Indices of extinction-induced "depression" after operant learning using a runway vs. a cued free-reward delivery schedule.

Loss of reward is one of the etiological factors leading to affective disorders, such as major depression. We have proposed several variants of an animal model of depression based on extinction of reinforced behavior of rats. A number of behaviors emitted during extinction trials were found to be attenuated by antidepressant treatment and, thus, qualified as indices of extinction-induced "despair". These include increases in immobility in the Morris water maze and withdrawal from the former source of reward as well as biting behavior in operant chambers. Here, we assess the effects of reward omission on behaviors after learning of (a) a cued free-reward delivery in an operant chamber and (b) food-reinforced runway behavior. Sixty adult male Wistar rats were either trained to receive food reinforcement every 90 s (s) after a 5s lasting cue light (FI 90), or to traverse an alley to gain food reward. Daily drug treatment with either the selective serotonin reuptake inhibitor citalopram or the tricyclic antidepressant imipramine (each 10mg/kg) or vehicle was begun either 25 days (operant chamber) or 3 days (runway) prior to extinction. The antidepressants suppressed rearing behavior in both paradigms specifically during the extinction trials, which indicates this measure as a useful marker of depression-related behavior, possibly indicating vertical withdrawal. In the operant chamber, only marginal effects on operant learning responses during extinction were found. In the runway, the operant learned responses run time and distance to the goal, as well as total distance moved, grooming and quiescence were also influenced by the antidepressants, providing a potential set of markers for extinction-induced "depression" in the runway. Both paradigms differ substantially with respect to the anticipation of reward, behaviors that are learned and that accompany extinction. Accordingly, antidepressant treatment influenced different sets of behaviors in these two learning tasks.