Excitatory amino acid receptor involvement in peripheral nociceptive transmission in rats

The involvement of excitatory amino acid receptors in peripheral nociceptive processing was assessed in two separate experiments. In the first, one knee joint cavity of rats was injected with 0.1 ml of L-glutamate (0.001 mM; 0.1 mM; 1.0 mM), L-aspartate (0.001 mM; 0.1 mM: 1.0 mM), L-arginine (0.1 mM) or different combinations of these amino acids. The animals tested for paw withdrawal latency to radiant heat and withdrawal threshold to von Frey filaments at different time points. Combinations of glutamate/aspartate, aspartate/arginine or glutamate/aspartate/arginine when injected into the joint, in the absence of any other treatment, reduced the paw withdrawal latency and withdrawal threshold immediately after the injection and persisting up to 5 h indicating the development of hyperalgesia and allodynia. Subsequent intra-articular injection of either an NMDA or a non-NMDA glutamate receptor antagonist ((+/-)-2-amino-7-phosphonoheptanoic acid (AP7), 0.2 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 0.1 mM) attenuated the thermal hyperalgesia and the mechanical allodynia produced by glutamate/aspartate/arginine. On the other hand, in a second experiment intra-articular injection of AP7, ketamine or CNQX reversed the hyperalgesia and allodynia produced by injection of a mixture of kaolin and carrageenan into the joint. These receptor antagonists, however, did not have an effect on the joint edema. These findings provide evidence for a potential role of peripheral NMDA and non-NMDA receptors in nociceptive transmission.