Laminar distribution of cholinergic markers in neocortex: Effects of lesions

The distribution of presynaptic markers for cholinergic and GABAergic neurons in the various layers of the lateral neocortex was determined with a techique in which sections of increasing depth horizontal to the pial surface were obtained on a frozen‐stage microtome and analyzed for the markers. The specific activities of choline acetyltransferase and acetylcholinesterase had an uneven distribution in the cortex with lowest levels of activity associated with lamina II and III, whereas glutamate decarboxylase activity was highest in lamina II, III, and IV. Fetally‐induced hypoplasia of the cortex resulted in an enrichment in both choline acetyltransferase and acetylcholinesterase activity in all layers with no significant change in the activity of glutamate decarboxylase. Lesion of the nucleus basalis–cortical cholinergic pathway resulted in marked reductions in the activities of acetylcholinesterase and choline acetyltranferase in all cortical layers and an elimination of the uneven distribution of these markers within the cortex. These results provide further evidence that acetylcholinesterase activity is specifically associated with cortical cholinergic afferents and support the existence of cortical intrinsic cholinergic neurons.