Potassium-coupled chloride cotransport controls intracellular chloride in rat neocortical pyramidal neurons

Cell type-specific properties of subicular GABAergic currents shape hippocampal output firing mode

GABAergic function of the subiculum is central to the regulation of hippocampal output activity. Subicular neuronal networks are indeed under potent control by local inhibition. However, information about the properties of GABAergic currents generated by neurons of this parahippocampal area in normal tissue is still missing. Here, we describe GABA_A receptor (GABA_AR)-mediated phasic and tonic currents generated by principal cells (PCs) and interneurons (INs) of the rat subiculum. We show that in spite of similar synaptic current densities, INs generate spontaneous IPSCs (sIPSCs) that occur less frequently and exhibit smaller charge transfer, thus receiving less synaptic total current than PCs. Further distinction of PCs between intrinsically bursting (IB) and regular-spiking (RS) neurons suggested that sIPSCs generated by the two PC sub-types are likely to be similar. PCs and INs are also controlled by a similar tonic inhibition. However, whereas a comparable tonic current density is found in RS cells and INs, IB neurons are constrained by a greater inhibitory tone. Finally, pharmacological blockade of GABA_AR did not promote functional switch of RS neurons to IB mode, but influenced the bursting propensity of IB cells and released fast spiking activity in INs. Our findings reveal differences in GABAergic currents between PCs and INs as well as within PC sub-types. We propose that GABAergic inhibition may shape hippocampal output activity by providing cell type-specific fine-tuning of subicular excitatory and inhibitory drives.

Rapid regulation of tonic GABA currents in cultured rat hippocampal neurons

Subacute and chronic changes in tonic GABAergic inhibition occur in human and experimental epilepsy. Less is known about how tonic inhibition is modulated over shorter time frames (seconds). We measured endogenous tonic GABA currents from cultured rat hippocampal neurons to evaluate how they are affected by 1) transient increases in extracellular GABA concentration ([GABA]), 2) transient postsynaptic depolarization, and 3) depolarization of presynaptic cells. Transient increases in [GABA] (1 μM) reduced tonic currents; this reduction resulted from GABA-induced shifts in the reversal potential for GABA currents (E(GABA)). Transient depolarization of postsynaptic neurons reversed the effects of exogenous GABA and potentiated tonic currents. The voltage-dependent potentiation of tonic GABA currents was independent of E(GABA) shifts and represented postdepolarization potentiation (PDP), an intrinsic GABA(A) receptor property (Ransom CB, Wu Y, Richerson GB. J Neurosci 30: 7672-7684, 2010). Inhibition of vesicular GABA release with concanamycin A (ConA) did not affect tonic currents. In ConA-treated cells, transient application of 12 mM K(+) to depolarize presynaptic neurons and glia produced a persistent increase in tonic current amplitude. The K(+)-induced increase in tonic current was reversibly inhibited by SKF89976a (40 μM), indicating that this was caused by nonvesicular GABA release from GABA transporter type 1 (GAT1). Nonvesicular GABA release due to GAT1 reversal also occurred in acute hippocampal brain slices. Our results indicate that tonic GABA currents are rapidly regulated by GABA-induced changes in intracellular Cl(-) concentration, PDP of extrasynaptic GABA(A) receptors, and nonvesicular GABA release. These mechanisms may influence tonic inhibition during seizures when neurons are robustly depolarized and extracellular GABA and K(+) concentrations are elevated.

Computational modeling reveals dendritic origins of GABA(A)-mediated excitation in CA1 pyramidal neurons

GABA is the key inhibitory neurotransmitter in the adult central nervous system, but in some circumstances can lead to a paradoxical excitation that has been causally implicated in diverse pathologies from endocrine stress responses to diseases of excitability including neuropathic pain and temporal lobe epilepsy. We undertook a computational modeling approach to determine plausible ionic mechanisms of GABA(A)-dependent excitation in isolated post-synaptic CA1 hippocampal neurons because it may constitute a trigger for pathological synchronous epileptiform discharge. In particular, the interplay intracellular chloride accumulation via the GABA(A) receptor and extracellular potassium accumulation via the K/Cl co-transporter KCC2 in promoting GABA(A)-mediated excitation is complex. Experimentally it is difficult to determine the ionic mechanisms of depolarizing current since potassium transients are challenging to isolate pharmacologically and much GABA signaling occurs in small, difficult to measure, dendritic compartments. To address this problem and determine plausible ionic mechanisms of GABA(A)-mediated excitation, we built a detailed biophysically realistic model of the CA1 pyramidal neuron that includes processes critical for ion homeostasis. Our results suggest that in dendritic compartments, but not in the somatic compartments, chloride buildup is sufficient to cause dramatic depolarization of the GABA(A) reversal potential and dominating bicarbonate currents that provide a substantial current source to drive whole-cell depolarization. The model simulations predict that extracellular K(+) transients can augment GABA(A)-mediated excitation, but not cause it. Our model also suggests the potential for GABA(A)-mediated excitation to promote network synchrony depending on interneuron synapse location - excitatory positive-feedback can occur when interneurons synapse onto distal dendritic compartments, while interneurons projecting to the perisomatic region will cause inhibition.

Taurine inhibits K+-Cl- cotransporter KCC2 to regulate embryonic Cl- homeostasis via with-no-lysine (WNK) protein kinase signaling pathway

GABA inhibits mature neurons and conversely excites immature neurons due to lower K(+)-Cl(-) cotransporter 2 (KCC2) expression. We observed that ectopically expressed KCC2 in embryonic cerebral cortices was not active; however, KCC2 functioned in newborns. In vitro studies revealed that taurine increased KCC2 inactivation in a phosphorylation-dependent manner. When Thr-906 and Thr-1007 residues in KCC2 were substituted with Ala (KCC2T906A/T1007A), KCC2 activity was facilitated, and the inhibitory effect of taurine was not observed. Exogenous taurine activated the with-no-lysine protein kinase 1 (WNK1) and downstream STE20/SPS1-related proline/alanine-rich kinase (SPAK)/oxidative stress response 1 (OSR1), and overexpression of active WNK1 resulted in KCC2 inhibition in the absence of taurine. Phosphorylation of SPAK was consistently higher in embryonic brains compared with that of neonatal brains and down-regulated by a taurine transporter inhibitor in vivo. Furthermore, cerebral radial migration was perturbed by a taurine-insensitive form of KCC2, KCC2T906A/T1007A, which may be regulated by WNK-SPAK/OSR1 signaling. Thus, taurine and WNK-SPAK/OSR1 signaling may contribute to embryonic neuronal Cl(-) homeostasis, which is required for normal brain development.

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).

Decreased Immunoreactivities and Functions of the Chloride Transporters, KCC2 and NKCC1, in the Lateral Superior Olive Neurons of Circling Mice

Objectives

We tested the possibility of differential expression and function of the potassium-chloride (KCC2) and sodium-potassium-2 chloride (NKCC1) co-transporters in the lateral superior olive (LSO) of heterozygous (+/cir) or homozygous (cir/cir) mice.

Methods

Mice pups aged from postnatal (P) day 9 to 16 were used. Tails from mice were cut for DNA typing. For Immunohistochemical analysis, rabbit polyclonal anti-KCC2 or rabbit polyclonal anti-NKCC1 was used and the density of immunolabelings was evaluated using the NIH image program. For functional analysis, whole cell voltage clamp technique was used in brain stem slices and the changes of reversal potentials were evaluated at various membrane potentials.

