Neurokinin A and the neurohypophysial response to melatonin: in vitro studies

The aim of this study was to investigate a possible role of neurokinin A (a member of a family of peptides known as tachykinins) in the pineal-neurohypophysial interrelationship. The effect of neurokinin A (NKA) alone or in the presence of pineal hormone - melatonin on basal and K(+)-stimulated vasopressin and oxytocin secretion from the hypothalamo-neurohypophysial system was studied in vitro. The present results show that NKA stimulated basal vasopressin and oxytocin release from the isolated hypothalamo-neurohypophysial system, when used at the concentration of 10(-7) M/L. Melatonin diminished basal release of the neurohypophysial hormones; it also significantly inhibited the NKA-stimulated secretion of vasopressin and oxytocin. Lower concentrations of NKA did not affect the neurohypophysial hormones basal release, however, when melatonin was added to the medium enriched with NKA at the concentration of 10(-9) M/L, the vasopressin secretion from the hypothalamo-neurohypophysial explants was decreased significantly. The K(+)-evoked release of neurohypophysial hormones was not further modified by either NKA or melatonin. The present results confirm previous reports as to the inhibitory effect of melatonin on both vasopressin and oxytocin secretion from the hypothalamo-neurohypophysial complex in vitro. However, under present experimental conditions, the contribution of NKA in the mechanisms of pineal-neurohypophysial interrelationships has not been demonstrated.