Serotonin (5-hydroxytryptamine; 5-HT) receptors have been implicated in a wide variety of neuropsychiatric disorders including schizophrenia, depression, anxiety, obsessive-compulsive behavior, and panic disorders. The diversity of these implications can be explained, at least in part, by the large number of 5-HT receptor populations that now have been identified: 5-HT1-5-HT7. Many of these seven populations have been shown to consist of subpopulations and at least 20 different subpopulations and splice variants of 5-HT receptors have been cloned. In some instances, the exact role(s) of these 5-HT populations is merely speculative due to the lack of selective sertonergic ligands. Indeed, very few truly subpopulation-selective agonists or antagonists have been developed. The purpose of the present investigation is to develop such agents. Specifically, we propose (a) to explain the 5-HT2A versus 5-HT2C selectivity of spiperone and to develop and test a comprehensive model that might eventually allow the de novo design and synthesis of novel 5-HT2A antagonists, (b) to exploit the first example of a 5-HT6-selective agonist, and (c) to explore a newly identified 5-HT1F ligand. 5-HT2A antagonists may be of value as atypical antipsychotics, there is evidence that 5-HT6 receptors may be involved in psychosis, depression, and mood disorders, and preliminary findings suggest that 5-HT1F receptors may play a role in migraine. Although there may be clinical value in developing such agents, their exact utility is far from clear at this time. Hence, a more immediate goal of this application is to develop such agents as tools to further investigate the pharmacologic significance of these receptor populations and to study basic neurochemical mechanisms. We have already identified in the course of our preliminary investigations, (a) a 5-HT2A versus 5-HT2C antagonist (i.e., KML-010), (b) a 5-HT6-selective agonist (i.e., 2-MMT), and (c) a novel 5-HT1F ligand. New analogs of KML-010 will be synthesized and examined in radioligand binding and behavioral studies in order to test a hypothetical model we have formulated to account for the 5-HT2A selectivity of spiperone and the lack of selectivity of ketanserin. The 5-HT6 agonist 2-MMT has not been thoroughly investigated, binds only with modest affinity, and likely will have difficulty in penetrating the blood-brain barrier due to its chemical composition. We propose to systematically exploit additional leads we have already identified in an attempt to optimize activity. Radioligand binding and functional assays will be conducted. The 5-HT1F ligand is not selective; we propose to explore its structure-activity relationships and functional activity in an attempt to develop a 5-HT1F-selective ligand.
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