Hypothalamic ventromedial nucleus and hormonal regulation of estrous rhythm in the rat.

Abstract

An experiment was performed to determine whether the ventromedial nucleus (VMN) lesions could modify the duration of the estrous cycle by altering ovarian progesterone secretion. In the 1st group of rats small bilateral stereotaxic lesions were performed on the VMN by passing a 2 mA cathodic current for 7 seconds through a .25 mm diameter teflon-insulated platinum unipolar electrode. 41 of 88 rats retained a 4-day cyclicity. 16 were sacrificed in the morning of estrus of their 3rd postoperative cycle to monitor estrous rhythm stability. The remaining rats were used for progesterone radioimmunoassay. 43 rats exhibited a 24-hour delay in cycle length postoperatively. A 2nd group was composed of 29 unoperated 4-day cyclers and 28 sham-VMN- and 18 DMN-lesioned 4-day cyclic females all of whom maintained 4-day cycles after surgery. Peripheral blood progesterone decreased from the morning of Diestrous 1 to that of Diestrous 2 in both unoperated and small VMN-lesioned 4-day cyclic females. Blood progesterone levels remained constant during the same period in unoperated 5-day cyclic females. The levels however increased in small VMN-lesioned 5-day cyclic animals reaching a higher level on the morning of Diestrous 2 than in unoperated 5-day cyclers (p less than .01). Results confirm that partial lesioning of the VMN causes a 24-hour cycle prolongation in about 50% of 4-day cyclic female rats.