Results

Immunohistochemical analysis revealed both KCC2 and NKCC1 immunoreactivities were more prominent in heterozygous (+/cir) than homozygous (cir/cir) mice on P day 16. In P9-P12 heterozygous (+/cir) mice, the reversal potential (E_gly) of glycine-induced currents was shifted to a more negative potential by 50 µM bumetanide, a known NKCC1 blocker, and the negatively shifted E_gly was restored by additional application of 1 mM furosemide, a KCC2 blocker (-58.9±2.6 mV to -66.0±1.5 mV [bumetanide], -66.0±1.5 mV to -59.8±2.8 mV [furosemide+bumetanide], n=11). However, only bumetanide was weakly, but significantly effective (-60.1±2.9 mV to -62.7±2.6 mV [bumetanide], -62.7±2.6 mV to -62.1±2.5 mV [furosemide+bumetanide], n=7) in P9-P12 homozygous (cir/cir) mice.

Conclusion

The less prominent immunoreactivities and weak or absent responses to bumetanide or furosemide suggest impaired function or delayed development of both transporters in homozygous (cir/cir) mice.

Components of neuronal chloride transport in rat and human neocortex

Considerable evidence indicates disturbances in the ionic gradient of GABAA receptor-mediated inhibition of neurones in human epileptogenic tissues. Two contending mechanisms have been proposed, reduced outward and increased inward Cl⁻ transporters. We investigated the properties of Cl⁻ transport in human and rat neocortical neurones (layer II/III) using intracellular recordings in slices of cortical tissue. We measured the alterations in reversal potential of the pharmacologically isolated inhibitory postsynaptic potential mediated by GABAA receptors (IPSPA) to estimate the ionic gradient and kinetics of Cl⁻ efflux after Cl⁻ injections before and during application of selected blockers of Cl⁻ routes (furosemide, bumetanide, 9-anthracene carboxylic acid and Cs+). Neurones from human epileptogenic cortex exhibited a fairly depolarized reversal potential of GABAA receptor-mediated inhibition (EIPSP-A) of -61.9 ± 8.3 mV. In about half of the neurones, the EIPSP-A averaged -55.2 ± 5.7 mV, in the other half, 68.6 ± 2.3 mV, similar to rat neurones (-68.9 ± 2.6 mV). After injections of Cl⁻, IPSPA recovered in human neurones with an average time constant (τ) of 19.0 ± 9.6 s (rat neurones: 7.2 ± 2.4 s). We calculated Cl⁻ extrusion rates (1/τ) via individual routes from the τ values obtained in different experimental conditions, revealing that, for example, the K+-coupled Cl⁻ transporter KCC2 comprises 45.3% of the total rate in rat neurones. In human neurones, the total rate of Cl⁻ extrusion was 63.9% smaller, and rates via KCC2, the Na+-K+-2Cl⁻ transporter NKCC1 and the voltage-gatedCl− channelClCwere smaller than in rat neurones by 80.0%, 61.7% and 79.9%, respectively. The rate via anion exchangers conversely was 14.4% larger in human than in rat neurones. We propose that (i) KCC2 is the major route of Cl⁻ extrusion in cortical neurones, (ii) reduced KCC2 is the initial step of disturbed Cl⁻ regulation and (iii) reductions in KCC2 contribute to depolarizing EIPSP-A of neurones in human epileptogenic neocortex.

The K+-Cl cotransporter KCC2 promotes GABAergic excitation in the mature rat hippocampus

GABAergic excitatory [K(+)](o) transients can be readily evoked in the mature rat hippocampus by intense activation of GABA(A) receptors (GABA(A)Rs). Here we show that these [K(+)](o) responses induced by high-frequency stimulation or GABA(A) agonist application are generated by the neuronal K(+)-Cl() cotransporter KCC2 and that the transporter-mediated KCl extrusion is critically dependent on the bicarbonate-driven accumulation of Cl() in pyramidal neurons. The mechanism underlying GABAergic [K(+)](o) transients was studied in CA1 stratum pyramidale using intracellular sharp microelectrodes and extracellular ion-sensitive microelectrodes. The evoked [K(+)](o) transients, as well as the associated afterdischarges, were strongly suppressed by 0.5-1 mm furosemide, a KCl cotransport inhibitor. Importantly, the GABA(A)R-mediated intrapyramidal accumulation of Cl(), as measured by monitoring the reversal potential of fused IPSPs, was unaffected by the drug. It was further confirmed that the reduction in the [K(+)](o) transients was not due to effects of furosemide on the Na(+)-dependent K(+)-Cl() cotransporter NKCC1 or on intraneuronal carbonic anhydrase activity. Blocking potassium channels by Ba(2+) enhanced [K(+)](o) transients whereas pyramidal cell depolarizations were attenuated in further agreement with a lack of contribution by channel-mediated K(+) efflux. The key role of the GABA(A)R channel-mediated anion fluxes in the generation of the [K(+)](o) transients was examined in experiments where bicarbonate was replaced with formate. This anion substitution had no significant effect on the rate of Cl() accumulation, [K(+)](o) response or afterdischarges. Our findings reveal a novel excitatory mode of action of KCC2 that can have substantial implications for the role of GABAergic transmission during ictal epileptiform activity.

Cell-type specific distribution of chloride transporters in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is a circadian oscillator and biological clock. Cell-to-cell communication is important for synchronization among SCN neuronal oscillators and the great majority of SCN neurons use GABA as a neurotransmitter, the principal inhibitory neurotransmitter in the adult CNS. Acting via the ionotropic GABA(A) receptor, a chloride ion channel, GABA typically evokes inhibitory responses in neurons via Cl(-) influx. Within the SCN GABA evokes both inhibitory and excitatory responses although the mechanism underlying GABA-evoked excitation in the SCN is unknown. GABA-evoked depolarization in immature neurons in several regions of the brain is a function of intracellular chloride concentration, regulated largely by the cation-chloride cotransporters NKCC1 (sodium/potassium/chloride cotransporter for chloride entry) and KCC1-4 (potassium/chloride cotransporters for chloride egress). It is well established that changes in the expression of the cation-chloride cotransporters through development determines the polarity of the response to GABA. To understand the mechanisms underlying GABA-evoked excitation in the SCN, we examined the SCN expression of cation-chloride cotransporters. Previously we reported that the K(+)/Cl(-) cotransporter KCC2, a neuron-specific chloride extruder conferring GABA's more typical inhibitory effects, is expressed exclusively in vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) neurons in the SCN. Here we report that the K(+)/Cl(-) cotransporter isoforms KCC4 and KCC3 are expressed solely in vasopressin (VP) neurons in the rat SCN whereas KCC1 is expressed in VIP neurons, similar to KCC2. NKCC1 is expressed in VIP, GRP and VP neurons in the SCN as is WNK3, a chloride-sensitive neuron-specific with no serine-threonine kinase which modulates intracellular chloride concentration via opposing actions on NKCC and KCC cotransporters. The heterogeneous distribution of cation-chloride cotransporters in the SCN suggests that Cl(-) levels are differentially regulated within VIP/GRP and VP neurons. We suggest that GABA's excitatory action is more likely to be evoked in VP neurons that express KCC4.

