Regional differences in the epileptiform electrical response of the unanesthetized cerebral cortex to acetylcholine

Abstract

There are marked regional differences in the epileptiform electric response of the unanesthetized monkey''s cerebral cortex to the topical application of acetylcholine. The occipital and posterior parietal cortex was much more sensitive to this form of stimulation than other cortical areas. The combined evidence from electrical and ACh stimulation proves that any cortical area is capable of responding with prolonged rhythmic epileptiform electric discharge to an adequate stimulus, but the cortical area from which such firing may be most easily precipitated varies with the nature of the stimulus. Stimulation of an area of cerebral cortex with acetylcholine causes a widespread lowering of the threshold (to ACh stimulation) of other areas throughout the cerebral cortex. The responsible mechanism is presumably mediated through projections to the brainstem or thalamic reticular areas. It is suggested that, in connection with the motor, premotor, and anterior temporal cortex, there is a mechanism that is more highly developed than in other regions which inhibits the effect of topical acetylcholine stimulation and prevents its tendency to produce synchronization, high-voltage rhythmic discharge, and epileptiform seizures. The present experiments on the regional differences in the threshold of normal cerebral cortex to topical acetylcholine stimulation have served as a valuable control for our previous observation that all cortical areas develop essentially the same (lowered) threshold (supersensitivity) to acetylcholine stimulation following chronic partial isolation. It is postulated that chronic partial isolation (denervation) of an area of cerebral cortex eliminates an inhibitory mechanism and will thus render any area of cerebral cortex equally susceptible to topical acetylcholine stimulation.