Early changes in KCC2 phosphorylation in response to neuronal stress result in functional downregulation

The Possible Role of Brain-derived Neurotrophic Factor in Epilepsy

Epilepsy is a neurological disease characterized by repeated seizures. Despite of that the brain-derived neurotrophic factor (BDNF) is implicated in the pathogenesis of epileptogenesis and epilepsy, BDNF may have a neuroprotective effect against epilepsy. Thus, the goal of the present review was to highlight the protective and detrimental roles of BDNF in epilepsy. In this review, we also try to find the relation of BDNF with other signaling pathways and cellular processes including autophagy, mTOR pathway, progranulin (PGN), and α-Synuclein (α-Syn) which negatively and positively regulate BDNF/tyrosine kinase receptor B (TrkB) signaling pathway. Therefore, the assessment of BDNF levels in epilepsy should be related to other neuronal signaling pathways and types of epilepsy in both preclinical and clinical studies. In conclusion, there is a strong controversy concerning the potential role of BDNF in epilepsy. Therefore, preclinical, molecular, and clinical studies are warranted in this regard.

Neuronal K+-Cl- cotransporter KCC2 as a promising drug target for epilepsy treatment

Epilepsy is a prevalent neurological disorder characterized by unprovoked seizures. γ-Aminobutyric acid (GABA) serves as the primary fast inhibitory neurotransmitter in the brain, and GABA binding to the GABAA receptor (GABAAR) regulates Cl- and bicarbonate (HCO3-) influx or efflux through the channel pore, leading to GABAergic inhibition or excitation, respectively. The neuron-specific K+-Cl- cotransporter 2 (KCC2) is essential for maintaining a low intracellular Cl- concentration, ensuring GABAAR-mediated inhibition. Impaired KCC2 function results in GABAergic excitation associated with epileptic activity. Loss-of-function mutations and altered expression of KCC2 lead to elevated [Cl-]i and compromised synaptic inhibition, contributing to epilepsy pathogenesis in human patients. KCC2 antagonism studies demonstrate the necessity of limiting neuronal hyperexcitability within the brain, as reduced KCC2 functioning leads to seizure activity. Strategies focusing on direct (enhancing KCC2 activation) and indirect KCC2 modulation (altering KCC2 phosphorylation and transcription) have proven effective in attenuating seizure severity and exhibiting anti-convulsant properties. These findings highlight KCC2 as a promising therapeutic target for treating epilepsy. Recent advances in understanding KCC2 regulatory mechanisms, particularly via signaling pathways such as WNK, PKC, BDNF, and its receptor TrkB, have led to the discovery of novel small molecules that modulate KCC2. Inhibiting WNK kinase or utilizing newly discovered KCC2 agonists has demonstrated KCC2 activation and seizure attenuation in animal models. This review discusses the role of KCC2 in epilepsy and evaluates its potential as a drug target for epilepsy treatment by exploring various strategies to regulate KCC2 activity.

Alpha-methyltyrosine reduces the acute cardiovascular and behavioral sequelae in a murine model of traumatic brain injury

BACKGROUND

Increased catecholamines contribute to heightened cardiovascular reactivity and behavioral deficits after traumatic brain injury (TBI); adrenergic receptor blockade has limited success in reducing adverse sequelae of TBI. Injury-induced increases in the synthesis of catecholamines could contribute to adverse outcomes in TBI. Inhibition of catecholamine synthesis with alpha-methyltyrosine (αMT) could offer a benefit after TBI.

METHODS

Original research trial in mice randomized to αMT (50 mg·kg -1 ·d -1 ) or vehicle for 1 week after TBI induced by controlled cortical impact. Primary outcomes of cardiovascular reactivity and behavioral deficits were assessed after 1 week. Secondary outcomes included blood brain barrier permeability and quantification of gene transcription whose products determine intraneuronal chloride concentrations, the release of catecholamines, and activation of the sympathetic nervous system. These genes were the alpha-2 adrenergic receptor ("Adra2c"), the sodium-potassium-chloride cotransporter ("Nkcc1"), and the potassium chloride cotransporter ("Kcc2"). We also assessed the effect of TBI and αMT on the neuronal chloride/bicarbonate exchanger ("Ae3").

RESULTS

Traumatic brain injury-induced increases in blood pressure and cardiac reactivity were blocked by αMT. Inhibition of catecholamine synthesis decreased blood brain barrier leakage and improved behavioral outcomes after TBI. Traumatic brain injury diminished the transcription of Adra2c and enhanced expression of Nkcc1 while reducing Kcc2 transcription; αMT prevented the induction of the Nkcc1 by TBI without reversing the effects of TBI on Kcc2 expression; αMT also diminished Ae3 transcription.

CONCLUSION

Traumatic brain injury acutely increases cardiovascular reactivity and induces behavioral deficits in an αMT-sensitive manner, most likely by inducing Nkcc1 gene transcription. Alpha-methyltyrosine may prove salutary in the treatment of TBI by attenuating the enhanced expression of Nkcc1, minimizing blood brain barrier leakage, and diminishing central catecholamine and sympathetic output. We also found an unreported relationship between Kcc2 and the chloride/bicarbonate exchanger, which should be considered in the design of trials planned to manipulate central intraneuronal chloride concentrations following acute brain injury.

Maternal taurine as a modulator of Cl- homeostasis as well as of glycine/GABAA receptors for neocortical development

During brain and spinal cord development, GABA and glycine, the inhibitory neurotransmitters, cause depolarization instead of hyperpolarization in adults. Since glycine and GABAA receptors (GABAARs) are chloride (Cl-) ion channel receptor, the conversion of GABA/glycine actions during development is influenced by changes in the transmembrane Cl- gradient, which is regulated by Cl- transporters, NKCC1 (absorption) and KCC2 (expulsion). In immature neurons, inhibitory neurotransmitters are released in a non-vesicular/non-synaptic manner, transitioning to vesicular/synaptic release as the neuron matures. In other word, in immature neurons, neurotransmitters generally act tonically. Thus, the glycine/GABA system is a developmentally multimodal system that is required for neurogenesis, differentiation, migration, and synaptogenesis. The endogenous agonists for these receptors are not fully understood, we address taurine. In this review, we will discuss about the properties and function of taurine during development of neocortex. Taurine cannot be synthesized by fetuses or neonates, and is transferred from maternal blood through the placenta or maternal milk ingestion. In developing neocortex, taurine level is higher than GABA level, and taurine tonically activates GABAARs to control radial migration as a stop signal. In the marginal zone (MZ) of the developing neocortex, endogenous taurine modulates the spread of excitatory synaptic transmission, activating glycine receptors (GlyRs) as an endogenous agonist. Thus, taurine affects information processing and crucial developmental processes such as axonal growth, cell migration, and lamination in the developing cerebral cortex. Additionally, we also refer to the possible mechanism of taurine-regulating Cl- homeostasis. External taurine is uptake by taurine transporter (TauT) and regulates NKCC1 and KCC2 mediated by intracellular signaling pathway, with-no-lysine kinase 1 (WNK1) and its subsequent kinases STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Through the regulation of NKCC1 and KCC2, mediated by the WNK-SPAK/OSR1 signaling pathway, taurine plays a role in maintaining Cl- homeostasis during normal brain development.

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.

Ionic Plasticity: Common Mechanistic Underpinnings of Pathology in Spinal Cord Injury and the Brain

The neurotransmitter GABA is normally characterized as having an inhibitory effect on neural activity in the adult central nervous system (CNS), which quells over-excitation and limits neural plasticity. Spinal cord injury (SCI) can bring about a modification that weakens the inhibitory effect of GABA in the central gray caudal to injury. This change is linked to the downregulation of the potassium/chloride cotransporter (KCC2) and the consequent rise in intracellular Cl^- in the postsynaptic neuron. As the intracellular concentration increases, the inward flow of Cl^- through an ionotropic GABA-A receptor is reduced, which decreases its hyperpolarizing (inhibitory) effect, a modulatory effect known as ionic plasticity. The loss of GABA-dependent inhibition enables a state of over-excitation within the spinal cord that fosters aberrant motor activity (spasticity) and chronic pain. A downregulation of KCC2 also contributes to the development of a number of brain-dependent pathologies linked to states of neural over-excitation, including epilepsy, addiction, and developmental disorders, along with other diseases such as hypertension, asthma, and irritable bowel syndrome. Pharmacological treatments that target ionic plasticity have been shown to bring therapeutic benefits.

