The use of opioids, such as morphine, as analgesics for the treatment of chronic pain is a common clinical practice. However, a major disadvantage of long-term opioid treatment is the development of opioid-induced bowel dysfunction, which commonly manifests with severe constipation, occurring in more than 40% of the patients. Analgesia and constipation are mediated by the interaction of opioids with ? opioid receptors (?ORs). However, constipation is mediated by ?ORs in enteric neurons, whereas analgesia is mediated by ?ORs in the central nervous system (CNS). Furthermore, analgesia undergoes tolerance with long-term treatment, whereas constipation does not subside over time and worsens with the increase of analgesic doses required to control pain. Activation of ?ORs initiates a cascade of events including endocytosis, desensitization and resensitization. Endocytosis and desensitization are key components in reducing signaling, and long-term treatment induces intracellular changes leading to adaptations in downstream signaling in heterologous cells and CNS neurons. In contrast, ?OR signaling events triggered by long-term opioids in enteric neurons are largely unknown and our initial studies indicate they differ from CNS neurons. The overarching hypothesis of this proposal is that long-term activation of ?OR in enteric neurons leads to signaling events mediating the development of constipation, which is resistant to tolerance. This is in contrast to induction of tolerance in the CNS. These studies will use rats and mice, and in vivo and in vitro approaches with cultured enteric neurons, patch clamp recording, lentiviral neuronal transfection to block trafficking proteins, and mice with deletion of signaling molecules or opioid receptors.
Specific Aim 1 will test the hypothesis that downstream signaling events in enteric neurons following long-term opioids are ligand dependent and tissue specific. cAMP, MAPK phosphorylation and Ca2+ channel currents will be evaluated as measures of receptor-effector coupling.
Specific Aim 2 will identify the relationship between ?OR internalization and desensitization in response to morphine and internalizing agonists, determine the importance of ?OR internalization in downstream signaling and the specific roles of trafficking proteins and kinases involved in ?OR regulation.
Specific Aim 3 will identify the relationship between ?OR signaling events and GI and colonic transit delay induced by long-term ?OR activation and determine whether disruption of ?OR signaling affects opioid-induced GI motility impairment. Understanding the mechanisms underlying the development and persistence of constipation will provide better targets for its prevention and treatment. This proposal will have high impact in improving the quality of life of patients experiencing the burden of severe constipation.
Morphine is the drug of choice for the management of chronic pain, but its clinical utility is hampered by the development of constipation, which can be so severe to dissuade the patients to reduce or interrupt pain treatment with negative impact on their quality of life. Constipation results from disruption of bowel motility, through the action of morphine on opioid receptors (ORs) in enteric neurons, which are located within the gut wall. The goal of this proposal is to understand how morphine acts on the ORs in enteric neurons and how this chronic activation impairs bowel transit, which will provide better targets for the development of efficacious and safe drugs to prevent and/or treat constipation, thus improving the quality of life of these patients and enhance compliance in taking pain medication.
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