Synaptic transmission modulates while non-synaptic processes govern the transition from pre-ictal to seizure activity in vitro

It is well established that non-synaptic mechanisms can generate electrographic seizures after blockade of synaptic function. We investigated the interaction of intact synaptic activity with non-synaptic mechanisms in the isolated CA1 region of rat hippocampal slices using the “elevated-K+” model of epilepsy. Elevated K+ ictal bursts share waveform features with other models of electrographic seizures, including non-synaptic models where chemical synaptic transmission is suppressed, such as the low-Ca2+model. These features include a prolonged (several seconds) negative field shift associated with neuronal depolarization and superimposed population spikes. When population spikes are disrupted for up to several seconds, intracellular recording demonstrated that the prolonged suppression of population spikes during ictal activity was due to depolarization block of neurons. Elevated-K+ ictal bursts were often preceded by a build-up of “pre-ictal” epileptiform discharges that were characterized as either “slow-transition” (localized and with a gradual increase in population spike amplitude, reminiscent non-synaptic neuronal aggregate formation, presumed mediated by extracellular K+ concentrations ([K+])o accumulation), or “fast-transition” (with a sudden increase in population spike amplitude, presumed mediated by field effects). When ictal activity had a fast-transition it was preceded by fast-transition pre-ictal activity; otherwise population spikes developed gradually at ictal event onset. Addition of bicuculline, a GABAA receptor antagonist, suppressed population spike generation during ictal activity, reduced pre-ictal activity, and increased the frequency of ictal discharges. Nipecotic acid and NNC-711, both of which block GABA re-uptake, increased population spike amplitude during ictal bursts and promoted the generation of preictal activity. By contrast, addition of ionotropic glutamate-receptor antagonists (NBQX, D-APV) had no consistent effect on ictal burst waveform or frequency and did not fully suppress pre-ictal activity. Similarly, CGP 55848, a GABAB receptor antagonist, has no significant effect on pre-ictal activity or burst frequency (although it did increase burst duration slightly). Our results are consistent with the hypothesis that non-synaptic mechanisms underpin the generation of ictal bursts in CA1 and that GABAA synaptic mechanisms can shape event development by delaying event initiation and counteracting depolarization block.

Chondroitin Sulfate Induces Depression of Synaptic Transmission and Modulation of Neuronal Plasticity in Rat Hippocampal Slices

It is currently known that in CNS the extracellular matrix is involved in synaptic stabilization and limitation of synaptic plasticity. However, it has been reported that the treatment with chondroitinase following injury allows the formation of new synapses and increased plasticity and functional recovery. So, we hypothesize that some components of extracellular matrix may modulate synaptic transmission. To test this hypothesis we evaluated the effects of chondroitin sulphate (CS) on excitatory synaptic transmission, cellular excitability, and neuronal plasticity using extracellular recordings in the CA1 area of rat hippocampal slices. CS caused a reversible depression of evoked field excitatory postsynaptic potentials in a concentration-dependent manner. CS also reduced the population spike amplitude evoked after orthodromic stimulation but not when the population spikes were antidromically evoked; in this last case a potentiation was observed. CS also enhanced paired-pulse facilitation and long-term potentiation. Our study provides evidence that CS, a major component of the brain perineuronal net and extracellular matrix, has a function beyond the structural one, namely, the modulation of synaptic transmission and neuronal plasticity in the hippocampus.

