Abstract

The anxiolytic and anticonvulsant effects of benzodiazepines, barbiturates, ethanol and neuroactive steroids are mediated by selective interactions with gamma-aminobutyric acidA (GABA(A)) receptors. Chronic ethanol exposure decreases the sensitivity of GABA(A) receptors to benzodiazepines, barbiturates and ethanol. Ethanol withdrawing rats are cross-tolerant to the anticonvulsant effects of benzodiazepines as shown by a 16% decrease in the anticonvulsant efficacy of diazepam compared to controls. In contrast, ethanol withdrawing rats are sensitized to the anticonvulsant effects of the neuroactive steroid 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-THP), exhibiting a 46% increase in the anticonvulsant effect against bicuculline-induced seizures compared to control rats. This effect may involve a change in the sensitivity of GABA(A) receptors to 3 alpha,5 alpha-THP because potentiation of GABA(A) receptor mediated chloride uptake into cerebral cortical synaptoneurosomes is enhanced by 3 alpha,5 alpha-THP up to 50% in ethanol withdrawing rats compared to controls. 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (THDOC) potentiation of GABA(A) receptor-mediated chloride uptake is also enhanced during ethanol withdrawal. Moreover, the plasma levels of 3 alpha,5 alpha-THP and progesterone did not differ in ethanol withdrawing rats compared to controls. These alterations in neurosteroid sensitivity were also accompanied by selective alterations in cortical GABA(A) receptor subunit mRNA levels. Levels for the alpha 1 and alpha 4 subunit showed only slight alteration during withdrawal whereas we had previously observed a significant decrease in alpha 1 and a significant increase in alpha 4 mRNA levels in ethanol dependent (not withdrawing) animals. beta 2, beta and gamma 1 mRNA levels significantly increased during ethanol withdrawal. Taken together, these results suggest that ethanol withdrawal produces alterations in GABA(A) receptors that sensitize rats to the pharmacological effects of neuroactive steroids. Because ethanol-dependent or withdrawing rats are tolerant to the intoxicating, anxiolytic and anticonvulsant effects of ethanol and cross-tolerant to many effects of benzodiazepines and barbiturates, sensitization to the effects of neuroactive steroids could have significant therapeutic potential.