Induction of NMDA and GABAA receptor-mediated Ca2+ oscillations with KCC2 mRNA downregulation in injured facial motoneurons

Reduced Gene Expression of KCC2 Accelerates Axonal Regeneration and Reduces Motor Dysfunctions after Tibial Nerve Severance and Suturing

Gamma-aminobutyric acid and glycine (GABA/Gly) are predominantly inhibitory neurotransmitters in the mature central nervous system; however, they mediate membrane potential depolarization during development. These differences in actions depend on intracellular Cl- concentrations ([Cl-]i), which are primarily regulated by potassium chloride cotransporter 2 (KCC2). After nerve injury, KCC2 expression markedly decreases and GABA/Gly mediate depolarization. Following nerve regeneration, KCC2 expression recovers and GABA/Gly become inhibitory, suggesting that KCC2 reduction and GABA/Gly excitation may be crucial for axonal regeneration. To directly clarify their involvement in regeneration, we analyzed recovery processes after tibial nerve severance and suturing between heterozygous KCC2 knockout mice (HT), whose KCC2 levels are halved, and their wild-type littermates (WT). Compared with WT mice, the sciatic functional index-indicating lower limb motor function-was significantly higher until 28 days after operation (D28) in HT mice. Furthermore, at D7, many neurofilament-positive fibers were elongated into the distal part of the sutured nerve in HT mice only, and myelinated axonal density was significantly higher at D21 and D28 in HT animals. Electron microscopy and galanin immunohistochemistry indicated a shorter nerve degeneration period in HT mice. Moreover, a less severe decrease in choline acetyltransferase was observed in HT mice. These results suggest that nerve degeneration and regeneration proceed more rapidly in HT mice, resulting in milder motor dysfunction. Via similar microglial activation, nerve surgery may reduce KCC2 levels more rapidly in HT mice, followed by earlier increased [Cl-]i and longer-lasting GABA/Gly excitation. Taken together, reduced KCC2 may accelerate nerve regeneration via GABA/Gly excitation.

KCC2 downregulation after sciatic nerve injury enhances motor function recovery

Injury to mature neurons induces downregulated KCC2 expression and activity, resulting in elevated intracellular [Cl^-] and depolarized GABAergic signaling. This phenotype mirrors immature neurons wherein GABA-evoked depolarizations facilitate neuronal circuit maturation. Thus, injury-induced KCC2 downregulation is broadly speculated to similarly facilitate neuronal circuit repair. We test this hypothesis in spinal cord motoneurons injured by sciatic nerve crush, using transgenic (CaMKII-KCC2) mice wherein conditional CaMKIIα promoter-KCC2 expression coupling selectively prevents injury-induced KCC2 downregulation. We demonstrate, via an accelerating rotarod assay, impaired motor function recovery in CaMKII-KCC2 mice relative to wild-type mice. Across both cohorts, we observe similar motoneuron survival and re-innervation rates, but differing post-injury reorganization patterns of synaptic input to motoneuron somas-for wild-type, both VGLUT1-positive (excitatory) and GAD67-positive (inhibitory) terminal counts decrease; for CaMKII-KCC2, only VGLUT1-positive terminal counts decrease. Finally, we recapitulate the impaired motor function recovery of CaMKII-KCC2 mice in wild-type mice by administering local spinal cord injections of bicuculline (GABA_A receptor blockade) or bumetanide (lowers intracellular [Cl^-] by NKCC1 blockade) during the early post-injury period. Thus, our results provide direct evidence that injury-induced KCC2 downregulation enhances motor function recovery and suggest an underlying mechanism of depolarizing GABAergic signaling driving adaptive reconfiguration of presynaptic GABAergic input.

Central Facial Nervous System Biomolecules Involved in Peripheral Facial Nerve Injury Responses and Potential Therapeutic Strategies

Peripheral facial nerve injury leads to changes in the expression of various neuroactive substances that affect nerve cell damage, survival, growth, and regeneration. In the case of peripheral facial nerve damage, the injury directly affects the peripheral nerves and induces changes in the central nervous system (CNS) through various factors, but the substances involved in these changes in the CNS are not well understood. The objective of this review is to investigate the biomolecules involved in peripheral facial nerve damage so as to gain insight into the mechanisms and limitations of targeting the CNS after such damage and identify potential facial nerve treatment strategies. To this end, we searched PubMed using keywords and exclusion criteria and selected 29 eligible experimental studies. Our analysis summarizes basic experimental studies on changes in the CNS following peripheral facial nerve damage, focusing on biomolecules that increase or decrease in the CNS and/or those involved in the damage, and reviews various approaches for treating facial nerve injury. By establishing the biomolecules in the CNS that change after peripheral nerve damage, we can expect to identify factors that play an important role in functional recovery from facial nerve damage. Accordingly, this review could represent a significant step toward developing treatment strategies for peripheral facial palsy.

Preservation of KCC2 expression in axotomized abducens motoneurons and its enhancement by VEGF

The potassium chloride cotransporter 2 (KCC2) is the main Cl- extruder in neurons. Any alteration in KCC2 levels leads to changes in Cl- homeostasis and, consequently, in the polarity and amplitude of inhibitory synaptic potentials mediated by GABA or glycine. Axotomy downregulates KCC2 in many different motoneurons and it is suspected that interruption of muscle-derived factors maintaining motoneuron KCC2 expression is in part responsible. In here, we demonstrate that KCC2 is expressed in all oculomotor nuclei of cat and rat, but while trochlear and oculomotor motoneurons downregulate KCC2 after axotomy, expression is unaltered in abducens motoneurons. Exogenous application of vascular endothelial growth factor (VEGF), a neurotrophic factor expressed in muscle, upregulated KCC2 in axotomized abducens motoneurons above control levels. In parallel, a physiological study using cats chronically implanted with electrodes for recording abducens motoneurons in awake animals, demonstrated that inhibitory inputs related to off-fixations and off-directed saccades in VEGF-treated axotomized abducens motoneurons were significantly higher than in control, but eye-related excitatory signals in the on direction were unchanged. This is the first report of lack of KCC2 regulation in a motoneuron type after injury, proposing a role for VEGF in KCC2 regulation and demonstrating the link between KCC2 and synaptic inhibition in awake, behaving animals.

Riluzole treatment modulates KCC2 and EAAT-2 receptor expression and Ca2+ accumulation following ventral root avulsion injury

Avulsion injury results in motoneuron death due to the increased excitotoxicity developing in the affected spinal segments. This study focused on possible short and long term molecular and receptor expression alterations which are thought to be linked to the excitotoxic events in the ventral horn with or without the anti-excitotoxic riluzole treatment. In our experimental model the left lumbar 4 and 5 (L4, 5) ventral roots of the spinal cord were avulsed. Treated animals received riluzole for 2 weeks. Riluzole is a compound that acts to block voltage-activated Na⁺ and Ca²⁺ channels. In control animals the L4, 5 ventral roots were avulsed without riluzole treatment. Expression of astrocytic EAAT-2 and that of KCC2 in motoneurons on the affected side of the L4 spinal segment were detected after the injury by confocal and dSTORM imaging, intracellular Ca²⁺ levels in motoneurons were quantified by electron microscopy. The KCC2 labeling in the lateral and ventrolateral parts of the L4 ventral horn was weaker compared with the medial part of L4 ventral horn in both groups. Riluzole treatment dramatically enhanced motoneuron survival but was not able to prevent the down-regulation of KCC2 expression in injured motoneurons. In contrast, riluzole successfully obviated the increase of intracellular calcium level and the decrease of EAAT-2 expression in astrocytes compared with untreated injured animals. We conclude that KCC2 may not be an essential component for survival of injured motoneurons and riluzole is able to modulate the intracellular level of calcium and expression of EAAT-2.

