Relation between extracellular potassium concentration and neuronal activities in cat thalamus (VPL) during projection of cortical epileptiform discharge.

Neuronal and potassium activities (ak) were measured in the nucleus ventro-posterolateralis thalami (VPL) during propagated epileptiform activity from the somatosensory cortex of cats. Seizures were induced by repetitive electrical stimulation of the cortical surface or by topical application of penicillin. The recruitment of VPL into a seizure resulted in large increases of ak to levels of up to 11.6 mmoles/l, accompanied by increased in neuronal discharge rate to 300/sec. Sometimes the rise in ak preceded active participation of a given thalamo-cortical relay (TCR) neuron in the seizure. After reaching a peak level, ak and neuronal discharge rate slowly declined during an ictal episode. After cessation of seizures all TCR neurons were inhibited, while ak fell to subnormal levels. The duration of these postictal depressions increased with the amplitude of preceding increases and subsequent undershoots in ak and could last up to 120 sec. During decay and undershoot in ak, relay capability of TCR neurons was reduced. Also the probability that action potentials elicited in intracortical endings of TCR cells would antidromically invade their cell bodies was decreased. The duration of these periods varied with the amplitude of undershoot in ak. Seizure threshold was increased during undershoots. These observations are consistent with a long-lasting postictal hyperpolarization of neuronal membranes. The hyperpolarization may be caused by the action of an electrogenic pump, which is probably involved in termination of seizure discharge.