The dopamine D4 receptor: one decade of research
Introduction
The dopaminergic system has received a significant amount of attention due to the important role it plays in the central nervous system (CNS) in motor control, cognition, reward and endocrine regulation. The importance of the dopaminergic system in these processes is best exemplified by (1) disorders with deficits in dopaminergic signaling, such as Parkinson's disease and l-DOPA (l-3,4-dihydroxyphenylalanine) responsive dystonia, (2) the addictive properties of drugs that enhance dopaminergic signaling such as cocaine and amphetamine and (3) the therapeutic efficacy of neuroleptic medication in controlling the symptoms of Gilles de la Tourette syndrome and the psychoses seen in schizophrenia, Huntington's disease and Alzheimer's disease. In addition, psychostimulant drugs which enhance dopamine release, such as methylphenidate, are effective in the treatment of attention deficit hyperactivity disorder (ADHD). Even though the dopaminergic system may not be essential for normal development, dopamine-deficient transgenic mice will not survive post-weaning due to motor impairment and abnormalities in feeding behavior, but can be rescued by life-long l-DOPA treatment (Zhou and Palmiter, 1995).
The presence of receptors for dopamine in the brain that could mediate intracellular signaling through the activation of adenylyl cyclase were recognized almost three decades ago Kebabian and Greengard, 1971, Kebabian et al., 1972. This notion was soon followed by the direct demonstration of the existence of binding sites for dopamine in brain and the identification of these sites as the target for neuroleptic medications Burt et al., 1975, Seeman et al., 1975, Seeman and Lee, 1975, Creese et al., 1976, Seeman et al., 1976. Soon thereafter it was realized that two dopamine receptor subtypes existed, termed dopamine D1 and D2, which coupled to the stimulation and blockade of adenylyl cyclase, respectively Kebabian and Calne, 1979, Stoof and Kebabian, 1981. The functional coupling of these receptors is mediated by GTP-binding proteins Maeno, 1982, Kilpatrick and Caron, 1983, Niznik et al., 1986, Senogles et al., 1987, hence dopamine receptors belong to the superfamily of G protein-coupled receptors.
After the initial cloning of several G protein-coupled receptors through expression cloning or protein purification strategies, it became clear that this class of receptors shared a relatively high homology. Structurally, this is characterized by seven conserved hydrophobic domains that were proposed to span the plasma membrane (Hanley and Jackson, 1987). Based on this observation, homology cloning strategies using the cloned β2-adrenoceptor sequence as a probe were employed to identify novel G protein-coupled receptors. This led to the cloning of the dopamine D2 receptor (Bunzow et al., 1988), and it was soon found that two dopamine D2 receptor subtypes are generated through alternative splicing Dal Toso et al., 1989, Giros et al., 1989, Grandy et al., 1989, Monsma et al., 1989, Selbie et al., 1989. Subsequently, the other major dopamine binding site in the brain, the dopamine D1 receptor, was cloned Dearry et al., 1990, Monsma et al., 1990, Sunahara et al., 1990, Zhou et al., 1990. The homology cloning approach rapidly resulted in the identification of three novel dopamine receptor subtypes, called D3 (Sokoloff et al., 1990), D4 (Van Tol et al., 1991) and D5 Grandy et al., 1991, Sunahara et al., 1991, Tiberi et al., 1991, Weinshank et al., 1991, the existence of which were unanticipated. Due to the similarity of these new dopamine receptor subtypes with either the dopamine D1 (for D5) or D2 (for D3 and D4) receptor and their relative low abundance, these novel receptors had evaded previous detection by classic pharmacological and biochemical approaches. No other functional dopamine receptors have been found in mammalian species to date, although two pseudogenes for the dopamine D5 receptor subtype are found in humans Grandy et al., 1991, Nguyen et al., 1991, Weinshank et al., 1991. Additional dopamine D1-like receptor subtypes have been identified in non-mammalian species Sugamori et al., 1994, Demchyshyn et al., 1995, Lamers et al., 1996, Cardinaud et al., 1997.