Developmental up-regulation of the potassium-chloride cotransporter type 2 in the rat lumbar spinal cord

The classical GABA/glycine hyperpolarizing inhibition is not observed in the immature spinal cord. GABA(A) and glycine receptors are anions channels and the efficacy of inhibitory transmission in the spinal cord is largely determined by the gradient between intracellular and extracellular chloride concentrations. The concentration of intracellular chloride in neurons is mainly regulated by two cation-chloride cotransporters, the potassium-chloride cotransporter 2 (KCC2) and the sodium-potassium-chloride co-transporter 1 (NKCC1). In this study, we measured the reversal potential of IPSPs (E(IPSP)) of lumbar motoneurons during the first postnatal week and we investigated the expression of KCC2 and NKCC1 in the ventral horn of the spinal cord from the embryonic day 17 to the postnatal day 20 in the rat. Our results suggest that the negative shift of E(IPSP) from above to below the resting membrane potential occurs during the first postnatal week when the expression of KCC2 increases significantly and the expression of NKCC1 decreases. KCC2 immunolabeling surrounded motoneurons, presumably in the plasma membrane and NKCC1 immunolabeling appeared outside this KCC2-labeled fine strip. Taken together, the present results indicate that maturation of chloride homeostasis is not completed at birth in the rat and that the upregulation of KCC2 plays a key role in the shift from depolarizing to hyperpolarizing IPSPs.

Inhibitory effects of amiloride on the current mediated by native GABA(A) receptors in cultured neurons of rat inferior colliculus

1. The diuretic amiloride is known to modulate the activity of several types of ion channels and membrane receptors in addition to its inhibitory effects on many ion transport systems. However, the effects of amiloride on some important ion channels and receptors, such as GABA(A) receptors, in the central nervous system have not been characterized. 2. In the present study, we investigated the functional action of amiloride on native GABA(A) receptors in cultured neurons of rat inferior colliculus using whole-cell patch-clamp recordings. 3. Amiloride reversibly inhibited the amplitude of the GABA-induced current (I(GABA)) in a concentration-dependent manner (IC(50) 454 +/- 24 micromol/L) under conditions of voltage-clamp with a holding potential at -60 mV. The inhibition depended on drug application mode and was independent of membrane potential. Amiloride did not change the reversal potential of I(GABA). Moreover, amiloride induced a parallel right-ward shift in the concentration-response curve for I(GABA) without altering the maximal value and Hill coefficient. 4. The present study shows that amiloride competitively inhibits the current mediated by native GABA(A) receptors in the brain region, probably via a direct action on GABA-binding sites on the receptor. The findings suggest that the functional actions of amiloride on GABA(A) receptors may result in possible side-effects on the central nervous system in the case of direct application of this drug into the cerebrospinal fluid for treatment of diseases such as brain tumours.

Differences in cortical versus subcortical GABAergic signaling: a candidate mechanism of electroclinical uncoupling of neonatal seizures

Electroclinical uncoupling of neonatal seizures refers to electrographic seizure activity that is not clinically manifest. Uncoupling increases after treatment with Phenobarbital, which enhances the GABA(A) receptor (GABA(A)R) conductance. The effects of GABA(A)R activation depend on the intracellular Cl(-) concentration ([Cl(-)](i)) that is determined by the inward Cl(-) transporter NKCC1 and the outward Cl(-) transporter KCC2. Differential maturation of Cl(-) transport observed in cortical versus subcortical regions should alter the efficacy of GABA-mediated inhibition. In perinatal rat pups, most thalamic neurons maintained low [Cl(-)](i) and were inhibited by GABA. Phenobarbital suppressed thalamic seizure activity. Most neocortical neurons maintained higher [Cl(-)](i), and were excited by GABA(A)R activation. Phenobarbital had insignificant anticonvulsant responses in the neocortex until NKCC1 was blocked. Regional differences in the ontogeny of Cl(-) transport may thus explain why seizure activity in the cortex is not suppressed by anticonvulsants that block the transmission of seizure activity through subcortical networks.

Expression of NKCC2 in the rat gastrointestinal tract

NKCC2, an isoform of Na+-K+-2Cl(-) cotransporter, is principally present in the kidney and plays a critical role in salt reabsorption. Expression of NKCC2 has been found in the apical membrane of intestinal epithelial cells in a number of marine fish, however, details for expression in the mammalian gastrointestinal tract are lacking. RT-PCR, Western blotting and immunohistochemistry were used to study the expression and localization of NKCC2 in the rat gastrointestinal tract. We found that mRNA transcripts, protein and immunoreactivity (IR) for NKCC2 were expressed in the stomach, small and large intestine of adult rats. NKCC2 IR was localized to the base of the gastric glands, intestinal epithelia, myenteric and submucosal plexuses. NKCC2 IR was expressed strongly in the apical membranes and weakly in the basolateral membranes of intestinal epithelial cells. In the enteric nervous system, NKCC2 IR was widely distributed and localized to enteric neurons with cholinergic, calretinin and nitrergic neuronal immunochemical codes in the myenteric plexus. It was localized to non-cholinergic secretomotor neurons in the submucosal plexus. In conclusion, this study for the first time clearly detected the expression of NKCC2 in the gastrointestinal tract of a mammalian species. Expression of NKCC2 in gastrointestinal epithelial cells suggested that this cation chloride cotransporter might be involved in gastrointestinal ion transport. Expression of NKCC2 in enteric neurons might contribute to the accumulation of Cl(-) and a more depolarized E(Cl)(-) in enteric neurons.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Presynaptic actions of propofol enhance inhibitory synaptic transmission in isolated solitary tract nucleus neurons

General anesthetics variably enhance inhibitory synaptic transmission that relies on (-aminobutyric acid (GABA) and GABAA receptor function with distinct differences across brain regions. Activation of "extra-synaptic" GABAA receptors produces a tonic current considered the most sensitive target for general anesthetics, particularly in forebrain neurons. To evaluate the contribution of poor drug access to neurons in slices, we tested the intravenous anesthetic propofol in mechanically isolated neurons from the solitary tract nucleus (NTS). Setting chloride concentrations to ECl=-29 mV made GABA currents inward at holding potentials of -60 mV. Propofol triggered pronounced but slowly-developing tonic currents that reversed with 5 min washing. Effective concentrations in isolated cells were lower than in slices and propofol enhanced phasic IPSCs more potently than tonic currents (1 microM increased phasic decay-time constant vs. >3 microM tonic currents). Propofol increased IPSC frequency (>3 microM), a presynaptic action. Bicuculline blocked all propofol actions. Gabazine blocked only phasic IPSCs. IPSCs persisted in TTX and/or cadmium but these agents prevented propofol-induced increases in IPSC frequency. Furosemide (>1 mM) reversibly blocked propofol-evoked IPSC frequency changes without altering waveforms. We conclude that presynaptic actions of propofol depend on a depolarizing chloride gradient across presynaptic inhibitory terminals. Our results in isolated neurons indicate that propofol pharmacokinetics intrinsically trigger the tonic currents slowly and the time course is not related to slow permeation or delivery. Unlike forebrain, phasic NTS GABAA receptors are more sensitive to propofol than tonic receptors but that presynaptic GABAA receptor mechanisms regulate GABA release.

