Abstract Employing a novel, rapid and sensitive method for evaluation of phospholipase C (PLC) activity, the pre- sent study characterized the actions of diverse agonists and antagonists at human (h)5-HT 2C receptors expressed in Chinese Hamster Ovary (CHO) cells. In addition, affini- ties and efficacies at these sites were compared with those obtained at h5-HT 2B receptors. 5-HT elicited a robust and rapid reduction in levels of the pre-labelled, membrane-bound substrate of PLC, [ 3 H]phosphatidylinositols ([ 3 H]PI). The time-course of [ 3 H]PI depletion paralleled that of [ 3 H]inositol phosphate ([ 3 H]IP) accumulation, as determined by conventional anion exchange chromatography. Inactivation of h5-HT 2C receptors with the alkylating agent, N-ethoxycarbonyl-2- ethoxy-1,2-dihydroquinoline (EEDQ), revealed a large re- ceptor reserve, with half-maximal PLC activation induced by a concentration of 5-HT occupying only 5% of sites. In analogy to 5-HT (E max =100%), DOI, MK212 and mCPP, as well as the novel ligands, Ro600332, Ro600175 and BW723C86, showed “full” efficacy at h5-HT 2C sites. Their efficacies were similar at h5-HT 2B sites, with the excep- tion of mCPP and MK212, which acted as partial agonists. Further, lisuride and Ro600869 behaved as partial ago- nists and antagonists at h5-HT 2C and h5-HT 2B receptors, respectively. As concerns functional selectivity (potency for induction of [ 3 H]PI depletion), only Ro600175 prefer- entially activated h5-HT 2B sites. In contrast, Ro600332 pref- erentially activated h5-HT 2C receptors. Amongst antago- nists, RS102221 and SB242084 displayed a marked pref- erence for h5-HT 2C sites, whereas LY266097, S33526 and SB204741 behaved as selective antagonists at h5-HT 2B receptors. At both h5-HT 2C and h5-HT 2B receptors, antag- onist potency (pK b ) and binding affinity (pK i ) were highly correlated. In conclusion, this rapid and innovative method for de- termination of PLC activity permitted characterization of an extensive range of novel ligands at h5-HT 2C receptors. Although several antagonists clearly differentiated h5-HT 2C from h5-HT 2B receptors under these conditions, highly se- lective agonists remain to be identified. Keywords 5-HT 2C receptors · 5-HT 2B receptors · Phospholipase C · Phosphatidylinositols · Receptor reserve Introduction 5-HT 2C receptors are currently attracting considerable in- terest in view of their broad implication in the control of mood, motor behaviour and cognition, and their potential importance as targets for the improved treatment of anxi- ety, depression and other psychiatric and neurologic dis- orders (Stefanski and Goldberg 1997; Jenck et al. 1998a; Roth et al. 1998; Fox and Brotchie 1999; Meltzer 1999; Millan et al. 2000a). Of particular interest, the potent an- tagonist properties of the atypical antipsychotic, cloza- pine, at 5-HT 2C receptors (Canton et al. 1994; Millan et al. 1998; Cussac et al. 2000a) may contribute to its low ex- trapyramidal potential, superior control of negative symp- toms and improvement of mood (Roth and Meltzer 1995; Meltzer 1999; Reavill et al. 1999). Further, blockade of 5-HT 2C receptors may participate in the therapeutic actions of several antidepressant agents, including nefazodone and mirtazapine (Millan et al. 2000a, 2000b). The role of closely related 5-HT 2B receptors remains unclear inasmuch as their density is low in the CNS (Foguet et al. 1992; Kursar et al. 1992, 1994; Bonhaus et al. 1995; Duxon et al. 1997a; Hoyer and Martin 1997). However, they may be involved, together with 5-HT 2C receptors, in the modu- lation of appetite (Kennett et al. 1997a). Further, activa- tion of 5-HT 2B receptors may, in contrast to 5-HT 2C recep- tor, attenuate anxious states (Kennett et al. 1996a, 1998a; Didier Cussac · Adrian Newman-Tancredi · Yann Quentric · Nathalie Carpentier · Guillaume Poissonnet · Jean-Gilles Parmentier · Solo Goldstein · Mark J. Millan Characterization of phospholipase C activity at h5-HT 2C compared with h5-HT 2B receptors: influence of novel ligands upon membrane-bound levels of [ 3 H]phosphatidylinositols Naunyn-Schmiedeberg’s Arch Pharmacol (2002) 365 : 242–252 DOI 10.1007/s00210-001-0505-y Received: 25 June 2001 / Accepted: 20 October 2001 / Published online: 16 January 2002 ORIGINAL ARTICLE D. Cussac (✉) · A. Newman-Tancredi · Y. Quentric · N. Carpentier · M.J. Millan Department of Psychopharmacology, Institut de Recherches Servier, 125 Chemin de Ronde, 78290 Croissy-Sur-Seine (Paris), France e-mail: didier.cussac@fr.netgrs.com, Fax: +33-155-722470 G. Poissonnet · J.-G. Parmentier · S. Goldstein Department of Chemistry A, Institut de Recherches Servier, 11 rue des Moulineaux, 92150 Suresnes (Paris), France © Springer-Verlag 2002