The K+/Cl- co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation

Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family

The electroneutral cation-chloride-coupled cotransporter gene family ( SLC12) was identified initially at the molecular level in fish and then in mammals. This nine-member gene family encompasses two major branches, one including two bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporters and the thiazide-sensitive Na(+):Cl(-) cotransporter. Two of the genes in this branch ( SLC12A1 and SLC12A3), exhibit kidney-specific expression and function in renal salt reabsorption, whereas the third gene ( SLC12A2) is expressed ubiquitously and plays a key role in epithelial salt secretion and cell volume regulation. The functional characterization of both alternatively-spliced mammalian Na(+)-K(+)-2Cl(-) cotransporter isoforms and orthologs from distantly related species has generated important structure-function data. The second branch includes four genes ( SLC12A4- 7) encoding electroneutral K(+)-Cl(-) cotransporters. The relative expression level of the neuron-specific SLC12A5 and the Na(+)-K(+)-2Cl(-) cotransporter SLC12A2 appears to determine whether neurons respond to GABA with a depolarizing, excitatory response or with a hyperpolarizing, inhibitory response. The four K(+)-Cl(-) cotransporter genes are co-expressed to varying degrees in most tissues, with further roles in cell volume regulation, transepithelial salt transport, hearing, and function of the peripheral nervous system. The transported substrates of the remaining two SLC12 family members, SLC12A8 and SLC12A9, are as yet unknown. Inactivating mutations in three members of the SLC12 gene family result in Mendelian disease; Bartter syndrome type I in the case of SLC12A1, Gitelman syndrome for SLC12A3, and peripheral neuropathy in the case of SLC12A6. In addition, knockout mice for many members of this family have generated important new information regarding their respective physiological roles.

Loss of K-Cl co-transporter KCC3 causes deafness, neurodegeneration and reduced seizure threshold

K-Cl co-transporters are encoded by four homologous genes and may have roles in transepithelial transport and in the regulation of cell volume and cytoplasmic chloride. KCC3, an isoform mutated in the human Anderman syndrome, is expressed in brain, epithelia and other tissues. To investigate the physiological functions of KCC3, we disrupted its gene in mice. This severely impaired cell volume regulation as assessed in renal tubules and neurons, and moderately raised intraneuronal Cl(-) concentration. Kcc3(-/-) mice showed severe motor abnormalities correlating with a progressive neurodegeneration in the peripheral and CNS. Although no spontaneous seizures were observed, Kcc3(-/-) mice displayed reduced seizure threshold and spike-wave complexes on electrocorticograms. These resembled EEG abnormalities in patients with Anderman syndrome. Kcc3(-/-) mice also displayed arterial hypertension and a slowly progressive deafness. KCC3 was expressed in many, but not all cells of the inner ear K(+) recycling pathway. These cells slowly degenerated, as did sensory hair cells. The present mouse model has revealed important cellular and systemic functions of KCC3 and is highly relevant for Anderman syndrome.

Homogenous glycine receptor expression in cortical plate neurons and cajal-retzius cells of neonatal rat cerebral cortex

Glycinergic membrane responses have been described in cortical plate neurons (CPn) and Cajal-Retzius cells (CRc) during early neocortical development. In order to elucidate the functional properties and molecular identity of glycine receptors in these two neuronal cell types, we performed whole-cell patch-clamp recordings and subsequent single-cell multiplex reverse transcriptase-polymerase chain reaction (RT-PCR) analyses on visually identified neurons in tangential and coronal slices as well as in situ hybridizations of coronal slices from neonatal rat cerebral cortex (postnatal days 0-4). In both CPn and CRc the glycinergic agonists glycine, beta-alanine and taurine induced inward currents with larger current densities in CRc. The functional properties of these currents were similar between CPn and CRc. In both cell types the glycine receptor showed a higher affinity for glycine than for the glycinergic agonists beta-alanine and taurine. The glycinergic responses of both cells were blocked by the glycinergic antagonist strychnine and were unaffected by the GABAergic antagonist bicuculline (100 microM), the N-methyl-D-aspartic acid receptor antagonist (+/-)-2-amino-5-phosphonopentatonic acid (60 microM) and by picrotoxin (30 microM), an antagonist of alpha homomeric glycine receptors. Single-cell multiplex RT-PCR revealed the expression of glycine receptor alpha(2) and beta subunits in CPn and CRc, while no alpha(1) and alpha(3) subunits were observed. In situ hybridization histochemistry showed the expression of mRNAs for alpha(2) and beta subunits within the cortical plate and in large neurons of the marginal zone, while there were no signals for alpha(1) and alpha(3) subunits. In summary, these results suggest that CPn and CRc express glycine receptors with similar functional and pharmacological properties. The correlation of pharmacological properties and mRNA expression suggests that the glycine receptors in both cell types may consist of alpha(2)/beta heteromeric receptors.

Taurine activates strychnine-sensitive glycine receptors in neurons freshly isolated from nucleus accumbens of young rats

Although functional glycine receptors (GlyRs) are present in the mature nucleus accumbens (NAcc), an important area of the mesolimbic dopamine system involved in drug addiction, their role has been unclear because the NAcc contains little glycine. However, taurine, an agonist of GlyRs, is abundant throughout the brain, especially during early development. In the present study on freshly dissociated NAcc neurons from young Sprague-Dawley rats (12- to 21-day old), we found that both glycine and taurine can strongly depolarize NAcc neurons and modulate their excitability. In voltage-clamped NAcc neurons, glycine and taurine elicited chloride currents (IGly and ITau) with an EC50 of 0.12 and 1.25 mM, respectively. The reversal potential of IGly or ITau was 0 mV in conventional whole cell mode and -30 mV in gramicidin-perforated mode. At concentrations <1 mM, both glycine and taurine were very effectively antagonized by strychnine and by picrotoxin (with an IC50 of 60 nM and 36.5 microM for IGly, and 40 nM and 42.2 microM for ITau) but were insensitive to 10 microM bicuculline. The currents elicited by taurine (< or =1 mM) showed complete cross-desensitization with IGly, but none with gamma-aminobutyric acid (GABA)-induced currents (IGABA). However, ITau elicited by very concentrated taurine (10 mM) showed partial cross-desensitization with IGABA, and it was substantially antagonized by 10 microM bicuculline. These results indicate that taurine binds mainly to GlyRs in NAcc, but it could be a partial agonist of GABAA receptors. By activating GlyRs, taurine may play an important physiological role in the control of NAcc function, especially during development.

Localization of the kcc4 potassium–chloride cotransporter in the nervous system

Potassium-chloride cotransporters (KCCs) collectively play a crucial role in the function and development of both the peripheral and central nervous systems. KCC4 is perhaps the least abundant KCC in the adult mammalian brain, where its localization is unknown. In the embryonic brain, KCC4 mRNA is found in the periventricular zone, cranial nerves and choroid plexus [Eur J Neurosci 16 (2002) 2358]. To investigate the distribution of KCC4 protein in the nervous system we developed a rabbit polyclonal antibody directed against a short N-terminal peptide. Western blot analysis of brain microsomal protein using purified antibody revealed the presence of a band at approximately 145 kDa, consistent with the size of a glycosylated K-Cl cotransporter. Western blot analysis of brain, spinal cord and peripheral nerves revealed high expression levels in peripheral nerves and spinal cord, with low levels in whole brain. Within the brain, the cerebral cortex, hippocampus, and cerebellum revealed minimal KCC4 expression, whereas midbrain and brainstem demonstrated higher levels. In the adult mouse brain, KCC4 staining was observed on the apical membrane of choroid plexus epithelial cells as well as in cranial nerves. All other brain structures, e.g. cortex, hippocampus, cerebellum showed no KCC4 immunoreactivity, suggesting very low or absent expression of the cotransporter in these regions. Co-staining of KCC4 with anti-MAP2, GFAP and CNPase revealed that KCC4 is expressed in peripheral neurons. Thus, KCC4 is expressed on the apical membrane of the choroid plexus, where it likely participates to K(+) reabsorption. KCC4 is also expressed in peripheral neurons, where its function remains to be determined.

