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Lack of correlation between neuronal hyperexcitability and electrocorticographic responsiveness in epileptogenic human neocortex.

OBJECT
The purpose of this study was to determine whether intrinsic neuronal properties and synaptic responses differed between interictally active and inactive tissue removed in neocortical resections from patients undergoing surgical treatment for epilepsy.


METHODS
Whole-cell patch recordings were performed in layer 2 or 3 and layer 5 pyramidal cells in neocortical slices obtained from tissue surgically removed from patients for the treatment of medically intractable seizures. Synaptic responses to stimulation at the layer 6-white matter border were used to classify cells as nonbursting if they responded with only a single action potential for all above-threshold stimuli (80%). These responses were usually followed by biphasic inhibitory postsynaptic potentials (IPSPs). Cells were classified as bursting if they fired at least three action potentials in response to synaptic stimulation (20%). These cells typically showed no IPSPs and responded in either an all-or-nothing or graded fashion. Approximately twice as many cells at layer 2 or 3 (29%) than cells at layer 5 (14%) fired synaptic bursts. Synaptic bursting was not associated with an alteration in a cell's response properties to gamma-aminobutyric acid. It was notable that, in tissue samples determined by electrocorticography (ECoG) to be either interictally active or not active, the proportion of cells that burst was exactly the same in both groups (24%). We found no cells with intrinsic burst firing.


CONCLUSIONS
We conclude that synaptic bursting was characteristic of a small proportion of cells from epileptic tissue; however, this did not correlate with interictal spikes on ECoG.