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Studies in vertebrates have shown that hormones can influence circadian rhythms of behavior. We investigated whether testosterone could induce rhythmicity in arrhythmic Japanese quail, kept in DD. The animals used were 3 1/2-week-old castrated males from a line of quail selected for the lack of the circadian rhythm of feeding activity. After 3 weeks in DD, 8 birds were implanted with an empty implant and 16 others with a testosterone implant. Two weeks later, the operation was repeated. After implantation, we noticed that 15 out of 16 testosterone-treated birds showed a circadian rhythm of feeding activity, in contrast to the control birds, which remained arrhythmic. The clarity of this rhythm increased significantly after each implantation. A positive correlation was found between the indexes of clarity of the rhythm (autocorrelation coefficient ratio and area of the peak of spectrum) and the plasma testosterone level. The period of the induced free-running rhythm was identical to the specific value of the endogenous circadian rhythm in immature quail. The circadian period showed a significant lengthening with the second implantation. This lengthening looks like the variation previously observed in maturing rhythmic or implanted quail. So, it would appear that testosterone can act on rhythmicity on at least two levels: by inducing the circadian rhythm and increasing its clarity and by modulating its period. To explain these results, several hypotheses can be considered. First, the observed arrhythmy may be the consequence of an internal desynchronization of oscillators, responsible for generating the circadian rhythm of feeding activity, and testosterone could play a role in the coupling of these oscillators. Alternatively, we suggest that testosterone could act on the transcription of genes implicated in the control of the rhythmicity or may regulate by rapid signals the cellular rhythmic activity. The possible functional values of the enhancing of circadian rhythmicity by testosterone at different stages of the bird's life were discussed.

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