5-Hydroxytryptamine sub(2A) (5-HT sub(2A)) receptor desensitization can occur without down-regulation

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 275; no. 3; pp. 1633 - 1646
• Main Authors: Roth, B L, Palvimaki, E-P, Berry, S, Khan, N, Sachs, N, Uluer, A, Choudhary, MS
• Format: Journal Article
• Language: English
• Published: 01.01.1995
• ISSN: 0022-3565

The connection between agonist-induced desensitization and down-regulation of 5-hydroxytryptamine sub(2A) (5-HT sub(2A)) receptors was examined in a clonal cell line that stably expresses the 5-HT sub(2A) receptor. Brief (2-hr) and prolonged (24-hr) exposure to the agonist quipazine or the agonist 4-iodo-(2,5-dimethoxy)-phenylisopropylamine (DOI) diminished 5-HT sub(2A) receptor-mediated phosphoinositide hydrolysis; no change in 5-HT sub(2A) receptor number or affinity was measured after 24 hr of exposure to DOI or quipazine. Immunohistochemical studies demonstrated that a 24-hr exposure to DOI did not alter surface 5-HT sub(2A) receptor immunoreactivity. Western blot analysis with G sub( alpha q)- and G sub( alpha 11)-selective antibodies indicate that a 24-hr agonist exposure did not alter the levels of phospholipase C-dependent G proteins. These results suggest that desensitization after prolonged DOI exposure can occur via a process independent of the levels of phospholipase C-coupled G proteins. Studies with a mutant 5-HT sub(2A) 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-12, 13-dibutyrate attenuated the intermediate phase (i.e., after 2-6 hr of agonist exposure) of DOI- and quipazine-induced desensitization. These results indicate that the intermediate phase of DOI-induced desensitization is mediated by the alpha - and/or epsilon -protein kinase C isozymes but that neither is involved in the later phase (i.e., after 24 hr of agonist exposure) of desensitization. Taken together, these results indicate that 24 hr of agonist exposure causes a desensitization of 5-HT sub(2A) receptors via two distinct processes, both of which are independent of receptor and G protein levels.