Both pineal and lateral eyes are needed to sustain daily circulating melatonin rhythms in sea bass

This study investigates the role of the pineal organ and lateral eyes (the two most important sources of melatonin in vertebrate species) on daily melatonin rhythms of sea bass, a fish exhibiting reversed melatonin profiles, as well as their contribution to circulating melatonin levels. To this aim, the pineal and/or the eyes were surgically removed (Exp. 1), the optic nerve sectioned and retinal dopaminergic neurons damaged with injections of 6-hydroxydopamine (Exp. 2), and the pineal or the eyes covered with aluminium foil (Exp. 3). The results show that plasma and ocular melatonin display opposing profiles. In Experiment 1, pinealectomized fish displayed lower nightly plasma melatonin levels (66+/-22 pg/ml) than intact or sham-operated groups (131+/-14 pg/ml), as it occurred in ophthalmectomized fish (64+/-12 pg/ml). Fish that were both pinealectomized and ophthalmectomized showed a further decrease in plasma melatonin levels (1.4+/-0.4 pg/ml), which approached daytime levels. In Experiment 2, plasma melatonin levels in both optic nerve-sectioned and ophthalmectomized fish were lower than control levels, while injection of 6-hydroxydopamine did not modify plasma melatonin concentrations. In Experiment 3, covering only the pineal made melatonin drop after a light pulse at MD, and covering only the eyes had a similar effect. In conclusion, these findings suggest that even though sea bass eyes do not directly contribute to plasma melatonin, the pineal organ, which unlike that of mammals is a direct photoreceptor in fish, requires light information from the lateral eyes to normally secrete melatonin into the bloodstream.