Fluoxetine and norfluoxetine stereospecifically facilitate pentobarbital sedation by increasing neurosteroids

Mice housed in social isolation exhibit a decreased response to gamma-aminobutyric acid-mimetic drugs [i.e., pentobarbital (PTB)] associated with a down-regulation of telencephalic allopregnanolone (Allo) levels. In these mice, the PTB-induced loss of righting reflex is greatly reduced. Fluoxetine (FLX) and norfluoxetine (NFLX) stereospecifically reverse the effect of social isolation on the PTB-induced loss of righting reflex and on the decrease of telencephalic Allo content. The S-isomers of FLX and NFLX are 2- and 7-fold more potent, respectively, than their respective R-isomers. The EC(50)s of FLX and NFLX required to normalize brain Allo content and PTB action are 10-50 times lower than the IC(50)s required for selective serotonin reuptake inhibitor activity. We conclude that normalization of PTB action elicited by the S-isomers of FLX and NFLX is related to the reversal of the down-regulation of brain Allo content and is independent of selective serotonin reuptake inhibitor activity.