Gunn et al. GABAA receptors, neurosteroids, and stress that δ-GABAARs are signiﬁcantly more sensitive to modulation by endogenous neurosteroids, than their γ2-containing counterparts (Belelli et al., 2002; Brown et al., 2002). However, the preferen- tial interaction with δ-GABAARs is secondary to GABA acting as a partial agonist at this receptor subtype, thus allowing for the magnitude of the steroid response to be signiﬁcantly greater than for the homologous γ2-containing receptors (Bianchi and Mac- donald, 2003). Consequently, neurosteroid modulation of extrasy- naptic δ-GABAARs has received considerable attention and these receptors have been proposed to play an important role in medi- ating the physiological effects of these compounds (Maguire and Mody, 2009). However, it should be noted that endogenous neu- rosteroids display neuron-speciﬁc actions upon GABAAR function and that certain synaptic (γ2-containing) GABAARs are similarly sensitive to physiological neurosteroid concentrations in speciﬁc brain regions (Cooper et al., 1999; Herd et al., 2007; Belelli et al., 2009). Recent site-directed mutagenesis studies have revealed that the activation and potentiation of GABAARs by endogenous neu- rosteroids are mediated by two discrete groups of amino acid residues, housed in the GABAAR transmembrane domains (Hosie et al., 2006). Thus, αT236 and βY284 regulate neurosteroid (i.e., 5α3α-THPROG and 5α3α-THDOC)-induced GABAAR activa- tion, whilst αQ241 and αN407 are implicated in the GABA- modulatory effects of these steroids (Hosie et al., 2006). How- ever, subsequent studies have suggested the neurosteroid bind- ing pocket to have a more complex structure with additional residues, e.g., α1S240 contributing to the modulatory actions of steroid molecules of different structure (Akk et al., 2008). Of relevance to the interaction of neurosteroids with δ-GABAARs, GABA-evoked currents mediated by recombinant α4β3δ recep- tors in which the critical glutamine residue of the α4 subunit was mutated (α4Q246L) are insensitive to 5α3α-THDOC. This obser- vation suggests that the δ subunit does not contribute directly to the GABA-modulatory neurosteroid binding site (Hosie et al., 2009). It has been postulated that the differential accumulation of neurosteroids within the lipid membrane (due to their lipophilic- ity) serves to increase their local concentration, and as such, these steroids may diffuse laterally, through the membrane, to access low afﬁnity binding site(s) on GABAARs, a proposal consistent with a putative transmembrane docking site (Akk et al., 2005; Chisari et al., 2010). Within the brain, the levels of endogenous steroids derived from either central, or peripheral sources, are sufﬁcient to mod- ulate GABAAR function (Paul and Purdy, 1992; Corpéchot et al., 1993; Mellon and Vaudry, 2001; Mellon and Grifﬁn, 2002). Initial studies suggested that steroid synthesizing enzymes were primar- ily expressed in glia (Melcangi et al., 1993). However, it is now apparent that the enzymes necessary for steroidogenesis are also expressed in certain neurons (Tsutsui, 2008; Do Rego et al., 2009) Therefore, as originally postulated, neurosteroids may function as endocrine messengers, but could additionally act in a paracrine, or indeed autocrine manner as local neuromodulators to “ﬁne tune” inhibitory transmission (Agis-Balboa et al., 2006). Interestingly, the endogenous levels of neurosteroids within the CNS are not static, but are dynamically regulated in response to a number of physiological states, including development, stress, puberty, preg- nancy, and during the ovarian cycle (Purdy et al., 1991; Paul and Purdy, 1992; Maguire and Mody, 2009; Shen et al., 2010). A number of psychological conditions, including panic attacks, major depression, postpartum depression, premenstrual tension, and schizophrenia, have been associated with perturbations in the levels of neurosteroids within the CNS (Purdy et al., 1991; Eser et al., 2006; Smith et al., 2006; Uzunova et al., 2006; Longone et al., 2008), while treatment with certain psychoactive drugs (e.g., ethanol, ﬂuoxetine) can perturb the levels of these neurosteroids (Uzunova et al., 1998; Sundstrom-Poromaa, 2004; Biggio et al., 2007). This review will focus on the impact of endogenous neuros- teroids upon inhibitory transmission within the CNS in both physiological and pathophysiological scenarios, with an empha- sis on their documented and potential role within the stress neurocircuitry. THE INFLUENCE OF “LOCAL” NEUROSTEROIDS ON NEURONAL INHIBITION: PHYSIOLOGICAL RELEVANCE As noted above and discussed further below, several lines of evi- dence are consistent with the proposal that GABAAR-mediated inhibition may be subject to ﬁne tuning by locally produced steroids. Although a signiﬁcant proportion of these neurosteroids are derived from peripheral sources including the adrenal cortex and the ovaries (Paul and Purdy, 1992), the brain is a steroido- genic organ, capable of de novo synthesis of these neuromod- ulators (Purdy et al., 1991; Mellon, 2004; Agis-Balboa et al., 2006; Do Rego et al., 2009). Thus, the enzymes and mitochon- drial transporters necessary for the synthesis of GABAAR-active pregnane steroids are expressed in the CNS. The synthesis of 5α3α-THPROG and 5α3α-THDOC from PROG and DOC respec- tively, requires two sequential reactions catalyzed by 5α-reductase and 3α-hydroxysteroid dehydrogenase (3α-HSD; Karavolas and Hodges, 1990; Figure 1). These key enzymes are expressed within the CNS in a region-speciﬁc manner and often, are co-localized (Agis-Balboa et al., 2006). Furthermore, the expression of these enzymes is not limited to GABAergic neurons. Thus, for example both enzymes are selectively expressed in glutamatergic principal neurons in the cortex and hippocampus, whilst in the cerebel- lum and thalamus their co-expression is restricted to Purkinje and nucleus reticularis thalami (nRT) neurons, which are GABAer- gic (Agis-Balboa et al., 2006; Tsutsui, 2008). These observations raise the prospect that the excitability of neurons, which co- express GABAARs and the steroid synthesizing enzymes may be self-regulated by the neurosteroid acting in an autocrine manner (Agis-Balboa et al., 2006; Herd et al., 2007; Figure 2). Recent experimental evidence lends considerable support to the notion that locally produced neurosteroids can modulate neu- ronal excitability. At synaptic and extrasynaptic GABAARs the effect of these neuromodulators is typically manifest as a prolonga- tion of the decay time of inhibitory postsynaptic currents (IPSCs) and an increase in the tonic conductance respectively. The treat- ment (5 h previously) of mice with the 5α-reductase inhibitor SKF 105111 (17β-17-[bis (1methylethyl) amino carbonyl]; Figure 1) resulted in a reduction in the decay time of spontaneous IPSCs (sIPSCs) recorded from cortical neurons, suggesting the presence Frontiers in Neuroscience | Neuroendocrine Science December 2011 | Volume 5 | Article 131 | 2