How Staying Negative Is Good for the (Adult) Brain: Maintaining Chloride Homeostasis and the GABA-Shift in Neurological Disorders

Excitatory-inhibitory (E-I) imbalance has been shown to contribute to the pathogenesis of a wide range of neurodevelopmental disorders including autism spectrum disorders, epilepsy, and schizophrenia. GABA neurotransmission, the principal inhibitory signal in the mature brain, is critically coupled to proper regulation of chloride homeostasis. During brain maturation, changes in the transport of chloride ions across neuronal cell membranes act to gradually change the majority of GABA signaling from excitatory to inhibitory for neuronal activation, and dysregulation of this GABA-shift likely contributes to multiple neurodevelopmental abnormalities that are associated with circuit dysfunction. Whilst traditionally viewed as a phenomenon which occurs during brain development, recent evidence suggests that this GABA-shift may also be involved in neuropsychiatric disorders due to the “dematuration” of affected neurons. In this review, we will discuss the cell signaling and regulatory mechanisms underlying the GABA-shift phenomenon in the context of the latest findings in the field, in particular the role of chloride cotransporters NKCC1 and KCC2, and furthermore how these regulatory processes are altered in neurodevelopmental and neuropsychiatric disorders. We will also explore the interactions between GABAergic interneurons and other cell types in the developing brain that may influence the GABA-shift. Finally, with a greater understanding of how the GABA-shift is altered in pathological conditions, we will briefly outline recent progress on targeting NKCC1 and KCC2 as a therapeutic strategy against neurodevelopmental and neuropsychiatric disorders associated with improper chloride homeostasis and GABA-shift abnormalities.

Furosemide prevents membrane KCC2 downregulation during convulsant stimulation in the hippocampus

In adults, γ-aminobutyric acid (GABA) type A receptor (GABA_AR)-mediated inhibition depends on the maintenance of low intracellular chloride anion concentration through neuron-specific potassium-chloride cotransporter-2 (KCC2). KCC2 has been widely reported to have a plasticity change during the course of epilepsy development, with an early downregulation and late recovery in neuronal cell membranes after epileptic stimulation, which facilitates epileptiform burst activity. Furosemide is a clinical loop diuretic that inhibits KCC2. Here, we first confirmed that furosemide pretreatment could effectively prevented convulsant stimulation-induced neuronal membrane KCC2 downregulation in the hippocampus in both in vivo and in vitro cyclothiazide-induced seizure model. Second, we verified that furosemide pretreatment rescued KCC2 function deficits, as indicated by E _GABA depolarizing shift and GABA_AR inhibitory function impairment induced via cyclothiazide treatment. Further, we demonstrated that furosemide also suppressed cyclothiazide-induced epileptiform burst activity in cultured hippocampal neurons and lowered the mortality rate during acute seizure induction. Overall, furosemide prevents membrane KCC2 downregulation during acute seizure induction, restores KCC2-mediated GABA inhibition, and interrupts the progression from acute seizure to epileptogenesis.

Oxytocin ameliorates KCC2 decrease induced by oral bacteria-derived LPS that affect rat primary cultured cells and PC-12 cells

Inflammation, especially neuroinflammation, which is caused by stress, leads to central nervous system (CNS) dysfunction. Because lipopolysaccharides (LPSs) cause neuroinflammation, we investigated the effect of LPSs to CNS. In PC-12 cells, LPSs derived from oral bacteria reduced the expression of KCC2, a Cl^- transporter. LPS derived from P. gingivalis (P. g) administered to rat primary cultured cells also reduced the KCC2 expression. However, LPSs derived from E. coli did not reduce the KCC2 expression. LPS treatment activated TLR4, IL-1β, and REST gene expressions, which led to KCC2 inactivation in PC-12 cells. The mechanism of KCC2 has been shown to play an important role in brain maturation, function (such as the GABA switch), and behavioral problems, we investigated the GABA function. We found that the GABA function was changed from inhibitory to excitatory by the LPS derived from P. g treatment. We demonstrated that the GSK3β also involved in the KCC2 reduction by LPS treatment. We show that oxytocin rescued the reduction in KCC2 expression caused by LPSs by inhibiting GSK3β signaling but vasopressin could not. Considered together, our results indicate that the LPSs from oral bacteria but not the LPS from E. coli increase the risk for brain disorders and oxytocin might be a candidate to overcome the abnormal behavior caused by brain disorders such as psychiatric disorders.

Gamma frequency light flicker regulates amyloid precursor protein trafficking for reducing β-amyloid load in Alzheimer's disease model

Inducing gamma oscillations with non‐invasive light flicker has been reported to impact Alzheimer's disease‐related pathology. However, it is unclear which signaling pathways are involved in reducing amyloid load. Here, we found that gamma frequency light flicker increased anchoring of amyloid precursor protein (APP) to the plasma membrane for non‐amyloidogenic processing, and then physically interacted with KCC2, a neuron‐specific K⁺‐Cl⁻ cotransporter, suggesting that it is essential to maintain surface GABA_A receptor α1 levels and reduce β‐amyloid (Aβ) production. Stimulation with such light flicker limited KCC2 internalization and subsequent degradation via both tyrosine phosphorylation and ubiquitination, leading to an increase in surface‐KCC2 levels. Specifically, PKC‐dependent phosphorylation of APP on a serine residue was induced by gamma frequency light flicker, which was responsible for maintaining plasma membrane levels of full‐length APP, leading to its reduced trafficking to endosomes and inhibiting the β‐secretase cleavage pathway. The activated PKC from the gamma frequency light flicker subsequently phosphorylated serine of KCC2 and stabilized it onto the cell surface, which contributed to the upregulation of surface GABA_A receptor α1 levels. Together, these data indicate that enhancement of APP trafficking to the plasma membrane via light flicker plays a critical modulatory role in reduction of Aβ load in Alzheimer's disease.

Disturbed Balance of Inhibitory Signaling Links Hearing Loss and Cognition

Neuronal hyperexcitability in the central auditory pathway linked to reduced inhibitory activity is associated with numerous forms of hearing loss, including noise damage, age-dependent hearing loss, and deafness, as well as tinnitus or auditory processing deficits in autism spectrum disorder (ASD). In most cases, the reduced central inhibitory activity and the accompanying hyperexcitability are interpreted as an active compensatory response to the absence of synaptic activity, linked to increased central neural gain control (increased output activity relative to reduced input). We here suggest that hyperexcitability also could be related to an immaturity or impairment of tonic inhibitory strength that typically develops in an activity-dependent process in the ascending auditory pathway with auditory experience. In these cases, high-SR auditory nerve fibers, which are critical for the shortest latencies and lowest sound thresholds, may have either not matured (possibly in congenital deafness or autism) or are dysfunctional (possibly after sudden, stressful auditory trauma or age-dependent hearing loss linked with cognitive decline). Fast auditory processing deficits can occur despite maintained basal hearing. In that case, tonic inhibitory strength is reduced in ascending auditory nuclei, and fast inhibitory parvalbumin positive interneuron (PV-IN) dendrites are diminished in auditory and frontal brain regions. This leads to deficits in central neural gain control linked to hippocampal LTP/LTD deficiencies, cognitive deficits, and unbalanced extra-hypothalamic stress control. Under these conditions, a diminished inhibitory strength may weaken local neuronal coupling to homeostatic vascular responses required for the metabolic support of auditory adjustment processes. We emphasize the need to distinguish these two states of excitatory/inhibitory imbalance in hearing disorders: (i) Under conditions of preserved fast auditory processing and sustained tonic inhibitory strength, an excitatory/inhibitory imbalance following auditory deprivation can maintain precise hearing through a memory linked, transient disinhibition that leads to enhanced spiking fidelity (central neural gain⇑) (ii) Under conditions of critically diminished fast auditory processing and reduced tonic inhibitory strength, hyperexcitability can be part of an increased synchronization over a broader frequency range, linked to reduced spiking reliability (central neural gain⇓). This latter stage mutually reinforces diminished metabolic support for auditory adjustment processes, increasing the risks for canonical dementia syndromes.

Dysregulation of GABAergic Signaling in Neurodevelomental Disorders: Targeting Cation-Chloride Co-transporters to Re-establish a Proper E/I Balance

The construction of the brain relies on a series of well-defined genetically and experience- or activity -dependent mechanisms which allow to adapt to the external environment. Disruption of these processes leads to neurological and psychiatric disorders, which in many cases are manifest already early in postnatal life. GABA, the main inhibitory neurotransmitter in the adult brain is one of the major players in the early assembly and formation of neuronal circuits. In the prenatal and immediate postnatal period GABA, acting on GABA_A receptors, depolarizes and excites targeted cells via an outwardly directed flux of chloride. In this way it activates NMDA receptors and voltage-dependent calcium channels contributing, through intracellular calcium rise, to shape neuronal activity and to establish, through the formation of new synapses and elimination of others, adult neuronal circuits. The direction of GABA_A-mediated neurotransmission (depolarizing or hyperpolarizing) depends on the intracellular levels of chloride [Cl⁻]_i, which in turn are maintained by the activity of the cation-chloride importer and exporter KCC2 and NKCC1, respectively. Thus, the premature hyperpolarizing action of GABA or its persistent depolarizing effect beyond the postnatal period, leads to behavioral deficits associated with morphological alterations and an excitatory (E)/inhibitory (I) imbalance in selective brain areas. The aim of this review is to summarize recent data concerning the functional role of GABAergic transmission in building up and refining neuronal circuits early in development and its dysfunction in neurodevelopmental disorders such as Autism Spectrum Disorders (ASDs), schizophrenia and epilepsy. In particular, we focus on novel information concerning the mechanisms by which alterations in cation-chloride co-transporters (CCC) generate behavioral and cognitive impairment in these diseases. We discuss also the possibility to re-establish a proper GABA_A-mediated neurotransmission and excitatory (E)/inhibitory (I) balance within selective brain areas acting on CCC.

