Gunn et al. GABAA receptors, neurosteroids, and stress reduction in the number of USVs compared to non-stressed con- trols when exposed to a subsequent maternal separation at PN8 (Zimmerberg et al., 1999). Interestingly, the adaptation exhibited by maternally separated pups to a subsequent separation corre- lated with enhanced 5α3α-THPROG levels in the brain following maternal separation (Kehoe et al., 2000). Thus, it is tempting to speculate that neurosteroid-induced enhancement of GABAer- gic inhibition may contribute to this hyporesponsive phenotype. However, maternally separated rats exhibited a more anxious phe- notype in response to a novel stressor than their non-stressed counterparts (Patchev et al., 1997; Zimmerberg et al., 1999). More- over, for adult rats that have been maternally separated (PN 2–10) mRNA levels of CRH and GR mRNA were increased and decreased in the PVN and the hippocampus, respectively. Interestingly, these behavioral and neuroendocrine alterations were attenuated when 5α3α-THDOC was concomitantly administered during mater- nal separation, suggesting that these 5α-reduced neurosteroids may exert persistent stress–protective effects during development (Patchev et al., 1997). Intriguingly, the level of 3α-HSD mRNA has been shown to be elevated in the dentate gyrus of male and female rats at P7, a time coinciding with the stress hyporesponsive period, further supporting a physiological role for endogenous neuros- teroids at this stage of development (Mitev et al., 2003). Behavioral and neuroendocrine dysregulation induced by neonatal mater- nal separation exhibits differences between genders (Mitev et al., 2003), although such effects may be additionally dependent on the maternal separation protocol utilized (Zimmerberg and Kajun- ski, 2004). Thus, adult male, but not female rats that experienced maternal separation (PN 5-10) exhibited a more anxious-like phe- notype on the EPM than their non-stressed counterparts (Mitev et al., 2003). However, ovariectomized (OVX) female rats exposed to maternal separation in infancy exhibited an anxious-like phe- notype on the EPM in adulthood cf control females, suggesting that peripheral steroids may normally mask the behavioral conse- quences of neonatal stress in females. Moreover, the concomitant administration of 5α3α-THPROG during maternal separation counteracted the behavioral and neuroendocrine consequences of neonatal stress in both genders (Mitev et al., 2003). Supporting the notion that abnormal GABAAR function is associated with exposure to early life adversities, early life stress induces changes to the expression of GABAARs that occur con- comitantly with the perturbations of behavior and neuroen- docrine function described above. These changes in expression are evident in brain regions that are important for the processing of “psychogenic” stressors, a function known to be signiﬁcantly altered in adulthood by early life stress (Caldji et al., 2003). In general, offspring receiving good quality maternal care displayed signiﬁcantly greater mRNA expression of α1 and β3-subunits in limbic and forebrain regions in comparison to those that received low quality maternal care (Caldji et al., 2003). Intriguingly, the majority of changes observed in GABAAR subunit expression appeared to be unique to the amygdala, a region important in the development of fear-related memory (Ehrlich et al., 2009; Roozendaal et al., 2009). Thus, in the central (CeA), basolat- eral (BLA), and lateral (LA) amygdaloid nuclei, adult offspring that had experienced high-quality maternal care expressed sig- niﬁcantly greater mRNA levels of both γ1 and γ2-subunits, the latter being important in mediating the majority of actions of benzodiazepines. However, in the central and basolateral regions of the amygdaloid nucleus, low-maternal care offspring exhibited relatively greater levels of α3 and α4 mRNA (Caldji et al., 2003). How these changes in the levels of subunit mRNAs translate to perturbations of synaptic/extrasynaptic inhibition, or to changes in the sensitivity of native GABAARs to neurosteroid modulation provides scope for future investigations. Changes in GABAAR subunit expression may also be asso- ciated with altered localization of GABAergic interneurons in a manner similar to that described by Grobin et al. (2003) for the prefrontal cortex following elevated levels of neurosteroids dur- ing development. Such a scenario in the amygdala, hippocampus, and prefrontal cortex may severely compromise the ability of an organism to process and cope with a variety of stressors. In this regard, it would be of interest to investigate whether the quality of maternal care inﬂuences neurosteroids levels and their actions in the developing brain. The proposal that extrasynaptic δ-GABAARs may mediate some of the physiological actions of neurosteroids may be relevant in this respect (Maguire and Mody, 2009). Thus, a recent study has indicated maternal care to be compromised in mice lacking the δ subunit (δ0/0 mice – Maguire and Mody, 2008). Moreover, genetic inactivation of this subunit blunts the anxiolytic actions of neurosteroids in an acute stress challenge paradigm in adult mice (Mihalek et al., 1999). Collectively, these ﬁndings raise the intriguing prospect that early life stress may encode long-lasting changes in network function, contributing to the loss of the anxiolytic action of neurosteroids in adult δ0/0 mice. Consistent with this suggestion, the output of neuroendocrine par- vocellular neurons of the hypothalamic PVN is insensitive to the inhibitory actions of the neurosteroid 5α3α-THPROG in neonatal (PN18–P24) mice lacking the δ subunit cf wild type counterparts (Gunn et al., 2010). However, complicating the interpretation of these ﬁndings, δ-GABAAR are not expressed in the PVN (Wisden et al., 1992; Gunn et al., 2010). This observation suggests that these receptors are unlikely to be the direct molecular target of 5α3α- THPROG within the PVN and caution should be exercised when directly implicating δ-GABAARs in the actions of neurosteroids based upon δ “knock out” studies. In contrast to long periods of maternal separation, short episodes (∼15 min) induce a neuroendocrine plasticity that is characterized by reduced stress responses (Plotsky and Meaney, 1993; Avishai-Eliner et al., 2001), enhanced resilience to depressive behaviors (Meaney et al., 1991), and improved learning and mem- ory function (Fenoglio et al., 2005). Augmented maternal care in the early postnatal period (PN 2–P8), results in reduced levels of CRH expression within the PVN during adulthood. Interestingly, this effect was associated with a reduction in glutamatergic inner- vation of neuroendocrine parvocellular neurons during postnatal development and the increased expression of the transcription repressor neuron-restrictive silencer factor (NRSF), an adapta- tion maintained through to adulthood (Korosi et al., 2010). Thus, the reduced glutamatergic excitation in the postnatally develop- ing PVN may be important in initiating the reprogramming of CRH expression, possibly via the induction of genomic and epi- genetic modiﬁcations, which, in turn, would result in increased stress resilience in later life (Korosi et al., 2010). Whether limbic Frontiers in Neuroscience | Neuroendocrine Science December 2011 | Volume 5 | Article 131 | 10