Agonist-induced functional desensitization of recombinant human 5-HT2 receptors expressed in CHO-K1 cells

The desensitization characteristics of recombinant human 5-HT 2A, 5-HT 2B, and 5-HT 2C receptors (VSV and INI isoforms) stably expressed in CHO-K1 (Chinese hamster ovary) cells was investigated by calcium fluorimetry. Comparative desensitization characteristics of the agonists 5-HT, m-chlorophenylpiperazine (mCPP), and 2,5-dimethoxy-4-iodoamphetamine hydrobromide (DOI) were performed. Human 5-HT 2C (INI) receptors exhibited a greater degree of desensitization to all agonists tested than edited 5-HT 2C (VSV) receptors. A 2-hr exposure to 5-HT resulted in a significantly larger reduction in response upon re-exposure to 5-HT at 5-HT 2C (INI) receptors, as compared to 5-HT 2C (VSV) receptors (72% and 47% respectively, P < 0.01). Both receptor isoforms were expressed at similar densities. Human 5-HT 2B receptors exhibited the most dramatic degree of desensitization, with prior exposure to 5-HT reducing subsequent response to 5-HT by 80%, with an extremely rapid time-course ( t 1 2 < 5 min). The response at 5-HT 2A receptors was reduced by 54%. The partial agonists mCPP and DOI also elicited desensitization, generally in line with their relative efficacies at each receptor, but exhibited more rapid kinetic profiles than 5-HT. Heterologous desensitization of an endogenously expressed G q/11-coupled purinergic receptor was also examined following preincubation of the cell lines with 10 μM 5-HT. Only stimulation of 5-HT 2C (VSV) receptors resulted in a profound attenuation of subsequent ATP mediated responses. These results demonstrate differing degrees of both homologous and heterologous desensitization of 5-HT 2 receptors. Additionally, the different desensitization profiles of 5-HT 2C (INI) and 5-HT 2C (VSV) receptor may be due to signal transduction differences caused by RNA editing.