Eyes--the second (and third) pineal glands?

The pineal gland, the retinas and perhaps other tissues as well may in some species produce melatonin that appears in significant quantities in the circulation. In at least one species, Japanese quail, the circadian rhythm in the levels of circulating melatonin reflects contributions from both the pineal and the retinas; in other species circulating melatonin may come exclusively from the pineal or perhaps only from the eyes. Comparative behavioural and physiological data from several bird and lizard species indicate that retinas and pineal glands fulfil similar endocrine roles. Current evidence suggests that in iguanid lizards either retinas or pineal glands, but not both in the same species, have important regulatory influences on circadian organization. This suggests that it should be relatively easy to influence the melatonin-forming ability of a tissue by natural selection, an interpretation bolstered by our finding that the ability to synthesize melatonin has been inadvertently eliminated in the pineal glands of laboratory mice, presumably by the selection involved in producing inbred strains. The genetics of melatonin synthesis in mice is briefly discussed.