Influence of Light Phase Exposure to LED Lighting on Circadian Levels of Neuroendocrine Hormones in Sprague-Dawley Rats

Abstract

Light and lighting protocols of animal research facilities are critically important to the outcomes of biomedical research that uses animals. Previous studies from our laboratory showed that the wavelength (color) of light in animal housing areas affects the nocturnal melatonin signal that temporally coordinates circadian rhythms in rodents. Here, we tested the hypothesis that exposure to LED light enriched in the blue-appearing portion (460-480 nm) of the visible spectrum during the light phase (bLAD) influences circadian concentrations of select neuroendocrine hormones in adolescent Sprague-Dawley rats. Male and female rats (4 to 5 wk old) were housed on a novel IVC system under a 12L:12D in either cool-white fluorescent (control, n = 72) or bLAD (experimental, n = 72) lighting. Every third day, body weight and food and water consumption were measured. On Day 30, rats were anesthetized with ketamine/xylazine and terminal collection of arterial blood was performed to quantify serum concentrations of melatonin, corticosterone, insulin, and glucose at 6 circadian time points (0400, 0800, 1200, 1600, 2000, 2400). As compared with male and female rats housed under cool white fluorescent (CWF) lighting, rats in bLAD lighting showed a 6-fold higher peak in dark phase serum melatonin (P < 0.05). Effects on serum corticosterone were sex dependent, as CWF and bLAD females had significantly higher corticosterone levels than did CWF and bLAD males, respectively. CWF and bLAD females had significantly higher serum glucose overall as compared with males. However, serum insulin was not affected by sex (M or F) or lighting conditions (CWF or bLAD). These data show that housing Sprague-Dawley rats under bLAD lighting conditions increases circadian peaks of melatonin without increasing serum levels of corticosterone, glucose or insulin, indicating less variation of circadian cycling of key neuroendocrine hormones in bLAD-exposed rats.

Figure 1.

Adolescent Sprague–Dawley rats (n = 36 males, n = 36 females) were housed in same-sex groups of 3 rats per cage under control CWF lighting on an IVC rack. Light contamination during the dark phase was controlled with blackout curtains at the single entrance to the animal room, in addition to covering any light-emitting sources within the room, like the cardboard-covered digital face to the IVC rack, as shown.

Figure 2.

Adolescent Sprague–Dawley rats (n = 36 males, n = 36 females) were housed in same-sex groups of 3 rats per cage on a novel IVC light-containment rack. Light contamination during the dark phase was controlled with the use of blackout panels on each unit and shelf of the light containment rack.

Figure 3.

This figure depicts the spectrum of the light used in this study compared with intensity for both CWF control (T8 fluorescent) and bLAD experimental (T8 blue-enriched LED). While both lights are broad spectrum in nature, they exhibit peak intensity at different wavelengths of light: CWF peaks on the higher end of the visible spectrum (550-650 nm), whereas bLAD peaks at the lower end of the visible spectrum (450 nm).

Figure 4.

Mean rat A) body weight ± SEM, B) daily food intake ± SEM, and C) daily water intake ± SEM between males and females in both CWF and bLAD groups over the 30-d housing period is shown.

Figure 5.

Mean serum melatonin (> 4 pg/mL detectable level) ± SD between male and female rats in CWF and bLAD groups. Serum concentrations denoted with an asterisk (*) denote statistical significance (P < 0.05).

Figure 6.

Mean serum corticosterone (> 15 ng/mL detectable level) ± SEM between male and female rats in CWF and bLAD groups. Serum concentrations denoted with an asterisk (*) denote statistical significance (P < 0.05).

Figure 7.

Mean serum insulin (> 0.2 ng/mL detectable level) ± SEM between male and female rats in CWF and bLAD groups.

Figure 8.

Mean serum glucose ± SEM between male and female rats in CWF and bLAD groups.