Repeated post trial administration of vasopressin impairs subsequent Differential Reinforcement of Low rates (DRL) performance.

Twenty-six male rats, maintained on a 23-h food deprivation regime, were trained on a DRL learning schedule. During pretraining (CRF) rats were placed in two groups according to speed in obtaining the criterion (good and poor learners). The performance of good and poor learners, injected with lysine vasopressin (LVP) immediately after each training session, was compared with that of control good and poor learners injected with saline. During the first DRL 20 stage, injected rats had fewer reinforcements than control rats. Fifty-five days later, during reacquisition of DRL 20, LVP rats again had fewer reinforcements, especially those which were formerly good learners in the CRF stage. Nonetheless, treated rats were able to shift their rate of responding from high to low frequency. A reduction of the minimal interresponse time to 16 s showed that LVP rats were able to adapt to this condition and that, in a further DRL 20 stage, the difference between the two groups was no longer significant. The results are discussed in terms of a modification of the behavioral expression of a learned response, without a specific action on memory processes.