Neurotrophins, acting through their high-affinity signal-transducing Trk receptors, are involved in the development, differentiation and maintenance of discrete neuron populations in the higher vertebrates. Furthermore, the presence of Trk receptors in some non-neuronal tissues, including the endocrine cells of the gut, could indicate an involvement of neurotrophins also in these tissues. Recently, neurotrophins and neurotrophin receptor proteins have been identified in the lower vertebrates and invertebrates, whose amino acid sequences are highly homologous with those found in mammals. The present study investigates the occurrence and distribution of Trk-like proteins in the neurons and gut endocrine cells in five species of teleost. Single and double immunolabeling was carried out on fresh and paraffin-embedded tissue using commercially available antibodies against sequences of the intracytoplasmic domain of the mammalian Trk. Western-blot analysis, carried out on samples of stomach and intestine of bass, identified proteins whose estimated molecular masses (140 kDa, 145 kDa and 143-145 kDa) were similar to those reported for full-length TrkA, TrkB and TrkC in the higher vertebrates. TrkA-like immunoreactivity was found in the enteric nervous system plexuses of three fish species. Trk-like immunoreactivity was observed in the endocrine cells as follows: sparse TrkA-like immunoreactive endocrine cells were detected only in the intestine: TrkB-like immunoreactive cells were detected only in the stomach; and TrkC-like immunoreactive cells were found both in the intestine of the carp and in the stomach of the bass, where they also showed TrkB-like immunoreactivity. These findings confirm the occurrence and distribution of Trk-like proteins in teleosts. These proteins are closely related to the Trk neurotrophin receptors of mammals. The functional significance of Trk-like proteins in both neuronal and non-neuronal cells of teleosts is still not clear.

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Neurotrophins are structurally related proteins that regulate the development, differentiation and maintenance of many neuronal populations. In higher vertebrates (reptiles, birds and mammals) four neurotrophins have been found: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin (NT) 3 and NT4/5. In the gut, experimental data and the occurrence of neurotrophin receptors in intestinal neurons and endocrine cells suggest neurotrophin involvement in intestinal physiology. However, very few data are available regarding the cellular localization and distribution of neurotrophins in the gut. In this study we report the presence of NGF, BDNF and NT3 in neurons and endocrine cells of mouse, duck and lizard intestine. In particular, immunoreactivity to NGF was observed: (a) in both endocrine and nerve cells of mouse and duck intestine, (b) in endocrine cells of lizard gut. Immunoreactivity to BDNF was seen: (a) in nerve cells of mouse intestine, (b) in very few endocrine cells of mouse and duck intestine. Immunoreactivity to NT3 was detected: (a) in nerve cells of the mouse intestine, (b) in endocrine and nerve cells of duck and lizard gut. Our results, together with data previously reported, on the distribution of specific neurotrophin receptors, seem to suggest a possible paracrine/autocrine mechanism of neurotrophin action in both the enteric nervous system and endocrine cells.

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Brain-derived neurotrophic factor (BDNF) is a growth factor that belongs to the group of neurotrophins. Its amino acid sequences are well conserved during vertebrate phylogenesis. Pancreatic tissue has recently been reported to be one of the physiological sources of BDNF in humans and mice. In this study we investigated the presence and localization of BDNF immunoreactivity (IR) in the pancreas of three species of higher vertebrates: mouse, duck and lizard. BDNF IR was present in the islets and in single cells scattered in the exocrine parenchyma of all three species examined. Using double staining, BDNF IR was seen to be colocalized with glucagon IR in all the species studied. There was a total overlap of BDNF and glucagon IR in duck and lizard pancreas, and partial overlap in mouse pancreas. Our findings suggest that, as well as the primary structure, the presence and pattern of distribution of BDNF in higher vertebrates is also well conserved. Moreover, the abundance of BDNF IR in the pancreas of the species studied leads us to the suggestion that these neurotrophins could regulate the function of pancreatic innervation and/or act on pancreatic cells in a paracrine/autocrine fashion.