Immunohistochemical and neurochemical evidence for GABAA receptor heterogeneity between the hypothalamus and cortex.

This study examined both the function of the GABAA receptor complex and the expression of its alpha 1, alpha 2 and alpha 3 subunits within the hypothalamus as compared to that of the cerebral cortex. A large number of different GABAA receptor subunit combinations potentially exist in various brain regions which, presumably, would intimate differing receptor structure and function. Here, we present evidence that the average functional characteristics of GABAA receptors within the rat hypothalamus are considerably different from those of the cerebral cortex. We assessed two neurochemical measures of GABAA receptor function: namely, chloride-facilitation of [3H]flunitrazepam binding and GABA-mediated 36chloride uptake. [3H]Flunitrazepam binding in the rat cortex was facilitated by increasing concentrations (12.5-500 mM) of chloride, and this facilitation was responsive to 15 min restraint. Yet, hypothalamic [3H]flunitrazepam binding was not responsive to increasing chloride-concentration in either the basal or restraint conditions. Also, maximal facilitation of GABA-mediated 36chloride uptake was significantly blunted in the hypothalamus relative to cortex (7.4 +/- 0.9 versus 35.8 +/- 1.5 nmoles/mg protein, respectively). While in vitro addition of 10 microM diazepam shifted GABA-mediated 36chloride uptake curves of the cortex to the left, diazepam addition appeared to be without effect in the hypothalamus. However, the blunted maximal facilitation of GABA on hypothalamic 36chloride uptake made accurate determination of the EC50 for the diazepam-potentiation difficult. In addition to these functional disparities between the regions, differences in subunit expression were also apparent. Distributions of alpha 1, alpha 2 and alpha 3 subunit immunoreactivities within cingulate, parietal and temporal cortices and 8 major hypothalamic regions were assessed. Staining of the alpha 1 subunit was prevalent throughout the hypothalamus and cortex, and dense in both regions. However, the alpha 2 and alpha 3 subunits, while of intermediate density in cortex, were of low density or absent (alpha 3) in the hypothalamus. The alpha 2-immunoreactivity was restricted to cell bodies of the arcuate nucleus, dorsomedial nucleus and overlying dorsal area and to neuropil staining of the median eminence. Thus, functional responsiveness of the GABAA receptor differs in the hypothalamus relative to the cortex and this would seem related to the presence of different receptor alpha subunits in homogenate preparations of the two regions.