5-HYDROXYTRYPTAMINE(2A) (5-HT2A) RECEPTOR DESENSITIZATION CAN OCCUR WITHOUT DOWN-REGULATION

The connection between agonist-induced desensitization and down-regula tion of 5-hydroxytryptamine(2A). (5-HT2A) receptors was examined in a clonal cell line that stably expresses the 5-HT2A receptor. Brief (2-h r) and prolonged (24-hr) exposure to the agonist quipazine or the agon ist 4-iodo-(2,5-dimethoxy)-phenylisopropylamine (DOI) diminished 5-HT2 A receptor-mediated phosphoinositide hydrolysis; no change in 5-HT2A r eceptor number or affinity was measured after 24 hr of exposure to DOI or quipazine. Immunohistochemical studies demonstrated that a 24-hr e xposure to DOI did not alter surface 5-Ht(2A) receptor immunoreactivit y. Western blot analysis with G(alpha)q- and G(alpha 11)-selective ant ibodies indicate that a 24-hr agonist exposure did not alter the level s of phospholipase C-dependent G proteins. These results suggest that desensitization after prolonged DOI exposure can occur via a process i ndependent of the levels of phospholipase C-coupled G proteins. Studie s with a mutant 5-HT2A receptor (F340L) indicated that binding per se is not sufficient for desensitization. Down-regulation of the protein kinase C isozymes alpha and epsilon by overnight exposure to phorbol-1 2,13-dibutyrate attenuated the intermediate phase (i.e., after 2-6 hr of agonist exposure) of DOI- and quipazine-induced desensitization. Th ese results indicate that the intermediate phase of DOI-induced desens itization is mediated by the alpha- and/or epsilon-protein kinase C is ozymes but that neither is involved in the later phase (i,e., after 24 hr of agonist exposure) of desensitization. Taken together, these res ults indicate that 24 hr of agonist exposure causes a desensitization of 5-HT2A receptors via two distinct processes, both of which are inde pendent of receptor and G protein levels.