Brain allopregnanolone regulates the potency of the GABA A receptor agonist muscimol

Allopregnanolone (ALLO), a potent positive-allosteric modulator of the action of GABA at GABA A receptors, is synthesized in the brain from progesterone by the sequential action of two enzymes: 5α-reductase and 3 α-hydroxysteroidoxidoreductase. The concentration of ALLO in various parts of the mouse brain varies substantially, from 15 pmol/g in the olfactory bulb, to approximately 6 pmol/g in the frontoparietal cortex, and 2.7 pmol/g in the cerebellum. The systemic administration of 48 μmol/kg of the Type I and Type II 5α-reductase inhibitor, (17β)-17-[bis (1-methylethyl) amino carbonyl)] androsta-3, 5-diene-3-carboxylic acid (SKF 105,111), reduced brain ALLO content by 80–90% in 30 min; the rate constant ( k) of ALLO decrease in each brain area can be utilized to establish the rate of ALLO biosynthesis, which is higher in the olfactory bulb (62 pmol/g/h) than in the frontoparietal cortex (24 pmol/g/h) or cerebellum (11 pmol/g/h). The duration of the righting reflex loss elicited by the potent GABA A receptor agonist muscimol was reduced in SKF 105,111-treated ALLO-depleted mice. SKF 105,111 treatment had no effect on muscimol metabolism or on brain levels of pregnenolone and progesterone; however, the brain levels of 5α-DHP, the precursor of ALLO, were also decreased. Administration of ALLO at a dose of 15 μmol/kg ip by itself did not alter the muscimol-induced loss of the righting reflex; but it completely blocked the effect of SKF 105,111. To elucidate the possible molecular mechanism by which a decrease of brain ALLO content can shorten the duration of the righting reflex loss elicited by muscimol, we patch-clamped neocortical pyramidal neurons of mice pretreated with SKF 105,111 or vehicle, and studied the efficiency of muscimol in eliciting Cl − currents. The current amplitude was significantly smaller in neurons from SKF 105,111-treated mice, especially at lower doses (0.1–1 μM) of muscimol, and the muscimol dose–response (0.1–10 μM) relationship displayed cooperativity ( n H=1.4). These data suggest that ALLO synthesized in brain plays an important physiological permissive role in the modulation of GABA-gated Cl − channel function.