Occurrence of neurotrophin receptors and transmitters in the developing Xenopus gut

The ontogeny of gut innervation in the anuran amphibian Xenopus laevis was studied using immunohistochemistry on sections of whole larvae from NF stag es 38-52. Immunoreactivity to acetylated tubulin confirmed the presence of nerve fibres as early as stages 38-39. Actin immunoreactivity was found at stage 41, indicating the presence of smooth muscle cells. Trk-like neurotro phin receptors were occasionally found in nerve fibres as soon as stages 38 -39. Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclas e-activating peptide (PACAP) immunoreactivities coexisted in nerves innerva ting the gut wall from stages 40-41, and nitric oxide synthase (NOS) from s tage 42. Substance P/neurokinin A (SP/NKA) occurred at stage 42. In all the se cases, the first fibres were observed in the oesophagus. Calcitonin gene -related peptide (CGRP) was first observed in nerves at stage 48. In genera l, VIP/PACAP and NOS innervation was denser than the tachykinin innervation . In conclusion, the development of nerve fibres in the Xenopus gut is prob ably dependent on neurotrophins that may act via Trk-like receptors and occ ur before the gut wall is fully organised morphologically. Feeding in Xenop us larvae starts at NF stage 45. The study demonstrates that several of the transmitters investigated are expressed in the gut innervation (and in end ocrine cells) prior to this stage.