Dim Light at Night Increases Immune Function in Nile Grass Rats, a Diurnal Rodent

Abstract

With the widespread adoption of electrical lighting during the 20th century, human and nonhuman animals became exposed to high levels of light at night for the first time in evolutionary history. This divergence from the natural environment may have significant implications for certain ecological niches because of the important influence light exerts on the circadian system. For example, circadian disruption and nighttime light exposure are linked to changes in immune function. The majority of studies investigating the effects of light exposure and circadian disruption on the immune system use nocturnal rodents. In diurnal species, many hormones and immune parameters vary with secretion patterns 180° out of phase to those of nocturnal rodents. Thus, the authors investigated the effects of nighttime light exposure on immunocompetence in diurnal Nile grass rats (Arvicanthis niloticus). Rats were housed in either standard 14-h light (L):10-h dark (D) cycles with L ∼150 lux and D 0 lux or dim light at night (dLAN) cycles of LD 14:10 with L ∼150 lux and D 5 lux for 3 wks, then tested for plasma bactericidal capacity, as well as humoral and cell-mediated immune responses. Rats exposed to dLAN showed increased delayed-type hypersensitivity pinna swelling, which is consistent with enhanced cell-mediated immune function. dLAN rats similarly showed increased antibody production following inoculation with keyhole lymphocyte hemocyanin (KLH) and increased bactericidal capacity. Daytime corticosterone concentrations were elevated in grass rats exposed to nighttime dim light, which may have influenced immunological measures. Overall, these results indicate nighttime light affects immune parameters in a diurnal rodent. (Author correspondence: [email protected])

Keywords:

• Arvicanthis niloticus
• Bacteria killing
• Delayed-type hypersensitivity
• Grass rats
• Keyhole lymphocyte
• Light at night

ACKNOWLEDGMENTS

We thank Laura Smale for providing the grass rats and for consultation on the study. We also thank Zachary M. Weil and James C. Walton for assistance with this study and Sallion Wolfe for excellent animal care. This research was supported by NSF grant IOS 04-16897 and by United States–Israel Binational Science Foundation grant 2005-337.

Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.