Abstract
Abstract: :
Purpose: To examine the effects of constant light on the retinal dark–light switch and eye growth in the chicken retina. Methods: White Leghorn x Black Australorp chickens were obtained from Bellchambers Produce Merchants, Canberra, and raised either in a 12:12 light:dark cycle or constant light. The light intensity in the light phase was from 100 to 300 Lux. After 6–7 days, chickens were killed over a 24 h light–dark cycle. Retinas processed for determination of N–acetyltransferase activity as a measure of melatonin release, or for determination of enkephalin levels. Vitreous bodies were also collected for DOPAC determination as a measure of dopamine release. Retinal morphology was examined by enkephalin immunohistochemistry, and OCT and light microscopic histology. Axial dimensions were measured with ultrasound and sulphate incorporation into scleral glycosaminoglycans was determined. Refractive error was also determined. Results: Constant light abolished the normal rhythms in dopamine, melatonin and enkephalin release. Melatonin release was suppressed, as expected, but the release of enkephalin in constant light was higher than expected, and that of dopamine was lower. Retinal morphology was otherwise normal, and normal functioning was restored when birds were returned to a light–dark cycle. Four to six hours of dark/night were required to prevent the "constant light" effect. In constant light, axial elongation was increased, but with increasing days of exposure, birds developed marked hypermetropic errors compared to control eyes. Conclusions: The functioning of the retinal dark–light switch was severely disrupted in constant light. Not only were rhythmic release patterns suppressed, but the functioning of thedopaminergic amacrine cells was suppressed, while that of the ENSLI amacrine cells was increased relative to expectation. The requirement for 4–6 hours of dark/night to prevent these effects is interesting, in view of the evidence that a similar period of dark/night is required to prevent the effects of constant light on eye growth (Li et al. 2000, Vision Research 40:238702393).
Keywords: amacrine cells • dopamine • melatonin