Cl(-)-mediated interaction between GABA and glycine currents in cultured rat hippocampal neurons

Cross-talk pattern between GABAA- and glycine-receptors in CNS neurons is shaped by their relative expression levels

GABA_A receptors (GABA_ARs) and glycine receptors (GlyRs) are two principal inhibitory chloride ion channels in the central nervous system. The two receptors do not function independently but cross-talk to each other, i.e., the activation of one receptor would inhibit the other. This cross-talk is present in different patterns across various regions in the central nervous system; however, the factor that determines these patterns is not understood. Here, we show that the pattern of cross-talk between the two receptors is shaped by their relative expression level in a neuron: a higher expression level correlates with louder talk. In line with a tendency of decrease in expression level of GlyRs and increase in expression level of GABA_ARs from the spinal cord, the brainstem to the neocortex, GlyRs talked much louder (i.e. produced greater inhibition) than GABA_ARs (one-way pattern) in spinal cord neurons, about equally loud as GABA_ARs (symmetric pattern) in inferior colliculus neurons and less loud (i.e. less inhibition) than GABA_ARs (asymmetric pattern) in auditory cortex neurons. Overexpression of GlyRs in inferior colliculus neurons produced an asymmetric pattern that should otherwise have been observed in spinal cord neurons. These expression level-dependent patterns of cross-talk between the two receptors may suggest how the central nervous system uses an alternative mechanism to maintain a delicate level of inhibition through adjusting the proportion of the two receptors in a neuron along its pathway.

Transient receptor potential vanilloid four channels modulate inhibitory inputs through differential regulation of GABA and glycine receptors in rat retinal ganglion cells

The transient receptor potential vanilloid 4 (TRPV4) channel is widely distributed in the retina. Activation of the TRPV4 channel enhances excitatory signaling from bipolar cells to retinal ganglion cells (RGCs), thereby increasing RGC firing rate and membrane excitability. In this study, we investigated the effect of TRPV4 channel activation on the miniature inhibitory postsynaptic current (mIPSC) in rat RGCs. Our results showed that perfusion with HC-067047, a TRPV4-channel antagonist, significantly reduced the amplitude of RGC mIPSCs. Extracellular application of the TRPV4 channel agonist GSK1016790A (GSK101) enhanced the frequency and amplitude of mIPSCs in ON- and OFF-type RGCs; pre-application of HC-067047 blocked the effect of GSK101 on mIPSCs. Furthermore, TRPV4 channels were able to enhance the frequency and amplitude of glycine receptor (GlyR)-mediated mIPSCs and inhibit the frequency of type A γ-aminobutyric acid receptor (GABA_A R)-mediated mIPSCs. Upon intracellular administration or intravitreal injection of GSK101, TRPV4 channel activation reduced the release of presynaptic glycine and enhanced the function and expression of postsynaptic GlyRs; however, it inhibited presynaptic release of GABA, but did not affect postsynaptic GABA_A Rs. Our study results provide insight regarding the effect of TRPV4 channel activation on RGCs and offer a potential interventional target for retinal diseases involving TRPV4 channels.

Synaptic Inhibition in Avian Interaural Level Difference Sound Localizing Neurons

Synaptic inhibition plays a fundamental role in the neural computation of the interaural level difference (ILD), an important cue for the localization of high-frequency sound. Here, we studied the inhibitory synaptic currents in the chicken posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), the first binaural level difference encoder of the avian auditory pathway. Using whole-cell recordings in brain slices, we provide the first evidence confirming a monosynaptic inhibition driven by direct electrical and chemical stimulation of the contralateral LLDp, establishing the reciprocal inhibitory connection between the two LLDps, a long-standing assumption in the field. This inhibition was largely mediated by GABAA receptors; however, functional glycine receptors were also identified. The reversal potential for the Cl- channels measured with gramicidin-perforated patch recordings was hyperpolarizing (-88 mV), corresponding to a low intracellular Cl- concentration (5.2 mm). Pharmacological manipulations of KCC2 (outwardly Cl- transporter) activity demonstrate that LLDp neurons can maintain a low intracellular Cl- concentration under a high Cl- load, allowing for the maintenance of hyperpolarizing inhibition. We further demonstrate that hyperpolarizing inhibition was more effective at regulating cellular excitability than depolarizing inhibition in LLDp neurons.

