Circadian and Ultradian Rhythms of Peripheral Cortisol Concentrations in Lactating Dairy Cows

Abstract To investigate possible circadian and ultradian periodicities for plasma cortisol in lactating dairy cows, integrated 15-min blood samples taken sequentially over 48h from six cows were analyzed by radioimmunoassay. The cows were housed in an environmental chamber at about 19°C, 50% relative humidity, and 16h of light and 8h of darkness (lights on at 0700h); fed daily at 0900h; and milked at 0830 and 2000h. Peripheral concentrations of cortisol for all six cows exhibited weak circadian rhythms—average minima were 3.1ng/ml at 1800h, maxima were 4.5ng/ml at 0530h—and strong ultradian rhythms with periods around 120min. Peak to trough amplitudes of ultradian rhythms, 1 to 17ng/ml, varied among and within cows over time. No direct relation between ultradian peaks and milking or feeding was apparent. Spectral analysis indicated that most power (variation) was centered around 1 cycle every 2h, i.e., around a period of 120min. This very strong, easily observable, ultradian rhythm indicates that an oscillation with a period around 120min is probably intrinsic to mechanisms regulating peripheral glucocorticoid concentrations.