Dexmedetomidine relieves formaldehyde-induced pain in rats through both α2 adrenoceptor and imidazoline receptor

Dexmedetomidine (DEX) is a highly selective α2 adrenergic receptor agonist. In this study, we investigated the analgesic effect and the underlying mechanisms of DEX on inflammatory visceral pain in rats. Twenty-five male Sprague Dawley (SD) rats were randomly divided into 5 groups, including control, sham, low dose DEX, medium dose DEX and high dose DEX group. Pain was induced with 10% formalin and scored every 15min till 2 h-post the induction. Hematoxylin-eosin (HE) staining was used to evaluate the toxicity of DEX on spinal cord neurons. Acetycholine (Ach) and noradrenaline (NA) levels were determined by using ELISA method. The expressions of natural nitric oxide synthase (nNOS), protein kinase γ (PKCγ) and protease-activated receptor 2 (PAR2) were determined by using western blot. DEX treatment relieved formaldehyde-induced pain in rats in a dose-dependent manner. Furthermore, DEX showed little neuro-toxicity on the spinal cord neurons, even at the highest dosage used in our study. Ach level was significantly increased in Sham group compared with control group. DEX treatment decreased NA levels and increased Ach levels in the incubation medium of spinal cord sections. Western blot analysis showed that the expression of nNOS, PKCγ and PAR2 was significantly decreased in DEX group compared with Sham group, whereas these effects of DEX on nNOS, PKCγ and PAR2 were blocked by both yohimbine and idazoxan, indicating that the analgesic effect of DEX is mediated by both α2 adrenergic receptor and imidazoline receptor. Yohimbine and idazoxan treatment significantly enhanced pain scores compared to DEX group, and which antagonizes the effects DEX. In conclusion, our study demonstrated that DEX could inhibit formaldehyde-induced pain by inhibiting nNOS, PKCγ and PAR2 expression through α2 adrenergic receptor and imidazoline receptor.