Effects of third ventricular injection of .BETA.-endorphin on luteinizing hormone surges in female rat: Sites and mechanisms of opioid actions in the brain.

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Abstract
The effects of 3rd ventricular injection of .beta.-endorphin (.beta.-EP) on spontaneous, brain stimulation-induced and estrogen-induced LH [luteinizing hormone] surges were studied in the adult female rat. .beta.-EP blocked the preovulatory surge of LH release and ovulation, while it did not affect LH release in response to LHRH injection. The site of the .beta.-EP blockade of ovulation was proved to be in the brain. .beta.-EP completely blocked ovulatory LH release induced by the electrochemical stimulation of the medial amygdaloid nucleus and medial septum-diagonal band of Broca, but failed to block ovulation due to the stimulation of the medial preoptic area (MPO) or median eminence, though serum LH levels after the MPO stimulation were inhibited by .beta.-EP. In the spayed rats treated with estradiol benzoate (EB) on Day 1 and 4 of experiment, .beta.-EP given on day 5 blocked the LH surge that normally occurred on that day and led to a compensatory surge of LH on the following day. Moreover, the LH surge on day 5 was inhibited by .beta.-EP given either on day 1 or day 4. .beta.-EP may act in inhibiting the preovulatory LH surges not only by suppressing the preoptic-tuberal LHRH activities but also by affecting the initiation and development of stimulatory feedback of estrogen in the CNS.