Region-specific modulation of electrically induced synchronous oscillations in the rat hippocampus and cerebral cortex

Columnar distribution of activity dependent gabaergic depolarization in sensorimotor cortical neurons

Background

GABA, the major inhibitory neurotransmitter in CNS, has been demonstrated to paradoxically produce excitation even in mature brain. However activity-dependent form of GABA excitation in cortical neurons has not been observed. Here we report that after an intense electrical stimulation adult cortical neurons displayed a transient GABA excitation that lasted for about 30s.

Results

Whole-cell patch recordings were performed to evaluate the effects of briefly applied GABA on pyramidal neurons in adult rodent sensorimotor cortical slice before and after 1 s, 20 Hz suprathreshold electrical stimulation of the junction between layer 6 and the underlying white matter (L6/WM stimulation). Immediately after L6/WM stimulation, GABA puffs produced neuronal depolarization in the center of the column-shaped region. However, both prior to or 30s after stimulation GABA puffs produced hyperpolarization of neurons. 2-photon imaging in neurons infected with adenovirus carrying a chloride sensor Clomeleon revealed that GABA induced depolarization is due to an increase in [Cl^-]_i after stimulation. To reveal the spatial extent of excitatory action of GABA, isoguvacine, a GABA_A receptors agonist, was applied right after stimulation while monitoring the intracellular Ca²⁺ concentration in pyramidal neurons. Isoguvacine induced an increase in [Ca²⁺]_i in pyramidal neurons especially in the center of the column but not in the peripheral regions of the column. The global pattern of the Ca²⁺ signal showed a column-shaped distribution along the stimulation site.

Conclusion

These results demonstrate that the well-known inhibitory transmitter GABA rapidly switches from hyperpolarization to depolarization upon synaptic activity in adult somatosensory cortical neurons.

Common time-frequency analysis of local field potential and pyramidal cell activity in seizure-like events of the rat hippocampus

To study cell-field dynamics, physiologists simultaneously record local field potentials and the activity of individual cells from animals performing cognitive tasks, during various brain states or under pathological conditions. However, apart from spike shape and spike timing analyses, few studies have focused on elucidating the common time-frequency structure of local field activity relative to surrounding cells across different periods of phenomena. We have used two algorithms, multi-window time frequency analysis and wavelet phase coherence (WPC), to study common intracellular-extracellular (I-E) spectral features in spontaneous seizure-like events (SLEs) from rat hippocampal slices in a low magnesium epilepsy model. Both algorithms were applied to 'pairs' of simultaneously observed I-E signals from slices in the CA1 hippocampal region. Analyses were performed over a frequency range of 1-100 Hz. I-E spectral commonality varied in frequency and time. Higher commonality was observed from 1 to 15 Hz, and lower commonality was observed in the 15-100 Hz frequency range. WPC was lower in the non-SLE region compared to SLE activity; however, there was no statistical difference in the 30-45 Hz band between SLE and non-SLE modes. This work provides evidence of strong commonality in various frequency bands of I-E SLEs in the rat hippocampus, not only during SLEs but also immediately before and after.

Nociceptive afferent activity alters the SI RA neuron response to mechanical skin stimulation

Procedures that reliably evoke cutaneous pain in humans (i.e., 5-7 s skin contact with a 47-51 °C probe, intradermal algogen injection) are shown to decrease the mean spike firing rate (MFR) and degree to which the rapidly adapting (RA) neurons in areas 3b/1 of squirrel monkey primary somatosensory cortex (SI) entrain to a 25-Hz stimulus to the receptive field center (RF(center)) when stimulus amplitude is "near-threshold" (i.e., 10-50 μm). In contrast, RA neuron MFR and entrainment are either unaffected or enhanced by 47-51 °C contact or intradermal algogen injection when the amplitude of 25-Hz stimulation is 100-200 μm (suprathreshold). The results are attributed to an "activity dependence" of γ-aminobutyric acid (GABA) action on the GABA(A) receptors of RA neurons. The nociceptive afferent drive triggered by skin contact with a 47-51 °C probe or intradermal algogen is proposed to activate nociresponsive neurons in area 3a which, via corticocortical connections, leads to the release of GABA in areas 3b/1. It is hypothesized that GABA is hyperpolarizing/inhibitory and suppresses stimulus-evoked RA neuron MFR and entrainment whenever RA neuron activity is low (as when the RF(center) stimulus is weak/near-threshold) but is depolarizing/excitatory and augments MFR and entrainment when RA neuron activity is high (when the stimulus is strong/suprathreshold).

Prototypic seizure activity driven by mature hippocampal fast-spiking interneurons

A variety of epileptic seizure models have shown that activation of glutamatergic pyramidal cells is usually required for rhythm generation and/or synchronization in hippocampal seizure-like oscillations in vitro. However, it still remains unclear whether GABAergic interneurons may be able to drive the seizure-like oscillations without glutamatergic transmission. Here, we found that electrical stimulation in rat hippocampal CA1 slices induced a putative prototype of seizure-like oscillations ("prototypic afterdischarge," 1.8-3.8 Hz) in mature pyramidal cells and interneurons in the presence of ionotropic glutamate receptor antagonists. The prototypic afterdischarge was abolished by GABA(A) receptor antagonists or gap junction blockers, but not by a metabotropic glutamate receptor antagonist or a GABA(B) receptor antagonist. Gramicidin-perforated patch-clamp and voltage-clamp recordings revealed that pyramidal cells were depolarized and frequently excited directly through excitatory GABAergic transmissions in each cycle of the prototypic afterdischarge. Interneurons that were actively spiking during the prototypic afterdischarge were mostly fast-spiking (FS) interneurons located in the strata oriens and pyramidale. Morphologically, these interneurons that might be "potential seizure drivers" included basket, chandelier, and bistratified cells. Furthermore, they received direct excitatory GABAergic input during the prototypic afterdischarge. The O-LM cells and most of the interneurons in the strata radiatum and lacunosum moleculare were not essential for the generation of prototypic afterdischarge. The GABA-mediated prototypic afterdischarge was observed later than the third postnatal week in the rat hippocampus. Our results suggest that an FS interneuron network alone can drive the prototypic form of electrically induced seizure-like oscillations through their excitatory GABAergic transmissions and presumably through gap junction-mediated communications.

