Use of the sensitive anterograde tracer cholera toxin fragment B reveals new details of the central retinal projections in turtles

Previous anterograde degeneration and autoradiographic studies have yielded inconsistent results on the extent and target areas of the central retinal projections in turtles. We used the highly sensitive anterograde and retrograde tracer, cholera toxin B fragment (CTB), to re-examine the central retinal projections in Pseudemys scripta elegans. In contrast to the results of the previous anterograde degeneration studies and autoradiographic studies, immunohistochemical detection of CTB-labeled central retinal axons and terminals revealed these projections with great morphological detail and clarity. In addition, the CTB labeling revealed much more extensive projections than previously realized to all known major retinorecipient areas, particularly in the dorsal and ventral lateral geniculate nuclei, the pretectal region, the nucleus of the basal optic root and the optic tectum. Although the contralateral projections were always much more extensive, ipsilateral projections were clearly present (and in some cases abundant) in all previously known retinorecipient areas. In addition, prominent contralateral (and lesser ipsilateral) retinal projections to several novel target, including the basal hypothalamus, the perirotundal nuclei, the suprapeduncular nucleus and the region between the pretectal nuclei and the tegmentum, were observed. Our results suggest that the use of CTB as a tracer to examine central retinal projections in diverse other species will show that central retinal projections are much more widespread and extensive than previously realized.