Gunn et al. GABAA receptors, neurosteroids, and stress At the level of the PVN, chronic variable stress signiﬁcantly increases the number of glutamatergic and noradrenergic termi- nals apposing CRH-immunopositive cell bodies and dendrites, whilst having no effect upon the number of GABAergic termi- nals (Flak et al., 2009). Thus, the balance between excitatory and inhibitory control of the HPA axis appears signiﬁcantly altered fol- lowing chronic stress exposure. Although the number of GABAer- gic terminals apposing parvocellular neurons was unchanged, a marked down-regulation of the β1 and β3 GABAAR subunits was observed following chronic variable stress (Cullinan and Wolfe, 2000). This effect may indirectly modulate the neuros- teroid sensitivity of these receptors (Fancsik et al., 2000; Brussaard and Koksma, 2003; Harney et al., 2003) via documented kinase- speciﬁc interactions with β subunit isoforms (Kittler and Moss, 2003). Electrophysiological recordings from parvocellular neu- rons derived from rats exposed to chronic variable stress revealed a reduction in the frequency of mIPSCs, supporting the notion of reduced GABAergic inhibition of these neurons (Verkuyl et al., 2004). However, in contrast to the morphological data, a reduction in the mIPSC frequency with no change to the paired pulse ratio would suggest a reduction in the number of GABAergic synapses apposing these neurons (Verkuyl et al., 2004). Clearly, the impact that these alterations in GABAAR expression in stress–sensitive brain structures have upon neurosteroid modulation of the stress response deserves further investigation. A number of preclinical and clinical studies have implicated 5α3α-THPROG and other 3α-reduced neurosteroids in depression and attributed the anxiolytic properties of some antidepressants of the selective serotonin reuptake inhibitors (SSRI) class (e.g., ﬂuoxetine or Prozac®) to these endogenous neuromodulators (Guidotti and Costa, 1998; Eser et al., 2006; Uzunova et al., 2006). In support, the brain levels of 5α3α-THPROG are signiﬁcantly decreased in animal models of depression or PTSD (Serra et al., 2000, 2008; Pinna, 2010) and similarly the levels of this neuros- teroid are also reduced in the plasma and CSF of depressed patients (Romeo et al., 1998; Uzunova et al., 1998). In addition, treatment with exogenous neurosteroids produces robust antidepressant-like actions in rodent models of depression (Pinna et al., 2009), while in humans elevated 5α3α-THPROG levels have been associated with a reduction in depressive behavior (reviewed in Uzunova et al., 2006). Intriguingly, animal studies have revealed that acute treatment with the SSRI ﬂuoxetine normalizes the brain levels of 5α3α-THPROG in a stereospeciﬁc manner (Uzunov et al., 1996; Matsumoto et al., 1999; Serra et al., 2001; Pinna et al., 2004; Figure 1). Similarly, the CSF levels of this neurosteroid are also restored in depressed patients following SSRI treatment (Romeo et al., 1998; Uzunova et al., 1998). The ﬂuoxetine-induced increase in neurosteroid levels occurs at concentrations signiﬁcantly lower than those required to block serotonin reuptake and on a faster time scale (i.e., ∼30 min) than that associated with the clinical improvement by classical antidepressants (i.e., 3 weeks; Uzunov et al., 1996; Guidotti and Costa, 1998; Pinna et al., 2004). Therefore, enhancement of GABAAR-mediated inhibition may contribute to the anxiolytic actions of this SSRI (Longone et al., 2008). Fluoxe- tine has been postulated to increase the levels of 5α3α-THPROG by either potentiating the reduction of 5α-DHP, or by inhibiting the oxidation of 5α3α-THPROG back to 5α-DHP, possibly via the modulation of 3α-HSD activity (Uzunov et al., 1996). In support, Grifﬁn and Mellon (1999) reported that ﬂuoxetine and other SSRIs (e.g., paroxetine) enhanced the reductive function of recombinant rat and human forms of 3α-HSD. However, a direct interaction of SSRIs with 3α-HSD remains controversial as a subsequent study did not support these ﬁndings (Trauger et al., 2002). Plasma and CSF levels of neurosteroids measured in clinical studies may not accurately reﬂect their brain levels, particularly if such steroids are acting in a paracrine or autocrine manner. Con- sequently, determining whether alterations in neurosteroid levels following SSRI treatment reduce depressive behaviors has proved difﬁcult. However, a number of non-pharmacological antidepres- sant treatments, including electroconvulsive therapy (ECT) and repetitive transcranial magnetic stimulation (rTMS), all failed to alter the levels of 5α3α-THPROG, although they produced clin- ical improvement of patients suffering from major depression (Padberg et al., 2002; Baghai et al., 2005). Moreover, although partial sleep deprivation (PSD) ameliorated symptoms in 60% of patients suffering from major depression, the concentrations of neurosteroids were not affected by PSD in either responders or non-responders (Schule et al., 2003). Similarly, responders and non-responders to treatment with the antidepressant mirtazapine exhibited similar neurosteroids levels (Schule et al., 2006). Collec- tively, although a dysregulation of neurosteroid equilibrium has been consistently reported in animal models and in humans suffer- ing from depression, it remains unclear whether the normalization of neurosteroid levels following ﬂuoxetine treatment is actively involved in alleviating depressive symptoms, or is simply a phar- macological by-product associated with clinical improvement, i.e., changes in neurosteroid production may reﬂect distinct proper- ties of certain antidepressants rather then alleviation of mood symptoms (Uzunova et al., 2006). PROGRAMMING OF THE STRESS RESPONSE: A ROLE FOR NEUROSTEROIDS? The maturation of the HPA axis is highly sensitive to prena- tal and postnatal stress, resulting in adverse alterations in the behavioral and neuroendocrine responses to stress in adulthood. In the accompanying section we will consider speciﬁcally the role of neurosteroids in animal models of prenatal and postna- tal stress and their potential relevance to the etiology of stress- associated pathologies, e.g., depression, Alzheimer’s disease, and drug addiction. PRENATAL STRESS Exposure of the fetal brain to stress, or glucocorticoids, can inﬂu- ence the development of physiological systems resulting in an increased susceptibility in adulthood to cardiovascular (Barker, 2002), metabolic (Levitt et al., 2000), and affective disorders (Wel- berg and Seckl, 2001). Indeed, the exposure of pregnant rats (gestation day 14, G14) to chronic restraint stress resulted in off- spring that displayed an anxious-like phenotype both as neonates (P7) and adults. Intriguingly, these behavioral effects of prenatal stress (PNS) were attenuated by the concomitant administration of 5α3α-THPROG to the pregnant dams (G14; Zimmerberg and Blaskey, 1998). Similarly, Brunton and Russell have explored a putative role for neurosteroids in a model of social PNS (social Frontiers in Neuroscience | Neuroendocrine Science December 2011 | Volume 5 | Article 131 | 8