Expression of the KCl cotransporter KCC2 parallels neuronal maturation and the emergence of low intracellular chloride

Fast synaptic inhibition in the adult central nervous system (CNS) is mediated by GABA and glycine. During early development GABA acts as an excitatory neurotransmitter, which is deemed to be important for the maturation of the CNS. During development GABAergic responses undergo a switch from excitatory to inhibitory. This switch is correlated with upregulation of KCC2, the neuronal isoform of the potassium-chloride cotransporter family. KCC2 lowers the intraneuronal chloride concentration below its electrochemical equilibrium. KCC2 activity is thought to depend on phosphorylation by endogenous tyrosine kinases. Here, we analyzed the expression pattern of KCC2 during murine embryonic and postnatal development by in situ hybridization and Western blot analysis. KCC2 expression paralleled neuronal differentiation and preceded the decline of the GABA reversal potential (EGABA) in spinal cord motoneurons and hippocampal pyramidal cells. The adult inhibitory response to GABA was established earlier in the spinal cord than in the hippocampus. Phosphorylated KCC2 protein was already present early in development when the functional GABA switch had not yet occurred. Thus, tyrosine-phosphorylation seems to be less important than the transcriptional upregulation of KCC2.

The effects of seizures on the connectivity and circuitry of the developing brain

Recurring seizures in infants and children are often associated with cognitive deficits, but the reason for the learning difficulties is unclear. Recent studies in several animal models suggest that seizures themselves may contribute in important ways to these deficits. Other studies in animals have shown that recurring seizures result in dendritic spine loss. This change, coupled with a down-regulation in NMDA receptor subunit expression, suggests that repetitive seizures may interrupt the normal development of glutamatergic synaptic transmission. We hypothesize that homeostatic, neuroprotective processes are induced by recurring early-life seizures. These processes, by diminishing glutamatergic synaptic transmission, are aimed at preventing the continuation of seizures. However, by preventing the normal development of glutamatergic synapses, and particularly NMDA receptor-mediated synaptic transmission, such homeostatic processes also reduce synaptic plasticity and diminish the ability of neuronal circuits to learn and store memories.

Developmental changes in rhythmic spinal neuronal activity in the rat fetus

In the developing rat spinal cord, formation and differentiation of the central pattern generator for locomotion occur during the prenatal period. Early on, excitatory synaptic transmission mediated by glycine receptors plays a leading role for rhythmogenesis, at a later stage, followed by glutamate-receptor-mediated synaptic transmission becoming dominant. The maturation of inhibitory circuitry in the spinal cord, mediated largely by glycinergic synapses, is crucial for the generation of alternating activity between left/right limbs and flexor/extensor muscles. Formation of left/right alternation is presumably due to developmental changes in the properties of the postsynaptic neurons, themselves, whereas flexor/extensor alternation requires the additional emergence of inhibitory synaptic functions in the spinal cord.

Interneuron Diversity series: Hippocampal interneuron classifications--making things as simple as possible, not simpler

The nervous system is made up of many specific types of neuron intricately intertwined to form complex networks. Identifying and defining the characteristic features of the many different neuronal types is essential for achieving a cellular understanding of complex activity from perception to cognition. So far, cortical GABAergic interneurons have represented the epitome of cellular diversity in the CNS. Despite the desperate need for effective classification criteria allowing a common language among neuroscientists, interneurons still evoke memories of Babel. Several approaches are now available to overcome the challenges and problems associated with the various classification systems used so far.

Role of Cl- co-transporters in the excitation produced by GABAA receptors in juvenile bovine adrenal chromaffin cells

GABA is the primary inhibitory neurotransmitter in the adult mammalian brain. However, in neonatal animals, activation of Cl(-)-permeable GABA receptors is excitatory and appears to depend on the expression of a Na(+)-K(+)-2Cl- cotransporter (NKCC) that elevates intracellular Cl- levels, leading to a depolarized Cl- equilibrium potential (ECl). The change from excitation to inhibition appears to involve the expression of the K+/Cl- co-transporter, KCC2, which lowers intracellular Cl- levels resulting in a hyperpolarized ECl. In this study, we show that bovine chromaffin cells from 4- to 5-mo-old animals are excited by GABA. Activation of GABAA receptors depolarizes the cells, opens voltage-dependent Ca2+ channels, elevates [Ca2+]i, and promotes the release of catecholamines. Blockade of voltage-dependent Ca2+ channels prevents the elevation of [Ca2+]i by GABA. The extrapolated anion reversal potential in these cells is approximately -28 mV, indicating a resting intracellular anion concentration of approximately 50 mM. Expression of KCC2 protein was not detected in the juvenile chromaffin cells. In contrast, clear expression of NKCC1 was observed. Blockade of NKCC1 should reduce the intracellular Cl- concentration and hyperpolarize ECl. Bumetanide, an NKCC1 blocker, reduced the elevation of [Ca2+]i by GABA. In some cells, activation of GABAA receptors inhibits responses to excitatory neurotransmitters, even though GABA itself is depolarizing. Co-activation of cholinergic and GABAA receptors in chromaffin cells produced elevations in [Ca2+]i that were comparable to those produced by cholinergic receptors alone. Our data showing the selective expression of chloride co-transporters and the resulting strongly depolarized anion reversal potential may help explain how activation of GABAA receptors causes sufficient excitation to elicit catecholamine release from chromaffin cells.

Role of sex hormones in the sexually dimorphic expression of KCC2 in rat substantia nigra

KCC2 is a neuronal-specific potassium chloride cotransporter. The level of KCC2 expression is a factor determining whether GABA(A) receptor agonists depolarize or hyperpolarize neurons. Substantia nigra reticulata (SNR) neurons of male postnatal day 15 (PN15) rats have low KCC2 mRNA expression and respond to GABA(A) receptor activation with depolarization and activation of calcium-regulated gene expression. Female PN15 SNR neurons have high KCC2 mRNA expression and GABA(A) receptor agonists cannot activate calcium-dependent signaling processes. We investigate whether sex hormones regulate KCC2 mRNA expression in PN15 rat SNR. Using in situ hybridization, we studied the effects of acute (4 h) or prolonged (52 h) subcutaneous (s.c.) administration of testosterone (100 microg), dihydrotestosterone (180 microg) or 17beta-estradiol benzoate (5 microg) on KCC2 mRNA expression in male and female PN15 rat SNR. Different doses of estradiol (1 and 10 microg s.c., 4 h) were also acutely administered in female PN15 rats. Controls received oil injections. Separate groups of PN15 male rats were pretreated with antagonists of L-type voltage-sensitive calcium channels (L-VSCCs) [nifedipine, 100 mg/kg s.c.] or GABA(A) receptors [bicuculline, 2 mg/kg intraperitoneally (i.p.)] or their vehicles, 30 min before estradiol (5 microg s.c., 4 h). Testosterone and dihydrotestosterone upregulated KCC2 mRNA in both sexes. Estradiol downregulated KCC2 mRNA in males but not in females. Both acute and prolonged hormonal administration had similar effects. In male PN15 SNR, nifedipine and bicuculline decreased KCC2 mRNA acutely and prevented further downregulation of KCC2 mRNA by estradiol. Estradiol therefore downregulates KCC2 mRNA in male PN15 SNR, by interacting with the GABA(A) receptor and L-VSCC signaling pathway.

Developmental up-regulation of KCC2 in the absence of GABAergic and glutamatergic transmission

Postsynaptic gamma-aminobutyric acid (GABA)A-mediated responses switch from depolarizing to hyperpolarizing during postnatal development of the rodent hippocampus. This is attributable to a decrease in the concentration of intracellular chloride set by the expression of the neuron-specific K+-Cl- co-transporter, KCC2. A recent in vitro study [Ganguly et al. (2001) Cell, 105, 521-532] showed that KCC2 expression may be under the trophic control of GABAA receptor-mediated transmission. Here we have studied the developmental expression of KCC2 protein in mouse hippocampal dissociated cultures as well as organotypic cultures. A low somatic expression level was found in neurons prior to the formation of the first synapses, as detected by synaptophysin immunoreactivity. Thereafter, KCC2 expression was strongly up-regulated during neuronal maturation. The developmental up-regulation of KCC2 expression was not altered by a chronic application (throughout the culturing period; 2-15 days in vitro) of the action-potential blocker TTX or the N-methyl-d-aspartate (NMDA) and non-NMDA antagonists APV and NBQX. Blockade of GABAA-mediated transmission with picrotoxin did not affect the expression levels of KCC2 protein either. These data show that neither neuronal spiking nor ionotropic glutamatergic and GABAergic transmission are required for the developmental expression of KCC2 in mouse hippocampal neurons in vitro.

