Activation of orexin‐1 receptors in the ventrolateral periaqueductal grey matter (vlPAG) modulates pulpal nociception and the induction of substance P in vlPAG and trigeminal nucleus caudalis

Aim To characterize the role of orexin‐1 receptors (OX1Rs) in ventrolateral periaqueductal grey matter (vlPAG) on modulation of capsaicin‐induced pulpal nociception in rats. Methodology Sixty‐six adult male Wistar rats (2 months old) weighing between 230 and 260 g were used. The animals were cannulated for microinjection of drugs into the vlPAG matter. Pulpalgia was induced by intradental application of capsaicin solution (100 μg) into the incisor teeth of the rats. Ten min prior to capsaicin application, orexin‐A (50, 100 and 150 pmol L−1 per rat) was administered. Orexin‐A (150 pmol L−1) was also co‐administrated with SB‐334867 (40 nmol L−1 per rat), an OX1Rs antagonist; or bicuculline (1 μg per rat), a GABAA receptors antagonist. Moreover, treatment effects on the release of pro‐nociceptive modulator substance P (SP) in vlPAG and trigeminal nucleus caudalis (Vc) of rats were explored using an immunofluorescence technique. One‐way analysis of variance was used for the statistical analysis. Results Orexin‐A dose‐dependently decreased capsaicin‐induced nociceptive behaviour. However, SB‐334867 (40 nmol L−1 per rat) pretreatment (P