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Diana C Lozano, Andrew T E Hartwick, Michael D Twa; Circadian Rhythms of Intraocular Pressure and Core Body Temperature Persist in Continuous Dim Light. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5821. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To compare and contrast the circadian rhythms of intraocular pressure (IOP) and body temperature in Brown Norway rats when housed in standard light-dark(LD) and continuous dim light(LL) conditions (40-90 lux).
An intraperitoneal temperature sensor was implanted and body temperature measurements were obtained every 5 minutes until the end of each experiment. IOP was measured every two-hours over 26-hours using a rebound tonometer. These two physiological functions were evaluated when the animals were housed in LD and after the animals had been housed in LL for 1 and 4 weeks (Group 1; n=4 animals) or for 7 weeks (Group 2; n=7 animals). Circular statistics (length of the mean resultant vector [R], circular variance [S], and Rayleigh’s test) were used to determine the distribution of temperature and IOP peak times.
Body temperature in LD was lowest during the light-phase (36.9±0.1°C), highest during the dark-phase (37.5±0.2°C), and peaked near the middle of the dark phase (17.5±1.9 Zeitgeber Time). IOP in LD was lowest during the light-phase (16±2 mmHg), highest during the dark-phase (30±7mmHg), and peaked near the middle of the dark-phase (16.6±1.2 Zeitgeber Time). In LD, the vector length for temperature and IOP were larger than 0.96, circular variances were less than 0.04, and Rayleigh’s test (P<.001) supported that the times were concentrated around the middle of the dark-phase. However, temperature and IOP peaked at different times when the animals were place in LL. In Group 1, the time difference was -4.6±1.0h after 1 week of LL and IOP peak times were still concentrated around the same time (R=0.98; S=0.02; Rayleigh’s P=.01). The time difference was +9.5±6.8h and IOP peak times were more spread around the clock after 4 weeks in LL (R=0.21; S=0.79; Rayleigh’s P=.85). In Group 2, the time difference was +6.2±8.4h after 7 weeks in LL and IOP peak times were spread out evenly around the clock (R=0.11; S=0.89; Rayleigh’s P=.92). The maximum range of IOP measurements was 14±3mmHg under LD conditions; this range dampened (8±1mmHg) after 1 week in LL and stayed dampened after 4 weeks in LL (8±2mmHg; Group 1). For Group 2, the maximum range dampened to 6±1mmHg after 7 weeks in LL.
Persistent, but dampened, circadian rhythms of IOP and temperature were measured in continuous dim light and the results showed that they are not synchronized by the same central oscillator.
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