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
N(G)-nitro-l-arginine methyl ester (
l-NAME), the highly selective neuronal NOS (nNOS) inhibitor
N(omega)-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.