A putative suprachiasmatic nucleus of birds responds to visual motion.

In mammals, the suprachiasmatic nucleus (SCN) is a pacemaker regulating daily rhythms. In birds, two retinorecipient nuclei have been called the avian SCN, one in the lateral hypothalamus and the other more medial and rostral. We asked whether the proto-oncogene c-fos is expressed in either nucleus after light exposure during subjective night, but not during subjective day, as is the case in the SCN of mammals. Chicks raised with one eye covered by a diffuser were exposed to vertically moving surroundings, after the diffuser had been switched to the other eye. Surprisingly, we saw strong Fos label only in the lateral nucleus contralateral to the eye newly exposed to visual motion, but not in the ipsilateral nucleus nor in either medial SCN. No label was seen in animals kept in darkness or if the diffuser was not switched. Fos labeling did not differ between subjective day and night. The sensitivity to "novel" motion is also seen in motion-processing nuclei of the accessory optic system and pretectum; this suggests either that the lateral SCN is not the SCN, but part of the motion pathway, or that the avian SCN may by motion-sensitive during both day and night.