Melatonin rhythms in quail: regulation by photoperiod and circadian pacemakers

The profile of melatonin in the eyes, pineal, and blood of Japanese quail was assessed in birds held under LD 16:8 and LD 6:18 photoperiods. Melatonin levels in all three tissues showed a robust daily rhythm with higher levels occurring at night. The amplitude of the rhythm was depressed and its duration lengthened on LD 6:18 relative to LD 16:8. The blood melatonin rhythm precisely reflected the rhythms shown by the pineal and eyes, supporting the idea that the blood rhythm is a result of melatonin secretion by both the eyes and pineal. The ocular melatonin rhythm continued after sectioning of the optic nerve, was reentrainable to a shift in the phase of the LD cycle, and persisted for at least 2 days in constant darkness. It was concluded that either an intraocular circadian clock drives the ocular melatonin rhythm, or an extraocular clock drives the ocular melatonin rhythm via a route other than the efferent innervation (which enters the eye via the optic tract).