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.

Abstract

To characterize the role of orexin-1 receptors (OX1Rs) in ventrolateral periaqueductal grey matter (vlPAG) on modulation of capsaicin-induced pulpal nociception in rats. Sixty-six adult male Wistar rats (2months old) weighing between 230 and 260g were used. The animals were cannulated for microinjection of drugs into the vlPAG matter. Pulpalgia was induced by intradental application of capsaicin solution (100μg) into the incisor teeth of the rats. Tenmin prior to capsaicin application, orexin-A (50, 100 and 150pmolL-1 per rat) was administered. Orexin-A (150pmolL-1 ) was also co-administrated with SB-334867 (40nmolL-1 per rat), an OX1Rs antagonist; or bicuculline (1μg 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. Orexin-A dose-dependently decreased capsaicin-induced nociceptive behaviour. However, SB-334867 (40nmolL-1 per rat) pretreatment (P<0.05), but not bicuculline (1μg per rat), attenuated the analgesic effect of orexin-A (150pmolL-1 ). The level of SP was significantly increased in Vc and decreased in vlPAG of capsaicin-treated rats (P<0.05). Capsaicin-induced changes in SP levels, however, were prohibited by orexin-A treatment (150pmolL-1 ) (P<0.05). Orexin-A administration into the vlPAG was associated with an inhibitory effect on capsaicin-induced pulpal nociception and bidirectional effects on the induction of SP in vlPAG and Vc of rats. Central activation of OX1Rs is a potential therapeutic tool for pulpalgia.