Glycinergic transmission modulates GABAergic inhibition in the avian auditory pathway

For all neurons, a proper balance of synaptic excitation and inhibition is crucial to effect computational precision. Achievement of this balance is remarkable when one considers factors that modulate synaptic strength operate on multiple overlapping time scales and affect both pre- and postsynaptic elements. Recent studies have shown that inhibitory transmitters, glycine and GABA, are co-released in auditory nuclei involved in the computation of interaural time disparities (ITDs), a cue used to process sound source location. The co-release expressed at these synapses is heavily activity dependent, and generally occurs when input rates are high. This circuitry, in both birds and mammals, relies on inhibitory input to maintain the temporal precision necessary for ITD encoding. Studies of co-release in other brain regions suggest that GABA and glycine receptors (GlyRs) interact via cross-suppressive modulation of receptor conductance. We performed in vitro whole-cell recordings in several nuclei of the chicken brainstem auditory circuit to assess whether this cross-suppressive phenomenon was evident in the avian brainstem. We evaluated the effect of pressure-puff applied glycine on synaptically evoked inhibitory currents in nucleus magnocellularis (NM) and the superior olivary nucleus (SON). Glycine pre-application reduced the amplitude of inhibitory postsynaptic currents (IPSCs) evoked during a 100 Hz train stimulus in both nuclei. This apparent glycinergic modulation was blocked in the presence of strychnine. Further experiments showed that this modulation did not depend on postsynaptic biochemical interactions such as phosphatase activity, or direct interactions between GABA and GlyR proteins. Rather, voltage clamp experiments in which we manipulated Cl⁻ flux during agonist application suggest that activation of one receptor will modulate the conductance of the other via local changes in Cl⁻ ion concentration within microdomains of the postsynaptic membrane.

Concomitant facilitation of GABAA receptors and KV7 channels by the non-opioid analgesic flupirtine

BACKGROUND AND PURPOSE

Flupirtine is a non-opioid analgesic that has been in clinical use for more than 20 years. It is characterized as a selective neuronal potassium channel opener (SNEPCO). Nevertheless, its mechanisms of action remain controversial and are the purpose of this study.

EXPERIMENTAL APPROACH

Effects of flupirtine on native and recombinant voltage- and ligand-gated ion channels were explored in patch-clamp experiments using the following experimental systems: recombinant K(IR)3 and K(V)7 channels and α3β4 nicotinic acetylcholine receptors expressed in tsA 201 cells; native voltage-gated Na(+), Ca(2+), inward rectifier K(+), K(V)7 K(+), and TRPV1 channels, as well as GABA(A), glycine, and ionotropic glutamate receptors expressed in rat dorsal root ganglion, dorsal horn and hippocampal neurons.

KEY RESULTS

Therapeutic flupirtine concentrations (≤10 µM) did not affect voltage-gated Na(+) or Ca(2+) channels, inward rectifier K(+) channels, nicotinic acetylcholine receptors, glycine or ionotropic glutamate receptors. Flupirtine shifted the gating of K(V)7 K(+) channels to more negative potentials and the gating of GABA(A) receptors to lower GABA concentrations. These latter effects were more pronounced in dorsal root ganglion and dorsal horn neurons than in hippocampal neurons. In dorsal root ganglion and dorsal horn neurons, the facilitatory effect of therapeutic flupirtine concentrations on K(V)7 channels and GABA(A) receptors was comparable, whereas in hippocampal neurons the effects on K(V)7 channels were more pronounced.

CONCLUSIONS AND IMPLICATIONS

These results indicate that flupirtine exerts its analgesic action by acting on both GABA(A) receptors and K(V)7 channels.

