Microinjection of opiates into the periaqueductal gray matter attenuates neuropathic pain symptoms in rats

We have previously demonstrated that electrical stimulation of the ventral periaqueductal gray matter (PAG) produced analgesia in neuropathic pain in rats. Opioids were also shown to be involved in analgesic effects. This stu dy sought to determine whether opiates microinjected into the ventral PAG p roduce analgesia. Male Sprague-Dawley rats were chronically implanted with a guide cannula in the PAG under pentobarbital anesthesia and both the tibi al and sural nerves were completely cut. Pain sensitivity was postoperative ly measured with a von Prey filament and acetone applied to the sensitive a rea for 1 week. Opioids such as [D-Ala(2),N-MePhe(4),Gly(ol)(5)]-enkephalin (DAMGO) and [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE) were injected into the PAG. DAMGO, a mu-opioid agonist, and DPDPE, a delta-opioid agonist, were hi ghly effective in reducing neuropathic pain. These effects were reversed by naloxone. These results suggest that the neurons in the ventral PAG are ac tivated by opioids to produce analgesia and that specific opioid receptors are involved in the descending pain inhibition system from the PAG. NeuroRe port 11:1413-1416 (C) 2000 Lippincott Williams & Wilkins.