A DAILY RHYTHM IN THE BEHAVIOURAL SENSITIVITY OF THE FEMALE RAT TO OESTRADIOL

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Abstract
Ovariectomized rats were implanted with oestradiol-filled elastomer capsules which were removed at various times after implantation. Sexual receptivity was tested after implantation of a progesterone-filled elastomer capsule 42 h after the onset of treatment with oestradiol. Exposure to oestradiol for about 32 h was required for induction of receptivity. Intact rats with regular 4 day oestrous cycles, and exposed to oestradiol-filled elastomer capsules for 6 h, showed sexual receptivity 24 h after the onset of oestradiol treatment. The behavioural effects of oestradiol in both ovariectomized and intact rats depended on when during the light–darkness (LD) cycle stimulation with oestradiol occurred; maximum effects were seen only if the oestradiol capsules were implanted at 16.00 h (4 h after lights off in the 12 h L: 12 h D cycle). The behavioural effect of progesterone implants, however, did not depend on the phase of the LD cycle. The LD-dependent rhythm in oestradiol sensitivity was eliminated by lesions in the suprachiasmatic nuclei of the hypothalamus.

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