair insert is organized as a separate exon. Analysis of the sequence of the 8-amino acid insert revealed that it contains a protein kinase C consensus phosphorylation site. Expression of the large cytoplasmic loop domains of gamma 2S and gamma 2L in Escherichia coli, followed by phosphorylation of the recombinant proteins by protein kinase C, demonstrated that gamma 2L, but not gamma 2S, could be phosphorylated. Thus the two forms of gamma 2 subunit differ by the presence or absence of a protein kinase C phosphorylation site. This mechanism for creating GABAA receptor diversity may allow differential regulation of the function of receptor subtypes.

Structure and function of G protein coupled receptors

The G protein coupled receptors (GPC-Rs) comprise a large superfamily of genes encoding numerous receptors which all show common structural features, e.g., seven putative membrane spanning domains. Their biological functions are extremely diverse, ranging from vision and olfaction to neuronal and endocrine signaling. The GPC-Rs couple via multiple G proteins to a growing number of recognized second messenger pathway, e.g., cAMP and phosphatidyl inositol turnover. This review summarizes our current knowledge of the molecular mechanisms by which the GPC-Rs activate second messenger systems, and it addresses their regulation and structure.

Both pertussis toxin-sensitive and insensitive g-proteins link melatonin receptor to inhibition of adenylate cyclase in the ovine pars tuberalis

Bordetella pertussis toxin (islet activating protein, IAP) has been used to investigate the G-proteins involved in mediating the action of the melatonin receptor. Melatonin inhibits iorskolin-stimulated cyclic AMP production in ovine pars tuberalis (PT) cells. In cells treated with IAP for 16 h this response is attenuated in a dose-dependent manner, but not abolished. IAP catalyses the incorporation of [(32) P-ADP]ribose into a 41 kd protein present in PT membranes, but this labelling can be reduced if PT cells are preincubated with IAP for 16 h. Treatment of crude membrane preparations with IAP (20 /ig/ml) suppresses the binding of 2-[(125) l]iodomelatonin by 20%, whereas 1 mM GTP alone reduces binding by 40%, and in combination with IAP its effect is additive (60% inhibition). Therefore, these results indicate that the melatonin receptor acts via two G-proteins, one pertussis toxin-sensitive and the other pertussis toxin-insensitive.

Molecular analysis of human argininosuccinate lyase: mutant characterization and alternative splicing of the coding region

Argininosuccinic acid lyase (ASAL) deficiency is a clinically heterogeneous autosomal recessive urea cycle disorder. We previously established by complementation analysis that 28 ASAL-deficient patients have heterogeneous mutations in a single gene. To prove that the ASAL structural gene is the affected locus, we sequenced polymerase chain reaction-amplified ASAL cDNA of a representative mutant from the single complementation group. Fibroblast strain 944 (approximately 1% of residual ASAL activity), from a late-onset patient who was the product of a consanguineous mating, had only a single base-pair change in the coding region, a C-283----T transition at a CpG dinucleotide in exon 3. This substitution converts Arg-95 to Cys (R95C), occurs in a stretch of 13 residues that is identical in yeast and human ASAL, and was present in both of the patient's alleles but not in 14 other mutant or 10 normal alleles. Expression in COS cells demonstrated that the R95C mutation produces normal amounts of ASAL mRNA but little protein and less than 1% ASAL activity. We observed that amplified cDNA from mutant 944 and normal cells (liver, keratinocytes, lymphoblasts, and fibroblasts) contained, in addition to the expected 5' 513-base-pair band, a prominent 318-base-pair ASAL band formed by the splicing of exon 2 from the transcript. The short transcript maintains the ASAL reading frame but removes Lys-51, a residue that may be essential for catalysis, since it binds the argininosuccinate substrate. We conclude (i) that the identification of the R95C mutation in strain 944 demonstrates that virtually all ASAL deficiency results from defects in the ASAL structural gene and (ii) that minor alternative splicing of the coding region occurs at the ASAL locus.

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.

