Intraocular pressure fluctuations of growing chick eyes are suppressed in constant light conditions

We report measurements of intraocular pressure (IOP) in growing domestic chicks at 12 h intervals, with three different lighting conditions. One group of chicks was raised in 12 h light and 12 h darkness (N), another in constant light (CL), and the third group was initially exposed to CL for three weeks then returned to N for either one week or four weeks (CLN). Pressures were measured in the middle of the light and dark periods (noon and midnight) for N and CLN birds, and at corresponding 12 h intervals for CL birds (also noon and midnight). The IOP of N chicks fluctuated from a light period average value of 25 mm Hg (+/- 1.3 SD), to a dark period average value of 17.5 mm Hg (+/- 1.1 SD mm Hg; P < 0.0001). These pressures were established by 4 days of age. At 7 weeks, (N) IOP continued to fluctuate: light values were 21.7 mm Hg (+/- 1.2 SD), and dark values were 18.3 mm Hg (+/- 0.7 SD). The IOP of CL birds did not fluctuate, remaining steady at 17 mm Hg (+/- 1.4 SD). Chicks exposed to CL for 3 weeks required more than one week in N to re-establish (N) lOP values. We conclude that lOP fluctuates in hatchling chicks under N light conditions, that fluctuation is suppressed in CL light conditions, and that IOP recovery from 3 weeks suppression in CL requires more than one week in N light conditions. (C) 2016 Elsevier Ltd. All rights reserved.