Immunohistochemical localization of endomorphin-1 and endomorphin-2 in immune cells and spinal cord in a model of inflammatory pain

Immunohistochemical localization of endomorphin-1 and endomorphin-2 in immune cells and spinal cord in a model of inflammatory pain

Mousa SA; Machelska H; Schafer M; Stein C

Journal Of Neuroimmunology

2002

Male; Animals; Rats; Biomarkers of Pain; Immunohistochemistry; Freund's Adjuvant; Hindlimb; Wistar; Lymph Nodes/cytology; Lymphocytes/chemistry; Macrophages/chemistry; Monocytes/chemistry; Oligopeptides/analysis; Pain/chemically induced/immunology; Posterior Horn Cells/chemistry; Skin/chemistry/immunology/innervation

Recently, two novel highly selective mu-opioid receptor (MOR) agonists, endomorphin-1 and endomorphin-2, have been isolated from bovine as well as human brains and were proposed to be the endogenous ligand for MOR. Later, endomorphin-1 and endomorphin-2 have been detected in the immune system of rats and humans using radioimmunoassay in combination with reverse-high-phase-liquid chromatography. In the present study, we analyzed the expression of endomorphin-1, endomorphin-2 and MOR by immunohistochemistry in a model of Freund's complete adjuvant (FCA)-induced painful inflammation. While MOR was upregulated on peripheral and central nerve terminals, inflammation did not alter endomorphin-2 expression in nerve fibers either in the dorsal horn of the spinal cord or in subcutaneous tissue. Endomorphin-1 and endomorphin-2 were expressed in immune cells (macrophage/monocytes) in the medullary region of the popliteal lymph nodes. The proportion of immunocytes (macrophage/monocytes, lymphocytes) containing endomorphin-1 and endomorphin-2 was increased in inflamed lymph nodes and subcutaneous paw tissue of animals with local inflammatory pain. Taken together, the upregulation of MOR and of its endogenous ligands endomorphin-1 and endomorphin-2 in immunocytes suggests an involvement of these opioid peptides in the peripheral control of inflammatory pain.

2002

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