Lesions in the preoptic area suppressed sexual receptivity in ovariectomized rats with estrogen implants in the ventromedial hypothalamus.

The objective of the experiment was to determine if electrolytic ablation of a portion of the preoptic area (POA) influenced the activation of female lordosis behavior by implants of estradiol benzoate in the ventromedial hypothalamus (VMH) of ovariectomized (OVX) rats. Two weeks after ovariectomy, rats received either bilateral electrolytic lesions (2 mA for 10 sec in Experiment 1, or 1 mA for 5 sec in Experiment 2) in the POA, or sham lesions (all procedures except passage of current). On the same day (day 0 of the experiment) thirty-gauge stainless steel cannulae containing crystalline estradiol benzoate were stereotaxically placed bilaterally into the VMH of all the rats. Subsequently, females were tested for the lordosis response to stud males on days 2, 4, 6, 8, and 10 in Experiment 1 or on days 7, 14 and 15 in Experiment 2. All rats received 0.5 mg progesterone (SC) only before the last test. A female was considered sexually receptive if she exhibited a lordosis quotient (LQ) greater than or equal to 10 (LQ = No. lordosis responses/10 mounts by male X 100). The frequencies for sexual receptivity in females with POA lesions were significantly lower than those for control females without lesions in both experiments. Additionally the degree of receptivity (lordosis quotient) was significantly lower on each test day for rats with POA lesions than that for rats without POA lesions. The results imply that the maintenance of the integrity of the POA under this experimental condition was important for the expression of the facilitative influence of the VMH on lordotic responsiveness.