Impact of oral melatonin on the electroretinogram cone response

Twelve healthy participants aged between 18 to 52 years old were submitted to a placebo-controlled, double-blind, crossover, and counterbalanced-order design. The subjects were tested on 2 sessions beginning first with a baseline ERG, followed by the administration of the placebo or melatonin condition and then, 30 min later, a second ERG to test the effect.Following oral melatonin administration, a significant decrease of about 8% of the cone maximal response was observed (mean 6.9 μV ± SEM 2.0; P = 0.0065) along with a prolonged b-wave implicit time of 0.4 ms ± 0.1, 50 minutes after ingestion.Oral melatonin appears to reach the eye through the circulation. When it is administered at a time of day when it is not usually present, melatonin appears to reduce input to retinal cones. We believe that the impact of melatonin on retinal function should be taken into consideration when used without supervision in chronic self-medication for sleep or circadian disorder treatment.Melatonin is a circulating hormone (N-acetyl-5 methoxytryptamine) produced mostly by the pineal gland at night [1]. Considering that melatonin feeds back to the suprachiasmatic nucleus (SCN), site of the internal clock, where melatonin receptors are also located [2], it is suggested that melatonin may be a regulatory hormone of darkness for the SCN. Of interest, melatonin can also be suppressed by light exposure to the eyes not only in animals but also in humans [3]. Because the production of this hormone is "light sensitive", it is not surprising that it is also produced in the retina by the photoreceptors [4] and that melatonin MT1 receptors have been localized in the mammalian eye including the human eye [5]. In fact, melatonin is produced by many structures of the eye including the lens [6], the iris, the ciliary body [7] and the lacrimal glands [8]. Moreover, there is strong evidence for the existence of an ocular circadian clock in mammals. This implies the possibility of interactions between