Increased substance P and tumor necrosis factor-alpha level in the paws following formalin injection in rat

We previously described a rat model where the injection of formalin in the tail induced a facilitation of the hindpaw withdrawal reflexes (hyperalgesia). In the present work, after injecting formalin in the tail, we measured the levels of pro-nociceptive mediators tumor necrosis factor-alpha (TNF) and substance P (SP) in the rat paws. A significant increase of SP levels was evident in the hindpaw, whereas no changes in SP were observed in the forepaw. Both in the hindpaw and in the forepaw the TNF levels were higher than normal at each stage of measurement. Our results indicate that a prolonged neuronal activation induced by formalin injection is associated with a change in nociceptive and inflammatory mediators in distal sites of the body. The fact that SP levels are changed in the hindpaw but not in the forepaw might point to the activation of a mechanism of retrograde signaling from central synapses to paw afferent nerves.