Neurotransmitters and putative neuromodulators in the gut of Anguilla anguilla (L.). Localizations in the enteric nervous and endocrine systems

The gut of silver eels (Anguilla anguilla L.) was investigated in order to describe both the cholinergic and adrenergic intramural innervations, and t he localization of possible accessory neuromediators. Histochemical reactio ns for the demonstration of nicotinamide adenine dinucleotide phosphate, re duced form(NADPH-)diaphorase and acetylcholinesterase (AChEase) were perfor med, as well as the immunohistochemical testing of tyrosine hydroxylase, me t-enkephalin, substance P, calcitonin gene-related peptide (CGRP), bombesin , vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), somatostatin, cholecystokinin-octapeptide (CCK-8), serotonin, cholineacetyl transferase. The results evidenced a different pattern in comparison with other vertebra tes, namely mammals, and with other fish. Both NADPH-diaphorase and AChEase activities were histochemically detected all along the gut in the myenteri c plexus, the inner musculature and the propria-submucosa. Tyrosine hydroxy lase immunoreactivity was observed in the intestinal tract only, both in th e myenteric plexus and in the inner musculature. Several neuropeptides (met enkephalin, CGRP, bombesin, substance P, VIP, NPY, somatostatin) were, in a ddition, detected in the intramural innervation; some of them also in epith elial cells of the diffuse endocrine system (met-enkephalin, substance P, N PY, somatostatin). Serotonin was only present in endocrine cells. Tyrosine hydroxylase immunoreactivity was present in localizations similar to those of NADPH-diaphorase-reactivity, and in the same nerve bundles in which subs tance P- and CGRP-like-immunoreactivities were detectable in the intestinal tract. In addition, NADPH-diaphorase-re active neurons showed an anatomica l relationship with AChEase-reactive nerve terminals, and a similar relatio nship existed between the latter and substance P-like immunoreactivity.