Decreased GABAA and GABAB receptor functional activity in cannabinoid CB1 receptor knockout mice

The interaction between brain GABAergic and endocannabinoid systems was evaluated by examining the quantitative and functional status of GABAergic receptors in cannabinoid CB1 receptor knockout (CB1-/-) mice. To this aim, GABA(A) ([H-3]-Muscimol binding assay), GABA(B) (baclofen-stimulated [S-35]-GTP gamma S binding assay), GABA(A)alpha(1), GABA(A)alpha(2) and GABA(A)gamma(2) receptors gene expression (real-time reverse transcriptase polymerase chain reaction [PCR]) were carried out in CB1-/- and wild-type mice (CB1+/+). [H-3]-Muscimol binding assays revealed significant reduction in the density of GABA(A) receptors in CA(2) (30%) and DG (28%) of the hippocampus, thalamus (40%), cingulate (28%) and motor cortex (35%) of CB1-/- mice. Functional activity of metabotropic GABA(B) receptors was measured by evaluating the ability of GABA(B) agonist baclofen to stimulate [35S]-GTP gamma S binding. The results showed significant reduced [S-35]-GTP gamma S binding in CA1 (61%), CA3 (51%) and DG (60%) of CB1-/- mice compared with CB1(+/+) mice. Real-time reverse transcriptase PCR was carried out for evaluating gene expression of alpha(1), alpha(2) and gamma(2) subunits of GABA(A) receptor in the amygdala. The results showed significant reduced GABA(A)alpha(1) (50%) and GABA(A)alpha(2) ( 40%) receptor subunits gene expression in the amygdala of CB1-/- mice. No difference was observed in GABA(A)gamma(2) receptor subunit gene expression. This study provides strong evidence of the involvement of CB1 receptors in the control of GABAergic responses mediated by GABA(A) and GABA(B) receptors, and suggests a possible role of the endocannabinoid system in the regulation of anxiety-related disorders.