The eyeless inbred mouse strain ZRDCT has long served as a spontaneous model for human anophthalmia and the evolutionary reduction of eyes that has occurred in some naturally blind mammals. ZRDCT mice have orbits but lack eyes and optic tracts and have hypothalamic abnormalities. Segregation data suggest that a small number of interacting genes are responsible, including at least one major recessive locus, ey1. Although predicted since the 1940s, these loci were never identified. We mapped ey1 to chromosome 18 using an F2 genome scan and there found a Met10-->Leu mutation in Rx/rax, a homeobox gene that is expressed in the anterior headfold, developing retina, pineal, and hypothalamus and is translated via a leaky scanning mechanism. The mutation affects a conserved AUG codon that functions as an alternative translation initiation site and consequently reduces the abundance of Rx protein. In contrast to a targeted Rx null allele, which causes anophthalmia, central nervous system defects, and neonatal death, the hypomorphic M10L allele is fully viable.

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The distribution of neuronal elements containing immunoreactive somatostatin (I-SRIF) in the rat central visual pathway was examined by light-microscopic immunocytochemistry. These studies were concerned with the location and morphology of neurons and innervated cells and the distribution of fiber and terminal plexuses in the primary visual cortex (area 17), visual association areas 18 and 18a, the superior colliculus, the lateral geniculate nucleus, and the pretectum. In the superior colliculus, I-SRIF-containing fibers and perikarya were distributed predominantly in the superficial, or visual, layers; these elements were moderately dense and occupied the entire mediolateral extent of these layers. In the intermediate and deep layers, immunoreactive neurons were widely scattered, and fibers were located mainly in the medial third. Immunoreactive cell populations in the superior colliculus included small bipolar neurons with fusiform perikarya and multipolar neurons with round to ovoid perikarya. In the pretectum, the peptide was demonstrable in large and small multipolar neurons of the nucleus of the optic tract and in the posterior and olivary pretectal nuclei. I-SRIF-containing neurons were also present in the nucleus of the posterior commissure, the nucleus of Edinger-Westphal, and the ventral division of the lateral geniculate nucleus. In the visual cortex, the peptide was present in all layers and in a variety of morphologically defined cell populations, including some which are presumed excitatory (pyramidal and bipolar cells) and others which are presumed inhibitory (bitufted and stellate cells). Our data suggest that somatostatin is involved in visual and visuomotor reflex pathways and in the horizontal optokinetic nystagmus reflex pathway. These results provide a foundation for further studies to evaluate the role of this peptide in visual processes.