Substance P and naloxone enhancement of avoidance conditioning in rats

Experiments were carried out to investigate the effects of peripheral post-training administration of substance P (SP) and naloxone on learning. Rats were trained in three different avoidance tasks (uphill, step-down and alcove) and tested 24 h later. Thirty minutes before each trial (training and testing) rats received naloxone (0.5, 1, 5, or 50 mg/kg, ip) or saline. SP (50 micrograms/kg) or vehicle was administered ip immediately after training. Animals that received SP and SP in combination with naloxone (5 and 50 mg/kg) showed significantly better retention test performance for the uphill and step-down avoidance (P less than 0.05 when compared to control rats treated with vehicle). In the alcove task no influence of SP and/or naloxone was demonstrable, probably due to a ceiling effect. These results suggest that the memory enhancement effects observed are mediated, at least in part, via interactions between substance P and the endogenous opioid systems.