Effects of Third Ventricular Injection of β-endorphin on Luteinizing Hormone Surges in Female Rat: Sites and Mechanisms of Opioid Actions in the Brain

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The effects of third ventricular injection of β-endorphin (β-EP) on spontaneous, brain stimulation-induced and estrogen-induced LH surges were studied in the adult female rat. It was found that β-EP blocked the preovulatory surge of LH release and ovulation, while it did not affect LH release in response to LH-RH injection. The site of the β-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 β-EP. In the spayed rats treated with estradiol benzoate (EB) on Day 1 and 4 of experiment, β-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 β-EP given either on Day 1 or Day 4. Present data suggest that β-EP may act in inhibiting the preovulatoy LH surges not only by suppressing the preoptic-tuberal LH-RH activities but also by affecting the initiation and development of stimulatory feedback of estrogen in the central nervous system.
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