The neurosteroid environment in the hippocampus exerts bi-directional effects on seizure susceptibility in mice

The progesterone derivative allopregnanolone (ALLO) rapidly potentiates γ-aminobutyric acid_A (GABA_A) receptor mediated inhibition. The present studies determined whether specific manipulation of neurosteroid levels in the hippocampus would alter seizure susceptibility in an animal model genetically susceptible to severe ethanol (EtOH) withdrawal, Withdrawal Seizure-Prone (WSP) mice. Male WSP mice were surgically implanted with bilateral guide cannulae aimed at the CA1 region of the hippocampus one week prior to measuring seizure susceptibility to the convulsant pentylenetetrazol (PTZ), given via timed tail vein infusion. Bilateral intra-hippocampal infusion of ALLO (0.1 μg/side) was anticonvulsant, increasing the threshold dose of PTZ for onset to myoclonic twitch and face and forelimb clonus by 2- to 3-fold. In contrast, infusion of the 5α-reductase inhibitor finasteride (FIN; 2 μg/side), which decreases endogenous ALLO levels, exhibited a proconvulsant effect. During withdrawal from chronic EtOH exposure, WSP mice were tolerant to the anticonvulsant effect of intra-hippocampal ALLO infusion, consistent with published results following systemic injection. Finally, administration of intra-hippocampal FIN given only during the development of physical dependence significantly increased EtOH withdrawal severity, measured by handling-induced convulsions. These findings are the first demonstration that bi-directional manipulation of hippocampal ALLO levels produces opposite behavioral consequences that are consistent with alterations in GABAergic inhibitory tone in drug-naive mice. Importantly, EtOH withdrawal rendered WSP mice less sensitive to ALLO's anticonvulsant effect and more sensitive to FIN's proconvulsant effect, suggesting an alteration in the sensitivity of hippocampal GABA_A receptors in response to fluctuations in GABAergic neurosteroids during ethanol withdrawal.