?-Aminobutyric AcidBReceptor Activation Modifies Agonist Binding to ?-Adrenergic Receptors in Rat Brain Cerebral Cortex

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Abstract
The interaction of isoproterenol with .beta.-adrenergic receptor (.beta.AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of .gamma.-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered .beta.AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between .beta.AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.