Therapeutic effects of KCC2 chloride transporter activation on detrusor overactivity in mice with spinal cord injury

This study aimed to clarify whether downregulation of K⁺-Cl^- cotransporter 2 (KCC2) in the sacral parasympathetic nucleus (SPN) of the lumbosacral spinal cord, from which the efferent pathway innervating the bladder originates, causes cellular hyperexcitability and triggers detrusor overactivity (DO) in spinal cord injury (SCI). SCI was produced by Th8-9 spinal cord transection in female C57BL/6 mice. At 4 wk after SCI, CLP290, a KCC2 activator, was administered, and cystometry was performed. Thereafter, neuronal activity with c-fos staining and KCC2 expression in cholinergic preganglionic parasympathetic neurons in the SPN was examined using immunohistochemistry. Firing properties of neurons in the SPN region were evaluated by extracellular recordings in the spinal cord slice preparations. DO evident as nonvoiding contractions was significantly reduced by CLP290 treatment in SCI mice. The number of c-fos-positive cells and coexpression of c-fos in choline acetyltransferase-positive cells were decreased in the SPN region of the SCI CLP290-treated group versus the SCI vehicle-treated group. KCC2 immunoreactivity was present on the cell membrane of SPN neurons and normalized fluorescence intensity of KCC2 in choline acetyltransferase-positive SPN neurons was decreased in the SCI vehicle-treated group versus the spinal intact vehicle-treated group but recovered in the SCI CLP290-treated group. Extracellular recordings showed that CLP290 suppressed the high-frequency firing activity of SPN neurons in SCI mice. These results indicated that SCI-induced DO is associated with downregulation of KCC2 in preganglionic parasympathetic neurons and that activation of KCC2 transporters can reduce DO, increase KCC2 expression in preganglionic parasympathetic neurons, and decrease neuronal firing of SPN neurons in SCI mice.NEW & NOTEWORTHY This study is the first report to suggest that activation of the Cl^- transporter K⁺-Cl^- cotransporter 2 may be a therapeutic modality for the treatment of spinal cord injury-induced detrusor overactivity by targeting bladder efferent pathways.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

Events Occurring in the Axotomized Facial Nucleus

Transection of the rat facial nerve leads to a variety of alterations not only in motoneurons, but also in glial cells and inhibitory neurons in the ipsilateral facial nucleus. In injured motoneurons, the levels of energy metabolism-related molecules are elevated, while those of neurofunction-related molecules are decreased. In tandem with these motoneuron changes, microglia are activated and start to proliferate around injured motoneurons, and astrocytes become activated for a long period without mitosis. Inhibitory GABAergic neurons reduce the levels of neurofunction-related molecules. These facts indicate that injured motoneurons somehow closely interact with glial cells and inhibitory neurons. At the same time, these events allow us to predict the occurrence of tissue remodeling in the axotomized facial nucleus. This review summarizes the events occurring in the axotomized facial nucleus and the cellular and molecular mechanisms associated with each event.

Slow progression of sciatic nerve degeneration and regeneration after loose ligation through microglial activation and decreased KCC2 levels in the mouse spinal cord ventral horn

Peripheral nerve injury affects motor functions. To reveal the mechanisms underlying motor dysfunction and recovery after nerve compression, which have not been precisely examined, we investigated the temporal relationship among changes in motor function, nerve histopathology, and marker molecule expression in the spinal cord after loose ligation of the mouse sciatic nerve. After ligation, sciatic motor function suddenly declined, and axons gradually degenerated. During degeneration, galanin was localized in motor neuron cell bodies. Then, in the ventral horn, microglia were activated, and expression of choline acetyltransferase (ChAT), a synthetic enzyme of acetylcholine, and potassium chloride co-transporter 2 (KCC2), which shifts the action of γ-amino butyric acid (GABA) and glycine to inhibitory, decreased. Motor function recovery was insufficient although axonal regeneration was complete. ChAT levels gradually recovered during axonal regeneration. When regeneration was nearly complete, microglial activation declined, and KCC2 expression started to increase. The KCC2 level sufficiently recovered when axonal regeneration was complete, suggesting that the excitatory action of GABA/glycine may participate in axonal regeneration. Furthermore, these changes proceeded slower than those after severance, suggesting that loose ligation, compression, may mediate slower progression of degeneration and regeneration than severance, and these changes may cause the motor dysfunction and its recovery.

Developmental Formation of the GABAergic and Glycinergic Networks in the Mouse Spinal Cord

Gamma-aminobutyric acid (GABA) and glycine act as inhibitory neurotransmitters. Three types of inhibitory neurons and terminals, GABAergic, GABA/glycine coreleasing, and glycinergic, are orchestrated in the spinal cord neural circuits and play critical roles in regulating pain, locomotive movement, and respiratory rhythms. In this study, we first describe GABAergic and glycinergic transmission and inhibitory networks, consisting of three types of terminals in the mature mouse spinal cord. Second, we describe the developmental formation of GABAergic and glycinergic networks, with a specific focus on the differentiation of neurons, formation of synapses, maturation of removal systems, and changes in their action. GABAergic and glycinergic neurons are derived from the same domains of the ventricular zone. Initially, GABAergic neurons are differentiated, and their axons form synapses. Some of these neurons remain GABAergic in lamina I and II. Many GABAergic neurons convert to a coreleasing state. The coreleasing neurons and terminals remain in the dorsal horn, whereas many ultimately become glycinergic in the ventral horn. During the development of terminals and the transformation from radial glia to astrocytes, GABA and glycine receptor subunit compositions markedly change, removal systems mature, and GABAergic and glycinergic action shifts from excitatory to inhibitory.

Normal Development and Pathology of Motoneurons: Anatomy, Electrophysiological Properties, Firing Patterns and Circuit Connectivity

This chapter will provide an introduction into motoneuron anatomy, electrophysiological properties, firing patterns focusing on development and also describing several pathological conditions that affect mononeurons. It starts with a historical retrospective describing the early landmark work into motoneurons. The next section lays out the various types of motoneurons (alpha, beta, and gamma) and their subclasses (fast-twitch fatigable, fast-twitch fatigue-resistant, and slow-twitch fatigue resistant), highlighting the functional relevance of this classification scheme. The third section describes the development of motoneurons' passive and active electrophysiological properties. This section also defines the major terms one uses in describing how a neuron functions electrophysiologically. The electrophysiological aspects of a neuron is critical to understanding how it behaves within a circuit and contributes to behavior since the firing of an action potential is how neurons communicate with each other and with muscles. The electrophysiological changes of motoneurons over development underlies how their function changes over the lifetime of an organism. After describing the properties of individual motoneurons, the chapter then turns to revealing how motoneurons interact within complex neural circuits, with other motoneurons as well as sensory neurons, and how these circuits change over development. Finally, this chapter ends with highlighting some recent advances made in motoneuron pathology, focusing on spinal muscular atrophy, amyotrophic lateral sclerosis, and axotomy.

