Effects of shifts in reinforcement level on both running speeds and corticosterone concentrations in rats

Bibliography: pages 55-60.The present study investigated the effects of shifts in reward magnitude on both the running speeds and corticosterone concentrations of Long-Evans rats. The behavioral contrast paradigm was used, which involves shifting subjects from one expected reinforcement condition to either a more or less favorable reinforcement condition. By examining both the running speeds throughout training and the corticosterone concentrations obtained immediately after the final session, the paradigm allowed an assessment of the affective state of the animal. The experiment involved two phases: an acquisition phase and a postshift phase. In the acquisition phase, rats were trained with either a 2- or 22-pellet reinforcement magnitude for 18 daily blocks of 3 trials each. The postshift phase involved shifting subjects to the opposite reinforcement magnitude on block 19 for either one additional block of trials (shift groups) or five additional blocks of trials (postshift groups). The control groups continued training with the original reinforcement magnitude (no shift groups). An 11 min inter-trial-interval was employed, so that the corticosterone concentrations (assayed from the plasma obtained immediately following the final session) would reflect the subject's emotional reaction to the session. Analysis of the acquisition data (running speeds) indicated that reward magnitude differentiated the subjects, in that 22-pellet groups ran faster than did 2-pellet groups. However, neither the 2- nor 22-pellet groups showed increased running speeds over blocks of trials during acquisition. In the postshift phase, the differences in running speeds suggested contrast effects; however, the difference between the shifted groups (postshift) and the nonshifted controls were not sufficiently large to attain statistical significance. Plasma corticosterone concentrations immediately after the final session were not different for the shifted and nonshifted groups, or for groups receiving 2 versus 22 pellets reinforcement. The failure to obtain contrast effects may be related to the failure of the rats to show acquisition effects. The failure to obtain corticosterone differences between groups may have resulted from the long intertrial-interval. That is, the long inter-trial-interval may have obscured the impact of the shift in reinforcement on the emotional response and pituitary-adrenal activity.M.A. (Master of Arts