Serotonergic efferent nerve fibers in the retinal plexiform layer of the abalone

Serotonin (5-HT)-immunohistochemistries at light- and electron-microscopic levels, using rabbit anti-5-HT serum (#1234), were applied to the whole head and only to the eye of the abalone, respectively. Peroxidase-antiperoxidase and fluorescein isothiocyanate methods were used for the light-microscopic immunohistochemistries. Many immunoreactive nerve fibers were demonstrated in the outer zone of the retinal plexiform layer, small optic nerve fiber bundles, the optic nerve trunk and the cerebral ganglion. Immunoreactive somata were observed only in the cerebral ganglion. Accordingly the immunoreactive fibers in the retinal plexiform layer are considered to be efferent. Cored vesicles in the retinal plexiform layer demonstrated by both conventional chemofixation and a rapid-freeze-substitution method showed strong immunoreactivities localized within their limiting membrane. The same fibers also contained small clear vesicles. They are considered to be different from larger clear vesicles in non-immunoreactive fibers reportedly containing acetylcholine. The function of the efferent fibers remains to be elucidated.