Choline acetyltransferase‐like immunoreactivity in the superior colliculus of the cat and its relation to the pattern of acetylcholinesterase staining

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Abstract
Choline acetyltransferase, the biosynthetic enzyme for acetylcholine, is thought to be a marker for cholinergic neurons. This report presents an analysis of the pattern of choline acetyltransferase‐like immunoreactivity in the superior colliculus of the cat, A dense network of highly varicose immunoreactive fibers pervaded the superficial gray and optical layer. The density of the fiber network in the superficial layers was heterogeneous, forming a mosaic pattern with a period of about 400 μm. The antigen was also located in numerous small perikarya embedded in this network. This neuronal population reached a density of 2,000 cells/mm3 of the superficial gray layer and suggested the presence of a substantial cholinergic system originating in the superior colliculus. A detailed comparison was made between the pattern of choline acetyltransferase‐like immunoreactivity and the distribution of acetylcholinesterase activity. By comparisons of adjacent sections, both staining patterns were found to be similar in all collicular layers. In particular, the compartmental distribution of immunoreactivity in the intermediate collicular layers seemed to mimic the pattern of acetylcholinesterase staining. A double‐staining technique demonstrated a near‐perfect correlation between the two patterns. In conclusion, there was no indication of heightened acetylcholinesterase activity without an associated elevation in choline acetyltransferase‐like immunoreactivity throughout the superior colliculus. In this part of the brain, the presence of the putative cholinergic terminals could fully account for the distribution of acetylcholinesterase activity.