Relationship between nociceptor activity, peripheral edema, spinal microglial activation and long-term hyperalgesia induced by formalin.

To determine whether initial nociceptive inputs caused by subcutaneous injection of formalin into the hindpaw are necessary and/or sufficient for allodynic behavior and microglial activation observed at one week following behavior, we examined Sprague-Dawley rats under five test conditions. Test condition 1. Formalin alone group (six rats), 5% formalin was injected subcutaneously into the dorsal side of the right hind paw. Test condition 2. Bupivacaine/Formalin group (six rats), bupivacaine was injected into the ankle area and into the site of formalin injection 10 min before formalin injection. Test condition 3. Saline/Formalin group (six rats), saline was injected 10min before formalin in the same manner as bupivacaine. Test condition 4. Formalin/Bupivacaine group 1 (six rats), bupivacaine was injected 10 min after formalin. Test condition 5. Formalin/Bupivacaine group 2 (six rats), bupivacaine was injected similarly 1h after formalin. The magnitude of paw edema and paw withdrawal thresholds to mechanical stimuli applied to the plantar surface of the injected paw and on the dorsal surface of the contralateral side were evaluated prior to and one week after formalin injection. The lumbar spinal cord was immunohistochemically processed at one week to assess the expression of a marker for activated microglia. The results showed: (i) pre-treatment with bupivacaine blocked both phases of formalin-evoked pain behaviors and the mechanical allodynia that developed one week post-formalin injection, but did not block microglial activation; (ii) treatment with bupivacaine 1h after formalin injection reduced paw edema and prevented skin ulceration, but one week allodynia and microglial activation were still present; and (iii) prolonged spinal microglial activation was not dependent on acute formalin-induced nociceptor activity, but was strongly associated with the amount of tissue destruction. Our studies suggest that: (i) the central sensitization associated with the phase II of formalin-evoked behaviors and spinal microglial activation are both necessary to permit the development of the long-term hyperalgesia produced by the subcutaneous administration of formalin into the rat's hindpaw; and (ii) acute nociceptive inputs following formalin injection are not necessary for central microglial activation that may be triggered by nerve damage or prolonged signals from peripherally inflamed tissue