Cl(-) concentration changes and desensitization of GABA(A) and glycine receptors

Desensitization of ligand-gated ion channels plays a critical role for the information transfer between neurons. The current view on γ-aminobutyric acid (GABA)_A and glycine receptors includes significant rapid components of desensitization as well as cross-desensitization between the two receptor types. Here, we analyze the mechanism of apparent cross-desensitization between native GABA_A and glycine receptors in rat central neurons and quantify to what extent the current decay in the presence of ligand is a result of desensitization versus changes in intracellular Cl⁻ concentration ([Cl⁻]_i). We show that apparent cross-desensitization of currents evoked by GABA and by glycine is caused by changes in [Cl⁻]_i. We also show that changes in [Cl⁻]_i are critical for the decay of current in the presence of either GABA or glycine, whereas changes in conductance often play a minor role only. Thus, the currents decayed significantly quicker than the conductances, which decayed with time constants of several seconds and in some cells did not decay below the value at peak current during 20-s agonist application. By taking the cytosolic volume into account and numerically computing the membrane currents and expected changes in [Cl⁻]_i, we provide a theoretical framework for the observed effects. Modeling diffusional exchange of Cl⁻ between cytosol and patch pipettes, we also show that considerable changes in [Cl⁻]_i may be expected and cause rapidly decaying current components in conventional whole cell or outside-out patch recordings. The findings imply that a reevaluation of the desensitization properties of GABA_A and glycine receptors is needed.

Anomalous levels of Cl- transporters cause a decrease of GABAergic inhibition in human peritumoral epileptic cortex

PURPOSE

Several factors contribute to epileptogenesis in patients with brain tumors, including reduced γ-aminobutyric acid (GABA)ergic inhibition. In particular, changes in Cl(-) homeostasis in peritumoral microenvironment, together with alterations of metabolism, are key processes leading to epileptogenesis in patients afflicted by glioma. It has been recently proposed that alterations of Cl(-) homeostasis could be involved in tumor cell migration and metastasis formation. In neurons, the regulation of intracellular Cl(-) concentration ([Cl(-) ](i) ) is mediated by NKCC1 and KCC2 transporters: NKCC1 increases while KCC2 decreases [Cl(-) ](i) . Experiments were thus designed to investigate whether, in human epileptic peritumoral cortex, alterations in the balance of NKCC1 and KCC2 activity may decrease the hyperpolarizing effects of GABA, thereby contributing to epileptogenesis in human brain tumors.

METHODS

Membranes from peritumoral cortical tissues of epileptic patients afflicted by gliomas (from II to IV WHO grade) and from cortical tissues of nonepileptic patients were injected into Xenopus oocytes leading to the incorporation of functional GABA(A) receptors. The GABA-evoked currents were recorded using standard two-microelectrode voltage-clamp technique. In addition, immunoblot analysis and immunohistochemical staining were carried out on membranes and tissues from the same patients.

KEY FINDINGS

We found that in oocytes injected with epileptic peritumoral cerebral cortex, the GABA-evoked currents had a more depolarized reversal potential (E(GABA) ) compared to those from nonepileptic healthy cortex. This difference of E(GABA) was abolished by the NKCC1 blocker bumetanide or unblocking of KCC2 with the Zn(2+) chelator TPEN. Moreover, Western blot analysis revealed an increased expression of NKCC1, and more modestly, of KCC2 transporters in epileptic peritumoral tissues compared to nonepileptic control tissues. In addition, NKCC1 immunoreactivity was strongly increased in peritumoral cortex with respect to nonepileptic cortex, with a prominent expression in neuronal cells.

SIGNIFICANCE

We report that the positive shift of E(GABA) in epileptic peritumoral human cortex is due to an altered expression of NKCC1 and KCC2, perturbing Cl(-) homeostasis, which might lead to a consequent reduction in GABAergic inhibition. These findings point to a key role of Cl(-) transporters KCC2 and NKCC1 in tumor-related epilepsy, suggesting a more specific drug therapy and surgical approaches for the epileptic patients afflicted by brain tumors.

Extrasynaptic and postsynaptic receptors in glycinergic and GABAergic neurotransmission: a division of labor?

