Differences in agonist-independent activity of 5-Ht2A and 5-HT2c receptors revealed by heterologous expression

5-Hydroxytryptamine 5-HT2A and 5-HT2C receptors share many properties, including a common ability to stimulate phospholipase C. Traditionally, this activation was thought to be initiated only after agonist binding, in accordance with the ternary complex model of receptor function. Recently, though, the 5-HT2C receptor was shown to deviate from this tenet by spontaneously isomerizing into the active receptor state, thereby activating G proteins in the absence of agonist. To determine if 5-HT2A receptors share this property of constitutive activity, 5-HT2A and 5-HT2C receptor function was evaluated in transiently transfected NIH 3T3 fibroblasts. In 3T3 cells expressing 5-HT2C receptors, agonist-independent phosphatidyl inositol hydrolysis was substantially elevated relative to mock-transfected cells. In contrast, expression of the 5-HT2A receptor at the same density caused only a marginal increase in basal signaling. Control experiments in the current and previous papers establish that basal activity does not reflect contaminating serotonin. In addition, the magnitude of serotonin-induced signaling was the same in cells expressing either receptor, suggesting that the intrinsic ability of the two receptors to couple to G proteins is comparable. These data indicate that the 5-HT2A receptor has a much lower intrinsic ability to spontaneously adopt or maintain the active receptor conformation than does the closely related 5-HT2C receptor.