Brain 5α-dihydroprogesterone and allopregnanolone synthesis in a mouse model of protracted social isolation

Abstract

Allopregnanolone (ALLO), is a brain endogenous neurosteroid that binds with high affinity to ␥-aminobutyric acid type A (GABAA) receptors and positively modulates the action of GABA at these receptors. Unlike ALLO, 5␣-dihydroprogesterone (5␣-DHP) binds with high affinity to intracellular progesterone receptors that regulate DNA transcription. To investigate the physiological roles of ALLO and 5␣-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␤)-17-(bis-1-methyl amino carbonyl) androstane-3,5-diene-3-carboxylic acid (SKF105,111), an inhibitor of the enzyme (5␣-reductase Type I and II) that converts progesterone into 5␣-DHP, the ALLO and 5␣-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 ؊1 ) of ALLO and 5␣-DHP decline multiplied by the ALLO and 5␣-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␣-DHP biosynthesis rates were decreased to 30% of the values calculated in grouphoused mice. Moreover, in socially isolated mice, the expression of 5␣-reductase Type I mRNA and protein was approximately 50% lower than in group-housed mice whereas 3␣-hydroxysteroid oxidoreductase mRNA expression was equal in the two groups. Protracted social isolation in mice may provide a model to investigate whether 5␣-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 GABAA receptors.A llopregnanolone (ALLO), (3␣5␣-tetrahydroprogesterone), and 5␣-dihydroprogesterone (5␣-DHP) are the most likely candidates among the endogenous neuroactive steroids for a neurophysiological role in maintaining GABAergic transmission efficacy. ALLO is synthesized in brain from progesterone by the sequential action of two enzymes: (i) 5␣-reductase, which reduces progesterone to 5␣-DHP; and (ii) 3␣-hydroxysteroidoxidoreductase (3␣-HSOR), which either converts 5␣-DHP into ALLO (reductive reaction) or converts ALLO into 5␣-DHP (oxidative reaction) (1, 2).Several lines of evidence have demonstrated that these neurosteroids in nanomolar concentrations regulate GABAergic transmission by two modalities: (i) nongenomic (ALLO), by facilitating ␥-aminobutyric acid (GABA) gating of GABA type A (GABA A ) receptors after ALLO-specific binding to an allosteric center expressed by these receptors; and (ii) genomic (5␣-DHP) by regulating DNA transcription, perhaps including genes encoding for GABA A receptor subunits, after 5␣-DHP binding to intracellular progesterone receptors (3, 4).The influence of these neurosteroids on the physiological efficacy of GABA A receptor function ...

Neurosteroids in the context of stress: Implications for depressive disorders

“…This is supported, in part, by evidence from animals demonstrating that stressors that produce behavioral depression and anxiety not only alter neurosteroid levels but also regulate GABA A receptor expression and result in insensitivity to benzodiazepines and neurosteroids (Deutsch et al, 1994;Serra et al, 2000;Dong et al, 2001). Such alterations in GABA A receptor function, which we will return to later, could set the stage for dysphoric mood potentially resulting from chronic stress.…”

“…Although no studies to date have specifically examined the association of chronic stress with neuroactive steroid function in depressed patients, it could be argued that clinical depression is itself a chronic stressor; the link between chronic stress and neurosteroid depletion is supported in part by animal models showing that protracted long-term adaptation to the stress of social isolation, a characteristic feature of human depression (Prince et al, 1997;Roberts et al, 1997), is associated with anxiety, aggression, and decreased response to GABAmimetic drugs and with significantly lower brain and plasma neuroactive steroid concentrations, including allopregnanolone and allotetrahydroDOC (Serra et al, 2000;Guidoti et al, 2001;Dong et al, 2001). The finding that the expression of 5α-reductase (mRNA and protein) in mouse brain, the rate limiting enzyme in the conversion of progesterone to allopregnanolone, is down-regulated during protracted social isolation supports the view that chronic stress could alter neuroactive steroid synthesis (Reddy, 2006).…”

Role of Neuroactive Steroid Allopregnanolone in Antipsychotic-like Action of Olanzapine in Rodents

“…A protracted social isolation paradigm was used to further investigate the role of GABA A receptors and of the neurosteroid ALLO in the actions of different antipsychotics. Social isolation of rodents for 6 weeks is known to downregulate the GABA A receptor and to selectively decrease ALLO levels without altering other neurosteroids in brain (Serra et al, 2000;Dong et al, 2001;Guidotti et al, 2001).…”

“…In addition, neuroleptic activity was evaluated in protracted social isolation. This paradigm has been widely employed to investigate the influences of decreased cortical ALLO content and GABA A receptor function (Serra et al, 2000;Dong et al, 2001;Guidotti et al, 2001). The aim of this study was to provide the specific behavioral evidences to support the hypothesis that neuroactive steroid ALLO-mediated GABAergic modulation is essential for antipsychotic-like action of olanzapine.…”

Serotonin 2C Receptor (5-HT2CR) mRNA Editing–Induced Down-Regulation of 5-HT2CR Function in Xenopus Oocytes: the Significance of Site C Editing

“…Templates for each isoform were generated by sitedirected mutagenesis using a PCR overlapping extension method (17,18) to introduce point mutated sites, where adenosine at the editing sites was displaced by guanosine, midway between the amplification primers. The two sets of primers for 5-HT2CR isoforms were as follows: 1) a T3 primer and a reverse primer including the point mutated site(s) and 2) a T7 primer and a forward primer complimentary to the aforementioned reverse primer including the point mutated sites.…”