Synaptic activation of GABAA receptors induces neuronal uptake of Ca2+ in adult rat hippocampal slices

Neuron matters: neuromodulation with electromagnetic stimulation must consider neurons as dynamic identities

Neuromodulation with electromagnetic stimulation is widely used for the control of abnormal neural activity, and has been proven to be a valuable alternative to pharmacological tools for the treatment of many neurological diseases. Tremendous efforts have been focused on the design of the stimulation apparatus (i.e., electrodes and magnetic coils) that delivers the electric current to the neural tissue, and the optimization of the stimulation parameters. Less attention has been given to the complicated, dynamic properties of the neurons, and their context-dependent impact on the stimulation effects. This review focuses on the neuronal factors that influence the outcomes of electromagnetic stimulation in neuromodulation. Evidence from multiple levels (tissue, cellular, and single ion channel) are reviewed. Properties of the neural elements and their dynamic changes play a significant role in the outcome of electromagnetic stimulation. This angle of understanding yields a comprehensive perspective of neural activity during electrical neuromodulation, and provides insights in the design and development of novel stimulation technology.

The Long and Winding Road to Gamma-Amino-Butyric Acid as Neurotransmitter

This review centers on the discoveries made during more than six decades of neuroscience research on the role of gamma-amino-butyric acid (GABA) as neurotransmitter. In doing so, special emphasis is directed to the significant involvement of Canadian scientists in these advances. Starting with the early studies that established GABA as an inhibitory neurotransmitter at central synapses, we summarize the results pointing at the GABA receptor as a drug target as well as more recent evidence showing that GABAA receptor signaling plays a surprisingly active role in neuronal network synchronization, both during development and in the adult brain. Finally, we briefly address the involvement of GABA in neurological conditions that encompass epileptic disorders and mental retardation.

Carbonic anhydrase inhibition by acetazolamide reduces in vitro epileptiform synchronization

Depolarizing GABAA receptor-mediated currents are contributed by HCO3(-) efflux, and play a role in initiating ictal-like epileptiform events in several cortical structures supporting the view that GABAA receptor signaling actively participates to epileptiform synchronization. We employed here field potential recordings to analyze the effects of the carbonic anhydrase inhibitor acetazolamide (10 μM) on the epileptiform activity generated in vitro by piriform and entorhinal cortices (PC and EC, respectively) during application of the K(+) channel blocker 4-aminopyridine (4AP, 50 μM). Under these experimental conditions ictal- and interictal-like discharges along with high-frequency oscillations (ripples: 80-200 Hz, fast ripples: 250-500 Hz) occurred in these two regions. In both PC and EC, acetazolamide: (i) reduced the duration and the interval of occurrence of ictal discharges along with the associated ripples and fast ripples; (ii) decreased the interval of occurrence of interictal discharges and the rates of associated fast ripples; and (iii) diminished the duration and amplitude of pharmacologically isolated GABAergic events while increasing their interval of occurrence. Our results indicate that acetazolamide effectively controls 4AP-induced epileptiform synchronization in PC and EC. We propose that this action may rest on decreased GABAA receptor-mediated HCO3(-) efflux leading to diminished depolarization of principal cells and, perhaps, of interneurons.

The role of glia in stress: polyamines and brain disorders

This review focuses on the roles of glia and polyamines (PAs) in brain function and dysfunction, highlighting how PAs are one of the principal differences between glia and neurons. The novel role of PAs, such as putrescine, spermidine, and spermine and their precursors and derivatives, is discussed. However, PAs have not yet been a focus of much glial research. They affect many neuronal and glial receptors, channels, and transporters. They are therefore key elements in the development of many diseases and syndromes, thus forming the rationale for PA-focused and glia-focused therapy for these conditions.

