Recently several psychiatric disorders, such as seasonal affective disorders, sleep-wake rhythm disorders and nocturnal behavioral disorders in demented patients have been paid attention, because those disorders are considered to have some causal relation with biological rhythm. Patients with these disorders show some disorders of circadian rhythms and are well treated with bright light, which is able to shift the phase of the circadian rhythm. In an attempt to develop an animal model for such rhythm disorders, we investigated the effect of various stresses on the biological rhythm in rats.
When blinded rats were given acute or chronic stresses, such as cold-restrain stress or foot-shock stress, the period of the free-running rhythm of the animal was elongated. The repeated stress given at the same time in a day for more than one week failed to entrain the free-running rhythm in adult rats.
On the other hand, forced exercise in the running wheel resulted in shortening of the free-running period. The motor activity determined by the number of the wheel revolution was found to be negatively correlated with the period of free-running rhythm.
We determined the content of serotonin in the suprschiasmatic nucleus (SCN), the center of the biological clock, in all animals subjected to stress experiments. Both cold-restrain stress and foot-shock stress caused a marked increase in serotonin content in the SCN. Furthermore, exercise also caused an increase of serotonin content. These results suggested that serotonin content itself has no causal relation with change of free-running period caused by stresses, because change of free-running period was different in spite of the similar change of serotonin content in the SCN.
Repeated mother deprivation for more than 10 days from blinded pups only during light period under 12: 12 light dark cycle caused a reversal of the phase of the N-acetyltransferase activity (NAT) in the pineal gland. In this experiment, warming pups by a heat plate during mother deprivation prevented the NAT rhythm to reverse caused by mother deprivation. Thus, low temperature stress seemed to play an important role in reversal of the NAT rhythm caused by mother deprivation in blinded pups. Since a thermo-regulating mechanism is still immature in pups, cold stress presumably exerts a significant effect. This may explain the difference in mode of entrainement of the rhythm by repeated stress between adult and infant rats.
In the former group repeated cold-restrain stress did not cause an entrainment of the circadian rhythms.
Further study will be necessary to clarify a mechanism involved in the change of the period of free-running rhythm caused by various stresses.
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