When Are Depolarizing GABAergic Responses Excitatory?

The membrane responses upon activation of GABA(A) receptors critically depend on the intracellular Cl⁻ concentration ([Cl⁻]_i), which is maintained by a set of transmembrane transporters for Cl⁻. During neuronal development, but also under several pathophysiological conditions, the prevailing expression of the Cl⁻ loader NKCC1 and the low expression of the Cl⁻ extruder KCC2 causes elevated [Cl⁻]_i, which result in depolarizing GABAergic membrane responses. However, depolarizing GABAergic responses are not necessarily excitatory, as GABA(A) receptors also reduces the input resistance of neurons and thereby shunt excitatory inputs. To summarize our knowledge on the effect of depolarizing GABA responses on neuronal excitability, this review discusses theoretical considerations and experimental studies illustrating the relation between GABA conductances, GABA reversal potential and neuronal excitability. In addition, evidences for the complex spatiotemporal interaction between depolarizing GABAergic and glutamatergic inputs are described. Moreover, mechanisms that influence [Cl⁻]_i beyond the expression of Cl⁻ transporters are presented. And finally, several in vitro and in vivo studies that directly investigated whether GABA mediates excitation or inhibition during early developmental stages are summarized. In summary, these theoretical considerations and experimental evidences suggest that GABA can act as inhibitory neurotransmitter even under conditions that maintain substantial depolarizing membrane responses.

Prochlorperazine Withdraws the Delayed Onset Tonic Activity of Unloaded Rat Soleus Muscle: A Pilot Study

A gradual increase in rat soleus muscle electromyographic (EMG) activity is known to occur after 3–4 days of hindlimb suspension/unloading (HS). The physiological significance and mechanisms of such activity of motoneurons under unloading conditions are currently unclear. Since hyperactivity of motoneurons and muscle spasticity after spinal cord injury are associated with KCC2 downregulation, we hypothesized that a decrease in potassium (K⁺)/chloride (Cl⁻) co-transporter 2 (KCC2) in motoneurons would be responsible for an increase in soleus muscle EMG activity during HS. We aimed to investigate the effect of prochlorperazine (KCC2 activator) on the electrical activity of rat soleus muscle under HS. Wistar rats were divided into the following groups: (1) vivarium control (C), (2) 7-day HS group (7HS) and (3) 7-day HS group plus intraperitoneal injections of prochlorperazine (10 mg/kg, daily) (7HS + P). Expression of proteins in the motoneurons of the lumbar spinal cord was determined by Western blotting. An electromyogram of the rat soleus muscle was recorded using intramuscular electrodes. KCC2 content after 7-day HS significantly decreased by 34% relative to the control group. HS-induced decrease in KCC2 protein content was prevented by prochlorperazine administration. HS also induced a significant 80% decrease in KCC2 Ser940 phosphorylation; however prochlorperazine did not affect KCC2 phosphorylation. The treatment of the rats with prochlorperazine prevented a HS-induced increase in Na(+)/K(+)/(Cl−) co-transporter 1 (KCC2 antagonist) protein content. In parallel with the restoration of KCC2 content, prochlorperazine administration during HS partially prevented an increase in the soleus muscle tonic EMG activity. Thus, prochlorperazine administration during 7-day HS prevents a decrease in KCC2 protein expression in motoneurons and significantly reduces the level of HS-induced soleus muscle electrical activity.

The Jun-dependent axon regeneration gene program: Jun promotes regeneration over plasticity

The regeneration-associated gene (RAG) expression program is activated in injured peripheral neurons after axotomy and enables long-distance axon re-growth. Over 1000 genes are regulated, and many transcription factors are upregulated or activated as part of this response. However, a detailed picture of how RAG expression is regulated is lacking. In particular, the transcriptional targets and specific functions of the various transcription factors are unclear. Jun was the first-regeneration-associated transcription factor identified and the first shown to be functionally important. Here we fully define the role of Jun in the RAG expression program in regenerating facial motor neurons. At 1, 4 and 14 days after axotomy, Jun upregulates 11, 23 and 44% of the RAG program, respectively. Jun functions relevant to regeneration include cytoskeleton production, metabolic functions and cell activation, and the downregulation of neurotransmission machinery. In silico analysis of promoter regions of Jun targets identifies stronger over-representation of AP1-like sites than CRE-like sites, although CRE sites were also over-represented in regions flanking AP1 sites. Strikingly, in motor neurons lacking Jun, an alternative SRF-dependent gene expression program is initiated after axotomy. The promoters of these newly expressed genes exhibit over-representation of CRE sites in regions near to SRF target sites. This alternative gene expression program includes plasticity-associated transcription factors and leads to an aberrant early increase in synapse density on motor neurons. Jun thus has the important function in the early phase after axotomy of pushing the injured neuron away from a plasticity response and towards a regenerative phenotype.

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.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Intra-axonal mechanisms driving axon regeneration

Traumatic injury to the peripheral and central nervous systems very often causes axotomy, where an axon loses connections with its target resulting in loss of function. The axon segments distal to the injury site lose connection with the cell body and degenerate. Axotomized neurons in the periphery can spontaneously mount a regenerative response and reconnect to their denervated target tissues, though this is rarely complete in humans. In contrast, spontaneous regeneration rarely occurs after axotomy in the spinal cord and brain. Here, we concentrate on the mechanisms underlying this spontaneous regeneration in the peripheral nervous system, focusing on events initiated from the axon that support regenerative growth. We contrast this with what is known for axonal injury responses in the central nervous system. Considering the neuropathy focus of this special issue, we further draw parallels and distinctions between the injury-response mechanisms that initiate regenerative gene expression programs and those that are known to trigger axon degeneration.

Development and persistence of neuropathic pain through microglial activation and KCC2 decreasing after mouse tibial nerve injury

Gamma-amino butyric acid (GABA) is an inhibitory neurotransmitter in the mature brain, but is excitatory during development and after motor nerve injury. This difference in GABAergic action depends on the intracellular chloride ion concentration ([Cl-]i), primarily regulated by potassium chloride co-transporter 2 (KCC2). To reveal precise processes of the neuropathic pain through changes in GABAergic action, we prepared tibial nerve ligation and severance models using male mice, and examined temporal relationships amongst changes in (1) the mechanical withdrawal threshold in the sural nerve area, (2) localization of the molecules involved in GABAergic transmission and its upstream signaling in the dorsal horn, and (3) histology of the tibial nerve. In the ligation model, tibial nerve degeneration disappeared by day 56, but mechanical allodynia, reduced KCC2 localization, and increased microglia density remained until day 90. Microglia density was higher in the tibial zone than the sural zone before day 21, but this result was inverted after day 28. In contrast, in the severance model, all above changes were detected until day 28, but were simultaneously and significantly recovered by day 90. These results suggested that in male mice, allodynia may be caused by reduced GABAergic synaptic inhibition, resulting from elevated [Cl-]i after the reduction of KCC2 by activated microglia. Furthermore, our results suggested that factors from degenerating nerve terminals may diffuse into the sural zone, whereby they induced the development of allodynia in the sural nerve area, while other factors in the sural zone may mediate persistent allodynia through the same pathway.

