Effects of anticonvulsant lactams on in vitro seizures in the hippocampal slice preparation

Neocortical seizure termination by focal cooling: temperature dependence and automated seizure detection

PURPOSE

The therapy for focal neocortical epilepsy remains suboptimal. We have, therefore, worked to develop techniques to cool small regions of the neocortical surface for seizure mapping and, ultimately, for long-term suppression of focal seizures.

METHODS

We induced focal neocortical seizures in halothane-anesthetized rats by the microinjection of 4-aminopyridine (4-AP) into the motor cortex. The dura over the injection site was cooled with a Peltier device, and the temperature at the interface between dura and Peltier was measured with a thermocouple. In some experiments, seizures were automatically detected by a computer program that activated the Peltier device.

RESULTS

Monopolar EEG indicated that our seizures were focal and suppressed when cooling was applied directly over the injection site. The threshold temperature required to observe any reduction in seizure duration was 24 degrees C. The temperature gradient across the cooled neocortex was sharp, with the temperature increasing to 31 degrees C at 4 mm below the Peltier, which was cooled to 20 degrees C. Automatic seizure detection reduced the total seizure duration from 43.4 +/- 33.6 s to 5.6 +/- 5.3 s.

CONCLUSIONS

Cooling terminates neocortical seizures when applied very close to the epileptogenic focus. The threshold for seizure termination (24 degrees C) may be lower than the threshold for termination of normal cortical activity, suggesting that this technique will not dissociate the anticonvulsant effect of cooling from the disruption of normal behavior. However, when coupled with automatic seizure detection, focal cooling remains an attractive option for development as a treatment for focal epilepsy.

Seizures and neurodegeneration induced by 4-aminopyridine in rat hippocampus in vivo: role of glutamate- and GABA-mediated neurotransmission and of ion channels

Infusion of the K(+) channel blocker 4-aminopyridine in the hippocampus induces the release of glutamate, as well as seizures and neurodegeneration. Since an imbalance between excitation and inhibition, as well as alterations of ion channels, may be involved in these effects of 4-aminopyridine, we have studied whether they are modified by drugs that block glutamatergic transmission or ion channels, or drugs that potentiate GABA-mediated transmission. The drugs were administered to anesthetized rats subjected to intrahippocampal infusion of 4-aminopyridine through microdialysis probes, with simultaneous collection of dialysis perfusates and recording of the electroencephalogram, and subsequent histological analysis. Ionotropic glutamate receptor antagonists clearly diminished the intensity of seizures and prevented the neuronal damage, but did not alter substantially the enhancement of extracellular glutamate induced by 4-aminopyridine. None of the drugs facilitating GABA-mediated transmission, including uptake blockers, GABA-transaminase inhibitors and agonists of the A-type receptor, was able to reduce the glutamate release, seizures or neuronal damage produced by 4-aminopyridine. In contrast, nipecotate, which notably increased extracellular levels of the amino acid, potentiated the intensity of seizures and the neurodegeneration. GABA(A) receptor antagonists partially reduced the extracellular accumulation of glutamate induced by 4-aminopyridine, but did not exert any protective action. Tetrodotoxin largely prevented the increase of extracellular glutamate, the electroencephalographic epileptic discharges and the neuronal death in the CA1 and CA3 hippocampal regions. Valproate and carbamazepine, also Na(+) channel blockers that possess general anticonvulsant action, failed to modify the three effects of 4-aminopyridine studied. The N-type Ca(2+) channel blocker omega-conotoxin, the K(+) channel opener diazoxide, and the non-specific ion channel blocker riluzole diminished the enhancement of extracellular glutamate and slightly protected against the neurodegeneration. However, the two former compounds did not antagonize the 4-aminopyridine-induced epileptiform discharges, and riluzole instead markedly increased the intensity and duration of the disharges. Moreover, at the highest dose tested (8mg/kg, i.p.), riluzole caused a 75% mortality of the rats. We conclude that 4-aminopyridine stimulates the release of glutamate from nerve endings and that the resultant augmented extracellular glutamate is directly related to the neurodegeneration and is involved in the generation of epileptiform discharges through the concomitant overactivation of glutamate receptors. Under these conditions, a facilitated GABA-mediated transmission may paradoxically boost neuronal hyperexcitation. Riluzole, a drug used to treat amyotrophic lateral sclerosis, seems to be toxic when combined with neuronal hyperexcitation.

Rapid cooling aborts seizure-like activity in rodent hippocampal-entorhinal slices

PURPOSE

As a preliminary step in the development of an implantable Peltier device to abort focal neocortical seizures in vivo, we have examined the effect of rapid cooling on seizures in rodent hippocampal-entorhinal slices.

METHODS

Seizure-like discharges were induced by exposing the slices to extracellular saline containing 4-aminopyridine (50 micromol/L).

RESULTS

When we manually activated a Peltier device that was in direct contact with the slice, seizures terminated within seconds of the onset of cooling, sometimes preceding a detectable decrease in temperature measured near the top of the slice. However, activation of the Peltier device did not stop seizures when slices were no longer in direct physical contact with the device, indicating that this was not a field effect. When cooling was shut off and temperature returned to 33 degrees C, bursting sometimes returned, but a longer-term suppressive effect on seizure activity could be observed. In two of our experiments, a custom computer program automatically detected seizure discharges and triggered a transistor-transistor logic pulse to activate the Peltier device. In these experiments, the Peltier device automatically terminated the slice bursting in less than 4 seconds. When the Peltier device was placed in contact with the normal, exposed cortex of a newborn pig, we found that the cortical temperature decreased rapidly from 36 degrees C to as low as 26 degrees C at a depth of 1.7 mm below the cooling unit.

CONCLUSIONS

These experiments show that local cooling may rapidly terminate focal paroxysmal discharges and might be adapted for clinical practice.