Ghrelin Exerts Analgesic Effects through Modulation of IL-10 and TGF-β Levels in a Rat Model of Inflammatory Pain

BACKGROUND Ghrelin is a peptide with attenuating effect on inflammatory pain. Both anti- and pro-inflammatory mediators have a role in the nociception and development of pain and hyperalgesia. IL-10 and TGF-β are anti-inflammatory cytokines and inhibit the expression of pro-inflammatory cytokines related to peripheral and central inflammatory pain. In this study, the effects of i.p. injection of ghrelin on the early and the late phases of pain, as well as serum levels of IL-10 and TGF-β, as anti-inflammatory cytokines, were investigated in formalin-induced pain in male rats. METHODS Adult male Wistar rats (n=48) were randomly divided into six groups: control, formalin+saline, ghrelin (40, 80, and 160 μg/kg), and morphine. Ghrelin was administered i.p. 30 min before inducing pain by formalin. Pain induced by intraplantar (i.pl.) injection of 50 µl formalin 5%, and pain behavior was studied for 60 min. Serum IL-10 and TGF-β levels were assessed by ELISA method. RESULTS The findings of the present study showed that ghrelin with high doses (80 and 160 μg/kg) significantly reduced pain intensity in both the early and the late phases of pain. The serum levels of cytokines, IL-10, and TGF-β1 showed a significant elevation with ghrelin at the dose of 160 μg/kg. CONCLUSION Ghrelin is effective in reducing the intensity of both the early and the late phases of inflammatory pain. It seems that ghrelin exerts its analgesic effects in part by increasing the serum levels of anti-inflammatory cytokines.