Distribution of radioactivity from topically applied [H³]acetylcholine in relation to seizure

A study was performed to determine if topically applied acetylcholine (ACh) could reach infragranular cortical layers by the time epileptiform activity appeared in intact and bilaterally chronically undercut cat cortex. At various times after bilateral application of neostigmine and [H³]ACh, the brain was frozen with liquid nitrogen. The cortex was sectioned parallel to the surface, placed in liquid sicntillation vials, and curves of radioactivity estimated as ACh vs. cortical depth constructed. In 2 min activity is found in white matter with maximum amounts of activity in all layers of the cortex reached in 3 to 5 min, the time when seizure begins when it occurs. It is concluded that topically applied ACh is not restricted to supragranular layers and probably acts throughout the cortical depth to induce seizure. Seizure occurred in nine of 24 foci, eight of which were in chronically undercut preparations. With these bilateral foci, seizure began earlier (x = 4.8 min) than seizure from unilateral foci (x = 8.7 min). However, no difference in depth distribution was found between seizure and nonseizure foci or between intact and chronically undercut cortex. Therefore, depth distribution alone is not sufficient to explain the seizure propensity of chronically undercut cortex.