Plasma melatonin secretion rhythms in cattle under varying seasonal conditions.

The purpose of the present study was to investigate the secretion cycles of melatonin (MEL) in cattle over the course of four seasons. Four female Japanese Shorthorn cattle under free-stall and ad libitum feeding conditions were used, and plasma MEL concentrations were measured over a 48 h period at 1 h intervals. The time-series data were analyzed by spectral analysis, and the cycle hour was determined. Data indicated that the secretion cycle for MEL was approximately 23.5 h for all four seasons. The area under the curve of MEL from start to end of experiment for 48 h did not differ significantly among the four seasons. However, the duration of high MEL secretion which defined the duration time of the values were more than 10 pg/mL and differed significantly among the four seasons. In conclusion, this study, which was the first to use spectral analysis to evaluate the cyclic rhythm of MEL in cattle, revealed that MEL secretion cycles did not differ among the seasons. These findings are inconsistent with previous study results in that previous reports suggested that the MEL secretion cycle differed under different lighting conditions.