Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters.
In mammals, pineal melatonin secretion is under the control of adrenergic and peptidergic inputs regulating serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase; AA-NAT) activity. In this study, the accumulation of AA-NAT mRNA induced by norepinephrine (NE) and peptides of the secretin superfamily (pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal peptide (VIP), growth hormone releasing factor (GRF), secretin) was investigated by a new quantitative reverse transcription-PCR (RT-PCR) assay. We demonstrated that PACAP was the most potent peptide to increase the expression of AA-NAT mRNA and to induce cAMP production in rat pinealocytes. VIP was also able to elevate the AA-NAT mRNA level and cAMP efflux in a dose-dependent manner; however, it was six- and threefold, respectively, less potent than PACAP. The maximal values of AA-NAT mRNA level after PACAP and VIP exposures were similar (523.1 +/- 52.5 amol to 640.7 +/- 68.8 amol vs 461.5 +/- 54.3 amol to 579.2 +/- 72.4 amol). These saturable peak values were approximately five- to eightfold less than that after NE (3.0 +/- 0.3 fmol to 3.6 +/- 0.4fmol). GRF and secretin were less potent than VIP in inducing AA-NAT gene expression and cAMP efflux. These data suggest that the peptides act mostly on VIP(1)/PACAP (VPAC(1)) receptors of pinealocytes with different affinity. The peak cAMP efflux always preceded the elevation of AA-NAT gene expression during the 3-h infusion of VIP or NE. The cAMP efflux had declined by the time of onset of maximal AA-NAT gene expression, but remained significantly higher than its basal values. Our data indicate that even a submaximal level of cAMP is sufficient for maintaining the maximal AA-NAT mRNA accumulation. These findings show that, in addition to NE, PACAP and VIP may have an important role in the regulation of AA-NAT mRNA levels in rat pinealocytes.