Changes in the effect of spinal prostaglandin E2 during inflammation:: Prostaglandin E (EP1-EP4) receptors in spinal nociceptive processing of input from the normal or inflamed knee joint

Inflammatory pain is caused by sensitization of peripheral and central nociceptive neurons. Prostaglandins substantially contribute to neuronal sensitization at both sites. Prostaglandin E-2 (PGE(2)) applied to the spinal cord causes neuronal hyperexcitability similar to peripheral inflammation. Because PGE(2) can act through EP1-EP4 receptors, we addressed the role of these receptors in the spinal cord on the development of spinal hyperexcitability. Recordings were made from nociceptive dorsal horn neurons with main input from the knee joint, and responses of the neurons to noxious and innocuous stimulation of the knee, ankle, and paw were studied after spinal application of recently developed specific EP1-EP4 receptor agonists. Under normal conditions, spinal application of agonists at EP1, EP2, and EP4 receptors induced spinal hyperexcitability similar to PGE(2). Interestingly, the effect of spinal EP receptor activation changed during joint inflammation. When the knee joint had been inflamed 7-11 hr before the recordings, only activation of the EP1 receptor caused additional facilitation, whereas spinal application of EP2 and EP4 receptor agonists had no effect. Additionally, an EP3alpha receptor agonist reduced responses to mechanical stimulation. The latter also attenuated spinal hyperexcitability induced by spinal PGE(2). In isolated DRG neurons, the EP3alpha agonist reduced the facilitatory effect of PGE(2) on TTX-resistant sodium currents. Thus pronociceptive effects of spinal PGE(2) can be limited, particularly under inflammatory conditions, through activation of an inhibitory splice variant of the EP3 receptor. The latter might be an interesting target for controlling spinal hyperexcitability in inflammatory pain states.