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We studied the influence of physical activity stress on the circadian rhythms of melatonin and corticosterone in 3-month old male Wistar rats. Every two hours for 24 h around the clock, an animal from the stressed group was first made to swim for two hours, and was then subjected to a further ten minutes of forced swimming using a modification of the apparatus employed in the Porsolt test. The capacity to resume swimming after the exhausting 2-hour swim was measured by the number of swimming movements that were made by the animal in the additional 10-min swimming period. Blood was collected immediately after the trial, and the plasma melatonin and corticosterone levels determined by RIA. Control group blood was collected at 1-h intervals in the periods from 22:00 to 06:00 and from 16:00 to 18:00, and at 2-h intervals during the remaining periods. The control rats presented plasma melatonin and corticosterone circadian rhythms with nocturnal (02:00) and diurnal (17:00) maxima, respectively. The pattern of these rhythms in the stressed rats was flatter, and the animals tested during hours of the night presented greater endurance than those tested during daytime hours. This suggests that, in evaluating an animals response to stress, it is important to take into account the co-ordination between the time of day when the physical stressing test is applied and the natural sleep/activity periods of the study species.

Changes in behaviour and in the circadian rhythms of melatonin and corticosterone in rats subjected to a forced-swimming test