Involvement of Endogenous Opioidergic System in Effects of Food Deprivation on Formalin-Induced Nociceptive Behaviors in Rats

Background and Aims: It is well-recognized that endogenous opioidergic system plays an important role in pain sensitivity, pain perception, and response to some stressing situations. The present study examined the probable role of endogenous opioidergic system in effects of food deprivation on formalin-induced nociceptive behaviors in male and female rats. Methods: To make short-term food deprivation, the rats did not receive food for 48 hours prior to the formalin test. However, they could freely access water. Afterward, 50 µl formalin 2% was injected into the bottom of the rats’ right paw. Naloxone was injected to evaluate the involvement of the endogenous opioidergic system in effects of food deprivation on formalin-induced nociceptive behaviors. Results: Formalin injection caused nociceptive behaviors in two phases The first and second phases were separated by a brief interphase where nociceptive behaviors decreased. As a result of 48-hour food deprivation, nociceptive behaviors in male and female rats were increased and the second phase was finished with delay. Naloxone administration blocked the pronociceptive effect of the 48-hour food deprivation in the interphase of male rats and in the first phase of female rats. Conclusion: The present study indicated that food deprivation increased formalin-induced nociceptive behaviors through affecting the opioidergic system.