Activation of peripheral NMDA-nitric oxide cascade in formalin test

Background: It has been suggested that peripheral glutamate and nitric oxid e (NO) released by tissue-damaging stimuli play an important role in periph eral nociceptive transmission. This study was conducted to determine whethe r NO was released in the periphery after subcutaneous injection of formalin and whether the peripheral N-methyl-D-aspartate (NMDA)-NO cascade was acti vated. Methods: During pentobarbital anesthesia, a microdialysis probe was implant ed subcutaneously into the glabrous skin of both hind paws of Sprague-Dawle y rats. After sample collection to obtain the basal level of NO metabolites (total amount of nitrite [NO2-] and nitrate [NO3-] [NO2--NO3-]), 5% formal in was injected into the plantar surface of the right hind paw during perfu sion of the dialysis catheters with artificial cerebrospinal fluid (ACSF), the NO synthase inhibitor N-G-monomethyl-L-arginine acetate, or the NMDA an tagonist D,L-2-amino-5-phosphonovaleric acid through a microdialysis probe. Formalin also was injected in the animals that underwent sciatic nerve sec tioning. In another series of experiments, NMDA was perfused through one pr obe. Samples for measurement of NO2--NO3- were collected and immediately an alyzed using high-performance liquid chromatography. Results: Subcutaneous formalin significantly increased the dialysate concen trations of NO2--NO3- (maximum increase 144 +/- 12% of baseline value 30 mi n after formalin administration; P < 0.05) on the side ipsilateral to the i njection. Both N-G-monomethyl-L-arginine acetate and D,L-2-amino-5-phosphon ovaleric acid significantly(P < 0.05) suppressed the formalin-induced incre ases in NO2--NO3- concentration. In the rats with denervation of the sensor y nerves, formalin did not change the NO2--NO3- concentration. In addition, NMDA perfusion significantly (P < 0.05) increased the concentrations of NO 2--NO3-. Conclusion: The results of the current study show that subcutaneous formali n injection induces peripheral release of NO, the production of which is me diated by activation of NMDA receptors in the peripheral nervous system.