Abstract
The site at which norethindrone (NE) acts to block ovulation in the rat was investigated by electrochemical stimulation of the preoptic hypothalamus and by the administration of crude bovine median eminence (ME) extracts. Single injections of 0.5 mg NE on different days of the oestrous cycle raised the preoptic threshold for the induction of ovulation. Using 20 <i>µA </i>for 90 sec, there was a significant increase in the number of rats ovulating as compared with NE-treatment alone, only when NE was given on day 1 (oestrus) of the cycle. NE-treatment did not significantly affect the number of rats ovulating in response to ME extract. Compared with NE-treatment alone, NE plus ME extract increased the number of rats ovulating when the NE was given on day 1 of the cycle but not on days 2 and 3. It was concluded that NE exerts its inhibitory action on ovulation in the rat at a site within the central nervous system.
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