Chloride Homeostasis in Developing Motoneurons

Maturation of GABA/Glycine chloride-mediated synaptic inhibitions is crucial for the establishment of a balance between excitation and inhibition. GABA and glycine are excitatory neurotransmitters on immature neurons that exhibit elevated [Cl^-]_i. Later in development [Cl^-]_i drops leading to the occurrence of inhibitory synaptic activity. This ontogenic change is closely correlated to a differential expression of two cation-chloride cotransporters that are the Cl^- channel K⁺/Cl^- co-transporter type 2 (KCC2) that extrudes Cl^- ions and the Na⁺-K⁺-2Cl^- cotransporter NKCC1 that accumulates Cl^- ions. The classical scheme built from studies performed on cortical and hippocampal networks proposes that immature neurons display high [Cl^-]_i because NKCC1 is overexpressed compared to KCC2 and that the co-transporters ratio reverses in mature neurons, lowering [Cl^-]_i. In this chapter, we will see that this classical scheme is not true in motoneurons (MNs) and that an early alteration of the chloride homeostasis may be involved in pathological conditions.

The Important Role of Ion Transport System in Cervical Cancer

Cervical cancer is a significant gynecological cancer and causes cancer-related deaths worldwide. Human papillomavirus (HPV) is implicated in the etiology of cervical malignancy. However, much evidence indicates that HPV infection is a necessary but not sufficient cause in cervical carcinogenesis. Therefore, the cellular pathophysiology of cervical cancer is worthy of study. This review summarizes the recent findings concerning the ion transport processes involved in cell volume regulation and intracellular Ca²⁺ homeostasis of epithelial cells and how these transport systems are themselves regulated by the tumor microenvironment. For cell volume regulation, we focused on the volume-sensitive Cl⁻ channels and K⁺-Cl⁻ cotransporter (KCC) family, important regulators for ionic and osmotic homeostasis of epithelial cells. Regarding intracellular Ca²⁺ homeostasis, the Ca²⁺ store sensor STIM molecules and plasma membrane Ca²⁺ channel Orai proteins, the predominant Ca²⁺ entry mechanism in epithelial cells, are discussed. Furthermore, we evaluate the potential of these membrane ion transport systems as diagnostic biomarkers and pharmacological interventions and highlight the challenges.

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.

Global transgenic upregulation of KCC2 confers enhanced diazepam efficacy in treating sustained seizures

Reduced anticonvulsant efficacy of benzodiazepines is a problem in the treatment of status epilepticus, with up to 50% of patients failing to respond to their first dose. KCC2 is a neuronal K⁺ -Cl^- co-transporter that helps set and maintain intracellular Cl^- concentrations. KCC2 functional downregulation is a potential contributor to benzodiazepine resistance. We tested this idea using male and female doxycycline-inducible, conditional transgenic mice to increase the functional expression of KCC2 in pyramidal neurons. We administered mice with two doses of the chemoconvulsant kainic acid (5 mg/kg, i.p.) 60 min apart and quantified the resultant seizures with electroencephalography (EEG) recordings. Overexpression of KCC2 prior to the chemoconvulsant challenge did not affect seizure latency or other measures of seizure severity, but it did increase diazepam's efficacy in stopping EEG seizures. Spike rate, time in seizure, and EEG spectral power following diazepam (5 mg/kg, i.p) were all significantly lower in KCC2 overexpression mice as compared to control mice. Our results indicate that, in the context of benzodiazepine resistance during sustained seizures, addressing impaired Cl^- homeostasis alone appreciably improves the efficacy of γ-aminobutyric acid (GABA)ergic inhibition. We therefore suggest the simultaneous targeting of KCC2 and GABA_A receptors as a pathway for improving current anticonvulsant therapeutic strategies.

BDNF shifts excitatory-inhibitory balance in the paraventricular nucleus of the hypothalamus to elevate blood pressure

Presympathetic neurons in the paraventricular nucleus of the hypothalamus (PVN) play a key role in cardiovascular regulation. We have previously shown that brain-derived neurotrophic factor (BDNF), acting in the PVN, increases sympathetic activity and blood pressure and serves as a key regulator of stress-induced hypertensive responses. BDNF is known to alter glutamatergic and GABA-ergic signaling broadly in the central nervous system, but whether BDNF has similar actions in the PVN remains to be investigated. Here, we tested the hypothesis that increased BDNF expression in the PVN elevates blood pressure by enhancing N-methyl-d-aspartate (NMDA) receptor (NMDAR)- and inhibiting GABA_A receptor (GABA_AR)-mediated signaling. Sprague-Dawley rats received bilateral PVN injections of AAV2 viral vectors expressing green fluorescent protein (GFP) or BDNF. Three weeks later, cardiovascular responses to PVN injections of NMDAR and GABA_AR agonists and antagonists were recorded under α-chloralose-urethane anesthesia. In addition, expressions of excitatory and inhibitory signaling components in the PVN were assessed using immunofluorescence. Our results showed that NMDAR inhibition led to a greater decrease in blood pressure in the BDNF vs. GFP group, while GABA_AR inhibition led to greater increases in blood pressure in the GFP group compared to BDNF. Conversely, GABA_AR activation decreased blood pressure significantly more in GFP vs. BDNF rats. In addition, immunoreactivity of NMDAR1 was upregulated, while GABA_AR-α1 and K⁺/Cl^- cotransporter 2 were downregulated by BDNF overexpression in the PVN. In summary, our findings indicate that hypertensive actions of BDNF within the PVN are mediated, at least in part, by augmented NMDAR and reduced GABA_AR signaling.NEW & NOTEWORTHY We have shown that BDNF, acting in the PVN, elevates blood pressure in part by augmenting NMDA receptor-mediated excitatory input and by diminishing GABA_A receptor-mediated inhibitory input to PVN neurons. In addition, we demonstrate that elevated BDNF expression in the PVN upregulates NMDA receptor immunoreactivity and downregulates GABA_A receptor as well as KCC2 transporter immunoreactivity.

Non-Cell-Autonomous Regulation of Optic Nerve Regeneration by Amacrine Cells

Visual information is conveyed from the eye to the brain through the axons of retinal ganglion cells (RGCs) that course through the optic nerve and synapse onto neurons in multiple subcortical visual relay areas. RGCs cannot regenerate their axons once they are damaged, similar to most mature neurons in the central nervous system (CNS), and soon undergo cell death. These phenomena of neurodegeneration and regenerative failure are widely viewed as being determined by cell-intrinsic mechanisms within RGCs or to be influenced by the extracellular environment, including glial or inflammatory cells. However, a new concept is emerging that the death or survival of RGCs and their ability to regenerate axons are also influenced by the complex circuitry of the retina and that the activation of a multicellular signaling cascade involving changes in inhibitory interneurons - the amacrine cells (AC) - contributes to the fate of RGCs. Here, we review our current understanding of the role that interneurons play in cell survival and axon regeneration after optic nerve injury.

Pharmacological Modulation of Rate-Dependent Depression of the Spinal H-Reflex Predicts Therapeutic Efficacy against Painful Diabetic Neuropathy

Impaired rate-dependent depression (RDD) of the spinal H-reflex occurs in diabetic rodents and a sub-set of patients with painful diabetic neuropathy. RDD is unaffected in animal models of painful neuropathy associated with peripheral pain mechanisms and diabetic patients with painless neuropathy, suggesting RDD could serve as a biomarker for individuals in whom spinal disinhibition contributes to painful neuropathy and help identify therapies that target impaired spinal inhibitory function. The spinal pharmacology of RDD was investigated in normal rats and rats after 4 and 8 weeks of streptozotocin-induced diabetes. In normal rats, dependence of RDD on spinal GABAergic inhibitory function encompassed both GABA_A and GABA_B receptor sub-types. The time-dependent emergence of impaired RDD in diabetic rats was preceded by depletion of potassium-chloride co-transporter 2 (KCC2) protein in the dorsal, but not ventral, spinal cord and by dysfunction of GABA_A receptor-mediated inhibition. GABA_B receptor-mediated spinal inhibition remained functional and initially compensated for loss of GABA_A receptor-mediated inhibition. Administration of the GABA_B receptor agonist baclofen restored RDD and alleviated indices of neuropathic pain in diabetic rats, as did spinal delivery of the carbonic anhydrase inhibitor acetazolamide. Pharmacological manipulation of RDD can be used to identify potential therapies that act against neuropathic pain arising from spinal disinhibition.

