An anterograde HRP study of retinal projections to the hypothalamus in the rat.

Retinal projections to the hypothalamus in the rat were examined using anterograde transport of horseradish peroxidase (HRP). After intraocular HRP injections, retinohypothalamic fibers were histochemically visualized by a highly sensitive HRP procedure using benzidine. Light microscopic evaluation of HRP labeling in the hypothalamus confirmed a retinal projection to the suprachiasmatic nucleus (SCN). In the rostral part of the SCN labeling was observed in the ventromedial, ventral and the ventrolateral regions and was heavier on the contralateral than the ipsilateral site to the injected eye. In the most caudal part of the SCN labeling was observed in the medial region and was almost equal in amount bilaterally. Further, this study clearly demonstrates a new retinal fiber trajectory to the lateral hypothalamic area (LHA). Some labeled retinal fibers were segregated from the dorsolateral margin of the main optic tract and continued into the LHA, supporting the notion of direct retino-LHA projections.