DEVELOPMENT OF THE CIRCADIAN RHYTHM OF CORTISOL IN THE GILT FROM WEANING UNTIL PUBERTY

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Abstract
In order to characterize the development of the adrenal axis by monitoring development of the circadian rhythm of cortisol, 10 gilts were blood-sampled serially for 24 h every 4 wk from 4 to 28 wk of age. Age at first estrus for sampled animals (199.6 ± 5.3 d, mean ± SEM) was not different (P > 0.05) from that of another group of 10 control gilts not blood sampled (203.4 ± 6.8 d). Cortisol profiles at 4 wk of age did not show a significant diurnal variation. At 8 and 12 wk, the profile showed the development of an afternoon trough. At 16 wk, an afternoon peak was present. At 20 and 24 wk, the continued development of peaks and a well-formed trough were evident and by 28 wk of age the profile was characterized by a large morning peak, small afternoon peak and an evening trough. Mean levels of total cortisol at 4, 8, 12, 16, 20, 24 and 28 wk were 25.3 ± 2.2, 37.4 ± 4.1, 40.1 ± 3.5, 26.8 ± 2.8, 20.6 ± 2.6, 38.3 ± 3.6 and 24.8 ± 2.9 ng mL−1, respectively. Profiles of biologically active cortisol appeared to closely mirror total cortisol profiles. Total cortisol levels at 8, 12 and 24 wk of age were higher (P < 0.05) than at other ages. These results indicate that in the gilt the circadian rhythm of cortisol undergoes gradual development reaching adult profiles at or shortly before first estrus. Key words: Adrenal, circadian rhythm, cortisol, cortisol binding globulin, development, gilt