A role for intracellular zinc in glioma alteration of neuronal chloride equilibrium

Glioma patients commonly suffer from epileptic seizures. However, the mechanisms of glioma-associated epilepsy are far to be completely understood. Using glioma-neurons co-cultures, we found that tumor cells are able to deeply influence neuronal chloride homeostasis, by depolarizing the reversal potential of γ-aminobutyric acid (GABA)-evoked currents (EGABA). EGABA depolarizing shift is due to zinc-dependent reduction of neuronal KCC2 activity and requires glutamate release from glioma cells. Consistently, intracellular zinc loading rapidly depolarizes EGABA in mouse hippocampal neurons, through the Src/Trk pathway and this effect is promptly reverted upon zinc chelation. This study provides a possible molecular mechanism linking glioma invasion to excitation/inhibition imbalance and epileptic seizures, through the zinc-mediated disruption of neuronal chloride homeostasis.

Astrocytic and neuronal accumulation of elevated extracellular K(+) with a 2/3 K(+)/Na(+) flux ratio-consequences for energy metabolism, osmolarity and higher brain function

Brain excitation increases neuronal Na⁺ concentration by 2 major mechanisms: (i) Na⁺ influx caused by glutamatergic synaptic activity; and (ii) action-potential-mediated depolarization by Na⁺ influx followed by repolarizating K⁺ efflux, increasing extracellular K⁺ concentration. This review deals mainly with the latter and it concludes that clearance of extracellular K⁺ is initially mainly effectuated by Na⁺,K⁺-ATPase-mediated K⁺ uptake into astrocytes, at K⁺ concentrations above ~10 mM aided by uptake of Na⁺,K⁺ and 2 Cl⁻ by the cotransporter NKCC1. Since operation of the astrocytic Na⁺,K⁺-ATPase requires K⁺-dependent glycogenolysis for stimulation of the intracellular ATPase site, it ceases after normalization of extracellular K⁺ concentration. This allows K⁺ release via the inward rectifying K⁺ channel K_ir4.1, perhaps after trans-astrocytic connexin- and/or pannexin-mediated K⁺ transfer, which would be a key candidate for determination by synchronization-based computational analysis and may have signaling effects. Spatially dispersed K⁺ release would have little effect on extracellular K⁺ concentration and allow K⁺ accumulation by the less powerful neuronal Na⁺,K⁺-ATPase, which is not stimulated by increases in extracellular K⁺. Since the Na⁺,K⁺-ATPase exchanges 3 Na⁺ with 2 K⁺, it creates extracellular hypertonicity and cell shrinkage. Hypertonicity stimulates NKCC1, which, aided by β-adrenergic stimulation of the Na⁺,K⁺-ATPase, causes regulatory volume increase, furosemide-inhibited undershoot in [K⁺]_e and perhaps facilitation of the termination of slow neuronal hyperpolarization (sAHP), with behavioral consequences. The ion transport processes involved minimize ionic disequilibria caused by the asymmetric Na⁺,K⁺-ATPase fluxes.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Field effects in the CNS play functional roles

An endogenous electrical field effect, i.e., ephaptic transmission, occurs when an electric field associated with activity occurring in one neuron polarizes the membrane of another neuron. It is well established that field effects occur during pathological conditions, such as epilepsy, but less clear if they play a functional role in the healthy brain. Here, we describe the principles of field effect interactions, discuss identified field effects in diverse brain structures from the teleost Mauthner cell to the mammalian cortex, and speculate on the function of these interactions. Recent evidence supports that relatively weak endogenous and exogenous field effects in laminar structures reach significance because they are amplified by network interactions. Such interactions may be important in rhythmogenesis for the cortical slow wave and hippocampal sharp wave-ripple, and also during transcranial stimulation.

