Vasoactive intestinal peptide in rat hypophysial portal blood: effects of electrical stimulation of various brain areas, the oestrous cycle and anaesthetics.

Vasoactive intestinal peptide (VIP) was measured by radioimmunoassay in hypophysial portal and peripheral blood from adult male and female Wistar rats. The results confirmed that the concentration of VIP in hypophysial portal blood was significantly greater than in peripheral blood and showed that VIP release into portal blood was not affected by removal of the gut, the largest peripheral source of VIP. Electrical stimulation of the median eminence, several hypothalamic nuclei, the amygdala or hippocampus had no significant effect on the release of VIP into portal blood, possibly because under the conditions of the experiment the spontaneous release of VIP is already at a maximum. In female rats, the VIP released into portal blood collected between 13.00 and 18.30 h of each day of the 4-day oestrous cycle varied under different anaesthetics, and there was no consistent increase in VIP release on pro-oestrus suggesting that VIP is not involved in the pro-oestrous surge of prolactin.