Evaluation of GABAergic neuroactive steroid 3α-hydroxy-5α-pregnane-20-one as a neurobiological substrate for the anti-anxiety effect of ethanol in rats

Rationale: Acute systemic ethanol administration is known to elevate plasma and cerebral levels of neuroactive steroid 3 alpha-hydroxy-5 alpha-pregnane-20-one (3 alpha, 5 alpha-THP; allopregnanolone) to a concentration sufficient to potentiate GABA(A) receptors. We have earlier demonstrated that 3 alpha, 5 alpha- THP mediates the antidepressant-like effect of ethanol in Porsolt forced swim test. Objective: The aim of the present study is to explain the relationship between endogenous GABAergic neurosteroids and anxiolytic effect of ethanol in Sprague - Dawley rats. Method: The mediation of 3 alpha, 5 alpha-THP in the anti-anxiety effect of ethanol was assessed by pharmacological interactions of ethanol with various endogenous neurosteroidal modulators and using simulated physiological conditions of altered neurosteroid content in elevated plus maze (EPM) test. Results: Pretreatment of 3 alpha, 5 alpha-THP (0.5 - 2.5 mu g/rat, i.c.v.) or neurosteroidogenic agents such as 3 alpha, 5 alpha-THP precursor progesterone ( 5 or 10 mg/kg, i. p.), 11-beta hydroxylase inhibitor metyrapone ( 50 or 100 mg/kg, i. p.) or the GABA(A) receptor agonist muscimol ( 25 ng/rat, i. c. v.) significantly potentiated the anti-anxiety effect of ethanol ( 1 g/ kg, i. p.). On the other hand, the GABAergic antagonistic neurosteroid dehydroepiandrosterone sulphate (DHEAS) ( 1 mg/kg, i. p.), the GABA(A) receptor blocker bicuculline ( 1 mg/kg, i.p.), the 5 alpha- reductase inhibitor finasteride ( 50 x 2 mg/kg, s.c.) or the mitochondrial diazepam binding inhibitory receptor antagonist PK11195 ( 1 mg/kg, i.p.) reduced ethanol-induced preference of time spent and number of entries into open arms. Anti-anxiety effect of ethanol was abolished in adrenalectomized (ADX) rats as compared to sham-operated control. This ADX-induced blockade was restored by prior systemic injection of progesterone, signifying the contribution of peripheral steroidogenesis in ethanol anxiolysis. Socially isolated animals known to exhibit decreased brain 3 alpha, 5 alpha-THP and GABA(A) receptor functions displayed reduced sensitivity to the effects of ethanol and 3 alpha, 5 alpha-THP in EPM test. Conclusions: Our results demonstrated the contributory role of neuroactive steroid 3 alpha, 5 alpha-THP in the anti-anxiety effect of ethanol. It is speculated that ethanol-induced modulation of endogenous GABAergic neurosteroids, especially 3 alpha, 5 alpha-THP, might be crucial pertinent to the etiology of 'trait' anxiety ( tension reduction) and ethanol abuse.