Physiological basis of phatophysiological brain rhythms

Focal epilepsy may be induced acutely in the brain in vivo , by measures which reduce inhibition or enhance excitation. Athough the various models involve different mechanisms causing the epilepsy, their epileptiform discharge patterns vary only little. Intracellular analyses in vivo and in vitro reveal that the cellular hallmark of epileptic discharge, the paroxysmal depolarization shift, is followed by a giant hyperpolarization. The latter ic comprised of several, overlapping, components with different durations. including calcium dependent potaccium currentc and GABA dependent inhibitionc. Relative reduction of one inhibitory component is compensated by other inhibitory componentc. In epilepcy caused by reduction of GABAergic inhibition. the absolute duration and amplitude of GABAergic inhibition may even be increased in comparison to the responses following afferent stimulation under control conditions since the excitatory drive of the paroxysmal discharges on the interneurons is strongly increased. In some interictal discharge patternc, the enhanced inhibitions within the focus determine the refractory periodc of the focus. The latter is paced by neurons from the perifocal area which show a shorter inibition accociated with the interictal epileptic event. The diccharge pattern of the focus may switch to other patterns. either cpontaneously, or as entrained by external ctimulation. Such changes are caused e.g. by progressive potassium accumulations in the extracellular space with critically cmall intervals of the epileptic events. It is concluded that the epileptiform diccharge patterns reflect intrinsic properties of the brain. and do not very well reflect the mechanism of action of the epileptogenic model. The brain is thus equipped with inherent mechanisms which favor rhythmic epileptiform discharges under certain conditions.