Reduced behavioral withdrawal responses during fear retrieval in adult mice and rats.

Pain triggers emotional changes in humans and animals, including fear and anxiety. Conversely, fear and anxiety may enhance suffering of patients with pain. However, in animal models of acute pain, it has been reported that fear may inhibit pain by activating endogenous inhibitory systems. In this study, we wanted to examine if behavioral withdrawal responses may be affected during fear retrieval, a condition where fear-associated tone is applied. We found that thermal pain thresholds were significantly increased during fear retrieval. Our results indicate that animals are suffering fear like-events, while their behavioral responses are inhibited. These results indicate that it will be important to evaluate both emotional and behavioral withdrawal responses for future development of new pain medicine.