Adrenocorticotrophin (ACTH) and corticosterone in the plasma of adult female rats were measured sequentially at 4 h intervals for 24 h before and after lesions of the suprachiasmatic nuclei or treatment with p-chlorophenylalanine (to inhibit serotonin synthesis). After lesions or p-chlorophenylalanine treatment, the concentrations of ACTH were diminished relative to those in control animals and rhythmic changes could not be detected. However, injection of animals, pretreated with p-chlorophenylalanine, with 5-hydroxytryptophan (60 mg/kg) 8 h before the time when plasma ACTH is maximal in intact animals, stimulated ACTH secretion up to control values. Mean corticosterone concentrations in plasma remained unchanged (after lesions) or increased (after p-chlorophenylalanine). This increase was associated with an increased minimal concentration of corticosterone. After both treatments there was evidence of continued circadian or ultradian rhythms of corticosterone concentration.
Locomotor activity of female rats given identical treatment, but without blood sampling, indicated that nocturnal activity was diminished after lesions whereas diurnal activity was enhanced after p-chlorophenylalanine treatment. Periodicity analysis detected the persistence of free-running circadian, and sometimes ultradian activity, rhythms. Adrenalectomy did not alter further the activity pattern observed in rats with lesions.
These results therefore support the proposition that both the suprachiasmatic nuclei and the serotoninergic system play an irreplaceable role in the mechanism of ACTH secretory rhythms. The suprachiasmatic nuclei are also important for synchronization of locomotor activity and corticosterone rhythms, which may both persist after the suppression of ACTH rhythms.
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