Effect of trigeminal nucleus caudalis orexin 1 receptor on capsaicin orofacial pain- induced and expression of C-fos in adult male rats

Orexin A and orexin B are hypothalamic neuropeptides that have critical roles in the regulation of physiological responses such as pain. C-fos is neural activity markers in painful situations. This study was designed to evaluate the role of trigeminal nucleus caudalis (TNC) orexin 1 receptor on capsaicin-induced orofacial pain and c-fos expression in TNC. In this experimental study, 70 male Wistar rats (200-250 g) were randomly allocated into 10 groups: control, pain, sham surgery, sham capsaicin, surgery and pain, sham orexin A and pain, orexin A (50 pM/rat) and pain, sham SB-334867 and pain, SB-334867 (40 nM/rat) as antagonist and pain, orexin A plus SB-334867 and pain. Following cannula implantation and recovery period, the drugs were administration 20 min before capsaicin injection. C-fos expression in TNC was evaluated using immunohistochemistry. The capsaicin-injected rats showed a significant pain behavior and expression c-fos (p<0.001). Orexin A significantly inhibited capsaicin-induced pain and decreased c-fos expression (p<0.001). Pretreatment with SB-334867 could prevent the effects of orexin A. SB-334867 alone exaggerated capsaicin-induced pain responses. The level of c-fos was high in SB-334867–treated rats some as capsaicin group. SB-334867 could partially inhibit the effect of orexin A on c-fos orexin A expression.