Delta subunit inhibits neurosteroid modulation of GABAA receptors

Neurosteroid modulation of GABAA receptors has been observed with all subunit combinations investigated; however, hetero-oligomeric GABAA receptors containing delta subunits were not studied previously. We describe the effect of delta subunit expression on 3alpha,21-dihydroxy-5alpha-pregnan-20-1 (THDOC)-induced potentiation of GABA-gated currents in transfected HEK 293 cells and in cerebellar granule cells in vitro. THDOC (100 nM) significantly potentiated GABA-gated currents in cells transfected with combinations of alpha1, alpha6, beta3, and gamma2 subunit cDNAs, whereas cotransfection of delta subunit cDNA inhibited this potentiation. In contrast, the direct Cl- channel activation by THDOC at higher concentrations (1-10 microM) was not significantly dependent on delta subunit cotransfection. These results suggest that the presence of the delta subunit inhibits GABAA receptor modulation but not the direct activation by neurosteroids. Cotransfection with delta subunit also affected the negative allosteric modulation by pregnenolone sulfate. THDOC potentiation of GABA-gated currents was greater in cerebellar granule cell cultures at 4 d in vitro (DIV) compared with those at 14 DIV. Single-cell reverse transcription-PCR analysis of the mRNAs expressed in cultured cerebellar granule cells shows that an increased number of granule cells at 14 DIV express delta subunit mRNAs as compared with 4 DIV granule cells. The presence of delta subunit mRNAs detected in individual cells correlated well with the lack of sensitivity to THDOC. These results suggest that developmental expression of GABAA receptor delta subunits may play an important role in determining the region-specific neurosteroid-induced modification of fast inhibitory synaptic function.