α-Aminobutyric Acid Receptors in Anterior Pituitary and Brain Areas after Median Eminence Lesions*

Abstract

Previous results have indicated that the median eminence (ME) plays a key role in the regulation of estrogen and dopamine receptors in the anterior pituitary (AP). Since 3H-labeled gamma-aminobutyric acid ([3H]GABA) receptors have been described in the pituitary as well as in the brain, the aim of the present work was to investigate whether those receptors are also under hypothalamic control. Ovariectomized adults rats were divided into two groups. In the first one, the ME was lesioned by an anodic current (MEL); the second group consisted of sham-operated controls. Animals were used 7-14 days thereafter to study [3H]GABA binding in a crude mitochondrial membrane fraction prepared from the anterior pituitary, the hypothalamic-preoptic suprachiasmatic area, or the frontoparietal cortex. Binding of [3H]GABA was a saturable process, with high affinity in all studied structures. In AP Scatchard analysis of the saturation curves indicated similar Kd values in sham and MEL rats; the maximal number of binding sites increased from 0.65 +/- 0.005 pmol/mg protein in controls to 1.05 +/- 0.006 pmol/mg protein in MEL rats (P less than 0.001). In the hypothalamus, Kd values were similar in both groups, but the maximum number of binding sites (Bmax) increased about 50% in MEL animals. In the frontoparietal cortex, Kd values were similar in control and lesioned animals; but the Bmax decreased by 40% after the lesion. Animals with lesions showed hyperprolactinemia and a reduction in serum levels of LH, FHS, and TSH. Thus, Kd values were of similar magnitude in all studied regions and were not altered by ME destruction. Bmax values in controls were higher in the cerebral cortex than in the hypothalamus and AP. After ME lesions, there was an increment of [3H]GABA binding in AP and hypothalamus and a reduction in cerebral cortex. The data indicate that the ME participates in the regulation of [3H]GABA binding in AP and also suggest that it plays a role in regulating GABA receptors in various regions of the brain.