Stimulation of Prolactin Secretion in Rhesus Monkeys by Vasoactive Intestinal Polypeptide

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Abstract
Vasoactive intestinal polypeptide (VIP) is a potent stimulus for prolactin (PRL) release in rats. The purpose of this study was to test the effect of VIP on PRL secretion in rhesus monkeys and to identify its site of action. Three experimental models were used: (1) intact monkeys during the follicular phase of the menstrual cycle; (2) female, hypophyseal stalk-transected, ovariectomized monkeys (ST-OVX), and (3) monkey pituitary tissue perifused in vitro. Initial serum concentrations of immunoreactive PRL were more than 10-fold higher for ST-OVX monkeys (52.4 ± 10.4 ng/ml, X ± SEM; n = 6) than for intact monkeys (4.79± 1.0 ng/ml; n = l0). Intravenous administration of VIP(20 μg/kg body weight) induced an elevation of circulating PRL in each of the intact and ST-OVX monkeys tested. On the average, VIP treatment evoked more than a 9-fold rise in serum PRL in intact monkeys (p < 0.01) and more than a 2-fold increase in ST-OVX monkeys (p < 0.01), while injection of vehicle alone did not affect PRL levels in either experimental group. Addition of VIP (5 × 10–9 – 5 × 10–6M) to medium perifusing monkey pituitary tissue in vitro stimulated PRL secretion both in the presence and in the absence of dopamine (2.5 × 10–7M).Furthermore, the in vitro potency of VIP was comparable, on a molar basis, with that of thyrotropin releasing hormone. Collectively, these results indicate that VIP is a potent stimulus for PRL secretion in monkeys which exerts its effect, at least in part, by a direct action at the pituitary level. Therefore, VIP should now be considered as a possible PRL releasing factor in primates.