Involvement of nitric oxide in the inhibition of bone cancer-induced hyperalgesia through the activation of peripheral opioid receptors in mice.
ABSTRACT Experiments were designed to elucidate the involvement of nitric oxide (NO) in the antihyperalgesic effect induced by the activation of peripheral mu-opioid receptors on osteosarcoma-induced thermal hyperalgesia in mice. Since this pathway has previously been shown to be involved in the antihyperalgesic effect induced by some drugs--including opiates--on inflammatory pain, experiments were also performed in inflamed mice. The intraplantar administration of loperamide (15 microg) abolishes the thermal hyperalgesia that appears 4 weeks after the intratibial inoculation of NCTC 2472 cells in C3H/HeJ mice. The blockade of this effect by coadministering a peripheral opioid receptor antagonist (naloxone methiodide), a nitric oxide synthase (NOS) inhibitor (L-NMMA), a soluble guanylyl cyclase inhibitor (ODQ), a PKG inhibitor (KT-5823) or a K(+)(ATP)-channel blocker (glibenclamide) shows the involvement of a NO/cGMP/K(+)(ATP)-channel pathway. Accordingly the administration of loperamide produced, in osteosarcoma-bearing mice, an increase in the concentrations of NO metabolites, nitrites and nitrates, extracted from paws. The selective inhibitor of eNOS L-NIO, but not the inhibitors of nNOS (N-omega-propyl-L-arginine) or iNOS (1400w), blocked the effect of loperamide on osteosarcoma-induced hyperalgesia and also the endogenous opioid peripheral hypoalgesia that appears during the initial stages of the development of this osteosarcoma. Although this pathway also participates in the inhibitory effect of loperamide on the thermal hyperalgesia induced by administration of complete Freund's adjuvant, only selective inhibitors of nNOS or iNOS antagonized this effect. Our results demonstrate that the activation of a NO/cGMP/K(+)(ATP)-channel triggered by eNOS participates in the peripheral antihyperalgesic of loperamide on osteosarcoma-induced thermal hyperalgesia.
Article: Peripheral effects of morphine and expression of μ-opioid receptors in the dorsal root ganglia during neuropathic pain: nitric oxide signaling.[show abstract] [hide abstract]
ABSTRACT: The local administration of μ-opioid receptor (MOR) agonists attenuates neuropathic pain but the precise mechanism implicated in this effect is not completely elucidated. We investigated if nitric oxide synthesized by neuronal (NOS1) or inducible (NOS2) nitric oxide synthases could modulate the local antiallodynic effects of morphine through the peripheral nitric oxide-cGMP-protein kinase G (PKG)-ATP-sensitive K+ (KATP) channels signaling pathway activation and affect the dorsal root ganglia MOR expression during neuropathic pain. In wild type (WT) mice, the subplantar administration of morphine dose-dependently decreased the mechanical and thermal allodynia induced by the chronic constriction of the sciatic nerve (CCI), which effects were significantly diminished after their co-administration with different subanalgesic doses of a selective NOS1 (N-[(4S)-4-amino-5-[(2-aminoethyl)amino]pentyl]-N'-nitroguanidine tris(trifluoroacetate) salt; NANT), NOS2 (L-N(6)-(1-iminoethyl)-lysine; L-NIL), L-guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ), PKG ((Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate; Rp-8-pCPT-cGMPs) inhibitor or a KATP channel blocker (glibenclamide). The evaluation of the expression of MOR in the dorsal root ganglia from sham-operated and sciatic nerve-injured WT, NOS1 knockout (KO) and NOS2-KO mice at 21 days after surgery demonstrated that, although the basal mRNA and protein levels of MOR were similar between WT and both NOS-KO animals, nerve injury only decreased their expression in WT mice. These results suggest that the peripheral nitric oxide-cGMP-PKG-KATP signaling pathway activation participates in the local antiallodynic effects of morphine after sciatic nerve injury and that nitric oxide, synthesized by NOS1 and NOS2, is implicated in the dorsal root ganglia down-regulation of MOR during neuropathic pain.Molecular Pain 01/2011; 7:25. · 3.53 Impact Factor