Acetylcholine epilepsy: Modification of DC shift in chronically undercut cat cortex

1. 1. Cortical depth penetrations were made with DC recording during Ach induced epileptiform activity in a series of cats with undercut suprasylvian gyrus acute to 82 days chronic. 2. 2. This epileptiform acitivity consists of rhythmic oscillations (bursts) superimposed on a sudden DC shift with periods of electrocorticographic "silence" (suppression) in between. 3. 3. Whereas in acutely undercut preparations, the surface DC shift was negative, in the chronic preparations it was positive. 4. 4. Histologica reconstruction of electrode tracts showed that the bursts in acute and chronic preparations, and the DC shift in chronic preparations reversed phase in cortical layers II-III and became maximally negative near layer IV. 5. 5. The appearance of the superficial positive DC shift in the chronics has led to an hypothesis concerning a supragranular inhibitory system in the genesis of Ach induced epileptiform activity. It has not provided further explanation in our analysis for the seizure propensity of chronically undercut cortex. 6. 6. Current density analysis using the second derivative of voltage with respect to depth (-α²V/αZ²) reveals that there is a superficial source and deep sink in both acute and chronic preparations which has a similar laminar depth distribution for both DC shift and burst components. Unit analysis would implicate neuronal depolarization in deeper layers in the genesis of the deep sink for both the DC shift and the bursts. The superficial source may be a passive phenomenon.