SENSITIVITY TO LIGHT OF THE BLIND MOLE RAT: BEHAVIORAL AND NEUROANATOMICAL STUDY

ABSTRACT

The blind mole rat, Spalax ehrenbergi, is a fossorial rodent which has extreme morphological, physiological, and behavioral adaptations to the subterranean environment. Its atrophied eyes are covered by skin and dark fur and are greatly degenerated. The mole rat's response to light is quite unique. On one hand, the animal is blind and strong light flashes do not elicit visual evoked potentials in its visual cortex. On the other hand, mole rats do respond to changes in photoperiods by altering their thermoregulatory capacities, as well as by changing their circadian locomotor activity. Using an anterograde tracing technique, with monocular injection of wheat-germ-agglutinin-horseradish-peroxidase we found that in the adult mole rat the optic tract, which terminates in the ventral lateral geniculate nucleus, the optic tract nucleus, the lateropos-terior nucleus, and the superior colliculus is highly degenerated, and the connection between the retina and the dorsal lateral geniculate nucleus is missing. This pattern of retinal projections provides the neuroanatomical basis for the mole rat's ability to discriminate between dark or light environments, and also for the animal's avoidance response to light during the first month in captivity. The absence of retinal-LGBd projections explains its “blindness” in the usual sense. In contrast to the primary visual system, the retinohypothalamic pathway is quite developed, and its existence which underlies light-affected rhythms and other cyclic biological phenomena, enables the animal to respond to photoperiodic changes by altering its thermoregulatory responses, and by changing its locomotor activity.