IMMUNOHISTOCHEMICAL AND NEUROCHEMICAL EVIDENCE FOR GABA(A) RECEPTOR HETEROGENEITY BETWEEN THE HYPOTHALAMUS AND CORTEX

This study examined both the function of the GABA(A) 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 GABA(A) receptor subunit combinations potentially exist in various brain regions which, presumably, would intimate diffe ring receptor structure and function. Here, we present evidence that t he average functional characteristics of GABA(A) receptors within the rat hypothalamus are considerably different from those of the cerebral cortex. We assessed two neurochemical measures of GABA(A) receptor fu nction: namely, chloride-facilitation of [H-3]flunitrazepam binding an d GABA-mediated (36)chloride uptake. [H-3]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 [H-3]flunitrazepam binding was not responsive to i ncreasing chloride-concentration in either the basal or restraint cond itions. Also, maximal facilitation of GABA-mediated (36)chloride uptak e 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 mu M diazepam shifted GABA-mediated (36)chlori de uptake curves of the cortex to the left, diazepam addition appeared to be without effect in the hypothalamus. However, the blunted maxima l facilitation of GABA on hypothalamic (36)chloride uptake made accura te determination of the EC(50) for the diazepam-potentiation difficult . In addition to these functional disparities between the regions, dif ferences in subunit expression were also apparent. Distributions of al pha 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 th e hypothalamus and cortex, and dense in both regions. However, the alp ha 2 and alpha 3 subunits, while of intermediate density in cortex, we re of low density or absent alpha 3 in the hypothalamus. The alpha 2-i mmunoreactivity 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 GABA(A ) receptor differs in the hypothalamus relative to the cortex and this would seem related to the presence of different receptor ct subunits in homogenate preparations of the two regions.