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

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(A) receptors (comprising alpha1beta2gamma2L or alpha2beta2gamma2L subunit assemblies) expressed in Xenopus oocytes. Exposure of cultured rat cerebellar granule cells to 1 microM ganaxolone for 5 days had no effect on the abundance of mRNAs encoding the alpha1, alpha2, alpha3, alpha4, alpha5, gamma2L, or gamma2S subunits of the GABA(A) receptor. Withdrawal of ganaxolone after such long-term treatment, however, induced an increase in the abundance of alpha2, alpha4, and alpha5 subunit mRNAs and a decrease in the amounts of alpha1, gamma2L, and gamma2S 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(A) receptor to several positive modulators. Nevertheless, the reduction in the amounts of the alpha1 and gamma2 subunit mRNAs together with the increase in the abundance of the alpha4 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.