Microinjection of opioid antagonist into periaqueductal gray blocks formalin-induced alterations of substance P in spinal dorsal horn

Using formalin injected into the rat's hind paw as nociceptive stimulation, the effect of naloxone and β-endorphin (β-EP) antiserum microinjected into the periaqueductal gray (PAG) on the level of substance P like immunorcactivity (SPLI) of the spinal dorsal horn was observed. After formalin injection, a significant increase in SPLI was found as compared with the control (P < 0.01). After naloxone or β-EP antiserum microinjection, SPLI level in the dorsal horn was decreased (P < 0.05). It is thus suggested that nociceptive stimulation induce the release of β-EP in the PAG, which, in turn, modulate SP release in the spinal dorsal horn.