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JOURNAL ARTICLE

Effects of Sodium Lighting on Circadian Rhythms in Rats

Xian Chen, Chang-Ning Liu, Judith E Fenyk-Melody
Journal of the American Association for Laboratory Animal Science: JAALAS 2019 April 10
30971327
Rodent studies often must be conducted during an animal's active phase (that is, in darkness) yet also during a typicalday shift for staff. Low-pressure sodium lighting (LPSL), to which human retinas are more sensitive than rodents' at lowintensity, has been used to facilitate study conduct in dark phase. The assumption was that LPSL would be equivalent tototal darkness due to low rodent retinal sensitivity but provide enough lighting for safe technical manipulations due to higher human retinal sensitivity. Unlike other light sources, LPSL has been tested for effects on circadian rhythm specific to locomotive activities in albino mice. Whether LPSL affects circadian rhythms in rats is unknown. In this study, circadianendpoints were derived from body temperature and locomotor activity via telemeters in 8 adult male Wistar rats. When movedfrom a 12:12-h white-light (that is, cold white fluorescent light):dark (LD) cycle to a 12:12-h white-light:sodium-light cycle, rats demonstrated free-running and disrupted circadian rhythms (that is, lengthened circadian period and reduced circadian robustness and amplitude). Body temperature and locomotor activity were significantly lower in the LPSL phase as compared with dark phase under the baseline condition. When exposed to a 12:12 h sodium-light:dark (SD) cycle, rats entrained with a circadian period similar to 12:12-h white-light:dark (LD), but significantly different from the period under constant darkness(DD). Circadian onset and acrophase were delayed under SD compared with LD. When illuminated with a LPSL pulse underDD, rats showed phase shifts similar to white-light pulse effects, consistent with the phase response curve. To determinewhether the image-forming photoreceptors are involved in this process, we used electroretinography. Compared with whitelight, 589-nm light generated during electroretinography elicited rod photoreceptors responses with longer latency and conephotoreceptor responses with lower amplitude. These results indicate that LPSL is a weaker zeitgeber than white light andmay alter the circadian system in rats. Furthermore, because LPSL appeared to be visible to rats, it may not be an appropriatesubstitute for actual darkness.

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