Neurotrophin and Trk receptor-like immunoreactivity in the frog gastrointestinal tract

Nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are members of the neurotrophin family, which is involved in the differentiation, growth, repair, plasticity and maintenance of many neuronal populations. They act through three tyrosin-kinase (Trk) specific receptors: NGF bind to TrkA, BDNF to TrkB and NT3 to TrkC. Despite increasing evidence regarding the presence of neurotrophin and their receptors in many vertebrate species, in amphibians there are very few data concerning them. Thus, the aim of this study was to extend the investigation to the presence of both neurotrophins and their Trk receptors in the gut of an anuran amphibian, Rana temporaria. In the frog gut NT-3- like immunoreactivity (IR) was observed in both the nervous system and endocrine cells of the stomach and intestine, while NGF-like IR was observed only in the enteric nervous system, and BDNF-like IR in the intestinal endocrine cells. TrkA- and TrkB-like IR was detected in both neurons and endocrine cells of the intestine, while TrkC-like IR was observed only in intestinal neurons. No Trk IR was detected in the stomach. The occurrence of the IR to neurotrophins and their receptors in the gut of the frog further confirms the well-conserved presence of this family of growth factors and Trk receptors during the evolution of vertebrates and suggests their complex involvement in the biology of the gastrointestinal neuro-endocrine system.