FUNCTIONAL MAPPING OF THE LATE STAGES OF STATUS EPILEPTICUS IN THE LITHIUM PILOCARPINE MODEL IN RAT - A (14)C-2-DEOXYGLUCOSE STUDY

Pilocarpine administration to lithium chloride-pretreated rats results initially in discrete convulsive seizures, each behaviorally and electroencephalographically terminated, which then progress to convulsive activity with waxing-and-waning behavioral and electrographic severity; finally, a continuous convulsive state ensues, associated electrographically with continuous fast spiking. This stage does not last indefinitely but is followed by a dramatic electrographic change to periodic epileptiform discharges. The purpose of the present study was to determine with the C-14-2-deoxyglucose functional mapping technique what changes occur in the seizure anatomic substrate during and after this transition, in order to enable inferences about underlying mechanisms. Behavior associated with early and late continuous fast spiking consisted of head twitching; corresponding deoxyglucose autoradiographs displayed seizure-induced intense glucose utilization in most forebrain areas; extranigral brainstem was normal. At 2-3 h of status, fast spiking became interrupted by flat periods; periodic complexes soon dominated the electroencephalogram. Behaviorally, convulsive severity increased. Despite this dramatic electrographic evolution, little change in generalized forebrain metabolic hyperactivation occurred, except that the zona incerta/pretectal/superior colliculus complex displayed markedly increased activity. Deoxyglucose studies in late stages of periodic epileptiform discharges established a sequence of further changes. In late periodic discharges with clonic jerks, at 4 h after status entry, generalized forebrain hyperactivation still prevailed, but to a lesser degree than in early periodic discharges with clonic jerks. At a still later stage, late periodic discharges, subtle convulsive, autoradiographs revealed constriction of the seizure-activated anatomic substrate: hyperactivation was lost in most of neocortex and thalamus, and in caudal olfactory structures, cortical amygdala, and entorhinal areas, but retained in deep occipital cortex and many limbic areas. In the last stage, late periodic discharges, electrical, not associated with convulsive behavior, autoradiographs revealed residual activation in only Ammon's horn; in contrast, much of the forebrain displayed below-normal glucose utilization. These results demonstrate that in the later stages of status epilepticus, the transition from fast spiking to periodic complexes is not associated with a reduction in the seizure anatomic substrate. The electrographic entity of periodic epileptiform discharges is not anatomically or behaviorally homogeneous, but proceeds through successive stages characterized initially by a reduction of glucose utilization within generalized seizure-activated forebrain, then a contraction of the seizure anatomic substrate. Possible mechanisms underlying the transition to periodic complexes are discussed.