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GABA plays a key role in both embryonic and neonatal brain development. For example, during early neonatal nervous system maturation, synaptic transmission, mediated by GABAA receptors (GABAARs), undergoes a temporally specific form of synaptic plasticity, to accommodate the changing requirements of maturing neural networks. Specifically, the duration of miniature inhibitory postsynaptic currents (mIPSCs), resulting from vesicular GABA activating synaptic GABAARs, is reduced, permitting neurons to appropriately influence the window for postsynaptic excitation. Conventionally, programmed expression changes to the subtype of synaptic GABAAR are primarily implicated in this plasticity. However, it is now evident that in developing thalamic and cortical principal- and inter-neurons an endogenous neurosteroid tone e.g. allopregnanolone, enhances synaptic GABAAR function. Furthermore, a cessation of steroidogenesis, due to a lack of substrate, or a co-factor, appears primarily responsible for early neonatal changes to GABA-ergic synaptic transmission, followed by further refinement, which results from subsequent alterations of the GABAAR subtype. The timing of this cessation of neurosteroid influence is neuron specific, occurring by postnatal day 10 (P10) in thalamus, but about a week later in cortex. Neurosteroid levels are not static, but change dynamically in a variety of physiological and pathophysiological scenarios. Given that GABA plays an important role in brain development, abnormal perturbations of neonatal GABAAR-active neurosteroids, may have a considerable immediate, but also longer term impact, upon neural network activity. Here we review recent evidence that changes in neurosteroidogenesis substantially influence neonatal GABA-ergic synaptic transmission. We discuss the physiological relevance of these findings and how interference of neurosteroid-GABAAR interaction early in life may contribute to psychiatric conditions later in life.

Key points During neuronal development synaptic events mediated by GABAA receptors are progressively reduced in their duration, allowing for rapid and precise network function. Here we focused on ventrobasal thalamocortical neurones, which contribute to behaviourally relevant oscillations between thalamus and cortex. We demonstrate that the developmental decrease in the duration of inhibitory phasic events results predominantly from a precisely timed loss of locally produced neurosteroids, which act as positive allosteric modulators of the GABAA receptor. The mature thalamus retains the ability to synthesise neurosteroids, thus preserving the capacity to enhance both phasic and tonic inhibition, mediated by synaptic and extrasynaptic GABAA receptors, respectively, in physiological and pathophysiological scenarios associated with perturbed neurosteroid levels. Our data establish a potent, endogenous mechanism to locally regulate the GABAA receptor function and thereby influence thalamocortical activity. Abstract During brain development the duration of miniature inhibitory postsynaptic currents (mIPSCs) mediated by GABAA receptors (GABAARs) progressively reduces, to accommodate the temporal demands required for precise network activity. Conventionally, this synaptic plasticity results from GABAAR subunit reorganisation. In particular, in certain developing neurones synaptic α2-GABAARs are replaced by α1-GABAARs. However, in thalamocortical neurones of the mouse ventrobasal (VB) thalamus, the major alteration to mIPSC kinetics occurs on postnatal (P) day 10, some days prior to the GABAAR isoform change. Here, whole-cell voltage-clamp recordings from VB neurones of mouse thalamic slices revealed that early in postnatal development (P7–P8), the mIPSC duration is prolonged by local neurosteroids acting in a paracrine or autocrine manner to enhance GABAAR function. However, by P10, this neurosteroid ‘tone’ rapidly dissipates, thereby producing brief mIPSCs. This plasticity results from a lack of steroid substrate as pre-treatment of mature thalamic slices (P20–24) with the GABAAR-inactive precursor 5α-dihydroprogesterone (5α-DHP) resulted in markedly prolonged mIPSCs and a greatly enhanced tonic conductance, mediated by synaptic and extrasynaptic GABAARs, respectively. In summary, endogenous neurosteroids profoundly influence GABAergic neurotransmission in developing VB neurones and govern a transition from slow to fast phasic synaptic events. Furthermore, the retained capacity for steroidogenesis in the mature thalamus raises the prospect that certain physiological or pathophysiological conditions may trigger neurosteroid neosynthesis, thereby providing a local mechanism for fine-tuning neuronal excitability.

