Fluoxetine and norfluoxetine stereospecifically facilitate pentobarbital sedation by increasing neurosteroids

Abstract

Mice housed in social isolation exhibit a decreased response to ␥-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 50s of FLX and NFLX required to normalize brain Allo content and PTB action are 10 -50 times lower than the IC 50s 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. M ale mice that have been housed in social isolation (SI) for 15-30 days develop increased anxiety-related behaviors, respond aggressively to an intruder of the same sex (1-3), and display decreased responsiveness to drugs that stimulate ␥-aminobutyric acid type A (GABA A ) receptors, as shown by the shorter duration of the loss of righting reflex (RRL) induced by pentobarbital (PTB) or muscimol (1, 4).Several independent lines of evidence suggest that these effects are maintained in part by SI-induced down-regulation of neurosteroid biosynthesis. First, protracted SI decreases the telencephalic levels of allopregnanolone (Allo) (5), a neurosteroid that positively modulates the action of GABA and other GABA receptor ligands (i.e., muscimol or barbiturates) on GABA A receptors (6-9). Thus, reduced levels of Allo could mediate the SI-induced decrease in GABA responsiveness.Second, administration of SKF 105,111, a potent type I 5␣-reductase inhibitor, to naïve mice depletes brain Allo content by 60-80% in Ϸ1 h and concurrently reduces the duration of PTB-or muscimol-induced RRL (1, 4). In SI or SKF 105,111-treated mice, administration of Allo in doses that per se fail to change gross behavior normalizes the reduced action of muscimol or PTB (1, 4).Third, electrophysiological studies using the patch-clamp technique to record GABA-evoked Cl Ϫ currents from neocortical pyramidal neurons obtained from brain slices of mice in which the concentration of endogenous brain Allo was decreased by Ϸ80% after the administration of SKF 105,111 showed a reduction in the potency of exogenously applied GABA or muscimol to evoke Cl Ϫ currents (10). Furthermore, direct application of Allo to these slices reverted the dose-response curve of GABA toward the control profile of non-SKF 105,111-treated slices (10).Fourth, the administration of fluoxetine (FLX) to SI mice in doses that normalize brain Allo content reverses the reduced duration of PTB-induced RRL (1). The effect of FLX on brain Allo content is stereospecific; the S-isomers of FLX (S-FLX) and no...

Neuroactive Steroids are Altered in Schizophrenia and Bipolar Disorder: Relevance to Pathophysiology and Therapeutics

“…Furthermore, recent data support the possibility that allopregnanolone potentiates olanzapine actions on dopamine-mediated behaviors in rodents (Ugale et al, 2004). In addition to certain antipsychotics, fluoxetine also elevates allopregnanolone levels in animal models (Uzunov et al, 1996;Pinna et al, 2003Pinna et al, , 2004, and these increases have been linked to its antidepressant-like effects in rodent behavioral models (Khisti and Chopde, 2000a;Khisti et al, 2000b) and to the mitigation of aggression in socially isolated rats (Pinna et al, 2003). We have also determined that olanzapine and fluoxetine elevate pregnenolone levels in rodent hippocampus (Trost et al, 2005).…”

Decreased corticolimbic allopregnanolone expression during social isolation enhances contextual fear: A model relevant for posttraumatic stress disorder

“…Importantly, drugs like the SBSSs fluoxetine and congeners are widely used for the treatment of PTSD (40,41). We have shown that S-NFLX, which is more potent than fluoxetine in increasing corticolimbic Allo levels (2,13,22), rapidly normalizes enhanced contextual fear-conditioned responses related to exposure to social isolation. Selective activation of neurosteroidogenesis may be an appropriate drugdevelopment target for the treatment of PTSD.…”

“…We show that in socially isolated mice Allo or S-NFLX, in doses that increase FC and hippocampus Allo levels at the time of the training session (2,22), also reduces or abolishes the expression of an enhanced contextual fear-conditioning response (Table 2). In contrast, injecting SKF 105,111 into group housed mice facilitates the enhancement of contextual fear responses by decreasing hippocampus, amygdala, and cortex Allo levels (Fig.…”

“…Allo doses (8-16 mol/kg, s.c.) that increased OB and FC Allo levels (13,21) reversed the social isolation-induced increase of contextual conditioned freezing time duration (Table 2). S-NFLX treatment, at doses below those that block serotonin reuptake (0.45-1.8 mol/kg, s.c.), increased corticolimbic Allo content (2,22) and reduced freezing time in socially isolated but not group housed mice (Table 2). …”

Neurosteroid regulation of central nervous system development

“…Changes in sensitivity of GABA A receptors toward allopregnanolone, due to alterations in subunit composition, are induced by neurosteroid withdrawal (Biggio et al, 2006;Gulinello et al, 2002;Mascia et al, 2002;Smith et al, 1998), long term steroid administration (Follesa et al, 2002) and in animal models of epilepsy (Banerjee et al, 1998;Brussaard and Herbison, 2000;Cagetti et al, 2003;Devaud et al, 1996;Follesa et al, 2000;Gulinello et al, 2002;Mtchedlishvili et al, 2001;Reddy and Rogawski, 2000), as well as to changes in neurosteroid concentrations resulting from pregnancy (Concas et al, 1998;Fujii and Mellon, 2001;Herbison, 2001), stress (Barbaccia et al, 1996), social isolation (Pinna et al, 2004;Serra et al, 2000), as well as due to age of the animal (Shen et al, 2007) …”