Periaqueductal Grey EP3 Receptors Facilitate Spinal Nociception in Arthritic Secondary Hypersensitivity.

After tissue damage, sensitivity to painful stimulation develops in undamaged areas (secondary hypersensitivity). This is found in many painful conditions, particularly arthritis. The periaqueductal gray (PAG) is an important center that controls spinal nociceptive processing, on which secondary hypersensitivity depends. Prostaglandins (PGs) are mediators of inflammation with pronociceptive actions within the PAG under normal conditions. We find that secondary hindpaw hypersensitivity in arthritic rats results from spinal sensitization to peripheral A-nociceptor inputs. In the PAG of arthritic, but not naive, rats, there is enhanced control of spinal A-nociceptor processing through PG EP3 receptors. The descending facilitatory actions of intra-PAG PGs play a direct and central role in the maintenance of inflammatory secondary hypersensitivity, particularly relating to the processing of A-fiber nociceptive information.