Effect of Substance P on Intravenous Self-Administration of Morphine in Different Rat Strains

We studied the effects of substance P on intravenous self-administration of morphine in WAG/G and Fischer-344 rats. By the end of week 2 the daily amount of self-administered morphine in WAG/G rats was higher than in Fischer-344 rats. Treatment with substance P markedly suppressed self-injection of morphine, particularly in low doses. The most pronounced effects were observed in Fischer-344 rats. Substance P did not change food-procuring behavior of animals in the same experimental chambers. Since the content of substance P in the hippocampus, hypothalamus, and midbrain of Fischer-344 rats is much lower than in WAG/G rats, and morphine addiction in Fischer-344 rats is less pronounced than in WAG/G rats, the degree of opiate addiction is not determined by the content of substance P in rat brain. However, in our experiments treatment with substance P abolished morphine addiction, particularly in animals with low content of this compound in the brain.