The regulation of receptors in brain is important in explaining certain effects of psychotherapeutic drugs. Historically, studies of receptor regulation have focused on the effect of chronic under- or over-exposure of a receptor to its neurotransmitter. The regulation of receptors and responses may also occur through interactions between receptor subtypes for a particular neurotransmitter. The overall objective of this proposal is to investigate whether interactions between the 5-HT1A and 5-HT2 receptors are involved in their regulation. Treatments that result in a desensitization of 5-HT1A receptor-mediated responses also regulate 5-HT2 receptor number and/or the sensitivity of 5-HT2 receptor-mediated responses. The studies proposed in the first specific aim are designed to explore, using cells maintained in culture, whether interactions between the second messenger systems linked to each subtype occur and contribute to regulatory phenomena. Activation of protein kinase C, an integral part of the second messenger system to which the 5-HT2 receptor is coupled, leads to the desensitization of the 5-HT1A receptor. Thus, 5-HT2 receptor activation may play a role in the desensitization of the 5-HT1A receptor. P11 cells, which express 5-HT2 receptors coupled to phosphoinositide (PI) hydrolysis, will be transfected with the 5-HT1A receptor in order to study the interactions between these serotonin receptor subtypes and their respective second messenger systems on the same cell. Whether activation of 5-HT2 receptors causes desensitization of 5-HT1A receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase and/or changes in 5-HT1A receptor binding will be determined. The effect of activation of the adenylyl cyclase cascade on 5-HT2 receptor stimulated PI hydrolysis and 5-HT2 receptor binding will also be investigated. The regulation of 5-HT1A receptors in P11 cells expressing both serotonin receptor subtypes will be compared to its regulation in Chinese Hamster Ovary (CHO) cells, transfected to express only the 5-HT1A receptor. In the second specific aim, studies are proposed to test in vivo whether the regulation of 5-HT1A receptors occurs as a result of activation of 5-HT2 receptors, or as a result of the desensitization or down regulation of 5-HT2 receptors and whether intact serotonergic neurons are required for this to occur. Rats will receive a single injection of the 5-HT2 receptor agonist DOI, or single injection of the antagonist mianserin, or chronic treatment with the 5- HT2 receptor antagonist ketanserin. Treatment with DOI causes activation of 5-HT2 receptors but no down regulation of 5-HT2 receptors at the time when measurements will be done. By contrast, these treatments with antagonists have been shown to down regulate 5-HT2 receptors. The sensitivity and function of both serotonin receptor subtypes will be assessed in the same animal and in the same brain region, specifically 5-HT1A receptor-mediated inhibition of adenylyl cyclase in cortical homogenates and 5-HT2 receptor stimulated PI hydrolysis in cortical slices. This combined approach should provide important information that will contribute to our understanding not only of the interactions between serotonin receptor subtypes and their respective second messenger systems but also of the regulation of the 5-HT1A receptor in vivo.
