Decreased GABA A and GABA B receptor functional activity in cannabinoid CB 1 receptor knockout mice

Abstract

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 ([³H]-Muscimol binding assay), GABA_B (baclofen-stimulated [³⁵S]-GTPγS binding assay), GABA_Aα₁, GABA_Aα₂ and GABA_Aγ₂ receptors gene expression (real-time reverse transcriptase polymerase chain reaction [PCR]) were carried out in CB₁ ^-/- and wild-type mice (CB₁ ^+/+). [³H]-Muscimol binding assays revealed significant reduction in the density of GABA_A receptors in CA2 (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]-GTPγS binding. The results showed significant reduced [³⁵S]-GTPγS 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 α₁, α₂ and γ₂ subunits of GABA_A receptor in the amygdala. The results showed significant reduced GABA_Aα₁ (50%) and GABA_Aα₂ (40%) receptor subunits gene expression in the amygdala of CB₁ ^-/- mice. No difference was observed in GABA_Aγ₂ 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.