Activity-dependent tuning of inhibitory neurotransmission based on GABAAR diffusion dynamics

Alterations in GABAA receptor-mediated inhibition triggered by status epilepticus and their role in epileptogenesis and increased anxiety

The triggers of status epilepticus (SE) in non-epileptic patients can vary widely, from idiopathic causes to exposure to chemoconvulsants. Regardless of its etiology, prolonged SE can cause significant brain damage, commonly resulting in the development of epilepsy, which is often accompanied by increased anxiety. GABAA receptor (GABAAR)-mediated inhibition has a central role among the mechanisms underlying brain damage and the ensuing epilepsy and anxiety. During SE, calcium influx primarily via ionotropic glutamate receptors activates signaling cascades which trigger a rapid internalization of synaptic GABAARs; this weakens inhibition, exacerbating seizures and excitotoxicity. GABAergic interneurons are more susceptible to excitotoxic death than principal neurons. During the latent period of epileptogenesis, the aberrant reorganization in synaptic interactions that follow interneuronal loss in injured brain regions, leads to the formation of hyperexcitable, seizurogenic neuronal circuits, along with disturbances in brain oscillatory rhythms. Reduction in the spontaneous, rhythmic "bursts" of IPSCs in basolateral amygdala neurons is likely to play a central role in anxiogenesis. Protecting interneurons during SE is key to preventing both epilepsy and anxiety. Antiglutamatergic treatments, including antagonism of calcium-permeable AMPA receptors, can be expected to control seizures and reduce excitotoxicity not only by directly suppressing hyperexcitation, but also by counteracting the internalization of synaptic GABAARs. Benzodiazepines, as delayed treatment of SE, have low efficacy due to the reduction and dispersion of their targets (the synaptic GABAARs), but also because themselves contribute to further reduction of available GABAARs at the synapse; furthermore, benzodiazepines may be completely ineffective in the immature brain.

GABA and astrocytic cholesterol determine the lipid environment of GABAAR in cultured cortical neurons

The γ-aminobutyric acid (GABA) type A receptor (GABAAR), a GABA activated pentameric chloride channel, mediates fast inhibitory neurotransmission in the brain. The lipid environment is critical for GABAAR function. How lipids regulate the channel in the cell membrane is not fully understood. Here we employed super resolution imaging of lipids to demonstrate that the agonist GABA induces a rapid and reversible membrane translocation of GABAAR to phosphatidylinositol 4,5-bisphosphate (PIP2) clusters in mouse primary cortical neurons. This translocation relies on nanoscopic separation of PIP2 clusters and lipid rafts (cholesterol-dependent ganglioside clusters). In a resting state, the GABAAR associates with lipid rafts and this colocalization is enhanced by uptake of astrocytic secretions. These astrocytic secretions enhance endocytosis and delay desensitization. Our findings suggest intercellular signaling from astrocytes regulates GABAAR location based on lipid uptake in neurons. The findings have implications for treating mood disorders associated with altered neural excitability.

UNC-43/CaMKII-triggered anterograde signals recruit GABAARs to mediate inhibitory synaptic transmission and plasticity at C. elegans NMJs

Disturbed inhibitory synaptic transmission has functional impacts on neurodevelopmental and psychiatric disorders. An essential mechanism for modulating inhibitory synaptic transmission is alteration of the postsynaptic abundance of GABA_ARs, which are stabilized by postsynaptic scaffold proteins and recruited by presynaptic signals. However, how GABAergic neurons trigger signals to transsynaptically recruit GABA_ARs remains elusive. Here, we show that UNC-43/CaMKII functions at GABAergic neurons to recruit GABA_ARs and modulate inhibitory synaptic transmission at C. elegans neuromuscular junctions. We demonstrate that UNC-43 promotes presynaptic MADD-4B/Punctin secretion and NRX-1α/Neurexin surface delivery. Together, MADD-4B and NRX-1α recruit postsynaptic NLG-1/Neuroligin and stabilize GABA_ARs. Further, the excitation of GABAergic neurons potentiates the recruitment of NLG-1-stabilized-GABA_ARs, which depends on UNC-43, MADD-4B, and NRX-1. These data all support that UNC-43 triggers MADD-4B and NRX-1α, which act as anterograde signals to recruit postsynaptic GABA_ARs. Thus, our findings elucidate a mechanism for pre- and postsynaptic communication and inhibitory synaptic transmission and plasticity.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Quantifying postsynaptic receptor dynamics: insights into synaptic function

The molecular composition of presynaptic and postsynaptic neuronal terminals is dynamic, and yet long-term stabilizations in postsynaptic responses are necessary for synaptic development and long-term plasticity. The need to reconcile these concepts is further complicated by learning- and memory-related plastic changes in the molecular make-up of synapses. Advances in single-particle tracking mean that we can now quantify the number and diffusive properties of specific synaptic molecules, while statistical thermodynamics provides a framework to analyse these molecular fluctuations. In this Review, we discuss the use of these approaches to gain quantitative descriptions of the processes underlying the turnover, long-term stability and plasticity of postsynaptic receptors and show how these can help us to understand the balance between local molecular turnover and synaptic structural identity and integrity.

The dependence of acetylcholine on dynamic changes in the membrane potential and an action potential during spike timing-dependent plasticity induction in the hippocampus

The hippocampus is an important area for memory encoding and retrieval and is the location of spike timing-dependent plasticity (STDP), a basic phenomenon of learning and memory. STDP is facilitated if acetylcholine (ACh) is released from cholinergic neurons during attentional processes. However, it is unclear how ACh influences postsynaptic changes during STDP induction and determines the STDP magnitude. To address these issues, we obtained patch clamp recordings from CA1 pyramidal neurons to evaluate the postsynaptic changes during stimuli injection in Schaffer collaterals by quantifying baseline amplitudes (i.e., the lowest values elicited by paired pulses comprising STDP stimuli) and action potentials. The results showed that baseline amplitudes were elevated if eserine was applied in the presence of picrotoxin. In addition, muscarinic ACh receptors (mAChRs) contributed more to the baseline amplitude elevation than nicotinic AChRs (nAChRs). Moreover, the magnitude of the STDP depended on the magnitude of the baseline amplitude. However, in the absence of picrotoxin, baseline amplitudes were balanced, regardless of the ACh concentration, resulting in a similar magnitude of the STDP, except under the nAChR alone-activated condition, which showed a larger STDP and lower baseline amplitude induction. This was due to broadened widths of action potentials. These results suggest that activation of mAChRs and nAChRs, which are effective for baseline amplitudes and action potentials, respectively, plays an important role in postsynaptic changes during memory consolidation.

Abnormal Expression of Synaptic and Extrasynaptic GABAA Receptor Subunits in the Dystrophin-Deficient mdx Mouse

Duchenne muscular dystrophy (DMD) is a neurodevelopmental disorder primarily caused by the loss of the full-length Dp427 dystrophin in both muscle and brain. The basis of the central comorbidities in DMD is unclear. Brain dystrophin plays a role in the clustering of central gamma-aminobutyric acid A receptors (GABA_ARs), and its loss in the mdx mouse alters the clustering of some synaptic subunits in central inhibitory synapses. However, the diversity of GABAergic alterations in this model is still fragmentary. In this study, the analysis of in vivo PET imaging of a benzodiazepine-binding site radioligand revealed that the global density of central GABA_ARs is unaffected in mdx compared with WT mice. In contrast, semi-quantitative immunoblots and immunofluorescence confocal imaging in tissue sections revealed complex and differential patterns of alterations of the expression levels and/or clustered distribution of a variety of synaptic and extrasynaptic GABA_AR subunits in the hippocampus, cerebellum, cortex, and spinal cord. Hence, dystrophin loss not only affects the stabilization of synaptic GABA_ARs but also influences the subunit composition of GABA_ARs subtypes at both synaptic and extrasynaptic sites. This study provides new molecular outcome measures and new routes to evaluate the impact of treatments aimed at compensating alterations of the nervous system in DMD.

