Dark-phase light contamination disrupts circadian rhythms in plasma measures of endocrine physiology and metabolism in rats

Abstract

Dark-phase light contamination can significantly disrupt chronobiologic rhythms, thereby potentially altering the endocrine physiology and metabolism of experimental animals and influencing the outcome of scientific investigations. We sought to determine whether exposure to low-level light contamination during the dark phase influenced the normally entrained circadian rhythms of various substances in plasma. Male Sprague-Dawley rats (n = 6 per group) were housed in photobiologic light-exposure chambers configured to create 1) a 12:12-h light:dark cycle without dark-phase light contamination (control condition; 123 μW/cm(2), lights on at 0600), 2) experimental exposure to a low level of light during the 12-h dark phase (with 0.02, 0.05, 0.06, or 0.08 μW/cm(2) light at night), or 3) constant bright light (123 μW/cm(2)). Dietary and water intakes were recorded daily. After 2 wk, rats underwent 6 low-volume blood draws at 4-h intervals (beginning at 0400) during both the light and dark phases. Circadian rhythms in dietary and water intake and levels of plasma total fatty acids and lipid fractions remained entrained during exposure to either control conditions or low-intensity light during the dark phase. However, these patterns were disrupted in rats exposed to constant bright light. Circadian patterns of plasma melatonin, glucose, lactic acid, and corticosterone were maintained in all rats except those exposed to constant bright light or the highest level of light during the dark phase. Therefore even minimal light contamination during the dark phase can disrupt normal circadian rhythms of endocrine metabolism and physiology and may alter the outcome of scientific investigations.

Figure 1.

sPlasma melatonin levels (pg/mL; mean ± 1 SD) of Sprague–Dawley rats (n = 6 per group) maintained for 6 wk on either a control 12:12-h light:dark cycle (300 lx; 123 μW/cm²; lights on, 0600; group 1); experimental light-at-night (LAN) lighting cycles (that is, a 12:12-h light:dark cycle with light contamination during the dark phase) with 0.02 (group 2); 0.04 (group 3), 0.06 (group 4), 0.08 (group 5) μW/cm² during the dark phase; or constant bright light (group 6). Plasma samples for controls were obtained at either daytime (1600, day control) or nighttime (0400, night control) and for experimental groups (2 through 6) during the dark phase at 0400. Melatonin levels in the control group at 0400 and 1600 differed significantly (P < 0.05); melatonin levels for groups 1 (1600), 5 (0400), and 6 (0400) did not differ between each other or with those of groups 2 through 4 (0400).

Figure 2.

Diurnal changes in plasma total fatty acids in the arterial blood of animals in groups 1 through 4 (closed circles), 5 (triangles), and constant bright light (open circles, group 6) fed normal chow ad libitum. Animals were subjected to dark-phase lighting cycles (as described in the legend to Figure 1) from 1800 to 0600. Values are expressed as μg/mL plasma, and total fatty acid values are the sums of myristic, palmitic, palmitoleic, stearic, oleic, linoleic, and arachidonic acids. Each point represents mean ± 1 SD (n = 6 per group). Data are plotted twice. Concentrations without asterisks are different (P < 0.05) from concentrations with asterisks.

Figure 3.

Diurnal changes in the blood plasma lipid concentrations in the arterial blood of adult male Sprague–Dawley rats fed normal chow ad libitum. Animals were subjected to dark-phase lighting cycles (as described in the legend to Figure 1) from 1800 to 0600 (indicated by dark bars). Total fatty acid values (μg/mL plasma; mean ± 1 SD; n = 6 per group) for groups 1 through 4 (closed circles), 5 (triangles), and 6 (constant bright light; open circles) were the sums of myristic, palmitic, palmitoleic, stearic, oleic, linoleic, and arachidonic acids in the different lipid classes of (A) triglycerides, (B) free fatty acids, (C) phospholipids, and (D) cholesterol esters collected at the various time points. Data are plotted twice. Concentrations without asterisks are different (P < 0.05) from concentrations with asterisks.

Figure 4.

Diurnal changes in plasma glucose levels (mg/dL; mean ± 1 SD; n = 6 per group) in the arterial blood of rats in groups 1 through 4 (closed circles), 5 (triangles), and 6 (constant bright light; open circles). Rats were exposed to dark-phase lighting cycles (as described in the legend to Figure 1) from 1800 to 0600 (dark bars). Data are plotted twice. Concentrations without asterisks are different (P < 0.05) from concentrations with asterisks.

Figure 5.

Diurnal changes in plasma lactic acid concentrations (mg/dL; mean ± 1 SD; n = 6 per group) in the arterial blood of rats in groups 1 through 4 (closed circles), 5 (triangles), and 6 (constant bright light; open circles). Rats were exposed to dark-phase lighting cycles (as described in the legend to Figure 1) from 1800 to 0600 (dark bars). Data are plotted twice. Concentrations without asterisks are different (P < 0.05) from concentrations with asterisks.

Figure 6.

Diurnal changes in plasma corticosterone concentrations (ng/dL; mean ± 1 SD; n = 6 per group) in the arterial blood of rats in groups 1 through 4 (closed circles), 5 (triangles), and 6 (constant bright light; open circles). Rats were exposed to dark-phase cycles (as described in the legend to Figure 1) from 1800 to 0600 (dark bars). Data are plotted twice. Concentrations without asterisks are different (P < 0.05) from concentrations with asterisks.