The role of the intracellular and extracellular serotonin in the regulation of melatonin production in rat pinealocytes

Miguez JM, Simonneaux V, Pevet P. The role of the intracellular and extracellular serotonin in the regulation of melatonin production in rat pinealocytes. J. Pineal Res. 1997; 23:63–71. © Munksgaard, Copenhagen This study investigated whether the activation of pinealocyte β‐adrenergic receptors is involved in the regulation of serotonin (5‐HT) synthesis and release, as it is for melatonin production. In addition, the role of the intra‐ and extra‐cellular 5‐HT in modulating the synthesis of melatonin induced by the β‐adrenergic agonist isoproterenol (ISO) was also studied. The incubation of dissociated pinealocytes with 0.1–10 IAM ISO resulted in a concentration‐dependent increase of melatonin synthesis. 5‐HT release and intracellular 5‐HT content were increased by 0.1 and 1 μM ISO but they were reduced after ISO 10 μM. Moreover, when incubated with the tryptophan hydroxylase inhibitor p‐chlorophenylalanine (PCPA), the secretion of 5‐HT as well as the intracellular 5‐HT levels were markedly reduced in both ISO‐stimulated and unstimulated conditions. Melatonin release was also inhibited by PCPA, although it responded in the expected manner to increasing concentrations of ISO. These data indicate that the release of 5‐HT from pinealocytes depends on the availability of cytoplasmic 5‐HT, which in turn is highly dependent on the tryptophan hydroxylase activity. In cells stimulated with moderate ISO concentrations, 5‐HT release may be an important regulatory process of pineal 5‐HT. After a large stimulation of N‐acetyltransferase (NAT) activity by ISO, the synthesis of melatonin prevails on 5‐HT release, whose decrease is associated to a deficit of intracellular 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, α‐methyl‐5‐hydroxytryptamine, reverses partially the inhibitory effect of PCPA on the ISO‐stimulated melatonin synthesis. In contrast the 5‐HT2 antagonist, ketanserin, results in an inhibiton of the release of melatonin following ISO stimulation. These results suggest that released 5‐HT may have a role in the full expression of the β‐adrenergically induced NAT activity and, thus, may contribute to the optimal melatonin synthesis at night.