Df(h22q11)/+ mouse model exhibits reduced binding levels of GABAA receptors and structural and functional dysregulation in the inhibitory and excitatory networks of hippocampus

The 22q11.2 hemizygous deletion confers high risk for multiple neurodevelopmental disorders. Inhibitory signaling, largely regulated through GABA_A receptors, is suggested to serve a multitude of brain functions that are disrupted in the 22q11.2 deletion syndrome. We investigated the putative deficit of GABA_A receptors and the potential substrates contributing to the inhibitory and excitatory dysregulations in hippocampal networks of the Df(h22q11)/+ mouse model of the 22q11.2 hemizygous deletion. The Df(h22q11)/+ mice exhibited impairments in several hippocampus-related functional domains, represented by impaired spatial memory and sensory gating functions. Autoradiography using the [³H]muscimol tracer revealed a significant reduction in GABA_A receptor binding in the CA1 and CA3 subregions, together with a loss of GAD67⁺ interneurons in CA1 of Df(h22q11)/+ mice. Furthermore, electrophysiology recordings exhibited significantly higher neuronal activity in CA3, in response to the GABA_A receptor antagonist, bicuculline, as compared with wild type mice. Density and volume of dendritic spines in pyramidal neurons were reduced and Sholl analysis also showed a reduction in the complexity of basal dendritic tree in CA1 and CA3 subregions of Df(h22q11)/+ mice. Overall, our findings demonstrate that hemizygous deletion in the 22q11.2 locus leads to dysregulations in the inhibitory circuits, involving reduced binding levels of GABA_A receptors, in addition to functional and structural modulations of the excitatory networks of hippocampus.

A Versatile Synthetic Affinity Probe Reveals Inhibitory Synapse Ultrastructure and Brain Connectivity**

Visualization of inhibitory synapses requires protocol tailoring for different sample types and imaging techniques, and usually relies on genetic manipulation or the use of antibodies that underperform in tissue immunofluorescence. Starting from an endogenous ligand of gephyrin, a universal marker of the inhibitory synapse, we developed a short peptidic binder and dimerized it, significantly increasing affinity and selectivity. We further tailored fluorophores to the binder, yielding “Sylite”—a probe with outstanding signal‐to‐background ratio that outperforms antibodies in tissue staining with rapid and efficient penetration, mitigation of staining artifacts, and simplified handling. In super‐resolution microscopy Sylite precisely localizes the inhibitory synapse and enables nanoscale measurements. Sylite profiles inhibitory inputs and synapse sizes of excitatory and inhibitory neurons in the midbrain and combined with complimentary tracing techniques reveals the synaptic connectivity.

Pierre J, Xiong X, Howland JG, Josselyn SA, Frankland PW, Parkinson J. Toxoplasma infection in male mice alters dopamine-sensitive behaviors and host gene expression patterns associated with neuropsychiatric disease

During chronic infection, the single celled parasite, Toxoplasma gondii, can migrate to the brain where it has been associated with altered dopamine function and the capacity to modulate host behavior, increasing risk of neurocognitive disorders. Here we explore alterations in dopamine-related behavior in a new mouse model based on stimulant (cocaine)-induced hyperactivity. In combination with cocaine, infection resulted in heightened sensorimotor deficits and impairment in prepulse inhibition response, which are commonly disrupted in neuropsychiatric conditions. To identify molecular pathways in the brain affected by chronic T. gondii infection, we investigated patterns of gene expression. As expected, infection was associated with an enrichment of genes associated with general immune response pathways, that otherwise limits statistical power to identify more informative pathways. To overcome this limitation and focus on pathways of neurological relevance, we developed a novel context enrichment approach that relies on a customized ontology. Applying this approach, we identified genes that exhibited unexpected patterns of expression arising from the combination of cocaine exposure and infection. These include sets of genes which exhibited dampened response to cocaine in infected mice, suggesting a possible mechanism for some observed behaviors and a neuroprotective effect that may be advantageous to parasite persistence. This model offers a powerful new approach to dissect the molecular pathways by which T. gondii infection contributes to neurocognitive disorders.

Regulation of Inhibitory Signaling at the Receptor and Cellular Level; Advances in Our Understanding of GABAergic Neurotransmission and the Mechanisms by Which It Is Disrupted in Epilepsy

Inhibitory signaling in the brain organizes the neural circuits that orchestrate how living creatures interact with the world around them and how they build representations of objects and ideas. Without tight control at multiple points of cellular engagement, the brain’s inhibitory systems would run down and the ability to extract meaningful information from excitatory events would be lost leaving behind a system vulnerable to seizures and to cognitive decline. In this review, we will cover many of the salient features that have emerged regarding the dynamic regulation of inhibitory signaling seen through the lens of cell biology with an emphasis on the major building blocks, the ligand-gated ion channel receptors that are the first transduction point when the neurotransmitter GABA is released into the synapse. Epilepsy association will be used to indicate importance of key proteins and their pathways to brain function and to introduce novel areas for therapeutic intervention.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

Why won't it stop? The dynamics of benzodiazepine resistance in status epilepticus

Status epilepticus is a life-threatening neurological emergency that affects both adults and children. Approximately 36% of episodes of status epilepticus do not respond to the current preferred first-line treatment, benzodiazepines. The proportion of episodes that are refractory to benzodiazepines is higher in low-income and middle-income countries (LMICs) than in high-income countries (HICs). Evidence suggests that longer episodes of status epilepticus alter brain physiology, thereby contributing to the emergence of benzodiazepine resistance. Such changes include alterations in GABA_A receptor function and in the transmembrane gradient for chloride, both of which erode the ability of benzodiazepines to enhance inhibitory synaptic signalling. Often, current management guidelines for status epilepticus do not account for these duration-related changes in pathophysiology, which might differentially impact individuals in LMICs, where the average time taken to reach medical attention is longer than in HICs. In this Perspective article, we aim to combine clinical insights and the latest evidence from basic science to inspire a new, context-specific approach to efficiently managing status epilepticus.

Inhibitory postsynaptic density from the lens of phase separation

To faithfully transmit and decode signals released from presynaptic termini, postsynaptic compartments of neuronal synapses deploy hundreds of various proteins. In addition to distinct sets of proteins, excitatory and inhibitory postsynaptic apparatuses display very different organization features and regulatory properties. Decades of extensive studies have generated a wealth of knowledge on the molecular composition, assembly architecture and activity-dependent regulatory mechanisms of excitatory postsynaptic compartments. In comparison, our understanding of the inhibitory postsynaptic apparatus trails behind. Recent studies have demonstrated that phase separation is a new paradigm underlying the formation and plasticity of both excitatory and inhibitory postsynaptic molecular assemblies. In this review, we discuss molecular composition, organizational and regulatory features of inhibitory postsynaptic densities through the lens of the phase separation concept and in comparison with the excitatory postsynaptic densities.

