A novel short isoform of the D3 dopamine receptor generated by alternative splicing in the third cytoplasmic loop

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Dopaminergic regulation of dopamine D3 and D3nf receptor mRNA expression

Dopamine D3 receptors have the highest dopamine affinity of all dopamine receptors, and may thereby regulate dopamine signaling mediated by volume transmission. Changes in D3 receptor isoform expression may alter D3 receptor function, however, little is known regarding coordination of D3 isoform expression in response to perturbations in dopaminergic stimulation. To determine the effects of dopamine receptor stimulation and blockade on D3 receptor alternative splicing, we determined D3 and D3nf isoform mRNA expression following treatment with the D3 receptor antagonist NGB 2904, and the indirect dopamine agonist amphetamine. Expression of tyrosine hydroxylase (TH) mRNA, the rate-limiting enzyme in dopamine synthesis, was also determined. The D3/D3nf mRNA expression ratio was increased in ventral striatum, prefrontal cortex, and hippocampus 6 h following D3 antagonist NGB 2904 treatment, and remained persistently elevated at 24 h in hippocampus and substantia nigra/ventral tegmentum. D3 mRNA decreased 65% and D3nf mRNA expression decreased 71% in prefrontal cortex 24 h following amphetamine treatment, however, these changes did not reach statistical significance. TH mRNA expression was unaffected by D3 antagonist NGB 2904, but was elevated by amphetamine in ventral striatum, hippocampus, and prefrontal cortex. These findings provide evidence for an adaptive response to altered D3 receptor stimulation involving changes in D3 receptor alternative splicing. Additionally, these data suggest D3 autoreceptor regulation of dopamine synthesis does not involve regulation of TH mRNA expression. Finally, the observation of regulated TH mRNA expression in dopamine terminal fields provides experimental support for the model of local control of mRNA expression in adaptation to synaptic activity.

Genes encoding putative biogenic amine receptors in the parasitic nematode Brugia malayi

Filarial nematodes, such as Brugia malayi, cause major health problems worldwide. The lack of a vaccine against B. malayi, combined with ineffective chemotherapy against the adult has prompted the examination of biogenic amine receptors (BARs) as possible targets for drug discovery. We employed bioinformatics to identify genes encoding putative B. malayi BARs. Surprisingly, the B. malayi genome contains half of the genes predicted to encode BARs in the genomes of free-living nematodes such as Caenorhabditis elegans or C. briggsae; however, all of the predicted B. malayi receptors have clear orthologues in C. elegans. The B. malayi genes encode each of the major BAR subclasses, including three serotonin, two dopamine and two tyramine/octopamine receptors and the structure of orthologous BAR genes is conserved. We find that potential G-protein coupling and ligand-specificity of individual BARs may be predicted by phylogenetic comparisons. Our results provide a framework for how G-protein coupled receptors may be targeted for drug development in medically important parasitic nematodes.

Arterial blood pressure and renal sodium excretion in dopamine D3 receptor knockout mice

Alterations in the dopaminergic system may contribute to the development of hypertension. Recently, it has been reported that pentobarbital-anesthetized mice with deficient dopamine D(3) receptors showed renin-dependent elevation in blood pressure. In a series of experiments, we evaluated the contribution of the dopamine D(3) receptor to the renal sodium excretion and arterial blood pressure behavior in conscious as well as anesthetized dopamine D(3) receptor knockout (-/-) mice. The blood pressure measuring study was designed as a cross-over trial to investigate the influence of different sodium loads. The animals were fed a normal salt diet (0.6% NaCl, NS) for 1 week and afterwards a low (0.2% NaCl, LS) or a high salt diet (4.6% NaCl, HS) for 2 weeks. After the third week, the animals were switched to the corresponding protocol. Systolic blood pressure in conscious (-/-) mice measured by tail-cuff plethysmography was not different from that of wild-type (+/+) animals, irrespective of the time course or the salt diet. In another experiment, challenge of an acute sodium loading per gavage in conscious D(3) receptor (-/-) and (+/+) animals on HS or NS diet did not show significant differences in renal sodium excretion between the two genotypes. Additionally, animals were fed an NS diet for 1 week and an HS diet for another week. As expected, sodium excretion significantly increased after the change from the NS to the HS diet. A slightly lower urinary sodium excretion was observed when comparing D(3) receptor (-/-) mice to their corresponding (+/+) mice, both on an HS diet. Clearance experiments with anesthetized D(3) receptor (-/-) and (+/+) mice were performed to investigate the renal sodium excretion capacity, when exposed to a moderate volume expansion (VE). Urinary sodium excretion increased in response to the VE; however, no difference were observed between the two genotypes. Taking these results together, we conclude that in the present animal model renal dopamine D(3) receptors are not significantly involved in the regulation of blood pressure associated with a deficiency in renal sodium elimination.

Behavioral sensitization, alternative splicing, and d3 dopamine receptor-mediated inhibitory function

