The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Postsynaptic scaffolding proteins ensure efficient neurotransmission by anchoring receptors and signaling molecules in synapsespecific subcellular domains. In turn, posttranslational modifications of scaffolding proteins contribute to synaptic plasticity by remodeling the postsynaptic apparatus. Though these mechanisms are operant in glutamatergic synapses, little is known about regulation of GABAergic synapses, which mediate inhibitory transmission in the CNS. Here, we focused on gephyrin, the main scaffolding protein of GABAergic synapses. We identify a unique phosphorylation site in gephyrin, Ser270, targeted by glycogen synthase kinase 3β (GSK3β) to modulate GABAergic transmission. Abolishing Ser270 phosphorylation increased the density of gephyrin clusters and the frequency of miniature GABAergic postsynaptic currents in cultured hippocampal neurons. Enhanced, phosphorylation-dependent gephyrin clustering was also induced in vitro and in vivo with lithium chloride. Lithium is a GSK3β inhibitor used therapeutically as mood-stabilizing drug, which underscores the relevance of this posttranslational modification for synaptic plasticity. Conversely, we show that gephyrin availability for postsynaptic clustering is limited by Ca 2+ -dependent gephyrin cleavage by the cysteine protease calpain-1. Together, these findings identify gephyrin as synaptogenic molecule regulating GABAergic synaptic plasticity, likely contributing to the therapeutic action of lithium.GABA A receptors | lithium chloride | postsynaptic density | PSD95 | homeostatic plasticity P lasticity of chemical synapses endows neuronal networks with the capacity to store information by adjusting their functional connectivity. Hence, understanding the molecular underpinnings of synaptic plasticity is a fundamental quest of neuroscience. These mechanisms have been characterized most extensively at glutamatergic synapses, in which a core scaffolding protein, PSD95, forms a signaling complex assembled by proteins interacting via specific PDZ domains (1). In contrast, little is known about signals regulating GABAergic synapses, despite their ubiquitous presence throughout the CNS and their key role in the control of network activity and synchronization. In particular, the postsynaptic density (PSD) of GABAergic synapses, localized primarily on neuronal somata and dendritic shafts, remains ill characterized. Gephyrin, a 93-kDa cytoplasmic polypeptide, has emerged as a multifunctional protein mediating postsynaptic aggregation of GABA A receptors (GABA A R) and glycine receptors by forming a scaffold anchored to the cytoskeleton (2-4). However, the mechanisms of gephyrin and GABA A R clustering are poorly understood, although evidence for direct interaction between gephyrin and GABA A R is slowly emerging (5, 6). Though gephyrin is a phosphoprotein (7,8), the relevance of gephyrin phosphorylation for regulating GABAergic transmission has not been addressed.In the present work, we focused on gephyrin posttranslational modification for regulating its postsyna...

Background: Molecular mechanisms of plasticity at GABAergic synapses are presently unclear. Results: ERK phosphorylates gephyrin at Ser-268 to regulate size of gephyrin postsynaptic scaffold and strength of GABAergic transmission. Ser-268 phosphorylation by ERK is functionally coupled to Ser-270 phosphorylation by GSK3␤ to determine calpain action on gephyrin. Conclusion: Multiple signaling cascades regulate gephyrin postsynaptic clustering. Significance: Dynamic modulation of gephyrin clustering by phosphorylation regulates GABAergic synaptic transmission.

