Localized Increase of GABA Levels in Brain Areas of the Rat and Inhibition of the Plasma LH Rise following Orchidectomy

Abstract

Previous studies have suggested that gamma-aminobutyric acid (GABA) exerts inhibitory actions on luteinizing hormone (LH) secretion that are likely to be mediated by modifications in noradrenergic transmission. To explore further this hypothesis we have studied the effect of increasing GABA contents in discrete areas of the brain on plasma LH levels in short-term orchidectomized rats. GABA accumulation was produced by the GABA transaminase inhibitor, gamma-vinyl-GABA (GVG). The locus coeruleus area (LC), where the noradrenaline (NA) cells projecting through the dorsal noradrenergic bundle are located, and several hypothalamic areas that are innervated by NA-containing fibers were microinjected with GVG. Most of these areas are known to be related to the neural control of LH secretion. GVG microinjected in the LC and medial preoptic area increased the GABA content and blunted significantly the acute increase of plasma LH produced by castration. Bicuculline prevented these effects. Delayed effects of GVG were observed when applied in the anterior hypothalamic area and ventromedial-arcuate nucleus area. In these latter areas, a single injection of GVG did not augment the GABA concentrations and was unable to prevent LH release, but a clear inhibitory effect took place after a second injection of GVG between 24 and 48 h after orchidectomy. Unresponsive areas to GVG treatment were the lateral preoptic area, the median eminence and the dorsal raphe. These results add support to the view that GABA inhibits LH release in rats, at discrete areas of the brain.