DIURNAL GROWTH RHYTHMS IN THE CHICKEN EYE - RELATION TO MYOPIA DEVELOPMENT AND RETINAL DOPAMINE LEVELS

1. If the eyes of young chickens are deprived of clear vision by translucent occluders, they develop considerable amounts of axial myopia within days. At the same time, the day time retinal dopamine levels drop by about 30%. Because the retinal dopamine levels of normally sighted chicks also differ diurnally and are low at night, we expected that the rate of axial eye growth might also differ during this time. 2. Unexpectedly, eyes grew in length only during the day (about 0.13 mm/day) and even shrank during the night (about -0.04 mm/night, average net growth +0.09 mm in 24 h). 3. If the eyes were occluded, they grew both during the day and also at night (average net growth: + 0.16 mm in 24 h). Therefore, development of deprivation myopia was a result of the lack of growth inhibition at night rather than of excessive growth during the day when the actual deprivation occurred. 4. Suppression of dopaminergic retinal pathways by intravitreal injections of the neurotoxin 6-hydroxy-dopamine (6-OHDA) also suppressed development of deprivation myopia and it restored the growth inhibition at night. With normal visual experience, the drug had no effect on axial eye growth and refractive state. 5. Diurnal growth rhythms of the eyes disappeared under continuous light. 6. Our results show that: (a) normal diurnal eye growth rhythms require a ''normal'' (12/12 h) light cycle and normal visual experience; with a degraded retinal image during the day, growth rates at night change so that they relate to retinal dopamine levels in the opposite way as with normal visual experience, (b) intact retinal dopaminergic pathways are necessary to mediate the deprivation-induced alterations in diurnal growth rhythms and myopia, (c) deprivation myopia is not simply a result of the lack of visual feedback control of eye growth during deprivation but rather of an active process related to abnormal diurnal dopamine rhythms.