RECIPROCAL BINDING-PROPERTIES OF 5-HYDROXYTRYPTAMINE TYPE 2C RECEPTORAGONISTS AND INVERSE AGONISTS

Expression of the 5-hydroxytryptamine type 2C (5-HT2C) receptor in NIH /3T3 fibroblasts results in agonist-independent 5-HT2C receptor activa tion. Some 5-HT2C receptor antagonists decrease this activation and ar e termed inverse agonists. The present study uses this system to evalu ate functional and receptor binding properties of other 5-HT2C recepto r antagonists. A number of inverse agonists, including clozapine, and a neutral antagonist (methysergide) were identified in a functional as say. Guanine nucleotides increased the affinity of a radiolabeled inve rse agonist ([H-3]mesulergine), suggesting that inverse agonists bind the G protein-uncoupled form of the 5-HT2C receptor with high affinity . Competition binding was performed using conditions that separately l abeled the G protein-coupled and -uncoupled forms of the receptor. The se studies demonstrated that inverse agonists bound the uncoupled form of the 5-HT2C receptor with higher affinity, compared with the G prot ein-coupled form. Agonists, on the other hand, had higher affinity for the coupled form, whereas neutral antagonists had equal affinity for both forms of the receptor. Thus, 5-HT2C receptor neutral antagonists exhibited functional and receptor binding properties consistent with t hose of classical receptor antagonists. However, 5-HT2C receptor inver se agonists displayed functional and receptor binding properties that were opposite those of agonists.