Rapid desensitization of the serotonin_2c receptor system: Effector pathway and agonist dependence

The serotonin_2c (5-HT_2c) receptor couples to multiple effector mechanisms, including phospholipase A₂-mediated arachidonic acid (AA) release and phospholipase C-mediated production of inositol phosphates (IP). Agonist relative efficacy differs depending upon which response (AA release or IP accumulation) is measured. In this study, we investigated the characteristics and agonist dependence of rapid desensitization of 5-HT_2c receptor- mediated AA release and IP accumulation measured simultaneously from the same cell population. Pretreatment with 5-HT reduced the ability of a maximal concentration of 5-HT to elicit AA release and IP accumulation by about 60%; however, the AA response desensitized more rapidly (t_1/2 ≊ 1.3 min) than the IP response (t_1/2 ≊ 6.9 min). In addition, desensitization of the IP response was more sensitive (occurred at lower receptor occupancy levels) than the AA response. Moreover, in response to submaximal 5-HT concentrations, after an initial transient desensitization, the AA response was enhanced by up to -250%. After maximal desensitization, both responses recovered, but recovery of the AA response was complete and faster than that for IP. Desensitization of both responses was also agonist-dependent, and the capacity of agonists to elicit desensitization was not related to their efficacy to activate signaling. These data suggest that desensitization of the 5-HT_2c receptor system is both agonist and effector pathway-dependent and underscore the need to study multiple cellular responses to multiple agonists to understand receptor-mediated signaling systems.