With the identification of five dopamine receptor subtypes, it was apparent that the distinct physiological and behavioral roles attributed to the dopamine D1 and D2 receptors were now less clear. The anatomical localization and pharmacological properties of the dopamine D4 receptor led to intense interest in this receptor as a possible target of neuroleptic drugs. Unfortunately, many of the pharmacological tools available at that time did not provide the necessary specificity to discriminate the dopamine D4 receptor from the other D2-like receptor subtypes. In the last decade, significant advances have been made in this respect, and a plethora of specific receptor agonists and antagonists have been developed. In addition, the development of transgenic mouse models deficient for the individual receptor subtypes have significantly contributed to the understanding of the functional roles of the different receptors. This review summarizes a decade of research into the molecular biology, biochemistry, human genetics and physiology of the dopamine D4 receptor to clarify what we have learned and to identify what questions remain unanswered.
Section snippets
Gene structure
The human dopamine D4 receptor gene contains four exons Van Tol et al., 1991, Van Tol et al., 1992, and this genomic organization is conserved within the mouse and rat homologues O'Malley et al., 1992, Asghari et al., 1994, Fishburn et al., 1995, Matsumoto et al., 1995a, Suzuki et al., 1995. This organization is also partially found in the dopamine D2 (Grandy et al., 1989) and D3 receptors Giros et al., 1990, Giros et al., 1991, Fishburn et al., 1993, Fu et al., 1995, Park et al., 1995, Griffon
Protein structure
The primary sequence of the dopamine D4 receptor displays highest homology to the dopamine D2-like receptor and α2-adrenoceptor families. This similarity is particularly evident in the postulated transmembrane domains of the receptor. The existence of a seven transmembrane topology is predicted by hydrophobicity analysis of the primary structure and the observed sequence similarities with other G protein-coupled receptors (Van Tol et al., 1991). The dopamine D4 receptor does not contain a
Expression
Northern blot and RT-PCR (reverse transcriptase-polymerase chain reaction) analyses have demonstrated that the dopamine D4 receptor is expressed in various brain areas, albeit at relatively low levels compared to dopamine D2 receptor levels found in striatum Van Tol et al., 1991, O'Malley et al., 1992, Matsumoto et al., 1995a, Matsumoto et al., 1996. Expression of the dopamine D4 receptors is most abundant in retina (Cohen et al., 1992), cerebral cortex, amygdala, hypothalamus and pituitary
Pharmacology
The pharmacological profile of the dopamine D4 receptor has been the topic of several extensive reviews by us Seeman and Van Tol, 1994, Seeman et al., 1996, Seeman et al., 1997, Wilson et al., 1998. In general, the dopamine D4 receptor displays a pharmacological profile that is very comparable to that of the dopamine D2 and D3 receptors. The most striking observations are that the dopamine D2/D3 receptor-specific ligands raclopride and S-sulpiride fail to recognize the dopamine D4 receptor with
Cellular signaling
The D2-like family of receptors couple to multiple intracellular effectors (reviewed by Huff, 1996). Inhibition of adenylyl cyclase by the dopamine D2-like receptor was first identified in the pituitary prior to its cloning De Camilli et al., 1979, Onalli et al., 1981, Stoof and Kebabian, 1981. In the mouse retina, the dopamine D4 receptor has been shown to reduce dark-adapted cAMP levels, indicating that this subtype is active in vivo (Cohen et al., 1992). Dopamine D2-like receptors also
Genetic association studies
Despite early optimism that the higher affinity of clozapine for the dopamine D4 receptor compared with the dopamine D2/D3 receptor may form the biochemical basis for its atypical antipsychotic profile, association and linkage studies failed to find evidence identifying DRD4 as a risk factor for schizophrenia Daniels et al., 1994, Shaikh et al., 1994, Petronis et al., 1995, Kohn et al., 1997, Serretti et al., 1999. Two recent publications report a relationship between the exon 3 polymorphism
Physiological role
The importance of dopamine in movement, mood/cognition, and pituitary hormone secretion is well established (reviewed by Emilien et al., 1999). However, insight into the function of the dopamine D4 receptor has been limited until very recently by the lack of selective antagonists and agonists. The search for these ligands has been driven by speculation that antagonism of this receptor may underlie the activity of atypical antipsychotics such as clozapine. It has been proposed that the ability
Conclusions
Almost a decade after it was cloned, the dopamine D4 receptor continues to be a source of intense interest. The critical function that the dopaminergic system plays in the CNS and the unique polymorphic structure of this receptor ensures that the drive to understand this receptor will continue. While the exon 3 VNTR polymorphism remains the most ubiquitous and structurally divergent polymorphism that has been identified in the dopamine receptor family, its physiological role, if any, remains
Acknowledgements
This work is supported by the Medical Research Council of Canada. Hubert H.M. Van Tol is a Career Scientist of the Ontario Ministry of Health.
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