Spontaneous sharp waves in human neocortical slices excised from epileptic patients

Human neocortical temporal lobe tissue resected for treatment of pharmacoresistant epilepsy was investigated. In slices prepared from this tissue, field potentials sometimes superimposed by population spikes were found to appear spontaneously, In individual slices, they were generalized or highly localized to a field of similar to 200 mu m in diameter. Synchronous with these potentials, hyperpolarizing and depolarizing postsynaptic potentials were recorded from neurons in the vicinity of the field potential electrode. Hyperpolarizing postsynaptic potentials appeared to be mainly chloride mediated. All potentials, i.e. sharp field potentials as well as postsynaptic potentials, were reversibly suppressed by blockade of the non-NMDA (non-N-methyl-D-aspartate) glutamate-subreceptor and of the GABA(A) (gamma-aminobutyric acid) receptor, and by application of the organic calcium channel blocker verapamil. By contrast, all potentials remained unaffected by blockade of the NMDA glutamate-subreceptor and the GABA(B) receptor. The antiepileptic drugs carbamazepine and phenytoin faded to suppress the spontaneous potentials at therapeutic concentrations. Washout of Mg2+ from the superfusate left the spontaneous potentials unchanged or converted them to ictal-type discharges. This epileptiform activity was not suppressed, but augmented by blockade of the GABA(A) receptor. As a whole, the spontaneously appearing field potentials may be assumed to reflect a state of increased neuronal synchronization.