Conditioning decreases in plasma corticosterone level in rats by pairing stimuli with daily feedings.

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Abstract
The nature of a stimulus-induced decrease in the plasma corticosterone levels of rats was examined. Rats maintained on a 23 h food and water deprivation regiment were fed each morning immediately upon entrance of the experimenter into the otherwise isolated animal quarters. After only 14 feeding trials, the rats showed a marked decrease in corticosterone levels within 10 min of the experimenter''s entrance whether fed and watered or not. The acquisition of this decrease was then examined over training trials and by comparing the conditioned animals (group CD) with a group designed to control for pseudoconditioning, sensitization and habituation (group PC) during the use of the more distinctive conditioning stimulus of placing the rat''s cage in a sound-attenuating chamber. During training, group CD was fed .5-6 min after placement in the chamber, and group PC was fed 1.5-2.5 h before or after placement. After 1 training trial, the CD and PC groups showed an increase in corticosterone levels in response to chamber placement when not fed or watered. After 14 training trials the CD group exhibited a significant decline, whereas the PC group exhibited a rise in corticosterone levels. External stimuli can cause a decrease in corticosterone level by virtue of prior association with feeding and drinking. This strongly suggests an acquired inhibition of the pituitary-adrenal system.