Brain derived neurotrophic factor and neurotrophic factor 3 modulate neurotransmitter receptor expressions on developing spiral ganglion neurons

Cochlear spiral ganglion neurons (SGN) provide the only pathway for transmitting sound evoked activity from the hair cells to the central auditory system. Neurotrophic factor 3 (NT-3) and brain derived neurotrophic factor (BDNF) released from hair cells and supporting cells exert a profound effect on SGN survival and neural firing patterns; however, it is unclear what the effects NT-3 and BDNF have on the type of neurotransmitter receptors expressed on SGN. To address this question, the whole-cell patch clamp recording technique was used to determine what effect NT-3 and BDNF had on the function and expression of glutamate, GABA and glycine receptors (GlyR) on SGN of cochlea from postnatal C57 mouse. Receptor currents induced by the agonist of each receptor were recorded from SGN cultured with or without BDNF or NT-3. NT-3 and BDNF exerted different effects. NT-3, and to a lesser extent BDNF, enhanced the expression of GABA receptors and had comparatively little effect on glutamate receptors. Absence of BDNF and NT-3 resulted in the emergence of glycine-induced currents; however, GlyR currents were absent from the short term cultured SGN. In contrast, NT-3 and BDNF suppressed GlyR expression on SGN. These results indicate that NT-3 and BDNF exert a profound effect on the types of neurotransmitter receptors expressed on postnatal SGN, results that may have important implications for neural development and plasticity.

Fabrication and use of high-speed, concentric h+- and Ca2+-selective microelectrodes suitable for in vitro extracellular recording

Ion-selective microelectrodes (ISMs) have been used extensively in neurophysiological studies. ISMs selective for H(+) and Ca(2+) are notable for their sensitivity and selectivity, but suffer from a slow response time, and susceptibility to noise because of the high electrical resistance of the respective ion exchange cocktails. These drawbacks can be overcome by using a "coaxial" or "concentric" inner micropipette to shunt the bulk of the ion exchanger resistance. This approach was used decades ago to record extracellular [Ca(2+)] transients in cat cortex, but has not been subsequently used. Here, we describe a method for the rapid fabrication of concentric pH- and Ca(2+)-selective microelectrodes useful for extracellular studies in brain slices or other work in vitro. Construction was simplified compared with previous implementations, by using commercially available, thin-walled borosilicate glass, drawing an outer barrel with a rapid taper (similar to a patch pipette), and by use of a quick and reliable silanization procedure. Using a piezoelectric stepper to effect a rapid solution change, the response time constants of the concentric pH and Ca(2+)-electrodes were 14.9 +/- 1.3 and 5.3 +/- 0.90 ms, respectively. Use of these concentric ISMs is demonstrated in rat hippocampal slices. Activity-dependent, extracellular pH, and [Ca(2+)] transients are shown to arise two- to threefold faster, and attain amplitudes two- to fourfold greater, when recorded by concentric versus conventional ISMs. The advantage of concentric ISMs for studies of ion transport and ion diffusion is discussed.

Layer-specific pyramidal cell oscillations evoked by tetanic stimulation in the rat hippocampal area CA1 in vitro and in vivo

Tetanic stimulation of axons terminating in the CA1 region of the hippocampus induces oscillations in the gamma-to-beta frequency band (13-100 Hz) and can induce long-term potentiation (LTP). The rapid pyramidal cell discharge is driven by a mainly GABA(A)-receptor-mediated slow depolarization and entrained mainly through ephaptic interactions. This study tests whether cellular compartmentalization can explain how cells, despite severely reduced input resistance, can still fire briskly and have IPSPs superimposed on the slow GABAergic depolarization, and whether this behaviour occurs in vivo. Oscillations induced in CA1 in vitro by tetanic stimulation of the stratum radiatum or oriens were analysed using intracellular and multichannel field potentials along the cell axis. Layer-specific effects of focal application of bicuculline indicate that the GABAergic depolarization is concentrated on tetanized dendrites. Current-source density analysis and characteristics of partial spikes indicate that early action potentials are initiated in the proximal nontetanized dendrite but cannot invade the tetanized dendrite, where recurrent EPSPs and evoked IPSPs were largely suppressed. As the oscillation progresses, IPSPs recover and slow the neuronal firing to beta frequencies, with a small subpopulation of neurons continuing to fire at gamma frequency. Carbonic anhydrase dependence, threshold intensity, frequency, field strength and spike initiation/propagation of tetanus-evoked oscillations in urethane-anaesthetized rats, validate our observations in vitro, and show that these mechanisms operate in healthy tissue. However, the disrupted electrophysiology of the tetanized dendrites will disable normal information processing, has implications for LTP induction and is likely to play a role in pathological synchronization as found during epileptic discharges.

