Chicken GABAA, receptor β4 subunits form robust homomeric GABA-gated channels in Xenopus oocytes

Chicken GABA, receptor beta 4L and beta 4S subunits were expressed in Xenopus oocytes by cRNA injection. Oocytes expressing either beta 4 subunit alone or in combination with the chicken alpha 1 subunit were studied using the two-electrode voltage-clamp technique. Both the beta 4L and beta 4S subunits form homomeric GABA-gated Cl- channels with similar efficiencies. In comparison, oocytes expressing either the chicken alpha 1 or beta 2S polypeptide show no or barely detectable GABA responses, as reported by others for most single-subunit vertebrate GABA(A) receptors. The GABA-gated currents due to the beta 4L-subunit homomer were not affected by the presence of actinomycin D during cRNA expression, indicating that nascent oocyte polypeptides are not required for channel formation. The homomeric beta 4L-subunit receptors show high affinity for GABA with an EC50 value of 4.3 +/- 0.4 mu M and a Hill coefficient of 1.1 +/- 0.1 (n = 6). In response to GABA application at the EC25 value, currents elicited from the beta 4L-subunit receptor are enhanced by 50 mu M pentobarbital (110 +/- 10%, n = 3) and 10 mu M loreclezole (60 +/- 3%, n = 3), inhibited by 10 mu M picrotoxinin (93 +/- 3%, n = 3), but not affected by 1 mu M diazepam. These properties are similar to those found for oocytes expressing heteromeric chicken alpha 1 beta 4L and alpha 1 beta 2S receptors. Since the beta subunits of GABA(A) receptors provide essential determinants for receptor assembly and subcellular localization, homomeric beta 4-subunit receptors are a useful model system for further study of the structure and function of GABA(A) receptors. (C) 1998 Elsevier Science B.V. All rights reserved.