Adrenal responses to reinforcement and extinction: role of expectancy versus instrumental responding

Plasma corticosterone levels were evaluated during operant conditioning in order to determine the effect of reinforced and nonreinforced responding (extinction) on adrenal activation. The influence of instrumental responding was assessed by comparing trained rats with yoked subjects that received a matched reward schedule in the absence of an operant task. Reinforcement sessions resulted in a significant decrease in adrenal secretion at 20 min, but not at 5 min, whereas extinction caused a rapid increase in corticosterone levels at 5 min and an even greater elevation by 20 min. Comparison of the operant and yoked subjects showed that this effect of reinforcement and extinction was not dependent on instrumental responding, but rather on the receipt or withdrawal of the expected reward.