Thalamocortical circuits govern cognitive, sensorimotor, and sleep-related network processes, and generate pathological activities during absence epilepsy. Inhibitory control of thalamocortical (TC) relay neurons is partially mediated by GABA released from neurons of the thalamic reticular nucleus (nRT), acting predominantly via synaptic α1β2γ2 GABAAreceptors (GABAARs). Importantly, TC neurons also express extrasynaptic α4β2δ GABAARs, although how they cooperate with synaptic GABAARs to influence relay cell inhibition, particularly during physiologically relevant nRT output, is unknown. To address this question, we performed paired whole-cell recordings from synaptically coupled nRT and TC neurons of the ventrobasal (VB) complex in brain slices derived from wild-type and extrasynaptic GABAAR-lacking, α4 “knock-out” (α40/0) mice. We demonstrate that the duration of VB phasic inhibition generated in response to nRT burst firing is greatly reduced in α40/0pairs, suggesting that action potential-dependent phasic inhibition is prolonged by recruitment of extrasynaptic GABAARs. Furthermore, the influence of nRT tonic firing frequency on VB holding current is also greatly reduced in α40/0pairs, implying that the α4-GABAAR-mediated tonic conductance of relay neurons is dynamically influenced, in an activity-dependent manner, by nRT tonic firing intensity. Collectively, our data reveal that extrasynaptic GABAARs of the somatosensory thalamus do not merely provide static tonic inhibition but can also be dynamically engaged to couple presynaptic activity to postsynaptic excitability. Moreover, these processes are highly sensitive to the δ-selective allosteric modulator, DS2 and manipulation of GABA transport systems, revealing novel opportunities for therapeutic intervention in thalamocortical network disorders.

Over the past 20 years it has become apparent that certain steroids, synthesised de novo in the brain, hence named neurosteroids, produce immediate changes (within seconds) in neuronal excitability, a time scale that precludes a genomic locus of action. Identified molecular targets underlying modulation of brain excitability include both the inhibitory GABA(A) and the excitatory NMDA receptor. Of particular interest is the interaction of certain neurosteroids with the GABA(A) receptor, the major inhibitory receptor in mammalian brain. During the last decade, compelling evidence has accrued to reveal that locally produced neurosteroids may selectively "fine tune" neuronal inhibition. A range of molecular mechanisms including the subunit composition of the receptor(s), phosphorylation and local steroid metabolism, underpin the region- and neuronal selectivity of action of neurosteroids at synaptic and extrasynaptic GABA(A) receptors. The relative contribution played by each of these mechanisms in a variety of physiological and pathophysiological scenarios is currently being scrutinised at a cellular and molecular level. However, it is not known how such mechanisms may act in concert to influence behavioural profiles in health and disease. An important question concerns the identification of the anatomical substrates mediating the repertoire of behaviours produced by neurosteroids. "Knock-in" mice expressing mutant GABA(A) subunits engineered to be insensitive to benzodiazepines or general anaesthetics have proved invaluable in evaluating the role of GABA(A) receptor subtypes in complex behaviours such as sedation, cognition and anxiety [Rudolph, U., Mohler, H., 2006. GABA-based therapeutic approaches: GABA(A) receptor subtype functions. Curr. Opin. Pharmacol. 6, 18-23]. However, the development of a similar approach for neurosteroids has been hampered by the limited knowledge that, until recently, has surrounded the identity of the amino acid residues contributing to the neurosteroid binding pocket. Here, we will review recent progress in identifying the neurosteroid binding site on the GABA(A) receptor, and discuss how these discoveries will impact on our understanding of the role of neurosteroids in health and disease.

