ORGANIZATION OF THE VISUAL RETICULAR THALAMIC NUCLEUS OF THE RAT

The visual sector of the reticular thalamic nucleus has come under som e intense scrutiny over recent years, principally because of the key r ole that the nucleus plays in the processing of visual information. De spite this scrutiny, we know very little of how the connections betwee n the reticular nucleus and the different areas of visual cortex and t he different visual dorsal thalamic nuclei are organized. This study e xamines the patterns of reticular connections with the visual cortex a nd the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and r etrograde tracer, was injected into different visual cortical areas [1 7; rostral 18a: presumed area AL (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into the different visual dorsal thalamic nuclei (posterior thalamic, lateral posterior, lateral geniculate nuclei), and the patterns of anterograde and retrog rade labelling in the reticular nucleus were examined. From the cortic al injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulti ng pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, e ach cortical area projects to a largely distinct subsector of the reti cular nucleus, as it does to a largely distinct dorsal thalamic nucleu s. As with each of the visual cortical areas, each of the visual dorsa l thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region of the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus en compasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that indiv idual cortical areas may influence the activity of different dorsal th alamic nuclei through their reticular connections.