Ligand‐directed activation of extracellular signal‐regulated kinases (ERK1/2) in enteric neurons (1114.1)

Opioids, acting at µ opioid receptors (µORs) are commonly used for chronic pain treatment. Opioids differ in their efficiency to induce µOR internalization, a key event in receptor signaling and function. Here, we tested the effect of opioids with different internalizing efficiency on the activation of the ERK family of Mitogen‐Activated Protein Kinases (MAPKs), a µOR downstream signaling, in enteric neurons, which mediate constipation, a major side effect of chronic opioids. Cultured rat myenteric neurons were stimulated with μOR‐internalizing agonists (the endogenous opioid analogue DAMGO or fentanyl at 1 µM), or morphine (10 µM) a poor internalizing agonist, for 0‐30 min at 37oC. ERK1/2 phosphorylation was determined by Western Blot with phospho‐p44/p42 and total ERK1/2 antibodies. DAMGO and fentanyl, but not morphine induced a rapid and transient ERK1/2 activation, which was blocked by naloxone, MAPK inhibitor, hypertonic sucrose, dynamin inhibition or mutation. To determine the effects of chronic µOR activation on ERK1/2 phosphorylation, cells were treated with 10 µM morphine or 1 µM fentanyl for 4 days before exposure to DAMGO, fentanyl or morphine. In enteric neurons chronically treated with morphine, DAMGO, fentanyl and morphine induced ERK1/2 activation, whereas in enteric neurons chronically treated with fentanyl only DAMGO and fentanyl induced ERK1/2 activation. Chronic morphine induced activation of the transcription factor, cAMP response element‐binding protein (CREB), which was abolished by MAPK pathway blockade. Chronic µOR activation induces ligand‐dependent changes of µOR downstream signaling, which are likely to contribute to the development of opioid‐induced constipation. Grant Funding Source : NIDDK54155 and 41301