Behavioral sensitization, the progressive and enduring augmentation of certain behaviors following repetitive drug use, alters rodent locomotion in a long-standing manner. The same dopamine pathways playing an important role in drug dependence and psychosis also play a critical role in sensitization. Individual dopamine receptor subtypes have markedly different functional responses to stimulation, with D3 dopamine receptor stimulation inhibiting rodent locomotion. The D3 receptor has highest affinity of the dopamine receptor subtypes for dopamine, and is occupied to a greater degree following stimulant drug administration. D3 receptor activity may be regulated through the expression of an alternatively spliced, truncated receptor isoform (termed 'D3nf') altering receptor localization and function via dimerization with the full-length subunit. The expected physiological response to repetitive drug administration is tolerance. Tolerance of D3 receptor inhibition of locomotion would contribute to sensitization to stimulant drugs. We hypothesize that repetitive D3 receptor stimulation contributes to the development of behavioral sensitization through decreased responsivity of D3-receptor-mediated locomotor inhibition. Increased D3nf expression may direct altered receptor localization and subsequent release of D3-receptor-mediated inhibition, contributing to the expression of sensitization. These hypotheses follow directly from the affinities of the receptor subtypes for dopamine; dopamine concentrations following stimulant administration; the effects of individual dopamine receptor subtype stimulation on locomotion; and the expected homeostatic response of the system to perturbation by drug. Clarifying these mechanisms underlying sensitization may suggest new interventions for neuropsychiatric conditions in which dopamine plays an important role, including psychosis, drug dependence, and Parkinson's disease. This information may also elucidate a previously unrecognized mechanism regulating receptor trafficking and desensitization.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Relative expression of D3 dopamine receptor and alternative splice variant D3nf mRNA in high and low responders to novelty

Studies in rodents suggest an important role for the D3 dopamine receptor in regulating locomotor responses to spatial novelty and psychostimulants. The D3 receptor alternatively spliced variant D3nf produces a non-dopamine binding protein that may alter D3 receptor localization by dimerizing with the full-length receptor. In the high responder/low responder (HR/LR) model, the locomotor response to an inescapable, novel spatial environment predicts individual differences in the locomotor and rewarding effects of psychostimulants. We hypothesized that individual differences in D3 receptor expression could contribute to individual differences in the locomotor response to novelty in the HR/LR model. To test this hypothesis, we screened rats for response to a novel spatial environment and analyzed brain tissue for mRNA levels of the D3 receptor and D3nf by real-time RT-PCR. The ratios of D3/D3nf mRNA in prefrontal cortex and substantia nigra/ventral tegmentum were significantly lower in HRs than in LRs. There were no differences in relative expression of D3/D3nf between HRs and LRs in nucleus accumbens. These data further support a role for the D3 dopamine receptor in behavioral responses to novelty and, given the established relationship between novelty and psychostimulant responses, suggest that the D3 receptor may be an important target for assessment of drug abuse vulnerability. Additionally, these findings are consistent with the hypothesis that alternative splicing may contribute to regulation of D3 dopamine receptor function.

Expression of D2 dopamine receptor in the mouse brain

The neurotransmitter, dopamine, binds to dopamine receptor (DR), and is involved in several functions of the brain, such as initiation and execution of movement, emotion, prolactin secretion, etc. Of all the five DRs, D2 dopamine receptor has maximal affinity for dopamine. D2 has a short isoform, D2S, and a long isoform D2L. D2L is longer than D2S by 29 amino acid residues. We studied the expression of the gene and protein of D2 receptor in the cerebral and cerebellar cortices of the brain of new born, developing, adult, and old male mice to find out: (i) at what stage of development, expression of the gene peaks and (ii) if it undergoes any changes as the animal ages, which may account for the neurodegenerative changes and symptoms of Parkinson's and other diseases seen in old age. RT-PCR and Western blot studies show that peak expression of D2 gene occurs in the cerebral and cerebellar cortices around 15-day after birth. We speculate that the majority of dopaminergic synapses are established and possibly become functional in the brain around 15-day after birth. The expression of D2 receptor is upregulated in the cerebral cortex in old mice. However, it is down-regulated in the cerebellar cortex.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Differential expression of D2-like dopamine receptors in the kidney of the spontaneously hypertensive rat

OBJECTIVE

To compare the expression and cellular distribution of D(2)-like dopamine receptors in the kidney of the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat.

DESIGN

Renal D(2)-like receptor protein expression and distribution has not been studied in the SHR. Since changes in D(2)-like receptor expression and/or distribution may contribute to the dysregulation of renal dopamine and D(1A) receptor function, we examined the expression of the three subtypes of D(2)-like receptors (D(2), D(3) and D(4)) in SHR and WKY rat kidneys.

METHODS

Western blot analysis and confocal immunocytochemistry with specific polyclonal antipeptide antibodies directed against the receptor subtypes, were used to assess protein expression.

RESULTS

There were no differences in protein expression and cellular immunolocalization of the D(2) receptor subtypes between SHR and WKY rats. Expression of the 50 kDa D(3) receptor was reduced in the cortex of the SHR; no differences in D(3) receptor levels were seen in the inner medulla of SHR and WKY rats. The D(4) receptor polypeptides were overexpressed in the cortex of SHR, while in the inner medulla no difference in expression of the D(4) receptor proteins was observed between SHR and WKY rats. Immunocytochemistry also showed increased immunostaining of D(4) receptors in tubular structures in the cortex, but diminished staining in the SHR inner medulla.

CONCLUSION

The observed differences in expression and distribution of D(3) and D(4) dopamine receptors between cortex and inner medulla of the kidneys of SHR and WKY rats may contribute to the aberrant state of dopaminergic-mediated natriuresis in SHR.

Expression of dopamine receptors and transporter in neuroendocrine gastrointestinal tumor cells

C-11- or F-18-DOPA positron emission tomography (DOPA PET) is a new sensitive imaging technique for small neuroendocrine gastrointestinal tumors which evaluates the decarboxylase activity. To further characterize the dopaminergic system in neuroendocrine gastrointestinal tumor cells, we investigated the expression of both dopamine receptors and the transmembrane dopamine transporter (DAT) in the human neuroendocrine pancreatic cell line BON and in the neuroendocrine gut cell line STC-1. Both BON and STC-1 cells expressed mRNA of the dopamine receptors D2-D5 and DAT. mRNA of the dopamine receptor D1 was detected in BON cells only. Both in BON and STC-1 cells, expression of D2 and D5 receptors and DAT was also demonstrated immunocytochemically. For functional receptor characterization intracellular cAMP levels ([cAMP]i) were determined. Whereas in STC-1 cells dopamine and the D1-like (D1/D5) receptor agonist SKF 38393 increased [cAMP]i, [cAMP]i was decreased by dopamine or the D2-like (D2-D4) receptor agonist quinpirole in BON cells. Functional DAT activity was, however, not detected in either cell line. The presence of both dopamine receptors and of the DAT suggests an autocrine and/or paracrine function of dopamine in neuroendocrine gastrointestinal tumor cells. Yet neither the transmembrane dopamine transporter nor dopamine receptors are likely to contribute to positive DOPA PET imaging of neuroendocrine gastrointestinal tumors. However, these molecules may be of diagnostic importance when applying other dopaminergic system tracers.

