Pharmacological assessments of nitric oxide synthase isoforms and downstream diversity of NO signaling in the maintenance of thermal and mechanical hypersensitivity after peripheral nerve injury in mice

Nitric oxide synthase (NOS) isoforms and NO downstream signal pathways involved spinally in the maintenance of thermal and mechanical hypersensitivity were assessed in a mouse model of neuropathic pain developing after partial ligation of the sciatic nerve. Intrathecal injection of the NOS inhibitor NG-nitro-l-arginine methyl ester (l-NAME), the highly selective neuronal NOS (nNOS) inhibitor Nω-propyl-l-arginine and the potent selective inducible NOS (iNOS) inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine hydrochloride (AMT) exerted dose-dependent analgesic effects on thermal and mechanical hypersensitivity, which were assessed by the plantar and von Frey tests, respectively, suggesting that both nNOS and iNOS participate in producing NO to maintain neuropathic pain. Since the selective inhibitor of NO-sensitive guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and the guanosine 3′,5′-cyclic monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor Rp-8-pCPT-cGMPS intrathecally exerted dose-dependent analgesic effects on thermal and mechanical hypersensitivity, spinally released NO most likely stimulates the NO–cGMP–PKG pathway. Moreover, the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), a potent superoxide scavenger, reduced thermal and mechanical hypersensitivity when administered intrathecally, suggesting that spinal release of superoxide, which can then react with NO to produce peroxynitrite, also appears to mediate neuropathic pain. Finally, intrathecal injection of phenyl-N-tert-butylnitrone (PBN), a reactive oxygen species (ROS) scavenger, ameliorated thermal and mechanical hypersensitivity, thus further confirming the importance of ROS including NO and superoxide in the maintenance of neuropathic pain. Together, the present results demonstrate that NO, produced presumably via nNOS and iNOS in the spinal cord, mediates the maintenance of neuropathic pain following peripheral nerve injury through both the NO–cGMP–PKG and the NO–peroxynitrite pathways.