No evidence for extraocular photoreceptors in the circadian system of the Syrian hamster

Campbell and Murphy reported recently that 3 h of bright light (13,000 lux) exposure to the area behind the knee caused phase shifts of the circadian rhythms of both body temperature and saliva melatonin in humans. The author s tested the hypothesis that extraocular photoreception is also involved in the circadian system of the Syrian hamster. Hamsters were bilaterally enuc leated (eyes removed), and their backs were shaved. Hamsters with stable fr ee-running rhythms in constant darkness were exposed to direct sunlight for 1 or 3 hours during their subjective night. Intact (control) animals showe d phase shifts as expected, but the locomotor activity of enucleated animal s was unaffected by the exposure to sunlight. The authors also measured the pineal melatonin content after exposure to sunlight. Pineal melatonin cont ent in intact animals declined markedly as expected, but no decline was obs erved in the enucleated hamsters. The authors conclude that extraocular pho totransduction is not capable of shifting the phase of the hamster's locomo tor activity rhythm or of suppressing pineal melatonin synthesis.