Small-molecule screen identifies inhibitors of the neuronal K-Cl cotransporter KCC2

KCC2, a neuronal-specific K-Cl cotransporter, plays a major role in maintaining intracellular Cl(-) concentration in neurons below its electrochemical equilibrium potential, thus favoring robust GABA hyperpolarizing or inhibitory responses. The pharmacology of the K-Cl cotransporter is dominated by loop diuretics such as furosemide and bumetanide, molecules used in clinical medicine because they inhibit the loop of Henle Na-K-2Cl cotransporter with much higher affinity. To identify molecules that affect KCC2 activity, we developed a fluorescence-based assay suitable for high-throughput screening (HTS) and used the assay to screen a library of 234,000 small molecules. We identified a large number of molecules that either decrease or increase the activity of the cotransporter. Here, we report the characterization of a small number of inhibitors, some of which inhibit KCC2 activity in the submicomolar range without substantially affecting NKCC1 activity. Using medicinal chemistry, we synthesized a number of variants, tested their effect on KCC2 function, and provide an analysis of structure/activity relationships. We also used one of the compounds to demonstrate competitive inhibition in regard to external [K(+)] versus noncompetitive inhibition in respect to external [Cl(-)].

Altered chloride homeostasis removes synaptic inhibitory constraint of the stress axis

In mammals, stress elicits a stereotyped endocrine response that requires an increase in the activity of hypothalamic parvocellular neuroendocrine neurons. The output of these cells is normally constrained by powerful GABA-mediated synaptic inhibition. We found that acute restraint stress in rats released the system from inhibitory synaptic drive in vivo by down-regulating the transmembrane anion transporter KCC2. This manifested as a depolarizing shift in the reversal potential of GABA(A)-mediated synaptic currents that rendered GABA inputs largely ineffective. Notably, repetitive activation of GABA synapses after stress resulted in a more rapid collapse of the anion gradient and was sufficient to increase the activity of neuroendocrine cells. Our data indicate that hypothalamic neurons integrate psychological cues to mount the endocrine response to stress by regulating anion gradients.

Potassium dynamics in the epileptic cortex: new insights on an old topic

The role of changes in the extracellular potassium concentration [K(+)](o) in epilepsy has remained unclear. Historically, it was hypothesized that [K(+)]( o) is the causal factor for epileptic seizures. This so-called potassium accumulation hypothesis led to substantial debate but subsequently failed to find wide acceptance. However, recent studies on the pathophysiology of tissue from epileptic human patients and animal epilepsy models revealed aberrations in [K(+)](o) regulation. Computational models of cortical circuits that include ion concentration dynamics have catalyzed a renewed interest in the role of [K(+)](o) in epilepsy. The authors here connect classical and more recent insights on [K(+)]( o) dynamics in the cortex with the goal of providing starting points for a next generation of [K(+)](o) research. Such research may ultimately lead to an entirely new class of antiepileptic drugs that act on the [K(+)](o) regulation system.

Cajal-Retzius cells fail to trigger the developmental expression of the Cl- extruding co-transporter KCC2

Cajal-Retzius (CR) cells are transient neurons of the developing cerebral cortex that play a pivotal role in the lamination and construction of neural circuits. One physiological feature of CR cells is the failure to switch GABAergic transmission from excitation to inhibition. To examine the mechanisms underlying the persistence of the depolarizing action of GABA we analyzed the mRNA expression of the K+/Cl- co-transporter type 2 (KCC2) in mouse CR by in situ hybridization. During the second postnatal week, the developmentally regulated expression of KCC2 reached adult levels in most neurons of the cerebral cortex. Double labeling with the CR-cell marker calretinin and KCC2 in situ hybridization showed that CR cells were consistently devoid of KCC2 expression in several cortical areas such as neocortex and hippocampus. Since most cortical calretinin- and calbindin-containing non-CR neurons did express KCC2 mRNA, we conclude that CR cells specifically fail to trigger the developmental expression of the K+/Cl- co-transporter KCC2. These results suggest that absence of KCC2 preserves the depolarizing action of GABA in CR cells and support the notion that KCC2 is a key factor controlling Cl- homeostasis and preventing hyperexcitability.

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1–P4 neonatal rats and inhibitory in P9–P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

NKCC1 and KCC2 prevent hyperexcitability in the mouse hippocampus

During postnatal development of the central nervous system (CNS), the response of GABA(A) receptors to its agonist undergoes maturation from depolarizing to hyperpolarizing. This switch in polarity is due to the developmental decrease of the intracellular Cl concentration in neurons. Here we show that absence of NKCC1 in P9-P13 CA3 pyramidal neurons, through genetic manipulation or through bumetanide inhibition, results in a significant increase in cell excitability. Furthermore, the pro-convulsant agent 4-aminopyridine induces seizure-like events in NKCC1-null mice but not in wild-type mice. Measurements of muscimol responses in the presence and absence of NKCC1 shows that the Na-K-2Cl cotransporter only marginally affects intracellular Cl(-) in P9-P13 CA3 principal neurons. However, large increases in intracellular Cl(-) are observed in CA3 pyramidal neurons following increased hyperexcitability, indicating that P9-P13 CA3 pyramidal neurons lack robust mechanisms to regulate intracellular Cl(-) during high synaptic activity. This increase in the Cl(-) concentration is network-driven and activity-dependent, as it is blocked by the non-NMDA glutamate receptor antagonist DNQX. We also show that expression of the outward K-Cl cotransporter, KCC2, prevents the development of hyperexcitability, as a reduction of KCC2 expression by half results in increased susceptibility to seizure under control and 4-AP conditions.

Reduced potassium-chloride co-transporter expression in spinal cord dorsal horn neurons contributes to inflammatory pain hypersensitivity in rats

Cation chloride co-transporters are important determinants for the efficacy of inhibitory neurotransmission in the spinal cord and alterations in their expression levels contribute to allodynia and hyperalgesia associated with neuropathy. However, it remains unknown whether these co-transporters contribute to chronic inflammatory pain. We investigated the expression of potassium-chloride co-transporter 2 (KCC2) and sodium-potassium-chloride co-transporter 1 (NKCC1) in the rat spinal cord after peripheral inflammation induced by complete Freund's adjuvant (CFA) injection. Our results suggest that the expression of KCC2, but not that of NKCC1, was significantly reduced in CFA-injected rats. We also found that blockade of endogenous brain-derived neurotrophic factor-tyrosine receptor kinase B pathway inhibited the inflammation-induced KCC2 downregulation. Moreover, intrathecal injection of KCC2 antisense oligodeoxynucleotides into naïve rats reduced KCC2 expression in the spinal cord, leading to behavioral hypersensitivity similar to the hyperalgesia induced by peripheral inflammation. Taken together, these results indicate that peripheral inflammation induces downregulation of KCC2 in the dorsal horn of the spinal cord, which may in turn facilitate the development and/or maintenance of chronic inflammatory pain. The data also support the notion that disinhibition in the spinal cord is a general feature of inflammatory and neuropathic pain conditions, and suggest new therapeutic intervention.

Chloride channels as drug targets

Chloride channels represent a relatively under-explored target class for drug discovery as elucidation of their identity and physiological roles has lagged behind that of many other drug targets. Chloride channels are involved in a wide range of biological functions, including epithelial fluid secretion, cell-volume regulation, neuroexcitation, smooth-muscle contraction and acidification of intracellular organelles. Mutations in several chloride channels cause human diseases, including cystic fibrosis, macular degeneration, myotonia, kidney stones, renal salt wasting and hyperekplexia. Chloride-channel modulators have potential applications in the treatment of some of these disorders, as well as in secretory diarrhoeas, polycystic kidney disease, osteoporosis and hypertension. Modulators of GABA(A) (gamma-aminobutyric acid A) receptor chloride channels are in clinical use and several small-molecule chloride-channel modulators are in preclinical development and clinical trials. Here, we discuss the broad opportunities that remain in chloride-channel-based drug discovery.

