GABAA receptor pharmacology and subtype mRNA expression in human neuronal NT2-N cells

Human NT2 teratocarcinoma cells differentiate into neuron-like NT2-N cells when treated with retinoic acid. GABA evoked concentration-dependent whole-cell currents in NT2-N cells with an EC50 of 21.8 mu M and a Hill slope of 1.2. GABA(A) receptor (GABAR) currents reversed at ECl- and did not display voltage-dependent rectification. GABAR single channels opened in bursts to a 23 pS main conductance level and a 19 pS subconductance level, with infrequent openings to a 27 pS conductance level. Kinetic properties of the main conductance level were similar to other native and recombinant GABAR channels. Diazepam and zolpidem enhanced GABAR currents with moderate affinity, whereas methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate inhibited GABAR currents. Loreclezole enhanced GABAR currents with high affinity, but furosemide antagonized GABAR currents with low affinity. The neurosteroids alphaxalone and pregnenolone sulfate appropriately modulated GABAR currents. Zinc blocked GABAR currents with low affinity, but lanthanum did not significantly alter NT2-N GABAR currents. Reverse transcription PCR (RT-PCR) performed on RNA from NT2-N cells clearly detected transcripts encoding human alpha 2, alpha 3, alpha 5, beta 3, gamma 3, and pi subtypes. The combined pharmacological and RT-PCR results are most consistent with a single or predominant GABAR isoform composed of an alpha 2 and/or alpha 3 subtype combined with the beta 3 and gamma 3 subtypes. The data do not rule out receptors containing combinations of alpha 2 and/or alpha 3 subtypes with the alpha 5 subtype or receptors with both pi and beta 3 subtypes. The presence or absence or the pi subunit in functionally expressed receptors could not be determined.