Evidence for the existence of a retino-hypothalamo-retinal loop in rabbits

Rabbits which have previously been exposed to 24-h light/dark alternations exhibit in their photic responses at all levels of the visual pathway, and in the electroretinogram, simultaneous circadian changes with a time course that is programmed by the preceding light/dark schedule. The occurrence of simultaneous changes in the response of the visual cortex to electrical stimulation at subcortical levels could not be established. After bilateral sectioning of the optic nerves, there were still changes in the electroretinogram which were now no longer programmed, but reflected the existence of a free-running rhythm. Bilateral sectioning of the cervical sympathetic nerves resulted in a complete absence of circadian changes in photic responses. The results suggest the possible existence of a retino-hypothalamo-retinal loop by means of which the responses to photic stimuli on a given day are modulated according to the time course of the light/dark alternations on the immediately preceding day. The possible role of this feedback loop in the origin of animal photoperiodism is discussed.