Slow GABA(A) mediated synaptic transmission in rat visual cortex

BACKGROUND

Previous reports of inhibition in the neocortex suggest that inhibition is mediated predominantly through GABA(A) receptors exhibiting fast kinetics. Within the hippocampus, it has been shown that GABA(A) responses can take the form of either fast or slow response kinetics. Our findings indicate, for the first time, that the neocortex displays synaptic responses with slow GABA(A) receptor mediated inhibitory postsynaptic currents (IPSCs). These IPSCs are kinetically and pharmacologically similar to responses found in the hippocampus, although the anatomical specificity of evoked responses is unique from hippocampus. Spontaneous slow GABA(A) IPSCs were recorded from both pyramidal and inhibitory neurons in rat visual cortex.

RESULTS

GABA(A) slow IPSCs were significantly different from fast responses with respect to rise times and decay time constants, but not amplitudes. Spontaneously occurring GABA(A) slow IPSCs were nearly 100 times less frequent than fast sIPSCs and both were completely abolished by the chloride channel blocker, picrotoxin. The GABA(A) subunit-specific antagonist, furosemide, depressed spontaneous and evoked GABA(A) fast IPSCs, but not slow GABA(A)-mediated IPSCs. Anatomical specificity was evident using minimal stimulation: IPSCs with slow kinetics were evoked predominantly through stimulation of layer 1/2 apical dendritic zones of layer 4 pyramidal neurons and across their basal dendrites, while GABA(A) fast IPSCs were evoked through stimulation throughout the dendritic arborization. Many evoked IPSCs were also composed of a combination of fast and slow IPSC components.

CONCLUSION

GABA(A) slow IPSCs displayed durations that were approximately 4 fold longer than typical GABA(A)fast IPSCs, but shorter than GABA(B)-mediated inhibition. The anatomical and pharmacological specificity of evoked slow IPSCs suggests a unique origin of synaptic input. Incorporating GABA(A) slow IPSCs into computational models of cortical function will help improve our understanding of cortical information processing.

Cellular and network mechanisms of electrographic seizures

Epileptic seizures constitute a complex multiscale phenomenon that is characterized by synchronized hyperexcitation of neurons in neuronal networks. Recent progress in understanding pathological seizure dynamics provides crucial insights into underlying mechanisms and possible new avenues for the development of novel treatment modalities. Here we review some recent work that combines in vivo experiments and computational modeling to unravel the pathophysiology of seizures of cortical origin. We particularly focus on how activity-dependent changes in extracellular potassium concentration affects the intrinsic dynamics of neurons involved in cortical seizures characterized by spike/wave complexes and fast runs.

Evidence for an extended duration of GABA-mediated excitation in the developing male versus female hippocampus

Gamma-aminobutyric acid (GABA) is as an excitatory neurotransmitter during brain development. Activation of GABA(A) receptors in neonatal rat hippocampus results in chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels. As development progresses, there is a decline in the magnitude of calcium influx subsequent to GABA(A) receptor activation and the number of cells that respond to GABA with excitation. By the second postnatal week in the rat, GABA action in the hippocampus is predominantly inhibitory. The functional consequences and endogenous regulation of developmental GABA-mediated excitation remains under-explored. Hippocampal neurons in the newborn male and female rat respond to GABA(A) receptor activation with increased intracellular calcium and are susceptible to GABA-mediated damage -- both being indicative of the excitatory nature of GABA. In the present study we observed that by postnatal day 7, only males are susceptible to GABA(A) agonist-induced damage and respond to GABA(A) agonist administration with elevated levels of intracellular calcium in cultured hippocampal neurons. By postnatal day 14, GABA(A) agonist administration was without effect on intracellular calcium in both males and females. The age-related sex difference in the impact of GABA(A) receptor activation correlates with a sex difference in chloride co-transporter expression. Males have elevated protein levels of pNKCC1 on PN0 and PN7, with no sex difference by PN14. In contrast, females displayed elevated levels of KCC2 on PN7. This converging evidence infers that sex affects the duration of GABA(A) receptor-mediated excitation during normal hippocampal development, and provides a mechanism by which the effect is mediated.

Zinc Enhances the Inhibitory Effects of Strychnine-Sensitive Glycine Receptors in Mouse Hippocampal Neurons

Although extracellular Zn2+ is an endogenous biphasic modulator of strychnine-sensitive glycine receptors (GlyRs), the physiological significance of this modulation remains poorly understood. Zn2+ modulation of GlyR may be especially important in the hippocampus where presynaptic Zn2+ is abundant. Using cultured embryonic mouse hippocampal neurons, we examined whether 1 μM Zn2+, a potentiating concentration, enhances the inhibitory effects of GlyRs activated by sustained glycine applications. Sustained 20 μM glycine (EC25) applications alone did not decrease the number of action potentials evoked by depolarizing steps, but they did in 1 μM Zn2+. At least part of this effect resulted from Zn2+ enhancing the GlyR-induced decrease in input resistance. Sustained 20 μM glycine applications alone did not alter neuronal bursting, a form of hyperexcitability induced by omitting extracellular Mg2+. However, sustained 20 μM glycine applications depressed neuronal bursting in 1 μM Zn2+. Zn2+ did not enhance the inhibitory effects of sustained 60 μM glycine (EC70) applications in these paradigms. These results suggest that tonic GlyR activation could decrease neuronal excitability. To test this possibility, we examined the effect of the GlyR antagonist strychnine and the Zn2+ chelator tricine on action potential firing by CA1 pyramidal neurons in mouse hippocampal slices. Co-applying strychnine and tricine slightly but significantly increased the number of action potentials fired during a depolarizing current step and decreased the rheobase for action potential firing. Thus Zn2+ may modulate neuronal excitability normally and in pathological conditions such as seizures by potentiating GlyRs tonically activated by low agonist concentrations.

In vitro and in vivo measures of evoked excitatory and inhibitory conductance dynamics in sensory cortices

In order to better understand the synaptic nature of the integration process operated by cortical neurons during sensory processing, it is necessary to devise quantitative methods which allow one to infer the level of conductance change evoked by the sensory stimulation and, consequently, the dynamics of the balance between excitation and inhibition. Such detailed measurements are required to characterize the static versus dynamic nature of the non-linear interactions triggered at the single cell level by sensory stimulus. This paper primarily reviews experimental data from our laboratory based on direct conductance measurements during whole-cell patch clamp recordings in two experimental preparations: (1) in vitro, during electrical stimulation in the visual cortex of the rat and (2) in vivo, during visual stimulation, in the primary visual cortex of the anaesthetized cat. Both studies demonstrate that shunting inhibition is expressed as well in vivo as in vitro. Our in vivo data reveals that a high level of diversity is observed in the degree of interaction (from linear to non-linear) and in the temporal interplay (from push-pull to synchronous) between stimulus-driven excitation (E) and inhibition (I). A detailed analysis of the E/I balance during evoked spike activity further shows that the firing strength results from a simultaneous decrease of evoked inhibition and increase of excitation. Secondary, the paper overviews the various computational methods used in the literature to assess conductance dynamics, measured in current clamp as well as in voltage clamp in different neocortical areas and species, and discuss the consistency of their estimations.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl(-) efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl(-) against an electrochemical gradient. In isolated olfactory receptor neurons, the Na(+)-K(+)-2Cl(-) cotransporter NKCC1 is essential for Cl(-) accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl(-) efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl(-) accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl(-) transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl(-) transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl(-) transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl(-)/HCO(3)(-) exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na(+)-Cl(-) cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl(-)/HCO(3)(-) exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl(-) component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl(-) accumulation in olfactory receptor neurons.

