Influence of Repeated Immobilization Stress upon the Circadian Rhythmicity of Adrenocorticoid Biosynthesis

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Abstract
Experiments were carried out to elucidate the influence of stress on the diurnal change in adrenal biosynthetic activity during controlled 24 h light-dark cycles. stress Groups of 10 rabbits were decapitated at 3 h intervals and the adrenal glands were rapidly removed for measurement of incorporation of 14C-l-acetate into corticosterone and 17-hydroxycorticosterone in their homogenates. Immobilization stress was applied for 6 h a day. The influence of repeated stress was studied in the rabbits after exposure to 7 immobilization stresses. A diurnal rhythm was observed in the activity of adrenocorticoid biosynthesis in non-stressed rabbits, with a maximum at 18:00 and a minimum at midnight. When the animals were exposed to 6 h of immobilization, the adrenocortical biosynthetic activity showed an 80 % increase at 3 h and a 40 % increase at 6 h; the normal rhythmicity was thus disturbed by exposure to this stressor. The lowest level was observed at 9:00 on the next morning, with a 9 h delay. Where rabbits were repeatedly exposed to immobilization for 7 days, the 7th immobilization did not affect the diurnal rhythmicity of adrenal biosynthetic activity or induce its facilitation.

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