Prototypic GABA_A receptor agonist muscimol acts preferentially through forebrain high-affinity binding sites

Muscimol has been regarded as a universal agonist for all γ-aminobutyric acid type A receptor (GABA_A-R) subtypes. However, brain regional distribution of muscimol's high-affinity binding sites greatly differs from those of other binding sites of the GABA_A-R. To test whether behavioral effects of muscimol correlated with the density of high-affinity 3 Hmuscimol binding, we examined several GABA A-R subunit gene-modified mouse lines: α1, α4, or-knockouts (KO), α4-double KO, and Thy1.2 promoter-driven α6 transgenic mice (Thy1α6). We determined the high-affinity 3 Hmuscimol binding in brain sections by quantitative autoradiography and sedative/ataxic effects induced in vivo by muscimol using a constant speed rotarod. α4-KO mice had reduced 3 Hmuscimol binding in the caudate-putamen, thalamus, and hippocampus, and were less sensitive to the behavioral impairment by muscimol. Similarly,-KO mice also had reduced binding to forebrain regions and a lower behavioral sensitivity to muscimol than their wild-type controls. In contrast, α1-KO mice had unaltered behavioral sensitivity to muscimol and unaltered 3 Hmuscimol binding, even though previous studies have demonstrated dramatically reduced binding to various other GABA_A-R sites in these mice. Finally, Thy1α6 mice exhibited increased behavioral sensitivity to muscimol, and to another direct GABA-site agonist gaboxadol, and increased 3 Hmuscimol binding in the cerebral cortex and hippocampus. Thus, the differences in sedative and motor-impairing actions of muscimol in various mouse models correlated with the level of forebrain high-affinity 3 Hmuscimol binding. These data suggest that a small special population of GABA_A-Rs, most likely extrasynaptic non-α1-containing receptors, strongly contributes to the in vivo pharmacological effects of muscimol.