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MU-opioid receptor-knockout mice: role of mu-opioid receptor in morphine mediated immune functions
1Department of Pharmacology, University of Minnesota Medical School, Minneapolis, MN 55455, USA. royxx002@gold.tc.umn.edu
Abstract:
The role of the mu-opioid receptor in immune function was investigated using mu-opioid receptor knockout mice (MOR-KO). Morphine modulation of several immune functions, including macrophage phagocytosis and macrophage secretion of TNF-alpha, was not observed in the MOR-KO animals, suggesting that these functions are mediated by the classical mu-opioid receptor. In contrast, morphine reduction of splenic and thymic cell number and mitogen-induced proliferation were unaffected in MOR-KO mice, as was morphine inhibition of IL-1 and IL-6 secretion by macrophages. These latter results are consistent with morphine action on a naloxone insensitive morphine receptor, a conclusion supported by previous studies characterizing a nonopioid morphine binding site on immune cells. Alternatively, morphine may act either directly or indirectly on these cells, by a mechanism mediated by either delta or kappa opioid receptors.
Insights
The mu-opioid receptor (MOR) mediates some morphine effects on immune cells, but not others. Non-classical pathways, possibly involving naloxone-insensitive receptors, explain morphine
Area of Science:
- Immunology
- Neuroscience
- Pharmacology
Background:
- The mu-opioid receptor (MOR) is a key target for opioid drugs like morphine.
- Opioid receptors are known to be expressed on immune cells, suggesting a role in neuro-immune interactions.
- Previous research indicates that morphine can modulate immune cell function, but the specific receptors involved are not fully elucidated.
Purpose of the Study:
- To investigate the specific role of the mu-opioid receptor in mediating morphine's effects on immune function.
- To determine whether observed morphine effects on immune cells are dependent on the classical mu-opioid receptor pathway.
Main Methods:
- Utilized mu-opioid receptor knockout mice (MOR-KO) to compare immune responses to morphine with those in wild-type animals.
- Assessed key immune functions including macrophage phagocytosis, cytokine secretion (TNF-alpha, IL-1, IL-6), splenic and thymic cell numbers, and mitogen-induced proliferation.
Main Results:
- Morphine's modulation of macrophage phagocytosis and TNF-alpha secretion was absent in MOR-KO mice, confirming MOR mediation.
- However, morphine's effects on reducing splenic/thymic cell numbers and inhibiting IL-1/IL-6 secretion were not dependent on MOR.
- These MOR-independent effects suggest the involvement of alternative morphine-binding sites, potentially naloxone-insensitive receptors or other opioid receptor types (delta, kappa).
Conclusions:
- The classical mu-opioid receptor is essential for certain morphine-induced immunomodulatory effects.
- Morphine exerts other effects on immune cell number and cytokine production through MOR-independent mechanisms.
- These findings support the existence of non-classical, potentially naloxone-insensitive, morphine targets on immune cells, broadening our understanding of opioid-immune system interactions.