Establishment of the FMRFamide-immunoreactive olfacto-retinalis pathway during ontogeny of the rainbow trout (Oncorhynchus mykiss).

Migration of neurons is one of the mechanisms establishing normal central nervous system connectivity during ontogeny. Proper timing of axonal sprouting is relevant in the same context. In the present study, we used the immunoreactivity of the tetrapeptide FMRFamide (Phe-Met-Arg-Phe-NH2) to visualize the olfacto-retinalis projection during trout ontogeny. It starts to innervate the retina two to four weeks after hatching, in contrast to reports on salmon where it only appears after the fish are imprinted on their natal stream.