The synaptic scaffold protein MPP2 interacts with GABAA receptors at the periphery of the postsynaptic density of glutamatergic synapses

Recent advances in imaging technology have highlighted that scaffold proteins and receptors are arranged in subsynaptic nanodomains. The synaptic membrane-associated guanylate kinase (MAGUK) scaffold protein membrane protein palmitoylated 2 (MPP2) is a component of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor–associated protein complexes and also binds to the synaptic cell adhesion molecule SynCAM 1. Using superresolution imaging, we show that—like SynCAM 1—MPP2 is situated at the periphery of the postsynaptic density (PSD). In order to explore MPP2-associated protein complexes, we used a quantitative comparative proteomics approach and identified multiple γ-aminobutyric acid (GABA)_A receptor subunits among novel synaptic MPP2 interactors. In line with a scaffold function for MPP2 in the assembly and/or modulation of intact GABA_A receptors, manipulating MPP2 expression had effects on inhibitory synaptic transmission. We further show that GABA_A receptors are found together with MPP2 in a subset of dendritic spines and thus highlight MPP2 as a scaffold that serves as an adaptor molecule, linking peripheral synaptic elements critical for inhibitory regulation to central structures at the PSD of glutamatergic synapses.

This study shows that the MAGUK scaffold protein MPP2 is located at the periphery of postsynaptic densities in excitatory neurons, where it interacts with GABA-A receptors, thereby serving as a functional adaptor that links excitatory and inhibitory components of synaptic transmission at glutamatergic synapses.

Spatial regulation of coordinated excitatory and inhibitory synaptic plasticity at dendritic synapses

The induction of synaptic plasticity at an individual dendritic glutamatergic spine can affect neighboring spines. This local modulation generates dendritic plasticity microdomains believed to expand the neuronal computational capacity. Here, we investigate whether local modulation of plasticity can also occur between glutamatergic synapses and adjacent GABAergic synapses. We find that the induction of long-term potentiation at an individual glutamatergic spine causes the depression of nearby GABAergic inhibitory synapses (within 3 μm), whereas more distant ones are potentiated. Notably, L-type calcium channels and calpain are required for this plasticity spreading. Overall, our data support a model whereby input-specific glutamatergic postsynaptic potentiation induces a spatially regulated rearrangement of inhibitory synaptic strength in the surrounding area through short-range heterosynaptic interactions. Such local coordination of excitatory and inhibitory synaptic plasticity is expected to influence dendritic information processing and integration.

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LTP of individual dendritic spines causes iLTD at neighboring GABAergic synapses

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Interaction between single-spine LTP and iLTD occurs in the spatial range of ±3 μm

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This iLTD depends on the local dendritic calcium increase and calpain activation

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iLTD is associated with reduced gephyrin clustering and increased GABAAR mobility

Stepwise disassembly of GABAergic synapses during pathogenic excitotoxicity

GABAergic synaptic inhibition controls neuronal firing, excitability, and synaptic plasticity to regulate neuronal circuits. Following an acute excitotoxic insult, inhibitory synapses are eliminated, reducing synaptic inhibition, elevating circuit excitability, and contributing to the pathophysiology of brain injuries. However, mechanisms that drive inhibitory synapse disassembly and elimination are undefined. We find that inhibitory synapses are disassembled in a sequential manner following excitotoxicity: GABAARs undergo rapid nanoscale rearrangement and are dispersed from the synapse along with presynaptic active zone components, followed by the gradual removal of the gephyrin scaffold, prior to complete elimination of the presynaptic terminal. GABAAR nanoscale reorganization and synaptic declustering depends on calcineurin signaling, whereas disassembly of gephyrin relies on calpain activation, and blockade of both enzymes preserves inhibitory synapses after excitotoxic insult. Thus, inhibitory synapse disassembly occurs rapidly, with nanoscale precision, in a stepwise manner and most likely represents a critical step in the progression of hyperexcitability following excitotoxicity.

Targeting autotaxin impacts disease advance in the SOD1-G93A mouse model of amyotrophic lateral sclerosis

A preclinical strategy to broaden the search of potentially effective treatments in amyotrophic lateral sclerosis (ALS) relies on identifying factors controlling motor neuron (MN) excitability. These partners might be part of still unknown pathogenic pathways and/or useful for the design of new interventions to affect disease progression. In this framework, the bioactive membrane‐derived phospholipid lysophosphatidic acid (LPA) affects MN excitability through LPA receptor 1 (LPA₁). Furthermore, LPA₁ knockdown is neuroprotective in transgenic ALS SOD1‐G93A mice. On this basis, we raised the hypothesis that the major LPA‐synthesizing ectoenzyme, autotaxin (ATX), regulates MN excitability and is a potential target to modulate disease development in ALS mice. We show here that PF‐8380, a specific ATX inhibitor, reduced intrinsic membrane excitability (IME) of hypoglossal MNs in brainstem slices, supporting that baseline ATX activity regulates MN IME. PF‐8380‐induced alterations were prevented by a small‐interfering RNA directed against mRNA for lpa₁. These outcomes support that impact of ATX‐originated lysophospholipids on MN IME engages, at least, the G‐protein‐coupled receptor LPA₁. Interestingly, mRNA_atx levels increased in the spinal cord of pre‐symptomatic (1–2 months old) SOD1‐G93A mice, thus preceding MN loss. The rise in transcripts levels also occurred in cultured spinal cord MNs from SOD1‐G93A embryos, suggesting that mRNA_atx upregulation in MNs is an etiopathogenic event in the ALS cell model. Remarkably, chronic administration in the drinking water of the orally bioavailable ATX inhibitor PF‐8380 delayed MN loss, motor deterioration and prolonged life span in ALS mice. Treatment also led to a reduction in LPA₁‐immunoreactive patches in transgenic animals mostly in MNs. These outcomes support that neuroprotective effects of interfering with ATX in SOD1‐G93A mice rely, at least in part, on LPA₁ knockdown in MNs. Therefore, we propose ATX as a potential target and/or a biomarker in ALS and highlight ATX inhibitors as reasonable tools with therapeutic usefulness for this lethal pathology.

Nogo-A Modulates the Synaptic Excitation of Hippocampal Neurons in a Ca2+-Dependent Manner

A tight regulation of the balance between inhibitory and excitatory synaptic transmission is a prerequisite for synaptic plasticity in neuronal networks. In this context, the neurite growth inhibitor membrane protein Nogo-A modulates synaptic plasticity, strength, and neurotransmitter receptor dynamics. However, the molecular mechanisms underlying these actions are unknown. We show that Nogo-A loss-of-function in primary mouse hippocampal cultures by application of a function-blocking antibody leads to higher excitation following a decrease in GABA_ARs at inhibitory and an increase in the GluA1, but not GluA2 AMPAR subunit at excitatory synapses. This unbalanced regulation of AMPAR subunits results in the incorporation of Ca²⁺-permeable GluA2-lacking AMPARs and increased intracellular Ca²⁺ levels due to a higher Ca²⁺ influx without affecting its release from the internal stores. Increased neuronal activation upon Nogo-A loss-of-function prompts the phosphorylation of the transcription factor CREB and the expression of c-Fos. These results contribute to the understanding of the molecular mechanisms underlying the regulation of the excitation/inhibition balance and thereby of plasticity in the brain.

Axonal GABAA stabilizes excitability in unmyelinated sensory axons secondary to NKCC1 activity

KEY POINTS

GABA depolarized sural nerve axons and increased the electrical excitability of C-fibres via GABAA receptor. Axonal excitability responses to GABA increased monotonically with the rate of action potential firing. Action potential activity in unmyelinated C-fibres is coupled to Na-K-Cl cotransporter type 1 (NKCC1) loading of axonal chloride. Activation of axonal GABAA receptor stabilized C-fibre excitability during prolonged low frequency (2.5 Hz) firing. NKCC1 maintains intra-axonal chloride to provide feed-forward stabilization of C-fibre excitability and thus support sustained firing.