Enhancing neuronal chloride extrusion rescues α2/α3 GABAA-mediated analgesia in neuropathic pain

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis.

Removal of the Potassium Chloride Co-Transporter from the Somatodendritic Membrane of Axotomized Motoneurons Is Independent of BDNF/TrkB Signaling But Is Controlled by Neuromuscular Innervation

The potassium-chloride cotransporter (KCC2) maintains the low intracellular chloride found in mature central neurons and controls the strength and direction of GABA/glycine synapses. We found that following axotomy as a consequence of peripheral nerve injuries (PNIs), KCC2 protein is lost throughout the somatodendritic membrane of axotomized spinal cord motoneurons after downregulation of kcc2 mRNA expression. This large loss likely depolarizes the reversal potential of GABA/glycine synapses, resulting in GABAergic-driven spontaneous activity in spinal motoneurons similar to previous reports in brainstem motoneurons. We hypothesized that the mechanism inducing KCC2 downregulation in spinal motoneurons following peripheral axotomy might be mediated by microglia or motoneuron release of BDNF and TrkB activation as has been reported on spinal cord dorsal horn neurons after nerve injury, motoneurons after spinal cord injury (SCI), and in many other central neurons throughout development or a variety of pathologies. To test this hypothesis, we used genetic approaches to interfere with microglia activation or delete bdnf from specifically microglia or motoneurons, as well as pharmacology (ANA-12) and pharmacogenetics (F616A mice) to block TrkB activation. We show that KCC2 dysregulation in axotomized motoneurons is independent of microglia, BDNF, and TrkB. KCC2 is instead dependent on neuromuscular innervation; KCC2 levels are restored only when motoneurons reinnervate muscle. Thus, downregulation of KCC2 occurs specifically while injured motoneurons are regenerating and might be controlled by target-derived signals. GABAergic and glycinergic synapses might therefore depolarize motoneurons disconnected from their targets and contribute to augment motoneuron activity known to promote motor axon regeneration.

Changes in the expression and localization of signaling molecules in mouse facial motor neurons during regeneration of facial nerves

After injury, peripheral axons usually re-extend toward their target, and neuronal functions recover. Previous studies have reported that expression of various molecules are transiently altered in motor neurons after nerve injury, but the time course of these changes and their relationship with functional recovery have not been clearly demonstrated. We used the mouse facial nerve transection and suturing model, and examined the changes in expression of five molecules, choline acetyl transferase (ChAT), galanin, calcitonin gene-related protein (CGRP), gephyrin, and potassium chloride co-transporter 2 (KCC2) in the facial motor neurons after surgery until recovery. Number of ChAT-positive neurons was markedly decreased at days 3 and 7, and recovered to the normal level by day 60, when facial motor functions recovered. Localization of two neuropeptides, CGRP and galanin, was increased in the perikarya and axons during regeneration, and returned to the normal levels by days 60 and 28, respectively. Expression of two postsynaptic elements of γ-amino butyric acid synapses, gephyrin and KCC2, was decreased at days 3 and 7, and recovered by day 60. These results suggest that ChAT, CGRP, and KCC2 may be objective indicators of regeneration, and altering their expression may be related to the functional recovery and axonal re-extension.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

Bumetanide promotes neural precursor cell regeneration and dendritic development in the hippocampal dentate gyrus in the chronic stage of cerebral ischemia

Bumetanide has been shown to lessen cerebral edema and reduce the infarct area in the acute stage of cerebral ischemia. Few studies focus on the effects of bumetanide on neuroprotection and neurogenesis in the chronic stage of cerebral ischemia. We established a rat model of cerebral ischemia by injecting endothelin-1 in the left cortical motor area and left corpus striatum. Seven days later, bumetanide 200 µg/kg/day was injected into the lateral ventricle for 21 consecutive days with a mini-osmotic pump. Results demonstrated that the number of neuroblasts cells and the total length of dendrites increased, escape latency reduced, and the number of platform crossings increased in the rat hippocampal dentate gyrus in the chronic stage of cerebral ischemia. These findings suggest that bumetanide promoted neural precursor cell regeneration, dendritic development and the recovery of cognitive function, and protected brain tissue in the chronic stage of ischemia.

BDNF Signaling Promotes Vestibular Compensation by Increasing Neurogenesis and Remodeling the Expression of Potassium-Chloride Cotransporter KCC2 and GABAA Receptor in the Vestibular Nuclei

Reactive cell proliferation occurs rapidly in the cat vestibular nuclei (VN) after unilateral vestibular neurectomy (UVN) and has been reported to facilitate the recovery of posturo-locomotor functions. Interestingly, whereas animals experience impairments for several weeks, extraordinary plasticity mechanisms take place in the local microenvironment of the VN: newborn cells survive and acquire different phenotypes, such as microglia, astrocytes, or GABAergic neurons, whereas animals eventually recover completely from their lesion-induced deficits. Because brain-derived neurotrophic factor (BDNF) can modulate vestibular functional recovery and neurogenesis in mammals, in this study, we examined the effect of BDNF chronic intracerebroventricular infusion versus K252a (a Trk receptor antagonist) in our UVN model. Results showed that long-term intracerebroventricular infusion of BDNF accelerated the restoration of vestibular functions and significantly increased UVN-induced neurogenesis, whereas K252a blocked that effect and drastically delayed and prevented the complete restoration of vestibular functions. Further, because the level of excitability in the deafferented VN is correlated with behavioral recovery, we examined the state of neuronal excitability using two specific markers: the cation-chloride cotransporter KCC2 (which determines the hyperpolarizing action of GABA) and GABAA receptors. We report for the first time that, during an early time window after UVN, significant BDNF-dependent remodeling of excitability markers occurs in the brainstem. These data suggest that GABA acquires a transient depolarizing action during recovery from UVN, which potentiates the observed reactive neurogenesis and accelerates vestibular functional recovery. These findings suggest that BDNF and/or KCC2 could represent novel treatment strategies for vestibular pathologies.

SIGNIFICANCE STATEMENT

In this study, we report for the first time that brain-derived neurotrophic factor potentiates vestibular neurogenesis and significantly accelerates functional recovery after unilateral vestibular injury. We also show that specific markers of excitability, the potassium-chloride cotransporter KCC2 and GABAA receptors, undergo remarkable fluctuations within vestibular nuclei (VN), strongly suggesting that GABA acquires a transient depolarizing action in the VN during the recovery period. This novel plasticity mechanism could explain in part how the system returns to electrophysiological homeostasis between the deafferented and intact VN, considered in the literature to be a key parameter of vestibular compensation. In this context, our results open new perspectives for the development of therapeutic approaches to alleviate the vestibular symptoms and favor vestibular function recovery.

Inhibition, Not Excitation, Drives Rhythmic Whisking

Sniffing and whisking typify the exploratory behavior of rodents. These actions involve separate oscillators in the medulla, located respectively in the pre-Bötzinger complex (preBötC) and the vibrissa-related region of the intermediate reticular formation (vIRt). We examine how these oscillators synergize to control sniffing and whisking. We find that the vIRt contains glycinergic/GABAergic cells that rhythmically inhibit vibrissa facial motoneurons. As a basis for the entrainment of whisking by breathing, but not vice versa, we provide evidence for unidirectional connections from the preBötC to the vIRt. The preBötC further contributes to the control of the mystacial pad. Lastly, we show that bilateral synchrony of whisking relies on the respiratory rhythm, consistent with commissural connections between preBötC cells. These data yield a putative circuit in which the preBötC acts as a master clock for the synchronization of vibrissa, pad, and snout movements, as well as for the bilateral synchronization of whisking.

