Dorsal lateral geniculate substructure in the Long-Evans rat: A cholera toxin B-subunit study

This study describes the substructure of the dorsal lateral geniculate nucleus of the thalamus of the pigmented rat (Rattus norvegicus) based on the eye-of-origin of its retinal ganglion cell inputs. We made monocular intra-ocular injections of the B-subunit of cholera toxin (CTB), a sensitive anterograde tracer, in three adult male Long-Evans rats. In four additional subjects, we injected fluorophor-conjugated CTB in both eyes, using a different fluorophor in each eye. Brains of these subjects were fixed and sectioned, and the labeled retinal ganglion cell termini were imaged with wide-field sub-micron resolution slide scanners. Retinal termination zones were traced to reconstruct a three dimensional model of the ipsilateral and contralateral retinal termination zones in the dLGN on both sides of the brain. The dLGN volume was 1.58 \pm0.094 mm^{3}, comprising 70 \pm 3% the volume of the entire retinorecipient LGN. We find the retinal terminals to be well-segregated by eye of origin. We consistently found three or four spatially separated ipsilateral-recipient zones within each dLGN, rather than the single compact zone expected. It remains to be determined whether these subdomains represent distinct functional sublaminae

Substructure within the dorsal lateral geniculate nucleus of the pigmented rat

Much can be learned about the functional organization of the visual system by examining its anatomy. In traditionally-studied cat and primate species, certain morphological types of retinal ganglion cells project to specific thalamic and then cortical layers, separated by eye in each region. In addition, physiology shows that information is segregated by functional cell type; each layer is a complete or near-complete map of some aspect of the world such as form and motion. This results in a system that can easily distribute specialized information without disrupting the larger visuo-spatial map. While the layers of the macaque retino-recipient thalamus are easily discernible due to the correlation between cytoarchitecture and function, the rat geniculate appears to have no such patterning. As the rat is a favorable research model, it is important to understand how its brain is set up to tackle the same visual problems as that of higher mammals. The purpose of this project was to describe substructure within the rat dorsal lateral geniculate nucleus of the thalamus, with the goal of learning more about how the anatomical organization might give insight into function. To visualize the anatomy, I intra-occularly injected retrograde tracer to map retinal termini throughout the nucleus. Based on three-dimensional reconstructions of termination zones from each eye, I identified multiple ipsilaterally-recipient zones within the largely contralateral thalamic nucleus. These ipsilateral subregions are more reminiscent of the layering in the primate thalamus than previously described in the rat. Furthermore, the nucleus appears to be well segregated by eye, suggesting that, like in these other species, the rat thalamus passes on information without binocular mixing. My findings also support the hypothesis that, unlike in the primate, rat cell bodies throughout the layers appear to be cytoarchitecturally homogeneous. While we can glean insight into the function of macaque thalamic layers by looking at cell-body statistics such as size and density, no such patterns can be seen in the rat. Among other possibilities, this could mean that rat projection neurons have similar morphologies, or that functional cell types are intermingled and not spatially organized as they are in other specie