Papers of the Week

Chronic use of mu-opioid receptor (MOR) agonists, such as morphine, for pain management can lead to the rapid development of analgesic tolerance. In contrast, tolerance to morphine effects in the gastrointestinal tract develops at a different rate. This can limit the therapeutic utility of morphine as the undesired gastrointestinal effects can persist even in the absence of analgesia. It is unclear if the discrete rates of morphine tolerance are due to different underlying mechanisms. β-arrestin-2 is a multi-functional protein implicated in the mechanism of antinociceptive tolerance to opioids. We have recently shown that morphine tolerance in dorsal root ganglia nociceptive neurons is mediated via two distinct mechanisms that are dependent on the duration of drug exposure: a β-arrestin-2-dependent mechanism for short-term (15-18 hours) tolerance and a β-arrestin-2-independent mechanism for long-term (7 days) tolerance (Muchhala et al., 2021, European Journal of Pharmacology). In the gastrointestinal tract, myenteric plexus neurons regulate intestinal motility. However, it is not clear if morphine engages the same molecular mechanisms for tolerance development in myenteric plexus neurons and dorsal root ganglia neurons. Therefore, in the present study we investigated the role of β-arrestin-2 in the development of morphine tolerance in myenteric plexus neurons of the mouse ileum. Here, we used whole-cell patch clamp electrophysiology to examine morphine tolerance in individual ileum myenteric plexus neurons treated with 10 µM morphine for 15-18 hours or isolated from mice treated with morphine for 7 days. Acute 3 µM morphine significantly increased the threshold to fire action potentials in naïve myenteric plexus neurons. However, this effect was not observed in wild-type neurons exposed to morphine for 15 -18 hours or isolated from mice treated with morphine for 7 days, indicating the development of tolerance. Furthermore, genetic deletion of β-arrestin-2 did not prevent the development of morphine tolerance in ileum neurons. In contrast, Bisindolylmaleimide XI, a selective protein kinase C inhibitor, reversed tolerance in ileum myenteric plexus neurons exposed to morphine in vivo for 7 days. In these neurons acute 3 µM morphine significantly increased action potential threshold. Thus, unlike dorsal root ganglia neurons, morphine tolerance in myenteric plexus neurons does not utilize β-arrestin-2. These findings reveal a potential mechanism for the differences in the rates of tolerance to morphine and highlight the need to investigate tolerance mechanisms for mu-opioid analgesics in different tissues/neurons. These results also indicate that mu-opioid analgesics that preferentially signal through G-proteins over β-arrestin-2 i.e. biased agonists, such as Oliceridine, may induce tolerance in the ileum.