Introduction.Deficits in biosynthesis of the GABAA receptor-active neurosteroid allopregnanolone (ALLO) have been implicated in debilitating psychiatric disorders.Decreased cerebrospinal fluid (CSF) levels of ALLO in patients with posttraurnatic stress disorder (PTSD) (Biol.Psychiatry 60, 704-713.2006), and with major depression and an increase ofALLO bioavailability after fluoxetine (FLX) and other SSRIs suggest that brain ALLO biosynthesis contributes to the anxiolytic and antidepressant actions of SSRIs.Corticolimbic circuits comprising the amygdala, hippocampus, and medial prefrontal cortex have been implicated in emotion, impulsivity, and fear regulation.PTSD-like behavior (aggression, fear, and anxiety-like behavior) associated with a corticolimbic ALLO level decrease can be modeled by exposing mice to protracted social isolation stress followed by a fear conditioning test.Social isolation-induced ALLO biosynthesis downregulation in glutamatergic neurons of the cortex/hippocampus/amygdala causes a GABAergic neurotransmission dysfunction by decreasing the efficacy of GABA action at post-and extra-synaptic GABAA receptors.Hence, socially isolated (SI) mice offer a suitable model to study anxiolytic and antidepressant drugs.To increase corticolimbic ALLO levels and thereby correct behavioral dysfunctions in SI mice, we administered FLX and norfluoxetine (NFLX).Since FLX is an S and R racemic mixture that is metabolized into S-or R-NFLX, we tested the stereospecificity of these drugs in upregulating corticolimbic ALLO content versus their ability to inhibit 5-HT reuptake.Part of this hypothesis was that FLX and NFLX stereoisomer doses that change corticolimbic ALLO content equal the doses that improve behavioral decifits and may differ from those that inhibit 5-HT reuptake.FLX and NFLX upregulate brain ALLO levels and improve behavior in submicromolar doses and in a stereospecific manner.The S-isomers are several-fold more potent than their respective R-stereoisomers.Importantly, the EC50s of S-FLX and S-NFLX that normalize brain ALLO content are at least 10-fold lower than the EC50s needed to inhibit 5-HT reuptake, which lacks stereospecificity.Traditionally, 5-HT reuptake inhibition was thought to be the molecular mechanism for the pharmacological action of SSRIs.However, we suggest another mechanism that may mediate the anxiolytic and antidepressant actions of SSRIs.This study clarified that S-FLX and S-NFLX facilitate GABAA receptor neurotransmission and improve behavioral deficits by normalizing corticolimbic ALLO content and not by their SSRI activity.Hence, rather than SSRI mechanisms, ALLO biosynthesis in glutamatergic corticolimbic neurons may offer a non-traditional pharmacological target for a new generation of drugs, the selective brain steroidogenic stimulants (SBSSs) that can be used to treat depression and PTSD symptoms.