NKCC1 does not accumulate chloride in developing retinal neurons

GABA excites immature neurons due to their relatively high intracellular chloride concentration. This initial high concentration is commonly attributed to the ubiquitous chloride cotransporter NKCC1, which uses a sodium gradient to accumulate chloride. Here we tested this hypothesis in immature retinal amacrine and ganglion cells. Western blotting detected NKCC1 at birth and its expression first increased, then decreased to the adult level. Immunocytochemistry confirmed this early expression of NKCC1 and localized it to all nuclear layers. In the ganglion cell layer, staining peaked at P4 and then decreased with age, becoming undetectable in adult. In comparison, KCC2, the chloride extruder, steadily increased with age localizing primarily to the synaptic layers. For functional tests, we used calcium imaging with fura-2 and chloride imaging with 6-methoxy-N-ethylquinolinium iodide. If NKCC1 accumulates chloride in ganglion and amacrine cells, deleting or blocking it should abolish the GABA-evoked calcium rise. However, at P0-5 GABA consistently evoked a calcium rise that was not abolished in the NKCC1-null retinas, nor by applying high concentrations of bumetanide (NKCC blocker) for long periods. Furthermore, intracellular chloride concentration in amacrine and ganglion cells of the NKCC1-null retinas was approximately 30 mM, same as in wild type at this age. This concentration was not lowered by applying bumetanide or by decreasing extracellular sodium concentration. Costaining for NKCC1 and cellular markers suggested that at P3, NKCC1 is restricted to Müller cells. We conclude that NKCC1 does not serve to accumulate chloride in immature retinal neurons, but it may enable Müller cells to buffer extracellular chloride.

Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity

It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.

Shunting and hyperpolarizing GABAergic inhibition in the high-potassium model of ictogenesis in the developing rat hippocampus

Ontogenesis of GABAergic signaling may play an important role in developmental changes in seizure susceptibility in the high-potassium model of ictogenesis in vitro. The age-dependent effects of [K(+)](o) on the reversal potential of the GABA(A)-mediated responses and membrane potential in hippocampal slices in vitro were compared with the effect of GABA(A)-receptors antagonists and GABA(A) modulators on high-potassium induced seizures in the CA3 pyramidal layer of rat hippocampus in vivo. GABA(A) responses were depolarizing at P8-12 and hyperpolarizing at P17-21. In P8-12 rats, GABA(A) responses switch their polarity from depolarizing to hyperpolarizing upon elevation of extracellular potassium. At approximately 10 mM [K(+)](o), activation of GABA(A) receptors produced an isoelectric, purely shunting response characterized by no changes in the membrane potential but an increase in the membrane conductance. In P17-21 rats, the hyperpolarizing GABA(A) driving force progressively increased with elevation of [K(+)](o). In P8-12 rats in vivo, GABA(A)-receptor antagonists did not affect the occurrence of ictal discharges induced by intrahippocampal injection of 10 mM [K(+)](o), but significantly increased seizure duration. Diazepam and isoguvacine completely prevented seizures induced by 10 mM [K(+)](o). In P17-21 rats, GABA(A)-receptor antagonists strongly increased the occurrence of ictal activity induced both by 10 mM [K(+)](o). Taken together, these results suggest that anticonvulsive effects of GABA are because of the combination of shunting and hyperpolarizing actions of GABA. Although shunting GABA is already efficient in the young age group, a developmental increase in the hyperpolarizing GABA(A) driving force likely contributes to the increase in the GABAergic control of seizures upon maturation.

Theta-bursts induce a shift in reversal potentials for GABA-A receptor-mediated postsynaptic currents in rat hippocampal CA1 neurons

Theta-burst stimulation of the stratum radiatum induces a negative shift in the reversal potential (RP) of gamma-aminobutyric acid (GABA)-ergic postsynaptic currents (PSCs) in hippocampal CA1 neurons in brain slices from rats of age groups 3-4 days, 6-9 days and 3-4 weeks. Furosemide reversed the shift in the RP. The amplitude of the evoked PSC appeared to increase following the theta-burst stimulation but this increase was secondary to the change in the RP. These results indicate that the RP for GABA-ergic PSCs undergoes an activity-dependent plasticity in not only neonatal but also adult neurons presumably through an up-regulation of a K(+)-Cl(-) co-transporter. This plasticity can have significant implications for neuronal network activity in the central nervous system. Furthermore, these results indicate that studies on GABA-ergic synaptic efficacy require a careful, parallel monitoring of the RP.

GABAergic control of substantia nigra dopaminergic neurons

At least 70% of the afferents to substantia nigra dopaminergic neurons are GABAergic. The vast majority of these arise from the neostriatum, the external globus pallidus and the substantia nigra pars reticulata. Nigral dopaminergic neurons express both GABA(A) and GABA(B) receptors, and are inhibited by local application of GABA(A) or GABA(B) agonists in vivo and in vitro. However, in vivo, synaptic responses elicited by stimulation of neostriatal or pallidal afferents, or antidromic activation of nigral pars reticulata GABAergic projection neurons are mediated predominantly or exclusively by GABA(A) receptors. The clearest and most consistent role for the nigral GABA(B) receptor in vivo is as an inhibitory autoreceptor that presynaptically modulates GABA(A) synaptic responses that originate from all three principal GABAergic inputs. The firing pattern of dopaminergic neurons is also effectively modulated by GABAergic inputs in vivo. Local blockade of nigral GABA(A) receptors causes dopaminergic neurons to shift to a burst firing pattern regardless of the original firing pattern. This is accompanied by a modest increase in spontaneous firing rate. The GABAergic inputs from the axon collaterals of the pars reticulata projection neurons seem to be a particularly important source of a GABA(A) tone to the dopaminergic neurons, inhibition of which leads to burst firing. The globus pallidus exerts powerful control over the pars reticulata input, and through the latter, disynaptically over the dopaminergic neurons. Inhibition of pallidal output leads to a slight decrease in firing of the dopaminergic neurons due to disinhibition of the pars reticulata neurons whereas increased firing of pallidal neurons leads to burst firing in dopaminergic neurons that is associated with a modest increase in spontaneous firing rate and a significant increase in extracellular levels of dopamine in the neostriatum. The pallidal disynaptic disinhibitory control of the dopaminergic neurons dominates the monosynaptic inhibitory influence because of a differential sensitivity to GABA of the two nigral neuron types. Nigral GABAergic neurons are more sensitive to GABA(A)-mediated inhibition than dopaminergic neurons, in part due to a more hyperpolarized GABA(A) reversal potential. The more depolarized GABA(A) reversal potential in the dopaminergic neurons is due to the absence of KCC2, the chloride transporter responsible for setting up a hyperpolarizing Cl(-) gradient in most mature CNS neurons. The data reviewed in this chapter have made it increasingly clear that in addition to the effects that nigral GABAergic output neurons have on their target nuclei outside of the basal ganglia, local interactions between GABAergic projection neurons and dopaminergic neurons are crucially important to the functioning of the nigral dopaminergic neurons.

