Temporal limits for copulation-induced ovulation in the pentobarbital-blocked proestrous rat

In two experiments, factors critical to copulation-induced ovulation (CIO) were examined. In Exp I cycling Sprague-Dawley rats were given 25 mg/kg sodium pentobarbital (PB) at 1345 h on proestrus and allowed overnight copulation starting at 1900 (dark onset), 2100, or 2300 h. Only the groups in which copulation began at 1900 or 2100 h displayed ovulation significantly more frequently than PB-treated controls. In Exp II rats received an atrial cannula on diestrus II and 25 mg/kg PB at 1345 h on proestrus. Four blood samples were taken over the course of an hour, starting at 1900 or 2300 h. Controls did not ovulate that night. Rats allowed to copulate for 30 or 60 min starting at 1900 h ovulated and showed elevated plasma LH levels during the period sampled. Copulation at 23 h or cervical probing at 1900 h did not induce LH release or ovulation. There were no differences in the behavior of the males or females during the first 30 min of cohabitation in either experiment; the females were highly receptive. It is concluded that a narrow temporal window exists for CIO, and that temporal changes in induced LH release but not behavior limit CIO.