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 study sought to determine whether opiates microinjected into the ventral PAG produce analgesia. Male Sprague-Dawley rats were chronically implanted with a guide cannula in the PAG under pentobarbital anesthesia and both the tibial and sural nerves were completely cut. Pain sensitivity was postoperatively measured with a von Frey filament and acetone applied to the sensitive area for I 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 μ-opioid agonist, and DPDPE, a δ-opioid agonist, were highly effective in reducing neuropathic pain. These effects were reversed by naloxone. These results suggest that the neurons in the ventral PAG are activated by opioids to produce analgesia and that specific opioid receptors are involved in the descending pain inhibition system from the PAG.