Stress-induced analgesia in μ-opioid receptor knockout mice reveals normal function of the δ-opioid receptor system

Abstract

Stress-induced analgesia (SIA) was examined in wildtype and μ-opioid receptor knockout mice. We used thermal paw withdrawal (TPW) latency following a continuous 3-min swim in 20°C water, and found a significant increase in TPW latency in both wild-type and knockout mice. Pre-treatment prior to the swim with naltrindole, a selective δ-opioid receptor antagonist, blocked the increase in TPW latency in knockout mice. These results demonstrate an intact δ-receptor-mediated function of a physiologically-released endogenous agonist in the μ-opioid receptor knockout mouse. The present findings are in contrast with previous reports that analgesia induced by exogenous delta agonists is reduced in the knockout mice.