Individual groups of rats were trained to leverpress for either a high or a low rate of food reward. Between-group comparisons of plasma and adrenal corticosterone concentrations were made for groups remaining on their respective reinforcement schedules and groups shifted to extinction. Plasma and adrenal corticosterone concentrations for groups receiving a low rate of food reward were elevated in comparison to those receiving a high rate of food reward. A shift to extinction from a high rate of reinforcement produced a significant increase in plasma and adrenal corticosterone concentration, but no change occurred when the shift was made from a schedule delivering a low rate of reinforcement. Hormone comparisons were also made between two groups of rats working on schedules that differed in preferential value (DRL and VI), but were equated for reinforcement frequency. There was no significant difference in corticosterone concentrations between these two groups, indicating that schedule preference does not influence pituitary-adrenal activity when reinforcement frequency is controlled.
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