The neurotransmitter GABA mediates the majority of rapid inhibition in the CNS. Inhibition can occur via the conventional mechanism, the transient activation of subsynaptic GABA A receptors (GABA A-Rs), or via continuous activation of high-affinity receptors by low concentrations of ambient GABA, leading to ''tonic'' inhibition that can control levels of excitability and network activity. The GABA A-R ␣4 subunit is expressed at high levels in the dentate gyrus and thalamus and is suspected to contribute to extrasynaptic GABAA-R-mediated tonic inhibition. Mice were engineered to lack the ␣4 subunit by targeted disruption of the Gabra4 gene. ␣4 Subunit knockout mice are viable, breed normally, and are superficially indistinguishable from WT mice. In electrophysiological recordings, these mice show a lack of tonic inhibition in dentate granule cells and thalamic relay neurons. Behaviorally, knockout mice are insensitive to the ataxic, sedative, and analgesic effects of the novel hypnotic drug, gaboxadol. These data demonstrate that tonic inhibition in dentate granule cells and thalamic relay neurons is mediated by extrasynaptic GABA A-Rs containing the ␣4 subunit and that gaboxadol achieves its effects via the activation of this GABA A-R subtype.ABA is the major inhibitory neurotransmitter in the mammalian CNS. Its primary target, GABA A receptors (GABA A -Rs), are pentameric complexes that function as ligandgated chloride ion channels. Two types of inhibitory neurotransmission are mediated via GABA A -Rs (1, 2). Phasic inhibition results from the activation of receptors at the synapse by intermittent release of high concentrations of GABA from presynaptic terminals. Tonic inhibition, in contrast, is mediated by the continuous activation of receptors located outside the synaptic cleft by low concentrations of ambient GABA. These ''extrasynaptic'' GABA A -Rs have a higher affinity for GABA and have faster channel deactivation rates (3, 4) and, more importantly, slower rates of desensitization (1-5), relative to the classical ''synaptic'' GABA A -Rs.There are a variety of subunit families that make up GABA ARs; a total of 19 distinct subunits have been cloned, ␣1-6, ␤1-3, ␥1-3, ␦, , , , and 1-3 (6). This diversity in subunit composition results in substantial anatomical, functional, and pharmacological heterogeneity. GABA A -Rs containing the ␣4 subunit are highly expressed in the thalamus and dentate gyrus, with lower levels in cortex, striatum, and other brain areas (7-9). GABA A -Rs containing ␣4 subunits often are found with the ␥2 or ␦ subunits, in combination with ␤ subunits; the ␣4␤␦ subtypes are proposed to be localized to extrasynaptic sites and contribute to tonic inhibition (5, 10-13). Other extrasynaptic receptor subtypes include ␣5␤3␥2 in hippocampal CA1 pyramidal cells (14) and ␣6␤␦ in cerebellar granule cells (15). Notably, the ␣4 subunit containing GABA A -Rs, especially ␣4␤␥2, are not exclusively extrasynaptic; some are found within dentate gyrus synapses and others are located perisynaptically, wher...

According to the rules of GABA(A) receptor (GABA(A)R) subunit assembly, alpha4 and alpha6 subunits are considered to be the natural partners of delta subunits. These GABA(A)Rs are a preferred target of low, sobriety-impairing concentrations of ethanol. Here we demonstrate a new naturally occurring GABA(A)R subunit partnership: delta subunits of hippocampal interneurons are coexpressed and colocalized with alpha1 subunits, but not with alpha4, alpha6 or any other alpha subunits. Ethanol potentiates the tonic inhibition mediated by such native alpha1/delta GABA(A)Rs in wild-type and in alpha4 subunit-deficient (Gabra4(-/-)) mice, but not in delta subunit-deficient (Gabrd(-/-)) mice. We also ruled out any compensatory upregulation of alpha6 subunits that might have accounted for the ethanol effect in Gabra4(-/-) mice. Thus, alpha1/delta subunit assemblies represent a new neuronal GABA(A)R subunit partnership present in hippocampal interneurons, mediate tonic inhibitory currents and are highly sensitive to low concentrations of ethanol.

GABAergic transmission regulates adult neurogenesis by exerting negative feedback on cell proliferation and enabling dendrite formation and outgrowth. Further, GABAergic synapses target differentiating dentate gyrus granule cells prior to formation of glutamatergic connections. GABAA receptors (GABAARs) mediating tonic (extrasynaptic) and phasic (synaptic) transmission are molecularly and functionally distinct, but their specific role in regulating adult neurogenesis is unknown. Using global and single-cell targeted gene deletion of subunits contributing to the assembly of GABAARs mediating tonic (α4, δ) or phasic (α2) GABAergic transmission, we demonstrate here in the dentate gyrus of adult mice that GABAARs containing α4, but not δ, subunits mediate GABAergic effects on cell proliferation, initial migration and early dendritic development. In contrast, α2-GABAARs cell-autonomously signal to control positioning of newborn neurons and regulate late maturation of their dendritic tree. In particular, we observed pruning of distal dendrites in immature granule cells lacking the α2 subunit. This alteration could be prevented by pharmacological inhibition of thrombospondin signaling with chronic gabapentin treatment, shown previously to reduce glutamatergic synaptogenesis. These observations point to homeostatic regulation of inhibitory and excitatory inputs onto newborn granule cells under the control of α2-GABAARs. Taken together, the availability of distinct GABAAR subtypes provides a molecular mechanism endowing spatiotemporal specificity to GABAergic control of neuronal maturation in adult brain.

Taurine is one of the most abundant free amino acids in the brain. In a number of studies, taurine has been reported to activate glycine receptors (Gly-Rs) at moderate concentrations (Ն100 M), and to be a weak agonist at GABA A receptors (GABA A -Rs), which are usually activated at high concentrations (Ն1 mM). In this study, we show that taurine reduced the excitability of thalamocortical relay neurons and activated both extrasynaptic GABA A -Rs and Gly-Rs in neurons in the mouse ventrobasal (VB) thalamus. Low concentrations of taurine (10 -100 M) decreased neuronal input resistance and firing frequency, and elicited a steady outward current under voltage clamp, but had no effects on fast inhibitory synaptic currents. Currents elicited by 50 M taurine were abolished by gabazine, insensitive to midazolam, and partially blocked by 20 M Zn 2ϩ , consistent with the pharmacological properties of extrasynaptic GABA A -Rs (␣4␤2␦ subtype) involved in tonic inhibition in the thalamus. Tonic inhibition was enhanced by an inhibitor of taurine transport, suggesting that taurine can act as an endogenous activator of these receptors. Taurine-evoked currents were absent in relay neurons from GABA A -R ␣4 subunit knock-out mice. The amplitude of the taurine current was larger in neurons from adult mice than juvenile mice. Taurine was a more potent agonist at recombinant ␣4␤2␦ GABA A -Rs than at ␣1␤2␥2 GABA A -Rs. We conclude that physiological concentrations of taurine can inhibit VB neurons via activation of extrasynaptic GABA A -Rs and that taurine may function as an endogenous regulator of excitability and network activity in the thalamus.

These data suggest that the global loss of alpha 4 subunits leads to region- and cell location-specific compensatory increases in gamma 2 subunits, which in turn alter the pharmacological sensitivity of synaptic and extrasynaptic GABA(A)R-currents. Our data also suggests that while enhancement of tonic inhibitory currents by gaboxadol, alphaxalone, and EtOH are reduced, and behavioral sensitivity to gaboxadol and alphaxalone may be reduced, compensatory changes in synaptic GABA(A)R subunits may prevent similar reductions in behavioral sensitivity to EtOH.