The Effect of MK-801 and Naloxone in a Sympathetically Independent Neuropathic Pain Rat Model

ABSTRACT

OBJECTIVE: The authors evaluate the effects of each spinally administered MK-801, naloxone and coadministerd both drugs on the neuropathic pain states in the sympathetically independent pain(SIP) model induced by unilateral sural and tibial nerve transection of the sciatic nerve branches in the rat. METHODS: Pain sensitivity was measured with a von Frey filament for mechanical allodynia and acetone applied to the sensitive area for cold hyperalgesia. We evaluated the ability of the spinally applied MK-801 to spinal cord after laminectomy on T12, T13 and L1 to alleviate neuropathic pain either by itself of when spinally or intravenously administerd together with naloxone and coincidentally measured the behavioral test. RESULTS: Twenty out of 25 rats in which the tibial and sural nerves were injured showed well-developed neuropathic pain behaviors. The response rates of the rat models to the von Frey filament and acetone spray in hindlimbs by MK-801 were significantly reduced but, by naloxone were not changed. The pharmacological inhibition to mechanical allodynia and cold hyperalgesia for MK-801 when co-adminstered naloxone was reversed. CONCLUSION: MK-801 has been shown to alleviate neuropathic pain in SIP but, co-administered naloxone reversed the N-methyl-D-aspartate blocking effects of MK-801. These findings suggest that naloxone, which does not specifically relieve neuropathic pain, can reverse the neuropathic pain-relieving action of MK-801.