THE CIRCADIAN-RHYTHM OF SALIVARY CORTISOL IN GROWING PIGS - EFFECTS OF AGE, GENDER, AND STRESS

This experiment was designed to examine circadian rhythmicity of corti sol in saliva of growing pigs, in relation to age, gender, and (time o f) stressor application. Additionally, the acute cortisol response to a stressor was studied. Five groups, each consisting of 3 barrows and 3 gilts, were involved in the experiment. In a Control Group, saliva s amples were taken at 1-h intervals at 12, 16, 20, and 24 weeks of age. Within 1 week, rhythmicity of cortisol was assessed during two 24-h s pans (Monday and Friday). Rhythm characteristics were evaluated by cos inor analysis, describing the rhythm by several parameters. In 2 group s at 12 weeks and 2 other groups at 20 weeks of age, a stressor was ap plied (4 h of isolation) on Thursday morning or evening. Again, rhythm icity was assessed on Monday and Friday by sampling at 2-h intervals. Acute cortisol effects were studied by sampling at several time-points during isolation. Between 12 and 24 weeks of age, basal cortisol conc entrations decreased and a rather stable and adult circadian rhythm wa s reached at 20 weeks of age. Average basal cortisol concentrations we re higher in barrows than in gilts. Furthermore, after isolation, the amplitude of the rhythm was increased in barrows but was unchanged in gilts. The rhythm was more unstable and the maximum value tended to sh ift only after evening isolation. Stressor timing, but also age, was f ound to affect average cortisol concentrations. Moreover, stressor tim ing was important for the acute cortisol response: the increase was hi gher in the morning. The results of this study emphasize the importanc e of considering the circadian rhythmicity of cortisol, in relation to age, gender, and (time of) stressor application, when studying the co rtisol response of animals to stressors. (C) 1997 Elsevier Science Inc .