Nitric oxide (NO): in vivo electrochemical monitoring in the dorsal horn of the spinal cord of the rat

NO synthase (NOS) is largely distributed in the superficial and deep laminae of the dorsal horn as well as in dorsal root ganglion cells. It has been proposed that nitric oxide (NO) participates in the transmission of sustained, and possibly brief, nociceptive, inputs at the spinal level. The aim of this study was to check the ability of in vivo electrochemical monitoring of NO within the dorsal horn of the lumbar spinal cord (L-3-L-4 level) of chloral hydrate anesthetized or decerebrated spinalized rats. 30 mu m diameter and 450 mu m length treated carbon fiber electrodes coated with nickel(II) tetrakis (3-methoxy-4-hydroxy-phenyl) porphyrine and Nafion(R), and associated with differential normal pulse voltammetry, gave a peak of oxidation current around 650 mV (vs. Ag-AgCl) in vitro in NO solutions between 0.125 and 1.25 mu M. In vivo, a 650 mV peak appeared which was stable (recording interval 2 min) for up to 3 h (+/- 6%). Comparison between in vitro calibration and in vivo voltammograms gave an estimated in vivo extracellular concentration of 0.50 mu M. In vivo, peaks decreased by 95% at 90 min and for up to 3 h after an i.p. injection of 100 mg/kg of the NOS inhibitor (NOSI) L-arginine-p-nitroanilide (L-ANA). At the same dose i.p., N-G-nitro-L-arginine methyl ester (L-NAME) was almost ineffective after 90 min in animals paralyzed with pancuronium bromate or gallamine trethiodide. However, in non-curarized decerebrated spinalized animals, L-NAME depressed the voltammograms by 36% at 90 min. S-Ethylthiourea (80 mg/kg i.p.), also decreased the voltammograms by 45% at 140 min, and finally, 7-nitroindazole (7-NI, 90 mg/kg i.p), induced a important decrease of the 650 mV peak (23% of control) at 120 min. These results are in agreement with biochemical data showing the decrease of NOS activity within the lumbar spinal cord by L-NAME (45% of central at 90 min) and 7-NI (20% of control at 90 min). The NO donor hydroxylamine (30 mg/kg i.p.) significantly increased the peaks (140% at 90 min), and sodium nitroprusside (SNP, 20 mM) when directly superfused upon the spinal cord (200-300 mu l min(-1)) induced a large increase in the peak (300% at 90 min). Moreover, SNP 60 min after L-ANA, or 90 min after L-NAME, rapidly restored the 650 mV peak up to control values. These results demonstrate the validity of electrochemical monitoring of NO within the dorsal horn of the spinal cord. The in vivo electrochemical detection of NO is in progress to study the implication of this messenger in the transmission of nociceptive messages at the spinal level. (C) 1997 Elsevier Science B.V.