Regional differences in the enhancement by GABA of [H]zolpidem binding to ω1 sites in rat brain membranes and sections

The potential heterogeneity in the allosteric coupling between GABA and ω₁ binding sites within the native GABA_A receptor complex has been evaluated in the rat by measuring the enhancement by GABA of [³H]zolpidem binding to ω₁ site in membranes from three rat brain structures (neocortex, cerebellum and hippocampus) and brain sections. The maximal stimulatory effect of GABA was significantly higher (265 ± 47%) in cortical membranes than in cerebellar (165 ± 48%) or in hippocampal (118 ± 17%) membranes. These differences are not due either to the presence of different amounts of residual GABA in the membrane preparations or to the labeling, in presence of GABA, of binding sites other than ω₁ since: (1) the pharmacological properties of the [³H]zolpidem binding sites were similar in the three regions; (2) the degree of allosteric enhancement was unrelated to the relative proportion of ω₁ sites in each structure; and (3) GABA did not increase the B_max for [³H]zolpidem. Regional differences in the effect of 100 μM GABA on [³H]zolpidem binding were also observed by quantitative autoradiography. Regions where the strongest (3–4-fold) effects of GABA in [³H]zolpidem binding were observed included the substantia nigra, lateral geniculate body, olfactory tubercule and red nucleus. A large increase in [³H]zolpidem binding was also demonstrated in the cingulate and frontoparietal cortices with higher effects in deep (4.2-fold) rather than in superficial layers (3.3-fold). Heterogeneous subregional increases in [³H]zolpidem binding in the presence of GABA were quantified within the cerebellum, hippocampus and superior colliculus. In the cerebellum the effect of this neurotransmitter was larger in the molecular (3.8-fold) than in the granular (2.2-fold) layer. In the hippocampus the effect of GABA was also heterogeneous with larger increases in CA1 and CA2 fields (3.5-fold) than in CA3 field (2.2-fold) and dentate gyrus (2.5-fold). Finally in the deep layers of the superior colliculus GABA stimulation of [³H]zolpidem binding was greater than the superficial layer. In the other structures examined the GABA-induced increase in [³H]zolpidem binding was less than 3-fold. The smallest stimulations were quantified in the entorhinal cortex (2.1-fold), amygdala (2.4-fold) and nucleus accumbens (1.7-fold). These results suggest that [³H]zolpidem sites are associated to, at least, two GABA_A receptor subtypes that can be differentiated by their allosteric interaction between GABA and [³H]zolpidem sites.