Hyperbaric hyperoxia and normobaric reoxygenation increase excitability and activate oxygen-induced potentiation in CA1 hippocampal neurons

Breathing hyperbaric oxygen (HBO) is common practice in hyperbaric and diving medicine. The benefits of breathing HBO, however, are limited by the risk of central nervous system O2 toxicity, which presents as seizures. We tested the hypothesis that excitability increases in CA1 neurons of the rat hippocampal slice (400 μm) over a continuum of hyperoxia that spans normobaric and hyperbaric pressures. Amplitude changes of the orthodromic population spike were used to assess neuronal O2 sensitivity before, during, and following exposure to 0, 0.6, 0.95 (control), 2.84, and 4.54 atmospheres absolute (ATA) O2. Polarographic O2 electrodes were used to measure tissue slice Po2 (PtO2). In 0.95 ATA O2, core PtO2 at 200 μm deep was 115 ± 16 Torr (mean ± SE). Increasing O2 to 2.84 and 4.54 ATA increased core PtO2 to 1,222 ± 77 and 2,037 ± 157 Torr, respectively. HBO increased the orthodromic population spike amplitude and usually induced hyperexcitability (i.e., secondary population spikes) and, in addition, a long-lasting potentiation of the orthodromic population spike that we have termed “oxygen-induced potentiation” (OxIP). Exposure to 0.60 ATA O2 and hypoxia (0.00 ATA) decreased core PtO2 to 84 ± 6 and 20 ± 4 Torr, respectively, and abolished the orthodromic response. Reoxygenation from 0.0 or 0.6 ATA O2, however, usually produced a response similar to that of HBO: hyperexcitability and activation of OxIP. We conclude that CA1 neurons exhibit increased excitability and neural plasticity over a broad range of PtO2, which can be activated by a single, hyperoxic stimulus. We postulate that transient acute hyperoxia stimulus, whether caused by breathing HBO or reoxygenation following hypoxia (e.g., disordered breathing), is a powerful stimulant for orthodromic activity and neural plasticity in the CA1 hippocampus.

Tissue Resistance Changes and the Profile of Synchronized Neuronal Activity During Ictal Events in the Low-Calcium Model of Epilepsy

Population spikes vary in size during prolonged epileptic (“ictal”) discharges, indicating variations in neuronal synchronization. Here we investigate the role of changes in tissue electrical resistivity in this process. We used the rat hippocampal slice, low-Ca2+ model of epilepsy and measured changes in pyramidal layer extracellular resistance during the course of electrographic seizures. During each burst, population spike frequency decreased, whereas amplitude and spatial synchronization increased; after the main discharge, there could be brief secondary discharges that, in contrast with those in the primary discharge, started with high-amplitude population spikes. Mean resistivity increased from 1,231 Ω.cm immediately before the burst to a maximum of 1,507 Ω.cm during the burst. There was no significant increase during the first 0.5–1 s of the field burst, but resistance then increased (τ ∼ 5 s), reaching its peak at the end of the burst, and then decayed slowly (τ ∼ 10 s). In further experiments, we modulated the efficacy of electrical field effects by changing perfusate osmolarity. Reducing osmolarity by 40–70 mOsm increased preburst resistivity by 19%; it reduced minimum population spike frequency (×0.6–0.7) and increased both maximum population spike amplitude (×1.5–2.3) and spatial synchronization (×1.4–2.5, cross-correlation over 0.5 mm) during bursts. Increasing osmolarity by 20–40 mOsm had the opposite effects. These results suggest that, during each field burst, field effects between neurons gradually become more effective as cells swell, thereby modulating burst dynamics and facilitating the rapid synchronization of secondary discharges.

Sleep Deprivation Impairs Long-Term Potentiation in Rat Hippocampal Slices

To determine if 12-h sleep deprivation disrupts neural plasticity, we compared long-term potentiation (LTP) in five sleep-deprived and five control rats. Thirty minutes after tetanus population spike amplitude increased 101 ± 15% in 16 slices from sleep deprived rats and 139 ± 14% in 14 slices from control rats. This significant ( P < 0.05) reduction of LTP, the first demonstration that the sleep deprivation protocol impairs plasticity in adult rats, may be due to several factors. Reduced LTP may indicate that sleep provides a period of recuperation for cellular processes underlying neural plasticity. Alternatively, the stress of sleep deprivation, as indicated by elevated blood corticosterone levels, or other non-sleep-specific factors of deprivation may contribute to the LTP reduction.

