This proposal continues a study of the mechanism of action of antidepressant treatments with a focus on the regulation of 5HT- 2 receptors. During the previous award period, studies of antidepressant-induced alterations in 5HT-2 receptors shifted from radioligand binding analyses of the 5H-T-2 recognition site to studies of the transmembrane signaling pathway, hydrolysis of inositol lipids. These studies have discovered a new site of action of antidepressant drugs at a point distal to the cell surface recognition site. In future studies, a cell culture model of 5HT-2 receptors will be developed in order to examine the mechanism of desensitization and down regulation of 5HT-2 receptor induced by mianserin and other atypical antidepressants which have a high affinity for the 5HT-2 site. The proposed experiments are designed to examine 5HT - 2 receptor signal transduction all along the signal cascade; from the cell surface recognition site ketanserin binding), to the transmembrane signaling pathway (phosphoinositide hydrolysis), to the formation of intracellular second messengers (IP3 and diacyglycerol), to intracellular responses mediated by the 5HT-2/phospholipase C pathway (mobilization of internal calcium stores). These studies will serve as a supplement and guide to continued in vivo studies of delayed, adaptive changes in the 5HT-2 receptor system after chronic antidepressant treatments. The in vivo studies will test the hypothesis that desensitization of the 5HT-2 receptor linked to phosphoinositide hydrolysis is a common action of the various classes of antidepressant treatments. Studies to date have demonstrated a desensitization of the 5HT-2 receptor after the tricyclic antidepressant amitriptyline, after the atypical antidepressant mianserin, and after sertraline, which is representative of a new class of antidepressants. Additional treatment classes to be evaluated are electroconvulsive shocks, MAO inhibitors, and the combination of lithium with tricyclic antidepressants and selective 5HT uptake inhibitors. The mechanism of desensitization of the 5HT-2 receptor after chronic antidepressant treatment will be explored by measuring the number and affinity of cell surface recognition sites in radioligand binding studies, the catalytic activity of phospholipase C and the coupling efficiency of the 5HT-2 recognition site and phospholipase C estimated by changes in agonists affinity states and in the GTP- induced shift in agonist affinity.
