Background: As the meningeally derived, fibroblast-rich, mass-produced by intrathecal morphine infusion is not produced by all opiates, but reduced by mast cell stabilizers, the authors hypothesized a role for meningeal mast cell/fibroblast activation. Using the guinea pig, the authors asked: (1) Are intrathecal morphine masses blocked by opiate antagonism?; (2) Do opioid agonists not producing mast cell degranulation or fibroblast activation produce masses?; and (3) Do masses covary with Mas-related G protein-coupled receptor signaling thought to mediate mast cell degranulation?
Methods: In adult male guinea pigs (N = 66), lumbar intrathecal catheters connected to osmotic minipumps (14 days; 0.5 µl/h) were placed to deliver saline or equianalgesic concentrations of morphine sulfate (33 nmol/h), 2',6'-dimethyl tyrosine-(Tyr-D-Arg-Phe-Lys-NH2) (abbreviated as DMT-DALDA; 10 pmol/h; μ agonist) or PZM21 (27 nmol/h; biased μ agonist). A second pump delivered subcutaneous naltrexone (25 µg/h) in some animals. After 14 to 16 days, animals were anesthetized and perfusion-fixed. Drug effects on degranulation of human cultured mast cells, mouse embryonic fibroblast activation/migration/collagen formation, and Mas-related G protein-coupled receptor activation (PRESTO-Tango assays) were determined.
Results: Intrathecal infusion of morphine, DMT-DALDA or PZM21, but not saline, comparably increased thermal thresholds for 7 days. Spinal masses proximal to catheter tip, composed of fibroblast/collagen type I (median: interquartile range, 0 to 4 scale), were produced by morphine (2.3: 2.0 to 3.5) and morphine plus naltrexone (2.5: 1.4 to 3.1), but not vehicle (1.2: 1.1 to 1.5), DMT-DALDA (1.0: 0.6 to 1.3), or PZM21 (0.5: 0.4 to 0.8). Morphine in a naloxone-insensitive fashion, but not PZM21 or DMT-DALDA, resulted in mast cell degranulation and fibroblast proliferation/collagen formation. Morphine-induced fibroblast proliferation, as mast cell degranulation, is blocked by cromolyn. Mas-related G protein-coupled receptor activation was produced by morphine and TAN67 (∂-opioid agonist), but not by PZM21, TRV130 (mu biased ligand), or DMT-DALDA.
Conclusions: Opiates that activate Mas-related G protein-coupled receptor will degranulate mast cells, activate fibroblasts, and result in intrathecal mass formation. Results suggest a mechanistically rational path forward to safer intrathecal opioid therapeutics.