Subclassification of peripheral dopamine receptors

1. There is convincing evidence to suggest that two DA-receptor subtypes exist in the periphery, that can at best be differentiated by the potency of the antagonists SCH 23390 and domperidone: at DA-1 receptors the order of potency is SCH 23390 much greater than domperidone, at DA-2 receptors it is domperidone much greater than SCH 23390. 2. As in the adrenergic, muscarinic or 5-HT-receptor field, future studies employing both molecular biology and pharmacological approaches will show whether such a subclassification is sufficient to explain all peripheral effects of DA or whether DA-1 and DA-2 receptors have to be subdivided into further subtypes.

Tissue distribution and quantitation of the mRNAs for three rat tachykinin receptors

The family of mammalian tachykinin receptors consists of substance P receptor (SPR), neuromedin K receptor (NKR) and substance K receptor (SKR). In this investigation, tissue and regional distributions of the mRNAs for the three rat tachykinin receptors were investigated by blot-hybridization and RNase-protection analyses using the previously cloned receptor cDNAs. SPR mRNA is widely distributed in both the nervous system and peripheral tissues and is expressed abundantly in the hypothalamus and olfactory bulb, as well as in the urinary bladder, salivary glands and small and large intestines. In contrast, NKR mRNA is predominantly expressed in the nervous system, particularly in the cortex, hypothalamus and cerebellum, whereas SKR mRNA expression is restricted to the peripheral tissues, being abundant in the urinary bladder, large intestine, stomach and adrenal gland. Thus, the mRNAs for the three tachykinin receptors show distinct patterns of expression between the nervous system and peripheral tissues. Blot-hybridization analysis in combination with S1 nuclease protection and primer-extension analyses revealed that there are two large forms of SKR mRNA expressed commonly in the peripheral tissues, and two additional small forms of the mRNA expressed specifically in the adrenal gland and eye. These analyses also showed that the multiple forms of SKR mRNA differ in the lengths of the 5' mRNA portions, and that the two small forms of the mRNA, if translated, encode a truncated SKR polypeptide lacking the first two transmembrane domains. This investigation thus provides the comprehensive analysis of the distribution and mode of expression of the mRNAs for the multiple peptide receptors and offers a new basis on which to interpret the diverse functions of multiple tachykinin peptides in the CNS and peripheral tissues.

D2 dopamine receptor gene expression by cholinergic neurons in the rat striatum

In situ hybridization with D2 receptor probe and immunohistochemistry with choline acetyltransferase (ChAT) antibody performed on adjacent sections demonstrate dopamine D2 receptor gene expression in cholinergic neurons of the rat caudate-putamen and nucleus accumbens. Eighty per cent of cholinergic neurons in the striatum contain detectable D2 receptor mRNA. The other neurons without detectable D2 mRNA do not display specific localization or aspect in the caudate-putamen and nucleus accumbens as compared to the other cholinergic neurons. The absence of detectable D2 mRNA in certain cholinergic neurons can be due to the limited sensitivity of the procedure that would not detect low mRNA levels, or alternatively can reflect the existence of two cholinergic cell populations in the striatum, one of which would not express the D2 receptor gene. The other forebrain cholinergic neurons do not contain D2 mRNA.

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.

Dopamine receptor subtypes: beyond the D1/D2 classification

The D1/D2 dopamine receptor classification is widely accepted. However, intense investigative efforts over the last several years using pharmacological, biochemical and behavioral approaches have produced results that are increasingly difficult to reconcile with the existence of only two dopamine receptor subtypes. Recent developments, including cloning of the cDNAs and/or genes for several members of the large family of G-protein-coupled receptors, have revealed that heterogeneity in the pharmacological or biochemical characteristics of individual receptors often indicates the presence of previously unsuspected molecular subtypes. In this article, Marc Caron and colleagues have assembled the main lines of evidence that suggest the presence of several novel subtypes for both D1 and D2 dopamine receptors and predict that molecular cloning will, in the near future, confirm their existence.