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Background While opioid drugs are used to treat pain in IBD patients, escalating doses can induce tolerance and paradoxical increased pain signaling. Endogenous opioids are released by immune cells during chronic colitis and have an analgesic action but it is unknown whether they can mitigate the development of tolerance to opioid drugs in IBD. Aims To evaluate whether endogenous opioids released during chronic inflammation prevent the development of tolerance and/or increased pain signaling following chronic morphine treatment. Methods Three groups of C57BL/6 mice were studied: 1) control, 2) chronic colitis induced with 3 cycles of 2% DSS (cDSS mice) and 3) chronic DSS mice injected with escalating doses of morphine on the last 7 days of DSS (10, 20, 30 mg/kg for the first 3 days, then 40mg/kg on 4 to 7 day) (cDSS-morphine mice). Ex vivo afferent nerve recordings were obtained from flat-sheet distal colon preparations (day 31) to determine their mechanosensitivity by probing with a 1g von Frey hair before and after acute morphine (1μM, 10 min perfusion). In parallel experiments, supernatants were obtained from the proximal colon and incubated overnight with isolated dorsal root ganglia (DRG) neurons from control mice. Neuronal excitability was then examined by measuring the rheobase using perforated patch clamp. Results In afferent nerve recordings, acute application of 1μM morphine inhibited the colonic afferent response to probing in control mice (17.86 vs 12.31 Hz; p&amp;lt;0.05), whereas no effect of morphine was seen in cDSS mice. In cDSS-morphine mice acute morphine perfusion paradoxically increased the mean afferent response to probing (13.99 vs 17.28 Hz, p&amp;lt;0.05). Single units were analyzed to identify how many mechanosensitive responses were increased, decreased or not affected by acute morphine perfusion. Compared to control mice, cDSS mice exhibited increased numbers of unaffected units (6/18 vs 1/14) whereas cDSS-morphine mice had decreased number of inhibited units (1/17 vs 11/14) and increased number of excited units (8/17 vs. 2/14) (Figure 1). In patch clamp recordings, colonic supernatants from cDSS mice reduced the excitability of DRG neurons (rheobase increased 28%; p&amp;lt;0.05), however, this effect was lost in neurons incubated with cDSS-morphine supernatants. Conclusions Endogenous opioids released during cDSS colitis do not mitigate tolerance to chronic morphine exposure and may diminish the response to acute morphine. Moreover, the paradoxical hyperexcitable response to acute morphine induced by chronic morphine was not blocked by endogenous opioids. Our patch clamp data suggest that endogenous opioid actions are lost following chronic morphine exposure and could result from inhibition of their release from immune cells. Funding Agencies CCC

A127 THE ROLE OF ENDOGENOUS OPIOIDS IN MITIGATING OPIOID TOLERANCE IN A PRE-CLINICAL MODEL OF IBD