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

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

Comparable GABAergic Mechanisms of Hippocampal Seizurelike Activity in Posttetanic and Low-Mg2+ Conditions

It is known that GABA is a major inhibitory neurotransmitter in mature mammalian brains, but the effect of this substance is sometimes converted into depolarizing or even excitatory when the postsynaptic Cl– concentration becomes high. Recently we have shown that seizurelike afterdischarge induced by tetanic stimulation in normal extracellular fluid (posttetanic afterdischarge) is mediated through GABAergic excitation in mature hippocampal CA1 pyramidal cells. In this study, we examined the possible contribution of similar depolarizing/excitatory GABAergic input to the CA1 pyramidal cells to the seizurelike afterdischarge induced in a low extracellular Mg2+ condition, another experimental model of epileptic seizure activity (low-Mg2+ afterdischarge). Perfusion of the GABAA antagonist bicuculline abolished the low-Mg2+ afterdischarge, but not the interictal-like activity, in most cases. Each oscillatory response during the low-Mg2+ afterdischarge was dependent on Cl– conductance and contained an F–-insensitive depolarizing component in the pyramidal cells, thus indicating that the afterdischarge response may be mediated through both GABAergic and nonGABAergic transmissions. In addition, local GABA application to the recorded cells revealed that GABA responses were indeed depolarizing during the low-Mg2+ afterdischarge. Furthermore, the GABAergic interneurons located in the strata pyramidale and oriens fired in oscillatory cycles more actively than those in other layers of the CA1 region. These results suggest that the depolarizing GABAergic input may facilitate oscillatory synchronization among the hippocampal CA1 pyramidal cells during the low-Mg2+ afterdischarge in a manner similar to the expression of the posttetanic afterdischarge.

Evolving into epilepsy: Multiscale electrophysiological analysis and imaging in an animal model

Epilepsy research for the design of seizure detection/prediction neuroprosthetics has been faced with the search for electrophysiologic control parameters that can be used to infer the epileptic state of the animal and be leveraged at a later time to deliver neurotherapeutic feedback. The analysis presented here uses multi-microelectrode array technology to provide an electrophysiologic quantification of a hippocampal neural ensemble during the latent period of epileptogenesis. Through the use of signal processing system identification methodologies, we were able to assess the spatial and temporal interrelations of ensembles of hippocampal neurons and relate them to the evolution of the epileptic condition. High-field magnetic resonance (MR) imaging was used to determine the location of electrode placement and to evaluate hippocampal pyramidal cell structural damage. Long-term single unit activity analysis suggests that hippocampal neurons in both CA1-2 and dentate regions increase the number of occurrences and duration of their bursting activity after injury to the contra-lateral hippocampus. The trends inferred from both single neuron and ensemble analysis suggests that the evolution into epilepsy is not abrupt but modulates gradually from the time of injury.

Transient rhythmic network activity in the somatosensory cortex evoked by distributed input in vitro

The initiation and maintenance of physiological and pathophysiological oscillatory activity depends on the synaptic interactions within neuronal networks. We studied the mechanisms underlying evoked transient network oscillation in acute slices of the adolescent rat somatosensory cortex and modeled its underpinning mechanisms. Oscillations were evoked by brief spatially distributed noisy extracellular stimulation, delivered via bipolar electrodes. Evoked transient network oscillation was detected with multi-neuron patch-clamp recordings under different pharmacological conditions. The observed oscillations are in the frequency range of 2-5 Hz and consist of 4-12 mV large, 40-150 ms wide compound synaptic events with rare overlying action potentials. This evoked transient network oscillation is only weakly expressed in the somatosensory cortex and requires increased [K+]o of 6.25 mM and decreased [Ca2+]o of 1.5 mM and [Mg2+]o of 0.5 mM. A peak in the cross-correlation among membrane potential in layers II/III, IV and V neurons reflects the underlying network-driven basis of the evoked transient network oscillation. The initiation of the evoked transient network oscillation is accompanied by an increased [K+]o and can be prevented by the K+ channel blocker quinidine. In addition, a shift of the chloride reversal potential takes place during stimulation, resulting in a depolarizing type A GABA (GABAA) receptor response. Blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA), N-methyl-D-aspartate (NMDA), or GABA(A) receptors as well as gap junctions prevents evoked transient network oscillation while a reduction of AMPA or GABA(A) receptor desensitization increases its duration and amplitude. The apparent reversal potential of -27 mV of the evoked transient network oscillation, its pharmacological profile, as well as the modeling results suggest a mixed contribution of glutamatergic, excitatory GABAergic, and gap junctional conductances in initiation and maintenance of this oscillatory activity. With these properties, evoked transient network oscillation resembles epileptic afterdischarges more than any other form of physiological or pathophysiological neocortical oscillatory activity.