Thalamic ventrobasal (VB) relay neurones express multiple GABA(A) receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA(A) receptors of developing VB neurones and investigate the role of the alpha(1) subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and alpha(1)(0/0) mice and mice engineered to express diazepam-insensitive receptors (alpha(1H101R), alpha(2H101R)). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the alpha(4) and delta subunit occurred independently of the alpha(1) subunit. Subsequently (> or = P15), synaptic alpha(2) subunit/gephyrin clusters of WT VB neurones were replaced by those containing the alpha(1) subunit. Surprisingly, in alpha(1)(0/0) VB neurones the frequency of mIPSCs decreased between P12 and P27, because the alpha(2) subunit also disappeared from these cells. The loss of synaptic GABA(A) receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA(A) receptors in VB follows a developmental programme independent of the alpha(1) subunit. Changes to synaptic GABA(A) receptor function and the increased expression of extrasynaptic GABA(A) receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

As neuronal development progresses, GABAergic synaptic transmission undergoes a defined program of reconfiguration. For example, GABAA receptor (GABAAR)-mediated synaptic currents, (miniature inhibitory postsynaptic currents; mIPSCs), which initially exhibit a relatively slow decay phase, become progressively reduced in duration, thereby supporting the temporal resolution required for mature network activity. Here we report that during postnatal development of cortical layer 2/3 pyramidal neurons, GABAAR-mediated phasic inhibition is influenced by a resident neurosteroid tone, which wanes in the second postnatal week, resulting in the brief phasic events characteristic of mature neuronal signalling. Treatment of cortical slices with the immediate precursor of 5α-pregnan-3α-ol-20-one (5α3α), the GABAAR-inactive 5α-dihydroprogesterone, (5α-DHP), greatly prolonged the mIPSCs of P20 pyramidal neurons, demonstrating these more mature neurons retain the capacity to synthesize GABAAR-active neurosteroids, but now lack the endogenous steroid substrate. Previously, such developmental plasticity of phasic inhibition was ascribed to the expression of synaptic GABAARs incorporating the α1 subunit. However, the duration of mIPSCs recorded from L2/3 cortical neurons derived from α1 subunit deleted mice, were similarly under the developmental influence of a neurosteroid tone. In addition to principal cells, synaptic GABAARs of L2/3 interneurons were modulated by native neurosteroids in a development-dependent manner. In summary, local neurosteroids influence synaptic transmission during a crucial period of cortical neurodevelopment, findings which may be of importance for establishing normal network connectivity., Highlights • Upon postnatal maturation GABAA receptor synaptic inhibition is reduced in duration. • Reduced synthesis of local neurosteroids contributes to this cortical plasticity. • The study reveals a potent mechanism to locally regulate cortical neuron activity.

Dysfunction of GABA(A) receptor-mediated inhibition is implicated in a number of neurological and psychiatric conditions including epilepsy and affective disorders. Some of these conditions have been associated with abnormal levels of certain endogenously occurring neurosteroids, which potently and selectively enhance the function of the brain's major inhibitory receptor, the GABA(A) receptor. Consistent with their ability to enhance neuronal inhibition, such steroids exhibit in animals and humans anxiolytic, anticonvulsant and anesthetic actions. Neurosteroids, exemplified by the potent progesterone metabolite, 5alpha-pregnan-3alpha-ol-20-one can be synthesized de novo in the CNS both in neurones and glia in levels sufficient to modulate GABA(A) receptor function. Neurosteroid levels are not static, but are subject to dynamic fluctuations, for example during stress, or the later stages of pregnancy. These observations suggest that these endogenous modulators may refine the function of the brain's major inhibitory receptor and thus, play an important physiological and pathophysiological role. However, given the ubiquitous expression of GABA(A) receptors throughout the mammalian CNS, changes in neurosteroid levels should be widely experienced, causing a generalized enhancement of neuronal inhibition. Such a non-specific action would seem incompatible with a physiological role. However, neurosteroid action is both brain region and neurone selective. This specificity results from a variety of molecular mechanisms including receptor subunit composition, local steroid metabolism and phosphorylation. This paper will evaluate the relative contribution these mechanisms play in defining the interaction of neurosteroids with synaptic and extra-synaptic GABA(A) receptors.

