Differential effects of photophase irradiance on metabolic and urinary stress hormone concentrations in blind and sighted rodents

The effects of different photophase irradiance levels on the daily rhythms of energy expenditure (DEE, calculated from oxygen consumption, VO2) and urinary metabolites of stress hormones in sighted (Microtus socialis) and blind (Spalax ehrenbergi) rodents were compared. Five groups of each species were exposed to different irradiance levels (73, 147, 293, 366, and 498 μWcm²) under short photoperiod (8L:16D) condition with constant ambient temperature 25±2°C for 21 days before assessments. As light intensity increased from 73 μWcm², both species reduced DEE, especially among M. socialis. Cosinor analysis revealed significant ultradian rhythms in VO2 of M. socialis with period length being inversely related to irradiance level. Conversely, in S. ehrenbergi, robust 24 h VO2 rhythms were detected at all irradiances. In M. socialis, significant 24 h rhythms in urinary output of adrenaline were detected only at 293 μWcm², whereas for cortisol, unambiguous rhythms were detected at 73 and 147 μWcm². Distinct adrenaline daily rhythms of S. ehrenbergi were observed at 73 and 293 μWcm², whereas this species exhibited significant rhythms in cortisol at 147 and 293 μWcm². Changes in photophase irradiance levels affected stress hormone concentrations in a dose-dependent manner. There were significant negative and positive correlations of M. socialis and S. ehrenbergi stress hormones, respectively, with increasing irradiance. Our results indicate photophase light intensity is another environmental factor that can significantly affect entrainment of mammalian daily rhythms. Both low and high irradiance conditions can trigger stress responses, depending on the species' natural habitat.