Corticosterone and melatonin are true internal chemical pacemakers of different physiological processes, with both hormones having immunoregulatory effects and a circadian rhythm in most vertebrates (Rodríguez et al. 1999). Also, melatonin, due to its physiological antioxidant properties, acts as a buffer in stressful situations by quenching the adverse immunological effects of stress by way of its circadian release (Maestroni, 1993), whilst corticosterone is thought to be the main glucocorticoid secreted in response to stressful exercise (Simon, 1991). The present work was aimed at evaluating if swimming exercise-induced stress changes the circadian rhythms of melatonin, corticosterone, and phagocytosis in 3-month-old male Wistar rats.
Animals (n = 6) were maintained under a 12 h light and 12 h dark photoperiod and subjected every 2 h over one circadian period to a physical activity, which consisted of free swimming for 2 h and then rotating rod exercise until exhaustion, based on a modified version of the automated Porsolt test suggested by Nomura et al. (1982). Afterwards, they were killed by decapitation, and peritoneal cavity macrophage and plasma samples were taken for comparison with samples from control animals (the same conditions as the stressed individuals but in the absence of stressor). Radioimmunoassay was used to determine plasma levels of melatonin (IBL) and corticosterone (DRG Diagnostics). Prior to the melatonin assay, haemoglobin level of samples was measured to avoid any interference it might cause in the melatonin values. All experiments were carried out according to the guidelines of the European Community Council Directive 89/6091 EEC. Data are expressed as mean values ± S.D. and were compared by Student’s unpaired t test. Values of P ²le³ 0.05 were considered significant.
The results showed that the circadian rhythm of the control animals for melatonin (peak at 02:00 h; 137 ± 14 pg ml-1) and corticosterone (peak at 17:00 h; 285 ± 19 ng ml-1) was lost by the stressed group, with the melatonin levels being lower (although the values were still higher at night than during the day) and the corticosterone levels much higher than those of the control group. Phagocytosis was determined as the latex-bead phagocytosis index (PI), i.e. the number of latex beads ingested by 100 macrophages, the phagocytosis percentage (PP), i.e. the percentage of cells that had phagocytosed at least one latex bead, and the phagocytosis efficiency (PE), i.e. the ratio PI:PP which indicates how effectively the phagocytes ingested the particles. The control animals’ peritoneal macrophage PI also showed a circadian rhythm with maximum values at 03:00 h (684 ± 40 PI) because of a greater phagocytic efficiency of those cells. The stressed group displayed higher values than the controls at most hours of the night. These data confirm that melatonin and corticosterone act as modulators of the innate immune response.
The authors thank Elena Circujano Vadillo for technical assistance. The research was supported by the Consejería de Bienestar Social and the Consejería de Educación, Ciencia y Tecnología – Fondo Social Europeo (Junta de Extremadura, 2002).