Changes in chloride homeostasis-regulating gene expressions in the rat hippocampus following amygdala kindling

In a rat kindling model, we examined expression patterns of NKCC1, KCC1, KCC2, and CLC-2. In the dentate granule cell layer, there was an activity-dependent increase in NKCC1 mRNA but significant decreases in KCC1 and CLC-2 mRNAs. In addition, CLC-2 mRNA expression was markedly decreased in CA1 pyramidal layer. These results suggest that an increase in [Cl-]i and a resultant reduction in GABAergic inhibition may occur in hippocampus of epileptic rats.

Postnatal development and migration of cholecystokinin-immunoreactive interneurons in rat hippocampus

The development of cholecystokinin-immunoreactive (CCK-IR) interneurons in the rat hippocampus was studied using immunocytochemical methods at the light and electron microscopic levels from early (P0-P8) to later postnatal (P12-P20) periods. The laminar distribution of CCK-IR cell bodies changed considerably during the studied period, which is suggested to be due to migration. CCK-IR cells appear to move from the molecular layer of the dentate gyrus to their final destination at the stratum granulosum/hilus border, and tend to concentrate in the distal third of stratum radiatum in CA1-3. The density of CCK-IR cells is rapidly decreasing during the first 4 postnatal days without any apparent reduction in their total number, therefore it is due to the pronounced growth of hippocampal volume in this period. Axons of CCK-IR interneurons formed symmetrical synapses already at P0, and by far the predominant targets were dendrites of presumed principal cells in all subfields of the hippocampus. These axon arbors began to concentrate around pyramidal cell bodies only at P8, at earlier ages CCK-IR axons crossed stratum pyramidale at right angles, and gave rise to varicose collaterals only outside this layer. The dendrites and somata of CCK-IR cells received synapses already at P0, but those were mostly symmetrical, apart from a few immature asymmetrical synapses. At P4, mature asymmetrical synapses with considerable amounts of synaptic vesicles were already commonly encountered. Thus, the innervation of CCK-IR interneurons apparently develops later than their output synapses, suggesting that they may be able to release transmitter before receiving any considerable excitatory drive. We conclude that CCK-IR cells represent one, if not the major, interneuron type that assists in the maturation of glutamatergic synapses (activation of N-methyl-D-aspartate receptors) via GABAergic depolarization of principal cell dendrites, and may contribute to the generation of giant depolarizing potentials. CCK-IR cells will change their function to perisomatic hyperpolarizing inhibition, as glutamatergic transmission in the network becomes operational.

Glycine receptors and glycinergic synaptic transmission in the deep cerebellar nuclei of the rat: a patch-clamp study

To clarify possible glycinergic transmission in the cerebellum, principal neurons in deep cerebellar nuclei (DCN) of sliced cerebella (200 microm in thickness) from rats (aged 2-14 days) were studied using whole cell patch-clamp techniques. When glycine (100 microM) was applied to the DCN neurons from a "Y tube," large outward currents were induced (average peak amplitude of about 600 pA at -40 mV). The currents were blocked by strychnine (1 microM) and showed a reversal potential of -62 mV, which was approximately the estimated Cl- equilibrium potential. The dose-response relation of the currents showed an apparent dissociation constant of 170 microM for glycine and Hill coefficient of 1.6. In the presence of 6-cyano-7-nitroquinoziline-2, 3-dione (CNQX), d-(-)-2-amino-5-phosphonovaleric acid (APV) and bicuculline, which antagonize amino-3-hydroxy-5-methyl-isoxazol-propionate (APMA), N-methyl-d-aspartate (NMDA), and GABAA receptors, respectively, postsynaptic currents sensitive to strychnine (1 microM) were induced in DCN neurons by external perfusion of 20 mM K+ saline. Electrical stimulation of surrounding tissues in DCN evoked definite inhibitory postsynaptic currents (IPSCs) in these neurons. The IPSCs had a reversal potential of -62 mV and showed sensitivities to strychnine and tetrodotoxin. Thus this study has revealed that strychnine-sensitive glycine receptors are expressed in neurons of the DCN of rats and that glycinergic transmission mediated by these receptors is functional in these neurons from stages immediately after birth. The glycinergic innervations are presumably supplied by small interneurons located in the DCN.

Metabolic regulation of fertility through presynaptic and postsynaptic signaling to gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction and are inhibited by negative energy balance. In normal adults, these neurons maintain elevated intracellular chloride so that GABA(A) receptor activation is excitatory. We hypothesized that fasting alters homeostatic mechanisms to eliminate excitatory responses to GABA but rejected this hypothesis when brief, local GABA application elicited action currents in GnRH neurons from fed and fasted mice. This response was specific to GABA(A) receptors, because glycine elicited no response. We next found that fasting reduced the frequency of spontaneous GABAergic postsynaptic currents (PSCs) and that this was reversed by in vivo treatment with leptin during the fast. In the presence of tetrodotoxin to minimize presynaptic actions, leptin also potentiated the postsynaptic response of these cells to GABA(A) receptor activation. Postsynaptic effects of leptin on GABAergic miniature PSCs were eliminated by inhibiting JAK2/3 (Janus kinase), the tyrosine kinase through which leptin receptors signal. In all experiments, elimination of PSCs at ECl or by treatment with the GABAA receptor antagonist bicuculline confirmed that PSCs were specifically mediated by GABA(A) receptor chloride channels. These data demonstrate that fasting and leptin act presynaptically and postsynaptically to alter GABAergic drive to GnRH neurons, providing evidence for GABAergic communication of metabolic cues to GnRH neurons, and suggest the possibility for functional leptin receptors on GnRH neurons. They further demonstrate cytokine modulation of the postsynaptic response to GABA in mammals, which may be important to central neural regulation in both healthy and diseased states.

Brain-derived neurotrophic factor modulation of GABAergic synapses by postsynaptic regulation of chloride transport

Brain-derived neurotrophic factor (BDNF) potentiates excitatory synapses in a variety of systems by promoting presynaptic transmitter release. The existing evidence indicates that BDNF attenuates inhibitory transmission, but reports differ considerably in their characterization of the effect and proposed mechanisms. We examined the effects of exogenously applied BDNF on EPSCs and IPSCs recorded from functionally identified neurons in dissociated rat hippocampal cultures. When recording from glutamatergic neurons, we found that BDNF exerted differential effects at excitatory versus inhibitory synapses: increasing amplitude of EPSCs but slightly decreasing that of IPSCs. Furthermore, when recording from GABAergic neurons, we found that BDNF increased the IPSC amplitude. That these differential BDNF effects reflect distinct presynaptic and postsynaptic mechanisms was suggested by the BDNF-induced changes in miniature EPSCs and IPSCs. An increased mini-frequency was found at all synapses, indicating elevated presynaptic transmitter secretion; a change in the amplitude of mini-IPSCs was found at GABAergic cells, suggesting postsynaptic modulation of GABA responses. Selective postsynaptic mechanisms were further examined by comparing the effect of BDNF on GABA-induced currents recorded from glutamatergic versus GABAergic cells. For GABAergic but not glutamatergic postsynaptic cells, BDNF induced a shift in the reversal potential (EIPSC) toward more positive levels, hence reducing the inhibitory action of IPSCs. This BDNF-induced effect correlates with the existing level of furosemide-sensitive K+-Cl- transport activity in the postsynaptic cell. Thus, BDNF may decrease the efficacy of inhibitory transmission by acute postsynaptic downregulation of Cl- transport, in addition to its well known presynaptic effect.

