Sustained microglial immunoreactivity in the caudal spinal trigeminal nucleus after formalin injection

Recent studies indicate that glia may be involved in altered nociceptive pr ocessing after a peripheral inflammatory lesion produced by injection of in flammatory reagents such as formalin and zymosan. Most of these studies, ho wever, confined their observations to a period shortly after the injections . This study investigated the immunohistochemical responses of microglia in the caudal part of the spinal trigeminal nucleus for up to 60 days after s ubcutaneous injection of formalin into the lateral faces of Wistar rats. Th e results showed obvious up-regulation of microglial markers such as OX-18, OX-42 and OX-6 up to 21 days after formalin injection. These were somewhat reduced at 30 days after injection. Electron microscope investigation reve aled no evidence of significant phagocytosis of degenerative neuronal eleme nts by microglia in the nucleus at the time-that is, 7 days after formalin injection, when microglial activation was at its peak. Significantly, howev er, the period of microglial activation corresponded closely to that showin g enhanced nociceptive behavior after perioral formalin injection (Cadet et al., 1995). This study indicates a microglial role in the genesis of enhan ced nociceptive behavior.