Inhibition of Luteinizing Hormone Release and Luteinizing Hormone Releasing Hormone Action by the Ovaries of Postpartum Lactating Rats*

The influence of the ovaries on LH secretion was studied in postpartum lactating rats. LH surges were induced in intact and ovariectomized lactating rats, and in ovariectomized virgin rats by injecting a single dose of estradiol benzoate (EB) on day 7 postpartum or postovariectomy, followed by a single dose of progesterone (P) 72 h later. Ovariectomy of lactating rats significantly increased the LH surges induced with EB and P. However, the absolute magnitude of LH surges induced with EB and P in ovariectomized virigin rats was even greater than in ovariectomized or intact lactating rats. LHRH-induced release of LH was studied in intact and ovariectomized lactating rats, and in ovariectomized virgin rats, after 10 days priming with 1 μg EB or corn oil. On the 10th day postpartum or postovariectomy, the rats were given 10 injections of LHRH, 50 ng/100 g BW, during 3 h, and blood samples were collected immediately before and during the LHRH injections. Ovariectomy increased LH release by LHRH in both corn oil- and EB-primed lactating rats. EB priming enhanced LHRH action more in ovariectomized lactating rats than in intact lactating rats, but LHRH stimulation of LH release was greater in EB-primed, nonlactating ovariectomized virgin rats than in either group of lactating rats. These results suggest that the ovaries may act at both the hypothalamic and pituitary levels to decrease LH secretion, because both the positive feedback of estrogen and progesterone on LH release and the pituitary response to LHRH were reduced in lactating rats by the presence of the ovaries. (Endocrinology102: 832, 1978)