KCl cotransport is an important modulator of human cervical cancer growth and invasion

Cervical cancer is a major world health problem for women, but the pathophysiology of this disease has received scant attention. Here we show that the growth and invasion of cervical cancer cells are strongly linked the expression and activity of the KCl cotransporter (KCC), an important regulator of the ionic and cellular osmotic homeostasis. Functional assays of KCl cotransport activation by osmotic swelling, staurosporine, and N-ethylmaleimide indicate that removal of the N-terminal 117 amino acids from KCC1 produces a dominant-negative loss-of-function phenotype for KCl cotransport in human cervical cancer cells. The capability for regulatory volume decrease is much attenuated in the loss-of-function KCC mutant cervical cancer cells. The loss-of-function KCC mutant cervical cancer cells exhibit inhibited cell growth accompanied by decreased activity of the cell cycle gene products retinoblastoma and cdc2 kinase. Reduced cellular invasiveness is in parallel by reduced expression of alpha v beta 3 and alpha 6 beta 4 integrins, accompanied by decreased activity of matrix metalloproteinase 2 and 9. Inhibition of tumor growth in SCID mice confirms the crucial role of KCC in promoting cervical cancer growth and invasion. Thus, blockade of KCl cotransport may be a useful therapeutic adjunctive strategy to retard or prevent cervical cancer invasion.

Cell type-specific differences in chloride-regulatory mechanisms and GABA(A) receptor-mediated inhibition in rat substantia nigra

The regulation of intracellular chloride has important roles in neuronal function, especially by setting the magnitude and direction of the Cl- flux gated by GABA(A) receptors. Previous studies have shown that GABA(A)-mediated inhibition is less effective in dopaminergic than in GABAergic neurons in substantia nigra. We studied whether this phenomenon may be related to a difference in Cl-regulatory mechanisms. Light-microscopic immunocytochemistry revealed that the potassium-chloride cotransporter 2 (KCC2) was localized only in the dendrites of nondopaminergic (primarily GABAergic) neurons in the substantia nigra, whereas the voltage-sensitive chloride channel 2 (ClC-2) was observed only in the dopaminergic neurons of the pars compacta. Electron-microscopic immunogold labeling confirmed that KCC2 is localized in the dendritic plasma membrane of GABAergic neurons close to inhibitory synapses. Confocal microscopy showed that ClC-2 was selectively expressed in the somatic and dendritic cell membranes of the dopaminergic neurons. Gramicidin-perforated-patch recordings revealed that the GABA(A) IPSP reversal potential was significantly less negative and had a much smaller hyperpolarizing driving force in dopaminergic than in GABAergic neurons. The GABA(A) reversal potential was significantly less negative in bicarbonate-free buffer in dopaminergic but not in GABAergic neurons. The present study suggests that KCC2 is responsible for maintaining the low intracellular Cl- concentration in nigral GABAergic neurons, whereas a sodium-dependent anion (Cl--HCO3-) exchanger and ClC-2 are likely to serve this role in dopaminergic neurons. The relatively low efficacy of GABAA-mediated inhibition in nigral dopaminergic neurons compared with nigral GABAergic neurons may be related to their lack of KCC2.

Structure, function and regulation of glycine neurotransporters

Glycine exerts multiple functions in the central nervous system, as an inhibitory neurotransmitter through activation of specific, Cl--permeable, ligand-gated ionotropic receptors and as an obligatory co-agonist with glutamate on the activation of N-methyl-D-aspartate (NMDA) receptors. In some areas of the central nervous system, glycine seems to be co-released with gamma-aminobutyric acid (GABA), the main inhibitory amino acid neurotransmitter. The synaptic action of glycine ends by active recapture through sodium- and chloride-coupled glycine transporters located in glial and neuronal plasma membranes, whose structure-function relationship is being studied. The trafficking and plasma membrane expressions of these proteins are controlled by regulatory mechanisms. Glycine transporter inhibitors may find application in the treatment of muscle tone defects, epilepsy, schizophrenia, pain and neurodegenerative disorders. This review deals on recent progress on localization, transport mechanisms, structure, regulation and pharmacology of the glycine transporters (GLYTs).

Deafness disrupts chloride transporter function and inhibitory synaptic transmission

Loss of sensory function leads to atrophy or death within the developing CNS, yet little is known about the physiology of remaining synapses. After bilateral deafening, gramicidin-perforated-patch recordings were obtained from gerbil inferior colliculus neurons in a brain slice preparation. Afferent-evoked IPSPs had a diminished ability to block current-evoked action potentials in deafened neurons. This change could be attributed, in part, to a loss of potassium-dependent chloride transport function, with little change in K-Cl cotransporter expression. Treatments that suppressed chloride cotransport (bumetanide, cesium, and genistein) had little or no effect on neurons from deafened animals. These same treatments depolarized the E(IPSC) of control neurons. Semiquantitative RT-PCR and immunohistochemical staining indicated no change in the expression of chloride cotransporter mRNA or protein after deafness. Therefore, profound hearing loss leads rapidly to the disruption of chloride homeostasis, which is likely attributable to the dysfunction of the potassium-dependent chloride cotransport mechanism, rather than a downregulation of its expression. This results in inhibitory synapses that are less able to block excitatory events.

Developmental modulation of retinal wave dynamics: shedding light on the GABA saga

Embryonic spontaneous activity, in the form of propagating waves, is crucial for refining visual connections. To study what aspects of this correlated activity are instructive, we must first understand how their dynamics change with development and what factors trigger their disappearance after birth. Here we report that in the turtle retina, GABA, rather than glutamate and acetylcholine, influences developmental changes in wave dynamics. Using calcium imaging of the ganglion cell layer, we report how waves switch from fast and broad, when they emerge, to slow and narrow a few days before hatching, coinciding with the emergence of excitatory GABA(A) receptor-mediated activity. Around hatching, waves gradually become stationary patches, whereas GABA(A) shifts from excitatory to inhibitory, coinciding with the upregulation of the cotransporter KCC2, suggesting that changes in intracellular chloride underlie the shift. Dark-rearing from hatching causes correlated spontaneous activity to persist, whereas GABA(A) responses remain excitatory, and KCC2 expression is weaker. We conclude that GABA plays an important regulatory role during the maturation of retinal neural activity. Using a simple and elegant mechanism, namely the switch from excitatory to inhibitory, GABA(A) receptor-mediated activity is necessary and sufficient to cause retinal waves to stop propagating, ultimately leading to the disappearance of correlated spontaneous activity. Moreover, our results suggest that visual experience modulates the GABAergic switch.

In vitro formation of a secondary epileptogenic mirror focus by interhippocampal propagation of seizures

We have determined whether seizures generate an epileptogenic focus in distal structures using an in vitro preparation composed of three independent chambers that accommodate two intact hippocampi and their connecting commissures. This enabled us to apply a convulsive agent to one hippocampus, allow the propagation of a given number of seizures to the other side and block the connections reversibly by applying tetrodotoxin (TTX) to the commissural chamber. The propagation of seizures from the kainate-treated side to the naive side transformed the latter into an independent epileptogenic focus that was capable of generating spontaneous and evoked seizures. The induction mechanism required activation of NMDA receptors and the epileptogenic transformation was associated with long-term alterations in GABAergic synapses, which became excitatory because of a shift in the chloride reversal potential, E(Cl). These data indicate that the excitatory actions of GABA may be a fundamental property of epileptogenic structures.

Differential development of cation-chloride cotransporters and Cl- homeostasis contributes to differential GABAergic actions between developing rat visual cortex and dorsal lateral geniculate nucleus

A recent study suggested that gamma-aminobutyric acid (GABA) plays differential roles in activity-dependent plasticity between the visual cortex (VC) and the dorsal lateral geniculate nucleus (dLGN). In the present study, to investigate differential GABAergic functions in postnatal visual system development, the development of [Cl(-)](i), cation-Cl(-) cotransporter expression, and the [Ca(2+)](i) responses evoked by GABA were compared between VC and dLGN during the early stages of development. Using rat brain slices from postnatal days (P) 0-17, GABA-evoked [Ca(2+)](i) responses and resting [Cl(-)](i) were measured by means of optical imaging of Ca(2+) and Cl(-), respectively. Changes in the expression of cation-Cl(-) cotransporters (viz. the outwardly-directed K(+)-Cl(-) cotransporter, KCC2, and the inwardly-directed Na(+),K(+)-2Cl(-) cotransporter, NKCC1) were examined in VC and dLGN by in situ hybridization. At birth, the excitatory actions of GABA were powerful in VC, but missing in dLGN (as indicated by neuronal [Ca(2+)](i) transients), and the resting [Cl(-)](i) was significantly higher in VC than in dLGN. Signals for KCC2 mRNA expression were significantly higher in dLGN than in VC at P0. This suggests that extrusion of Cl(-) from neurons is stronger in dLGN than in VC at P0, so that a GABAergic excitatory effect was not observed in dLGN because of more negative equilibrium potential for Cl(-). The present study indicates clear differences in the molecular and physiological bases of Cl(-) homeostasis and GABA actions between the developing VC and dLGN. Such differential GABAergic actions may underlie the distinct mechanisms involved in VC and dLGN development within the visual system.

