Changes in focal epileptic activity produced by brainstem sections in the rabbit

Abstract

The propagation of both single epileptiform spike discharges and extended after-discharges (ADs) induced by penicillin was studied in rabbits with sections made through the caudal mesencephalon at either the midcollicular-prepontine or midcollicular-postpontine level, as well as in intact animals. Penicillin was placed either on the frontal cortex, the occipital cortex or in the dorsal hippocampus. When compared to intact controls, sectioning at the midcollicular-prepontine level increased the time required for the spike discharges to develop in the primary focus in all placements and decreased the frequency of single spike discharges when penicillin was placed in the hippocampus or frontal cortex. A transection at this level increased the duration of ADs in animals with occipital foci and made the duration of ADs originating from the frontal cortex more variable. Sectioning at the midcollicular-postpontine level decreased time required for the appearance of the spike discharges in the primary focus in subjects with occipital foci, decreased the amplitude of spike discharges in subjects with occipital or hippocampal foci, decreased duration of ADs in animals with either frontal or occipital foci, but increased duration of ADs in animals with hippocampal foci. Sections at either the pre- or postpontine level decreased the percentage of secondary ADs which continued to discharge after the AD in the primary focus had stopped. These data are taken to indicate that: (1) different mechanisms underly the development of the single spike discharge and the elaboration of these discharges into an extended AD; (2) the duration of the ADs is influenced by the ongoing background electrical activity of the brain structures involved in the AD; (3) it is likely that the same pontine and medullary structures which regulate normal synchronous electrical activity of the forebrain can also influence the hypersynchronous oscillations of the AD; and (4) an intact brain-stem may be necessary for the formation of independent secondary epileptogenic foci.