A role of periaqueductal grey NMDA receptors in mediating formalin-induced pain in the rat.

In the present study we examined the role of periaqueductal grey (PAG) N-methyl-D-aspartate (NMDA) receptors in the perception of tonic and phasic pain. Under sodium pentobarbital anesthesia rats were implanted unilaterally with a guide cannula aimed at the PAG. Following a 7-14 day recovery period rats received an infusion of the NMDA antagonist, 2-amino-5-phosponopentanoic acid (AP5), or saline into the PAG. Five minutes after the infusion of AP5 rats were tested for analgesia in the formalin test, or in the hotplate test. AP5 injections into the PAG reduced pain in the formalin test, but not the hotplate test. These data show that NMDA receptors within the PAG are involved in the perception of tonic, inescapable pain as measured in the formalin test, but not phasic, escapable pain as measured in the hotplate test.