D2/D3 dopamine receptor heterodimers exhibit unique functional properties

Evidence for heterodimerization has recently been provided for dopamine D(1) and adenosine A(1) receptors as well as for dopamine D(2) and somatostatin SSTR(5) receptors. In this paper, we have studied the possibility that D(2) and D(3) receptors interact functionally by forming receptor heterodimers. Initially, we split the two receptors at the level of the third cytoplasmic loop into two fragments. The first, containing transmembrane domains (TM) I to V and the N-terminal part of the third cytoplasmic loop, was named D(2trunk) or D(3trunk), and the second, containing the C-terminal part of the third cytoplasmic loop, TMVI and TMVII, and the C-terminal tail, was named D(2tail) or D(3tail). Then we defined the pharmacological profiles of the homologous (D(2trunk)/D(2tail) and D(3trunk)/D(3tail)) as well as of the heterologous (D(2trunk)/D(3tail) and D(3trunk)/D(2tail)) cotransfected receptor fragments. The pharmacological profile of the cross-cotransfected fragments was different from that of the native D(2) or D(3) receptors. In most cases, the D(3trunk)/D(2tail) was the one with the highest affinity for most agonists and antagonists. Moreover, we observed that all of these receptor fragments reduced the expression of the wild type dopamine D(2) and D(3) receptors, suggesting that D(2) and D(3) receptors can form complexes with these fragments and that these complexes bind [(3)H]nemonapride less efficiently or are not correctly targeted to the membrane. In a second set of experiments, we tested the ability of the split and the wild type receptors to inhibit adenylyl cyclase (AC) types V and VI. All of the native and split receptors inhibited AC-V and AC-VI, with the exception of D(3), which was unable to inhibit AC-VI. We therefore studied the ability of D(2) and D(3) to interact functionally with one another to inhibit AC-VI. We found that with D(2) alone, R-(+)-7-hydroxydypropylaminotetralin hydrobromide inhibited AC-VI with an IC(50) of 2.05 +/- 0.15 nm, while in the presence of D(2) and D(3) it inhibited AC-VI with an IC(50) of 0.083 +/- 0.011 nm. Similar results were obtained with a chimeric cyclase made from AC-V and AC-VI. Coimmunoprecipitation experiments indicate that D(2) and D(3) receptors are capable of physical interaction.

Characterization of the 5' flanking region of the rat D(3) dopamine receptor gene

The D(3) dopamine receptor has a restricted regional distribution in brain and is regulated by dopaminergic agents. Additionally, the D(3) gene is implicated in the pathogenesis of several neuropsychiatric disorders or in their response to pharmacological agents. Elucidating its transcription control mechanisms is therefore of interest in order to explain these biological features of the D(3) gene. In this study, the 5' flanking region of the rat D(3) gene was characterized by isolating the 5' end of its cDNA as well as 4.6 kb of genomic sequence. Analysis of this region revealed the presence of two new exons 196-bp and 120-bp long, separated by an 855-bp intron, located several kilobases upstream of the previously published coding exons. Thus, current evidence indicates that the rat D(3) gene is organized into eight exons. Transcription initiation site was determined by primer extension analysis and repeated rounds of 5' RACE and was found to localize at a pyrimidine-rich consensus 'initiator' sequence, similar to the rat D(2) gene. The D(3) promoter lacks TATA or CAAT boxes but unlike that of other dopamine receptor genes has only 52% GC content. Functional analysis of D(3) promoter deletion mutants fused to a reporter gene in TE671 cells, which endogenously express this gene, revealed strong transcriptional activity localized within 36 nucleotides upstream of transcription start site, and a potent silencer between bases --37 and --537. The D(3) promoter is inactive in C6 and COS7 cells. We conclude that the D(3) gene, similar to the closely related D(2) gene, is transcribed from a tissue specific promoter which is under intense negative control.

Pharmacology of [3H]R(+)-7-OH-DPAT binding in the rat caudate-putamen

Dopamine D3 receptors may be involved in drug addiction and in disorders such as schizophrenia and Parkinson's disease. To determine the pharmacological properties of dopamine D3 receptors in the rat caudate-putamen, we have investigated R(+)-[3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]R(+)-7-OH-DPAT) binding to membrane preparations from the rat caudate-putamen. Kinetic analyses showed that [3H]R(+)-7-OH-DPAT binding reached equilibrium in approximately 1 h and that both association and dissociation curves were composed of at least two components. Likewise, saturation curves showed at least two binding components with a combined Bmax value of about 600 fmol/mg protein, which is three times higher than what is present in the subcortical limbic area. Competition curves were performed with agonists such as R(-)-propylnorapomorphine, dopamine, PD 128907, quinpirole, and bromocriptine, and antagonists such as haloperidol, raclopride, clozapine, GR 218231x, remoxipride, and U99194A. These experiments revealed that [3H]R(+)-7-OH-DPAT binding could be resolved into three specific binding sites (R1-R3) and one nonspecific binding site, with R1-R2 probably representing D3 receptor binding and the minor R3 representing D2 receptor binding. The low affinities of (+/-)-8-OH-DPAT and 1,3-di(2-tolyl)guanidine to inhibit [3H]R(+)-7-OH-DPAT binding indicate negligible involvement of 5-HT1A or sigma binding sites, respectively. The pharmacological profile of [3H]R(+)-7-OH-DPAT (2 nM) binding in the caudate-putamen was similar to that of dopamine on [125I]iodosulpride binding in the cerebellar lobule X, which contain D3 but not D2 receptors. Mg2+ increased and GTP and Na+ decreased the binding of [3H]R(+)-7-OH-DPAT, suggesting a coupling of endogenous D3 receptors to G proteins. Taken together, these results suggest that dopamine D3 receptors display multiple agonist binding states, and that D3 receptors are present in high concentrations in the rat caudate-putamen. These results may have implications for the physiological and pathological roles of dopamine D3 receptors in the brain.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

