Stimulation of peripheral nociceptor endings by low dose morphine and its signaling mechanism

In this report, we demonstrated that peripheral application of very low dose (amol ranges) of morphine induced flexor response through a substance P (SP) release at the nociceptor endings in mice. The intraplantar (i.pl.) application of morphine produced flexor response in a dose-dependent manner from 0.1 to 1000amol. The μ-opioid receptor (MOP-R) agonist [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) also produced dose-dependent flexor response in same dose ranges. Morphine-induced flexor responses were markedly inhibited by naloxone and D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP) both MOP-R antagonists and by intrathecal injection of antisense oligodeoxynucleotide (AS-ODN) for MOP-R which is expected to reduce the receptor expression in sensory nerve endings. Prior incubation with capsaicin, a depletor of SP from polymodal C fibers and [(+)-(2S,3S)-(2-methoxybenzylamino)-2-phenylpiperidine] (CP-99994), a tachykinin 1 receptor antagonist, also blocked the morphine-induced flexor responses. Moreover, pertussis toxin (PTX) which inactivates Gα_i/o; [(1-[6-([(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl]-1H- pyrrole-2,5-dione)] (U-73122), an inhibitor of phospholipase C (PLC); ethyleneglycol-bis(β-aminoethyl ether) N,N,N′,N′-tetraacetic acid (EGTA), a Ca²⁺ chelating agent; xestospongin C, a membrane-permeable inositol trisphosphate (InsP₃) receptor antagonist inhibited the morphine-flexor responses. However, thapsigargin, a depletor of intracellular Ca²⁺ concentration and diphenhydramine, a histamine (His) H1 receptor antagonist, were unable to block the morphine-induced flexor responses. These results suggest that extremely low doses of morphine can stimulate sensory nerve endings through activation of peripheral MOP-R and its downstream mechanisms include activation of PLC through a SP release from polymodal C fibers.