Glycine and GABA mediate inhibitory neurotransmission in the spinal cord and central nervous system. The general concept of neurotransmission is now challenged by the contribution of both phasic activation of postsynaptic glycine and GABA(A) receptors (GlyRs and GABA(A)Rs, respectively) and tonic activity of these receptors located at extrasynaptic sites. GlyR and GABA(A)R kinetics depend on several parameters, including subunit composition, subsynaptic localization and activation mode. Postsynaptic and extrasynaptic receptors display different subunit compositions and are activated by fast presynaptic and slow paracrine release of neurotransmitters, respectively. GlyR and GABA(A)R functional properties also rely on their aggregation level, which is higher at postsynaptic densities than at extrasynaptic loci. Finally, these receptors can co-aggregate at mixed inhibitory postsynaptic densities where they cross-modulate their activity, providing another parameter of functional complexity. GlyR and GABA(A)R density at postsynaptic sites results from the balance between their internalization and insertion in the plasma membrane, but also on their lateral diffusion from and to the postsynaptic loci. The dynamic exchange of receptors between synaptic and extrasynaptic sites and their functional adaptation in terms of kinetics point out a new adaptive process of inhibitory neurotransmission.

Characterization of the inhibitory glycine receptor on entorhinal cortex neurons

In addition to the well-established functional description of the glycine receptor (GlyR) in the spinal cord, GlyR expression has recently been found in higher brain regions, such as the striatum or hippocampus. In this study we have investigated the electrophysiological response of glycine in the rat entorhinal cortex slice. In all recorded cells we found significant current responses to glycine with an EC(50) value of about 100 micro m. Most importantly, we detected a cross-inhibition of glycine responses by GABA but not vice versa. These findings are in line with recent published data of cross-talks between GABA(A)R and GlyR but indicate a novel type of cross-inhibition of these receptors in the entorhinal cortex.

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.

Asymmetric cross-inhibition between GABAA and glycine receptors in rat spinal dorsal horn neurons

Presynaptic nerve terminals of inhibitory synapses in the dorsal horn of the spinal cord and brain stem can release both GABA and glycine, leading to coactivation of postsynaptic GABAA and glycine receptors. In the present study we have analyzed functional interactions between GABAA and glycine receptors in acutely dissociated neurons from rat sacral dorsal commissural nucleus. Although the application of GABA and glycine activates pharmacologically distinct receptors, the current induced by a simultaneous application of these two transmitters was less than the sum of currents induced by applying two transmitters separately. Sequential application of glycine and GABA revealed that the GABA-evoked current is more affected by glycine than glycine-evoked responses by GABA. Activation of glycine receptors decreased the amplitude and accelerated the rate of desensitization of GABA-induced currents. This asymmetric cross-inhibition is reversible, dependent on the agonist concentration applied, but independent of both membrane potential and intracellular calcium concentration or changes in the chloride equilibrium potential. During sequential applications, the asymmetric cross-inhibition was prevented by selective GABAA or glycine receptor antagonists, suggesting that occupation of binding sites did not suffice to induce glycine and GABAA receptors functional interaction, and receptor channel activation is required. Furthermore, inhibition of phosphatase 2B, but not phosphatase 1 or 2A, prevented GABAA receptor inhibition by glycine receptor activation, whereas inhibition of phosphorylation pathways rendered cross-talk irreversible. Taken together, our results demonstrated that there is an asymmetric cross-inhibition between glycine and GABAA receptors and that a selective modulation of the state of phosphorylation of GABAA receptor and/or mediator proteins underlies the asymmetry in the cross-inhibition.

State-dependent cross-inhibition between anionic GABA(A) and glycine ionotropic receptors in rat hippocampal CA1 neurons

Using whole-cell patch-clamp recording of acutely isolated rat hippocampal CA1 neurons, we studied the state-dependent cross-inhibition between anionic GABA(A) and glycine (Gly) ionotropic receptors. Co-application of relatively high concentrations of GABA and Gly produced a total current (IGABA + Gly) much smaller than the linear summation of the two individual responses. The mutual inhibition between IGABA and IGly was voltage-independent. However, no current inhibition was observed with co-application of low concentrations of these two agonists. The hippocampal neurons lacking Gly response (IGly) showed no current inhibition with GABA current (IGABA), whereas the neurons losing GABA response (IGABA) (after complete rundown of IGABA) showed no current inhibition with IGly. The current inhibition was also absent in the presence of Gly or GABA(A) receptor antagonists, strychnine or bicuculline, respectively. The results indicate that cross-inhibition between GABA(A) and Gly receptors is state-dependent and a direct receptor-receptor interaction might enable the cross-inhibition to occur.