The current understanding about ethanol effects on the ligand-gated ion channel (LGIC) superfamily has been restricted to identify potential binding sites within transmembrane (TM) domains in the Cys-loop family. Here, we demonstrate a key role of the TM3-4 intracellular loop and G␤␥ signaling for potentiation of glycine receptors (GlyRs) by ethanol. We discovered 2 motifs within the large intracellular loop of the GlyR ␣1 subunit that are critical for the actions of pharmacological concentrations of ethanol. Significantly, the sites were ethanol-specific because they did not alter the sensitivity to general anesthetics, neurosteroids, or longer n-alcohols. Furthermore, G␤␥ scavengers selectively attenuated the ethanol effects on recombinant and native neuronal GlyRs. These results show a selective mechanism for low-ethanol concentration effects on the GlyR and provide a mechanism on ethanol pharmacology, which may be applicable to other LGIC members. Moreover, these data provide an opportunity to develop new genetically modified animal models and novel drugs to treat alcohol-related medical concerns.pharmacology ͉ signal transduction ͉ glycine receptor ͉ alcoholism ͉ G proteins

Glycine receptors (GlyRs), together with GABA(A) and nicotinic acetylcholine (ACh) receptors, form part of the ligand-activated ion channel superfamily and regulate the excitability of the mammalian brain stem and spinal cord. Here we report that the ability of the neurotransmitter glycine to gate recombinant and native ionotropic GlyRs is modulated by the G protein betagamma dimer (Gbetagamma). We found that the amplitude of the glycine-activated Cl- current was enhanced after application of purified Gbetagamma or after activation of a G protein-coupled receptor. Overexpression of three distinct G protein alpha subunits (Galpha), as well as the Gbetagamma scavenger peptide ct-GRK2, significantly blunted the effect of G protein activation. Single-channel recordings from isolated membrane patches showed that Gbetagamma increased the GlyR open probability (nP(o)). Our results indicate that this interaction of Gbetagamma with GlyRs regulates both motor and sensory functions in the central nervous system.

Potentially noxious stimuli are sensed by specialized nerve cells named nociceptors, which convey nociceptive signals from peripheral tissues to the central nervous system. The spinal dorsal horn and the trigeminal nucleus serve as first relay stations for incoming nociceptive signals. At these sites, nociceptor terminals contact a local neuronal network consisting of excitatory and inhibitory interneurons as well as of projection neurons. Blockade of neuronal inhibition in this network causes an increased sensitivity to noxious stimuli (hyperalgesia), painful sensations occurring after activation of non-nociceptive fibers (allodynia), and spontaneous pain felt in the absence of any sensory stimulation. It thus mimics the major characteristics of chronic pain states. Diminished inhibitory pain control in the spinal dorsal horn occurs naturally, e.g., through changes in the function of inhibitory neurotransmitter receptors or through altered chloride homeo-stasis in the course of inflammation or nerve damage. This review summarizes our current knowledge about endogenous mechanisms leading to diminished spinal pain control and discusses possible ways that could restore proper inhibition through facilitation of fast inhibitory neurotransmission.

