Facilitation of conditioned inhibitory avoidance by post-trial peripheral injection of substance P

Experiments were undertaken to investigate the effects of the neuropeptide Substance P (SP) on performance of a conditioned inhibitory avoidance response in rats. A single-trial inhibitory avoidance task was employed. In Experiment 1 SP was injected IP immediately after the training trial in doses of 0.5, 5, 50, 100, 250 or 500 micrograms/kg; control animals were injected with diluent vehicle. The group treated with 50 micrograms SP/kg exhibited better avoidance than the other groups. In Experiment 2 the doses of SP used were 1, 50, 250 micrograms/kg, and the control animals were injected with vehicle or not injected at all. Only the 50 micrograms SP/kg treatment group showed significantly better performance. In Experiment 3 50 micrograms/kg SP or vehicle was injected post-trial immediately or 5 hr after the trial. Only the group in which SP was injected immediately after the training trial showed significantly better performance when tested 24 hr later. This result rules out the possibility that SP exerts its effect by a long lasting proactive action on performance during the testing trial 24 hr later.