Mesencephalic pathway to the retina exhibits enkephalin-like immunoreactivity

Using immunocytochemical methods it was possible to demonstrate a small number of fibers within the optic nerve of the turtle Pseudemys scripta elegans exhibiting Met-enkephalin-like immunoreactivity. The fibers project to the retina where they ramify within the inner plexiform layer in the region of the visual streak. Retrograde labelling with Nuclear Yellow in combination with immunocytochemical staining revealed the origin of these fibers within visceral nuclei of the caudal mesencephalon. The existence of a mesencephalic, enkephalinergic pathway to the retina will facilitate further studies of opiate action in the retina by offering the possibility of selectively stimulating the endogenous release.