Effect of single or repeated estrogen administration on tuberoinfundibular GABA neurons and anterior pituitary GABA receptors: biochemical and functional studies.
The effect of single or protracted administration of estradiol valerate on the hypothalamo-pituitary gamma-aminobutyric acid (GABA)ergic system and on plasma prolactin levels has been evaluated in female rats 2 months after the last (chronic treatment) or the single dose of the steroid. In the group of animals receiving one dose of estrogen, no modifications were detected in the activity of the tuberoinfundibular GABAergic neurons as implied by unchanged GABA accumulation either in the median eminence or the anterior pituitary after blockade of GABA catabolism with ethanolamine-O-sulphate. However, a complete disappearance of the low affinity population of GABA receptors in the anterior pituitary was observed. In this experimental condition, where baseline prolactin levels were 3-fold higher than in control rats, muscimol, a potent GABA agonist, was effective in significantly lowering plasma prolactin concentrations. Chronic estradiol valerate administration reduced GABA accumulation in the median eminence and the anterior pituitary at 4, but not at 2 h, after intracerebroventricular injection of ethanolamine-O-sulphate. Moreover, in this instance, a complete disappearance of the high affinity population of GABA receptors in the anterior pituitary was detected. Long-term estrogen administration induced also a 55-fold increase of plasma prolactin titers and muscimol was ineffective in reducing prolactin concentrations in plasma. The ability of muscimol to inhibit prolactin release only in single-estrogen-treated animals strongly suggests that the high affinity population of anterior pituitary GABA receptors is that involved in the mechanisms whereby GABA inhibits prolactin release from anterior pituitary.
D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D001667:Binding, Competitive; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D005260:Female; D007030:Hypothalamo-Hypophyseal System / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D009118:Muscimol / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000378:metabolism; D011388:Prolactin / Q000097:blood; D051381:Rats; D011919:Rats, Inbred Strains; D011963:Receptors, GABA-A / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D011963:Receptors, GABA-A; D009118:Muscimol; D004958:Estradiol; D005680:gamma-Aminobutyric Acid; D011388:Prolactin; D001640:Bicuculline