Preliminary study of analgesic effect of bumetanide on neuropathic pain in patients with spinal cord injury

BACKGROUND/OBJECTIVE

The current study evaluated the analgesic effects of bumetanide as an adjunctive treatment in managing neuropathic pain following spinal cord injury. The peripheral expression level of Na-K-Cl-cotransporter-1 (NKCC1) and K-Cl-cotransporter-2 (KCC2) genes in polymorphonuclear lymphocytes (PMLs) assessed as a possible biomarker indicating central underlying mechanisms.

METHODS

This open-label, single-arm, pilot trial of bumetanide (2 mg/day) is an add-on treatment conducted in 14 SCI patients for 19 weeks. The whole duration consisted of three phases: pre-treatment (1 month), titration (3 weeks), and active treatment (4 months). Ultimately, nine patients completed the study. The primary outcome variables were the endpoint pain score measured by the numeric rating scale (NRS), and the short-form McGill Pain Questionnaire. Secondary endpoints included the Short-Form Health Survey that measures the quality of life. Blood samples were collected and used for determining the expression of NKCC1 and KCC2 genes in transcription and translation levels.

RESULTS

Bumetanide treatment significantly reduced average pain intensity according to the NRS and the short form of the McGill Pain Questionnaire scores. The baseline expression of KCC2 protein was low between groups and increased significantly following treatment (P < 0.05). Through the current study, pain improvement accompanied by the more significant mean change from the baseline for the overall quality of life.

CONCLUSION

These data might be a piece of preliminary evidence for the analgesic effect of bumetanide on neuropathic pain and could support the potential role of the upregulation of KCC2 protein and involvement of GABAergic disinhibition in producing neuropathic pain.

The emerging role of the BDNF-TrkB signaling pathway in the modulation of pain perception

The brain derived neurotrophic factor (BDNF) is a crucial neuromodulator in pain transmission both in peripheral and central nervous system (CNS). Despite evidence of a pro-nociceptive role of BDNF, recent studies have reported contrasting results, including anti-nociceptive and anti-inflammatory activities. Moreover, BDNF polymorphisms can interfere with BDNF role in pain perception. In Val66Met carriers, the Met allele may have a dual role, with anti-nociceptive actions in normal condition and pro-nociceptive effects during chronic pain. In order to elucidate the main effects of BDNF in nociception, we reviewed the main characteristics of this neurotrophin, focusing on its involvement in pain.

Novel Treatment Targets Based on Insights in the Etiology of Depression: Role of IL-6 Trans-Signaling and Stress-Induced Elevation of Glutamate and ATP

Inflammation and psychological stress are risk factors for major depression and suicide. Both increase central glutamate levels and activate the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system. Both factors also affect the function of the chloride transporters, Na-K-Cl-cotransporter-1 (NKCC1) and K-Cl-cotransporter-2 (KCC2), and provoke interleukin-6 (IL-6) trans-signaling. This leads to measurable increases in circulating corticosteroids, catecholamines, anxiety, somatic and psychological symptoms, and a decline in cognitive functions. Recognition of the sequence of pathological events allows the prediction of novel targets for therapeutic intervention. Amongst others, these include blockade of the big-K potassium channel, blockade of the P2X4 channel, TYK2-kinase inhibition, noradrenaline α2B-receptor antagonism, nicotinic α7-receptor stimulation, and the Sgp130Fc antibody. A better understanding of downstream processes evoked by inflammation and stress also allows suggestions for tentatively better biomarkers (e.g., SERPINA3N, MARCKS, or ¹³C-tryptophan metabolism).

Amyloid Precursor Protein (APP) and GABAergic Neurotransmission

The amyloid precursor protein (APP) is the parent polypeptide from which amyloid-beta (Aβ) peptides, key etiological agents of Alzheimer's disease (AD), are generated by sequential proteolytic processing involving β- and γ-secretases. APP mutations underlie familial, early-onset AD, and the involvement of APP in AD pathology has been extensively studied. However, APP has important physiological roles in the mammalian brain, particularly its modulation of synaptic functions and neuronal survival. Recent works have now shown that APP could directly modulate γ-aminobutyric acid (GABA) neurotransmission in two broad ways. Firstly, APP is shown to interact with and modulate the levels and activity of the neuron-specific Potassium-Chloride (K+-Cl-) cotransporter KCC2/SLC12A5. The latter is key to the maintenance of neuronal chloride (Cl-) levels and the GABA reversal potential (EGABA), and is therefore important for postsynaptic GABAergic inhibition through the ionotropic GABAA receptors. Secondly, APP binds to the sushi domain of metabotropic GABAB receptor 1a (GABABR1a). In this regard, APP complexes and is co-transported with GABAB receptor dimers bearing GABABR1a to the axonal presynaptic plasma membrane. On the other hand, secreted (s)APP generated by secretase cleavages could act as a GABABR1a-binding ligand that modulates presynaptic vesicle release. The discovery of these novel roles and activities of APP in GABAergic neurotransmission underlies the physiological importance of APP in postnatal brain function.

Crossing the Chloride Channel: The Current and Potential Therapeutic Value of the Neuronal K+-Cl- Cotransporter KCC2

Chloride (Cl-) homeostasis is an essential process involved in neuronal signalling and cell survival. Inadequate regulation of intracellular Cl- interferes with synaptic signalling and is implicated in several neurological diseases. The main inhibitory neurotransmitter of the central nervous system is γ-aminobutyric acid (GABA). GABA hyperpolarises the membrane potential by activating Cl- permeable GABAA receptor channels (GABAAR). This process is reliant on Cl- extruder K+-Cl- cotransporter 2 (KCC2), which generates the neuron's inward, hyperpolarising Cl- gradient. KCC2 is encoded by the fifth member of the solute carrier 12 family (SLC12A5) and has remained a poorly understood component in the development and severity of many neurological diseases for many years. Recent advancements in next-generation sequencing and specific gene targeting, however, have indicated that loss of KCC2 activity is involved in a number of diseases including epilepsy and schizophrenia. It has also been implicated in neuropathic pain following spinal cord injury. Any variant of SLC12A5 that negatively regulates the transporter's expression may, therefore, be implicated in neurological disease. A recent whole exome study has discovered several causative mutations in patients with epilepsy. Here, we discuss the implications of KCC2 in neurological disease and consider the evolving evidence for KCC2's potential as a therapeutic target.

Double-edged GABAergic synaptic transmission in seizures: The importance of chloride plasticity

GABAergic synaptic inhibition, which is a critical regulator of neuronal excitability, is closely involved in epilepsy. Interestingly, fast GABAergic transmission mediated by Cl- permeable GABAA receptors can bi-directionally exert both seizure-suppressing and seizure-promoting actions. Accumulating evidence suggests that chloride plasticity, the driving force of GABAA receptor-mediated synaptic transmission, contributes to the double-edged role of GABAergic synapses in seizures. Large amounts of Cl- influx can overwhelm Cl- extrusion during seizures not only in healthy tissue in a short-term "activity-dependent" manner, but also in chronic epilepsy in a long-term, irreversible "pathology-dependent" manner related to the dysfunction of two chloride transporters: the chloride importer NKCC1 and the chloride exporter KCC2. In this review, we address the importance of chloride plasticity for the "activity-dependent" and "pathology-dependent" mechanisms underlying epileptic events and provide possible directions for further research, which may be clinically important for the design of GABAergic synapse-targeted precise therapeutic interventions for epilepsy.

Effect of Co-administration of Bumetanide and Phenobarbital on Seizure Attacks in Temporal Lobe Epilepsy

Introduction:

The resistance of temporal lobe epilepsy to classic drugs is thought to be due to disruption in the excitation/inhibition of this pathway. Two chloride transporters, NKCC1 and KCC2, are expressed differently for the excitatory state of Gamma-Amino Butyric Acid (GABA). The present study explored the effect of bumetanide as a selective NKCC1 inhibitor either alone or in combination with the phenobarbital in the pilocarpine model of epilepsy.

Methods:

An animal model of Status Epilepticus (SE) was induced with pilocarpine in Wistar male rats followed by phenobarbital and or bumetanide or saline administration for 45 days after the induction of SE by Intraperitoneal (IP) injection. The rats were monitored, their behavior was recorded, and after 24 hours they were sacrificed to study the expression of NKCC1 and KCC2 using real time PCR.

Results:

The data showed that the effects of a combination of bumetanide with phenobarbital on frequency rate and duration of seizure attack were more than those of the phenobarbital alone. In addition, in the bumetanide and combined treatment groups, NKCC1 expression decreased significantly, compared with untreated epileptic animals. A delayed decrement in NKCC1/KCC2 expression ratio after bumetanide application was also observed.

