GABAergic inhibition and the induction of spontaneous epileptiform activity by low chloride and high potassium in the hippocampal slice

doi:10.1016/0006-8993(88)91068-2

Brain Research

Volume 445, Issue 1, 29 March 1988, Pages 12-18

https://doi.org/10.1016/0006-8993(88)91068-2 Get rights and content

Abstract

Intracellular recordings from CA3b/c neurons in rat hippocampal slices showed that reduction of the extracellular Cl⁻ concentration from 136 to 53 mM produced a positive (+ 10 mV) shift in the reversal potential of GABAergic inhibitory postsynaptic potentials (IPSPs). This shift was not significantly different from the shift produced by raising K⁺ from 3.5 to 8.5 mM. Spontaneous interictal bursting occurred in both low Cl⁻ and high K⁺. Extracellular recordings from the pyramidal cell layer in the CA3b/c region of hippocampal slices showed that bursts in 56 mM Cl⁻ were of the same waveform and intensity as bursts produced by high K⁺. However the frequency of spontaneous bursting was much lower (6.6 ± 1.2/min, n = 10) in low Cl⁻ compared to high K⁺ (42.2 ± 3.0/min, n = 33). Burst frequency was a linear function of the shift in IPSP reversal potential produced by high K⁺, but not low Cl⁻. Replacing 60% of the Cl⁻ with methylsulfate or isethionate was sufficient to produce spontaneous bursting, whereas it was necessary to replace 80% of the Cl⁻ when propionate was used as a substitute. All 3 Cl⁻ substitutes lowered the ionized Ca²⁺ concentration, but raising the extracellular Ca²⁺ concentration back to normal did not change the burst frequency. Since the amplitude of IPSPs is reduced to a similar extent in low Cl⁻ and high K⁺ solutions, whereas bursting is much faster in high K⁺, we suggest that impaired GABAergic inhibition is insufficient to fully account for spontaneous interictal bursting that is produced in hippocampal slices by raised extracellular K⁺.

