Naloxone blocks the formalin-induced increase of substance P in the dorsal horn

Substance P (SP) has been proposed as a mediator of nociception in the dorsal horn of the spinal cord. Activation of nociceptive pathways by stimuli such as formalin injected into the hind paw has been shown to produce an increase in the amount of immunoreactive SP in primary afferent neurons. Opiate agonist and antagonist binding in the dorsal horn has been shown to affect the SP levels and release. In order to determine the effects of opiates on SP mediated nociception in the spinal cord, anesthetized rats pretreated subcutaneously with morphine, naloxone, or saline were injected in the right hind paw with 0.4 ml of either saline of 5% formalin. After 1 h, the animals were perfused and the lumbar enlargement of the spinal cord removed. SP-like immunoreactivity (SPLI) in the dorsal horns was quantitated using immunohistochemical staining and manual photometry. The results show that formalin injection increases the SPLI in the dorsal horn after 1 h, as does pretreatment with morphine. Morphine pretreatment combined with formalin injection further increases SPLI, but not significantly higher than either treatment alone. The morphine-induced increases could be blocked by naloxone, which had no effects on saline-treated controls. Most importantly, naloxone was able to block the formalin-induced increase in SPLI, implying that endogenous opioid systems play a role in the SP increases seen during formalin-induced nociception.