Nitric Oxide in the Spinal Cord Is Involved in the Hyperalgesia Induced by Tetrahydrobiopterin in Chronic Restraint Stress Rats

It has been well recognized that exposure to chronic stress could increase pain responding and exacerbate pain symptoms, resulting in stress-induced hyperalgesia. However, the mechanisms underlying stress-induced hyperalgesia are not yet fully elucidated. To this end, we observed that restraint as a stressful event exacerbated mechanical and thermal hyperalgesia, accompanied with up-regulation of nitric oxide (NO) ( < 0.001), GTP cyclohydrolase 1 (GCH1) (GCH1 mRNA: = 0.001; GCH1 protein: = 0.001), and tetrahydrobiopterin (BH4) concentration (plasma BH4: < 0.001; spinal BH4: < 0.001) on Day 7 in restraint stress (RS) rats. Intrathecal injection of -nitro-L-arginine methyl ester (L-NAME), a non-specific NO synthase inhibitor, or -([3-(aminomethyl)phenyl]methyl) ethanimidamide, a special inhibitor of inducible NO synthase (iNOS), for seven consecutive days attenuated stress-induced hyperalgesia and decreased the production of NO ( < 0.001). Interestingly, 7-nitro indazole, a special inhibitor of neuronal NO synthase, alleviated stress-induced hyperalgesia but did not affect spinal NO synthesis. Furthermore, intrathecal injection of BH4 not only aggravated stress-induced hyperalgesia but also up-regulated the expression of spinal iNOS (iNOS mRNA: = 0.015; iNOS protein: < 0.001) and NO production ( < 0.001). These findings suggest that hyperalgesia induced by RS is associated with the modulation of the GCH1-BH4 system and constitutively expressed spinal iNOS. Thus, the GCH1-BH4-iNOS signaling pathway may be a new novel therapeutic target for pain relief in the spinal cord.