GABA(A) receptors (GABA(A)Rs) are usually formed by alpha, beta, and gamma or delta subunits. Recently, delta-containing GABA(A)Rs expressed in Xenopus oocytes were found to be sensitive to low concentrations of ethanol (1-3 mM). Our objective was to replicate and extend the study of the effect of ethanol on the function of alpha4beta3delta GABA(A)Rs. We independently conducted three studies in two systems: rat and human GABA(A)Rs expressed in Xenopus oocytes, studied through two-electrode voltage clamp; and human GABA(A)Rs stably expressed in the fibroblast L(tk-) cell line, studied through patch-clamp electrophysiology. In all cases, alpha4beta3delta GABA(A)Rs were only sensitive to high concentrations of ethanol (100 mM in oocytes, 300 mM in the cell line). Expression of the delta subunit in oocytes was assessed through the magnitude of the maximal GABA currents and sensitivity to zinc. Of the three rat combinations studied, alpha4beta3 was the most sensitive to ethanol, isoflurane, and 5alpha-pregnan-3alpha,21-diol-20-one (THDOC); alpha4beta3delta and alpha4beta3gamma(2S) were very similar in most aspects, but alpha4beta3delta was more sensitive to GABA, THDOC, and lanthanum than alpha4beta3gamma(2S) GABA(A)Rs. Ethanol at 30 mM did not affect tonic GABA-mediated currents in dentate gyrus reported to be mediated by GABA(A)Rs incorporating alpha4 and delta subunits. We have not been able to replicate the sensitivity of alpha4beta3delta GABA(A)Rs to low concentrations of ethanol in four different laboratories in independent studies. This suggests that as yet unidentified factors may play a critical role in the ethanol effects on delta-containing GABA(A)Rs.

Neurosteroids typified by 5α-pregnan-3α-ol-20-one (5α3α) have emerged as the most potent endogenous positive modulators of the GABAAreceptor, the principal mediator of fast inhibitory transmission within the CNS. Neurosteroids can be synthesizedde novoin the brain in levels sufficient to modulate GABAAreceptor function and, thus, might play an important physiological-pathophysiological role. Indirect support for this proposal comes from the observation that neurosteroid action is region and neuron selective. However, the mechanism(s) that imparts specificity of action remains primarily elusive. Although neurosteroids are relatively promiscuous toward different GABAAreceptor isoforms, the contribution of local neurosteroid metabolism has been relatively unexplored. Here, we investigate the role of neurosteroid metabolism by using electrophysiological techniques to compare the actions of 5α3α and its metabolically stable synthetic analog ganaxolone on inhibitory neurotransmission in CA1 and dentate gyrus neurons. Furthermore, we evaluate the contribution of a key enzyme in neurosteroid metabolism [i.e., 3α-hydroxysteroidoxidoreductase (3α-HSOR)] to the inactivation of endogenous, or exogenously applied 5α3α. We show that low concentrations of ganaxolone, but not of 5α3α, enhance inhibitory transmission in dentate gyrus, whereas both steroids are similarly effective in CA1 neurons. Furthermore, inhibition of 3α-HSOR by the contraceptive agent Provera results in enhanced synaptic and extrasynaptic GABAAreceptor-mediated inhibition in the dentate gyrus but not in the CA1 region. Collectively, these findings advocate a crucial role for local steroid metabolism in shaping GABAAreceptor-mediated inhibition in a regionally dependent manner and suggest a novel action by the contraceptive agent on inhibitory centers in the CNS.

During neuronal development synaptic events mediated by GABAA receptors are progressively reduced in their duration, allowing for rapid and precise network function. Here we focused on ventrobasal thalamocortical neurones, which contribute to behaviourally relevant oscillations between thalamus and cortex. We demonstrate that the developmental decrease in the duration of inhibitory phasic events results predominantly from a precisely timed loss of locally produced neurosteroids, which act as positive allosteric modulators of the GABAA receptor. The mature thalamus retains the ability to synthesise neurosteroids, thus preserving the capacity to enhance both phasic and tonic inhibition, mediated by synaptic and extrasynaptic GABAA receptors, respectively, in physiological and pathophysiological scenarios associated with perturbed neurosteroid levels. Our data establish a potent, endogenous mechanism to locally regulate the GABAA receptor function and thereby influence thalamocortical activity. During brain development the duration of miniature inhibitory postsynaptic currents (mIPSCs) mediated by GABAA receptors (GABAA Rs) progressively reduces, to accommodate the temporal demands required for precise network activity. Conventionally, this synaptic plasticity results from GABAA R subunit reorganisation. In particular, in certain developing neurones synaptic α2-GABAA Rs are replaced by α1-GABAA Rs. However, in thalamocortical neurones of the mouse ventrobasal (VB) thalamus, the major alteration to mIPSC kinetics occurs on postnatal (P) day 10, some days prior to the GABAA R isoform change. Here, whole-cell voltage-clamp recordings from VB neurones of mouse thalamic slices revealed that early in postnatal development (P7-P8), the mIPSC duration is prolonged by local neurosteroids acting in a paracrine or autocrine manner to enhance GABAA R function. However, by P10, this neurosteroid 'tone' rapidly dissipates, thereby producing brief mIPSCs. This plasticity results from a lack of steroid substrate as pre-treatment of mature thalamic slices (P20-24) with the GABAA R-inactive precursor 5α-dihydroprogesterone (5α-DHP) resulted in markedly prolonged mIPSCs and a greatly enhanced tonic conductance, mediated by synaptic and extrasynaptic GABAA Rs, respectively. In summary, endogenous neurosteroids profoundly influence GABAergic neurotransmission in developing VB neurones and govern a transition from slow to fast phasic synaptic events. Furthermore, the retained capacity for steroidogenesis in the mature thalamus raises the prospect that certain physiological or pathophysiological conditions may trigger neurosteroid neosynthesis, thereby providing a local mechanism for fine-tuning neuronal excitability.

