Melatonin and deprivation myopia in chickens.

Chicken eyes elongate and become myopic if they are covered with translucent diffusors which degrade the retinal image ('deprivation myopia'). Since it has been shown that dopamine D2/D4 receptors (which mediate inhibition of melatonin synthesis) are also implicated in deprivation myopia, we have studied the role of melatonin in the visual control of eye growth. We have found that (1) diurnal melatonin rhythms and melatonin content in the retina are unchanged during deprivation myopia development despite the breakdown of both diurnal growth rhythms of the eye and diurnal rhythms in retinal dopamine metabolism, (2) diurnal melatonin rhythms and melatonin content in the retina remain unchanged after application of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) and presumably also after 6-hydroxydopamine (6-OHDA) application which both have a suppressive effect on deprivation myopia and (3) deprivation myopia was slightly reduced in both eyes after unilateral intravitreal injection of melatonin, despite that deprivation myopia is based on a mechanism intrinsic to the eye. We conclude that melatonin is not involved in the retinal signaling pathway translating visual experience to deprivation myopia.