Post-natal development of type 1 cannabinoid receptor immunoreactivity in the rat hippocampus

Type 1 cannabinoid receptors, selectively located on axon terminals of GABAergic interneurons in the hippocampus, are known to be involved in endocannabinoid-mediated retrograde synaptic signalling. The question arises whether type 1 cannabinoid receptors appear on these axons during early post-natal life, when GABAergic transmission is still depolarizing, and whether there are any developmental changes in the cellular or subcellular expression pattern. Here we demonstrate, using single and double immunocytochemical methods at the light and electron microscopic levels, that type 1 cannabinoid receptors are expressed only on the membrane of axon terminals and pre-terminal axons but not on the soma-dendritic membrane at all examined timepoints between post-natal days 0 and 20, similar to the adult distribution. All type 1 cannabinoid receptor-positive boutons formed symmetric synapses. Granular labelling in the somata was already strong at post-natal day 0 and corresponded to multivesicular bodies, lysosomes, Golgi apparatus and rough endoplasmic reticulum. The type 1 cannabinoid receptor-positive axons were shown to originate largely from cholecystokinin-immunoreactive basket and bistratified neurons throughout the hippocampus (90% of all type 1 cannabinoid receptor-containing cells) and dentate gyrus (70% of all type 1 cannabinoid receptor-containing cells). The remaining cells have not been identified but probably belong to the somatostatin- and/or neuropeptide Y-containing subsets, as cholecystokinin-negative, type 1 cannabinoid receptor-positive axons have been observed in strata moleculare and lacunosum-moleculare of the dentate gyrus and CA1-3, respectively, where these neurons are known to arborize. No cell types were found that expressed type 1 cannabinoid receptors transiently at some developmental stage. We conclude that the cellular and subcellular pattern of type 1 cannabinoid receptor expression during early post-natal life is similar to the adult pattern and type 1 cannabinoid receptors are expressed on the cholecystokinin-containing axons as soon as synapse formation begins. This suggests that retrograde synaptic signalling by endocannabinoids is required for the normal operation of GABAergic neurotransmission even before it becomes hyperpolarizing.

Use-dependent shift from inhibitory to excitatory GABAA receptor action in SP-O interneurons in the rat hippocampal CA3 area

Cortical inhibitory interneurons set the pace of synchronous neuronal oscillations implicated in synaptic plasticity and various cognitive functions. The hyperpolarizing nature of inhibitory postsynaptic potentials (IPSPs) in interneurons has been considered crucial for the generation of oscillations at beta (15-30 Hz) and gamma (30-100 Hz) frequency. Hippocampal basket cells and axo-axonic cells in stratum pyramidale-oriens (S-PO) play a central role in the synchronization of the local interneuronal network as well as in pacing of glutamatergic principal cell firing. A lack of conventional forms of plasticity in excitatory synapses onto interneurons facilitates their function as stable neuronal oscillators. We have used gramicidin-perforated and whole cell clamp recordings to study properties of GABAAR-mediated transmission in CA3 SP-O interneurons and in CA3 pyramidal cells in rat hippocampal slices during electrical 5- to 100-Hz stimulation and during spontaneous activity. We show that GABAergic synapses onto SP-O interneurons can easily switch their mode from inhibitory to excitatory during heightened activity. This is based on a depolarizing shift in the GABAA reversal potential (EGABA-A), which is much faster and more pronounced in interneurons than in pyramidal cells. We also found that the shift in interneuronal function was frequency dependent, being most prominent at 20- to 40-Hz activation of the GABAergic synapses. After 40-Hz tetanic stimulation (100 pulses), GABAA responses remained depolarizing for approximately 45 s in the interneurons, promoting bursting in the GABAergic network. Hyperpolarizing EGABA-A was restored >60 s after the stimulus train. Similar but spontaneous GABAergic bursting was induced by application of 4-aminopyridine (100 microM) to slices. A shift to depolarizing IPSPs by the GABAAR permeant weak acid anion formate provoked interneuronal population bursting, supporting the role of GABAergic excitation in burst generation. Furthermore, depolarizing GABAergic potentials and synchronous interneuronal bursting were enhanced by pentobarbital (100 microM), a positive allosteric modulator of GABAARs, and were blocked by picrotoxin (100 microM). Intriguingly, GABAergic bursts displayed short (<1 s) oscillations at 15-40 Hz, even though only depolarizing GABAA responses were seen in the SP-O interneurons. This beta-gamma rhythmicity in the interneuron network was dependent on electrotonic coupling, and was abolished by blockade of gap junctions with carbenoxolone (200 microM). Results here implicate the rapid activity-dependent degradation of hyperpolarizing IPSPs in SP-O interneurons in setting the temporal limits for a given interneuron to participate in beta-gamma oscillations synchronized by GABAergic synapses. Furthermore, they imply that mutual GABAergic excitation provided by interneurons may be an integral part in the function of neuronal networks. We suggest that the use-dependent change in EGABA-A could represent a form of short-term plasticity in interneurons promoting coherent and sustained activation of local GABAergic networks.

GABAergic control of neuropeptide gene expression in parvocellular neurons of the hypothalamic paraventricular nucleus

To assess the functional impact of local inhibitory gamma-aminobutyric acid (GABA)ergic interneuron population on the cellular and transcriptional activity of parvocellular neurosecretory neurons in the hypothalamic paraventricular nucleus (PVH), we followed the expression of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) genes along with the activation marker c-fos in response to the blockade of GABA-A receptors. First, we analysed the effect of the GABA-A receptor antagonist bicuculline methiodide (BMI) in organotypic cultures of hypothalamic slices. These preparations preserve the cytoarchitecture of CRH-synthesizing cell populations and elements of local interneuronal networks, while remote connections originating from limbic- and brainstem areas are missing. In vitro, BMI resulted in a selective induction of c-Fos immunoreactivity that was localized exclusively to the PVH and upregulated both CRH mRNA and AVP hnRNA levels. Local microinjection of BMI into the paraventricular region of freely moving rats increased the adrenocorticotropin secretion and activated PVH neurons ipsilateral to the injection. c-Fos immunoreactivity was distributed within the PVH and in the perinuclear region, where it appeared in GABAergic and also in non-GABAergic profiles. This treatment induced AVP hnRNA expression in the parvocellular compartment without any reliable stimulation of CRH transcription in the parvocellular- and AVP hnRNA levels in the magnocellular neurons. These results reveal an intrinsic GABAergic mechanism in the PVH microenvironment that by itself, without limbic contribution, impinges a tonic inhibitory influence on the parvocellular CRH neurons in vitro. In vivo, remote inputs are superimposed on the local circuit, allowing differential transcriptional regulation of CRH and AVP genes in the hypophysiotropic neurons.

A tale of timing and transport

Excitatory and inhibitory synapses show long-term plasticity, but spike timing-dependent plasticity was seen only at excitatory connections. No longer. In this issue of Neuron, Woodin et al. demonstrate that coincident pre- and postsynaptic activity acts on the neuronal K+/Cl- cotransporter KCC2 to shift the reversal potential for Cl- and thus alters the effectiveness of GABAergic synapses.

