DIFFERENCES IN AGONIST-INDEPENDENT AND AGONIST-DEPENDENT 5-HYDROXYTRYPTAMINE(2C) RECEPTOR-MEDIATED CELL-DIVISION

Previous studies have shown that agonist activation of the 5-hydroxytr yptamine(2C) (5-HT2C) receptor expressed in NIH-3T3 fibroblasts result s in development of a transformed phenotype. In light of recent eviden ce from our laboratory demonstrating constitutive 5-HT2C receptor acti vity, we examined the contribution of this agonist-independent activit y to basal cell division. 5-HT2C receptor ligands modulated [H-3]thymi dine incorporation, DNA amounts, and cell number in serum-starved NIH- 3T3 fibroblasts transfected with 5-HT2C receptor cDNA. Three classes o f 5-HT2C receptor ligands were distinguished in transfected, but not n ontransfected, fibroblasts. Basal [H-3]thymidine incorporation was inc reased by agonists and decreased by inverse agonists, whereas neutral antagonists had little or no effect alone. Neutral antagonists did, ho wever, block the effects of both agonists and inverse agonists, The ra nk order of potencies of inverse agonists to decrease basal [H-3]thymi dine incorporation was consistent with their rank order to decrease ba sal 5-HT2C receptor-mediated phosphoinositide hydrolysis. However, two antagonists previously classified as inverse agonists based on their ability to eliminate basal phosphoinositide hydrolysis did not elicit comparable reductions in basal [H-3]thymidine incorporation. For examp le, mesulergine had no effect on basal cell division, even though it e liminates the phosphoinositide hydrolysis response. Pertussis toxin, w hich inactivates G proteins in the G(i) and G(o) families, had no effe ct on basal [H-3]thymidine incorporation or basal phosphoinositide hyd rolysis but partially inhibited these responses when elicited by an ag onist. Thus, agonist occupation of the 5-HT2C receptor apparently acti vates different or additional G proteins compared with constitutive 5- HT2C receptor activation. In conclusion, our findings suggest that con stitutively active 5-HT2C receptors stimulate cell division in transfe cted fibroblasts in the absence of an agonist. In addition, the 5-HT2C receptor may use multiple signaling pathways to mediate its effects.