7S,8R,17S-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic
acid (
resolvin D1 [RvD1]) is biosynthesized from
docosahexaenoic acid (DHA), and belongs to a novel family of
lipid mediators showing remarkable anti-inflammatory effects; however, the effect of RvD1 on
inflammation-induced hyperexcitability of nociceptive neurons under in vivo conditions remains to be determined. The present study, therefore, investigated whether under in vivo conditions, systemic administration of RvD1 could attenuate the
inflammation-induced hyperexcitability of spinal trigeminal nucleus caudalis (SpVc) wide-dynamic range (WDR) neurons associated with
hyperalgesia in rats. The threshold of escape from mechanical stimulation applied to the orofacial area in rats with complete
Freund's adjuvant-induced
inflammation was significantly lower than in naïve rats. The lowered mechanical threshold in rats with
inflammation was returned to control levels following administration of RvD1 (3 ng/kg, i.p.) for 3 days. The mean discharge frequency of SpVc WDR neurons in rats with
inflammation was significantly decreased after RvD1 administration for both non-noxious and noxious mechanical stimuli. Increased spontaneous discharge of SpVc WDR neurons in rats with
inflammation was also significantly decreased after RvD1 administration. Noxious pinch-evoked afterdischarge frequency and occurrence in rats with
inflammation was significantly diminished after RvD1 administration. Expansion of the receptive field in rats with
inflammation also returned to control levels after RvD1 administration. These results suggest that administration of RvD1 attenuates
inflammation-induced hyperexcitability of SpVc WDR neurons associated with inflammatory
hyperalgesia. These findings support the idea that RvD1, derived from
DHA, as well as DHA itself, are potential complementary or alternative therapeutic agents for the alleviation of inflammatory
hyperalgesia.