Activation of GABAAreceptors (GABAARs) produces two forms of inhibition: phasic inhibition generated by the rapid, transient activation of synaptic GABAARs by presynaptic GABA release, and tonic inhibition generated by the persistent activation of perisynaptic or extrasynaptic GABAARs, which can detect extracellular GABA. Such tonic GABAAR-mediated currents are particularly evident in dentate granule cells in which they play a major role in regulating cell excitability. Here we show that in rat dentate granule cells inex vivohippocampal slices, tonic currents are predominantly generated by GABA-independent GABAAreceptor openings. This tonic GABAAR conductance is resistant to the competitive GABAAR antagonist SR95531 (gabazine), which at high concentrations acts as a partial agonist, but can be blocked by an open channel blocker, picrotoxin. When slices are perfused with 200 nmGABA, a concentration that is comparable to CSF concentrations but is twice that measured by us in the hippocampusin vivousing zero-net-flux microdialysis, negligible GABA is detected by dentate granule cells. Spontaneously opening GABAARs, therefore, maintain dentate granule cell tonic currents in the face of low extracellular GABA concentrations.

The sedative and hypnotic agent 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol (THIP) is a GABA(A) receptor (GABA(A)R) agonist that preferentially activates delta-subunit-containing GABA(A)Rs (delta-GABA(A)Rs). To clarify the role of delta-GABA(A)Rs in mediating the sedative actions of THIP, we utilized mice lacking the alpha(1)- or delta-subunit in a combined electrophysiological and behavioural analysis. Whole-cell patch-clamp recordings were obtained from ventrobasal thalamic nucleus (VB) neurones at a holding potential of -60 mV. Application of bicuculline to wild-type (WT) VB neurones revealed a GABA(A)R-mediated tonic current of 92 +/- 19 pA, which was greatly reduced (13 +/- 5 pA) for VB neurones of delta(0/0) mice. Deletion of the delta- but not the alpha(1)-subunit dramatically reduced the THIP (1 mum)-induced inward current in these neurones (WT, -309 +/- 23 pA; delta(0/0), -18 +/- 3 pA; alpha(1) (0/0), -377 +/- 45 pA). Furthermore, THIP selectively decreased the excitability of WT and alpha(1) (0/0) but not delta(0/0) VB neurones. THIP did not affect the properties of miniature inhibitory post-synaptic currents in any of the genotypes. No differences in rotarod performance and locomotor activity were observed across the three genotypes. In WT mice, performance of these behaviours was impaired by THIP in a dose-dependent manner. The effect of THIP on rotarod performance was blunted for delta(0/0) but not alpha(1) (0/0) mice. We previously reported that deletion of the alpha(1)-subunit abolished synaptic GABA(A) responses of VB neurones. Therefore, collectively, these findings suggest that extrasynaptic delta-GABA(A)Rs vs. synaptic alpha(1)-subunit-containing GABA(A)Rs of thalamocortical neurones represent an important molecular target underpinning the sedative actions of THIP.

