Comparison of immune parameters of sheep with naturally high or low plasma concentrations of melatonin

Studies in rodents and humans have suggested that the pineal gland and its secretory product melatonin play an important role in the modulation of the immune system. In this study, we tested the hypothesis of a difference in immune parameters between ewes with naturally high vs. low circulating melatonin. Thus, two comparable groups of 10 Ile-de-France sheep were selected from a large flock, for their naturally high and low plasma concentrations of melatonin. The mean plasma melatonin concentrations during daytime (09:00 hr) and nighttime (24:00 hr) were, respectively, 9 and 664 pg/mL (high group) and 5 and 169 pg/mL (low group; P < 0.01). Animals from both groups were subjected to various in vitro and in vivo measurements of the characteristics of their immune system. The total number of white blood cells (lymphocytes, polymorphonuclears, and monocytes) and the assessment of the sub-populations of blood lymphocytes (T4, T8, T19, B, and monocytes) did not show any significant differences between the two groups, sampled during day or night. The level of blood leukocytes proliferation after in vitro culture with ConA, LPS, or CWF stimulation, before or after experimental immunization, did not reveal any differences. No significant differences were registered in the production of antibodies between the two groups of animals. The results of the present experiment suggest that in natural conditions a high level of circulating melatonin does not modify the activity of the immune system in sheep.