The splice variant D3nf reduces ligand binding to the D3 dopamine receptor: evidence for heterooligomerization

The D3 dopamine receptor belongs to the D2-like family of dopamine receptors. As with other members of this group, the D3 dopamine receptor gene contains introns which allow for alternative splicing of gene products. The best characterized of the human D3 dopamine receptor mRNA splice variants encodes a truncated protein called D3nf. The D3 dopamine receptor and D3nf were epitope-tagged and expressed in Sf9 insect cells by recombinant baculovirus infection. The D3 dopamine receptor showed saturable, high affinity binding of agonists and antagonists, consistent with reported D3 dopamine receptor pharmacology. When the D3 dopamine receptor and D3nf were co-expressed, the apparent density of D3 dopamine receptor expression, as determined by radioligand binding, was significantly lowered compared to D3 dopamine receptor expressed alone. This effect of D3nf was specific for the D3 dopamine receptor, since co-expression with the D2 dopamine receptor or beta2-adrenoceptor had no effect on binding. Confocal immunofluorescence studies were used to confirm that both D3 dopamine receptor and D3nf were well expressed on the cell surface and densitometric analysis of cell surface membrane protein confirmed that D3nf did not significantly alter the amount of D3 dopamine receptor expressed. Photoaffinity labelling with [125I]azidonemonapride showed that the amount of ligand bound by membranes co-expressing D3 dopamine receptor and D3nf was significantly less than that bound by membranes expressing D3 dopamine receptor alone. The greatest decrease in binding was observed in the D3 dopamine receptor oligomeric forms. Ligand binding to dimers and tetramers was reduced by 69 and 46%, respectively, indicating effects of a protein-protein interaction. Co-immunoprecipitation confirmed that the D3DR and D3nf interact with each other. These data indicate that D3nf heterodimerizes with the D3 dopamine receptor and decreases the capacity of D3 dopamine receptor to bind ligand.

Cloning and expression of the alpha9 nicotinic acetylcholine receptor subunit in cochlear hair cells of the chick

Hair cells of the vertebrate inner ear are subject to efferent control by the release of acetylcholine (ACh) from brainstem neurons. While ACh ultimately causes the hair cell to hyperpolarize through the activation of small conductance Ca(2+)-activated K(+) channels, the initial effect is to open a ligand-gated cation channel that briefly depolarizes the hair cell. The hair cell's ligand-gated cation channel has unusual pharmacology that is well matched to that of the nicotinic subunit alpha9 expressed in Xenopus oocytes. We used sequence-specific amplification to identify the ortholog of alpha9 in the chick's cochlea (basilar papilla). Chick alpha9 is 73% identical to rat alpha9 at the amino acid level. A second transcript was identified that differed by the loss of 132 base pairs coding for 44 amino acids near the putative ligand-binding site. RT-PCR on whole cochlear ducts suggested that this short variant is less abundant than the full length alpha9 mRNA. In situ hybridization revealed alpha9 mRNA in sensory hair cells of the chick cochlea. The pattern of expression was consistent with the efferent innervation pattern. The alpha9 label was strongest in short (outer) hair cells on which large calyciform efferent endings are found. Tall (inner) hair cells receiving little or no efferent innervation had substantially less label. The cochlear ganglion neurons were not labeled, consistent with the absence of axo-dendritic efferent innervation in birds. These findings suggest that alpha9 contributes to the ACh receptor of avian hair cells and supports the generality of this hypothesis among all vertebrates.

Structure and function of dopamine receptors

Dopamine (DA) is the most abundant catecholamine in the brain. The involvement and importance of DA as a neurotransmitter in the regulation of different physiological functions in the central nervous system (CNS) is well known. Deregulation of the dopaminergic system has been linked with Parkinson's disease, Tourette's syndrome, schizophrenia, attention deficit hyperactive disorder (ADHD) and generation of pituitary tumours. This review focuses on the pharmacological and biochemical features shared by the dopamine receptors. We address their coupling to secondary messenger pathways and their physiological function based upon studies using pharmacological tools, specific brain lesions and, more recently, genetically modified animal models.

Comparison of the ability of dopamine receptor agonists to inhibit forskolin-stimulated adenosine 3'5'-cyclic monophosphate (cAMP) accumulation via D2L (long isoform) and D3 receptors expressed in Chinese hamster ovary (CHO) cells

The pharmacological properties of the human D2L (long isoform) and rat D3 dopamine receptors in functional assays were examined. A range of dopamine agonists were assessed for their ability to inhibit adenosine 3'5'-cyclic monophosphate (cAMP) accumulation via the two receptors expressed stably in Chinese hamster ovary cells. Dopamine caused a significantly greater maximal inhibition (P < 0.05) of cAMP accumulation via the D2L receptor (approximately 70%) as compared to the D3 receptor (approximately 50%). The pattern of agonist effects was different at the two receptors. The absolute and relative potencies for inhibition of cAMP accumulation were different for a range of agonists acting at the two receptors. Similarly, the maximal inhibitions achieved by a range of agonists were different for the two receptors.