The role of GABA in the regulation of GnRH neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. Gamma-amino butyric acid (GABA) has long been implicated as one of the major players in the regulation of GnRH neurons. Although GABA is typically an inhibitory neurotransmitter in the mature adult central nervous system, most mature GnRH neurons show the unusual characteristic of being excited by GABA. While many reports have provided much insight into the contribution of GABA to the activity of GnRH neurons, the precise physiological role of the excitatory action of GABA on GnRH neurons remains elusive. This brief review presents the current knowledge of the role of GABA signaling in GnRH neuronal activity. We also discuss the modulation of GABA signaling by neurotransmitters and neuromodulators and the functional consequence of GABAergic inputs to GnRH neurons in both the physiology and pathology of reproduction.

Exercise modulates chloride homeostasis after spinal cord injury

Activity-based therapies are routinely integrated in spinal cord injury (SCI) rehabilitation programs because they result in a reduction of hyperreflexia and spasticity. However, the mechanisms by which exercise regulates activity in spinal pathways to reduce spasticity and improve functional recovery are poorly understood. Persisting alterations in the action of GABA on postsynaptic targets is a signature of CNS injuries, including SCI. The action of GABA depends on the intracellular chloride concentration, which is determined largely by the expression of two cation-chloride cotransporters (CCCs), KCC2 and NKCC1, which serve as chloride exporters and importers, respectively. We hypothesized that the reduction in hyperreflexia with exercise after SCI relies on a return to chloride homeostasis. Sprague Dawley rats received a spinal cord transection at T12 and were assigned to SCI-7d, SCI-14d, SCI-14d+exercise, SCI-28d, SCI-28d+exercise, or SCI-56d groups. During a terminal experiment, H-reflexes were recorded from interosseus muscles after stimulation of the tibial nerve and the low-frequency-dependent depression (FDD) was assessed. We provide evidence that exercise returns spinal excitability and levels of KCC2 and NKCC1 toward normal levels in the lumbar spinal cord. Acutely altering chloride extrusion using the KCC2 blocker DIOA masked the effect of exercise on FDD, whereas blocking NKCC1 with bumetanide returned FDD toward intact levels after SCI. Our results indicate that exercise contributes to reflex recovery and restoration of endogenous inhibition through a return to chloride homeostasis after SCI. This lends support for CCCs as part of a pathway that could be manipulated to improve functional recovery when combined with rehabilitation programs.

Accumulation of GABAergic neurons, causing a focal ambient GABA gradient, and downregulation of KCC2 are induced during microgyrus formation in a mouse model of polymicrogyria

Although focal cortical malformations are considered neuronal migration disorders, their formation mechanisms remain unknown. We addressed how the γ-aminobutyric acid (GABA)ergic system affects the GABAergic and glutamatergic neuronal migration underlying such malformations. A focal freeze-lesion (FFL) of the postnatal day zero (P0) glutamic acid decarboxylase-green fluorescent protein knock-in mouse neocortex produced a 3- or 4-layered microgyrus at P7. GABAergic interneurons accumulated around the necrosis including the superficial region during microgyrus formation at P4, whereas E17.5-born, Cux1-positive pyramidal neurons outlined the GABAergic neurons and were absent from the superficial layer, forming cell-dense areas in layer 2 of the P7 microgyrus. GABA imaging showed that an extracellular GABA level temporally increased in the GABAergic neuron-positive area, including the necrotic center, at P4. The expression of the Cl(-) transporter KCC2 was downregulated in the microgyrus-forming GABAergic and E17.5-born glutamatergic neurons at P4; these cells may need a high intracellular Cl(-) concentration to induce depolarizing GABA effects. Bicuculline decreased the frequency of spontaneous Ca(2+) oscillations in these microgyrus-forming cells. Thus, neonatal FFL causes specific neuronal accumulation, preceded by an increase in ambient GABA during microgyrus formation. This GABA increase induces GABAA receptor-mediated Ca(2+) oscillation in KCC2-downregulated microgyrus-forming cells, as seen in migrating cells during early neocortical development.

Overexpression of BDNF increases excitability of the lumbar spinal network and leads to robust early locomotor recovery in completely spinalized rats

Strategies to induce recovery from lesions of the spinal cord have not fully resulted in clinical applications. This is a consequence of a number of impediments that axons encounter when trying to regrow beyond the lesion site, and that intraspinal rearrangements are subjected to. In the present study we evaluated (1) the possibility to improve locomotor recovery after complete transection of the spinal cord by means of an adeno-associated (AAV) viral vector expressing the neurotrophin brain-derived neurotrophic factor (BDNF) in lumbar spinal neurons caudal to the lesion site and (2) how the spinal cord transection and BDNF treatment affected neurotransmission in the segments caudal to the lesion site. BDNF overexpression resulted in clear increases in expression levels of molecules involved in glutamatergic (VGluT2) and GABAergic (GABA, GAD65, GAD67) neurotransmission in parallel with a reduction of the potassium-chloride co-transporter (KCC2) which contributes to an inhibitory neurotransmission. BDNF treated animals showed significant improvements in assisted locomotor performance, and performed locomotor movements with body weight support and plantar foot placement on a moving treadmill. These positive effects of BDNF local overexpression were detectable as early as two weeks after spinal cord transection and viral vector application and lasted for at least 7 weeks. Gradually increasing frequencies of clonic movements at the end of the experiment attenuated the quality of treadmill walking. These data indicate that BDNF has the potential to enhance the functionality of isolated lumbar circuits, but also that BDNF levels have to be tightly controlled to prevent hyperexcitability.

Current view on the functional regulation of the neuronal K(+)-Cl(-) cotransporter KCC2

In the mammalian central nervous system (CNS), the inhibitory strength of chloride (Cl(-))-permeable GABAA and glycine receptors (GABAAR and GlyR) depends on the intracellular Cl(-) concentration ([Cl(-)]i). Lowering [Cl(-)]i enhances inhibition, whereas raising [Cl(-)]i facilitates neuronal activity. A neuron's basal level of [Cl(-)]i, as well as its Cl(-) extrusion capacity, is critically dependent on the activity of the electroneutral K(+)-Cl(-) cotransporter KCC2, a member of the SLC12 cation-Cl(-) cotransporter (CCC) family. KCC2 deficiency compromises neuronal migration, formation and the maturation of GABAergic and glutamatergic synaptic connections, and results in network hyperexcitability and seizure activity. Several neurological disorders including multiple epilepsy subtypes, neuropathic pain, and schizophrenia, as well as various insults such as trauma and ischemia, are associated with significant decreases in the Cl(-) extrusion capacity of KCC2 that result in increases of [Cl(-)]i and the subsequent hyperexcitability of neuronal networks. Accordingly, identifying the key upstream molecular mediators governing the functional regulation of KCC2, and modifying these signaling pathways with small molecules, might constitute a novel neurotherapeutic strategy for multiple diseases. Here, we discuss recent advances in the understanding of the mechanisms regulating KCC2 activity, and of the role these mechanisms play in neuronal Cl(-) homeostasis and GABAergic neurotransmission. As KCC2 mediates electroneutral transport, the experimental recording of its activity constitutes an important research challenge; we therefore also, provide an overview of the different methodological approaches utilized to monitor function of KCC2 in both physiological and pathological conditions.