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Mannitol decreases neocortical epileptiform activity during early brain development via cotransport of chloride and water
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Seizures and brain injury lead to water and Cl⁻ accumulation in neurons. The increase in intraneuronal Cl⁻ concentration ([Cl⁻]_i) depolarizes the GABA_A reversal potential (E_GABA) and worsens seizure activity. Neocortical neuronal membranes have a low water permeability due to the lack of aquaporins necessary to move free water. Instead, neurons use cotransport of ions including Cl⁻ to move water. Thus, increasing the extracellular osmolarity during seizures should result in an outward movement of water and salt, reducing [Cl⁻]_i and improving GABA_A receptor-mediated inhibition. We tested the effects of hyperosmotic therapy with a clinically relevant dose of mannitol (20 mM) on epileptiform activity, spontaneous multiunit activity, spontaneous inhibitory post-synaptic currents (sIPSCs), [Cl⁻]_i, and neuronal volume in layer IV/V of the developing neocortex of C57BL/6 and Clomeleon mice. Using electrophysiological techniques and multiphoton imaging in acute brain slices (post-natal day 7–12) and organotypic neocortical slice cultures (post-natal day 14), we observed that mannitol: 1) decreased epileptiform activity, 2) decreased neuronal volume and [Cl⁻]_i through CCCs, 3) decreased spontaneous multi-unit activity frequency but not amplitude, and 4) restored the anticonvulsant efficacy of the GABA_A receptor modulator diazepam. Increasing extracellular osmolarity by 20 mOsm with hypertonic saline did not decrease epileptiform activity. We conclude that an increase in extracellular osmolarity by mannitol mediates the efflux of [Cl⁻]_i and water through CCCs, which results in a decrease in epileptiform activity and enhances benzodiazepine actions in the developing neocortex in vitro. Novel treatments aimed to decrease neuronal volume may concomitantly decrease [Cl⁻]_i and improve seizure control.
High frequency stimulation or elevated K<sup>+</sup> depresses neuronal activity in the rat entopeduncular nucleus
2007, Neuroscience
Citation Excerpt :
They attributed this depression to axonal conduction failure. Their findings were contrary to others who showed an increase in neuronal activity (Chamberlin and Dingledine, 1988; McBain and Dingledine, 1992), but they suggested that their use of young animals could contribute to this difference. At the cellular level, increased [K+]e increases cell excitability (Pumain and Heinemann, 1985; Kreisman and Smith, 1993; Jensen et al., 1994; Jefferys, 1995) via a GHK-based depolarizing effect on the resting membrane potential.
High frequency stimulation (HFS) is applied to many brain regions to treat a variety of neurological disorders/diseases, yet the mechanism(s) underlying its effects remains unclear. While some studies showed that HFS inhibits the stimulated nucleus, others report excitation. In this in vitro study, we stimulated the rat globus pallidus interna (entopeduncular nucleus, EP), a commonly stimulated area for Parkinson’s disease, to investigate the effect of HFS-induced elevation of extracellular potassium (K⁺_e) on rat EP neuronal activity. Whole-cell patch-clamp recordings and [K⁺]_e measurements were obtained in rat EP brain slices before, during and after HFS. After HFS (150 Hz, 10 s), [K⁺]_e increased from 2.5–9.6±1.4 mM, the resting membrane potential of EP neurons depolarized by 11.1±2.5 mV, spiking activity was significantly depressed, and input resistance decreased by 25±6%. The GABA_A receptor blocker, gabazine, did not prevent these effects. The bath perfusion of 6 or 10 mM K⁺, with or without synaptic blockers, mimicked the HFS-mediated effects: inhibition of spike activity, a 20±9% decrease in input resistance and a 17.4±3.0 mV depolarization. This depolarization exceeded predicted values of elevated [K⁺]_e on the resting membrane potential. A depolarization block did not fully account for the K⁺-induced inhibition of EP neuronal activity. Taken together, our results show that HFS-induced elevation of [K⁺]_e decreased EP neuronal activity by the activation of an ion conductance resulting in membrane depolarization, independent of synaptic involvement. These findings could explain the inhibitory effects of HFS on neurons of the stimulated nucleus.
Sodium-activated potassium conductance participates in the depolarizing afterpotential following a single action potential in rat hippocampal CA1 pyramidal cells
2004, Brain Research
Citation Excerpt :
High [K+]o increased DAP amplitude more while synaptic inputs were intact than while they were blocked (Fig. 2C). The reason for this is not clear, but it may be partly explained by a reduction of the tonic or evoked GABA-A receptor mediated inhibition because of the decrease of Cl− gradient across the membrane induced by high [K+]o[4,22]. The blockade of GABA-A receptor mediated inhibition during high [K+]o may increase the DAP, but only in control aCSF in which GABA-A inhibition was intact.
The depolarizing afterpotential (DAP) following an action potential increases the excitability of a neuron. Mechanisms related to the DAP following an antidromic or current-induced spike were studied in CA1 pyramidal cells by whole-cell recordings in hippocampal slices in vitro. In DAP-holding voltage curves, the DAP at 10 ms after the spike peak (DAP10) was extrapolated to reverse at about −50 mV. Increase of extracellular K⁺ concentration increased DAP and neuronal bursting. DAP10 reversal potential shifted positively with an increase in [K⁺]_o and with the blockade of K⁺ conductance using pipettes filled with Cs⁺. Similarly, extracellular tetraethylammonium (TEA; 10 mM), 4-aminopyridine (3–10 mM) increased DAP and shifted the DAP10 reversal potential to a depolarizing direction. Decrease of [Ca²⁺]_o did not alter DAP significantly, suggesting a nonessential role of Ca²⁺ in the DAP. Perfusion of tetrodotoxin (TTX; 0.1–1 μM) and replacement of extracellular Na⁺ by choline⁺ suppressed both spike height and DAP simultaneously. Replacement of extracellular Na⁺ by Li⁺ increased DAP and spike bursts, and caused a positive shift of the DAP10 reversal potential. It is suggested that Li⁺ increased DAP by blocking an Na⁺-activated K⁺ current. In summary, multiple K⁺ conductances are normally active during the DAP following a single action potential.
Potassium and calcium channel dependence of bursting in cultured neuronal networks
1994, Brain Research
