Progesterone action on estrous rhythm in the rat following ventromedial nucleus lesions

Small bilateral lesions of the hypothalamic ventromedial nucleus (SVMN) were produced in 4 day cyclic female Wistar rats by passing a 2 mA cathodic current through stereotaxically oriented platinum unipolar electrodes for 7 sec. The 24 h lengthening of cycle which occurred in 111 our ot 208 females was shown to depend upon lesion placement. Early s.c. injection of 0.75 or 1.5 mg progesterone caused 24 h cycle prolongation more frequently in SVMN-lesioned females with maintained 4 day cycles than in 4 day cyclic unoperated, sham VMN-lesioned, or dorsomedial nucleus (DMN) lesioned females. This effect proved to be related to lesion placement. It was therefore concluded that the VMN is involved in the mechanisms whereby progesterone controls estrous cycle duration in the rat.