Differences in rapid desensitization of 5-Hydroxytryptamine_2A and 5-Hydroxytryptamine_2C receptor-mediated phospholipase C activation

The serotonin (5-HT)_2A and 5-HT_2C receptors share a high degree of sequence homology and have very similar pharmacological profiles. Although it is generally believed that the cellular signal transduction mechanisms activated by these receptors are indistinguishable, recent data suggest significant differences in their signaling cascades. In this study we explored differences in the characteristics and mechanisms of rapid desensitization between the 5-HT_2A and 5-HT_2C receptor systems. For both receptor systems, pretreatment with 5-HT reduced the ability of a maximal concentration of 5-HT to stimulate phospholipase C-mediated inositol phosphate accumulation by about 65%, although the 5-HT_2C receptor system was more sensitive to the desensitizing stimulus. Differences in the concentration dependence of the rate constant for desensitization (kdes) suggested different mechanisms of desensitization for the 5-HT_2A and 5-HT_2C receptor systems. At very high receptor occupancy (>99%), the responsiveness of the 5-HT_2A, but not the 5-HT_2C, receptor system returned to control levels despite the continued presence of the agonist. This resensitization was dependent upon the activity of protein kinase C (PKC). Agonist-induced desensitization of the 5-HT_2A, but not the 5-HT_2C, receptor system was reduced by the PKC inhibitors staurosporine and bisindolylmateimide, and by downregulation of PKC. In addition, inhibitors of calmodulin (W-7) or of calmodulin-dependent protein kinase II, reduced 5-HT_2A, but not 5-HT_2C, desensitization. Desensitization of the 5-HT_2C, but not the 5-HT_2A, receptor system was dependent on G protein receptor kinase activity. These data further emphasize the major differences in the signaling systems coupled to 5-HT_2A/2C receptors.