Retinal projections to the subcortical nuclei in the Japanese field vole (Microtus montebelli)

Retinal projections in the Japanese field vole (Microtus montebelli) were determined by anterograde transport of horseradish peroxidase (HRP). Injection of HRP into the unilateral vitreous body demonstrated that the terminal labeling of the optic projections was seen bilaterally in the suprachiasmatic nucleus (SCH), the ventral (GLv) and dorsal (GLd) lateral geniculate nuclei, the intergeniculate leaflet (IGL), the medial pretectal nucleus (NTOM) of the pretectum (PT) and the superficial layer of the superior colliculus (CS), with contralateral predominance, and only contralaterally labeled terminals were found in the lateroposterior thalamic nucleus (LP), the lateral pretectal nucleus (NTOL) of the PT, the dorsal (DTN) and medial (MTN) terminal nuclei of the accessory optic system (AOS). The distribution area of the retinofugal terminals was divided into a three laminar arrangement in the GLd, i.e., layers 1 and 3 and layer 2, received the retinal input from contralateral and ipsilateral eye, respectively, as in arboreal squirrels. The contralateral CS received retinal fibers in the superficial layer, while ipsilateral optic fibers projected sparsely to the stratum opticum of the colliculi. Retinal connections to the DTN and MTN of the AOS were clearly discerned but no lateral terminal nucleus with retinal afferents was found. In addition, the AOS had no inferior fasciculus. These findings indicate that the vole has a contradictory features of a well- and a less-developed sense of vision. Namely, the image forming visual system such as the retino-GLd was as well-developed as in a squirrel, on the other hand, the non-image forming visual system such as the retino-AOS was less-developed as in an insectivore's brain.