ABSTRACT

GABAA receptor (GABAA R)-mediated depolarization of dorsal root ganglia (DRG) axonal projections in the spinal dorsal horn is implicated in pre-synaptic inhibition. Inhibition, in this case, is predicated on an elevated intra-axonal chloride concentration and a depolarizing GABA response. In the present study, we report that the peripheral axons of DRG neurons are also depolarized by GABA and this results in an increase in the electrical excitability of unmyelinated C-fibre axons. GABAA R agonists increased axonal excitability, whereas GABA excitability responses were blocked by GABAA R antagonists and were absent in mice lacking the GABAA R β3 subunit selectively in DRG neurons (AdvillinCre or snsCre ). Under control conditions, excitability responses to GABA became larger at higher rates of electrical stimulation (0.5-2.5 Hz). However, during Na-K-Cl cotransporter type 1 (NKCC1) blockade, the electrical stimulation rate did not affect GABA response size, suggesting that NKCC1 regulation of axonal chloride is coupled to action potential firing. To examine this, activity-dependent conduction velocity slowing (activity-dependent slowing; ADS) was used to quantify C-fibre excitability loss during a 2.5 Hz challenge. ADS was reduced by GABAA R agonists and exacerbated by either GABAA R antagonists, β3 deletion or NKCC1 blockade. This illustrates that activation of GABAA R stabilizes C-fibre excitability during sustained firing. We posit that NKCC1 acts in a feed-forward manner to maintain an elevated intra-axonal chloride in C-fibres during ongoing firing. The resulting chloride gradient can be utilized by GABAA R to stabilize axonal excitability. The data imply that therapeutic strategies targeting axonal chloride regulation at peripheral loci of pain and itch may curtail aberrant firing in C-fibres.

Stochastic self-assembly of reaction-diffusion patterns in synaptic membranes

Synaptic receptor and scaffold molecules self-assemble into membrane protein domains, which play an important role in signal transmission across chemical synapses. Experiment and theory have shown that the formation of receptor-scaffold domains of the characteristic size observed in nerve cells can be understood from the receptor and scaffold reaction and diffusion processes suggested by experiments. We employ here kinetic Monte Carlo (KMC) simulations to explore the self-assembly of synaptic receptor-scaffold domains in a stochastic lattice model of receptor and scaffold reaction-diffusion dynamics. For reaction and diffusion rates within the ranges of values suggested by experiments we find, in agreement with previous mean-field calculations, self-assembly of receptor-scaffold domains of a size similar to that observed in experiments. Comparisons between the results of our KMC simulations and mean-field solutions suggest that the intrinsic noise associated with receptor and scaffold reaction and diffusion processes accelerates the self-assembly of receptor-scaffold domains, and confers increased robustness to domain formation. In agreement with experimental observations, our KMC simulations yield a prevalence of scaffolds over receptors in receptor-scaffold domains. Our KMC simulations show that receptor and scaffold reaction-diffusion dynamics can inherently give rise to plasticity in the overall properties of receptor-scaffold domains, which may be utilized by nerve cells to regulate the receptor number at chemical synapses.

Nanoscale synapse organization and dysfunction in neurodevelopmental disorders

Neurodevelopmental disorders such as those linked to intellectual disabilities or autism spectrum disorder are thought to originate in part from genetic defects in synaptic proteins. Single gene mutations linked to synapse dysfunction can broadly be separated in three categories: disorders of transcriptional regulation, disorders of synaptic signaling and disorders of synaptic scaffolding and structures. The recent developments in super-resolution imaging technologies and their application to synapses have unraveled a complex nanoscale organization of synaptic components. On the one hand, part of receptors, adhesion proteins, ion channels, scaffold elements and the pre-synaptic release machinery are partitioned in subsynaptic nanodomains, and the respective organization of these nanodomains has tremendous impact on synaptic function. For example, pre-synaptic neurotransmitter release sites are partly aligned with nanometer precision to postsynaptic receptor clusters. On the other hand, a large fraction of synaptic components is extremely dynamic and constantly exchanges between synaptic domains and extrasynaptic or intracellular compartments. It is largely the combination of the exquisitely precise nanoscale synaptic organization of synaptic components and their high dynamic that allows the rapid and profound regulation of synaptic function during synaptic plasticity processes that underlie adaptability of brain function, learning and memory. It is very tempting to speculate that genetic defects that lead to neurodevelopmental disorders and target synaptic scaffolds and structures mediate their deleterious impact on brain function through perturbing synapse nanoscale dynamic organization. We discuss here how applying super-resolution imaging methods in models of neurodevelopmental disorders could help in addressing this question.

Regulation of GABAA Receptors Induced by the Activation of L-Type Voltage-Gated Calcium Channels

GABA_A receptors are pentameric ion channels that mediate most synaptic and tonic extrasynaptic inhibitory transmissions in the central nervous system. There are multiple GABA_A receptor subtypes constructed from 19 different subunits in mammals that exhibit different regional and subcellular distributions and distinct pharmacological properties. Dysfunctional alterations of GABA_A receptors are associated with various neuropsychiatric disorders. Short- and long-term plastic changes in GABA_A receptors can be induced by the activation of different intracellular signaling pathways that are triggered, under physiological and pathological conditions, by calcium entering through voltage-gated calcium channels. This review discusses several mechanisms of regulation of GABA_A receptor function that result from the activation of L-type voltage gated calcium channels. Calcium influx via these channels activates different signaling cascades that lead to changes in GABA_A receptor transcription, phosphorylation, trafficking, and synaptic clustering, thus regulating the inhibitory synaptic strength. These plastic mechanisms regulate the interplay of synaptic excitation and inhibition that is crucial for the normal function of neuronal circuits.

Endogenous β-neurexins on axons and within synapses show regulated dynamic behavior

Neurexins are key organizer molecules that regulate synaptic function and are implicated in autism and schizophrenia. β-neurexins interact with numerous cell adhesion and receptor molecules, but their neuronal localization remains elusive. Using single-molecule tracking and high-resolution microscopy to detect neurexin1β and neurexin3β in primary hippocampal neurons from knockin mice, we demonstrate that endogenous β-neurexins are present in fewer than half of excitatory and inhibitory synapses. Moreover, we observe a large extrasynaptic pool of β-neurexins on axons and show that axonal β-neurexins diffuse with higher surface mobility than those transiently confined within synapses. Stimulation of neuronal activity further increases the mobility of synaptic and axonal β-neurexins, whereas inhibition causes the opposite. Blocking ectodomain cleavage by metalloproteases also reduces β-neurexin mobility and enhances glutamate release. These findings suggest that the surface mobility of endogenous β-neurexins inside and outside of synapses is dynamically regulated and linked to neuronal activity.

Kainate Receptor Activation Shapes Short-Term Synaptic Plasticity by Controlling Receptor Lateral Mobility at Glutamatergic Synapses

Kainate receptors (KARs) mediate postsynaptic currents with a key impact on neuronal excitability. However, the molecular determinants controlling KAR postsynaptic localization and stabilization are poorly understood. Here, we exploit optogenetic and single-particle tracking approaches to study the role of KAR conformational states induced by glutamate binding on KAR lateral mobility at synapses. We report that following glutamate binding, KARs are readily and reversibly trapped at glutamatergic synapses through increased interaction with the β-catenin/N-cadherin complex. We demonstrate that such activation-dependent synaptic immobilization of KARs is crucial for the modulation of short-term plasticity of glutamatergic synapses. Thus, the present study unveils the crosstalk between conformational states and lateral mobility of KARs, a mechanism regulating glutamatergic signaling, particularly in conditions of sustained synaptic activity.

