Pituitary-adrenal hormones and effects of partial reinforcement on appetitive behaviour in the rat.

Rats ran in a straight alley with food reward on continuous (CR) or partial (PR) reinforcement. They received during training placebo, 80 μg ACTH 4–10, 80 μg ACTH 4–10 (7-D-Phe), 1 or 2.5 mg corticosterone. None of these treatments affected performance during training itself. The rats were tested during extinction without hormone treatment. The partial reinforcement extinction effect (PREE) was decreased by ACTH 4–10 and increased by ACTH 4–10 (7-D-Phe), both these changes being due to altered running speeds in the peptide-treated PR groups. One mg corticosterone was without effect on the PREE. 2.5 mg corticosterone caused the PREE to be larger early in extinction (due to changed running speeds in the hormone-treated PR group). These and previous results suggest that ACTH 4–10 attenuates the behavioural effects of nonreward, ACTH 4–10 (7-D-Phe) increases them, and corticosterone has little influence on them.