Cell-attached recordings of responses evoked by photorelease of GABA in the immature cortical neurons

We present a novel non-invasive technique to measure the polarity of GABAergic responses based on cell-attached recordings of currents activated by laser-uncaging of GABA. For these recordings, a patch pipette was filled with a solution containing RuBi-GABA, and GABA was released from this complex by a laser beam conducted to the tip of the patch pipette via an optic fiber. In cell-attached recordings from neocortical and hippocampal neurons in postnatal days P2-5 rat brain slices in vitro, we found that laser-uncaging of GABA activates integral cell-attached currents mediated by tens of GABA(A) channels. The initial response was inwardly directed, indicating a depolarizing response to GABA. The direction of the initial response was dependent on the pipette potential and analysis of its slope-voltage relationships revealed a depolarizing driving force of +11 mV for the currents through GABA channels. Initial depolarizing responses to GABA uncaging were inverted to hyperpolarizing in the presence of the NKCC1 blocker bumetanide. Current-voltage relationships of the currents evoked by RuBi-GABA uncaging using voltage-ramps at the peak of responses not only revealed a bumetanide-sensitive depolarizing reversal potential of the GABA(A) receptor mediated responses, but also showed a strong voltage-dependent hysteresis. Upon desensitization of the uncaged-GABA response, current-voltage relationships of the currents through single GABA(A) channels revealed depolarizing responses with the driving force values similar to those obtained for the initial response. Thus, cell-attached recordings of the responses evoked by local intrapipette GABA uncaging are suitable to assess the polarity of the GABA(A)-Rs mediated signals in small cell compartments.

Excitatory actions of GABA in the intact neonatal rodent hippocampus in vitro

The excitatory action of gamma-aminobutyric acid (GABA) is considered to be a hallmark of the developing nervous system. However, in immature brain slices, excitatory GABA actions may be secondary to neuronal injury during slice preparation. Here, we explored GABA actions in the rodent intact hippocampal preparations and at different depths of hippocampal slices during the early post-natal period [post-natal days (P) 1–7]. We found that in the intact hippocampus at P1–3: (i) GABA exerts depolarizing action as seen in cell-attached single GABA(A) channel recordings; (ii) GABA(A) receptor (GABA(A)-R) agonist isoguvacine and synaptic activation of the GABA(A)-Rs increase the frequency of multiple unit activity and the frequency of the network-driven giant depolarizing potentials (GDPs); and that (iii) Na⁺–K⁺–2Cl^- cotransporter (NKCC1) antagonist bumetanide suppresses GDPs and the excitatory actions of isoguvacine. In the hippocampal slices at P2–5, isoguvacine and synaptic activation of GABA(A)-Rs-evoked excitatory responses at all slice depths, including surface and core. Thus, GABA exerts excitatory actions in the intact hippocampus (P1–3) and at all depths of hippocampal slices (P2–5). Therefore, the excitatory actions of GABA in hippocampal slices during the first post-natal days are not due to neuronal injury during slice preparation, and the trauma-related excitatory GABA actions at the slice surface are a fundamentally different phenomenon observed during the second post-natal week.

Pre- and post-synaptic switches of GABA actions associated with Cl- homeostatic changes are induced in the spinal nucleus of the trigeminal nerve in a rat model of trigeminal neuropathic pain

Although trigeminal neuropathic pain is one of the most common chronic pain syndromes, the etiology is still unknown. Here, a rat model was generated using chronic constrictive injury (CCI) with ligation of the infraorbital nerve to test the hypothesis that collapse of chloride homeostasis in trigeminal neurons causes impairment of γ-aminobutyric acid-ergic (GABAergic) inhibition and induces trigeminal allodynia. Rats showed a reduction and increase in pain threshold and pain response scores, respectively, to mechanical stimulation, 1 and 3weeks after CCI. In situ hybridization and immunohistochemical analysis showed that inward-directed Na(+), K(+)-2Cl(-) cotransporter (NKCC1) mRNA and protein were upregulated in the small-sized and large-sized primary neurons in the injured side of the trigeminal ganglion and in the peripherin-positive terminal, respectively, for the first 2weeks, while outward-directed K(+)-Cl(-) cotransporter (KCC2) mRNA and protein were downregulated in secondary relay neurons on the injured side of the spinal trigeminal nucleus caudalis (Sp5C). Optical imaging of evoked synaptic responses using a voltage-sensitive dye revealed that pre- and post-synaptic GABA actions were disinhibited and excitatory in the injured side, respectively, but inhibited in the sham-operated side of the Sp5C. This downregulation of KCC2 in the Sp5C may result in an excitatory switch by impairing postsynaptic GABA inhibition. GABA-mediated presynaptic disinhibition was attenuated by bumetanide, suggesting that NKCC1 upregulation in primary neurons may facilitate pain transmission by presynaptic GABAergic depolarization. Such Cl(-) homeostatic disruption resulting in perturbation of the inhibitory system possibly increases pain transmission, which may underlie the pathophysiology of trigeminal neuropathic pain.

Refuting the challenges of the developmental shift of polarity of GABA actions: GABA more exciting than ever!

During brain development, there is a progressive reduction of intracellular chloride associated with a shift in GABA polarity: GABA depolarizes and occasionally excites immature neurons, subsequently hyperpolarizing them at later stages of development. This sequence, which has been observed in a wide range of animal species, brain structures and preparations, is thought to play an important role in activity-dependent formation and modulation of functional circuits. This sequence has also been considerably reinforced recently with new data pointing to an evolutionary preserved rule. In a recent “Hypothesis and Theory Article,” the excitatory action of GABA in early brain development is suggested to be “an experimental artefact” (Bregestovski and Bernard, 2012). The authors suggest that the excitatory action of GABA is due to an inadequate/insufficient energy supply in glucose-perfused slices and/or to the damage produced by the slicing procedure. However, these observations have been repeatedly contradicted by many groups and are inconsistent with a large body of evidence including the fact that the developmental shift is neither restricted to slices nor to rodents. We summarize the overwhelming evidence in support of both excitatory GABA during development, and the implications this has in developmental neurobiology.

Identification of pharmacological targets in amyotrophic lateral sclerosis through genomic analysis of deregulated genes and pathways

Amyotrophic Lateral Sclerosis (ALS) is a progressive and disabling neurodegenerative disorder characterized by upper and lower motor neuron loss, leading to respiratory insufficiency and death after 3-5 years. Riluzole is currently the only FDA approved drug for ALS, but it has only modest effects on survival. The majority of ALS cases are sporadic and probably associated to a multifactorial etiology. With the completion of genome sequencing in humans and model organisms, together with the advent of DNA microarray technology, the transcriptional cascades and networks underlying neurodegeneration in ALS are being elucidated providing new potential pharmacological targets. The main challenge now is the effective screening of the myriad of targets to identify those with the most therapeutic utility. The present review will illustrate how the identification, prioritization and validation of preclinical therapeutics can be achieved through genomic analysis of critical pathways and networks deregulated in ALS pathology.

