Identification of retinal ganglion cells projecting to the lateral hypothalamic area of the rat.

The objective of the present study was to identify the retinal ganglion cells projecting to the lateral hypothalamic area of the rat. The retinohypothalamic tract has been divided into a medial and a lateral component on anatomical and developmental grounds. The medial component projects to the suprachiasmatic nucleus and adjacent structures such as the anterior hypothalamic and retrochiasmatic areas. The lateral component terminates in the lateral hypothalamic are dorsal to the supraoptic nucleus. Injections of the retrograde tracer FluoroGold were made into the retinorecipient region of the lateral hypothalamic area and retinal whole mounts were immunohistochemically processed for retrogradely labeled retinal ganglion cells. With FluoroGold injections confined to the lateral hypothalamic area, retrogradely labeled retinal ganglion cells are located almost exclusively in the superior temporal quadrant of the retina. Their size and morphology indicates that they are a homogeneous subset of type III cells, but a definitive classification would require a more complete fill of dendritic arbors than is available in our retrograde material. In contrast, injections involving fibers of passage in the optic tract, or centered in the medial terminal nucleus of the accessory optic system, label cells distributed across the entire retinal surface. Unlike the retinal ganglion cells projecting to the suprachiasmatic nucleus [Moore et al., J. Comp. Neurol., 352 (1995) 351-366], the cells labeled after restricted lateral hypothalamic injections are not distributed evenly across the retinal surface. The difference in location of the retinal ganglion cells projecting to the lateral hypothalamic area supports the view that this retinohypothalamic projection is anatomically and functionally distinct from the projection to the suprachiasmatic nucleus and adjacent medial hypothalamus.