Behavioral effects and anatomic correlates after brain injury: a progesterone dose-response study

Evidence suggests that progesterone enhances functional recovery in rats after medial frontal cortical contusions; however, a high dose of progesterone exacerbates tissue loss in a stroke model when administered chronically (7-10 days) prior to injury [Stroke 31 (2000) 1173)]. This study attempts to determine progesterone's dose-response effects on behavioral performance and GABA-A receptor expression following a cortical contusion. Male rats received injections of 0, 8, 16, or 32 mg/kg progesterone in 22.5% 2-hydroxypropyl-beta-cyclodextrin following cortical impact. Lesion 8 mg/kg and lesion 16 mg/kg groups displayed less thigmotaxis in the Morris water maze (MWM) than 0 and 32 mg/kg groups and were not significantly impaired relative to shams on other water maze measures. Increased variability in the 32 mg/kg group during somatosensory neglect testing was the only evidence indicating that a high dose of progesterone was disruptive to a few animals. These results suggest that low and moderate doses of progesterone are optimal for facilitating recovery of select behaviors and that postinjury progesterone treatment permits a wider dose range than preinjury treatment. Progesterone did not affect lesion size, but a strong negative correlation was observed between thalamic GABA-A receptor density and water maze performance. Future studies could explore causes for this relationship.

Inhibition of α1E Ca2+ Channels by Carbonic Anhydrase Inhibitors

We examined if a range of carbonic anhydrase inhibitors (CAIs) interacted with the high-voltage activated voltage-sensitive calcium channels (VSCCs) encoded by the human alpha(1E) subunit. Whole-cell recordings were made from HEK293 cells stably expressing human alpha(1E)beta(3)-mediated calcium channels. SNX-482 (an alpha(1E) inhibitor) blocked alpha(1E)-mediated VSCCs with an IC(50) close to 10 nM. The anticonvulsant CAI ethoxyzolamide also inhibited these currents, with an IC(50) close to 1 microM, and produced an accompanying 20-mV hyperpolarizing shift in the steady-state inactivation profile. Other structurally diverse CAIs (e.g., acetazolamide and benzolamide) produced approximately 30 - 40% inhibition of alpha(1E)beta(3)-mediated Ca(2+) currents at 10 microM. Topiramate (10 microM), an anticonvulsant with CAI activity, inhibited these currents by 68 +/- 7%. This off-target activity of CAIs at VSCCs may contribute to some of the effects they produce both in vitro and in vivo.

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.

Regulation of Cl--HCO3- exchangers by cAMP-dependent protein kinase in adult rat hippocampal CA1 neurons

