The role of trigeminal nucleus caudalis orexin 1 receptor in orofacial pain-induced anxiety in rat

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The relationship between anxiety and pain has received special attention. Orexins (A and B) are hypothalamic neuropeptides that have diverse functions in the regulation of different physiological and behavioral responses. This study was designed to evaluate the role of orexin 1 receptors (OX1R) within trigeminal nucleus caudalis (TNC) in anxiety following the induction of orofacial pain. The subcutaneous injection of capsaicin (CAP) into the rat upper lip region produced pain responses. OX1R agonist (orexin A) and antagonist (SB-334867) were microinjected into the TNC before the administration of CAP. Anxiety behaviors were investigated using elevated plus maze (EPM) and open-field tests. The results showed that CAP injection significantly decreases the percentage of time spent in the open arms of the EPM and the time spent in the center of the open field. Surprisingly, orexin (50, 100, and 150 pM/rat) significantly exaggerated the CAP effects, whereas SB-334867 (20, 40 nM/rat) significantly inhibited the CAP-induced anxiety. The CAP-injected group showed a significant decrease in the percentage of entries to open arms in the EPM and the number of visits in the center area of the open field compared with the control group. Orexin significantly potentiated the mentioned effects of CAP, whereas SB-334867 (40, 80 nM/rat) exerted a significant inhibitory effect on CAP-induced anxiety. The overall results indicated that the TNC OX1Rs play an important role in orofacial pain-induced anxiety.

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