Emmetropisation under continuous but non-constant light in chicks.
It has been suggested that ambient lighting at night influences eye growth and might play a causal role in human myopia. To test this hypothesis, we reared newly hatched chicks under 12 hr light-dark or light-dim cycles with a light phase intensity of 1500 microW/cm(2) and variable dim phase intensities between 0.01 and 500 microW/cm(2). Other chicks were reared under constant light conditions with intensities between 1 and 1500 microW/cm(2). After three weeks, the chicks were examined by refractometry, ultrasound and caliper measurements of enucleated eyes. To relate ocular parameters with a retinal neurotransmitter likely involved in eye growth control, retinal and vitreal levels of dopamine and its principal metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were measured by high performance liquid chromatography with electrochemical detection in the light, dark and dim phases. Diurnal fluctuations in axial length and choroidal thickness also were measured twice daily by partial coherence interferometry (PCI) in chicks under light-dark and the two brightest light-dim conditions. The eyes of chicks reared under most light-dim conditions had refractions and ocular dimensions comparable to those reared under light-dark conditions. At dim phase light intensities of 10 microW/cm(2) and above, the day-night changes in retinal dopamine metabolism were not observed. The daily fluctuations of axial length and choroidal thickness were altered with rearing under the two brightest dim light intensities, compared to the light-dark condition. Rearing under constant light with intensities ranging between 1 and 1500 microW/cm(2) produced a shallow anterior chamber and other eye alterations previously described for constant light rearing even though rearing under continuous light that fluctuated between these same intensities generally permitted normal eye growth. Thus, continuous but fluctuating light exerts different developmental effects on the eye than constant non-fluctuating light. Light-dim rearing may be more relevant to daily human light exposures than other laboratory lighting conditions and may provide an opportunity to study developmental interactions of visual quality (e.g. blur, defocus, etc.) and features of the light-dark cycle under conditions that perturb daily rhythms in dopamine metabolism and ocular dimensions. Such studies also could provide mechanistic insights into whether and how daily rhythms in retinal dopamine metabolism, axial length or choroidal thickness modulate refractive development.