Trk neurotrophin receptor-like proteins in the teleost Dicentrarchus labrax

In recent years, data have accumulated suggesting that the rule of neurotro phins and Trk receptors may not be limited to the nervous system, and the p resence of these substances has been detected in a variety of vertebrate an d invertebrate non-nervous tissues. This study was designed to map the expr ession of immunoreactivity (IR) for Trk-like proteins in alevins of the tel eost Dicentrarchus labrax, with particular emphasis on nonnervous structure s. We used antibodies against specific epitopes of the intracellular domain of these proteins, a region that is highly conserved in phylogeny. Trk-lik e IR was seen ill segregate cell populations of the nervous system, and non -nervous tissues. In the central nervous system TrkA-like and TrkC-like IR was abundant, whereas TrkB-like IR was restricted to a low number of brain areas. Expression of Trk-like protein IR was observed in the peripheral ner vous system and sensory organs, with the exception of the lateral line orga n. Outside the nervous system, TrkB-like IR was mainly found in different e pithelial, TrkB-like IR in the endocrine and digestive system, and TrkC-lik e IR in the cardiovascular and immune systems. The Sills showed IR for all three Trk-like proteins, whereas they were absent from the gonads. Furtherm ore, scattered cells positive for Trk-like proteins were found in most of t he investigated tissues, The distribution of Trk-like IR in this teleost is compared with that of mammals and birds, which it often paralleled, and th e possible role of neurotrophins and Trk-like receptor proteins in differen t non-neuronal tissues is discussed.