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Anchoring of KARs at glutamatergic synapses depends on receptor-glutamate binding

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KARs activation/desensitization promotes receptors trapping at glutamatergic synapses

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N-cadherins mediate the KAR activation/desensitization-dependent anchoring at synapses

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Synaptic trapping of desensitized KARs affects short-term synaptic plasticity

Artemisinin-treatment in pre-symptomatic APP-PS1 mice increases gephyrin phosphorylation at Ser270: a modification regulating postsynaptic GABAAR density

Artemisinins, a group of plant-derived sesquiterpene lactones, are efficient antimalarial agents. They also share anti-inflammatory and anti-viral activities and were considered for treatment of neurodegenerative disorders like Alzheimer's disease (AD). Additionally, artemisinins bind to gephyrin, the multifunctional scaffold of GABAergic synapses, and modulate inhibitory neurotransmission in vitro. We previously reported an increased expression of gephyrin and GABA_A receptors in early pre-symptomatic stages of an AD mouse model (APP-PS1) and in parallel enhanced CDK5-dependent phosphorylation of gephyrin at S270. Here, we studied the effects of artemisinin on gephyrin in the brain of young APP-PS1 mice. We detected an additional increase of gephyrin protein level, elevated gephyrin phosphorylation at Ser270, and an increased amount of GABA_AR-γ2 subunits after artemisinin-treatment. Interestingly, the CDK5 activator p35 was also upregulated. Moreover, we demonstrate decreased density of postsynaptic gephyrin and GABA_AR-γ2 immunoreactivities in cultured hippocampal neurons expressing gephyrin with alanine mutations at two CDK5 phosphorylation sites. In addition, the activity-dependent modulation of synaptic protein density was abolished in neurons expressing gephyrin lacking one or both of these phosphorylation sites. Thus, our results reveal that artemisinin modulates expression as well as phosphorylation of gephyrin at sites that might have important impact on GABAergic synapses in AD.

[123I]Iomazenil SPECT Detects a Reversible Lesion of the Left Medial Temporal Lobe in a Case of Global Autobiographical Amnesia

Global autobiographical amnesia is a rare disorder that is characterized by a sudden loss of autobiographical memories covering many years of an individual's life. Generally, routine neuroimaging studies such as CT and MRI yield negative findings in individuals with global autobiographical amnesia. However, in recent case reports, functional analyses such as SPECT and fMRI have revealed changes in activity in various areas of the brain when compared with controls. Studies using iomazenil (IMZ) SPECT with individuals with global autobiographical amnesia have not been reported. We report the case of a 62-year-old Japanese woman with global autobiographical amnesia who had disappeared for ∼4 weeks. [123I]-IMZ SPECT showed reduced IMZ uptake in her left medial temporal lobe and no significant reduction on N-isopropyl-[123I] p-iodoamphetamine (IMP) SPECT in the identical region. Because IMZ binds to the central benzodiazepine receptor, this dissociation between IMZ and IMP SPECT was thought to reflect the breakdown of inhibitory neurotransmission in the left medial temporal lobe. Moreover, when the woman recovered most of her memory 32 months after fugue onset, the IMZ SPECT-positive lesion had decreased in size. Because the woman had long suffered verbal abuse from her former husband's sister and brother, which can also cause global autobiographical amnesia, it is difficult to conclude whether the IMZ SPECT-positive lesion in the left medial temporal lobe was the cause or the result of her global autobiographical amnesia. Although only one case, these observations suggest that IMZ SPECT may be useful in uncovering the mechanisms underlying global autobiographical amnesia.

Postsynaptic Targeting and Mobility of Membrane Surface-Localized hASIC1a

Acid-sensing ion channels (ASICs), the main H+ receptors in the central nervous system, sense extracellular pH fluctuations and mediate cation influx. ASIC1a, the major subunit responsible for acid-activated current, is widely expressed in brain neurons, where it plays pivotal roles in diverse functions including synaptic transmission and plasticity. However, the underlying molecular mechanisms for these functions remain mysterious. Using extracellular epitope tagging and a novel antibody recognizing the hASIC1a ectodomain, we examined the membrane targeting and dynamic trafficking of hASIC1a in cultured cortical neurons. Surface hASIC1a was distributed throughout somata and dendrites, clustered in spine heads, and co-localized with postsynaptic markers. By extracellular pHluorin tagging and fluorescence recovery after photobleaching, we detected movement of hASIC1a in synaptic spine heads. Single-particle tracking along with use of the anti-hASIC1a ectodomain antibody revealed long-distance migration and local movement of surface hASIC1a puncta on dendrites. Importantly, enhancing synaptic activity with brain-derived neurotrophic factor accelerated the trafficking and lateral mobility of hASIC1a. With this newly-developed toolbox, our data demonstrate the synaptic location and high dynamics of functionally-relevant hASIC1a on the surface of excitatory synapses, supporting its involvement in synaptic functions.

Looking for Novelty in an "Old" Receptor: Recent Advances Toward Our Understanding of GABAARs and Their Implications in Receptor Pharmacology

Diverse populations of GABA_A receptors (GABA_ARs) throughout the brain mediate fast inhibitory transmission and are modulated by various endogenous ligands and therapeutic drugs. Deficits in GABA_AR signaling underlie the pathophysiology behind neurological and neuropsychiatric disorders such as epilepsy, anxiety, and depression. Pharmacological intervention for these disorders relies on several drug classes that target GABA_ARs, such as benzodiazepines and more recently neurosteroids. It has been widely demonstrated that subunit composition and receptor stoichiometry impact the biophysical and pharmacological properties of GABA_ARs. However, current GABA_AR-targeting drugs have limited subunit selectivity and produce their therapeutic effects concomitantly with undesired side effects. Therefore, there is still a need to develop more selective GABA_AR pharmaceuticals, as well as evaluate the potential for developing next-generation drugs that can target accessory proteins associated with native GABA_ARs. In this review, we briefly discuss the effects of benzodiazepines and neurosteroids on GABA_ARs, their use as therapeutics, and some of the pitfalls associated with their adverse side effects. We also discuss recent advances toward understanding the structure, function, and pharmacology of GABA_ARs with a focus on benzodiazepines and neurosteroids, as well as newly identified transmembrane proteins that modulate GABA_ARs.

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

Nitric oxide controls excitatory/inhibitory balance in the hypoglossal nucleus during early postnatal development

Synaptic remodeling during early postnatal development lies behind neuronal networks refinement and nervous system maturation. In particular, the respiratory system is immature at birth and is subjected to significant postnatal development. In this context, the excitatory/inhibitory balance dramatically changes in the respiratory-related hypoglossal nucleus (HN) during the 3 perinatal weeks. Since, development abnormalities of hypoglossal motor neurons (HMNs) are associated with sudden infant death syndrome and obstructive sleep apnea, deciphering molecular partners behind synaptic remodeling in the HN is of basic and clinical relevance. Interestingly, a transient expression of the neuronal isoform of nitric oxide (NO) synthase (NOS) occurs in HMNs at neonatal stage that disappears before postnatal day 21 (P21). NO, in turn, is a determining factor for synaptic refinement in several physiopathological conditions. Here, intracerebroventricular chronic administration (P7–P21) of the broad spectrum NOS inhibitor l-NAME (N(ω)-nitro-l-arginine methyl ester) differentially affected excitatory and inhibitory rearrangement during this neonatal interval in the rat. Whilst l-NAME led to a reduction in the number of excitatory structures, inhibitory synaptic puncta were increased at P21 in comparison to administration of the inactive stereoisomer d-NAME. Finally, l-NAME decreased levels of the phosphorylated form of myosin light chain in the nucleus, which is known to regulate the actomyosin contraction apparatus. These outcomes indicate that physiologically synthesized NO modulates excitatory/inhibitory balance during early postnatal development by acting as an anti-synaptotrophic and/or synaptotoxic factor for inhibitory synapses, and as a synaptotrophin for excitatory ones. The mechanism of action could rely on the modulation of the actomyosin contraction apparatus.