Interactions between GABA and glycine at inhibitory amino acid receptors on rat olfactory bulb neurons

Whole cell voltage-clamp electrophysiology was used to examine interactions between GABA and glycine at inhibitory amino acid receptors on rat olfactory bulb neurons in primary culture. Membrane currents evoked by GABA and glycine were selectively inhibited by low concentrations of bicuculline and strychnine, respectively, suggesting that they activate pharmacologically distinct receptors. However, GABA- and glycine-mediated currents showed cross-inhibition when the two amino acids were applied sequentially. Application of one amino acid inhibited the response to immediate subsequent application of the other. In the majority of neurons, GABA inhibited subsequent glycine-evoked currents and glycine inhibited subsequent GABA-evoked currents. In a small proportion of neurons, however, GABA inhibited glycine-evoked currents but glycine had little effect on GABA-evoked currents. The reverse was true in other neurons, suggesting that alterations in chloride gradients alone did not account for the cross-inhibition. Furthermore, no cross-inhibition was observed between GABA- and glycine-evoked currents in some neurons. The amplitude of the current evoked by the coapplication of saturating concentrations of GABA and glycine in these neurons was nearly the sum of the currents evoked by GABA and glycine alone. In contrast, the currents were not additive in neurons demonstrating cross-inhibition. These results suggest that olfactory bulb neurons heterogeneously express a population of inhibitory amino acid receptors that can bind either GABA or glycine. Interactions between GABA and glycine at inhibitory amino acid receptors may provide a mechanism to modulate inhibitory synaptic transmission.

Strong synergism between GABA(A) and glycine receptors on isolated carp third-order neurons

A strong synergistic interaction between the bicuculline-sensitive GABA receptor (GABA(A) receptor) and the strychnine-sensitive glycine receptor was observed in third-order neurons acutely isolated from crucian carp retina, with the use of the whole-cell patch-clamp recording technique. In 58 of 153 cells, 10 microM GABA or glycine separately applied to amacrine/ganglion cells failed to induce any responses or only induced small currents (<20 pA), while co-application of these two chemicals resulted in much larger responses (403.05+/-319.98 pA). The current induced by the co-application was mediated by chloride channels, and both GABA(A) and glycine receptors were involved in the potentiation. The underlying mechanisms of this interaction and its possible physiological role are discussed.

Cell-specific dendritic localization of glycine receptor alpha subunit messenger RNAs

The regional and subcellular localizations of glycine receptor complex messenger RNAs were determined in the adult rat central nervous system using non-radioactive in situ hybridization. The present investigation focused on glycine receptors alpha1 and alpha2 subunit messenger RNAs. Within the central nervous system we observed that the glycine receptor alpha1 and alpha2 subunit messenger RNAs are widely expressed. At the subcellular level, these messenger RNAs are present either in neuronal somata and dendrites or somata only. Furthermore, among different regions as well as within the same region the subcellular localizations of both alpha subunit messenger RNAs are cell type-dependent. In contrast, the regional distributions of beta subunit and gephyrin messenger RNAs are essentially as previously described [Fujita M. (1991) Brain Res. 560, 23-37; Malosio M.-L. et al. (1991) Eur. molec. Biol. Org. J. 9, 2401-2409; Kirsch J. et al. (1993) Eur. J. Neurosci. 5, 1109-1117] and their messenger RNAs are confined predominantly within the somata of neurons [Kirsch J. et al. (1993); Racca et al. (1997) J. Neurosci. 17, 1691-1700]. These results demonstrate that the glycine receptor complex messenger RNAs are broadly expressed in the central nervous system and that the glycine receptor alpha1 and alpha2 subunit messenger RNAs differ in their subcellular localization depending on the neuronal population. The latter finding suggests that different mechanisms for the localization of glycine receptor alpha1 and alpha2 subunit messenger RNAs are used by distinct populations of neurons.