Conclusion:

The combination of bumetanide with phenobarbital increases the inhibition of SE and maximizes the potential of GABA signaling pathway, and can be considered as an effective therapeutic strategy in patients with epilepsy.

Mechanism of BDNF Modulation in GABAergic Synaptic Transmission in Healthy and Disease Brains

In the mature healthy mammalian neuronal networks, γ-aminobutyric acid (GABA) mediates synaptic inhibition by acting on GABA_A and GABA_B receptors (GABA_AR, GABA_BR). In immature networks and during numerous pathological conditions the strength of GABAergic synaptic inhibition is much less pronounced. In these neurons the activation of GABA_AR produces paradoxical depolarizing action that favors neuronal network excitation. The depolarizing action of GABA_AR is a consequence of deregulated chloride ion homeostasis. In addition to depolarizing action of GABA_AR, the GABA_BR mediated inhibition is also less efficient. One of the key molecules regulating the GABAergic synaptic transmission is the brain derived neurotrophic factor (BDNF). BDNF and its precursor proBDNF, can be released in an activity-dependent manner. Mature BDNF operates via its cognate receptors tropomyosin related kinase B (TrkB) whereas proBDNF binds the p75 neurotrophin receptor (p75^NTR). In this review article, we discuss recent finding illuminating how mBDNF-TrkB and proBDNF-p75^NTR signaling pathways regulate GABA related neurotransmission under physiological conditions and during epilepsy.

Postnatal Changes in K+/Cl- Cotransporter-2 Expression in the Forebrain of Mice Bearing a Mutant Nicotinic Subunit Linked to Sleep-Related Epilepsy

The Na+/K+/Cl- cotransporter-1 (NKCC1) and the K+/Cl- cotransporter-2 (KCC2) set the transmembrane Cl- gradient in the brain, and are implicated in epileptogenesis. We studied the postnatal distribution of NKCC1 and KCC2 in wild-type (WT) mice, and in a mouse model of sleep-related epilepsy, carrying the mutant β2-V287L subunit of the nicotinic acetylcholine receptor (nAChR). In WT neocortex, immunohistochemistry showed a wide distribution of NKCC1 in neurons and astrocytes. At birth, KCC2 was localized in neuronal somata, whereas at subsequent stages it was mainly found in the somatodendritic compartment. The cotransporters' expression was quantified by densitometry in the transgenic strain. KCC2 expression increased during the first postnatal weeks, while the NKCC1 amount remained stable, after birth. In mice expressing β2-V287L, the KCC2 amount in layer V of prefrontal cortex (PFC) was lower than in the control littermates at postnatal day 8 (P8), with no concomitant change in NKCC1. Consistently, the GABAergic excitatory to inhibitory switch was delayed in PFC layer V of mice carrying β2-V287L. At P60, the amount of KCC2 was instead higher in mice bearing the transgene. Irrespective of genotype, NKCC1 and KCC2 were abundantly expressed in the neuropil of most thalamic nuclei since birth. However, KCC2 expression decreased by P60 in the reticular nucleus, and more so in mice expressing β2-V287L. Therefore, a complex regulatory interplay occurs between heteromeric nAChRs and KCC2 in postnatal forebrain. The pathogenetic effect of β2-V287L may depend on altered KCC2 amounts in PFC during synaptogenesis, as well as in mature thalamocortical circuits.

Oxytocin and Vasopressin, and the GABA Developmental Shift During Labor and Birth: Friends or Foes?

Oxytocin (OT) and vasopressin (AVP) are usually associated with sociability and reduced stress for the former and antidiuretic agent associated with severe stress and pathological conditions for the latter. Both OT and AVP play major roles during labor and birth. Recent contradictory studies suggest that they might exert different roles on the GABA excitatory/inhibitory developmental shift. We reported (Tyzio et al., 2006) that at birth, OT exerts a neuro-protective action mediated by an abrupt reduction of intracellular chloride levels ([Cl-]i) that are high in utero, reinforcing GABAergic inhibition and modulating the generation of the first synchronized patterns of cortical networks. This reduction of [Cl-]i levels is abolished in rodent models of Fragile X Syndrome and Autism Spectrum Disorders, and its restoration attenuates the severity of the pathological sequels, stressing the importance of the shift at birth (Tyzio et al., 2014). In contrast, Kaila and co-workers (Spoljaric et al., 2017) reported excitatory GABA actions before and after birth that are modulated by AVP but not by OT, challenging both the developmental shift and the roles of OT. Here, I analyze the differences between these studies and suggest that the ratio AVP/OT like that of excitatory/inhibitory GABA depend on stress and pathological conditions.

Perinatal Pb2+ exposure alters the expression of genes related to the neurodevelopmental GABA-shift in postnatal rats

BACKGROUND

Lead (Pb2+) is an environmental neurotoxicant that disrupts neurodevelopment, communication, and organization through competition with Ca2+ signaling. How perinatal Pb2+ exposure affects Ca2+-related gene regulation remains unclear. However, Ca2+ activates the L-Type voltage sensitive calcium channel β-3 subunit (Ca-β3), which autoregulates neuronal excitability and plays a role in the GABA-shift from excitatory-to-inhibitory neurotransmission.

METHOD

A total of eight females (n = 4 Control and n = 4 Perinatal) and four males (n = 2 Control and n = 2 Perinatal) rats were used as breeders to serve as Dams and Sires. The Dam's litters each ranged from N = 6-10 pups per litter (M = 8, SD = 2), irrespective of Pb2+ treatment, with a majority of males over females. Since there were more males in each of the litters than females, to best assess and equally control for Pb2+- and litter-effects across all developmental time-points under study, female pups were excluded due to an insufficient sample size availability from the litter's obtained. From the included pup litters, 24 experimentally naïve male Long Evans hooded rat pups (Control N = 12; Pb2+ N = 12) were used in the present study.  Brains were extracted from rat prefrontal cortex (PFC) and hippocampus (HP) at postnatal day (PND) 2, 7, 14 and 22, were homogenized in 1 mL of TRIzol reagent per 100 mg of tissue using a glass-Teflon homogenizer. Post-centrifugation, RNA was extracted with chloroform and precipitated with isopropyl alcohol. RNA samples were then re-suspended in 100 μL of DEPC treated H2O. Next, 10 μg of total RNA was treated with RNase-free DNase (Qiagen) at 37 °C for 1 h and re-purified by a 3:1 phenol/chloroform extraction followed by an ethanol precipitation. From the purified RNA, 1 μg was used in the SYBR GreenER Two-Step qRT-PCR kit (Invitrogen) for first strand cDNA synthesis and the quantitative real-time PCR (qRT-PCR). The effects of perinatal Pb2+ exposure on genes related to early neuronal development and the GABA-shift were evaluated through the expression of: Ca-β3, GABAAR-β3, NKCC1, KCC2, and GAD 80, 86, 65, and 67 isoforms.

RESULTS

Perinatal Pb2+ exposure significantly altered the GABA-shift neurodevelopmental GOI expression as a function of Pb2+ exposure and age across postnatal development. Dramatic changes were observed with Ca-β3 expression consistent with a Pb2+ competition with L-type calcium channels. By PND 22, Ca-β3 mRNA was reduced by 1-fold and 1.5-fold in PFC and HP respectively, relative to controls. All HP GABA-β3 mRNA levels were particularly vulnerable to Pb2+ at PND 2 and 7, and both PFC and HP were negatively impacted by Pb2+ at PND 22. Additionally, Pb2+ altered both the PFC and HP immature GAD 80/86 mRNA expression particularly at PND 2, whereas mature GAD 65/67 were most significantly affected by Pb2+ at PND 22.

CONCLUSIONS

Perinatal Pb2+ exposure disrupts the expression of mRNAs related to the GABA-shift, potentially altering the establishment, organization, and excitability of neural circuits across development. These findings offer new insights into the altered effects Pb2+ has on the GABAergic system preceding what is known regarding Pb2+ insults unto the glutamatergic system.

Chloride transporters and GABA polarity in developmental, neurological and psychiatric conditions

Neuronal chloride regulation is a determinant factor for the dynamic tuning of GABAergic inhibition during and beyond brain development. This regulation is mainly dependent on the two co-transporters K⁺/Cl^- co-transporter KCC2 and Na⁺/K⁺/Cl^- co-transporter NKCC1, whose activity can decrease or increase neuronal chloride concentrations respectively. Altered expression and/or activity of either of these co-transporters has been associated with a wide variety of brain disorders including developmental disorders, epilepsy, schizophrenia and stroke. Here, we review current knowledge on chloride transporter expression and activity regulation and highlight the intriguing potential for existing and future interventions to support chloride homeostasis across a wide range of mental disorders and neurological conditions.

