Cellular mechanisms of pharmacoresistance in slices from epilepsy surgery

Slices of human cortical tissue from epilepsy surgery were investigated with intracellular recordings to elucidate the mechanisms contributing to augmented synaptic excitation and to repetitive activity. The analysis of single synaptic potentials revealed, amongst other differences to rodent cortex, a disturbance of GABAA inhibition, namely depolarizing responses. A tentative ionic mechanism, impaired KCl outward-transport (KCC2), was evaluated in a rat model (0-Mg hyperexcitability). The observed down-regulation of KCC2 mRNA after 0-Mg-ACSF exposure of slices may contribute to the depolarizations by GABA. The factors enabling repetitive activity were addressed with a paired-pulse paradigm. In slices from epilepsy surgery, synaptic responses were virtually constant with interstimulus intervals between 100 and 1000 ms. Tiagabine markedly prolonged the effects of released GABA at GABAA receptors, but paired-pulse behaviour was only slightly affected. We demonstrate that bicuculline-induced paroxysmal activity of rat cortex is frequency-limited (to about <1 Hz) by presynaptic GABAB receptors. The lack of frequency limitation of synaptic events suggests an impaired GABAB receptor function in the human epileptogenic cortex. The data are discussed regarding the pivotal role of KCl transport in epileptic disorders of various origins and the role of GABAB receptors in the frequency limitation of paroxysmal activity.