Trk-neurotrophin receptor-like immunoreactivity in the gut of teleost species

Neurotrophins, acting through their high-affinity signal-transducing Trk re ceptors, are involved in the development, differentiation and maintenance o f discrete neuron populations in the higher vertebrates. Furthermore, the p resence of Trk receptors in some non-neuronal tissues, including the endocr ine cells of the gut, could indicate an involvement of neurotrophins also i n these tissues. Recently, neurotrophins and neurotrophin receptor proteins have been identified in the lower vertebrates and invertebrates, whose ami no acid sequences are highly homologous with those found in mammals. The pr esent study investigates the occurrence and distribution of Trk-like protei ns in the neurons and gut endocrine cells in five species of teleost. Singl e and double immunolabeling was carried out on fresh and paraffin-embedded tissue using commercially available antibodies against sequences of the int racytoplasmic domain of the mammalian Trk. Western-blot analysis, carried o ut on samples of stomach and intestine of bass, identified proteins whose e stimated molecular masses (140 kDa, 145 kDa and 143-145 kDa) were similar t o those reported for full-length TrkA, TrkB and TrkC in the higher vertebra tes. TrkA-like immunoreactivity was found in the enteric nervous system ple xuses 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 d etected 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 the y also showed TrkB-like immunoreactivity. These findings confirm the occurr ence and distribution of Trk-like proteins in teleosts. These proteins are closely related to the Trk neurotrophin receptors of mammals. The functiona l significance of Trk-like proteins in both neuronal and non-neuronal cells of teleosts is still not clear.