Coincident pre- and postsynaptic activity modifies GABAergic synapses by postsynaptic changes in Cl- transporter activity

Coincident pre- and postsynaptic activation is known to induce long-term modification of glutamatergic synapses. We report here that, in both hippocampal cultures and acute hippocampal slices, repetitive postsynaptic spiking within 20 ms before and after the activation of GABAergic synapses also led to a persistent change in synaptic strength. This synaptic modification required Ca2+ influx through postsynaptic L-type Ca2+ channels and was due to a local decrease in K+-Cl- cotransport activity, effectively reducing the strength of inhibition. Thus, GABAergic synapses can detect and be modified by coincident pre- and postsynaptic spiking, allowing the level of inhibition to be modulated in accordance to the temporal pattern of postsynaptic excitation.

Trans-synaptic shift in anion gradient in spinal lamina I neurons as a mechanism of neuropathic pain

Modern pain-control theory predicts that a loss of inhibition (disinhibition) in the dorsal horn of the spinal cord is a crucial substrate for chronic pain syndromes. However, the nature of the mechanisms that underlie such disinhibition has remained controversial. Here we present evidence for a novel mechanism of disinhibition following peripheral nerve injury. It involves a trans-synaptic reduction in the expression of the potassium-chloride exporter KCC2, and the consequent disruption of anion homeostasis in neurons of lamina I of the superficial dorsal horn, one of the main spinal nociceptive output pathways. In our experiments, the resulting shift in the transmembrane anion gradient caused normally inhibitory anionic synaptic currents to be excitatory, substantially driving up the net excitability of lamina I neurons. Local blockade or knock-down of the spinal KCC2 exporter in intact rats markedly reduced the nociceptive threshold, confirming that the reported disruption of anion homeostasis in lamina I neurons was sufficient to cause neuropathic pain.

Glycine facilitates transmitter release at developing synapses: a patch clamp study from Purkinje neurons of the newborn rat

Synaptic currents in immature Purkinje cells from rats on postnatal days 0-14 (P0-P14) were studied using whole-cell patch-electrodes applied to cerebellar slices (200 micro m in thickness). Purkinje cells (held at -40 mV) showed excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) spontaneously. From P2 to P12 the frequencies of miniature EPSCs and miniature IPSCs in the Purkinje cells increased by 10-fold or more, suggesting progressive formation of functional synapses during this period. Application of glycine (100 micro M) to an immature Purkinje cell at P3-10 immediately increased the frequencies of both EPSCs and IPSCs. The effects of glycine showed maximum at P5-6 for EPSCs and at P9-10 for IPSCs and decreased thereafter. Facilitatory effects of glycine were suppressed by strychnine (1 micro M), a specific blocker of the ionotropic glycine receptor, while the effects were also induced by other glycinergic agonists, including alpha-L-alanine (1 mM), L-serine (1 mM) and taurine (500 micro M). The site of glycinergic effects was studied by removing the action potential generation in cerebellar slices. Following the addition of tetrodotoxin (TTX, 1 micro M), the glycine-induced facilitation of EPSC almost disappeared, while that of IPSC remained (i.e. miniature IPSCs) and reached more than half of the value without TTX. These findings suggest that the ionotropic glycinergic receptors are expressed transiently but profoundly in the developing cerebellum, and that the distributions of these receptors causing excitation are different at excitatory and inhibitory presynaptic neurons. The glycine receptors may play distinct roles in the maturation and organization of cerebellar neural circuits.

Chloride influx aggravates Ca2+-dependent AMPA receptor-mediated motoneuron death

AMPA receptor-mediated excitotoxicity has been implicated in the pathogenesis of stroke, neurotrauma, epilepsy, and many neurodegenerative diseases such as motoneuron disease. We studied the role of Cl- in AMPA receptor-mediated Ca2+-dependent excitotoxicity in cultured rat spinal motoneurons. Using the gramicidin perforated patch-clamp technique, the intracellular Cl- concentration could be calculated from the reversal potential of the GABA-induced current. The membrane depolarization caused by AMPA receptor stimulation resulted in Cl- influx through 5-nitro-2(3-phenylpropyl-amino) benzoic acid- and niflumic acid-sensitive Cl- channels. Cl- influx during AMPA receptor stimulation aggravated excitotoxic motoneuron death by two mechanisms: an increase of AMPA receptor conductance and an elevation of the Ca2+ driving force through a partial repolarization. The Cl- influx during AMPA receptor stimulation was enhanced by coadministration of GABA. This resulted in an increased Ca2+ influx and an enhanced cell death, suggesting that concomitant GABAergic stimulation may aggravate excitotoxic motoneuron death.

Antagonistic modulation of a hyperpolarization-activated Cl(-) current in Aplysia sensory neurons by SCP(B) and FMRFamide

Whole cell voltage-clamp recordings from Aplysia mechanosensory neurons obtained from the pleural ganglion were used to investigate the actions on membrane currents of the neuropeptides SCP(B) and FMRFamide. At the start of whole cell recording, SCP(B) typically evoked an inward current at a holding potential of -40 mV, due to the cAMP-mediated closure of the S-type K+ channel, whereas FMRFamide evoked an outward current, due to the opening of the S-type K+ channels mediated by 12-lipoxygenase metabolites of arachidonic acid. However, after several minutes of whole cell recording with a high concentration of chloride in the whole cell patch pipette solution, the responses to SCP(B) and FMRF-amide at -40 mV were inverted; SCP(B) evoked an outward current, whereas FMRFamide and YGGFMRFamide evoked inward currents. Ion substitution experiments and reversal potential measurements revealed that these responses were due to the opposing regulation of a Cl(-) current, whose magnitude was greatly enhanced by dialysis with the high Cl(-) - containing pipette solution. SCP(B) inhibited this Cl(-) current through production of cAMP and activation of PKA. YGGFMRFamide activated this Cl(-) current by stimulating a cGMP-activated phosphodiesterase that hydrolyzed cAMP. Thus a cAMP-dependent Cl(-) current undergoes antagonistic modulation by two neuropeptides in Aplysia sensory neurons.

Sexual dimorphism in the GABAergic control of gonadotropin release in intact rats

GABA is a potent regulator of gonadotropin release both in male and female rats. We reported 24 h profiles of GABA release in the medial preoptic area (MPO) where gonadotropin-releasing hormone (GnRH) surge generator resides in female rats. In this article, we review the sex difference in 24 h profiles of GABA release. GABA release is high and episodic in male rats without any time dependency, but female rats showed a surge-like secretion of GABA in the early morning of the proestrous day. GABA release rapidly decreased until the afternoon of the day of proestrus followed by the preovulatory luteinizing hormone (LH) surge. The peak time of GABA episodes changes with estrous cycle in female rats. Fitting with the double cosinor method demonstrated that the acrophase of the GABA release in proestrous female rats occurs in the early morning, whereas the acrophases in diestrous females, estrous females and males occur at various time of day. Proestrous female rats showed significant difference in the peak time and acrophase of the GABA release compared with other estrous stages of female and male rats. These results demonstrated further sexual dimorphism of GABA release in the MPO, suggesting that coupling between the GABA release and the circadian clock may be a determining factor in the sex difference of the hypothalamo-pituitary-gonadal (HPG) axis in rats.

Hyperpolarizing inhibition develops without trophic support by GABA in cultured rat midbrain neurons

During a limited period of early neuronal development, GABA is depolarizing and elevates [Ca2+]i, which mediates the trophic action of GABA in neuronal maturation. We tested the attractive hypothesis that GABA itself promotes the developmental change of its response from depolarizing to hyperpolarizing (Ganguly et al. 2001). In cultured midbrain neurons we found that the GABA response changed from depolarizing to hyperpolarizing, although GABAA receptors had been blocked throughout development. In immature neurons prolonged exposure of the cells to nanomolar concentrations of GABA or brief repetitive applications of GABA strongly diminished the elevation of [Ca+]i by GABA. As revealed by gramicidin perforated-patch recording, reduced [Ca2+]i responses were due to a diminished driving force for Cl-. This suggests that immature neurons do not have an efficient inward transport that can compensate the loss of cytosolic Cl-resulting from sustained GABAA receptor activation by ambient GABA. Transient increases in external K+, which can induce voltage-dependent Cl- entry, restored GABA-induced [Ca2+]i elevations. In mature neurons, GABA reduced [Ca2+]i provided that background [Ca2+]i was elevated by the application of an L-type Ca2+ channel agonist. This was probably due to a hyperpolarization of the membrane by Cl- currents. K(+)-Cl- cotransport maintained the gradient for hyperpolarizing Cl-currents. We conclude that in immature midbrain neurons an inward Cl- transport is not effective although the GABA response is depolarizing. Further, GABA itself is not required for the developmental switch of GABAergic responses from depolarizing to hyperpolarizing in cultured midbrain neurons.