Neuroprotection by Propofol Post-Conditioning: Focus on PKMζ/KCC2 Pathway Activity

Critical and major operations are often accompanied by brain ischemic complications. Previous studies found that propofol post-conditioning provided neuroprotective functions through upregulating the expression of potassium chloride cotransporter 2 (KCC2) in gamma-aminobutyric acid (GABA) interneurons. Membrane expression and phosphorylation represents KCC2 activity, which were modulated by a protein kinase C (PKC)-dependent mechanism. However, the role of propofol in increasing KCC2 phosphorylation and the involvement of protein kinase Mζ (PKMζ), a major subtype of PKC, in the KCC2 pathway remained unclear. In this study, we established middle cerebral artery occlusion model in rats to evaluate the long-term recovery of brain functions using behavioral experiments. KCC2 and PKMζ were assessed via western blot. We used the selective inhibitor, zeta inhibitory peptide (ZIP), to investigate the relationship between KCC2 and PKMζ. Intracellular chloride concentration in the hippocampal CA1 area was measured to determine KCC2 activity. We found that propofol, infused at a speed of 20 mg kg-1 h-1 for 2 h at the onset of reperfusion, improved neurological deficits and cognitive dysfunction following ischemia/reperfusion injury. PKMζ expression was significantly upregulated, which improved KCC2 membrane expression and phosphorylation in the ischemic hippocampal CA1 area, and these effects could last up to 28 days. But ZIP inhibited this process. Ultimately, we showed that propofol increased KCC2 phosphorylation and PKMζ was the upstream of KCC2. Propofol led to long-term recovery of brain functions by upregulating the activity of the PKMζ/KCC2 pathway.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

The Role of K+-Cl--Cotransporter-2 in Neuropathic Pain

The pain sensory system normally functions under a fine balance between excitation and inhibition. When this balance is perturbed for some reason, it leads to neuropathic pain. There is accumulating evidence that attributes this pain generation to specific dysfunctions of the inhibitory system in the spinal cord. One possible mechanism leading to the induction of these dysfunctions is the down-regulation of K+-Cl--cotransporter-2 (KCC2) expression. In fact, various neuropathic pain models indicate a decrease of KCC2 expression in the spinal cord. The alteration of KCC2 expression affects GABAergic and glycinergic neurotransmissions, because KCC2 is a potassium-chloride exporter and serves to maintain intracellular chloride concentration. When there is a low level of KCC2 expression, GABAergic and glycinergic neurotransmissions transform from inhibitory signals to excitatory signals. In this review, the hypothesis that an alteration of KCC2 expression has a crucial influence on the initiation/development or maintenance of neuropathic pain is discussed. In addition, it is suggested that the alteration of inhibitory signals is dependent on the time after peripheral nerve injury.

K+-Cl- co-transporter 2 (KCC2) - a membrane trafficking perspective

K⁺-Cl^- co-transporter 2 (KCC2/SLC12A5) is a neuronal specific cation chloride co-transporter which is active under isotonic conditions, and thus a key regulator of intracellular Cl^- levels. It also has an ion transporter-independent structural role in modulating the maturation and regulation of excitatory glutamatergic synapses. KCC2 levels are developmentally regulated, and a postnatal upregulation of KCC2 generates a low intracellular chloride concentration that allows the neurotransmitters γ-aminobutyric acid (GABA) and glycine to exert inhibitory neurotransmission through its Cl^- permeating channel. Functional expression of KCC2 at the neuronal cell surface is necessary for its activity, and impairment in KCC2 cell surface transport and/or internalization may underlie a range of neuropathological conditions. Although recent advances have shed light on a range of cellular mechanisms regulating KCC2 activity, little is known about its membrane trafficking itinerary and regulatory proteins. In this review, known membrane trafficking signals, pathways and mechanisms pertaining to KCC2's functional surface expression are discussed.

Native KCC2 interactome reveals PACSIN1 as a critical regulator of synaptic inhibition

KCC2 is a neuron-specific K+-Cl- cotransporter essential for establishing the Cl- gradient required for hyperpolarizing inhibition in the central nervous system (CNS). KCC2 is highly localized to excitatory synapses where it regulates spine morphogenesis and AMPA receptor confinement. Aberrant KCC2 function contributes to human neurological disorders including epilepsy and neuropathic pain. Using functional proteomics, we identified the KCC2-interactome in the mouse brain to determine KCC2-protein interactions that regulate KCC2 function. Our analysis revealed that KCC2 interacts with diverse proteins, and its most predominant interactors play important roles in postsynaptic receptor recycling. The most abundant KCC2 interactor is a neuronal endocytic regulatory protein termed PACSIN1 (SYNDAPIN1). We verified the PACSIN1-KCC2 interaction biochemically and demonstrated that shRNA knockdown of PACSIN1 in hippocampal neurons increases KCC2 expression and hyperpolarizes the reversal potential for Cl-. Overall, our global native-KCC2 interactome and subsequent characterization revealed PACSIN1 as a novel and potent negative regulator of KCC2.

Neuronal Chloride Regulation via KCC2 Is Modulated through a GABAB Receptor Protein Complex

GABAB receptors are G-protein-coupled receptors that mediate inhibitory synaptic actions through a series of downstream target proteins. It is increasingly appreciated that the GABAB receptor forms part of larger signaling complexes, which enable the receptor to mediate multiple different effects within neurons. Here we report that GABAB receptors can physically associate with the potassium-chloride cotransporter protein, KCC2, which sets the driving force for the chloride-permeable ionotropic GABAA receptor in mature neurons. Using biochemical, molecular, and functional studies in rodent hippocampus, we show that activation of GABAB receptors results in a decrease in KCC2 function, which is associated with a reduction in the protein at the cell surface. These findings reveal a novel "crosstalk" between the GABA receptor systems, which can be recruited under conditions of high GABA release and which could be important for the regulation of inhibitory synaptic transmission.SIGNIFICANCE STATEMENT Synaptic inhibition in the brain is mediated by ionotropic GABAA receptors (GABAARs) and metabotropic GABAB receptors (GABABRs). To fully appreciate the function and regulation of these neurotransmitter receptors, we must understand their interactions with other proteins. We describe a novel association between the GABABR and the potassium-chloride cotransporter protein, KCC2. This association is significant because KCC2 sets the intracellular chloride concentration found in mature neurons and thereby establishes the driving force for the chloride-permeable GABAAR. We demonstrate that GABABR activation can regulate KCC2 at the cell surface in a manner that alters intracellular chloride and the reversal potential for the GABAAR. Our data therefore support an additional mechanism by which GABABRs are able to modulate fast synaptic inhibition.

APP modulates KCC2 expression and function in hippocampal GABAergic inhibition

Amyloid precursor protein (APP) is enriched at the synapse, but its synaptic function is still poorly understood. We previously showed that GABAergic short-term plasticity is impaired in App knock-out (App^-/-) animals, but the precise mechanism by which APP regulates GABAergic synaptic transmission has remained elusive. Using electrophysiological, biochemical, moleculobiological, and pharmacological analysis, here we show that APP can physically interact with KCC2, a neuron-specific K⁺-Cl^- cotransporter that is essential for Cl^- homeostasis and fast GABAergic inhibition. APP deficiency results in significant reductions in both total and membrane KCC2 levels, leading to a depolarizing shift in the GABA reversal potential (E_GABA). Simultaneous measurement of presynaptic action potentials and inhibitory postsynaptic currents (IPSCs) in hippocampal neurons reveals impaired unitary IPSC amplitudes attributable to a reduction in α1 subunit levels of GABA_AR. Importantly, restoration of normal KCC2 expression and function in App^-/- mice rescues E_GABA, GABA_AR α1 levels and GABA_AR mediated phasic inhibition. We show that APP functions to limit tyrosine-phosphorylation and ubiquitination and thus subsequent degradation of KCC2, providing a mechanism by which APP influences KCC2 abundance. Together, these experiments elucidate a novel molecular pathway in which APP regulates, via protein-protein interaction with KCC2, GABA_AR mediated inhibition in the hippocampus.

DOI:http://dx.doi.org/10.7554/eLife.20142.001

Alzheimer’s disease is the most common form of dementia. One of the hallmarks of the disease is the formation of sticky protein clumps called amyloid plaques in the brain. These plaques are formed from specific fragments of a protein called APP. The intact form of APP is essential for synapses (the junctions across which neurons transmit signals) to form and work correctly.

The hippocampus is one of the first brain regions to be affected in Alzheimer’s disease and is important for forming memories and emotions. In the hippocampus, GABA_A receptors at synapses normally tightly regulate synaptic signaling by reducing the ability of the receiving neuron to respond, but this inhibition is disrupted in Alzheimer’s disease. Studies suggest that APP can affect how GABA_A receptors transmit signals, but it is not known how it does so. One possibility is that APP regulates a protein called KCC2 that helps to maintain the inhibitory effect of GABA_A receptors.

