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Possible Role of the -Aminobutyric Acid-A Receptor System in the Timing of the Proestrous Luteinizing Hormone Surge in Rats

Department of Physiology, Yokohama City University School of Medicine, Fukuura, Kanazawa-ku, Yokohama, Japan

Address all correspondence and requests for reprints to: Dr. Dai Mitsushima, Department of Physiology, Yokohama City University School of Medicine, 3–9 Fukuura, Kanazawa-ku, Yokohama 236, Japan.

To examine the role of the -aminobutyric acid-A receptor-mediated system in the timing of the proestrous LH surge, we observed the free running activity rhythm and the timing of the LH surge simultaneously in blinded cycling female rats. Blood samples were obtained from unanesthetized freely moving rats through an intraatrial cannula. Five hours after the activity offset on the day of proestrus, bicuculline methiodide (BIC; 50 mg/kg·h) or saline was infused iv for 3 h into the freely moving rats. In the BIC group, the peak time of the surge occurred at 7.9 ± 0.2 h after the activity offset, with a significant advance compared to the peak time in the saline group (i.e. 9.9 ± 0.4 h), but neither BIC nor saline induced a significant phase shift in the circadian activity rhythm. We found that the infusion of BIC on the subjective morning of the proestrous day dissociates the timing of the LH surge from the circadian activity rhythm in rats.

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