Retinohypothalamic projections and the suprachiasmatic nucleus in birds.

Although many investigators have examined retinal projections in birds, there is uncertainty as to the location and identity of a retinorecipient nucleus in the hypothalamus of Aves. A direct retinohypothalamic projection to the medial hypothalamic suprachiasmatic nucleus in mammals is believed to play a role in the entrainment of circadian rhythms. A direct retinohypothalamic projection could subserve a similar function in birds. In the present experiment, horseradish peroxidase, injected intravitreously or applied to the cut optic nerve, consistently labeled a retinorecipient nucleus in the lateral hypothalamus in ringdoves, budgerigars, quails, starlings, and song sparrows, while a significant retinal input to a medial hypothalamic nucleus was absent in these species. These results are interpreted in the light of reports of two retinorecipient nuclei, one in the medial and the other in the lateral hypothalamus, in several classes of vertebrates.