Potentiation of the D2 mutant motor phenotype in mice lacking dopamine D2 and D3 receptors

Within the D2-class of dopamine receptors, the D2 and D3 subtypes share the highest degree of similarity in their primary structure. However, the extent to which these two receptor subtypes have similar or different functional properties is unclear. The present study used gene targeting to generate mice deficient for D2, D3, and D2/D3 receptors. A comparative analysis of D2 and D3 single mutants and D2/D3 double mutants revealed that D2/D3 double mutants develop motor phenotypes that, although qualitatively similar to those seen in D2 single mutants, are significantly more severe. Furthermore, increased levels of the dopamine metabolites dihydroxyphenyl acetic acid and homovanillic acid are found in the dorsal striatum of D2 single mutants. The levels of these metabolites, however, are significantly higher in mice lacking D2 and D3 receptors. In addition, results of immunoprecipitation experiments revealed that D2 single mutants express higher levels of D3 receptor proteins during later stages of their postnatal development. These results suggest that D3 receptors compensate for some of the lacking D2 receptor functions and that these functional properties of D3 receptors, detected in mice with a D2 mutant genetic background, remain masked when the abundant D2 receptor is expressed.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

7TM receptors: the splicing on the cake

Within a given family of seven transmembrane domain (7TM) receptors, functional diversity is most often afforded by the existence of multiple receptor subtypes, each encoded by a distinct gene. However, it is now clear that the existence of introns in genes encoding some members of a receptor family provides scope for additional diversity by virtue of splicing events that result in the formation of different receptor mRNAs and consequently distinct receptor isoforms. A large number of 7TM receptor splice variants have now been shown to exist. In this article, the current data on alternatively spliced variants for hormone and neurotransmitter 7TMs are reviewed, their potential physiological importance considered and some of the issues pertaining to the classification and nomenclature of receptor isoforms produced in this way are addressed.

Human D3 dopamine receptor in the medulloblastoma TE671 cell line: cross-talk between D1 and D3 receptors

In search of a cell line in which the D3 dopamine receptor is expressed endogenously, we found that the neuron-derived human medulloblastoma cell line TE671 expresses the human D3 (hD3) and D1 (hD1) receptor, but neither the D2 or D4 receptors. Exposure of TE671 cells to the D3 agonist 7-OH-DPAT (DPAT), or to the D1 agonist SKF-38393 (SKF) increased the expression of hD3 or hD1 mRNA, respectively. Moreover, whereas DPAT had no effect on hD1 mRNA levels, stimulating the cells with SKF caused an increase in both hD1 and hD3 transcript levels. These results suggest (i) that following ligand stimulation, hD3 and hD1 receptors are upregulated to enhance their own receptor expression, and (ii) that upregulation of hD1 receptor transcripts leads to a stimulation of the hD3 dopamine receptor transcripts.

Development and characterization of antibodies directed against the mouse D4 dopamine receptor

Polyclonal antibodies against the mouse D4 dopamine receptor have been developed in order to investigate the anatomical localization of this receptor in the mouse brain. Two antibodies were generated against specific peptides corresponding to predicted extracellular and intracellular regions of the D4 protein. Specificity of these antibodies was demonstrated on human embryonic kidney 293 (HEK 293) cells transfected with different dopamine receptor subtypes; immunoreactivity was detected only in cells transfected with the mouse D4 dopamine receptor cDNA. Following in vitro transcription/translation of the mouse D4 cDNA, a single protein band of 36 kDa was selectively immunoprecipitated with the anti-D4 antibodies. The antibodies also detected a single protein of 36 kDa in Western blot of HEK 293 cells transiently transfected with the mouse D4 receptor. These antibodies were able to detect the D4 receptor in several regions of the mouse brain. In the regions examined, D4 immunoreactivity was found in neurones located in layers II-VI of the frontal and piriform cortices, with the highest concentration in layer II; in scattered neurones in the caudate putamen and in larger neurones in the globus pallidus. In all experiments, both antibodies exhibit the same specificity, and all immunoreactivity could be abolished by preincubation with the corresponding peptide antigen.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Functional expression of the murine D2, D3, and D4 dopamine receptors in Xenopus laevis oocytes

The different murine D2-type dopamine receptors (D2L, D2S, D3L, D3S, and D4) were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficiently expressed in Xenopus oocytes and were shown to bind the D2 antagonist [125I]sulpride. They were all shown to activate Cl- influx upon agonist stimulation. Using the diagnostic inhibitor bumetanide, we were able to separate the Na+/K+/2Cl- cotransporter component of the Cl- influx from the total unidirectional Cl- influx. The D3L subtype was found to operate exclusively through the bumetanide-insensitive Cl- influx whereas the other D2-type receptors acted on the Na+/K+/2Cl- cotransporter as well. The pertussis toxin sensitivity of the receptor-activated chloride influx via the Na+/K+/2Cl- cotransporter varied between the various D2-type receptors showing that they may couple to different G proteins, and activate different second messenger systems.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

The preferential dopamine D3 receptor agonist cis-8-OH-PBZI induces limbic Fos expression in rat brain

The affinity, selectivity and agonistic properties of a constrained dopaminergic compound, the benz[e]indole cis-8-hydroxy-3-(n-propyl)1,2,3a.4,5,9b-hexahydro-1H-benz[e]indole (cis-8-OH-PBZI), for the dopamine D3 receptor were evaluated in competition binding experiments with cloned human dopamine receptor subtypes and, to further extend its profile, in in vitro radioligand binding assays. The Ki value measured for competition binding of this compound to the dopamine D3 receptor was 27.4+/-3.1 nM; this was 775-fold, 550-fold, 90-fold and 10-fold higher affinity than that measured at dopamine D1A, D5, D2s and D4 receptors, respectively. Evidence of dopamine receptor activation by cis-8-OH-PBZI was obtained by measuring dose-dependent increases in extracellular acidification rates and decreases in cAMP synthesis. In vivo, cis-8-OH-PBZI potently induced Fos protein immunoreactivity in the rat medial prefrontal cortex and shell region of the nucleus accumbens, but only marginally in the motor dorsolateral striatum, indicating a selective limbic site of action. In conclusion, the present data identify cis-8-OH-PBZI as having preference for the dopamine D3 receptor in vitro, and as having dopamine agonist activity and limbic sites of action in vivo.