The Epilepsy-Related Protein PCDH19 Regulates Tonic Inhibition, GABAAR Kinetics, and the Intrinsic Excitability of Hippocampal Neurons

PCDH19 encodes for protocadherin-19 (PCDH19), a cell-adhesion molecule of the cadherin superfamily preferentially expressed in the brain. PCDH19 mutations cause a neurodevelopmental syndrome named epileptic encephalopathy, early infantile, 9 (EIEE9) characterized by seizures associated with cognitive and behavioral deficits. We recently reported that PCDH19 binds the alpha subunits of GABA_A receptors (GABA_ARs), modulating their surface availability and miniature inhibitory postsynaptic currents (mIPSCs). Here, we investigated whether PCDH19 regulatory function on GABA_ARs extends to the extrasynaptic receptor pool that mediates tonic current. In fact, the latter shapes neuronal excitability and network properties at the base of information processing. By combining patch-clamp recordings in whole-cell and cell-attached configurations, we provided a functional characterization of primary hippocampal neurons from embryonic rats of either sex expressing a specific PCDH19 short hairpin (sh)RNA. We first demonstrated that PCDH19 downregulation reduces GABA_AR-mediated tonic current, evaluated by current shift and baseline noise analysis. Next, by single-channel recordings, we showed that PCDH19 regulates GABA_ARs kinetics without altering their conductance. In particular, GABA_ARs of shRNA-expressing neurons preferentially exhibit brief openings at the expense of long ones, thus displaying a flickering behavior. Finally, we showed that PCDH19 downregulation reduces the rheobase and increases the frequency of action potential firing, thus indicating neuronal hyperexcitability. These findings establish PCDH19 as a critical determinant of GABA_AR-mediated tonic transmission and GABA_ARs gating, and provide the first mechanistic insights into PCDH19-related hyperexcitability and comorbidities.

Fast Regulation of GABAAR Diffusion Dynamics by Nogo-A Signaling

Precisely controlling the excitatory and inhibitory balance is crucial for the stability and information-processing ability of neuronal networks. However, the molecular mechanisms maintaining this balance during ongoing sensory experiences are largely unclear. We show that Nogo-A signaling reciprocally regulates excitatory and inhibitory transmission. Loss of function for Nogo-A signaling through S1PR2 rapidly increases GABA_AR diffusion, thereby decreasing their number at synaptic sites and the amplitude of GABAergic mIPSCs at CA3 hippocampal neurons. This increase in GABA_AR diffusion rate is correlated with an increase in Ca²⁺ influx and requires the calcineurin-mediated dephosphorylation of the γ2 subunit at serine 327. These results suggest that Nogo-A signaling rapidly strengthens inhibitory GABAergic transmission by restricting the diffusion dynamics of GABA_ARs. Together with the observation that Nogo-A signaling regulates excitatory transmission in an opposite manner, these results suggest a crucial role for Nogo-A signaling in modulating the excitation and inhibition balance to restrict synaptic plasticity.

Leveraging epigenetics to examine differences in developmental trajectories of social attention: A proof-of-principle study of DNA methylation in infants with older siblings with autism

Preliminary evidence suggests that changes in DNA methylation, a widely studied epigenetic mechanism, contribute to the etiology of Autism Spectrum Disorder (ASD). However, data is primarily derived from post-mortem brain samples or peripheral tissue from adults. Deep-phenotyped longitudinal infant cohorts are essential to understand how epigenetic modifications relate to early developmental trajectories and emergence of ASD symptoms. We present a proof-of-principle study designed to evaluate the potential of prospective epigenetic studies of infant siblings of children with ASD. Illumina genome-wide 450 K DNA methylation data from buccal swabs was generated for 63 male infants at multiple time-points from 8 months to 2 years of age (total N = 107 samples). 11 of those infants received a diagnosis of ASD at 3 years. We conducted a series of analyses to characterize DNA methylation signatures associated with categorical outcome and neurocognitive measures from parent-report questionnaire, eye-tracking and electro-encephalography. Effects observed across the entire genome (epigenome-wide association analyses) suggest that collecting DNA methylation samples within infant-sibling designs allows for the detection of meaningful signals with smaller sample sizes than previously estimated. Mapping networks of co-methylated probes associated with neural correlates of social attention implicated enrichment of pathways involved in brain development. Longitudinal modelling found covariation between phenotypic traits and DNA methylation levels in the proximity of genes previously associated with cognitive development, although larger samples and more complete datasets are needed to obtain generalizable results. In conclusion, assessment of DNA methylation profiles at multiple time-points in infant-sibling designs is a promising avenue to comprehend developmental origins and mechanisms of ASD.

Silencing of spontaneous activity at α4β1/3δ GABAA receptors in hippocampal granule cells reveals different ligand pharmacology

BACKGROUND AND PURPOSE

The δ-subunit-containing GABA_A receptors, α₄ β₁ δ and α₄ β₃ δ, in dentate gyrus granule cells (DGGCs) are known to exhibit both spontaneous channel openings (i.e. constitutive activity) and agonist-induced current. The functional implications of spontaneous gating are unclear. In this study, we tested the hypothesis that constitutively active α₄ β_1/3 δ receptors limit agonist efficacy.

EXPERIMENTAL APPROACH

Whole-cell electrophysiological recordings of adult male rat and mouse hippocampal DGGCs were used to characterize known agonists and antagonists at δ-subunit-containing GABA_A receptors. To separate constitutive and agonist-induced currents, different recording conditions were employed.

KEY RESULTS

Recordings at either 24°C or 34°C, including the PKC autoinhibitory peptide (19-36) intracellularly, removed spontaneous gating by GABA_A receptors. In the absence of spontaneous gating, DGGCs responded to the α₄ β_1/3 δ orthosteric agonist Thio-THIP with a four-fold increased efficacy relative to recording conditions favouring constitutive activity. Surprisingly, the neutral antagonist gabazine was unable to antagonize the current by Thio-THIP. Furthermore, a current was elicited by gabazine alone only when the constitutive current was silenced (EC₅₀ 2.1 μM). The gabazine-induced current was inhibited by picrotoxin, potentiated by DS2, completely absent in δ^-/- mice and reduced in β₁ ^-/- mice, but could not be replicated in human α₄ β_1/3 δ receptors expressed heterologously in HEK cells.

CONCLUSION AND IMPLICATIONS

Kinase activity infers spontaneous gating in α₄ β_1/3 δ receptors in DGGCs. This significantly limits the efficacy of GABA_A agonists and has implications in pathologies involving aberrant excitability caused by phosphorylation (e.g. addiction and epilepsy). In such cases, the efficacy of δ-preferring GABA_A ligands may be reduced.

