[Retino-hypothalamic pathways in vertebrates].

Forty years ago Hollwich (1948) introduced the conception of an "energetic portion of the visual pathway". Contributions to this conception of a direct connection of the retina with the hypothalamus accumulated since then and summarized here in tabular form give rise to the following conclusions: In fish the main hypothalamic termination of retinofugal axons is the nucleus hypothalamicus opticus. It may pass for the suprachiasmatic nucleus of fishes. In amphibians retino-hypothalamic fibres project to the area praeoptica. In reptiles retinofugal fibres innervate hypothalamic neuronal populations called either Nucleus suprachiasmaticus or Nucleus praeopticus. In birds retinohypothalamic axons project to a circumscribed anterior hypothalamic area termed "suprachiasmatic nucleus" by some authors. In mammals at last the main part of the retinohypothalamic tract terminates in the suprachiasmatic nuclei, especially favouring their caudal and ventrolateral parts. The interneuronal connections are axo-dendritic synapses of the Gray Types-I and II. Connections of retinal neurons, especially with suprachiasmatic hypothalamic nuclei or their homologues, are by now well established facts. They represent relatively constant and phylogenically stable components of the centripetal retinal projection. These projections are probably in all, certainly in most of the vertebrates bilateral. Some former but also newer methods of research (Stumpf and Sar, 1975) also depicted optic fibers which terminate in hypothalamic sites apart from the nucleus suprachiasmaticus and the area hypothalamica anterior (Conrad and Stumpf, 1975). A review of the literature on the existence of nerve fibers directly connecting the retina with the hypothalamus is tabulated.