Pain sensitivity in dietary-induced obese rats.

Previous literature indicates possible interrelationships between the endogenous opioids or endorphins, pain response, and obesity or eating behaviour. The pain response was, therefore, examined in a rat model of obesity induced by palatable food high in unsaturated fats. Pellet-fed control and energy-dense obese and nonobese rats were tested for latency of response to a thermal stimulus using the tail flick test. Obese rats exhibited a statistically significant increase in tail flick latency compared to controls. In addition, the observed latencies were significantly correlated to the body weight of the rats (r = 0.52, p < 0.01). These data suggest that dietary-induced obese rats are similar to obese humans in being less sensitive to painful stimuli, consistent with an increase in endogenous opioids in obesity.