Molecular characteristics of mammalian dopamine receptors

Dopamine receptors belong to a large super-gene family of receptors which are linked to their signal transduction pathways through heterotrimeric G proteins. A variety of signalling events are known to be regulated by dopamine receptors including adenylate cyclase and phospholipase activities and various ion channels. Prior to the advent of molecular cloning technology, dopamine receptors were believed to belong to two subtypes, D1 and D2. This distinction was based on both pharmacological and functional criteria. We now know that at least five different dopamine receptors exist although they can still be described as to belonging within "D1" and "D2" subfamilies. The D1 subfamily consists of two receptors-the D1 and D5, whereas the D2, D3 and D4 receptors comprise the D2 subfamily. The cloning and molecular characteristics of these five receptors are described in this review.

DAD1- and DAD2-like agonist effects on motor activity of C57 mice: differences compared to rats

Studies on rats indicate that DAD1- and DAD2-like agonists produce a biphasic action on motor activity, with low doses reducing activity below control levels, and higher doses initially reducing, then elevating, activity for a prolonged period. Although some of the reported effects of DAD1- and DAD2-like receptor agonists on motor activity of mice are consistent with their effects on rats, the possibility of species differences is also apparent. In the current study the effects of DAD1- and DAD2-like agonists on motor activity of C57BL/6 (C57) mice were determined to establish species consistencies and differences with respect to their effects on rats. The partial DAD1-like agonist SKF 38393 reduced the activity of C57 mice at low doses and elevated activity above control levels at higher doses, if the mice were thoroughly habituated to the test chamber. The full DAD1 agonist SKF 82958 also increased the activity of C57 mice, and along with the SKF 38393 results indicates a response to DAD1 receptor stimulation similar to that reported for rats. In contrast to the species similarity in response to DAD1 stimulation, the DAD2-like agonist quinpirole produced only a dose-responsive monotonic reduction in the activity of C57 mice, whether the animals were nonhabituated or well-habituated to the testing environment, male or female, young or mid-aged, injected intraperitoneally (i.p.) or subcutaneously (s.c.), and with either low or high doses. This apparent species difference in response to quinpirole might reflect distinguishable functional properties of the DA subreceptor systems.

Antisense strategies in dopamine receptor pharmacology

Recent advances in molecular biology have provided pharmacologists the opportunity of developing an entirely new type of agent for studying and treating a variety of biological disorders. These agents, termed antisense oligodeoxynucleotides, have as their target the messenger RNAs encoding specific proteins. They act by binding to selected portions of these mRNAs through complimentary interactions and thereby prevent the synthesis of these proteins. These novel pharmacological tools have the promise of being easier to design and being more selective and predictable in their actions. In addition, insofar as agents targeted to receptors for neurotransmitters are concerned, unlike the classical pharmacological agents, these new compounds may not lead to the upregulation of the very receptors the drugs are designed to inhibit. The present review summarizes briefly studies on the effect of oligodeoxynucleotides antisense to the mRNAs encoding the various subtypes of the dopamine receptor. The studies show that oligodeoxynucleotides antisense to the D2 dopamine receptor when intracerebroventricularly into brains of rodents are rapidly taken up into the brain tissue, distributed to brain cells, and produce effects characteristic of highly selective D2 dopamine antagonists. The compounds also produced specific reductions in the levels of D2 dopamine receptor mRNA and D2 dopamine receptors. Similarly, injecting an antisense oligodeoxynucleotide targeted to the D1 dopamine receptor mRNA produces effects characteristic of D1 dopamine receptor antagonists. Other studies using these agents has produced evidence that there is a small pool of receptors that turn over very rapidly and which constitute the functional pool of these receptors. The evidence suggests further that antisense oligodeoxynucleotides inhibit the synthesis of this small functional pool of dopamine receptors, thereby providing an explanation of why there is often a discordance between changes in dopaminergic function and changes in the levels of dopamine receptors. Studies of antisense oligodeoxynucleotides targeted to the other subtypes of dopamine receptor may help reveal the biological roles that these and other newly discovered subtypes of neurotransmitter receptors have. They may also provide an entirely new and potentially more selective therapeutic regimen for altering the functions of these receptors.

Molecular attributes of dopamine receptors: new potential for antipsychotic drug development

The neurotransmitter dopamine (DA) plays a central role in the control of motor function, emotional states, and endocrine physiology. The discovery that schizophrenic symptoms can be alleviated by neuroleptic drugs and the finding that these drugs interact at dopamine receptors has indicated involvement of the dopamine system in schizophrenia. The dopamine system has recently been shown to rely on the activation of five distinct subtypes of DA receptors (D1-D5) identified by molecular cloning, and pharmacological studies have specifically implicated the D2-like receptors (D2R, D3R and D4R) in antipsychotic action. In addition, the localization of D3R and D4R expression in the mesolimbic/mesocortical DA pathways is consistent with their proposed involvement in affective behaviour, and suggests that drugs developed specifically for these receptor subtypes might have potent antipsychotic activity with a lower propensity for extrapyramidal, endocrine, and cognitive side-effects.

