Central effect of xylazine and yohimbin on pain sensitivity of rat during estrous cycle in formalin test

Sex hormone is supposed to modify pain sensitivity. The aim of the present study was to investigat, the role of 2 adrenergic receptors in nociception using formalin test. Formalin test was performed by subcutaneous injection of 50 ml formalin (%2.5) solution into hind paw. Animals were divided into 4 groups as control group (intact animal), sham group (received 2µl of artificial CSF by ICV route), agonist group (received 2µl of xylazine 5 and 10 mg / rat by ICV route) and antagonist group (received 2µl of yohimbin 5 and 10 mg /rat by ICV route). Data were analyzed by two way ANOVA and post- hoc test of tukey's test. Data showed that xylazine significantly decreased (p<0.05) pain sensitivity during all stages of estrous cycle; analgesic effect of xylazine was high in estrus stage of estrous cycle and low in metestrous stage of estrous cycle. Yohimbin in all stages of estrous cycle significantly increased (p<0.05) pain sensitivity. Hyperalgesic effect of yohimbin was high during metestrous stage of estrous cycle and it was low in proestrous stage of estrous cycle. Results of present study indicated that interaction of alpha2 adrenergic system with endogenous sex steroid is important in the modulation of pain sensitivity.