Axon regeneration pathways identified by systematic genetic screening in C. elegans

The mechanisms underlying the ability of axons to regrow after injury remain poorly explored at the molecular genetic level. We used a laser injury model in Caenorhabditis elegans mechanosensory neurons to screen 654 conserved genes for regulators of axonal regrowth. We uncover several functional clusters of genes that promote or repress regrowth, including genes classically known to affect axon guidance, membrane excitability, neurotransmission, and synaptic vesicle endocytosis. The conserved Arf Guanine nucleotide Exchange Factor (GEF), EFA-6, acts as an intrinsic inhibitor of regrowth. By combining genetics and in vivo imaging, we show that EFA-6 inhibits regrowth via microtubule dynamics, independent of its Arf GEF activity. Among newly identified regrowth inhibitors, only loss of function in EFA-6 partially bypasses the requirement for DLK-1 kinase. Identification of these pathways significantly expands our understanding of the genetic basis of axonal injury responses and repair.

Chloride cotransporters, chloride homeostasis, and synaptic inhibition in the developing auditory system

The role of glycine and GABA as inhibitory neurotransmitters in the adult vertebrate nervous system has been well characterized in a variety of model systems, including the auditory, which is particularly well suited for analyzing inhibitory neurotransmission. However, a full understanding of glycinergic and GABAergic transmission requires profound knowledge of how the precise organization of such synapses emerges. Likewise, the role of glycinergic and GABAergic signaling during development, including the dynamic changes in regulation of cytosolic chloride via chloride cotransporters, needs to be thoroughly understood. Recent literature has elucidated the developmental expression of many of the molecular components that comprise the inhibitory synaptic phenotype. An equally important focus of research has revealed the critical role of glycinergic and GABAergic signaling in sculpting different developmental aspects in the auditory system. This review examines the current literature detailing the expression patterns and function (chapter 1), as well as the regulation and pharmacology of chloride cotransporters (chapter 2). Of particular importance is the ontogeny of glycinergic and GABAergic transmission (chapter 3). The review also surveys the recent work on the signaling role of these two major inhibitory neurotransmitters in the developing auditory system (chapter 4) and concludes with an overview of areas for further research (chapter 5).

Altered GABA signaling in early life epilepsies

The incidence of seizures is particularly high in the early ages of life. The immaturity of inhibitory systems, such as GABA, during normal brain development and its further dysregulation under pathological conditions that predispose to seizures have been speculated to play a major role in facilitating seizures. Seizures can further impair or disrupt GABA_A signaling by reshuffling the subunit composition of its receptors or causing aberrant reappearance of depolarizing or hyperpolarizing GABA_A receptor currents. Such effects may not result in epileptogenesis as frequently as they do in adults. Given the central role of GABA_A signaling in brain function and development, perturbation of its physiological role may interfere with neuronal morphology, differentiation, and connectivity, manifesting as cognitive or neurodevelopmental deficits. The current GABAergic antiepileptic drugs, while often effective for adults, are not always capable of stopping seizures and preventing their sequelae in neonates. Recent studies have explored the therapeutic potential of chloride cotransporter inhibitors, such as bumetanide, as adjunctive therapies of neonatal seizures. However, more needs to be known so as to develop therapies capable of stopping seizures while preserving the age- and sex-appropriate development of the brain.

Development of cortical GABAergic innervation

The mature neocortex contains many different classes of GABAergic inhibitory interneurons, distributed, with some degree of selectivity, through six layers, and through many different regions. Some of the events in the early lives of these neurones that may determine their ultimate destination, their maturation and their selective innervation of targets appropriate for each subtype, are discussed. Both time and place of birth influence the class of interneuron that an early post-mitotic interneuronal precursor will become, driven by the selective expression of different combinations of transcription factors in different regions of their birth places in the ganglionic eminence and ventricular zone. The long distance migration of these precursors along tangential routes in marginal, subventricular, and intermediate zones and their final radial movement, into the developing cortex, is regulated by chemical cues, both attractant and repellent. Once they arrive at their final destination, they must integrate into the developing circuitry. As they mature within the cortex, their axons grow and branch in highly specific patterns that may be partially determined by the genetic blueprint for each interneuronal class and partly by the environment in which they find themselves. Finally, as each interneuron class begins to form synapses with only certain postsynaptic targets, cell–cell recognition, most probably via protein–protein interactions across the synaptic cleft, facilitate the formation of appropriate synapses.

NMDA receptor activity downregulates KCC2 resulting in depolarizing GABAA receptor-mediated currents

KCC2 is a neuron-specific K(+)-Cl(-) co-transporter that maintains a low intracellular Cl(-) concentration that is essential for hyperpolarizing inhibition mediated by GABA(A) receptors. Deficits in KCC2 activity occur in disease states associated with pathophysiological glutamate release. However, the mechanisms by which elevated glutamate alters KCC2 function are unknown. The phosphorylation of KCC2 residue Ser940 is known to regulate its surface activity. We found that NMDA receptor activity and Ca(2+) influx caused the dephosphorylation of Ser940 in dissociated rat neurons, leading to a loss of KCC2 function that lasted longer than 20 min. Protein phosphatase 1 mediated the dephosphorylation events of Ser940 that coincided with a deficit in hyperpolarizing GABAergic inhibition resulting from the loss of KCC2 activity. Blocking dephosphorylation of Ser940 reduced the glutamate-induced downregulation of KCC2 and substantially improved the maintenance of hyperpolarizing GABAergic inhibition. Reducing the downregulation of KCC2 therefore has therapeutic potential in the treatment of neurological disorders.

KCC2 was downregulated in small neurons localized in epileptogenic human focal cortical dysplasia

Focal cortical dysplasia (FCD), which is characterized histologically by disorganized cortical lamination and large abnormal cells, is one of the major causes of intractable epilepsies. γ-aminobutyric acid (GABA)(A) receptor-mediated synchronous depolarizing potentials have been observed in FCD tissue. Since alterations in Cl(-) homeostasis might underlie these depolarizing actions of GABA, cation-Cl(-) cotransporters could play critical roles in the generation of these abnormal actions. We examined the expression patterns of NKCC1 and KCC2 by in situ hybridization histochemistry and immunohistochemistry in FCD tissue obtained by surgery from patients with intractable epilepsy. KCC2 mRNA and protein were expressed not only in non-dysplastic neurons in histologically normal portions located in the periphery of the excised cortex, but also in dysplastic cells in FCD tissue. The levels of KCC2 mRNA and protein were significantly decreased in the neurons around large abnormal neurons (giant neurons), but not in giant neurons, compared with non-dysplastic neurons. The neurons localized only around giant neurons significantly smaller than non-dysplastic neurons. However NKCC1 expression did not differ among these cell types. These results suggest that the intracellular Cl(-) concentration ([Cl(-)](i)) of small neurons might increase, so that depolarizing GABA actions could occur in the FCD tissue of epileptic foci.

