Jr. Inglefield et al., IMMUNOHISTOCHEMICAL AND NEUROCHEMICAL EVIDENCE FOR GABA(A) RECEPTOR HETEROGENEITY BETWEEN THE HYPOTHALAMUS AND CORTEX, Journal of chemical neuroanatomy, 7(4), 1994, pp. 243-252
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.