Blockade of Ovulation in Rabbits by Hypothalamic Implants of Norethindrone

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Abstract
A study has been made of the ovulation-blocking effects of localized implants of norethindrone in the hypothalamus and hypophysis of mature New Zealand female rabbits. A minute amount of the antifertility steroid was implanted stereotaxically into the hypothalamus or hypophysis, and copulation-induced ovulation was tested 10 days, 5 weeks and 8 weeks after implantation. Pituitary LH content was determined by the ovarian ascorbic acid depletion method. When norethindrone was implanted into the posterior median eminence-posterior tuberal region of the hypothalamus (PME) ovulation was blocked in 5 of 7 rabbits 10 days later and in 4 of 5 rabbits 5 weeks after implantation. Five of 7 rabbits with 5-week-old PME implants also failed to ovulate post coitum when they were primed with estogen. However, by 8 weeks after implantation 3 of 4 previously blocked rabbits ovulated in response to the coital stimulus. The ovulatory stimulus was not blocked by control implants in the PME nor by norethindrone implants in other parts of the hypothalamus or the hypophysis. Ovulation-blocking norethindrone implants did not alter pituitary LH content nor the morphology of the ovary or uterus. Norethindrone implants slightly caudad to the ovulation-blocking sites, in the posterior tuberal, rostral mamillary body region, induced an anestrous behavioral condition not reversed by exogenous estrogen. (Endocrinology76: 691, 1965)