Pattern of retinal projections in the California ground squirrel (Spermophilus beecheyi): anterograde tracing study using cholera toxin.

ABSTRACT

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.