Retinal projections to the subcortical visual system in congenic albino and pigmented rats

Abstract

The primary visual pathway in albino mammals is characterized by an increased decussation of retinal ganglion cell axons at the optic chiasm and an enhanced contralateral projection to the dorsal lateral geniculate nucleus. In contrast to the primary visual pathway, little is known about the organization of retinal input to most nuclei of the subcortical visual system in albino mammals. The subcortical visual system is a large group of retinorecipient nuclei in the diencephalon and mesencephalon. These areas mediate a range of behaviors that include both circadian and acute responses to light. We used a congenic strain of albino and pigmented rats with a mutation at the c locus for albinism (Fischer 344-c/+; LaVail MM, Lawson NR (1986) Development of a congenic strain of pigmented and albino rats for light damage studies. Exp Eye Res 43:867-869) to quantitatively assess the effects of albinism on retinal projections to a number of subcortical visual nuclei including the ventral lateral hypothalamus (VLH), ventral lateral preoptic area (VLPO), olivary pretectal nucleus (OPN), posterior limitans (PLi), commissural pretectal area (CPA), intergeniculate leaflet (IGL), ventral lateral geniculate nucleus (vLGN) and superior colliculus (SC). Following eye injections of the neuroanatomical tracer cholera toxin-beta, the distribution of anterogradely transported label was measured. The retinal projection to the contralateral VLH, PLi, CPA and IGL was enhanced in albino rats. No significant differences were found between albino and pigmented rats in retinal input to the VLPO, OPN and vLGN. These findings raise the possibility that enhanced retinofugal projections to subcortical visual nuclei in albinos may underlie some light-mediated behaviors that differ between albino and pigmented mammals.

Figure 1

Photomicrograph of cholera toxin β (CTβ) label in the contralateral diencephalon. This section is representative of labeled retinal input to this region in an albino rat. The dashed lines indicate areas of interest (AOI) used for quantitation in nuclei of the subcortical visual system. OPN, olivary pretectal nucleus; CPA, commissural pretectal area; PLi, posterior limitans; IGL, intergeniculate nucleus; vLGN, ventral lateral geniculate nucleus. Scale bar = 600 μm.

Figure 2

Photomicrographs showing the distribution of CTβ-labeled retinal axons in matched sections through albino and pigmented rats. A, B: ipsilateral superior colliculus (SC) of albino (A) and pigmented (B) rats. Sections were selected through a region that showed the greatest number of patches. Note the presence of uniform label across the mediolateral extent of the colliculus in the albino, and distinct patches of dense label (arrows) in the pigmented animal. C, D: contralateral ventral lateral hypothalamus (VLH) of albino (C) and pigmented (D) rats. Sections were selected through the region of densest label in the nucleus. E, F: contralateral posterior limitans (PLi) nucleus of albino (E) and pigmented (F) rats. Sections were selected through a region that showed the clearest patches. Note the presence of patchy dense label in the PLi (arrows) in the albino rat (E). Scale bar for A, B, E, F = 500 μm; C, D = 250 μm.