Downregulation of the potassium chloride cotransporter KCC2 in vulnerable motoneurons in the SOD1-G93A mouse model of amyotrophic lateral sclerosis

The balance between excitatory and inhibitory synaptic inputs is critical for the physiological control of motoneurons. The maintenance of a low-intracellular chloride concentration by the potassium chloride cotransporter 2 (KCC2) is essential for the efficacy of fast synaptic inhibition of mature motoneurons in response to the activation of ionotropic γ-aminobutyric acid A and glycine receptors. Altered synaptic balance and excitotoxicity have been proposed as candidate pathophysiological processes in amyotrophic lateral sclerosis (ALS). Therefore, we investigated the expression patterns of KCC2 and its functional opponent, the chloride influx-mediating sodium-potassium chloride cotransporter 1 (NKCC1), in the superoxide dismutase 1 (SOD1-G93A) mouse model of ALS. We detected reduced KCC2 messenger RNA levels and less membrane-bound KCC2 immunoreactivity in ALS-vulnerable motoneurons in lumbar spinal cord and hypoglossal nuclei of SOD1-G93A mice but not in degeneration-resistant oculomotor nuclei. Downregulation of KCC2 started during late presymptomatic stages and accelerated in parallel to hind limb and tongue motor function deficits. In contrast, NKCC1 messenger RNA levels were unaltered in postnatal lumbar spinal cord motoneurons. Our data indicate that reductions in KCC2 gene expression may contribute to selective motor deficits and disease progression in vulnerable motoneurons in a mouse model of ALS.

GABAergic synaptic transmission regulates calcium influx during spike-timing dependent plasticity

Coincident pre- and postsynaptic activity of hippocampal neurons alters the strength of gamma-aminobutyric acid (GABA(A))-mediated inhibition through a Ca(2+)-dependent regulation of cation-chloride cotransporters. This long-term synaptic modulation is termed GABAergic spike-timing dependent plasticity (STDP). In the present study, we examined whether the properties of the GABAergic synapses themselves modulate the required postsynaptic Ca(2+) influx during GABAergic STDP induction. To do this we first identified GABAergic synapses between cultured hippocampal neurons based on their relatively long decay time constants and their reversal potentials which lay close to the resting membrane potential. GABAergic STDP was then induced by coincidentally (±1 ms) firing the pre- and postsynaptic neurons at 5 Hz for 30 s, while postsynaptic Ca(2+) was imaged with the Ca(2+)-sensitive fluorescent dye Fluo4-AM. In all cases, the induction of GABAergic STDP increased postsynaptic Ca(2+) above resting levels. We further found that the magnitude of this increase correlated with the amplitude and polarity of the GABAergic postsynaptic current (GPSC); hyperpolarizing GPSCs reduced the Ca(2+) influx in comparison to both depolarizing GPSCs, and postsynaptic neurons spiked alone. This relationship was influenced by both the driving force for Cl(-) and GABA(A) conductance (which had positive correlations with the Ca(2+) influx). The spike-timing order during STDP induction did not influence the correlation between GPSC amplitude and Ca(2+) influx, which is likely accounted for by the symmetrical GABAergic STDP window.

Downregulation of potassium chloride cotransporter KCC2 after transient focal cerebral ischemia

BACKGROUND AND PURPOSE

The potassium chloride cotransporter 2 (KCC2) is the main neuronal chloride extruder in the adult nervous system. Therefore, KCC2 is responsible for an inwardly directed electrochemical gradient of chloride that leads to hyperpolarizing GABA-mediated responses. Under some pathophysiological conditions, GABA has been reported to be depolarizing because of a downregulation of KCC2. This is the first study to our knowledge analyzing the expression of KCC2 after a focal cerebral ischemia.

METHODS

Mild and severe ischemia were induced in rats by a transient occlusion of the middle cerebral artery for 30 and 120 minutes, respectively. KCC2 mRNA and protein expression were studied in the ischemic hemisphere after different reperfusion times (2 hour, 1 day, 7 days, 30 days, 168 days) by using quantitative polymerase chain reaction, Western blotting, and immunohistological staining.

RESULTS

We found a substantial decrease of KCC2 mRNA and protein levels in the ischemic hemisphere, with a stronger downregulation of KCC2 after severe vs mild ischemia. Long-term surviving cells expressing KCC2 could be detected in the infarct core. These cells were identified as GABAergic interneurons mainly expressing parvalbumin.

CONCLUSIONS

Our study revealed a substantial neuron-specific downregulation of KCC2 after focal cerebral ischemia.

Down-regulation of the potassium-chloride cotransporter KCC2 contributes to spasticity after spinal cord injury

Hyperexcitability of spinal reflexes and reduced synaptic inhibition are commonly associated with spasticity after spinal cord injury (SCI). In adults, the activation of gamma-aminobutyric acid(A) (GABAA) and glycine receptors inhibits neurons as a result of low intracellular chloride (Cl-) concentration, which is maintained by the potassium-chloride cotransporter KCC2 (encoded by Slc12a5). We show that KCC2 is downregulated after SCI in rats, particularly in motoneuron membranes, thereby depolarizing the Cl- equilibrium potential and reducing the strength of postsynaptic inhibition. Blocking KCC2 in intact rats reduces the rate-dependent depression (RDD) of the Hoffmann reflex, as is observed in spasticity. RDD is also decreased in KCC2-deficient mice and in intact rats after intrathecal brain-derived neurotrophic factor (BDNF) injection, which downregulates KCC2. The early decrease in KCC2 after SCI is prevented by sequestering BDNF at the time of SCI. Conversely, after SCI, BDNF upregulates KCC2 and restores RDD. Our results open new perspectives for the development of therapeutic strategies to alleviate spasticity.

Thyroxin regulates BDNF expression to promote survival of injured neurons

A growing amount of evidence indicates that neuronal trauma can induce a recapitulation of developmental-like mechanisms for neuronal survival and regeneration. Concurrently, ontogenic dependency of central neurons for brain-derived neurotrophic factor (BDNF) is lost during maturation but is re-acquired after injury. Here we show in organotypic hippocampal slices that thyroxin, the thyroid hormone essential for normal CNS development, induces up-regulation of BDNF upon injury. This change in the effect of thyroxin is crucial to promote survival and regeneration of damaged central neurons. In addition, the effect of thyroxin on the expression of the K-Cl cotransporter (KCC2), a marker of neuronal maturation, is changed from down to up-regulation. Notably, previous results in humans have shown that during the first few days after traumatic brain injury or spinal cord injury, thyroid hormone levels are often diminished. Our data suggest that maintaining normal levels of thyroxin during the early post-traumatic phase of CNS injury could have a therapeutically positive effect.

Roles of channels and receptors in the growth cone during PNS axonal regeneration

Neurons in the peripheral nervous system (PNS) are known to maintain a regenerative capacity and will normally regenerate their axons within a permissive growth environment. The success of regeneration in the PNS largely depends on maintenance of the supportive basal lamina membrane, efficient removal of axonal and myelin debris by macrophages and Schwann cells, expression of neurotrophic factors by Schwann cells, and up-regulation of the intrinsic growth program in PNS neurons. The PNS regenerative process is well characterized through initial Wallerian degeneration followed by axonal sprouting, formation of neuronal growth cones, active axonal growth to the target, and finally sensory and motor functional recovery. The initiation and maintenance of active growth cones during peripheral nerve regeneration recapitulate many aspects of early neural development and are achieved through the activation of complex signaling cascades, involving various receptors, channels, cytoplasmic signaling cascades, as well as transcriptional and translational programs. This review focuses on roles of cell surface ion channels and receptors in the growth cone during Wallerian degeneration and axon regeneration in the PNS.