Regulation of KCC2 and NKCC during development: membrane insertion and differences between cell types

The developmental switch of GABA's action from excitation to inhibition is likely due to a change in intracellular chloride concentration from high to low. Here we determined if the GABA switch correlates with the developmental expression patterns of KCC2, the chloride extruder K+-Cl- cotransporter, and NKCC, the chloride accumulator Na+-K+-Cl- cotransporter. Immunoblots of ferret retina showed that KCC2 upregulated in an exponential manner similar to synaptophysin (a synaptic marker). In contrast, NKCC, which was initially expressed at a constant level, upregulated quickly between P14 and P28, and finally downregulated to an adult level that was greater than the initial phase. At the cellular level, immunocytochemistry showed that in the inner plexiform layer KCC2's density increased gradually and its localization within ganglion cells shifted from being primarily in the cytosol (between P1-13) to being in the plasma membrane (after P21). In the outer plexiform layer, KCC2 was detected as soon as this layer started to form and increased gradually. Interestingly, however, KCC2 was initially restricted to photoreceptor terminals, while in the adult it was restricted to bipolar dendrites. Thus, the overall KCC2 expression level in ferret retina increases with age, but the time course differs between cell types. In ganglion cells the upregulation of KCC2 by itself cannot explain the relatively fast switch in GABA's action; additional events, possibly KCC2's integration into the plasma membrane and downregulation of NKCC, might also contribute. In photoreceptors the transient expression of KCC2 suggests a role for this transporter in development.

Protein expression and mRNA cellular distribution of the NKCC1 cotransporter in the dorsal root and trigeminal ganglia of the rat

Primary afferent neurons maintain depolarizing responses to GABA into adulthood. The molecular basis for this GABAergic response appears to be the Na+K+2Cl- cotransporter NKCC1 that contributes to the maintenance of a high intracellular chloride concentration. Recently, a role for NKCC1 has been proposed in nociceptive processing which makes it timely to gain a better understanding of the distribution of NKCC1 in sensory ganglia. Here, we describe that, in the rat, NKCC1 mRNA is predominately expressed by small and medium diameter dorsal root (DRG) and trigeminal (TG) ganglion neurons. The colocalization of NKCC1 mRNA with sensory neuron population markers was assessed. In the DRG, many NKCC1 mRNA-expressing neurons colocalized peripherin (57.0+/-2.5%), calcitonin-gene-related peptide (CGRP, 39.2+/-4.4%) or TRPV1 immunoreactivity (50.0+/-1.9%) whereas only 8.7+/-1.2% were co-labeled with a marker for large diameter afferents (N52). Similarly, in the TG, NKCC1 mRNA-expressing neurons frequently colocalized peripherin (50.0+/-3.0%), CGRP (35.4+/-2.6%) or TRPV1 immunoreactivity (44.7+/-1.2%) while 14.8+/-1.3% were co-labeled with the N52 antibody. NKCC1 mRNA was also detected in satellite glial (SGCs) in both the DRG and TG. Colocalization of NKCC1 protein with the SGC marker NG2 confirmed the phenotype of these NKCC1-expressing glial cells. In contrast to in situ hybridization experiments, we did not observe NKCC1 immunoreactivity in primary afferent somata. These findings suggest that NKCC1 is expressed in anatomically appropriate cells in order to modulate GABAergic responses in nociceptive neurons. Moreover, these results suggest the possibility of a functional role of NKCC1 in the glial cells closely apposed to primary sensory afferents.

Neurosteroid modulation of respiratory rhythm in rats during the perinatal period

Neurosteroids regulate neuronal excitability and are expressed at particularly high levels in the CNS during the perinatal period. Further, neurosteroid levels are increased by a variety of stressors including hypoxia, asphyxia, parturition, ethanol exposure and infection. One mechanism by which neurosteroids regulate neuronal activity is by negative or positive modulation of GABA(A) receptor function. Perinatal respiration is strongly modulated by GABAergic synaptic drive, and GABA release is increased during hypoxia to contribute to hypoxia-induced depression of neonatal ventilation. Here, we use in vitro and in vivo rat models to test the hypothesis that GABA(A) receptor-mediated modulation of perinatal respiration is markedly influenced by the presence of neurosteroids. The principal finding of this study was that the efficacy of GABA(A) receptor-mediated modulation of respiratory membrane potential and rhythmogenesis is markedly enhanced by allopregnanolone and depressed by dehydroepiandrosterone sulphate. These data demonstrate that the modulation of breathing via GABA(A) receptor activation will be determined by the overall balance of negative and positive neurosteroid modulators within respiratory nuclei. This adds a level of complexity that must be considered when examining the depression of breathing in mammals associated with various behavioural states and pathogenic conditions such as apnoea and sudden death suspected to be associated with central respiratory dysfunction.

A C-terminal domain in KCC2 confers constitutive K+-Cl- cotransport

The neuron-specific K(+)-Cl(-) cotransporter KCC2 plays a crucial role in determining intracellular chloride activity and thus the neuronal response to gamma-aminobutyric acid and glycine. Of the four KCCs, KCC2 is unique in mediating constitutive K(+)-Cl(-) cotransport under isotonic conditions; the other three KCCs are exclusively swelling-activated, with no isotonic activity. We have utilized a series of chimeric cDNAs to localize the determinant of isotonic transport in KCC2. Two generations of chimeric KCC4-KCC2 cDNAs initially localized this characteristic to within a KCC2-specific expansion of the cytoplasmic C terminus, between residues 929 and 1043. This region of KCC2 is rich in prolines, serines, and charged residues and encompasses two predicted PEST sequences. Substitution of this region in KCC2 with the equivalent sequence of KCC4 resulted in a chimeric KCC that was devoid of isotonic activity, with intact swelling-activated transport. A third generation of chimeras demonstrated that a domain just distal to the PEST sequences confers isotonic transport on KCC4. Mutagenesis of this region revealed that residues 1021-1035 of KCC2 are sufficient for isotonic transport. Swelling-activated K(+)-Cl(-) cotransport is abrogated by calyculin A, whereas isotonic transport mediated by KCC chimeras and KCC2 is completely resistant to this serine-threonine phosphatase inhibitor. In summary, a 15-residue C-terminal domain in KCC2 is both necessary and sufficient for constitutive K(+)-Cl(-) cotransport under isotonic conditions. Furthermore, unlike swelling-activated transport, constitutive K(+)-Cl(-) cotransport mediated by KCC2 is completely independent of serine-threonine phosphatase activity, suggesting that these two modes of transport are activated by distinct mechanisms.

Brain-type creatine kinase activates neuron-specific K+-Cl- co-transporter KCC2

GABA, a major inhibitory neurotransmitter in the adult CNS, is excitatory at early developmental stages as a result of the elevated intracellular Cl- concentration ([Cl-]i). This functional switch is primarily attributable to the K+-Cl- co-transporter KCC2, the expression of which is developmentally regulated in neurons. Previously, we reported that KCC2 interacts with brain-type creatine kinase (CKB). To elucidate the functional significance of this interaction, HEK293 cells were transfected with KCC2 and glycine receptor alpha2 subunit, and gramicidin-perforated patch-clamp recordings were performed to measure the glycine reversal potential (Egly), giving an estimate of [Cl-]i. KCC2-expressing cells displayed the expected changes in Egly following alterations in the extracellular K+ concentration ([K+]o) or administration of an inhibitor of KCCs, suggesting that the KCC2 function was being properly assessed. When added into KCC2-expressing cells, dominant-negative CKB induced a depolarizing shift in Egly and reduced the hyperpolarizing shift in Egly seen in response to a lowering of [K+]o compared with wild-type CKB. Moreover, 2,4-dinitrofluorobenzene (DNFB), an inhibitor of CKs, shifted Egly in the depolarizing direction. In primary cortical neurons expressing CKB, the GABA reversal potential was also shifted in the depolarizing direction by DNFB. Our findings suggest that, in the cellular micro-environment, CKB activates the KCC2 function.

