Permissive role of brain allopregnanolone content in the regulation of pentobarbital-induced righting reflex loss

Allopregnanolone [3 alpha-hydroxy-5 alpha-pregnan-20-one] (ALLO), a potent neurosteroid that positively modulates gamma-aminobutyric acid (GABA) actio n at various GABA(A) receptor subtypes is synthesized in nanomolar concentr ations and stored non uniformly in various brain structures of mammals. We have measured brain ALLO content and its precursors by negative ion chemica l ionization-mass-spectrometry after purification and separation of the dif ferent steroids with HPLC and gas chromatography. Our procedure measures st eroids in the femtomolar range with structural information and unsurpassed selectivity. We were able to establish an association between the decrease in content of ALLO in mouse brain cortex elicited by either long-lasting so cial isolation or by the administration of 17 beta-17 [bis (1-methylethyl) amino carbonyl] and rostane-3,5-dilene-3-carboxylic acid (SKF 105 111), an inhibitor of Types I and II 5 alpha: reductases, and the shortening of the righting reflex loss elicited by pentobarbital (PBT). SKF 105 111 added to cortical brain slices in concentrations up to 10(-5) M failed per se to alt er GABAergic currents or their potentiation by PTB recorded from pyramidal neurons. Fluoxetine (1.45 or 2.9 mu mol/kg i.p.) doses that fail to change the PTB-induced loss of righting reflex and the level of brain ALLO in grou p-housed mice normalized both parameters in socially-isolated mice. In addi tion, we could detect both fluoxetine actions in socially isolated mice pre treated with doses of p-chlorophenylalanine (1.2 mmol/kg i.p. at 72, 48, an d 24 h) that substantially inhibit brain serotonin 5HT synthesis as shown b y an 80% drop of brain 5HT content. These studies for the first time have p rovided evidence suggesting that the endogenous cortical stores of ALLO phy siologically upregulate GABAergic tone and by such a mechanism play a permi ssive or facilitatory role on the PTB-induced loss of the righting;reflex. In the absence of such a permissive physiological influence by endogenous A LLO, the righting reflex inhibition by PTB is down regulated. (C) 1999 Publ ished by Elsevier Science Ltd. All rights reserved.