New Facts Concerning the Role Played by the Ventromedial Nucleus in the Control of Estrous Cycle Duration and Sexual Receptivity in the Rat

Bilateral lesions of the ventromedial nucleus(VMN) were performed in the 3rd cycle of a sequence of 4-day cycles in Wistar rats by means of stereotaxically-oriented platinum unipolar electrodes. Small (SVMN) or large (LVMN) lesions were produced by the passage of a cathodic current for 7 or 14 sec, respectively. Groups of intact 4-day cyclic and 5-day cyclic rats, and of dorsomedial nucleus (DMN) lesioned and VMN sham (SH) operated females served as controls. A modification of the duration of one or both of the 2 first post-operative cycles was found in 31 out of 60 SVMN and in 11 out of 12 LVMN lesioned females. Only 2 out of 16 DMN lesioned and none of the 17 SH operated rats exhibited a change of post-operative cycle length. No modification of the duration of the 4th and/or the 5th cycles occurred in control non-operated females after a sequence of three 4-day cycles. Further study of vaginal cycles in animals with modified cycle duration showed that estrous rhythm changed from 4- to 5-day or to alternate 4- and 5-day cycles following VMN lesions. Ovulation regularly occurred during the night following proestrus of either 4- or 5-day cycles in VMN lesioned animals. Neither frequency of ovulation nor mean number of corpora lutea/rat differed when compared with DMN lesioned or SH operated females. On the contrary, a significant diminution of sexual receptivity resulted from SVMN lesions in comparison with DMN and SH animals (4-day cycles) or with intact control animals (5-day cycles). LVMN lesions almost completely abolished mating receptivity.