It is now believed that the allosteric modulation produced by ethanol in glycine receptors (GlyRs) depends on alcohol binding to discrete sites within the protein structure. Thus, the differential ethanol sensitivity of diverse GlyR isoforms and mutants was explained by the presence of specific residues in putative alcohol pockets. Here, we demonstrate that ethanol sensitivity in two ligand-gated ion receptor members, the GlyR adult ␣ 1 and embryonic ␣ 2 subunits, can be modified through selective mutations that rescued or impaired G␤␥ modulation. Even though both isoforms were able to physically interact with G␤␥, only the ␣ 1 GlyR was functionally modulated by G␤␥ and pharmacological ethanol concentrations. Remarkably, the simultaneous switching of two transmembrane and a single extracellular residue in ␣ 2 GlyRs was enough to generate GlyRs modulated by G␤␥ and low ethanol concentrations. Interestingly, although we found that these TM residues were different to those in the alcohol binding site, the extracellular residue was recently implicated in conformational changes important to generate a preopen-activated state that precedes ion channel gating. Thus, these results support the idea that the differential ethanol sensitivity of these two GlyR isoforms rests on conformational changes in transmembrane and extracellular residues within the ion channel structure rather than in differences in alcohol binding pockets. Our results describe the molecular basis for the differential ethanol sensitivity of two ligand-gated ion receptor members based on selective G␤␥ modulation and provide a new mechanistic framework for allosteric modulations of abuse drugs.Glycine receptors (GlyRs) 4 are members of the ligand-gated ion receptor (LGIC) superfamily, which includes the Cys-loop family composed of the inhibitory ␥-aminobutyric acid receptors and GlyRs and the excitatory nicotinic acetylcholine (nAChR) and 5-hydroxytryptamine receptors. These ionotropic receptors mediate fast synaptic transmission in the central nervous system (1, 2). Specifically, inhibitory GlyRs are critical for the control of excitability in the mammalian spinal cord and brain stem, regulating important physiological functions such as pain transmission, respiratory rhythms, motor coordination, and neuronal development (3-7).Like all Cys-loop receptors, GlyRs are heteropentameric complexes composed of ␣ and ␤ subunits, which can assemble to form homomeric (5␣) or heteromeric (2␣3␤) channels. To date, molecular cloning studies have demonstrated four isoforms of the ␣ GlyRs (␣ 1-4 ) and one ␤ isoform. Homomeric and heteromeric receptors share most of the GlyR general features, including a high percentage of identity between ␣ GlyRs (Ϸ75%). Nevertheless, biochemical, immunocytochemical, and in situ hybridization studies have shown that the expression of the subunits are developmentally and regionally regulated (3,4,8). For example, the ␣ 1 subunit expression increases after birth, whereas expression of the ␣ 2 subunit appears mainly restricted to early...

Despite the fact that ethanol is one of the most widely used psychoactive agents, the mechanisms and sites of action by which it modifies brain functions are only now being elucidated. Studies over the last decade have shown that ethanol can specifically alter the function of several ligand-activated ion channels including N-methyl-D-aspartate (NMDA), serotonin (5-HT(3)), glycine and GABA(A) receptors. After several years of extensive research in this field, the resolution of what, where and how ethanol modifies GABA(A) receptors continues to be controversial. For example, after demonstrating that ethanol was able to alter Cl(-) flux in synaptoneurosomes and cultured neurons, several electrophysiological studies were unable to show enhancement of the GABA(A) receptor current in single neurons. The lack of positive results with low ethanol concentrations was interpreted as being due to receptor heterogeneity and differences in intracellular modulation by protein kinases and calcium. The existence of high receptor heterogeneity with respect to ethanol sensitivity has been supported by studies done in a variety of cell types which showed that ethanol potentiated some, but not other neurons. Adding to this complexity, it was shown that while some hippocampal GABA(A) receptors can be affected by ethanol concentrations between 1 and 100 mM, others are only sensitive to concentrations above 200 mM. The curve of the relationship between low ethanol concentrations and current enhancement suggests a high degree of complexity in the molecular interaction because of its steepness and "inverted" U shape. Similarly, the effects of ethanol on GABA(A) receptors seems much more complex than those of benzodiazepines, barbiturates and neurosteroids. The major problem encountered in advancing understanding of the mechanism of ethanol action in native neuronal receptors has been the large variability detected in ethanol sensitivity. For example, several studies have shown that only some groups of neurons are sensitive to pharmacologically relevant concentrations of ethanol (1-100 mM). This receptor sensitivity variability has not been resolved using recombinant expression systems. For example, studies performed in recombinant receptors, although important for elucidating molecular requirements, have shown that they are less sensitive to ethanol suggesting that neuronal substrates are important for ethanol actions. In this review, we discuss the possibility that ethanol's action on the GABA(A) receptor may not be due solely to a direct interaction with the receptor protein, but that its effects could also be modulated by intracellular regulation, and that this latter effect is the more physiologically relevant one. Data in cortical and hippocampal neurons suggest that ethanol action on the receptor is labile, and that it also depends on repetitive stimulation and neuron integrity. In addition, the action of ethanol can be modified by activation of protein kinases and neuronal development. Finally, we discuss that the best approach for...