ABSENCE OF A DIURNAL RHYTHM IN LORDOSIS BEHAVIOUR INDUCED BY OESTROGEN IN GONADECTOMIZED RATS

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Abstract
Gonadectomized rats bearing s.c. Silastic capsules containing crystalline oestradiol-17β diluted with cholesterol, or oestradiol-17β dissolved in sesame oil were tested for the presence of a diurnal rhythm in the display of lordotic behaviour. In experiment 1, female rats received four consecutive tests at intervals of 8 h in a lighting regimen of 12 h light: 12 h darkness beginning 4, 14 and 28 days after implantation of 5 mm capsules of oestradiol. After a single test on day 4, male rats were tested on days 14–15 only, at the same times as the female rats. Female animals were tested while vaginal–cervical stimulation was prevented by vaginal masking beginning 35 days after implantation of oestradiol. In experiment 2, lordotic responsiveness of female rats was assessed beginning 4 days after implantation of oestradiol once on each of 3 consecutive days, with each test occurring at a different time of day. Finally, in experiment 3, female rats were tested as in experiment 1 beginning 4 days after implantation of lower threshold amounts of oestradiol in oil-filled capsules. In no experiment were changes in lordotic behaviour observed as a function of the time of day. These findings failed to support recent reports of a sexually dimorphic rhythm in lordotic responsiveness to oestradiol in the rat.