Nitric oxide implicates c-Fos expression in the cuneate nucleus following electrical stimulation of the transected median nerve

Elevated galanin receptor type 2 primarily contributes to mechanical hypersensitivity after median nerve injury

In this study, we investigated temporal changes in galanin receptor type 2 (GalR2) expression in NF200-, galanin-, neuropeptide Y (NPY)-, and neuronal nitric oxide synthase (nNOS)-like immunoreactive (LI) dorsal root ganglion (DRG) neurons after median nerve chronic constriction injury (CCI), and the effects of GalR2 on c-Fos expression in the cuneate nucleus (CN). Double immunofluorescence labeling methods were used to appraise changes in GalR2 expression in NF200-LI, galanin-LI, NPY-LI, and nNOS-LI DRG neurons after CCI. The von Frey assay was used to assess the efficiency of intraplantar administration of saline, M871 (a GalR2 antagonist), or AR-M1896 (a GalR2 agonist) on neuropathic signs of rats with CCI. The effects of alterations in c-Fos expression were assessed in all treatments. The percentage of GalR2-LI neurons in lesioned DRGs increased and peaked at 1 week after CCI. We further detected that percentages of GalR2-LI neurons labeled for NF200, galanin, NPY, and nNOS significantly increased following CCI. Furthermore, M871 remarkably attenuated tactile allodynia, but the sensation was slightly aggravated by AR-M1896 after CCI. Consequentially, after electrical stimulation of the CCI-treated median nerve, the number of c-Fos-LI neurons in the cuneate nucleus (CN) was significantly reduced in the M871 group, whereas it increased in the AR-M1896 group. These results suggest that activation of GalR2, probably through NPY or nitric oxide, induces c-Fos expression in the CN and transmits mechanical allodynia sensations to the thalamus.

Potential role of nitric oxide synthase isoforms in pathophysiology of neuropathic pain

Neuropathic pain triggers a cascade of events in the sensory neurons. It is the main complication of diabetes after cardiovascular disease. Nitric oxide (NO) produced from nitric oxide synthases (NOS) is an important signaling molecule which is crucial for many physiological processes such as synaptic plasticity, neuronal survival, vasodilation, vascular homeostasis, immune regulation. Overproduction of NO due to changes in NOS isoforms level involves pathological processes such as neurotoxicity, septic shock and neuropathic pain. All three isoforms of NOS as well as their end product, NO have modulatory effect on neuropathic pain. Overactivation of the N-Methyl-D-Aspartate receptor and peroxynitrite formation results in high levels of neuronal NOS (nNOS) and endothelial NOS (eNOS) which suggest that nNOS and eNOS are critical for pain hypersensitivity. Inducible NOS induced in glia by inflammation due to activation of Tumor Necrosis Factor α, Calcitonin Gene Regulating Peptide, Mitogen Activated Protein Kinases, Extracellular signal Regulated Kinase, c-Jun N-terminal kinases can induce neuronal death. This review focuses on different nitric oxide synthases and their role in pathophysiology of neuropathic pain considering NOS as an important therapeutic target.