Ultradian, circadian and seasonal rhythms in cortisol secretion and adrenal responsiveness to ACTH and yarding in unrestrained red deer (Cervus elaphus) stags

Seasonal changes in the activity and responsiveness of the adrenal gland in red deer (Cervus elaphus) stags were quantified by measuring 24 h endogeno us cortisol secretory profiles and plasma cortisol responses to either admi nistration of exogenous ACTH or a standardised stressor during November (pe riod of velvet growth), February (pre-rut), April (mid-rut) and July (post- rut) (southern hemisphere) using a remote blood sampling device (DracPac). Ultradian rhythms in the concentration of plasma cortisol were observed res ulting from the episodic secretion of cortisol from the adrenal cortex at a mean rate of 0.8 pulses/h. Circadian rhythms in plasma cortisol concentrat ions were also found in 11 out of the 20 complete 24 h profiles (mean ampli tude, 3.8 +/- 1.4 ng/ml). Seasonal rhythms in mean 24 h plasma cortisol concentrations and cortisol p ulse parameters were also observed. Mean 24 h plasma cortisol concentration s were higher in November (12.5 +/- 1.0 ng/ml) than in February (6.3 +/- 1. 0 ng/ml), April (4.0 +/- 1.0 ng/ml) or July (4.2 +/- 1.0 ng/ml). Cortisol p ulse height, nadir and amplitude were all significantly higher in November than at other times of the year (P<0.01). Peak cortisol concentrations following infusion of ACTH(1-24) (0.04 IU kg(- 1)) were higher (P<0.05) in November (55.8 +/- 2.7 ng/ml) and lower (P<0.00 1) in April (33.7 +/- 1.8 ng/ml) than those in February and July (48.7 +/- 2.0 ng/ml and 45.4 +/- 2.0 ng/ml respectively). The area under the cortisol response curve was significantly smaller (P<0.05) in April (266.6 +/- 15.3 ng/ml/190 min) than at other times of the year (February, 366.1 +/- 15.3 n g/ml/190 min; July, 340.7 +/- 15.3 ng/ml/190 min and November, 387.8 +/- 21 .2 ng/ml/190 min). These data demonstrate that the adrenal gland of the red deer stag exhibits ultradian, circadian and seasonal rhythms in activity, and that its respon siveness to ACTH varies with season. November, a period of reproductive qui escence in the southern hemisphere, with new antler growth and rapid weight gain, is associated with higher mean :plasma cortisol concentrations and a greater responsiveness to exogenous ACTH. In contrast, the breeding season is associated with lower adrenal activity and responsiveness.