Location of neurons projecting to the retina in mammals.

In this study, horseradish peroxidase (HRP) was used as a retrograde tracer in order to investigate the existence of centrifugal pathways to the retina in mammals and to locate the somas of the retinopetal neurons. After the application of HRP to the stump of the cut optic nerve in monkeys, cats, guinea-pigs and rabbits, labeled neurons were located in various areas. The largest number of labeled neurons was found bilaterally in the hypothalamus in the premammillary area, and some neurons were also found slightly more rostral and dorsally towards the posterior hypothalamic area. At the mesencephalic-metencephalic junction, a few labeled neurons were observed in the dorsal raphe nucleus and more ventrally in the tegmental area situated between the medial longitudinal fascicle and the superior cerebellar peduncle. Furthermore, labeled neurons were found in the contralateral medial pretectal area in guinea-pigs, in the ipsilateral tegmental mesencephalic reticular formation in monkeys, in the laterodorsal tegmental nucleus in rabbits, and in the dorsal hypothalamic area near the nucleus of the anterior commissure in monkeys. We were unable to find labeled cells in the dorsal raphe nucleus or in the hypothalamus in rabbits or in the above-mentioned tegmental area in guinea-pigs. These divergences may be due to species differences or may simply be false-negative results due to the known difficulty of using the currently available tracers to label retinopetal neurons.