Acetylcholinesterase in the alimentary canal of fish: light and electron microscopic detection, quantitative distribution in different segments of the gut.

The acetylcholinesterase activity was measured and histochemically localized in the alimentary tract of 2 fish species, carp (Cyprinus carpio) and tench (Tinca tinca). A comparison was made of the activities in the different gut segments. Light and electron microscopic histochemistry revealed acetylcholinesterase-positive cell bodies along the entire length of the alimentary canal in both species, between the muscular layers. Acetylcholinesterase-positive, cholinergic motor endplates were frequent in the esophagus of both carp and tench, and they were also present in the striated muscular layers of the tench stomach and midgut. The enzyme activity detected by the method of Ellman et al. (1961) was highest (16.3 U/mg protein) in the tench foregut and midgut, while it was at the same lower level (9.5 U/mg protein) in each segment of the carp gut and in the tench hindgut. The morphological findings and the higher acetylcholinesterase activity in the tench foregut and midgut suggest that the enteric striated musculature is endowed with denser cholinergic innervation than the enteric smooth musculature.