The contributions of HCO(3)(-)-dependent, DIDS-sensitive mechanisms to the maintenance of steady-state pH(i), and the regulation of their activities by cAMP-dependent protein kinase (PKA), were investigated in CA1 neurons with the H(+)-sensitive fluorophore, BCECF. The addition of HCO(3)(-)/CO(2) to neurons with "low" (pH(i) < or = 7.20) and "high" (pH(i) > 7.20) initial pH(i) values under Hepes-buffered conditions, increased and decreased steady-state pH(i), respectively. Conversely, under HCO(3)(-)/CO(2)-buffered conditions, DIDS caused pH(i) to decrease and increase in neurons with low and high initial pH(i) values, respectively. In the presence, but not the absence, of HCO(3)(-), the PKA inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate (Rp-cAMPS; 50 microM) evoked DIDS-sensitive increases and decreases in pH(i) in neurons with low and high initial pH(i) values, respectively. In contrast, in neurons with low initial pH(i) values, activation of PKA with the Sp isomer of cAMPS (Sp-cAMPS; 25 microM) elicited increases in pH(i) that were smaller in the presence than in the absence of HCO(3)(-), whereas in neurons with high initial pH(i) values, Sp-cAMPS-evoked rises in pH(i) were larger in the presence than in the absence of HCO(3)(-); the differences between the effects of Sp-cAMPS on pH(i) under the different buffering conditions were attenuated by DIDS. Consistent with the possibility that changes in the activities of HCO(3)(-)-dependent, DIDS-sensitive mechanisms contribute to the steady-state pH(i) changes evoked by the PKA modulators, in neurons with initial pH(i) values < or = 7.20, Rp-cAMPS concurrently inhibited Na(+)-independent Cl(-)-HCO(3)(-) exchange and stimulated Na(+)-dependent Cl(-)-HCO(3)(-) exchange; in contrast, Sp-cAMPS concurrently stimulated Na(+)-independent Cl(-)-HCO(3)(-) exchange and inhibited Na(+)-dependent Cl(-)-HCO(3)(-) exchange. Data from a limited number of neurons with initial pH(i) values > 7.20 suggested that the directions of the reciprocal changes in anion exchange activities (inhibition or stimulation) evoked by Rp- and Sp-cAMPS may be opposite in cells with low vs. high resting pH(i) values. Taken together, the results indicate that the effects of modulating PKA activity on steady-state pH(i) in rat CA1 neurons under HCO(3)(-)/CO(2)-buffered conditions reflect not only changes in Na(+)-H(+) exchange activity but also changes in Na(+)-dependent and Na(+)-independent Cl(-)-HCO(3)(-) exchange activity that, in turn, may be dependent upon the initial pH(i).

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

Optical imaging of long-lasting depolarization on burst stimulation in area CA1 of rat hippocampal slices

Postsynaptic depolarization of dendrites paired with spike generation at the soma is considered to be a central mechanism of long-term potentiation (LTP) induction and a prime example of a Hebbian synapse. This pairing, however, has never been actually demonstrated on tetanic stimulation. Optical imaging of neural activity with a voltage-sensitive dye (VSD) is one potentially suitable method for examining this pairing. It is possible with optical recording to examine simultaneously the excitation of postsynaptic neurons at multiple sites. Thus the pairing of spike generation at the soma and dendritic depolarization can be examined with population level optical recording in highly laminar structures such as the hippocampal slice preparation. For example, one can correlate the optical signals obtained from cell layers with the activity of the soma, and, similarly, optical signals from stratum radiatum can be correlated with the activity of the apical dendrite, even though one cannot calibrate the optical signals in terms of actual membrane potential. Using the VSD aminonaphthylethenylpyridinium in rat hippocampal slices, we aimed to examine the pairing. Standard tetanic stimulation (100 Hz, 1 s) that elicited LTP in the field excitatory postsynaptic potential (fEPSP) resulted in a long-lasting depolarizing optical signal (about 2 s) that spread progressively along the known input pathway of CA1. The time course of this long-lasting depolarization was similar to that recorded intracellularly and to that reflected in the fEPSP. The long-lasting depolarization was insensitive to D,L-2-amino-5-phosphonovaleric acid (D,L-APV, 50 microM), but D,L-APV inhibited the induction of LTP; this allowed us to increase the signal-to-noise ratio of the optical signal by averaging several trials. Using this improved optical signal, we confirmed that postsynaptic cells practically "missed" spikes during tetanic stimulation in most parts of CA1, which had been suggested in the intracellular recordings. Intracellular recordings revealed a 23% reduction in input resistance, which might explain the failed spike generation at the soma via shunting. A steep spatial convergence of the depolarization along the transverse axis of area CA1 was observed. In contrast to the response resulting from a standard 100-Hz tetanus, broader activation, and paired depolarization with somatic spikes was observed on theta-burst stimulation. Overall we concluded that postsynaptic spike generation, at least in synchronous form, has less effect on LTP induction with standard tetanic stimulation, while theta-burst tetanic stimulation can elicit pairing of dendritic depolarization and somatic discharge.