Depressed responses to applied and synaptically-released GABA in CA1 pyramidal cells, but not in CA1 interneurons, after transient forebrain ischemia

Transient cerebral ischemia kills CA1 pyramidal cells of the hippocampus, whereas most CA1 interneurons survive. It has been proposed that calcium-binding proteins, neurotrophins, and/or inhibitory neuropeptides protect interneurons from ischemia. However, different synaptic responses early after reperfusion could also underlie the relative vulnerabilities to ischemia of pyramidal cells and interneurons. In this study, we used gramicidin perforated patch recording in ex vivo slices to investigate gamma-aminobutyric acid (GABA) synaptic function in CA1 pyramidal cells and interneurons 4 h after a bilateral carotid occlusion accompanied by hypovolemic hypotension. At this survival time, the amplitudes of both miniature inhibitory postsynaptic currents (mIPSCs) and GABA-evoked currents were reduced in CA1 pyramidal cells, but not in CA1 interneurons. In addition, the mean rise time of mIPSCs was reduced in pyramidal cells. The reversal potential for the GABA current (E(GABA)) did not shift toward depolarizing values in either cell type, indicating that the driving force for chloride was unchanged at this survival time. We conclude that early during reperfusion GABAergic neurotransmission is attenuated exclusively in pyramidal neurons. This is likely explained by reduced GABAA receptor sensitivity or clustering and possibly also reduced GABA release, rather than by an elevation of intracellular chloride. Impaired GABA function may contribute to ischemic neuronal death by enhancing the excitability of CA1 pyramidal cells and facilitating N-methyl-D-aspartic acid channel opening. Therefore, normalizing GABAergic function might be a useful pharmacological approach to counter excessive, and potentially excitotoxic, glutamatergic activity during the postischemic period.

Theta bursts set up glutamatergic as well as GABA-ergic plasticity in neonatal rat hippocampal CA1 neurons

gamma-Aminobutyric acid (GABA) is inhibitory in adult, but excitatory in neonatal, neurons. The switch from excitatory to inhibitory action is due to a negative shift in the equilibrium potential for the GABA(A) receptor-mediated postsynaptic current (E(GABA-PSC)). Here, we report that, in neonatal rat hippocampal CA1 neurons, presynaptic theta-burst activation induces not only a shift in E(GABA-PSC) towards that in adult neurons, but also a recruitment of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor-mediated postsynaptic currents.

Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat

The median preoptic nucleus (MnPO) is an integrative structure of the hypothalamus receiving periphery-derived information pertinent to hydromineral and cardiovascular homeostasis. In this context, excitability of MnPO neurones is controlled by fast GABAergic, glutamatergic and angiotensinergic projection from the subfornical organ (SFO). Taking advantage of a brain slice preparation preserving synaptic connection between the SFO and the MnPO, and appropriate bicarbonate-free artificial cerebrospinal fluid (CSF), we investigated a possible implication of an active outward Cl- transport in regulating efficacy of the GABA(A) receptor-mediated inhibitory response at the SFO-MnPO synapse. When somata of the MnPO neurones was loaded with 18 mm chloride, stimulation of the SFO evoked outward inhibitory postsynaptic currents (IPSCs) in 81% of the MnPO neurones held at -60 mV. Accordingly, E(IPSC) was found 25 mV hyperpolarized from the theoretical value calculated from the Nernst equation, indicating that IPSC polarity and amplitude were driven by an active Cl- extrusion system in these neurones. E(IPSC) estimated with gramicidin-based perforated-patch recordings amounted -89.2 +/- 4.3 mV. Furosemide (100 microm), a pharmacological compound known to block the activity of the neurone-specific K(+)-Cl- cotransporter, KCC2, reversed IPSC polarity and shifted E(IPSC) towards its theoretical value. Presence of the KCC2 protein in the MnPO was further detected with immunohistochemistry, revealing a dense network of KCC2-positive intermingled fibres. In the presence of a GABA(B) receptor antagonist, high-frequency stimulation (5 Hz) of the SFO evoked a train of IPSCs or inhibitory postsynaptic potentials (IPSPs), whose amplitude was maintained throughout the sustained stimulation. Contrastingly, similar 5 Hz stimulation carried out in the presence of furosemide (50 microm) evoked IPSCs/IPSPs, whose amplitude collapsed during the high-frequency stimulation. Similar reduction in inhibitory neurotransmission was also observed in MnPO neurones lacking the functional Cl- extrusion mechanism. We conclude that a majority of MnPO neurones were characterized by a functional Cl- transporter that ensured an efficient activity-dependent Cl- transport rate, allowing sustained synaptic inhibition of these neurones. Pharmacological and anatomical data strongly suggested the involvement of KCC2, as an essential postsynaptic determinant of the inhibitory neurotransmission afferent to the MnPO, a key-structure in the physiology of the hydromineral and cardiovascular homeostasis.

Endogenous acetylcholine enhances synchronized interneuron activity in rat neocortex

Application of 4-aminopyridine (4-AP) along with EAA) receptor antagonists produces gamma-aminobutyric acid (GABAA) receptor-dependent synchronized activity in interneurons. This results in waves of activity propagating through upper cortical layers. Because interneurons in the neocortex are excited by nicotinic acetylcholine receptor (nAChR) agonists, ACh may influence synchronization of these local neocortical interneuronal networks. To study this possibility, we have used voltage-sensitive dye imaging using the fluorescent dye RH 414 (30 microM) in rat neocortical slices. Recordings were obtained in the presence of 4-AP (100 microM) and the EAA receptor antagonists D-2-amino-5-phosphonvaleric acid (20 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM). In response to intracortical stimulation, localized or propagated activity restricted to upper cortical layers was seen. Bath application of the ACh esterase inhibitor neostigmine (10 microM) and the nAChR agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 10 microM) increased the response amplitude, the extent of spread, and the duration of this activity. These changes were seen in 13 of 16 slices tested with neostigmine (10 microM) and 4 of 7 slices tested with DMPP (10 microM). Application of the muscarinic AChR antagonist atropine (1 microM) did not block the enhancement of activity by neostigmine (n = 7). Application of dihydro-beta-erythroidine (10 microM), known, at this concentration, to selectively antagonize alpha4beta2-like nAChRs, blocked the effect of neostigmine (n = 5). The selective alpha7-like nAChR antagonist methyllycaconitine (50 nM) was ineffective (n = 5). These results suggest that activation of alpha4beta2-like nAChRs by endogenously released ACh can enhance synchronized activity in local neocortical inhibitory networks.

Activity-mediated shift in reversal potential of GABA-ergic synaptic currents in immature neurons

Gamma-aminobutyric acid (GABA), which is inhibitory in the adult central nervous system, can be excitatory in the developing brain. The change from excitatory to inhibitory action of GABA during development is caused by a negative shift in its reversal potential. Here, we report a presynaptic activity-mediated negative shift in the reversal potential of the GABA-mediated synaptic currents in immature deep cerebellar nuclei neurons. This shift appears to be due to an increased expression and activation of the K+-Cl- co-transporter type 2 (KCC-2) through the activation of protein kinase A, protein synthesis and activation of protein phosphatases. Thus, maturation of the GABA response may rely on an activity-dependent up-regulation of KCC-2.