THE ROLE OF THE INTRACELLULAR AND EXTRACELLULAR SEROTONIN IN THE REGULATION OF MELATONIN PRODUCTION IN RAT PINEALOCYTES

This study investigated whether the activation of pinealocyte beta-adr energic receptors is involved in the regulation of serotonin (5-HT) sy nthesis and release, as it is for melatonin production. In addition, t he role of the intra-and extra-cellular 5-HT in modulating the synthes is of melatonin induced by the beta-adrenergic agonist isoproterenol ( ISO) was also studied. The incubation of dissociated pinealocytes with 0.1-10 mu M ISO resulted in a concentration-dependent increase of mel atonin synthesis. 5-HT release and intracellular 5-HT content were inc reased by 0.1 and 1 mu M ISO but they were reduced after ISO 10 mu M. Moreover, when incubated with the tryptophan hydroxylase inhibitor p-c hlorophenylalanine (PCPA), the secretion of 5-HT as well as the intrac ellular 5-HT levels were markedly reduced in both ISO-stimulated and u nstimulated conditions. Melatonin release was also inhibited by PCPA, although it responded in the expected manner to increasing concentrati ons of ISO. These data indicate that the release of 5-HT from pinealoc ytes depends on the availability of cytoplasmic 5-HT which in turn is highly dependent on the tryptophan hydroxylase activity. In cells stim ulated with moderate ISO concentrations, 5-HT release may be an import ant regulatory process of pineal 5-HT. After a large stimulation of N- acetyltransferase (NAT) activity by ISO, the synthesis of melatonin pr evails on 5-HT release, whose decrease is associated to a deficit of i ntracellular 5-HT. On the other hand, the present study shows that the incubation of pineal cells with high concentrations of 5-HT or with a selective 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine, r everses partially the inhibitory effect of PCPA on the ISO-stimulated melatonin synthesis. In contrast the 5-HT2 antagonist, ketanserin, res ults in an inhibiton of the release of melatonin following ISO stimula tion, These results suggest that released 5-HT may have a role in the full expression of the beta-adrenergically induced NAT activity and, t hus, may contribute to the optimal melatonin synthesis at night.