Exogenous and Endogenous Opioid-induced Pain Hypersensitivity in Different Rat Strains

Background: Opioid-induced hyperalgesia (OIH) is a recognized complication of opioid use that may facilitate the development of exaggerated postoperative pain.

Objective: To examine the role of genetic factors on OIH by comparing four rat strains. Because the authors previously reported that the endogenous opioids released during non-nociceptive environmental stress induce latent pain sensitization, genetic and environmental factor interactions were also evaluated.

Methods: First, the propensity of Sprague Dawley, Wistar, Lewis and Fischer rats to develop OIH following single or repeated fentanyl exposures was compared by measuring the nociceptive threshold using the paw pressure vocalization test. Second, Sprague Dawley and Fischer rats were exposed to a series of three non-nociceptive environmental stress sessions to evaluate the ability of endogenous opioids to enhance hyperalgesia associated with a carrageenan-induced hind-paw inflammation test performed two weeks later.

Results: Sprague Dawley, Wistar and Lewis rats exhibited OIH, although differences were observed. OIH was not observed in Fischer rats. Inflammatory hyperalgesia enhancement induced through previous stress in Sprague Dawley rats was not observed in Fischer rats.

Conclusions: The pain level not only reflects nociceptive inputs but also depends on both the history and genetic factors of the individual. Genetic and environmental models may provide new insights into the mechanisms that underlie individual differences observed in postoperative pain.