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In cortex and hippocampus, protracted (>4 weeks) social isolation of adult male mice alters the subunit expression of GABA type A receptors (GABAA-Rs) as follows: (i) the mRNAs encoding GABAA-R [alpha]1, [alpha]2, and [gamma]2 subunits are decreased by [approximately equal to] 50%, whereas those encoding [alpha]4 and [alpha]5 subunits are increased by [approximately equal to] 100%; (ii) similarly, the synaptic membrane expression of the [alpha]1 subunit protein is down-regulated, and that of the [alpha]5 subunit protein is up-regulated; and (ii) the binding of [[sup.3]H]flumazenil to hippocampal synaptic membranes is decreased. Behaviorally, socially isolated (Sl) mice are resistant to the sedative effects of the positive allosteric GAB[A.sub.A]-R modulators diazepam (DZP) and zolpidem. This resistance seems to be attributable to the decrease of [alpha]1-containing GAB[A.sub.A]Rs. Paradoxically, DZP, which, unlike zolpidem, acts at [alpha]5-containing GAB[A.sub.A]-Rs, increases the locomotor activity of Sl mice. Imidazenil, which fails to modulate [alpha]1-, [alpha]4-, and [alpha]6-containing GAB[A.sub.A]-RS but is a selective positive allosteric modulator of [alpha]5containing GAB[A.sub.A]-Rs, also increases locomotor activity in SI mice. Importantly, SI mice responded to muscimol, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one, and allopregnanolone similar to group-housed mice. These data suggest that a switch (a decrease in [alpha]1/[alpha]2 and [gamma]2 and an increase in [alpha]4 and [alpha]5 subunits) in the composition of the heteropentameric GAB[A.sub.A]-R subunit assembly without a change in total GAB[A.sub.A]-R number occurs during social isolation. Thus, the repertoire of DZP and imidazenil actions in SI mice appears to be elicited by the allosteric modulation of GAB[A.sub.A]-Rs overexpressing [alpha]5 subunits. Benzodiazepine response mediated by [alpha]1-containing GAB[A.sub.A]-Rs is expected to be silent or reduced. GABAA receptors | nested RT-PCR

Social isolation (SI) of male mice lasting >4 weeks is associated with aggression toward intruders and a down-regulation of brain allopregnanolone (Allo) content. SI of female mice fails to down-regulate brain Allo content or to induce aggressiveness. Fluoxetine (Prozac in clinical use) is an S- and R-fluoxetine (FLX) mixture, which in mammals is metabolized into S- and R-norfluoxetine (NFLX). The S isomers of FLX and NFLX are more active than their respective R isomers in normalizing brain Allo down-regulation and in reducing the aggressiveness induced by SI. Thus, FLX stereospecifically reduces brain Allo down-regulation and the aggressiveness induced by SI, whereas serotonin (5-HT) uptake inhibition lacks stereospecificity. The doses of S-FLX and S-NFLX that reduce aggressiveness and Allo brain content down-regulation induced by SI are at least one order of magnitude lower than the doses that block 5-HT reuptake. Doses of imipramine that inhibit 5-HT uptake neither reduce aggressiveness nor normalize brain Allo down-regulation. We conclude that Allo brain content normalization is a better candidate than 5-HT reuptake inhibition to explain the reduction of aggressiveness elicited by S-FLX and S-NFLX.

Allopregnanolone (ALLO), is a brain endogenous neurosteroid that binds with high affinity to [Gamma]-aminobutyric acid type A ([GABA.sub.A]) receptors and positively modulates the action of GABA at these receptors. Unlike ALLO, 5[Alpha]-dihydroprogesterone (5[Alpha]-DHP) binds with high affinity to intracellular progesterone receptors that regulate DNA transcription. To investigate the physiological roles of ALLO and 5[Alpha]-DHP synthesized in brain, we have adopted a mouse model involving protracted social isolation. In the frontal cortex of mice, socially isolated for 6 weeks, both neurosteroids were decreased by approximately 50%. After administration of (17[Beta])-17-(bis-1-methyl amino carbonyl) androstane-3,5-diene-3-carboxylic acid (SKF105,111), an inhibitor of the enzyme (5[Alpha]-reductase Type I and II) that converts progesterone into 5[Alpha]-DHP, the ALLO and 5[Alpha]-DHP content of frontal cortex of both grouphoused and socially isolated mice decreased exponentially to 10%-20% of control values in about 30 min. The fractional rate constants (k [h.sup.-1]) of ALLO and 5[Alpha]-DHP decline multiplied by the ALLO and 5[Alpha]-DHP concentrations at any given steady-state estimate the rate of synthesis required to maintain that steady state:. After 6 weeks of social isolation, ALLO and 5[Alpha]-DHP biosynthesis rates were decreased to 30% of the values calculated in grouphoused mice. Moreover, in socially isolated mice, the expression of 5[Alpha]-reductase Type I mRNA and protein was approximately 50% lower than in group-housed mice whereas 3[Alpha]-hydroxysteroid oxidoreductase mRNA expression was equal in the two groups. Protracted social isolation in mice may provide a model to investigate whether 5[Alpha]-DHP by a genomic action, and ALLO by a nongenomic mechanism down-regulate the action of drugs acting as agonists, partial agonists, or positive allosteric modulators of the benzodiazepine recognition sites expressed by [GABA.sub.A] receptors.