Author links open overlay panel James N Oak a b c, John Oldenhof a b, Hubert H.M Van Tol a b c
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 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 D 1 and D 2, 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 β 2-adrenoceptor sequence as a probe were employed to identify novel G protein-coupled receptors. This led to the cloning of the dopamine D 2 receptor (Bunzow et al., 1988), and it was soon found that two dopamine D 2 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 D 1 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 D 3 (Sokoloff et al., 1990), D 4 (Van Tol et al., 1991) and D 5 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 D 1 (for D 5) or D 2 (for D 3 and D 4) 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 D 5 receptor subtype are found in humans Grandy et al., 1991, Nguyen et al., 1991, Weinshank et al., 1991. Additional dopamine D 1-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 D 1 and D 2 receptors were now less clear. The anatomical localization and pharmacological properties of the dopamine D 4 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 D 4 receptor from the other D 2-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 D 4 receptor to clarify what we have learned and to identify what questions remain unanswered.
The human dopamine D 4 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 D 2 (Grandy et al., 1989) and D 3 receptors Giros et al., 1990, Giros et al., 1991, Fishburn et al., 1993, Fu et al., 1995, Park et al., 1995, Griffon
The primary sequence of the dopamine D 4 receptor displays highest homology to the dopamine D 2-like receptor and α 2-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 D 4 receptor does not contain a
Northern blot and RT-PCR (reverse transcriptase-polymerase chain reaction) analyses have demonstrated that the dopamine D 4 receptor is expressed in various brain areas, albeit at relatively low levels compared to dopamine D 2 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 D 4 receptors is most abundant in retina (Cohen et al., 1992), cerebral cortex, amygdala, hypothalamus and pituitary
The pharmacological profile of the dopamine D 4 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 D 4 receptor displays a pharmacological profile that is very comparable to that of the dopamine D 2 and D 3 receptors. The most striking observations are that the dopamine D 2/D 3 receptor-specific ligands raclopride and S-sulpiride fail to recognize the dopamine D 4 receptor with
The D 2-like family of receptors couple to multiple intracellular effectors (reviewed by Huff, 1996). Inhibition of adenylyl cyclase by the dopamine D 2-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 D 4 receptor has been shown to reduce dark-adapted cAMP levels, indicating that this subtype is active in vivo (Cohen et al., 1992). Dopamine D 2-like receptors also
Despite early optimism that the higher affinity of clozapine for the dopamine D 4 receptor compared with the dopamine D 2/D 3 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 publica
