Is there more than one prostaglandin E receptor subtype mediating hyperalgesia in the rat hindpaw?

Five synthetic prostaglandin E analogs (11-deoxyPGE 1, 17-phenyl- ol-trinor prostaglandin E 2, enisoprost, MB28767 and misoprostol) have been evaluated for their ability to produce mechanical hyperalgesia in rats. The Randall-Selitto paw withdrawal model of mechanical hyperalgesia was used. Following intradermal injections (2.5 μl) into the dorsal surface of the hindpaw, each prostaglandin E analog produced a dose-dependent (1–1000 ng) decrease in nociceptive threshold (i.e. hyperalgesia). Hyperalgesia produced by 17-phenyl- ol-trinor prostaglandin E 2 and MB28767, was inhibited by the prostaglandin E 1 antagonist SC19220 (7.5 ng), while the hyperalgesia produced by 11-deoxyprostaglandin E 1, enisoprost and misoprostol was not inhibited by this antagonist. Hyperalgesia produced by all five analogs was significantly attenuated or completely blocked by inhibiting stimulatory guanine nucleotide-binding regulatory protein with guanosine 5′-O-(2-thiodiphosphate), adenyl cyclase with 2′5′-dideoxyadenosine and protein kinase A with WIPTIDE. These results suggest the presence of more than one prostaglandin E-receptor subtype, which mediate hyperalgesia, predominantly via the cAMP second messenger system, in the hindpaw of the rat.