DEMONSTRATION OF DIRECT INPUT FROM THE RETINA TO THE LATERAL HABENULAR NUCLEUS IN THE ALBINO-RAT

The projection from the retina to the habenular complex was studied us ing fluorescent retrograde tracers in the albino rat (Wistar, Japan Cl ea). Following separate unilateral injections of Fluoro-Gold (FG), Flu oro-Ruby (FR), or -acetamido,4'-isothiocyanostilbene-2,2'-disulfonic a cid (SITS) into the lateral habenular nucleus (LHB), a small populatio n of ganglion cells was labeled sporadically, predominantly those in t he nasal retina contralateral to each injection site. Most of them wer e small cells, ranging from 9 to 16 mu m in diameter, roughly correspo nding to the type III ganglion cell in the rat retina. Additionally, a ll of the structures previously described as regions projecting to the LHB were confirmed. Upon re-examination of previous brain sections of albino rats which had undergone monocular enucleation, degenerating r etinal nerve axons and/or their terminals, stained by a modified selec tive silver impregnation method, were observed in the well-documented end regions of retinal afferents as well as the LHB. The degenerating retino-habenular nerve terminals were distributed sparsely and restric ted mainly to the caudal part of the LHB contralateral to the side of ocular enucleation. The present experimental data provide evidence for the existence of a non-image forming retino-habenular pathway in the albino rat. We suggest that, besides serving as a point of convergence for some of the major conduction channels of the limbic and striatal systems, the LHB may play more general integrative roles, including pa rticipation in the integration of visual information.