Neurohypophysial response to peptides of the cholecystokinin family: in vivo and in vitro studies.

I.c.v. injection of CCK-8 (50 ng/day) to euhydrated rats significantly decreased vasopressin and oxytocin content in the hypothalamo-neurohypophysial system. In rats drinking hypertonic saline and simultaneously treated with CCK-8, the decrease of vasopressin content was more marked in hypothalamus but somewhat restrained in the neurohypophysis. The decrease of hypothalamic and neurohypophysial oxytocin content (as brought about by the salt load) was significantly less marked in animals treated simultaneously with CCK-8. Incubation of neurointermediate lobes in medium enriched with CCK-33/CCK-39 or CCK-8 did not change significantly the vasopressin and oxytocin release from the neurointermediate lobes both under basal conditions and during potassium stimulation. It is suggested that afferent impulses of osmoreceptor origin may modify the response of vasopressinergic and oxytocinergic neurons to CCK-8. The events, related to the influence of CCK peptides on vasopressin or oxytocin release, do not seem to be localized at the neurohypophysial level.