Correlation of plasma corticosterone levels with running activity in the blinded rat

Blinded female reats were placed in running-wheel cages to monitor the phase of their activity cycle. After approximately a month of adaptation to the wheels, jugular vein blood samples were withdrawn at the beginning and end of the running phase of activity and analyzed for plasma corticosterone. By the time blood samples were obtained, the blined rats' activity cycles had desynchronized from the lighting cycle of the room in an apparently free-running fashion, and were out of phase with each other. The corticosterone levels were high at the beginning of the running phase and low at the end: begin run corticosterone was 34.5 +/- 13, end run corticosteronewas 14.2 +/- 10 (mean +/-SD in mug/100 ml of plasma; t = 7.93, df = 82, P less than 0.001). It was concluded that blinded rats do have an adrenocortical rhythm and that it is in phase with the activity cycle of each individual rat.