Effect of tonic pain on schedule specific feeding behaviour

Food deprivation produces analgesia. This response is reversed i.e. pain sensitivity is lowered, when the food deprived rats are fed. In the present study the effect of chronic pain on the motivation to get food, in food deprived rats, was observed. In ten rats the effect of formalin and morphine plus formalin on the motivation to get food was studied. Injection of formalin significantly (P < 0.01) decreased the number of lever presses from 450 +/- 30 to 225 +/- 25. However, after injecting morphine the effect was reversed. The present study shows reduced internal drive to procure food by the food deprived animals, when they were under chronic pain. The effect was blocked by morphine, suggesting the role of opioids in modulating the motivation for getting food.