The effect of CA1 administration of orexin-A on hippocampal expression of COX-2 and BDNF in a rat model of orofacial pain

The neuropeptide orexin-A and its receptors are widely distributed in both hippocampal circuitry and pain transmission pathways. Objective: Involvement of the CA1 orexin 1 receptor (OX1R) on the modulation of orofacial pain and pain-induced changes in hippocampal expression of cyclooxygenase-2 (COX-2) and brain-derived neurotrophic factor (BDNF) was investigated. Methods: Orofacial pain was induced by an intra-lip injection of capsaicin (100 mu g). Reverse transcription polymerase chain reaction and immunoblot analysis were used to indicate changes in hippocampal BDNF and COX-2 expression, respectively. Results: Capsaicin induces a significant pain response, which is not affected by either orexin-A or SB-334867-A, an OX1R antagonist. However, an increased expression of COX-2 and decreased expression of BDNF was observed in the hippocampus of animals that received capsaicin or SB-334867-A (80 nM) plus capsaicin. Meanwhile, orexin-A (40 pM) attenuated the effects of capsaicin on the expression of COX-2 and BDNF. Conclusions: CA1 OX1R activation moderates capsaicin-induced neuronal inflammation and neurotrophic deficiency.