THE RETINOHYPOTHALAMIC TRACT ORIGINATES FROM A DISTINCT SUBSET OF RETINAL GANGLION-CELLS

The retinal ganglion cells giving rise to retinohypothalamic projectio ns in the rat were identified using retrograde transport of horseradis h peroxidase (HRP) or FluoroGold injected into the suprachiasmatic nuc leus (SCN), and using transneuronal transport of the Bartha strain of the swine herpesvirus (PRV-Bartha). When PRV-Bartha is injected into o ne eye, it is taken up by retinal ganglion cells, replicated, transpor ted to axon terminals in the SCN, and released. There the virus may ta ke one, or both, of two paths to retinal ganglion cells in the contral ateral eye: 1) uptake by SCN neurons, replication, and release from th e neurons with uptake and retrograde transport in retinal afferents or iginating in the contralateral retina; 2) transneuronal passage throug h axo-axonic appositions between retinal afferents in the SCN with sub sequent retrograde transport of virus to the contralateral retina. The ganglion cells thus labeled are a homogeneous population of small neu rons (mean diameter, 12.8 +/- 2.2 mu m and mean area, 81.8 +/- 21.8 mu m(2)) with sparsely branching dendrites that are widely distributed o ver the retina. This population is best identified when virus labeling of retinal projections in areas beyond the hypothalamus is eliminated by lateral geniculate lesions that transect the optic tract at its en try into the geniculate complex. The same population is labeled with r etrograde tracers but, with both HRP and FluoroGold, other ganglion ce lls are labeled, presumably from uptake by fibers of passage, indicati ng that the virus is a more reliable marker far ganglion cells giving rise to retinohypothalamic projections. The ganglion cells identified correspond to a subset of type III, or W, cells. (C) 1995 Wiley-Liss, Inc.