Circadian rhythm of melatonin, corticosterone and phagocytosis: effect of stress

Melatonin has a functional connection with the immune system. Phagocyte fun ction is altered by extirpation of the pineal gland, one source of melatoni n, or by in vitro incubation of phagocytes with pharmacological concentrati ons of melatonin. Given that its synthesis by pinealocytes is under the con trol of the noradrenaline released by the sympathetic postganglionaric nerv e endings, the present work was aimed at evaluating the circadian rhythm of melatonin, corticosterone, and phagocytosis in BALB/c mice in basal and st ress situations. Peritoneal macrophages were used as phagocytes, latex bead s as the particles to be ingested, and forced swimming to exhaustion as the stress situation. Radioimmunoassay was used to determine the animals ' ser um hormone levels. Samples were taken every 3 hr in the period from 04:00 t o 22:00 hr, and every 30 min during the remaining period from 22:00 to 04:0 0 hr. Control mice presented a short-term melatonin peak at 33:30 hr, while the maximum inert-particle ingestion capacity of the peritoneal macrophage s also occurred during the night but at 03:30 hr. The corticosterone levels in control mice presented a circadian rhythm with a day-time maximum peak (16:00 hr). Compared with the controls, the animals subjected to stress mai ntained, although at lower values, the melatonin peak at 23:30 hr, but they presented a loss of the rhythm of serum corticosterone levels, and the cor ticosterone levels and the macrophage phagocytic capacity were greater at a ll hours of the day.