Follicular growth, ovulatory phenomena and ventromedial nucleus lesions in the rat.

Small ventromedial nucleus lesions were placed in 4-day cyclic female rats in order to determine whether cycle lengthening - previously observed in some females following such lesions - could result from changes in follicular growth. A slowing up in follicle development was observed in lesioned females with a 24-hour cycle lengthening as compared to those with a maintained 4-day cyclicity. Similarly, natural 5-day cyclers displayed slower follicular growth than natural 4-day cyclers. A rebound effect in follicular growth was observed on proestrus in either natural or VMN lesioned 5-day cyclers compared to either natural or VMN lesioned 4-day cyclers. Concomitantly blood FSH level appeared to be higher on proestrus morning and afternoon in 5-day than in 4-day cyclers. The pattern of LH release differed between natural 4-day and natural 5-day cyclers on proestrus at 17:00-19:00 h. LH release appeared to be delayed by one hour in VMN lesioned rats with 24 hour cycle lengthening as compared to natural 5-day cyclers. The duration of LH release in VMN-lesioned rats with a maintained 4-day cyclicity was shorter than in natural 4-day cyclers.