Increases in extracellular potassium concentrations reliably increase burst rates in cultured fetal murine spinal cord networks. This effect could be mimicked by either blocking voltage-gated potassium conductances or facilitating excitatory synaptic interactions, but not by blocking specific calcium-dependent potassium conductances or tonic depolarization. Spontaneous bursting in cultured networks is apparently dependent on potassium currents and intracellular calcium levels, but not on the pharmacologically characterized calcium-dependent potassium conductances.
Disinhibition of hippocampal CA3 neurons induced by suppression of an adenosine A<inf>1</inf> receptor-mediated inhibitory tonus: Pre- and postsynaptic components
1993, Neuroscience
Intracellular recordings were performed on hippocampal CA3 neuronsin vitro to investigate the inhibitory tonus generated by endogenously produced adenosine in this brain region. Bath application of the highly selective adenosine A₁ receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine at concentrations up to 100 nM induced both spontaneous and stimulus-evoked epileptiform burst discharges. Once induced, the 1,3-dipropyl-8-cyclopentylxanthine-evoked epileptiform activity was apparently irreversible even after prolonged superfusion with drug-free solution. The blockade of glutamatergic excitatory synaptic transmission by preincubation of the slices with the amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 μM), but not with theN-methyl-d-aspartate receptor antagonistd-2-amino-5-phosphonovaleric acid (50/μM), prevented the induction of epileptiform activity by 1,3-dipropyl-8-cyclopentylxanthine. The generation of the burst discharges was independent of the membrane potential, and the amplitude of the slow component of the paroxysmal depolarization shift increased with hyperpolarization, indicating that the 1,3-dipropyl-8-cyclopentylxanthine-induced bursts were synaptically mediated events. Recordings from tetrodotoxin-treated CA3 neurons revealed a strong postsynaptic component of endogenous adenosinergic inhibition. Both 1,3-dipropyl-8-cyclopentylxanthine and the adenosine-degrading enzyme adenosine deaminase produced an apparently irreversible depolarization of the membrane potential by about 20 mV. Sometimes, this depolarization attained the threshold for the generation of putative calcium spikes, but no potential changes resembling paroxysmal depolarization shift-like events were observed.
At the concentrations used in electrophysiological experiments (30–100 nM), 1,3-dipropyl-8-cyclopentylxanthine displayed only a negligible inhibitory action on total cyclic nucleotide phosphodiesterase activity measured by means of a radiochemical assay in a homogenate of the rat cerebral cortex. Furthermore, even high concentrations of the selective phosphodiesterase inhibitor rolipram (10 μM), which displays no affinity to adenosine receptors, did not mimic the electrophysiological actions of 1,3-dipropyl-8-cyclopentylxanthine, thus excluding the possibility that the effects of the A₁ receptor antagonist on neuronal discharge behavior can be ascribed to an inhibition of phosphodiesterases.
The present data demonstrate that endogenously released adenosine exerts a vigorous control on the excitability of hippocampal CA3 neurons on both the pre- and postsynaptic sites. The long-lasting disinhibition following a transient suppression of adenosinergic inhibition strongly suggests that, besides its well-known short-term effects on neuronal activity, adenosine might also contribute to the long-term control of hippocampal excitability.
Potassium-induced changes in excitability in the hippocampal CA1 region of immature and adult rats
1993, Developmental Brain Research
Orthodromic and spontaneous population spike activity was measured in vitro in the CA1 region of rat hippocampal slices to determine maturational differences in excitability and susceptibility to K⁺-induced seizures. Several indices of excitability in the CA1 region changed in a non-monotonic fashion during maturation, in response to step-wise increases in bath [K⁺]. Slices from rats 18–22 days old, showed a greater probability of both spontaneous epileptiform activity and episodes of seizure-like activity followed by spreading depression, and more prolonged durations of evoked seizure-like events. Elevation of [K⁺] in the bathing medium increased these indices in a similar manner in older rats but not to the same degree as in 18- to 22-day-old rats. However, the threshold level of bath [K⁺] resulting in evoked bursts of population spikes was lower in adult and 28- to 32-day-old rats than in 18- to 22-day-old rats, suggesting that excitability is not uniformly greater at any given age. In 10- to 15-day-old rats, elevation of bath [K⁺] either produced persistent blockade of population responses, or increased the amplitude of the initial population spike, without producing bursts. Basal levels of [K⁺] in the interstitium of the slices corresponded to the various levels of [K⁺] placed in the bathing medium and there were no differences among age groups. Therefore, differences in basal [K⁺]_o, cannot account for the maturational changes in excitability and seizure activity. The period from 18–22 days of age in the rat is a useful focal point for investigating mechanisms underlying maturational changes in propensity to develop seizures.

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Research reportGABAergic inhibition and the induction of spontaneous epileptiform activity by low chloride and high potassium in the hippocampal slice

Abstract

Brain Research

Ammonium action on postsynaptic inhibition in crayfish neurones: implications for the mechanism of chloride extrusion

J. Physiol. (Lond.)

Gamma-aminobutyric acid hyperpolarizes rat hippocampal pyramidal cells trough a calcium-dependent potassium conductance

J. Physiol. (Lond.)

Spontaneous epileptiform activity in hippocampal slices bathed in high potassium or low chloride

Soc. Neurosci. Abstr.

The role of intracellular chloride in hyperpolarizing post-synaptic inhibition of crayfish stretch receptor neurones

J. Physiol. (Lond.)

Possible mechanisms of enkephalin action on hippocampal CA1 pyramidal neurons

J. Neurosci.

The hippocampus

Physiol. Rev.

Anion transport in the nervous system

Epileptiform burst activity induced by potassium in the hippocampus and its regulation by GABA-mediated inhibition

J. Neurophysiol.

GABA-mediated inhibitory mechanisms in relation to epileptic discharges

Research report
GABAergic inhibition and the induction of spontaneous epileptiform activity by low chloride and high potassium in the hippocampal slice