SUMMARY Systemic inflammation causes learning and memory deficits through mechanisms that remain poorly understood. Here, we studied the pathogenesis of memory loss associated with inflammation and found that we could reverse memory deficits by pharmacologically inhibiting α5-subunit-containing γ-aminobutyric acid type A (α5GABAA) receptors and deleting the gene associated with the α5 subunit. Acute inflammation reduces long-term potentiation, a synaptic correlate of memory, in hippocampal slices from wild-type mice, and this reduction was reversed by inhibition of α5GABAA receptor function. A tonic inhibitory current generated by α5GABAA receptors in hippocampal neurons was increased by the key proinflammatory cytokine interleukin-1β through a p38 mitogen-activated protein kinase signaling pathway. Interleukin-1β also increased the surface expression of α5GABAA receptors in the hippocampus. Collectively, these results show that α5GABAA receptor activity increases during inflammation and that this increase is critical for inflammation-induced memory deficits.

Tonic inhibition in the brain is mediated largely by specialized populations of extrasynaptic receptors, ␥-aminobutyric acid receptors (GABA A Rs). In the dentate gyrus region of the hippocampus, tonic inhibition is mediated primarily by GABA A R subtypes assembled from ␣4␤2/3 with or without the ␦ subunit. Although the gating of these receptors is subject to dynamic modulation by agents such as anesthetics, barbiturates, and neurosteroids, the cellular mechanisms neurons use to regulate their accumulation on the neuronal plasma membrane remain to be determined. Using immunoprecipitation coupled with metabolic labeling, we demonstrate that the ␣4 subunit is phosphorylated at Ser 443 by protein kinase C (PKC) in expression systems and hippocampal slices. In addition, the ␤3 subunit is phosphorylated on serine residues 408/409 by PKC activity, whereas the ␦ subunit did not appear to be a PKC substrate. We further demonstrate that the PKC-dependent increase of the cell surface expression of ␣4 subunit-containing GABA A Rs is dependent on Ser 443 . Mechanistically, phosphorylation of Ser 443 acts to increase the stability of the ␣4 subunit within the endoplasmic reticulum, thereby increasing the rate of receptor insertion into the plasma membrane. Finally, we show that phosphorylation of Ser 443 increases the activity of ␣4 subunit-containing GABA A Rs by preventing current run-down. These results suggest that PKC-dependent phosphorylation of the ␣4 subunit plays a significant role in enhancing the cell surface stability and activity of GABA A R subtypes that mediate tonic inhibition. ␥-Aminobutyric acid type A receptors (GABA A Rs)2 constitute the major inhibitory ligand-gated receptors in the adult central nervous system and are responsible for both phasic and tonic forms of inhibition (1). These receptors are pentameric, anion-selective ion channels that can be assembled from eight subunit classes: ␣(1-6), ␤(1-3), ␥(1-3), ␦, ⑀, , , and (1-3)(2-3). This large number of receptor subunits provides the basis for a significant degree of heterogeneity of GABA A R structure and function. However, previous studies suggest that in the brain, the majority of phasic inhibition is dependent upon a few GABA A Rs subunits, namely the ␣, ␤, and ␥2 subunits located within synaptic sites (2, 3). In the adult brain, these receptors are specific targets for brief exposures to high concentrations of GABA, resulting in short lived, but significant, hyperpolarization. In contrast, tonic inhibition is characterized by a sustained reduction in the cell's input resistance, effectively reducing the probability of action potential generation (1, 4). Tonic inhibition is the result of persistent activation by GABA A Rs consisting primarily of ␣, ␤, and ␦ subunits located within peri-or extrasynaptic sites (1). With respect to specific brain regions, extrasynaptic GABA A Rs that mediate tonic inhibition in the thalamus and dentate gyrus of the hippocampus are composed of the ␣4 and ␤2/3 subunits with or without the ␦ subunit (5-9). Verificati...

