FREE-RUNNING RHYTHMS OF ADRENOCORTICOTROPIC HORMONE (ACTH), CORTISOL AND MELATONIN IN PIGS

In the domestic pig, a circadian rhythm of plasma cortisol occurs, with greatest concentrations in the morning and lowest concentrations in the afternoon. However, photic entrainment of the rhythms of ACTH and melatonin in pigs have not been defined clearly. This experiment was designed to evaluate free-running rhythms of ACTH, cortisol and melatonin in pigs housed in constant light (LL) and constant darkness (DD). Twelve crossbred barrows, maintained under ambient photoperiod, were catheterized and tethered individually in two environmentally controlled rooms, one with LL and the other with DD. For animals in LL, fluorescent lights provided 202 +/- 15 (mean +/- standard deviation) lux of light at 65 cm above the floors. Incandescent nightlights equipped with 7 watt red bulbs provided 7 +/- 2 lux and were illuminated continuously in both rooms. Pigs were given at least 14 d exposure to LL and DD, then samples of plasma and serum were obtained at hourly intervals for 48 hr. Plasma was assayed for ACTH, and serum for cortisol and melatonin. Periodograms were constructed to analyze the data. For this type of analysis, a statistic, Q(p), is calculated, and circadian periodicity is suggested if maximum Q(p) (Q(p max)) occurs at or near 24 hr. The period of the free-running rhythms (tau) at Q(p max) for ACTH, cortisol and melatonin for pigs in LL (23.80 +/- .01, 23.78 +/- .01, and 23.21 +/- .02 hr, respectively) did not differ significantly from those for pigs in DD (23.39 +/- .01, 23.20 +/- .01, and 22.55 +/- .02 hr, respectively). For both treatments combined, Q(p) at tau = 24 hr was greater (P < .05) than Q(p) at tau = 20, 21, 25, 26, 27 and 28 hr for all three hormones. Unexpectedly, mean concentration of ACTH was greater (P < .005) and melatonin was less (P < .005) for pigs in DD (119.1 +/- 4.2 pg/ml ACTH and 7.4 +/- 1.5 pg/ml melatonin) compared to LL (67.9 +/- 4.0 pg/ml ACTH and 26.1 +/- 1.5 pg/ml melatonin). However, mean concentration of cortisol was similar for pigs in LL (22.7 +/- .8 ng/ml) and DD (21.6 +/- .8 ng/ml). In conclusion, rhythms of ACTH, cortisol and melatonin exist in constant photic conditions in pigs, with periodicity close to 24 hr. In addition, secretion and(or) disposition of ACTH and melatonin may be altered in pigs deprived of a light:dark cycle.