Excitatory GABA input directly drives seizure-like rhythmic synchronization in mature hippocampal CA1 pyramidal cells

GABA, which generally mediates inhibitory synaptic transmissions, occasionally acts as an excitatory transmitter through intense GABA(A) receptor activation even in adult animals. The excitatory effect results from alterations in the gradients of chloride, bicarbonate, and potassium ions, but its functional role still remains a mystery. Here we show that such GABAergic excitation participates in the expression of seizure-like rhythmic synchronization (afterdischarge) in the mature hippocampal CA1 region. Seizure-like afterdischarge was induced by high-frequency synaptic stimulation in the rat hippocampal CA1-isolated slice preparations. The hippocampal afterdischarge was completely blocked by selective antagonists of ionotropic glutamate receptors or of GABA(A) receptor, and also by gap-junction inhibitors. In the CA1 pyramidal cells, oscillatory depolarizing responses during the afterdischarge were largely dependent on chloride conductance, and their reversal potentials (average -38 mV) were very close to those of exogenously applied GABAergic responses. Moreover, intracellular loading of the GABA(A) receptor blocker fluoride abolished the oscillatory responses in the pyramidal cells. Finally, the GABAergic excitation-driven afterdischarge has not been inducible until the second postnatal week. Thus, excitatory GABAergic transmission seems to play an active functional role in the generation of adult hippocampal afterdischarge, in cooperation with glutamatergic transmissions and possible gap junctional communications. Our findings may elucidate the cellular mechanism of neuronal synchronization during seizure activity in temporal lobe epilepsy.

Heterogeneous expression of the potassium-chloride cotransporter KCC2 in gonadotropin-releasing hormone neurons of the adult mouse

In mature central neurons, chloride extrusion mediated by the K-Cl cotransporter KCC2 appears to be largely responsible for the Cl(-) driving force that allows gamma-aminobutyric acid(A) (GABA(A)) receptor activation to trigger a hyperpolarization. In its absence, GABA's effect is typically depolarizing and often excitatory. We examined the colocalization of KCC2 and GnRH in adult male and female mice using a combined in situ hybridization-immunofluorescence procedure. We found that KCC2 was localized to approximately 34% of GnRH neurons. This proportion was similar in females and males. However, females exhibited a marked rostrocaudal gradient of colocalization that was not seen in males. By contrast, KCC2 was localized to nearly all vasopressin neurons of the supraoptic nucleus. These results indicate that a substantial fraction of GnRH neurons may be depolarized and excited by GABA(A) receptor activation throughout life, supporting the existence of functionally heterogeneous subpopulations.

Expression and function of chloride transporters during development of inhibitory neurotransmission in the auditory brainstem

Glycine and GABA, the dominant inhibitory neurotransmitters in the CNS, assume a depolarizing role in early development, leading to increased cytoplasmic Ca2+ levels and action potentials. The effect is thought to be of some significance for maturation. The depolarization is caused by Cl- efflux, and chloride transporters contribute to the phenomenon by raising the intracellular Cl- concentration ([Cl-]i) above equilibrium, thereby generating an outward-directed electrochemical gradient for Cl-. In mature neurons, the [Cl-]i is reduced below equilibrium, thus rendering glycine activity hyperpolarizing. Here, we investigated the temporal expression of the K-Cl cotransporter KCC2 and the Na-K-Cl cotransporter NKCC1 in the lateral superior olive (LSO) of rats and mice. The two cation cotransporters normally extrude and accumulate Cl-, respectively. As evidenced by several methods, KCC2 mRNA was present in LSO neurons during both the depolarizing and hyperpolarizing periods. Western blots confirmed a constant level of KCC2 in the brainstem, and immunohistochemistry showed that the protein is diffusely distributed within neonatal LSO neurons, becoming integrated into the plasma membrane only with increasing age. The glycine reversal potential in KCC2 knock-out mice differed significantly from that determined in wild-type controls at postnatal day 12 (P12) but not at P3, demonstrating that KCC2 is not active in neonates, despite its early presence. NKCC1 mRNA was not detected during the depolarizing phase in the LSO, implying that this transporter does not contribute to the high [Cl-]i. Our results reveal major differences in the development of [Cl-]i regulation mechanisms seen in brainstem versus forebrain regions.

Aberrant chloride transport contributes to anoxic/ischemic white matter injury

Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl-. We used the in vitro rat optic nerve to study the role of aberrant Cl- transport in anoxia/ischemia. After 30 min of anoxia (NaN3, 2 mm), axonal membrane potential (V(m)) decreased to 42 +/- 11% of control and to 73 +/- 11% in the presence of tetrodotoxin (TTX) (1 microm). TTX + 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid disodium salt (500 microm), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 +/- 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 microm) together with TTX was also more effective than TTX alone (84 +/- 14%). The compound action potential (CAP) area recovered to 26 +/- 6% of control after 1 hr anoxia. KCC blockade (10 microm furosemide) improved outcome (40 +/- 4%), and TTX (100 nm) was even more effective (74 +/- 12%). In contrast, the Cl- channel blocker niflumic acid (50 microm) worsened injury (6 +/- 1%). Coapplication of TTX (100 nm) + furosemide (10 microm) was more effective than either agent alone (91 +/- 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl- gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl- fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.

Sex differences in the developing brain: crossroads in the phosphorylation of cAMP response element binding protein

Although it is widely known that steroid hormones differentiate the brain, little is known about the signal transduction pathways that are influenced by steroid hormones during development. This review focuses on divergence in the phosphorylation of cAMP response element binding protein (CREB) in the developing male and female rat brain. At birth, males have an increased phosphorylation of CREB compared to females. As CREB mediates changes in cellular morphology, function and survival rates, its activation may underlie an important event in steroid-mediated sexual differentiation of the brain. The importance of CREB is further supported by a sex difference in the expression of the nuclear receptor coactivator, CREB-binding protein, a critical factor involved in the genomic actions of CREB. This suggests that the developing male brain may be in a hyper-responsive state to factors that lead to increased phosphorylation of CREB, resulting in divergent responses in males versus females. An example of this divergence is the response to GABA. In the male rat brain, GABA action leads to increased phosphorylation of CREB; whereas GABA action in the female brain leads to decreased phosphorylation of CREB. The potential consequences of this divergence in the regulation of CREB are discussed in relation to adult sexually dimorphic brain morphology, physiology and behaviour.

Autocrine/paracrine activation of the GABA(A) receptor inhibits the proliferation of neurogenic polysialylated neural cell adhesion molecule-positive (PSA-NCAM+) precursor cells from postnatal striatum

GABA and its type A receptor (GABA(A)R) are present in the immature CNS and may function as growth-regulatory signals during the development of embryonic neural precursor cells. In the present study, on the basis of their isopycnic properties in a buoyant density gradient, we developed an isolation procedure that allowed us to purify proliferative neural precursor cells from early postnatal rat striatum, which expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). These postnatal striatal PSA-NCAM+ cells were shown to proliferate in the presence of epidermal growth factor (EGF) and formed spheres that preferentially generated neurons in vitro. We demonstrated that PSA-NCAM+ neuronal precursors from postnatal striatum expressed GABA(A)R subunits in vitro and in situ. GABA elicited chloride currents in PSA-NCAM+ cells by activation of functional GABA(A)R that displayed a typical pharmacological profile. GABA(A)R activation in PSA-NCAM+ cells triggered a complex intracellular signaling combining a tonic inhibition of the mitogen-activated protein kinase cascade and an increase of intracellular calcium concentration by opening of voltage-gated calcium channels. We observed that the activation of GABA(A)R in PSA-NCAM+ neuronal precursors from postnatal striatum inhibited cell cycle progression both in neurospheres and in organotypic slices. Furthermore, postnatal PSA-NCAM+ striatal cells synthesized and released GABA, thus creating an autocrine/paracrine mechanism that controls their proliferation. We showed that EGF modulated this autocrine/paracrine loop by decreasing GABA production in PSA-NCAM+ cells. This demonstration of GABA synthesis and GABA(A)R function in striatal PSA-NCAM+ cells may shed new light on the understanding of key extrinsic cues that regulate the developmental potential of postnatal neuronal precursors in the CNS.

