Consequences of reward and nonreward conditions: Runaway behavior, neurotransmitters and physiological indicators of stress

Abstract

Rats were subjected to continuous reinforcement (CR), partial reinforcement (PR) or extinction (EX) schedules following food-motivated conditioning in a straight runway. On the final day of testing, the animals were removed from the runaway after the fifth trial and were killed in order to determine the neurochemical consequences of differential reward. Corticosterone and adrenal ascorbic acid were also taken as indicators of immediate and long-term environmental stress, respectively. From the results it would appear that PR and EX caused a prolonged period of stress as indicated by the educed ascorbic acid concentrations. However, only the PR group showed elevated concentrations of corticosterone, which would suggest that the behavioral conditions were still stressful to the PR, but not to the EX group at the time killing. Changes in the septal concentrations of GABA and hippocampal 5-HT following PR and EX were attributed to the effects of nonreward conditions. Furthermore, the catecholaminergic-olfactory system may be necessary for appropriate extinction of food-motivated behavior, while the midbrain dopaminergic system may function to maintain motivation under conditions of variable reward.