Tuning GABAergic Inhibition: Gephyrin Molecular Organization and Functions

To be highly reliable, synaptic transmission needs postsynaptic receptors (Rs) in precise apposition to the presynaptic release sites. At inhibitory synapses, the postsynaptic protein gephyrin self-assembles to form a scaffold that anchors glycine and GABAARs to the cytoskeleton, thus ensuring the accurate accumulation of postsynaptic receptors at the right place. This protein undergoes several post-translational modifications which control protein-protein interaction and downstream signaling pathways. In addition, through the constant exchange of scaffolding elements and receptors in and out of synapses, gephyrin dynamically regulates synaptic strength and plasticity. The aim of the present review is to highlight recent findings on the functional role of gephyrin at GABAergic inhibitory synapses. We will discuss different approaches used to interfere with gephyrin in order to unveil its function. In addition, we will focus on the impact of gephyrin structure and distribution at the nanoscale level on the functional properties of inhibitory synapses as well as the implications of this scaffold protein in synaptic plasticity processes. Finally, we will emphasize how gephyrin genetic mutations or alterations in protein expression levels are implicated in several neuropathological disorders, including autism spectrum disorders, schizophrenia, temporal lobe epilepsy and Alzheimer's disease, all associated with severe deficits of GABAergic signaling. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Neuronal Plasticity: Neuronal Organization is Associated with Neurological Disorders

Stimuli from stressful events, attention in the classroom, and many other experiences affect the functionality of the brain by changing the structure or reorganizing the connections between neurons and their communication. Modification of the synaptic transmission is a vital mechanism for generating neural activity via internal or external stimuli. Neuronal plasticity is an important driving force in neuroscience research, as it is the basic process underlying learning and memory and is involved in many other functions including brain development and homeostasis, sensorial training, and recovery from brain injury. Indeed, neuronal plasticity has been explored in numerous studies, but it is still not clear how neuronal plasticity affects the physiology and morphology of the brain. Thus, unraveling the molecular mechanisms of neuronal plasticity is essential for understanding the operation of brain functions. In this timeline review, we discuss the molecular mechanisms underlying different forms of synaptic plasticity and their association with neurodegenerative/neurological disorders as a consequence of alterations in neuronal plasticity.

Quantification of native mRNA dynamics in living neurons using fluorescence correlation spectroscopy and reduction-triggered fluorescent probes

RNA localization in subcellular compartments is essential for spatial and temporal regulation of protein expression in neurons. Several techniques have been developed to visualize mRNAs inside cells, but the study of the behavior of endogenous and nonengineered mRNAs in living neurons has just started. In this study, we combined reduction-triggered fluorescent (RETF) probes and fluorescence correlation spectroscopy (FCS) to investigate the diffusion properties of activity-regulated cytoskeleton-associated protein (Arc) and inositol 1,4,5-trisphosphate receptor type 1 (Ip3r1) mRNAs. This approach enabled us to discriminate between RNA-bound and unbound fluorescent probes and to quantify mRNA diffusion parameters and concentrations in living rat primary hippocampal neurons. Specifically, we detected the induction of Arc mRNA production after neuronal activation in real time. Results from computer simulations with mRNA diffusion coefficients obtained in these analyses supported the idea that free diffusion is incapable of transporting mRNA of sizes close to those of Arc or Ip3r1 to distal dendrites. In conclusion, the combined RETF-FCS approach reported here enables analyses of the dynamics of endogenous, unmodified mRNAs in living neurons, affording a glimpse into the intracellular dynamics of RNA in live cells.

Inhibitory synaptic transmission tuned by Ca2+ and glutamate through the control of GABAA R lateral diffusion dynamics

The GABAergic synapses, a primary inhibitory synapse in the mammalian brain, is important for the normal development of brain circuits, and for the regulation of the excitation‐inhibition balance critical for brain function from the developmental stage throughout life. However, the molecular mechanism underlying the formation, maintenance, and modulation of GABAergic synapses is less understood compared to that of excitatory synapses. Quantum dot‐single particle tracking (QD‐SPT), a super‐resolution imaging technique that enables the analysis of membrane molecule dynamics at single‐molecule resolution, is a powerful tool to analyze the behavior of proteins and lipids on the plasma membrane. In this review, we summarize the recent application of QD‐SPT in understanding of GABAergic synaptic transmission. Here we introduce QD‐SPT experiments that provide further insights into the molecular mechanism supporting GABAergic synapses. QD‐SPT studies revealed that glutamate and Ca²⁺ signaling is involved in (a) the maintenance of GABAergic synapses, (b) GABAergic long‐term depression, and GABAergic long‐term potentiation, by specifically activating signaling pathways unique to each phenomenon. We also introduce a novel Ca²⁺ imaging technique to describe the diversity of Ca²⁺ signals that may activate the downstream signaling pathways that induce specific biological output.

Antidepressant mechanisms of ketamine: Focus on GABAergic inhibition

There has been much recent progress in understanding of the mechanism of ketamine's rapid and enduring antidepressant effects. Here we review recent insights from clinical and preclinical studies, with special emphasis of ketamine-induced changes in GABAergic synaptic transmission that are considered essential for its antidepressant therapeutic effects. Subanesthetic ketamine is now understood to exert its initial action by selectively blocking a subset of NMDA receptors on GABAergic interneurons, which results in disinhibition of glutamatergic target neurons, a surge in extracellular glutamate and correspondingly elevated glutamatergic synaptic transmission. This surge in glutamate appears to be corroborated by the rapid metabolism of ketamine into hydroxynorketamine, which acts at presynaptic sites to disinhibit the release of glutamate. Preclinical studies indicate that glutamate-induced activity triggers the release of BDNF, followed by transient activation of the mTOR pathway and increased expression of synaptic proteins, along with functional strengthening of glutamatergic synapses. This drug-on phase lasts for approximately 2h and is followed by a period of days characterized by structural maturation of newly formed glutamatergic synapses and prominently enhanced GABAergic synaptic inhibition. Evidence from mouse models with constitutive antidepressant-like phenotypes suggests that this phase involves strengthened inhibition of dendrites by somatostatin-positive GABAergic interneurons and correspondingly reduced NMDA receptor-mediated Ca²⁺ entry into dendrites, which activates an intracellular signaling cascade that converges with the mTOR pathway onto increased activity of the eukaryotic elongation factor eEF2 and enhanced translation of dendritic mRNAs. Newly synthesized proteins such as BDNF may be important for the prolonged therapeutic effects of ketamine.

Autism and Schizophrenia-Associated CYFIP1 Regulates the Balance of Synaptic Excitation and Inhibition

Altered excitatory/inhibitory (E/I) balance is implicated in neuropsychiatric and neurodevelopmental disorders, but the underlying genetic etiology remains poorly understood. Copy number variations in CYFIP1 are associated with autism, schizophrenia, and intellectual disability, but its role in regulating synaptic inhibition or E/I balance remains unclear. We show that CYFIP1, and the paralog CYFIP2, are enriched at inhibitory postsynaptic sites. While CYFIP1 or CYFIP2 upregulation increases excitatory synapse number and the frequency of miniature excitatory postsynaptic currents (mEPSCs), it has the opposite effect at inhibitory synapses, decreasing their size and the amplitude of miniature inhibitory postsynaptic currents (mIPSCs). Contrary to CYFIP1 upregulation, its loss in vivo, upon conditional knockout in neocortical principal cells, increases expression of postsynaptic GABA_A receptor β2/3-subunits and neuroligin 3, enhancing synaptic inhibition. Thus, CYFIP1 dosage can bi-directionally impact inhibitory synaptic structure and function, potentially leading to altered E/I balance and circuit dysfunction in CYFIP1-associated neurological disorders.

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CYFIP1 and CYFIP2 are enriched at inhibitory synapses.

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CYFIP1 upregulation differentially disrupts inhibitory and excitatory synapses.

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Conditional loss of CYFIP1 alters neuroligin 3 and GABA_AR β-subunits expression.

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Loss of CYFIP1 increases inhibitory synaptic clusters and hence mIPSC amplitude.

