[Effect of the time of administration of 5-hydroxytryptophan on the restoration of circadian stimulation of ACTH secretion in rats treated with p-chlorophenylalanine].

In female rats kept under a photoperiod of 12L-12D (50 lux from 07.00-19.00 h) the pharmacological blockade of serotonin synthesis by pCPA (2 X 300 mg/kg i.p.) obliterated the diel ACTH stimulation, which could, however be restored by an additionnal 5-HTP injection (60 mg/kg i.p.), provided that the serotonin precursor was administered at 11.00 h. If injected at 23.00 h the same dosage of 5-HTP failed to elicit any increase in plasma ACTH. The circadian ACTH rhythm appears, therefore to depend upon a daily activation of the serotoninergic system occurring 4 h after the onset of the light phase.