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

Allopregnanolone (ALLO), a potent positive-allosteric modulator of the acti on of GABA at GABA(A) receptors, is synthesized in the brain from progester one by the sequential action of two enzymes: 5 alpha-reductase and 3 alpha- 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 mu mol/kg of the Type I and Type II 5 alpha-reductase inhibitor, (17 beta)-17-[bis (1-methylethyl) amino carbonyl)] androsta-3, 5-diene-3-carboxylic acid (SW 105,111), reduce d brain ALLO content by 80-90% in 30 min; the rate constant (k) of ALLO dec rease in each brain area can be utilized to establish the rate of ALLO bios ynthesis, which is higher in the olfactory bulb (62 pmol/g/h) than in the f rontoparietal cortex (24 pmol/g/h) or cerebellum (11 pmol/g/h). The duratio n of the righting reflex loss elicited by the potent GABA(A) receptor agoni st 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 pr egnenolone and progesterone; however, the brain levels of 5 alpha-DHP, the precursor of ALLO, were also decreased. Administration of ALLO at a dose of 15 mu mol/kg ip by itself did not alter the muscimol-induced loss of the r ighting reflex; but it completely blocked the effect of SKF 105,111. To elu cidate the possible molecular mechanism by which a decrease of brain ALLO c ontent can shorten the duration of the righting reflex loss elicited by mus cimol, we patch-clamped neocortical pyramidal neurons of mice pretreated wi th SKF 105,111 or vehicle, and studied the efficiency of muscimol in elicit ing Cl- currents. The current amplitude was significantly smaller in neuron s from SKF 105,111-treated mice, especially at lower doses (0.1-1 mu M) of muscimol, and the muscimol dose-response (0.1-10 mu M) relationship display ed 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. (C) 2000 Elsevier Science Ltd. All rights reserved.