Distinct signal transduction pathways for GABA-induced GABA(A) receptor down-regulation and uncoupling in neuronal culture: a role for voltage-gated calcium channels

Changes in GABA receptor (GABA(A)R) gene expression are detected in animal models of epilepsy, anxiety and in post-mortem schizophrenic brain, suggesting a role for GABA(A)R regulation in neurological disorders. Persistent (48 h) exposure of brain neurons in culture to GABA results in down-regulation of GABA(A)R number and uncoupling of GABA and benzodiazepine (BZD) binding sites. Given the central role of GABA(A)Rs in fast inhibitory synaptic transmission, GABA(A)R down-regulation and uncoupling are potentially important mechanisms of regulating neuronal excitability, yet the molecular mechanisms remain unknown. In this report we show that treatment of brain neurons in culture with tetrodotoxin, glutamate receptor antagonists, or depolarization with 25 mM K(+) fails to alter GABA(A)R number or coupling. Changes in neuronal activity or membrane potential are therefore not sufficient to induce either GABA(A)R down-regulation or uncoupling. Nifedipine, a voltage-gated Ca(2+) channel (VGCC) blocker, inhibits both GABA-induced increases in [Ca(2+)](i) and GABA(A)R down-regulation, suggesting that VGCC activation is required for GABA(A)R down-regulation. Depolarization with 25 mM K(+) produces a sustained increase in intracellular [Ca(2+)] without causing GABA(A)R down-regulation, suggesting that activation of VGCCs is not sufficient to produce GABA(A)R down-regulation. In contrast to GABA(A)R down-regulation, nifedipine and 25 mM K(+) fail to inhibit GABA-induced uncoupling, demonstrating that GABA-induced GABA(A)R down-regulation and uncoupling are mediated by independent molecular events. Therefore, GABA(A)R activation initiates at least two distinct signal transduction pathways, one of which involves elevation of intracellular [Ca(2+)] through VGCCs.

Depolarizing/hyperpolarizing effects of the GABA(A) synapse have a beneficial role in synaptic weight resetting in the hippocampus

To eliminate undesirable memory traces from the short-term memory system, it is a crucial question to reorganize the activity induced synaptic efficiency. Here we suggest a possible solution for this mechanism. In the first part of the paper we demonstrate, that under different physiological conditions even in the case of adult animals, the GABA(A) synapse may mediate a depolarizing effect on the membrane potential. Considering the GABA(A) receptor-mediated complex effects on the membrane potential of adult mammalian central neurons we raised the question of the properties of the network, which has elements in the synaptic weight matrix with changing signs. The second part pinpoints the beneficial learning ability of such a network.

Ictal epileptiform activity is facilitated by hippocampal GABAA receptor-mediated oscillations

The cellular and network mechanisms of the transition of brief interictal discharges to prolonged seizures are a crucial issue in epilepsy. Here we used hippocampal slices exposed to ACSF containing 0 Mg(2+) to explore mechanisms for the transition to prolonged (3-42 sec) seizure-like ("ictal") discharges. Epileptiform activity, evoked by Shaffer collateral stimulation, triggered prolonged bursts in CA1, in 50-60% of slices, from both adult and young (postnatal day 13-21) rats. In these cases the first component of the CA1 epileptiform burst was followed by a train of population spikes at frequencies in the gamma band and above (30-120 Hz, reminiscent of tetanically evoked gamma oscillations). The gamma burst in turn could be followed by slower repetitive "tertiary" bursts. Intracellular recordings from CA1 during the gamma phase revealed long depolarizations, action potentials rising from brief apparent hyperpolarizations, and a drop of input resistance. The CA1 gamma rhythm was completely blocked by bicuculline (10-50 microm), by ethoxyzolamide (100 microm), and strongly attenuated in hyperosmolar perfusate (50 mm sucrose). Subsequent tertiary bursts were also blocked by bicuculline, ethoxyzolamide, and in hyperosmolar perfusate. In all these cases intracellular recordings from CA3 revealed only short depolarizations. We conclude that under epileptogenic conditions, gamma band oscillations arise from GABA(A)ergic depolarizations and that this activity may lead to the generation of ictal discharges.

