Involvement of GABA in the feedback action of estradiol on gonadotropin and prolactin release: hypothalamic GABA and catecholamine turnover rates.

The concentrations and turnover rates of norepinephrine (NE), dopamine (DA) and gamma-aminobutyric acid (GABA) were measured in discrete brain areas of diestrous (D), proestrous (P), ovariectomized (OVX) and ovariectomized rats treated with estradiol-benzoate (EB) 12 or 24 h before decapitation. The turnover of NE in the medial preoptic area (MPO) correlates well with plasma LH levels under all endocrine conditions showing high NE turnover in P and OVX and low NE turnover in D and OVX-EB animals. The DA turnover shows no hormone-dependent changes in the MPO. In those animals where estrogens exert no (OVX) or a negative feedback action (D, OVX-EB) on LH the GABA turnover correlates inversely with LH and preoptic NE turnover showing low GABA turnover values in OVX and high values in D and OVX-EB. For P animals the inverse correlation cannot be confirmed. It is concluded that GABA mediates the negative feedback action of estrogens to LH-RH perikarya located in the MPO. GABA might act by presynaptic inhibition of NE axon terminals. This hypothesis is supported by morphological findings which indicate that axon terminals in the MPO are in close contact without separating glial lamellae. In the anterior mediobasal hypothalamus (AMBH) NE turnover correlates best with serum prolactin levels being high in P and OVX animals 24 h after EB treatment. The DA turnover is increased in OVX rats 24 h after EB. It is not yet clear if this increase might be a consequence of the elevated prolactin levels. GABA turnover in the AMBH shows no significant changes. GABA concentrations and turnover rates were also determined in the mediocortical amygdala where estrogen receptors have been reported and in the nucleus accumbens. No significant changes could be observed in these regions.