Cloning and functional expression of the bovine GABA_C ρ2 subunit: Molecular evidence of a widespread distribution in the CNS

GABA_C receptors were first described as a non-desensitizing, bicuculline- and baclofen-insensitive component in Xenopus oocytes expressing bovine retina mRNA. However, the expression, tissue distribution and functional properties of GABA_C receptors from other areas of the CNS still remain controversial. In previous experiments, the injection of rat cerebellum mRNA into Xenopus oocytes induced the expression of receptors that generated currents with both GABA_A and GABA_C characteristics; the latter component apparently being given by the ρ2 subunit, suggesting the expression of GABA_C receptors in the CNS and the formation of homooligomeric receptors. In this study, using RT-PCR, we found that the ρ1 and ρ2 subunits are widely expressed in the CNS including areas where they have not been previously described such as the bulb, pons and the caudate nucleus. To determine if the GABA_C component of the GABA-currents elicited by oocytes expressing cerebellum mRNA was caused by activation of homomeric GABA ρ2 receptors, we cloned the corresponding cDNA and expressed it in Xenopus oocytes. It was found that oocytes injected with ρ2 cDNA, efficiently formed GABA-gated homooligomeric receptors. The GABA-dose-current response gave an EC50 = 1.19 μM and the currents were resistant to bicuculline and reversibly antagonized by the specific GABA_C receptor antagonist TPMPA. Altogether, our results indicate a widespread distribution of both ρ1 and ρ2 subunits in the bovine CNS and show further that the ρ2 subunit cDNA isolated from cerebellum, forms fully functional receptors when expressed in Xenopus oocytes.