5-Hydroxytryptamine2-Family Receptors (5-Hydroxytryptamine2A, 5-Hydroxytryptamine2B, 5-Hydroxytryptamine2C): Where Structure Meets Function
Introduction
Serotonin (5-hydroxytryptamine; 5-HT) is a major neurotransmitter involved in a large number of CNS processes, including the regulation of feeding behavior, aggression, mood, perception, pain, and anxiety Bradley et al. 1986, Roth 1994. In the periphery, 5-HT is important for regulating vascular and nonvascular smooth muscle contraction, platelet aggregation, uterine smooth muscle growth, and gastrointestinal functioning Hoyer et al. 1994, Roth 1994. To mediate these functions, a family of receptors, divided into 7 main types comprising at least 15 distinct receptors, has evolved (Hoyer et al., 1994). Not surprisingly, with so many potential targets, a large number of psychopharmaceuticals that mediate their actions, at least in part, by modulating the number, activity, or both of one or more 5-HT receptors have been developed. It is likely that these changes in receptor number and activity are involved in the therapeutic actions of many drugs used in treating various CNS disorders such as schizophrenia (Mikuni and Meltzer, 1984), depression Blier and de Montigny 1980, Peroutka and Snyder 1980a, Peroutka and Snyder 1980b, Barbaccia et al. 1983, Chaput et al. 1986, mania (Mizuta and Segawa, 1988), and anxiety (Robinson et al., 1989).
This review focuses on the 5-HT2 family of receptors for the following reasons. First, drugs active at 5-HT2 receptors are used in the treatment of many of the aforementioned disorders, including schizophrenia Matsubara and Meltzer 1989, Meltzer et al. 1989, Canton et al. 1990, Roth et al. 1992, Schmidt et al. 1995 and depression Peroutka and Snyder 1980a, Peroutka and Snyder 1980b, Marek et al. 1989, Fontaine 1993, Palvimaki et al. 1996. Second, allelic variations in 5-HT2-family receptors have been associated with the responses to various serotonergic drugs, including clozapine, although not all investigators have noted these associations Arranz et al. 1995, Nothen et al. 1995, Malhotra et al. 1996, Arranz et al. 1996, Erdmann et al. 1996. Third, 5-HT2 receptors represent the site of action of hallucinogens such as lysergic acid diethylamide (LSD), 4-bromo-2,5-dimethoxyphenylisopropylamine, and N,N′-dimethyltryptamine (DMT) Aghjanian et al. 1968, Glennon et al. 1984, Titeler et al. 1988. Fourth, the large body of information relating to the structure-function properties of 5-HT2-family receptors makes this receptor family a convenient model system for probing these properties.
What follows, then, is a review of the structure-activity relations important for 5-HT2 family receptors. After a general introduction to the 5-HT2 family of receptors, there is a discussion of the molecular mechanisms responsible for binding of ligands to, and activation of, these receptors. Site-directed mutagenesis, molecular modeling, and structural studies are discussed in detail. Studies addressing the regulation of 5-HT2-family receptors by various drugs, including agonists and antagonists, are then discussed. Finally, potential mechanisms of receptor regulation are discussed and integrated into an overview of the mechanisms responsible for regulation of G-protein-coupled receptors (GPCRs) in general.
Section snippets
Serotonin, Lysergic Acid Diethylamide, and Schizophrenia
5-HT was chemically identified by Rapport and Page at the Cleveland Clinic in 1948 (Rapport et al., 1948) and identified as one of the major vasoconstricting substances found in defibrinated blood. In 1954, Wooley and Shaw observed striking structural similarities between LSD and 5-HT. Based on the observations that LSD may induce hallucinations and that schizophrenia is characterized in part by hallucinosis, Wooley and Shaw (1954) proposed that 5-HT might be involved in the pathogenesis of
Structural requirements for ligand recognition at 5-hydroxytryptamine2a receptors: agonist binding
The 5-HT2-family receptors comprise three members: 5-HT2A, 5-HT2B, and 5-HT2C. Theoretically, each receptor has the prototypical heptahelical structure of GPCRs in which the seven helical domains are embedded in the plasma membrane. For the following discussion, the numbering scheme for the rat 5-HT2A receptor is followed, because it is this receptor for which the most detailed analyses have been published.
Direct G-Protein Coupling
Several studies have attempted to identify the region(s) of the 5-HT2A and 5-HT2C receptors essential for coupling to Gαq-family G-proteins. By constructing chimeric 5-HT1B/2A receptors, Oksenberg et al. (1995) found that the third intracellular loop (i3) was important for Gαq coupling. These authors were able to change the G-protein specificity of the 5-HT2A receptor from activation of PI hydrolysis to inhibition of adenylate cyclase activity by swapping i3 of the 5-HT2A receptor for i3 of the
Molecular Modeling and Mutagenesis of Ketanserin-like Antagonists to the 5-Hydroxytryptamine2A and 5-Hydroxytryptamine2C Receptors
Dahl’s group has performed the most extensive modeling studies of 5-HT2A and 5-HT2C receptors, by using both bacteriorhodopsin Edvardsen et al. 1992, Kristiansen et al. 1993 and rhodopsin-based (Kristiansen and Dahl, 1996) models. In initial studies (Edvardsen et al., 1992), they predicted that ritanserin, a 5-HT2A/2C antagonist, interacted with a large number of conserved (D120, D155, W151, W200, F240, F243, W336, F339, F340) and nonconserved (Leu123, Ser131, Ile135) residues located in
Unanswered issues regarding mechanisms of ligand binding to 5-hydroxytryptamine2-family receptors
At present, all the available structure-function studies of 5-HT2-family receptors have used molecular modeling, site-directed mutagenesis, analogue binding, or some combination of these approaches. Currently, no direct studies have determined how ligands bind to 5-HT2 receptors. In this regard, with the exception of rhodopsin, there is no structural information about the helical packing of 5-HT2-family receptors or other members of the GPCR superfamily. In the future, as technological advances
5-Hydroxytryptamine2A Receptors Are Down-Regulated and Desensitized by Agonists in Vitro and in Vivo
As is the case for the majority of GPCRs, 5-HT2A receptors can be down-regulated after exposure to an agonist. Thus, for instance, daily LSD administration down-regulates 5-HT2A receptors Buckholtz et al. 1985, Buckholtz et al. 1988 in vivo, as does DOI administration Buckholtz et al. 1988, McKenna et al. 1989, Pranzatelli 1991.
Similar results have been found for some, but not all, in vitro model systems. Thus, for instance, Leysen and co-workers found that DOM caused a rapid down-regulation
Unanswered issues for 5-hydroxytryptamine2- receptor regulation
At present, there is almost no information about the structural features of 5-HT2-family receptors essential for regulation by either agonists or antagonists. 5-HT2A and 5-HT2C receptors are unique in comparison with other GPCRs in that they are down-regulated by both agonists and antagonists. The molecular mechanisms responsible for this apparent down-regulation are unknown. Because a large number of psychiatrically important drugs, including atypical antipsychotics, antidepressants, and
Acknowledgements
This work was supported in part by National Intitutes of Health grants GM52213 and MH57635 and by a grant from the Scottish Rite Schizophrenia Research Foundation to BLR. BLR was also supported by a Research Scientist Development Award KO2MH01366 from the National Institute of Mental Health. DLW was supported in part by an NARSAD Young Investigator Award.
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