Early expression of D3 dopamine receptors in murine embryonic development

In order to determine whether the D2 and D3 dopamine receptors may have a role in prenatal development, we have studied the mRNA expression and distribution of these receptors during murine embryonic development. Using RT-PCR on RNA from embryos taken at progressive stages of development, we have shown that the D3 receptor is expressed significantly earlier than the D2 receptor, being detectable at day 9.5 post-conception (p.c.) compared with day 13.5 p.c. for the D2 subtype. We have also examined the mRNA distribution of the two receptors by whole mount in situ hybridisation. In agreement with the PCR assays, the D3 receptor was expressed earlier than the D2 subtype. D3 receptor transcripts were first detected at day 9.5 p.c. in the ventral aspect of the anterior neural tube, whereas D2 receptor transcripts first appeared a day later. By day 10.5-11.5 p.c. both D3 and D2 receptor transcripts were present in the developing forebrain, and later also in the branchial arches and along the prospective vertebral column. The early appearance of the D3 subtype in murine development and its predominance over the D2 subtype suggest that the D3 receptor may have a functional role in prenatal development.

Dopamine D3 receptor gene: organization, transcript variants, and polymorphism associated with schizophrenia

DNA fragments from a genomic library were used to establish the partial structure of the human dopamine D3 receptor gene (DRD3). Its coding sequence contains 6 exons and stretches over 40,000 base pairs. The complete DRD3 transcript and three shorter variants, in which the second and/or third exon are deleted, were detected in similar proportions in brains from four controls and three psychiatric patients. The Msp I polymorphism was localized in the fifth intron of the gene, 40,000 base pairs downstream the Bal I polymorphism and a PCR-based method was developed for genotyping this polymorphism. The distribution of the Msp I and Bal I genotypes were not independent in 297 individuals (chi 2 = 10.5, df = 4, P = 0.03), but only a weak association was found between allele 1 of the Bal I polymorphism and allele 2 of the Msp I polymorphism (chi 2 = 3.99, df = 1, P = 0.04). The previously reported association between homozygosity at both alleles of the Bal I polymorphism and schizophrenia was presently maintained in an extended sample, comprising 119 DSM-III-R chronic schizophrenics and 85 controls (chi 2 = 5.3, df = 1, P = 0.02) and found more important in mal than in females. The presence of the Bal I allele 2 is associated with an early age at onset, particularly in males (df = 35, t value = 2.6, P = 0.014). In the same sample, allelic frequencies, genotype counts, and proportion of homozygotes for the Msp I polymorphism did not differ between schizophrenics and controls (chi 2 = 0.06, df = 1, P = 0.80, chi 2 = 0.22, df = 1, P = 0.90 and chi 2 = 0.16, df = 1, P = 0.69, respectively). The large distance of the Msp I polymorphism from the Bal I polymorphism and its localization in the 3' part of the gene may explain the discrepant results obtained with the two polymorphisms.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Association study of dopamine D3 receptor gene and schizophrenia

Several groups have reported an association between schizophrenia and the MscI polymorphism in the first exon of the dopamine D3 receptor gene (DRD3). We studied this polymorphism using a North American sample (117 patients plus 188 controls) and an Italian sample (97 patients plus 64 controls). In the first part of the study, we compared allele frequencies of schizophrenia patients and unmatched controls and observed a significant difference in the total sample (P = 0.01). The second part of the study involved a case control approach in which each schizophrenia patient was matched to a control of the same sex, and of similar age and ethnic background. The DRD3 allele frequencies of patients and controls revealed no significant difference between the two groups in the Italian (N = 53) or the North American (N = 54) matched populations; however, when these two matched samples were combined, a significant difference was observed (P = 0.026). Our results suggest that the MscI polymorphism may be associated with schizophrenia in the populations studied.

Differential glycosylation and intracellular trafficking for the long and short isoforms of the D2 dopamine receptor

The D2 dopamine receptor exists in two alternatively spliced isoforms, "long" and "short" (D2L and D2S), which differ by 29 amino acids in the third cytoplasmic domain. The functional differences between these two isoforms are still obscure. We have performed pulse-chase studies on the D2L and D2S receptors expressed in CHO cells in order to follow the post-translational processing of the two isoforms. Both isoforms are present in three post-translational states: a newly synthesized protein, a partially glycosylated product, and a fully glycosylated mature 70-kDa receptor. However, the processing to the mature receptor differs between the two isoforms. First, the D2S receptor is processed to the mature 70-kDa species faster than the D2L receptor. Second, at 20 degrees C the D2S isoform is fully processed to the 70-kDa species, whereas the D2L isoform persists in its partially processed 45-kDa state. Finally, a significant portion of the D2L receptor remains in its partially processed form in an intracellular compartment and does not reach the plasma membrane. These results give rise to the suggestion that the difference observed between the two alternatively spliced isoforms of the D2 receptor may lie in their post-translational processing and intracellular trafficking.

Novel dopamine receptors half a decade later

Only half a decade ago, the effects of dopamine were all attributed to activation of two receptor subtypes, the D1 and D2, with opposing effects on adenylate cyclase, and for which apparently selective ligands were available. From the end of 1988, however, the application of homology cloning techniques starting from sequences of the seven transmembrane domain catecholamine receptors, particularly that of the D2 receptor, led to the identification of 'novel', previously uncharacterized dopamine receptors. In this article, Pierre Sokoloff and Jean-Charles Schwartz discuss the functional significance of such diversity, as well as the new therapeutic perspectives it offers.