Thalamic relay neurons have well-characterized dual firing modes: bursting and tonic spiking. Studies in brain slices have led to a model in which rhythmic synchronized spiking (phasic firing) in a population of relay neurons leads to hyper-synchronous oscillatory cortico-thalamo-cortical rhythms that result in absence seizures. This model suggests that blocking thalamocortical phasic firing would treat absence seizures. However, recent in vivo studies in anesthetized animals have questioned this simple model. Here we resolve this issue by developing a real-time mode-switching approach to drive thalamocortical neurons into or out of a phasic firing mode in two freely-behaving genetic rodent models of absence epilepsy. Toggling between phasic and tonic firing in thalamocortical neurons launched and aborted absence seizures, respectively. Thus a synchronous thalamocortical phasic firing state is required for absence seizures and switching to tonic firing rapidly halts absences. This approach should be useful for modulating other networks that have mode-dependent behaviors.

Neurosteroids are synthesized within the brain and act as endogenous anxiolytic, anticonvulsant, hypnotic, and sedative agents, actions that are principally mediated via their ability to potentiate phasic and tonic inhibitory neurotransmission mediated by γ-aminobutyric acid type A receptors (GABA A Rs). Although neurosteroids are accepted allosteric modulators of GABA A Rs, here we reveal they exert sustained effects on GABAergic inhibition by selectively enhancing the trafficking of GABA A Rs that mediate tonic inhibition. We demonstrate that neurosteroids potentiate the protein kinase C-dependent phosphorylation of S443 within α4 subunits, a component of GABA A R subtypes that mediate tonic inhibition in many brain regions. This process enhances insertion of α4 subunit-containing GABA A R subtypes into the membrane, resulting in a selective and sustained elevation in the efficacy of tonic inhibition. Therefore, the ability of neurosteroids to modulate the phosphorylation and membrane insertion of α4 subunit-containing GABA A Rs may underlie the profound effects these endogenous signaling molecules have on neuronal excitability and behavior.PKC | tonic current | receptor insertion | current rundown N eurosteroids are synthesized de novo in the brain from cholesterol, or steroid hormone precursors. Raising neurosteroid levels in the CNS causes anxiolysis, sedation/hypnosis, anticonvulsant action, and anesthesia and reduces depressivelike behaviors (1-3). Accordingly, dysregulation of neurosteroid signaling is associated with premenstrual dysphoric disorder, panic disorder, depression, schizophrenia, and bipolar disorder. Neurosteroids exert the majority of their actions via potentiating the activity of γ-aminobutyric acid receptors (GABA A Rs), which mediate the majority of fast synaptic inhibition in the adult brain. Accordingly, at low nanomolar concentrations they potentiate GABA-dependent currents, whereas at micromolar concentrations they directly activate GABA A Rs (4-8).GABA A Rs are Cl − -preferring pentameric ligand-gated ion channels that assemble from eight families of subunits: α(1-6), β(1-3), γ(1-3), δ, e, ө, π, and ρ(1-3) (9, 10). Receptor subtypes composed of α1-3βγ subunits largely mediate synaptic or phasic inhibition, whereas those constructed from α4-6β1-3, with or without γ/δ subunits, are principal determinants of tonic inhibition (11-13). Neurosteroids have been shown to bind GABA A Rs at an allosteric site distinct from that of GABA, benzodiazepines, or barbiturates (9, 14). Hosie et al. identified residues located within the transmembrane domain of GABA A R α and β subunits that are critical for the direct activation (α1-6; Threonine 236, β1-3; Tyrosine 284) and allosteric potentiation (α1-6 Asparagine 407, and α1-6 Glutamine 246) of neurosteroids (15-17). Accordingly, mutation of glutamine 241 (Q241) within the α1-6 subunits prevents allosteric potentiation of GABA A R composed of αβγ and αβδ subunits by neurosteroids (15,16).In addition to modulating channel gating, neurosteroids exert...