Inhibitory Mechanism of Periaqueductal Gray Matter on Neuropathic Pain in Rat

ABSTRACT

OBJECTIVE: Using Lee et al (1996) model, we assessed the effect of opioid within the PAG on the manifestations of the neuropathic pain, and we studied the effects of naloxone on the analgesic effects of opioid. METHOD: Under pentobarbital anesthesia, male Sprague-Dawley rats were implanted with cannula in the ventral (n=10) and dorsal (n=6) PAG after the unilateral tibial and sural nerves were ligated and cut, leaving the common peroneal nerve intact. Pain sensitivity was assessed using the von Frey filament (8 mN) and acetone applied to the sensitive area for 1 week postoperatively. Rats with neuropathic pain were intracerebrally microinjected with DAMGO (0.1microgram/5microliter) and enkephaline (20microgram/5microliter) into the ventral and dorsal PAG and the pain sensitivity was assessed. Naloxone was injected to assess the observed change of pain sensitivity. RESULTS: Intracerebral microinjection of DAMGO and enkephaline into the ventral PAG, but not the dorsal PAG, increased the pain threshold which was reversed by naloxone. CONCLUSION: The results suggest that stimulation of the ventral PAG in neuropathic rats may reduce neuropathic pain via opioid-mediating pathway of the descending pain inhibition system.