Suprachiasmatic nucleus and retinohypothalamic projections in moles.

The suprachiasmatic nucleus of the hypothalamus (SCN) and the retinohypothalamic projections were identified in one species of old-world moles, all of whom are blind as a result of natural loss of vision. A cyto-architectonic study revealed that the SCN is well developed, even though other visual nuclei in the dorsal thalamus and the midbrain are not. An immunohistochemical study showed that vasoactive intestinal polypeptide (VIP)-like immunoreactive cell bodies and fibers were distributed in the SCN, as has been reported in other mammals. Following intraocular injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP), the central retinal projections were examined. The results indicated that the SCN receives a direct projection from the retina, as seen in many other mammals. In addition to the projection to the SCN, retinal fibers were seen to terminate in the anterior hypothalamic region and the retrochiasmatic area, as observed in some other mammals. In moles, retinohypothalamic projections are bilateral, with an ipsilateral predominance. Considering that the retinogeniculate and retinotectal projections are vestigial, it is highly probable that the optic pathway in moles primarily consists of retinohypothalamic projections, which are devoted to the entrainment of circadian and circannual rhythms.