Evoked Responses of the Dentate Gyrus During Seizures in Developing Gerbils With Inherited Epilepsy

When they are 1–2 mo old, domesticated Mongolian gerbils begin having initially mild seizures which become more severe with age. To evaluate the development of this increasing seizure severity, we obtained field potential responses of the dentate gyrus to paired-pulse stimulation of the perforant path during seizures. In 18 gerbils that were 1.5–8.0 mo old, 73 seizures were analyzed. We measured population spike amplitude, the slope of the field excitatory postsynaptic potential (fEPSP), and the population spike amplitude ratio (2nd/1st) to evaluate excitatory and inhibitory synaptic processes. In gerbils <2 mo old, exposure to a novel environment was followed by an increase in population spike amplitude and then by seizure onset, but population spike amplitude ratio and fEPSP slope remained at baseline levels, and multiple population spikes were never evoked. As previously reported for chronically epileptic gerbils, these findings provide little evidence of a disinhibitory seizure-initiating mechanism in the dentate gyrus when young gerbils begin having seizures. In young gerbils evoked responses changed little during the behaviorally mild seizures. In contrast, most seizures in older gerbils included generalized convulsions, postictal depression, and evoked responses that changed dramatically. In older gerbils, shortly after seizure onset the dentate gyrus became hyperexcitable. Population spike amplitude and fEPSP slope peaked, and multiple population spikes were evoked, suggesting that mechanisms for seizure amplification and spread are more developed in older gerbils. Next, dentate gyrus excitability decreased precipitously, and population spike amplitude and fEPSP slope diminished. This period of hypoexcitability began before the end of the seizure, suggesting it may contribute to seizure termination. After the convulsive phase of the seizure, older gerbils remained motionless during a period of postictal depression, and population spike amplitude remained suppressed until the abrupt switch to normal exploratory activity. These findings suggest that the mechanisms of postictal depression may suppress granule cell excitability. The population spike amplitude ratio peaked after the convulsive phase and then gradually returned to the baseline level an average of 12 min after seizure onset, suggesting that granule cell inhibition recovers within minutes after a spontaneous seizure. Although it is unclear whether the seizure-related changes in evoked responses are a cause or an effect of increased seizure severity in older gerbils, their analysis provides clues about developmental changes in the mechanisms of seizure spread and termination.

Altered Dentate Filtering During the Transition to Seizure in the Rat Tetanus Toxin Model of Epilepsy

The dentate gyrus is thought to be a key area in containing the spread of seizure discharges in temporal lobe epilepsy. We investigated whether it actively contributes to the transition to seizure in vivo using the tetanus toxin chronic experimental epilepsy. Brief epileptic discharges lasted <2 s in freely moving animals and were clearly distinguishable from spontaneous seizures that lasted tens of seconds. This suggested that the changes underpinning the transition to seizure started within the first few seconds of seizure onset. During this period, we found that the amplitude of dentate gyrus population spikes depressed initially, but from 1.1 s after seizure onset, they potentiated. The amplitude and number of CA3 population spikes paralleled the pattern found in the dentate gyrus. We used hippocampal slices to study dentate filtering in more detail. The perforant pathway was stimulated repetitively at the frequency of field postsynaptic potentials found during epileptic discharges in vivo. The amplitude of dentate gyrus population spikes decreased to a steady state in naı̈ve hippocampal slices. In hippocampal slices prepared from rats previously injected with tetanus toxin, population spike amplitude decreased transiently and then potentiated. We found that the biphasic profile and rate of potentiation of dentate population spikes in vivo can be reproduced in naı̈ve hippocampal slices by blocking GABAB receptors. We conclude that the filtering properties of the dentate gyrus are altered in the tetanus toxin model of epilepsy and propose how this contributes to the transition to seizure in our animals.