Peripheral noxious stimulation releases spinal PGE2 during the first phase in the formalin assay of the rat

Injection of formalin (5%; 50 microl) into the dorsal surface of the hind paw of rats evoked a characteristic biphasic flinching behaviour of the injured paw accompanied by a significant increase in the interstitial prostaglandin E2 (PGE2) concentration of the dorsal lumbar spinal cord. Interestingly, the increase in PGE2 concentration was only observed during the first phase of the formalin behavioural response (during the 0-10 and 10-20 min microdialysis-sample). Saline paw injection did not have a significant effect on behaviour or on PGE2 concentration. These data suggest that spinal release of PGE2 is involved in nociceptive processing in the formalin-induced hyperalgesia model of the rat during the first but not second phase.