Rates of release of GABA and catecholamines in the mediobasal hypothalamus of ovariectomized and ovariectomized estrogen-treated rats: correlation with blood prolactin levels.

Push-pull cannulae were implanted into the mediobasal hypothalamus of ovariectomized (ovx) rats. After recovery animals were treated with estradiolbenzoate (E2B) or oil and they were perfused 3 days later. Only the E2B-treated animals which exhibited prolactin surges in the afternoon without concomitant LH surges were used in this study. In ovx animals hypothalamic GABA release, measured in 5-min intervals, was pulsatile, with pulses occurring every 37 min. This pattern was profoundly affected by E2B treatment: the pulse frequency was significantly reduced to 1 pulse every 117 min in steroid-treated rats. No differences in overall mean GABA release rates and pulse amplitudes were observed in ovx vs. E2B-treated rats. Our earlier demonstration of the existence of a large number of estrogen-receptive, GABAergic neurons in the MBH of rats is suggestive that these neurons change their secretory pattern in response to estrogen treatment. Estrogen-induced prolactin surges were accompanied by increased hypothalamic NE release. Concomitant changes in DA or E release rates were not demonstrable since catecholamine concentrations were too low to be reliable. However, the daily overall release rates of these two catecholamines were lower in E2B-treated rats compared to ovx animals. It is concluded that the positive feedback action of estradiol on prolactin release involves a stimulatory noradrenergic mechanism and may also involve estrogen-receptive, GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)