Comparison of cortisol and rectal temperature circadian rhythms in horses: the role of light/dark cycle and constant darkness

To investigate the daily rhythm of cortisol in horse and to establish whether this rhythm is endogenously generated, we assessed serum cortisol concentrations in five clinically healthy Italian Saddle horses housed in individual boxes under natural light/dark (L/D) cycle followed by constant darkness, and we compared our results with the good established body temperature circadian rhythm. Blood samples and rectal temperature were collected every 3 h during all the experimental period. Two-way for repeated measure analysis of variance showed statistically significant effect of time and no effect of experimental conditions on both studied parameters. In the two experimental conditions, both parameters showed robust circadian rhythmicity, with diurnal acrophase for cortisol and nocturnal acrophase for rectal temperature. In conclusion, serum cortisol daily fluctuation showed oppose trend respect to daily fluctuation of rectal temperature, and its rhythm is endogenously generated.