Plasma corticosterone levels in the rat following discrete-trial discrimination or a single extinction trial

Male Sprague-Dawley rats received either successive discrimination training or continuous reward in a straight alley with a 24-h intertriai interval. Plasma corticosterone levels were determined after moderate (18 trials) and extended (46 trials) acquisition in Experiment I and after 20 trials in Experiment II. Increases in plasma corticosterone levels following nonreward in the discrimination groups or in subjects previously given 45 continuous reward trials were not significant relative to two control groups. These results fail to support the proposal that the pituitary-adrenal system is sensitive to downward reinforcement shifts (frustration effects) under widely spaced trials.