Core temperature in the female rat: effect of ovariectomy and induction of pseudopregnancy

Nocturnal peaks in core body temperature of rats during the estrous cycle were highest during the night of ovulation (2300-0200 h, proestrus-estrus) and lowest during the night before (diestrus 2-proestrus). Less dramatic diurnal secondary peaks, absent only during estrus, occurred 3-4 h after the onset of daylight. After induction of pseudopregnancy, mean temperature declined, but both daily peaks persisted until the first postluteal estrus, when the secondary peak was again absent transiently. Ovariectomy reduced mean core temperature and abolished all secondary peaks. In contrast, castration during pseudopregnancy did not abolish the secondary peaks. When cyclic rats were gonadectomized (abolishing the secondary rhythm) it was possible to re-establish this rhythm by stimulating the uterine cervix (as if to induce pseudopregnancy). However, in animals exposed to darkness (which also abolishes the secondary rhythm) reinduction by cervical stimulation was ineffective. These results indicated that the integrity of the secondary peak, though dependent on photoperiod, nevertheless was influenced by a neuroendocrine reflex arc.