Evidence for the Theory of Dual Hypothalamic Control of Ovulation

Abstract

THE hypothalamus is rich in monoamines including serotonin (5-HT) and catecholamines (dopamine, adrenaline and noradrenaline). It has been reported that an injection of 5-HT into the third ventricle inhibits the secretion of gonadotrophins in cyclic and bilaterally ovariectomized rats^1–3 and elevated levels of 5-HT in the hypothalamus interfere with both gonadotrophically induced^4,5 and spontaneous ovulation^6,7. On the other hand, intraventricular injection of dopamine¹ or its metabolite adrenaline⁸ promotes gonadotrophin secretion and/or ovulation. On the basis of these and other results it has been postulated that the hypothalamus exerts a dual control over ovulation—its inhibitory influence being transmitted via a 5-HT pathway and a stimulatory one through a catecholaminergic (CA) pathway. It is believed that the balance between these contrasting influences determines the occurrence of ovulation—a certain critical balance in favour of the CA pathway promoting ovulation, and the opposite inhibiting it^6–11. How these contrasting influences interact to regulate ovulation during the reproductive cycle is not known. We therefore injected monoamines or their antagonists into the third ventricle of immature rats primed with gonadotrophin and given a single injection of progesterone, and studied the effects of these procedures on ovulation. It is well documented that the facilitatory effect of progesterone on ovulation in such rats is exerted at the hypothalamic level, because barbiturates^12,13 or hypothalamic lesions¹⁴ can block ovulation induced by progesterone.