Effects of repeated administration of estradiol benzoate on tubero-infundibular GABAergic activity in male rats

Repeated (once a day for 8 days) but not single administration of estradiol benzoate (10 μg/kg, s.c.) induced a sevenfold increase in anterior pituitary γ-aminobutyric acid (GABA) concentration in male rats. GABA concentration also increased in the median eminence whereas no changes or decreases were observed in other brain regions including hypothalamic arcuate nucleus, lateral septum, hippocampus, caudate nucleus, and substantia nigra. Eight-day estradiol benzoate injection also enhanced the V(max) of median eminence glutamate decarboxylase activity without affecting the K(m) of the enzyme for glutamic acid. Taken together, these results suggest that repeated administration of estradiol benzoate increases the activity of the tubero-infundibular GABAergic system in male rats.