Abstract
Purpose:
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).
Methods:
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.
Results:
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.
Conclusions:
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.