Glycine and GABAA receptor-mediated chloride fluxes in synaptoneurosomes from different parts of the rat brain

Strychnine-sensitive, inhibitory glycine receptors have not until lately been considered to play a significant role in neurotransmission in mammalian forebrain regions. In order to investigate the role of glycine as a neurotransmitter in brain we have measured glycine induced chloride fluxes in different adult rat forebrain areas using synaptoneurosomes and a chloride-sensitive fluorescent indicator. The results have been compared to those obtained with GABA. The synaptoneurosomes from every brain area investigated responded to both glycine and GABA with chloride fluxes in a picrotoxin sensitive manner. The effect of glycine was inhibited by strychnine, which had no effect on the GABA-induced Cl-flux. Bicuculline inhibited the effect of GABA, but had no effect on the glycine-induced Cl-flux. Addition of GABA did not affect the response to glycine and vice versa. The endogenous content of glycine and GABA in the synaptoneurosome preparations was about the same and synaptoneurosomes from every brain area investigated released both glycine and GABA upon depolarisation with KCl. The depolarisation induced release of both GABA and glycine was partly Ca(2+)-dependent and partly Ca(2+)-independent. These results indicate that glycine can induce inhibitory Cl- fluxes distinct from GABA induced fluxes in every investigated brain area and that glycine can be released upon depolarisation.

Proton modulation of functionally distinct GABAA receptors in acutely isolated pyramidal neurons of rat hippocampus

We have studied the effect of extracellular pH (pHo) on the GABAA receptor-mediated chloride conductance in acutely isolated pyramidal neurons from area CA1 of the rat hippocampus under whole-cell voltage clamp in bicarbonate-free solutions. The conductance evoked by saturating or near-saturating concentrations (200-1000 microM) of GABA showed a marked sensitivity to variations of pHo around 7.4. A decrease in pHo between 8.4 and 6.4 increased the GABAA receptor-mediated chloride conductance by about two-fold per pH unit. In contrast, when evoked by a low agonist concentration (1-10 microM) the conductance showed an equally marked decrease upon a decrease in pHo. The half-time for desensitization of the conductance induced by 500 microM GABA was around 900 ms at pHo 6.4 and 7.4, but decreased to 650 ms at pHo 8.4. A fall in pHo decreased the amount of desensitization of the conductance evoked by a 5 s application of 5 microM, but not of 500 microM, GABA. The concentration-response relationship of the GABA-induced conductance showed a local plateau between 50 and 100 microM of GABA, which was particularly evident at high pHo. Assuming two receptor populations with a high and a low affinity for GABA, the effect of H+ on the GABAA receptors could be explained as an increase in the EC50 of the high affinity receptor, and an apparently non-competitive potentiation of both the high and the low affinity receptors. The GABAA receptor-mediated conductance was markedly inhibited by 20-50 microM Zn2+. In addition, Zn2+ reverted the down-modulation by H+ observed at low GABA concentrations to up-modulation. Diazepam (1-10 microM) had only a marginal effect on the GABA-gated conductance. Taken together, the results suggest the coexistence in individual hippocampal neurons of two distinct GABAA receptor populations having differential sensitivities to H+. In the light of the inhibitory action of Zn2+ and the virtual absence of an effect of diazepam it is probable that a significant fraction of the GABAA receptors lack the gamma 2 subunit. The observation that an elevated pH has a strong suppressing effect on the conductance evoked by high concentrations of GABA may at least partly explain why an extracellular alkalosis leads to neuronal hyperexcitability.

TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured rat hippocampal neurons

Tumor necrosis factor-alpha (TNF-alpha) is a cytokine secreted by activated astrocytes and is known to alter evoked synaptic activity in slices of adult rat hippocampus. In this paper we show that TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured hippocampal neurons, acting at nanomolar concentrations. In addition, we show that the mRNA for the 55 kDa TNF-alpha receptor (TNF-R1) is detected in embryonic rat hippocampal cultures, as well as in acutely dissected embryonic and adult rat hippocampi. Possible transduction pathways mediating the TNF-alpha effect are discussed.