Influence of MSH and corticosterone on stimulus control over an operant response

Several previous papers have indicated that MSH/ACTH-like neuropeptides facilitate the reversal of brightness discrimination in a runway task. The present experiment was undertaken to explore the development during both acquisition and reversal of stimulus control over an operant response. Male albino adult rats were treated daily with either 10 micrograms of MSH, 500 micrograms of corticosterone or a placebo in a double blind procedure during the establishment of an on-light control over availability of reinforcement. Having completed this regime the conditions were reversed so that reinforcement was present only when the chamber-light was off. The results indicated that treatment with MSH during acquisition produced a significant decrease in % correct responses compared to animals treated with either the placebo or corticosterone with neither of these latter groups differing from the other. During the reversal phase of training there was a trend for both MSH and corticosterone to lower % correct responses. The discussion focuses on other literature which has shown opposite effects of corticosterone on aversive and appetitively motivated tasks.