To investigate this, Chen et al. genetically modified mice to lack the gene that produces APP. These mice had a lower level of KCC2 in their hippocampus than normal mice, and their GABA_A receptors were less able to inhibit synaptic signaling. Further experiments demonstrated that APP physically interacts with KCC2 and maintains normal levels of the protein by preventing it from being chemically modified and degraded.

Chen et al. also showed that treating mice that lack APP with specific compounds can restore KCC2 activity and return the behavior of synaptic GABA_A receptors to normal. Future studies in mice (and eventually people) that exhibit symptoms of Alzheimer's disease will help to determine whether KCC2 is important in the development of the disease. If so, modifying the levels of the KCC2 protein in the brain could potentially help to slow down memory loss in Alzheimer’s disease.

DOI:http://dx.doi.org/10.7554/eLife.20142.002

Chronic Unpredictable Mild Stress Induces Loss of GABA Inhibition in Corticotrophin-Releasing Hormone-Expressing Neurons through NKCC1 Upregulation

INTRODUCTION

Prolonged and repeated stresses cause hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. The corticotrophin-releasing hormone (CRH)-expressing neurons in the hypothalamic paraventricular nucleus (PVN) are an essential component of the HPA axis.

MATERIALS AND METHODS

Chronic unpredictable mild stress (CUMS) was induced in Sprague-Dawley rats. GABA reversal potentials (EGABA) were determined by using gramicidin-perforated recordings in identified PVN-CRH neurons through expressing enhanced green fluorescent protein driven by the CRH promoter. Plasma corticosterone (CORT) levels were measured in rats implanted with a cannula targeting the lateral ventricles and PVN.

RESULTS

Blocking the GABAA receptor in the PVN with gabazine significantly increased plasma CORT levels in unstressed rats but did not change CORT levels in CUMS rats. CUMS caused a depolarizing shift in EGABA in PVN-CRH neurons compared with EGABA in PVN-CRH neurons in unstressed rats. Furthermore, CUMS induced a long-lasting increase in expression levels of the cation chloride cotransporter Na+-K+-Cl--Cl- (NKCC1) in the PVN but a transient decrease in expression levels of K+-Cl--Cl- in the PVN, which returned to the basal level 5 days after CUMS treatment. The NKCC1 inhibitor bumetanide decreased the basal firing activity of PVN-CRH neurons and normalized EGABA and the gabazine-induced excitatory effect on PVN-CRH neurons in CUMS rats. In addition, central administration of bumetanide decreased basal circulating CORT levels in CUMS rats.

CONCLUSIONS

These data suggest that chronic stress impairs GABAergic inhibition, resulting in HPA axis hyperactivity through upregulation of NKCC1.

Insufficient developmental excitatory neuronal activity fails to foster establishment of normal levels of inhibitory neuronal activity

The nervous system is composed of excitatory and inhibitory neurons. One major class of inhibitory neurons release the neurotransmitter γ-Aminobutyric acid (GABA). GABAergic inhibitory activity maintains the balance that is disrupted in conditions such as epilepsy. At least some GABAergic neurons are initially excitatory and undergo a developmental conversion to convert to inhibitory neurons. The mechanism(s) behind this conversion are thought to include a critical developmental increase in excitatory activity. To test this hypothesis, we subjected ex vivo developing neuronal networks on multi-electrode arrays to various stimulation and pharmacological regimens. Synaptic activity of networks initially consists of epileptiform-like high-amplitude individual "spikes", which convert to organized bursts of activity over the course of approximately 1 month. Stimulation of networks with a digitized synaptic signal for 5days hastened the decrease of epileptiform activity. By contrast, stimulation for a single day delayed the appearance of bursts and instead increased epileptiform signaling. GABA treatment reduced total signals in unstimulated networks and networks stimulated for 5days, but instead increased signaling in networks stimulated for 1day. This increase was prevented by co-treatment with (2R)-amino-5-phosphonopentanoate and 6-cyano-7-nitroquinoxaline-2,3-dione, confirming that GABA invoked excitatory activity in networks stimulated for 1day. Glutamate increased signals in networks subjected to all stimulation regimens; the GABA receptor antagonist bicuculline prevented this increase only in networks stimulated for 1day. These latter findings are consistent with the induction of so-called "mixed" synapses (which release a combination of excitatory and inhibitory neurotransmitters) in networks stimulated for 1day, and support the hypothesis that a critical level of excitatory activity fosters the developmental transition of GABAergic neurons from excitatory to inhibitory.

Spinal Plasticity and Behavior: BDNF-Induced Neuromodulation in Uninjured and Injured Spinal Cord

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family of signaling molecules. Since its discovery over three decades ago, BDNF has been identified as an important regulator of neuronal development, synaptic transmission, and cellular and synaptic plasticity and has been shown to function in the formation and maintenance of certain forms of memory. Neural plasticity that underlies learning and memory in the hippocampus shares distinct characteristics with spinal cord nociceptive plasticity. Research examining the role BDNF plays in spinal nociception and pain overwhelmingly suggests that BDNF promotes pronociceptive effects. BDNF induces synaptic facilitation and engages central sensitization-like mechanisms. Also, peripheral injury-induced neuropathic pain is often accompanied with increased spinal expression of BDNF. Research has extended to examine how spinal cord injury (SCI) influences BDNF plasticity and the effects BDNF has on sensory and motor functions after SCI. Functional recovery and adaptive plasticity after SCI are typically associated with upregulation of BDNF. Although neuropathic pain is a common consequence of SCI, the relation between BDNF and pain after SCI remains elusive. This article reviews recent literature and discusses the diverse actions of BDNF. We also highlight similarities and differences in BDNF-induced nociceptive plasticity in naïve and SCI conditions.

Bidirectional Homeostatic Regulation of a Depression-Related Brain State by Gamma-Aminobutyric Acidergic Deficits and Ketamine Treatment

BACKGROUND

Major depressive disorder is increasingly recognized to involve functional deficits in both gamma-aminobutyric acid (GABA)ergic and glutamatergic synaptic transmission. To elucidate the relationship between these phenotypes, we used GABAA receptor γ2 subunit heterozygous (γ2(+/-)) mice, which we previously characterized as a model animal with construct, face, and predictive validity for major depressive disorder.

METHODS

To assess possible consequences of GABAergic deficits on glutamatergic transmission, we quantitated the cell surface expression of N-methyl-D-aspartate (NMDA)-type and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors and the function of synapses in the hippocampus and medial prefrontal cortex of γ2(+/-) mice. We also analyzed the effects of an acute dose of the experimental antidepressant ketamine on all these parameters in γ2(+/-) versus wild-type mice.

RESULTS

Modest defects in GABAergic synaptic transmission of γ2(+/-) mice resulted in a strikingly prominent homeostatic-like reduction in the cell surface expression of NMDA-type and AMPA-type glutamate receptors, along with prominent functional impairment of glutamatergic synapses in the hippocampus and medial prefrontal cortex. A single subanesthetic dose of ketamine normalized glutamate receptor expression and synaptic function of γ2(+/-) mice to wild-type levels for a prolonged period, along with antidepressant-like behavioral consequences selectively in γ2(+/-) mice. The GABAergic synapses of γ2(+/-) mice were potentiated by ketamine in parallel but only in the medial prefrontal cortex.

CONCLUSIONS

Depressive-like brain states that are caused by GABAergic deficits involve a homeostatic-like reduction of glutamatergic transmission that is reversible by an acute, subanesthetic dose of ketamine, along with regionally selective potentiation of GABAergic synapses. The data merge the GABAergic and glutamatergic deficit hypotheses of major depressive disorder.

Glial Na(+) -dependent ion transporters in pathophysiological conditions

Sodium dynamics are essential for regulating functional processes in glial cells. Indeed, glial Na(+) signaling influences and regulates important glial activities, and plays a role in neuron-glia interaction under physiological conditions or in response to injury of the central nervous system (CNS). Emerging studies indicate that Na(+) pumps and Na(+) -dependent ion transporters in astrocytes, microglia, and oligodendrocytes regulate Na(+) homeostasis and play a fundamental role in modulating glial activities in neurological diseases. In this review, we first briefly introduced the emerging roles of each glial cell type in the pathophysiology of cerebral ischemia, Alzheimer's disease, epilepsy, Parkinson's disease, Amyotrophic Lateral Sclerosis, and myelin diseases. Then, we discussed the current knowledge on the main roles played by the different glial Na(+) -dependent ion transporters, including Na(+) /K(+) ATPase, Na(+) /Ca(2+) exchangers, Na(+) /H(+) exchangers, Na(+) -K(+) -Cl(-) cotransporters, and Na(+) - HCO3- cotransporter in the pathophysiology of the diverse CNS diseases. We highlighted their contributions in cell survival, synaptic pathology, gliotransmission, pH homeostasis, and their role in glial activation, migration, gliosis, inflammation, and tissue repair processes. Therefore, this review summarizes the foundation work for targeting Na(+) -dependent ion transporters in glia as a novel strategy to control important glial activities associated with Na(+) dynamics in different neurological disorders. GLIA 2016;64:1677-1697.

