Clinically used opioids are effective analgesics but are highly associated with many adverse effects including physical dependence, tolerance development, rewarding properties and respiratory depression, the cause of opioid‐related overdoses and deaths. The goal of this study is to evaluate tolerance development to centrally and peripherally‐acting mu‐opioid receptor (MOR) agonists. In this study, we used a model of acute, peripherally‐mediated visceral pain, the acetic acid stretch assay (AASA) and an inflammatory model of pain measuring mechanical allodynia using von Frey monofilaments. In the AASA, loperamide and morphine displayed acute antinociceptive effects with ED50 values of 0.62 and 0.15 mg/kg, respectively. Naloxone pretreatment attenuated the acute antinociceptive effects of both loperamide and morphine. Following 5 days of 2x daily drug administration, the ED50 for the loperamide and morphine dose effect curves were shifted to the right. In the model of inflammatory pain, acute administration of loperamide was ineffective at doses up to 3.2mg/kg. Morphine produced dose‐dependent acute antinociceptive effects in this assay with an ED50 of 4.2mg/kg. This effect was attenuated by naloxone but was not altered by naloxone‐methiodide. Following 5 days of 3x daily administration of morphine, the ED50 of the morphine dose effect curve was shifted 3.5 fold to the right. Interestingly, pretreatments of naloxone‐methiodide prior to each chronic morphine injection prevented the rightward shift in the morphine dose effect curve following chronic morphine treatment. These results suggest that repeated activation of peripheral opioid receptors produce tolerance and that peripheral opioid receptors may be involved in the development of tolerance to morphine.

Opioids are regarded as the most effective drugs for pain therapy. However, adverse effects, such as tolerance development, hinder the effectiveness of opioids during chronic treatment. Therefore, elucidating mechanisms involved in tolerance may aid in the design of novel treatments of chronic pain. This study evaluates the role of peripheral opioid receptors in tolerance development to mu‐opioid receptor (MOR) agonists. All animals used in this study were male, C57BL/6N, and weighed between 20‐30 g at 7‐15 weeks old. Antinociceptive effects of MOR agonists were evaluated in a state of inflammatory pain (CFA‐induced mechanical allodynia) and a centrally‐mediated, thermal reflex assay (50°C warm water tail withdrawal (WWTW)). Following five days of 3x daily administration of morphine (10mg/kg), the ED50 of the morphine dose‐effect curve was shifted 2.5‐fold to the right in both assays. Interestingly, in the same two assays, naloxone‐methiodide (10mg/kg) pretreatments before each chronic morphine injection prevented the rightward shift in the morphine dose‐effect curve without altering the acute antinociceptive effects of morphine in mice treated with chronic saline. In the WWTW, five days of 4x daily administration of loperamide (3.2mg/kg, an ineffective dose in the WWTW) was sufficient to produce cross‐tolerance to the antinociceptive effects of morphine, as displayed by a rightward shift in the morphine dose‐effect curve. Pretreatments of naloxone‐methiodide (10mg/kg) completely attenuated loperamide‐induced cross‐tolerance to the antinociceptive effects of morphine. These results suggest that repeated activation of peripheral opioid receptors may be involved in developing tolerance to the central antinociceptive effects of MOR agonists.