Involvement of glutamate receptors on hyperexcitability of wide dynamic range neurons in the gracile nucleus of the rats with experimental mononeuropathy

In order to clarify the functional role of glutamate receptors of the gracile nucleus neurons in rats with nerve injury-induced hyperalgesia, pharmacological, electrophysiological and in situ hybridization techniques were used in rats with chronic constriction nerve injury (CCI) of the sciatic nerve. A total of 54 wide dynamic range neurons were recorded from the gracile nucleus in the rats with CCI. Mechanical evoked responses were significantly depressed following application of AMPA receptor antagonist, CNQX, with noxious and non-noxious responses being similarly affected. AP-5, an NMDA receptor antagonist, induced depression of the pressure-evoked response only after application of the 1-μM concentration of this drug. The size of the receptive fields was significantly decreased after CNQX, but not MK-801 or AP-5, application. Afterdischarge was significantly depressed following the application of CNQX (1000 μM). The expression of ionotropic glutamate receptor subunit mRNAs in the gracile nucleus was studied using the in situ hybridization technique. The signals for NMDA subunits, NR2A, -2B and -2C, in the gracile nucleus neurons were not prominent, suggesting a low level expression of functional NMDA receptor complex. AMPA receptor subunits GluR1, -R2, -R3 and -R4 mRNAs were expressed in a large number of gracile nucleus neurons. These data are consistent with the pharmacological results that AMPA receptor antagonists depressed nociceptive neuronal activity, but NMDA receptor antagonists showed limited effects. These results suggest that the ionotropic glutamate receptors, i.e. the AMPA and NMDA receptors, are differentially involved in modulation of the wide dynamic range neuronal activity in the gracile nucleus following peripheral nerve injury.