Altered expression of KCC2 in GABAergic interneuron contributes prenatal stress-induced epileptic spasms in infant rat

Long-term stress during pregnancy causes neurologic deficits to offspring with altered gamma-aminobutyric acid (GABA) system in the brain. However, it is not clear how prenatal stress affects the maturing GABAergic interneurons and the resulting abnormalities in infantile seizures. Here, we showed that prenatal stress alters the maturation of GABA inhibitory system using a seizure model induced by prenatal stress. Prenatal stress with betamethasone or acute immobilization stress (AIS) on gestational day 15 increased the seizure susceptibility to N-methyl-d-aspartate-triggered spasms on postnatal day 15. The expression of GABA was lower in the prenatally stressed group, which compromise the decrease of glutamate decarboxylase 67-immunopositive cells. Prenatal stress markedly decreased the expression of K(+)/Cl(-) co-transporter (KCC2) in the cortex. GABA induced membrane depolarization demonstrated prenatal stress models had significant higher membrane depolarization compared to control. GABA increased KCC2 expression in cultured cortex-containing slices. Taken together, our results showed that prenatal stress with betamethasone or AIS altered the maturation of GABAergic progenitors and resulted in the lack of GABA input, which in turn, decreased KCC2 expression and lowered seizure threshold. We conclude that delayed GABA excitatory/inhibitory shift would render the cortical neuronal circuit more susceptible to excitatory input in prenatal stress induced seizure.

The K(+)-Cl(-) Cotransporter KCC2 and Chloride Homeostasis: Potential Therapeutic Target in Acute Central Nervous System Injury

The K(+)-Cl(-) cotransporter-2 (KCC2) is a well-known member of the electroneutral cation-chloride cotransporters with a restricted expression pattern to neurons. This transmembrane protein mediates the efflux of Cl(-) out of neurons and exerts a critical role in inhibitory γ-aminobutyric acidergic (GABAergic) and glycinergic neurotransmission. Moreover, KCC2 participates in the regulation of various physiological processes of neurons, including cell migration, dendritic outgrowth, spine morphology, and dendritic synaptogenesis. It is important to note that down-regulation of KCC2 is associated with the pathogenesis of multiple neurological diseases, which is of particular relevance to acute central nervous system (CNS) injury. In this review, we aim to survey the pathogenic significance of KCC2 down-regulation under the condition of acute CNS injuries. We propose that further elucidation of the molecular mechanisms regarding KCC2 down-regulation after acute CNS injuries is necessary because of potential promising avenues for prevention and treatment of acute CNS injury.

KCC2 Gates Activity-Driven AMPA Receptor Traffic through Cofilin Phosphorylation

Expression of the neuronal K/Cl transporter KCC2 is tightly regulated throughout development and by both normal and pathological neuronal activity. Changes in KCC2 expression have often been associated with altered chloride homeostasis and GABA signaling. However, recent evidence supports a role of KCC2 in the development and function of glutamatergic synapses through mechanisms that remain poorly understood. Here we show that suppressing KCC2 expression in rat hippocampal neurons precludes long-term potentiation of glutamatergic synapses specifically by preventing activity-driven membrane delivery of AMPA receptors. This effect is independent of KCC2 transporter function and can be accounted for by increased Rac1/PAK- and LIMK-dependent cofilin phosphorylation and actin polymerization in dendritic spines. Our results demonstrate that KCC2 plays a critical role in the regulation of spine actin cytoskeleton and gates long-term plasticity at excitatory synapses in cortical neurons.

Prenatal stress, regardless of concurrent escitalopram treatment, alters behavior and amygdala gene expression of adolescent female rats

Depression during pregnancy has been linked to in utero stress and is associated with long-lasting symptoms in offspring, including anxiety, helplessness, attentional deficits, and social withdrawal. Depression is diagnosed in 10-20% of expectant mothers, but the impact of antidepressant treatment on offspring development is not well documented, particularly for females. Here, we used a prenatal stress model of maternal depression to test the hypothesis that in utero antidepressant treatment could mitigate the effects of prenatal stress. We also investigated the effects of prenatal stress and antidepressant treatment on gene expression related to GABAergic and serotonergic neurotransmission in the amygdala, which may underlie behavioral effects of prenatal stress. Nulliparous female rats were implanted with osmotic minipumps delivering clinically-relevant concentrations of escitalopram and mated. Pregnant dams were exposed to 12 days of mixed-modality stressors, and offspring were behaviorally assessed in adolescence (postnatal day 28) and adulthood (beyond day 90) to determine the extent of behavioral change. We found that in utero stress exposure, regardless of escitalopram treatment, increased anxiety-like behavior in adolescent females and profoundly influenced amygdala expression of the chloride transporters KCC2 and NKCC1, which regulate GABAergic function. In contrast, prenatal escitalopram exposure alone elevated amygdala expression of 5-HT1A receptors. In adulthood, anxiety-like behavior returned to baseline and gene expression effects in the amygdala abated, whereas deficits emerged in novel object recognition for rats exposed to stress during gestation. These findings suggest prenatal stress causes age-dependent deficits in anxiety-like behavior and amygdala function in female offspring, regardless of antidepressant exposure.

Enhanced expression of potassium-chloride cotransporter KCC2 in human temporal lobe epilepsy

Synaptic reorganization in the epileptic hippocampus involves altered excitatory and inhibitory transmission besides the rearrangement of dendritic spines, resulting in altered excitability, ion homeostasis, and cell swelling. The potassium-chloride cotransporter-2 (KCC2) is the main chloride extruder in neurons and hence will play a prominent role in determining the polarity of GABAA receptor-mediated chloride currents. In addition, KCC2 also interacts with the actin cytoskeleton which is critical for dendritic spine morphogenesis, and for the maintenance of glutamatergic synapses and cell volume. Using immunocytochemistry, we examined the cellular and subcellular levels of KCC2 in surgically removed hippocampi of temporal lobe epilepsy (TLE) patients and compared them to control human tissue. We also studied the distribution of KCC2 in a pilocarpine mouse model of epilepsy. An overall increase in KCC2-expression was found in epilepsy and confirmed by Western blots. The cellular and subcellular distributions in control mouse and human samples were largely similar; moreover, changes affecting KCC2-expression were also alike in chronic epileptic human and mouse hippocampi. At the subcellular level, we determined the neuronal elements exhibiting enhanced KCC2 expression. In epileptic tissue, staining became more intense in the immunopositive elements detected in control tissue, and profiles with subthreshold expression of KCC2 in control samples became labelled. Positive interneuron somata and dendrites were more numerous in epileptic hippocampi, despite severe interneuron loss. Whether the elevation of KCC2-expression is ultimately a pro- or anticonvulsive change, or both-behaving differently during ictal and interictal states in a context-dependent manner-remains to be established.

Regulation of GABA Equilibrium Potential by mGluRs in Rat Hippocampal CA1 Neurons

The equilibrium potential for GABA-A receptor mediated currents (E_GABA) in neonatal central neurons is set at a relatively depolarized level, which is suggested to be caused by a low expression of K⁺/Cl^- co-transporter (KCC2) but a relatively high expression of Na⁺-K⁺-Cl^- cotransporter (NKCC1). Theta-burst stimulation (TBS) in stratum radiatum induces a negative shift in E_GABA in juvenile hippocampal CA1 pyramidal neurons. In the current study, the effects of TBS on E_GABA in neonatal and juvenile hippocampal CA1 neurons and the underlying mechanisms were examined. Metabotropic glutamate receptors (mGluRs) are suggested to modulate KCC2 and NKCC1 levels in cortical neurons. Therefore, the involvement of mGluRs in the regulation of KCC2 or NKCC1 activity, and thus E_GABA, following TBS was also investigated. Whole-cell patch recordings were made from Wistar rat hippocampal CA1 pyramidal neurons, in a slice preparation. In neonates, TBS induces a positive shift in E_GABA, which was prevented by NKCC1 antisense but not NKCC1 sense mRNA. (RS)-a-Methyl-4-carboxyphenylglycine (MCPG), a group I and II mGluR antagonist, blocked TBS-induced shifts in both juvenile and neonatal hippocampal neurons. While blockade of mGluR1 or mGluR5 alone could interfere with TBS-induced shifts in E_GABA in neonates, only a combined blockade could do the same in juveniles. These results indicate that TBS induces a negative shift in E_GABA in juvenile hippocampal neurons but a positive shift in neonatal hippocampal neurons via corresponding changes in KCC2 and NKCC1 expressions, respectively. mGluR activation seems to be necessary for both shifts to occur while the specific receptor subtype involved seems to vary.