Inhibition in the brain is dominated by the neurotransmitter gamma-aminobutyric acid (GABA); operating through GABA(A) receptors. This form of neural inhibition was presumed to be mediated by synaptic receptors, however recent evidence has highlighted a previously unappreciated role for extrasynaptic GABA(A) receptors in controlling neuronal activity. Synaptic and extrasynaptic GABA(A) receptors exhibit distinct pharmacological and biophysical properties that differentially influence brain physiology and behavior. Here we used a fluorescence-based assay and cell lines expressing recombinant GABA(A) receptors to identify a novel series of benzamide compounds that selectively enhance, or activate alpha4beta3delta GABA(A) receptors (cf. alpha4beta3gamma2 and alpha1beta3gamma2). Utilising electrophysiological methods, we illustrate that one of these compounds, 4-chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS1) potently (low nM) enhances GABA-evoked currents mediated by alpha4beta3delta receptors. At similar concentrations DS1 directly activates this receptor and is the most potent known agonist of alpha4beta3delta receptors. 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS2) selectively potentiated GABA responses mediated by alpha4beta3delta receptors, but was not an agonist. Recent studies have revealed a tonic form of inhibition in thalamus mediated by the alpha4beta2delta extrasynaptic GABA(A) receptors that may contribute to the regulation of thalamocortical rhythmic activity associated with sleep, wakefulness, vigilance and seizure disorders. In mouse thalamic relay cells DS2 enhanced the tonic current mediated by alpha4beta2delta receptors with no effect on their synaptic GABA(A) receptors. Similarly, in mouse cerebellar granule cells DS2 potentiated the tonic current mediated by alpha6betadelta receptors. DS2 is the first selective positive allosteric modulator of delta-GABA(A) receptors and such compounds potentially offer novel therapeutic opportunities as analgesics and in the treatment of sleep disorders. Furthermore, these drugs may be valuable in elucidating the physiological and pathophysiological roles played by these extrasynaptic GABA(A) receptors.

Thalamic ventrobasal (VB) relay neurones express multiple GABA(A) receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA(A) receptors of developing VB neurones and investigate the role of the alpha(1) subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and alpha(1)(0/0) mice and mice engineered to express diazepam-insensitive receptors (alpha(1H101R), alpha(2H101R)). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the alpha(4) and delta subunit occurred independently of the alpha(1) subunit. Subsequently (or = P15), synaptic alpha(2) subunit/gephyrin clusters of WT VB neurones were replaced by those containing the alpha(1) subunit. Surprisingly, in alpha(1)(0/0) VB neurones the frequency of mIPSCs decreased between P12 and P27, because the alpha(2) subunit also disappeared from these cells. The loss of synaptic GABA(A) receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA(A) receptors in VB follows a developmental programme independent of the alpha(1) subunit. Changes to synaptic GABA(A) receptor function and the increased expression of extrasynaptic GABA(A) receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

Certain naturally occurring pregnane steroids act in a nongenomic manner to potently and selectively enhance the interaction of the inhibitory neurotransmitter GABA with the GABA(A) receptor. Consequently such steroids exhibit anxiolytic, anticonvulsant, analgesic, sedative, hypnotic, and anesthetic properties. In both physiological and pathophysiological scenarios, the pregnane steroids may function as endocrine messengers (e.g., produced in the periphery and cross the blood-brain barrier) to influence behaviour. However, additionally "neurosteroids" can be synthesised in the brain and spinal cord to act in a paracrine or autocrine manner and thereby locally influence neuronal activity. Given the ubiquitous expression of the GABA(A) receptor throughout the mammalian central nervous system (CNS), physiological, pathophysiological, or drug-induced pertubations of neurosteroid levels may be expected to produce widespread changes in brain excitability. However, the neurosteroid/GABA(A) receptor interaction is brain region and indeed neuron specific. The molecular basis of this specificity will be reviewed here, including (1) the importance of the subunit composition of the GABA(A) receptor; (2) how protein phosphorylation may dynamically influence the sensitivity of GABA(A) receptors to neurosteroids; (3) the impact of local steroid metabolism; and (4) the emergence of extrasynaptic GABA(A) receptors as a neurosteroid target.