Behavioral control, aversive stimulus frequency, and pituitary-adrenal response

Six females rats were trained to lever press under a free operant shock-postponement schedule. Each subject had a yoked control which received shock whenever the subject did. The shock-postponement interval (R-S) was varied from 5, 10, 20, 40, to 80 s over blocks of sessions. Corticosteroid levels were taken prior to training and before and after selected sessions. The rate of responding and the rate of shock were inversely related to the R-S interval. Corticosteroid levels were unrelated to the R-S parameter, although steroid samples taken during experimental periods were significantly elevated above preexperimental basal levels. Corticosteroid levels of avoidance subjects were significantly higher than basal levels at the beginning of a session. By the end of a session, these levels were significantly reduced, although still above basal levels. The results were reversed for the yoked control subjects. Their presession corticosteroid levels, although elevated over basal levels, were lower than postsession levels. The implication of these results for the notion that anticipation and control affect levels of arousal is discussed.