Nitric oxide (NO) is involved in many pathophysiological processes, including the
important role that it serves as a neurotransmitter/neuromodulator involved in nociceptive
processes in the spinal cord dorsal horn.1) It has been well demonstrated that NO is a
diffusible gas synthesized in biological systems by enzymes known as nitric oxide synthases
(NOS) (neuronal/nNOS, inducible/iNOS, or endothelial/eNOS).1) Interestingly, nNOS colocalizes
with N-methyl-D-aspartic acid (NMDA) receptors and is functionally coupled to
this glutamate receptor subtype via binding with postsynaptic density protein-95.2) Several
studies suggest that activation of NMDA receptors modulates spinal nociceptive transmission
via nNOS, which is dephosphorylated and activated by the Ca2+/CaM-dependent phosphatase
calcineurin, resulting from Ca2+ influx through NMDA receptors.3-5) In a previous study, we
showed that blockade of spinal nNOS using intrathecal (i.t.) administration of the nNOS
inhibitor, 7-nitroindazole, reduces sigma-1 receptor (Sig-1R)-induced pain hypersensitivity
and suppresses the Sig-1R-induced increase in PKC-dependent phosphorylation of GluN1
subunit (pGluN1) resulting in the potentiation of NMDA receptor function.6) These findings
raise the possibility that the action of NO on nociceptive transmission at spinal synapses
could be closely linked to Sig-1R-induced functional activation of NMDA receptors.