Development of the Circadian Adrenocortical Rhythm in Rats: Studied by Determination of 24- or 48-Hour Patterns of Blood Corticosterone Levels in Individual Pups*

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Abstract
To study blood corticosterone rhythm, a micromethod of blood sampling from individual infant rats by tail tip incision was developed. Twenty microliters of whole blood were enough for the determination of corticosterone levels. Using this method, 48-h patterns were determined at 4-h intervals in individual pups of 2–5 weeks of age. Values during the dark period tended to be higher than those during the light period at 3 weeks of age. However, a distinct circadian rhythm was observed around the end of the fourth week after birth. Time sequence analysis revealed that the circadian adrenocortical rhythm was established at this age. Well nurtured pups tended to develop the rhythm earlier than poorly nurtured ones. However, nutrition condition did not appear to be essential for the development of the adrenocortical rhythm, because the manifestation of the rhythm in the poorly nurtured group was delayed only 3 days. The intact pups reared by blinded mothers showed normal circadian rhythms entrained to lighting condition at 4 weeks of age, indicating that the blinding of the mother did not influence the development of the normal rhythm. On the other hand, in the pups blinded on the day of birth, a free-running rhythm was observed. The peaks in mean blood corticosterone levels were 2300 h at 4 weeks and 0300 h at 6 weeks of age. These facts indicate that the eyes and light are not essential for the development of the endogenous circadian adrenocortical rhythm in pups.

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