Early establishment of multiple release site connectivity between interneurons and pyramidal neurons in the developing hippocampus

The strength of the synaptic transmission between two neurons critically depends on the number of release sites connecting the neurons. Here we examine the development of connectivity between gamma-aminobutyric acid (GABA)ergic interneurons and CA1 pyramidal neurons in the hippocampus. GABAergic postsynaptic currents (PSCs) were recorded in whole-cell voltage-clamped CA1 pyramidal neurons. By comparing spontaneous and miniature (action potential-independent) GABAergic PSCs, we found that multiple release site connectivity is established already at the first postnatal day and that the degree of connectivity remains unaltered into adulthood. During the same time there is a dramatic increase in the number of GABAergic synapses on each pyramidal neuron as indicated by the increase in frequency of miniature GABAergic PSCs. These results indicate that during development a given interneuron contacts an increasing number of target pyramidal neurons but with the same multiple release site connectivity. It has been shown previously that the connectivity between CA3 and CA1 pyramidal neurons is initially restricted to one release site, and develops gradually. The present result thus suggests different mechanisms to govern the maturation of excitatory and inhibitory synaptic transmissions.

The action of BDNF on GABA(A) currents changes from potentiating to suppressing during maturation of rat hippocampal CA1 pyramidal neurons

During the development of the hippocampus, the action of GABA shifts from depolarizing to hyperpolarizing, and brain-derived neurotrophic factor (BDNF) has important roles in GABAergic transmission. We demonstrate that BDNF (20 ng ml-1) rapidly and reversibly potentiates postsynaptic GABAA receptor-mediated currents (by 80.5 +/- 14.3 %, n = 10) in hippocampal CA1 pyramidal neurons isolated from postnatal day (P)6 rats, using nystatin-perforated patch-clamp recordings. This potentiation is caused by an elevation of intracellular Ca2+ that occurs in response to the activation of Trk B receptor tyrosine kinase and phospholipase C-gamma. The modulation of the GABAA responses by BDNF in hippocampal CA1 pyramidal neurons isolated from P10 rats was more diverse (from potentiating to inhibitory), and at P14, BDNF induced a long-lasting inhibition. In addition, Ca2+/calmodulin-dependent protein kinase 2 plays important roles in the potentiating, but not in the inhibitory effect, of BDNF on the GABAA responses. These results suggest that changes in the intracellular signalling pathway could contribute to the developmental shift of the actions of BDNF on inhibitory systems.

Differential expression pattern of chloride transporters NCC, NKCC2, KCC1, KCC3, KCC4, and AE3 in the developing rat auditory brainstem

During development of inhibitory synapses, the action of the two neurotransmitters GABA and glycine shifts from depolarizing to hyperpolarizing. The shift is due to an age-dependent regulation of the intracellular free chloride concentration ([Cl(-)](i)) in postsynaptic neurons. A model system to study this maturation process is a glycinergic projection in the mammalian auditory brainstem. It is formed in the superior olivary complex (SOC) by neurons of the medial nucleus of the trapezoid body, whose axons terminate in the lateral superior olive (LSO). LSO neurons of perinatal rats and mice are depolarized upon glycine application, whereas older cells (>postnatal day (P) 8) are hyperpolarized. Here we examined the expression of six secondary active chloride transporter genes ( NCC, NKCC2, KCC1, KCC3, KCC4, and AE3) in the rat SOC to unravel the molecular mechanisms underlying this change. RT-PCR analysis demonstrated brainstem expression of KCC1, KCC3, KCC4, and AE3, but not of NCC and NKCC2. RNA in situ hybridization showed that only AE3 is highly expressed both at P3 (high [Cl(-)](i)) and P12 (low [Cl(-)](i)) in LSO neurons. KCC1 and KCC4 are weakly expressed in LSO neurons at P3 and P12, respectively. This study completes the expression analysis of all known chloride transporters sensitive to loop diuretic drugs in the SOC and demonstrates differences in the maturation between hippocampal and brainstem inhibitory synapses.

Coexistence of excitatory and inhibitory GABA synapses in the cerebellar interneuron network

Functional GABA synapses are usually assumed to be inhibitory. However, we show here that inhibitory and excitatory GABA connections coexist in the cerebellar interneuron network. The reversal potential of GABAergic currents (E(GABA)) measured in interneurons is relatively depolarized and contrasts with the hyperpolarized value found in Purkinje cells (-58 and -85 mV respectively). This finding is not correlated to a specific developmental stage and is maintained in the adult animal. E(GABA) in interneurons is close to the mean membrane potential (-56.5 mV, as measured with a novel "equal firing potential" method), and both parameters vary enough among cells so that the driving force for GABA currents can be either inward or outward. Indeed, using noninvasive cell-attached recordings, we demonstrate inhibitory, excitatory, and sequential inhibitory and excitatory responses to interneuron stimulation [results obtained both in juvenile (postnatal days 12-14) and subadult (postnatal days 20-25) animals]. In hyperpolarized cells, single synaptic GABA currents can trigger spikes or trains of spikes, and subthreshold stimulations enhance the responsiveness to subsequent excitatory stimulation over at least 30 msec. We suggest that the coexistence of excitatory and inhibitory GABA synapses could either buffer the mean firing rate of the interneuron network or introduce different types of correlation between neighboring interneurons, or both.

BDNF regulates spontaneous correlated activity at early developmental stages by increasing synaptogenesis and expression of the K+/Cl- co-transporter KCC2

Spontaneous neural activity is a basic property of the developing brain, which regulates key developmental processes, including migration, neural differentiation and formation and refinement of connections. The mechanisms regulating spontaneous activity are not known. By using transgenic embryos that overexpress BDNF under the control of the nestin promoter, we show here that BDNF controls the emergence and robustness of spontaneous activity in embryonic hippocampal slices. Further, BDNF dramatically increases spontaneous co-active network activity, which is believed to synchronize gene expression and synaptogenesis in vast numbers of neurons. In fact, BDNF raises the spontaneous activity of E18 hippocampal neurons to levels that are typical of postnatal slices. We also show that BDNF overexpression increases the number of synapses at much earlier stages (E18) than those reported previously. Most of these synapses were GABAergic, and GABAergic interneurons showed hypertrophy and a 3-fold increase in GAD expression. Interestingly, whereas BDNF does not alter the expression of GABA and glutamate ionotropic receptors, it does raise the expression of the recently cloned K(+)/Cl(-) KCC2 co-transporter, which is responsible for the conversion of GABA responses from depolarizing to inhibitory, through the control of the Cl(-) potential. Together, results indicate that both the presynaptic and postsynaptic machineries of GABAergic circuits may be essential targets of BDNF actions to control spontaneous activity. The data indicate that BDNF is a potent regulator of spontaneous activity and co-active networks, which is a new level of regulation of neurotrophins. Given that BDNF itself is regulated by neuronal activity, we suggest that BDNF acts as a homeostatic factor controlling the emergence, complexity and networking properties of spontaneous networks.

Cation-chloride co-transporters in neuronal communication, development and trauma

Electrical signaling in neurons is based on the operation of plasmalemmal ion pumps and carriers that establish transmembrane ion gradients, and on the operation of ion channels that generate current and voltage responses by dissipating these gradients. Although both voltage- and ligand-gated channels are being extensively studied, the central role of ion pumps and carriers is largely ignored in current neuroscience. Such an information gap is particularly evident with regard to neuronal Cl- regulation, despite its immense importance in the generation of inhibitory synaptic responses by GABA- and glycine-gated anion channels. The cation-chloride co-transporters (CCCs) have been identified as important regulators of neuronal Cl- concentration, and recent work indicates that CCCs play a key role in shaping GABA- and glycine-mediated signaling, influencing not only fast cell-to-cell communication but also various aspects of neuronal development, plasticity and trauma.