Abstract

In the present study, malnutrition was induced in adult rats by feeding a protein deficient (7% protein) diet for 36 d in order to study its effect on morphine analgesia and on brain serotonergic activity. In the early phase of malnutrition, both, morphine analgesia and serotonergic neuronal activity were increased concomitantly when compared to that of their respective control values from nourished adult rats. On the contrary, both morphine analgesia and serotonergic neuronal activity in brain were decreased in the late phase of protein deprivation. These results indicate that morphine analgesia in malnourished adult rats is primarily governed by serotonergic neuronal activity of the brain. In contrast to above, malnourished adult rats also showed an exposure dependent progressive decrease in the analgesic effect of morphine while malnourished rats rehabilitated on protein-rich diet for a period of two weeks did not show such anomaly. The decrease in morphine analgesia on its repeated exposure in malnourished adult rats may be related to the decrease in formation of N-normorphine, an active metabolite of morphine responsible for its analgesic effect, due to decrease in microsomal mixed function oxidase activity in the brain and liver.