Quantification of optical signals with electrophysiological signals in neural activities of Di-4-ANEPPS stained rat hippocampal slices

We have quantified the optical signals of synaptically induced neural activities in an in vitro brain slice preparation in terms of electrophysiological signals. The qualification was done using electrophysiologically well known neural activities in the CA1 area of rat hippocampal slices stained with externally applied fluorescent voltage-sensitive dye (VSD; Di-4-ANEPPS). Together with a newly designed CCD-based digital high-speed camera system and epi-fluorescent optics, our improvements were made on a protocol for staining using a newly designed chamber system. These improvements enabled us to make stable and reliable recordings of optical signals and electrophysiological measurements without affecting the physiological status and to make a quantitative comparison between them. The time course and amplitude of the optical signal showed fair agreement with intracellular and extracellular recordings, and was stable over 2 h. The optical signal followed synaptically induced long-term potentiation (LTP) as monitored by the electrophysiological signals. A regional difference in the amount of LTP was found in optical signals and was confirmed in the electrophysiological signals. These results demonstrate the capabilities of our improved method as an alternative but more potent tool to measure the neuronal activities of brain slice in addition to electrophysiological method.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Comparison of changes evoked by GABA (γ-aminobutyric acid) and anoxia in [K+]o, [Cl-]o, and [Na+]o in stratum pyramidale and stratum radiatum of the guinea pig hippocampus

Ion-selective microelectrode recordings were made to assess a possible contribution of extracellular γ-aminobutyric acid (GABA) accumulation to early responses evoked in the brain by anoxia and ischemia. Changes evoked by GABA or N2 in [K+]o, [Cl-]o, [Na+]o, and [TMA+]o were recorded in the cell body and dendritic regions of the stratum pyramidale (SP) and stratum radiatum (SR), respectively, of pyramidal neurons in CA1 of guinea pig hippocampal slices. Bath application of GABA (1-10 mM) for approximately 5 min evoked changes in [K+]o and [Cl-]o with respective EC50 levels of 3.8 and 4.1 mM in SP, and 4.7 and 5.6 mM in SR. In SP 5 mM GABA reversibly increased [K+]o and [Cl-]o and decreased [Na+]o; replacement of 95% O2 -5% CO2 by 95% N2 -5% CO2 for a similar period of time evoked changes which were for each ion in the same direction as those with GABA. In SR both GABA and N2 caused increases in [K+]o and decreases in [Cl-]o and [Na+]o. The reduction of extracellular space, estimated from levels of [TMA+]o during exposures to GABA and N2, was 5-6% and insufficient to cause the observed changes in ion concentration. Ion changes induced by GABA and N2 were reversibly attenuated by the GABAA receptor antagonist bicuculline methiodide (BMI, 100 µM). GABA-evoked changes in [K+]o in SP and SR and [Cl-]o in SP were depressed by >=90%, and of [Cl-]o in SR by 50%; N2-evoked changes in [K+]o in SP and SR were decreased by 70% and those of [Cl-]o by 50%. BMI blocked Δ [Na+]o with both GABA and N2 by 20-30%. It is concluded that during early anoxia: (i) accumulation of GABA and activation of GABAA receptors may contribute to the ion changes and play a significant role, and (ii) responses in the dendritic (SR) regions are greater than and (or) differ from those in the somal (SP) layers. A large component of the [K+]o increase may involve a GABA-evoked Ca2+-activated gk, secondary to [Ca2+]i increase. A major part of [Cl-]o changes may arise from GABA-induced gCl and glial efflux, with strong stimulation of active outward transport and anion exchange at SP, and inward Na+/K+/2Cl- co-transport at SR. Na+ influx is attributable mainly to Na+-dependent transmitter uptake, with only a small amount related to GABAA receptor activation. Although the release and (or) accumulation of GABA during anoxia might be viewed as potentially protectant, the ultimate role may more likely be an important contribution to toxicity and delayed neuronal death.

