Influence of Estrogen and Progesterone on Glutamic Acid Decarboxylase Activity in Discrete Regions of Rat Brain

Abstract: Female Charles River rats were ovariectomized and treated for three days with 17/8-estradiol benzoate (E) (1.0 /μg/day), progesterone (P) (500 μg/day), vehicle, or a combined treatment (2 days E, one day P). Animals were killed on day 3 and the brains were dissected by the micropunch technique. Glutamic acid decarboxylase (GAD) activity was measured by collection of ¹⁴CO². Estradiol benzoate and progesterone were potent inhibitors of GAD activity in regions such as the arcuate nucleus, ventromedial hypothalamus and corticomedial amygdala. Estrogen reduced the V_max of GAD for glutamate as a substrate without changing the K_m. Estrogen also failed to change the K_m for pyridoxal phosphate. Combined treatment with estrogen and progesterone did not reduce GAD from ovariectomized levels except in the septum, indicating an interaction of the two hormones at the level of GAD. The suggestion is made that under conditions that inhibit LH secretion GAD activity is low, but when LH secretion is stimulated GAD activity may be comparatively high.