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

Abstract

This study investigated whether the activation of pinealocyte beta-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 beta-adrenergic agonist isoproterenol (ISO) was also studied. The incubation of dissociated pinealocytes with 0.1-10 microM 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 microM ISO but they were reduced after ISO 10 microM. 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, alpha-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 beta-adrenergically induced NAT activity and, thus, may contribute to the optimal melatonin synthesis at night.

Generation of the Melatonin Endocrine Message in Mammals: A Review of the Complex Regulation of Melatonin Synthesis by Norepinephrine, Peptides, and Other Pineal Transmitters

“…It is noteworthy, however, that there is a high level of NAS release in vitro from NEstimulated cultured pinealocytes (Miguez et al, 1997) and in vivo in the extracellular medium of microdialyzed nocturnal pineal glands (Azekawa et al, 1991;Sun et al, 2002). These observations suggest that part of the NAS synthesized by AA-NAT is not used by HIOMT to produce MEL, and thus in conditions of marked pineal stimulation HIOMT, rather than AA-NAT, limits MEL synthesis.…”

“…Consequently, any variation in nighttime HIOMT activity should modulate the rate of MEL synthesis (the amplitude of the nocturnal MEL peak). This hypothesis is strengthened by the following observations: 1) an excess of NAS is released in the extracellular compartment at night (Azekawa et al, 1991;Sun et al, 2002) or following NAergic stimulation (Berg and Klein, 1971;Miguez et al, 1997); 2) the NAS concentration can be up to 2-fold greater than that of MEL in the rat and the Siberian hamster (Steinlechner et al, 1995); 3) when Siberian hamsters are transferred from LP to SP, the increase in the amplitude of the nocturnal MEL peak is not related to a similar increase in nocturnal AA-NAT activity, but in contrast to a decrease (Illnerova et al, 1984;Ribelayga et al, 2000); and 4) in the Siberian hamster, parallelism between daily variations of NAS and MEL is not always observed throughout along the year (especially in September; Steinlechner et al, 1995).…”

“…These variations are opposite to that of MEL, and are therefore supposed to reflect the nocturnal use of 5-HT to synthesize MEL (Mefford et al, 1983). Recent results, however, have shown that nocturnal synthesis and release of 5-HT is more complex and is required for maximal NAergic stimulation of MEL synthesis (Miguez et al, 1997;Sun et al, 2002;see Section VI.B.1. ).…”

“…Several studies have reported that 5-HT is also a secretory product of the pinealocytes (Shein et al, 1967;Walker and Aloyo, 1985;Chuluyan et al, 1989;Miguez et al, 1997). Furthermore, using pineal microdialysis, it has been shown that 5-HT is released in the pineal extracellular medium during the day with a significant increase at the beginning of the night followed by a marked decrease later in the night (Azekawa et al, 1991;Sun et al, 2002).…”

“…Furthermore, using pineal microdialysis, it has been shown that 5-HT is released in the pineal extracellular medium during the day with a significant increase at the beginning of the night followed by a marked decrease later in the night (Azekawa et al, 1991;Sun et al, 2002). These observations, suggesting that pineal 5-HT may display auto/paracrine effects on pineal metabolism, have triggered several studies to elucidate the mechanisms regulating 5-HT release and the role of 5-HT in the rat pineal gland (Sugden, 1990a;Miguez et al, 1997). We have found that there is a high basal release of 5-HT (approximately 10 to 15 ng/h/7 ϫ 10 4 pineal cells) compared to that of MEL (0.1 to 0.2 ng/h/7 ϫ 10 4 pineal cells) in cultured pineal cells (Miguez et al, 1997).…”