Circadian rhythms in adrenal cortical activity in the quail

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Abstract
Circadian rhythms in plasma (P.C.) and adrenal corticosterone (A.C.) content, together with general locomotor activity (L.A.) and body temperature (B.T.) have been evidenced, and mathematically analysed, in a predominantly diurnal laboratory animal, the male quail, under various photoperiods: 12L(7.00 hr)/12D (19.00 hr) (130 lux); 18L(7.00 hr)/6D1.00 nr) (130 lux); 6L(7.00 hr)/18D( 13.00 hr) (130 lux); 6L( 19.00 hr)/18D(1.00 hr) (130 lux); 12L(7.00hr)/12D(19.00hr)(3–5 1ux). The circadian amplitude (C) and the level (Co) of the corticosterone rhythms are depressed as the light span or light pituitary increases. A possible testo‐adrenal cortical interaction might in part account for this feature. The C and Co of the BC. and A.C. rhythms also varies if a same light span (6L) is given at various clock hours on the 24 hr scale, suggesting that cyclic fluctuations of the sensitivity of some structure involved in the light‐darkness synchronized adrenal cortical rhythmicity may occur. Whatever the photoperiod may be, the steep increase, and generally the peak in P.C. and A.P., occur during the second half of the dark span of the photoperiod, which is also the resting time of the animals — a situation similar to that of predominantly diurnal mammals. The possible rôle of some triggering structures in the Central Nervous System, activated during the late sleep hours is discussed. Shifting the onset of light by 12 hr in a 6L‐18D photoperiod, leads to a grossly similar shift in the four rhythms investigated. On the other hand, whereas chronic 6 hr lengthening of the light‐span has no repercussion on the circadian parameters of the investigated physiological functions, a 6 hr shortening of the light‐span leads to a marked phase advance of these rhythms. Under all photoperiods investigated, the phase‐relationships between the circadian P.C., A.P., L.A. and B.T. rhythms remain nearly constant. The possibility of an internal synchronization between these rhythms is discussed.