Finasteride, a 5α-reductase inhibitor, blocks the anticonvulsant activity of progesterone in mice

Progesterone is an effective anticonvulsant against pentylenetetrazol (PTZ) seizures. This action is hypothesized to require the metabolic conversion of progesterone to the γ-aminobutyric acid(A) receptor potentiating neuroactive steroid allopregnanolone by 5α-reductase isoenzymes followed by 3α-hydroxy oxidoreduction. We evaluated this possibility using the competitive 5α-reductase inhibitor finasteride. Progesterone (50-200 mg/kg, i.p.) protected mice against PTZ-induced seizures in a dose-dependent manner (ED₅₀, 94 mg/kg). Pretreatment with finasteride (50-300 mg/kg, i.p.) produced a dose-dependent (ED₅₀, 146 mg/kg) reversal of the protective effects of progesterone (2 x ED₅₀ dose = 188 mg/kg). In contrast, finasteride (up to 300 mg/kg) failed to affect the anticonvulsant activity of allopregnanolone (10-30 mg/kg, i.p.; ED₅₀, 12 mg/kg). Finasteride (up to 300 mg/kg) did not block the protective effect of high doses of progesterone (250-350 mg/kg) on tonic hindlimb extension in the maximal electroshock seizure test (progesterone ED₅₀, 235 mg/kg). The anticonvulsant activity of progesterone against PTZ-induced seizures can be blocked by 5α-reductase inhibition, providing strong evidence that the anticonvulsant effect of the steroid in this model is mediated by its active metabolite allopregnanolone.