Changes in GABA_A Receptor Gene Expression Induced by Withdrawal of, but Not by Long-Term Exposure to, Ganaxolone in Cultured Rat Cerebellar Granule Cells

Abstract

The effects of ganaxolone, a synthetic analog of the endogenous neuroactive steroid allopregnanolone, on the function and expression of GABA_A receptors were determined. Electrophysiological recordings demonstrated that ganaxolone potentiated with a potency and efficacy similar to those of allopregnanolone the Cl⁻currents evoked by GABA at recombinant human GABA_Areceptors (comprising α1β2γ2L or α2β2γ2L subunit assemblies) expressed in Xenopus oocytes. Exposure of cultured rat cerebellar granule cells to 1 μM ganaxolone for 5 days had no effect on the abundance of mRNAs encoding the α1, α2, α3, α4, α5, γ2L, or γ2S subunits of the GABA_A receptor. Withdrawal of ganaxolone after such long-term treatment, however, induced an increase in the abundance of α2, α4, and α5 subunit mRNAs and a decrease in the amounts of α1, γ2L, and γ2S subunit mRNAs. These changes were maximal 3 to 6 h after drug withdrawal and were reversible, being no longer apparent after 24 h. These results suggest that long-term exposure of cerebellar granule cells to ganaxolone does not affect the sensitivity of the GABA_Areceptor to several positive modulators. Nevertheless, the reduction in the amounts of the α1 and γ2 subunit mRNAs together with the increase in the abundance of the α4 subunit mRNA induced by abrupt discontinuation of long-term treatment with ganaxolone suggest that withdrawal of this drug might result in a reduced response to classic benzodiazepines.