Relationship between inhibition of cyclic AMP production in Chinese hamster ovary cells expressing the rat D2(444) receptor and antagonist/agonist binding ratios

1. Radioligand binding assays using [3H]-(-)-sulpiride, in the presence of 1 mM ethylenediaminetetraacetic acid (EDTA) and 100 microM guanylylimidodiphosphate (GppNHp) and [3H]-N0437 were developed to label the low and high agonist affinity states of the rD2(444) receptor (long form of the rat D2 receptor) respectively. The ratios of the affinities of compounds in these two assays (Kapp [3H]-(-)-supiride/Kapp [3H]-N-0437) were then calculated. 2. The prediction that the binding ratio reflected the functional efficacy of a compound was supported by measurement of the ability of a number of compounds acting at dopamine receptors to inhibit rD2(444)-mediated inhibition of cyclic AMP production. When the rank order of the ratios of a number of these compounds was compared to their ability to inhibit the production of cyclic AMP, a significant correlation was seen (Spearman rank correlation coefficient = 0.943, P = 0.01). 3. In conclusion, the sulpiride/N-0437 binding ratio reliably predicted the efficacy of compounds acting at dopamine receptors to inhibit cyclic AMP production mediated by the rD2(444) receptor.

Cholecystokinin and gastrin are not equally sensitive to GTP gamma S at CCKB receptors: importance of the sulphated tyrosine

We have shown that gastrin and cholecystokinin octapeptide (CCK-8) are differently coupled to G protein (GTP-binding protein) through type B cholecystokinin receptors in guinea-pig brain membranes and Jurkat cells. Indeed, the gastrin-13 binding affinity is strongly reduced by stable guanyl nucleotides, whereas CCK-8 binding is only slightly affected. In order to determine the structural requirements regulating such coupling, we have synthesized several gastrin and cholecystokinin fragments (sulphated or unsulphated) elongated at the N-terminus of the common C-terminal tetrapeptide. We investigated their interaction with CCKB receptors in guinea pig brain membranes and Jurkat cells and their involvement in the G protein coupling. Their apparent binding affinities to CCKB receptors were measured by inhibition of [125I]Bolton Hunter-CCK-8 (3-[125I]iodo-4-hydroxyphenyl)propionyl-CCK-8) binding in the presence or absence of GTP gamma S (guanosine 5'-O-(3-thio)triphosphate) or aluminum tetrafluoride (AlF4-). Activation of the G proteins by GTP gamma S or AlF4- led to a decrease in binding affinity for the gastrin related peptides, the common CCK-gastrin C-terminal forms, the cholecystokinin hexapeptide and the unsulphated cholecystokinin heptapeptide. Sulphated CCK-7, CCK-8, and cionin apparent binding affinities were not affected. These finding indicated that the sulphated tyrosine in position 7 in CCK (as counted from the C-terminus), provides the cholecystokinin selectivity for the CCKB receptor compared to gastrin. The results are discussed with the aim to better clarify the physiological relevance of gastrin and cholecystokinin toward CCKB receptors and their related intracellular events.

Molecular cloning and characterization of the mouse dopamine D3 receptor gene: an additional intron and an mRNA variant

The intron-exon organization for the murine dopamine D3 receptor gene was determined. A novel intron of approximately 1 kb was identified in both rat and mouse D3 receptor genes. This intron (termed intron 4) is situated between coding nucleotides 723 and 724, resulting in a split of former exon 4 (containing nucleotides 527-801) into two separate exons (exon 4 and exon 5). Thus, the coding regions of the D2 and D3 receptor genes contain an identical number of exons (seven exons) and share a very similar gene structure. Reverse transcription-PCR experiments revealed a short form of mouse D3 mRNA (D3Short) that lacks the first 63 nucleotides from exon 6, and results from a splicing event occurring within this exon. However, this mRNA variant was not found in either rat or human brain. No dopamine D3 receptor mRNA variants were found deriving from the alternative splicing of exon 5, although its counterpart, exon 6 in the D2 receptor gene, is spliced out to produce the D2Short mRNA. These data suggest that, although the intron-exon organizations of the D2 and D3 receptor genes are similar, the encoded transcripts may be processed differently.

Alternative splicing in human retinal mRNA transcripts of an opsin-related protein

An opsin-related gene encodes a putative RPE-retinal G-protein-coupled receptor (RGR) that is most homologous to the visual pigments and invertebrate retinochrome. A splice variant of human RGR mRNA can be demonstrated by the sequence of isolated cDNA clones and by the amplification and analysis of human retinal mRNA. The shortened transcript contains a deletion of 114 nucleotides that correspond exactly to the sequence of exon 6 in the human rgr gene. The predicted RGR variant lacks the putative sixth transmembrane domain and has a calculated molecular weight of 27,726. Variable amounts of a 28-kDa protein were found in the retinas of some individuals by immunoblot assay. Since a similar shortened RGR transcript was not detected in bovine retina or RPE, the RGR variant is not essential for vertebrate vision. Analysis of the structure of the rgr gene and of the sequences of cDNA clones indicates that the truncated mRNA may be produced through alternative splicing of pre-mRNA from which a cassette exon is removed and the predicted RGR variant is radically altered in primary structure.

Molecular cloning and characterisation of the gene encoding the murine D4 dopamine receptor

The murine D4 dopamine receptor was isolated from a murine genomic DNA library. The receptor's entire coding region was contained within a 6 kb EcoRI genomic fragment, indicating that the murine D4 receptor gene is significantly smaller than the corresponding D2 and D3 receptor genes, the coding regions of which each stretch over 30 kb. The murine D4 receptor gene has three introns and four exons, in common with the rat and human D4 receptor genes. RT-PCR on mRNA from different brain regions shows that the D4 receptor mRNA is expressed in various areas of the brain, with some differences from the rat and human receptor homologues.