Circadian rhythm of melatonin release in pineal gland culture: arg-vasopressin inhibits melatonin release.

The mammalian pineal gland is known to receive a noradrenergic sympathetic efferent signal from the suprachiasmatic nucleus (SCN) via the superior cervical ganglion. Arg-vasopressin (AVP) containing neurons in the SCN is one of the output paths of circadian information to the other brain areas. AVP release from the SCN is suppressed by melatonin. In turn, we determined the direct effect of AVP on melatonin release using pineal gland explant culture. AVP (1 microM) suppressed melatonin release. Noradrenaline stimulated melatonin release was attenuated by AVP. In turn, the expression of the melatonin synthesis enzyme arylalkylamine N-acetyltransferase mRNA in the rat SCN was reported. We measured melatonin content in the SCN in rats kept under the light-dark cycle and constant dim light. Melatonin in the SCN was higher during the dark period than that in the light. A similar tendency was also observed in the SCN of animals kept under a constant dim light. It was suggested that the reciprocal regulation of melatonin release and AVP release occurs in the SCN and pineal gland.