Key words: brain slices, ion-selective microelectrodes, stratum pyramidale, stratum radiatum, bicuculline methiodide, extracellular space shrinkage.

Pharmacological isolation of the synaptic and nonsynaptic components of the GABA-mediated biphasic response in rat CA1 hippocampal pyramidal cells

High-frequency stimulation (HFS) applied to stratum radiatum of a rat hippocampal slice in the presence of ionotropic glutamate receptor antagonists evokes a biphasic GABA(A) receptor-dependent response in CA1 pyramidal neurons, with a brief hyperpolarizing IPSP (hIPSP) followed by a long-lasting depolarization. We show now that it is possible to pharmacologically separate the hIPSP and late depolarization from one another. In neurons intracellularly perfused for 1-2 hr with F(-) as the major anion and no ATP, the hIPSP (and the corresponding current, hIPSC) evoked by HFS was blocked, whereas neither the late depolarization nor its underlying current was attenuated. In contrast, internal perfusion with a high concentration (5 mM) of the impermeant lidocaine derivative QX-314 selectively abolished the depolarizing component of the biphasic response and also strongly reduced depolarizations evoked by extracellular microinjection of K(+). Bath application of quinine (0. 2-0.5 mM) or quinidine (0.1 mM) resulted in a pronounced inhibition of the HFS-induced extracellular K(+) concentration ([K(+)](o)) transient but not of the bicarbonate-dependent alkaline shift in extracellular pH. The attenuation of the [K(+)](o) transient was closely paralleled by a suppression of the HFS-evoked depolarization but not of the hIPSP. Quini(di)ne did not affect depolarizations induced by exogenous K(+) either. These data provide direct pharmacological evidence for the view that the HFS-induced biphasic response of the pyramidal neuron is composed of mechanistically distinct components: a direct GABA(A) receptor-mediated phase, which is followed by a slow, nonsynaptic [K(+)](o)-mediated depolarization. The bicarbonate-dependent, activity-induced [K(+)](o) transient can be blocked by quini(di)ne, whereas its depolarizing action in the pyramidal neuron is inhibited by internal QX-314. The presence of fundamentally distinct components in GABA(A) receptor-mediated actions evoked by HFS calls for further investigations of their functional role(s) in standard experimental maneuvers, such as those used in studies of synaptic plasticity and induction of gamma oscillations.

On the synchronizing mechanisms of tetanically induced hippocampal oscillations

gamma (30-100 Hz) and beta (10-30 Hz) oscillations follow tetanic stimulation in the CA1 region of the rat hippocampal slice. Pyramidal neurons undergo a slow depolarization after the tetanus and generate synchronous action potentials. The slow depolarization was previously attributed to metabotropic glutamate receptor (mGluR) activation. However, we found that this event was mediated by GABA(A) receptors, being blocked by bicuculline (50 microM) and accompanied by a dramatic drop in input resistance. Experiments with NMDA and non-NMDA glutamate receptor antagonists revealed that fast synaptic excitation was not necessary for oscillations. IPSPs were strongly depressed during the oscillations. Instead, synchronization was caused by field effects, as shown by: (1) Action potentials of pyramidal neurons proximal (<200 micrometer) to the stimulation site were often preceded by negative deflections of the intracellular potential that masked a net transmembrane depolarization caused by the population spike. (2) Pyramidal neurons located on the surface of the slice, where field effects are weak, fired repetitively but were not synchronized to the network activity. (3) A moderate decrease (50 mOsm) in artificial CSF (ACSF) osmolality did not affect the slow depolarization but increased oscillation amplitude and duration and recruited previously silent neurons into oscillations. (4) 50 mOsm increase in ACSF osmolality dramatically reduced, or abolished, post-tetanic oscillations. Phasic IPSPs, not detectable in proximal neurons, were present, late in the oscillation, in cells located 200-400 micrometer from the stimulation site and possibly contributed to slowing the rhythm during the gamma to beta transition.