Hypothalamic-pituitary interactions during the periovulatory secretion of follicle-stimulating hormone in the rat

We investigated the importance of anterior afferents to the medial basal hypothalamus (MBH) on the increases in plasma FSH during the periovulatory period in the 4-day cyclic rat. We served the anterior connections to the MBH either at 1200 h on proestrus (before the time of onset of the normal spontaneous LH surge in plasma and the associated first phase of FSH release) or near the end of the LH surge and first phase of FSH release at 2000 h on proestrus (before the onset of the second or selective phase of FSH release). Analyses of FSH and LH in blood collected through indwelling atrial catheters or from the trunk after decapitation showed that anterior deafferentation of the MBH at 1200 h on proestrus blocked the proestrous LH surge, the elevations in plasma FSH during proestrus and estrus, and ovulation. In contrast, when brain surgery was delayed until 2000 h on proestrus, the second phase of FSH release and ovulation occurred. In rats with retrochiasmatic transections made at 1200 h, a constant rate iv infusion of LHRH from 1500-1800 h on proestrus restored the LH surge, both phases of increased plasma FSH, and ovulation. The results suggest that 1) the prevolutory LH surge and the first phase of FSH release are dependent on rostral afferents to the MBH which result in hypothalamic LHRH release and 2) the role of rostral afferents to the MBH in the second phase of FSH release is solely to result in hypothalamic LHRH release during proestrus.