Dynamic Changes in Synaptic Plasticity Genes in Ipsilateral and Contralateral Inferior Colliculus Following Unilateral Noise-induced Hearing Loss

Unilateral noise-induced hearing loss reduces the input to the central auditory pathway disrupting the excitatory and inhibitory inputs to the inferior colliculus (IC), an important binaural processing center. Little is known about the compensatory synaptic changes that occur in the IC as a consequence of unilateral noise-induced hearing loss. To address this issue, Sprague-Dawley rats underwent unilateral noise exposure resulting in severe unilateral hearing loss. IC tissues from the contralateral and ipsilateral IC were evaluated for acute (2-d) and chronic (28-d) changes in the expression of 84 synaptic plasticity genes on a PCR array. Arc and Egr1 genes were further visualized by in situ hybridization to validate the PCR results. None of the genes were upregulated, but many were downregulated post-exposure. At 2-d post-exposure, more than 75% of the genes were significantly downregulated in the contralateral IC, while only two were downregulated in the ipsilateral IC. Many of the downregulated genes were related to long-term depression, long-term potentiation, cell adhesion, immediate early genes, neural receptors and postsynaptic density. At 28-d post-exposure, the gene expression pattern was reversed with more than 85% of genes in the ipsilateral IC now downregulated. Most genes previously downregulated in the contralateral IC 2-d post-exposure had recovered; less than 15% remained downregulated. These time-dependent, asymmetric changes in synaptic plasticity gene expression could shed new light on the perceptual deficits associated with unilateral hearing loss and the dynamic structural and functional changes that occur in the IC days and months following unilateral noise-induced hearing loss.

Inhibitory Plasticity: From Molecules to Computation and Beyond

Synaptic plasticity is the cellular and molecular counterpart of learning and memory and, since its first discovery, the analysis of the mechanisms underlying long-term changes of synaptic strength has been almost exclusively focused on excitatory connections. Conversely, inhibition was considered as a fixed controller of circuit excitability. Only recently, inhibitory networks were shown to be finely regulated by a wide number of mechanisms residing in their synaptic connections. Here, we review recent findings on the forms of inhibitory plasticity (IP) that have been discovered and characterized in different brain areas. In particular, we focus our attention on the molecular pathways involved in the induction and expression mechanisms leading to changes in synaptic efficacy, and we discuss, from the computational perspective, how IP can contribute to the emergence of functional properties of brain circuits.

Heritability of interpack aggression in a wild pedigreed population of North American grey wolves

Aggression is a quantitative trait deeply entwined with individual fitness. Mapping the genomic architecture underlying such traits is complicated by complex inheritance patterns, social structure, pedigree information and gene pleiotropy. Here, we leveraged the pedigree of a reintroduced population of grey wolves (Canis lupus) in Yellowstone National Park, Wyoming, USA, to examine the heritability of and the genetic variation associated with aggression. Since their reintroduction, many ecological and behavioural aspects have been documented, providing unmatched records of aggressive behaviour across multiple generations of a wild population of wolves. Using a linear mixed model, a robust genetic relationship matrix, 12,288 single nucleotide polymorphisms (SNPs) and 111 wolves, we estimated the SNP-based heritability of aggression to be 37% and an additional 14% of the phenotypic variation explained by shared environmental exposures. We identified 598 SNP genotypes from 425 grey wolves to resolve a consensus pedigree that was included in a heritability analysis of 141 individuals with SNP genotype, metadata and aggression data. The pedigree-based heritability estimate for aggression is 14%, and an additional 16% of the phenotypic variation was explained by shared environmental exposures. We find strong effects of breeding status and relative pack size on aggression. Through an integrative approach, these results provide a framework for understanding the genetic architecture of a complex trait that influences individual fitness, with linkages to reproduction, in a social carnivore. Along with a few other studies, we show here the incredible utility of a pedigreed natural population for dissecting a complex, fitness-related behavioural trait.

Aberrant expression of S-SCAM causes the loss of GABAergic synapses in hippocampal neurons

The duplication and deletion mutations of the S-SCAM/MAGI-2 gene are associated with schizophrenia and infantile spasms, respectively. S-SCAM is a unique synaptic scaffolding protein that localizes to both excitatory and GABAergic synapses. However, consequences of aberrant S-SCAM expression on GABAergic synapses is little studied. Here we report the effect of S-SCAM knockdown and overexpression on GABAergic synapses. S-SCAM knockdown in cultured hippocampal neurons caused a drastic loss of both pre- and post-synaptic components of GABAergic synapses, indicating its essential role in GABAergic synapse formation and maintenance. Surprisingly, S-SCAM overexpression also attenuated GABAergic synapses, but the effect is mediated by the loss of postsynaptic GABA_A receptors, gephyrin, and neuroligin 2 and does not involve presynaptic component vesicular GABA transporters. Overexpression studies using S-SCAM mutants with various domain deletions indicated that GABAergic synapse loss correlates with their ability to increase excitatory synaptic function. Consistently, AMPA receptor antagonist CNQX or calcineurin inhibitor FK506 abolished the S-SCAM overexpression-induced loss of GABA_A receptors, supporting that GABAergic synapse loss by S-SCAM overexpression is due to the activity-induced dispersal of synaptic GABA_A receptors. These results suggest that abnormal S-SCAM protein levels disrupt excitation/inhibition balance in neurons, which may explain the pathogenic nature of S-SCAM copy number variations.

Inhibitory Receptor Diffusion Dynamics

The dynamic modulation of receptor diffusion-trapping at inhibitory synapses is crucial to synaptic transmission, stability, and plasticity. In this review article, we will outline the progression of understanding of receptor diffusion dynamics at the plasma membrane. We will discuss how regulation of reversible trapping of receptor-scaffold interactions in combination with theoretical modeling approaches can be used to quantify these chemical interactions at the postsynapse of living cells.

Preserving the balance: diverse forms of long-term GABAergic synaptic plasticity

Cellular mechanisms that regulate the interplay of synaptic excitation and inhibition are thought to be central to the functional stability of healthy neuronal circuits. A growing body of literature demonstrates the capacity for inhibitory GABAergic synapses to exhibit long-term plasticity in response to changes in neuronal activity. Here, we review this expanding field of research, focusing on the diversity of mechanisms that link glutamatergic signalling, postsynaptic action potentials and inhibitory synaptic strength. Several lines of evidence indicate that multiple, parallel forms of plasticity serve to regulate activity at both the input and output domains of individual neurons. Overall, these varied phenomena serve to promote both stability and flexibility over the life of the organism.

NMDA receptor activation induces long-term potentiation of glycine synapses

Of the fast ionotropic synapses, glycinergic synapses are the least well understood, but are vital for the maintenance of inhibitory signaling in the brain and spinal cord. Glycinergic signaling comprises half of the inhibitory signaling in the spinal cord, and glycinergic synapses are likely to regulate local nociceptive processing as well as the transmission to the brain of peripheral nociceptive information. Here we have investigated the rapid and prolonged potentiation of glycinergic synapses in the superficial dorsal horn of young male and female mice after brief activation of NMDA receptors (NMDARs). Glycinergic inhibitory postsynaptic currents (IPSCs) evoked with lamina II-III stimulation in identified GABAergic neurons in lamina II were potentiated by bath-applied Zn2+ and were depressed by the prostaglandin PGE2, consistent with the presence of both GlyRα1- and GlyRα3-containing receptors. NMDA application rapidly potentiated synaptic glycinergic currents. Whole-cell currents evoked by exogenous glycine were also readily potentiated by NMDA, indicating that the potentiation results from altered numbers or conductance of postsynaptic glycine receptors. Repetitive depolarization alone of the postsynaptic GABAergic neuron also potentiated glycinergic synapses, and intracellular EGTA prevented both NMDA-induced and depolarization-induced potentiation of glycinergic IPSCs. Optogenetic activation of trpv1 lineage afferents also triggered NMDAR-dependent potentiation of glycinergic synapses. Our results suggest that during peripheral injury or inflammation, nociceptor firing during injury is likely to potentiate glycinergic synapses on GABAergic neurons. This disinhibition mechanism may be engaged rapidly, altering dorsal horn circuitry to promote the transmission of nociceptive information to the brain.