Switch to Ca2+-permeable AMPA and reduced NR2B NMDA receptor-mediated neurotransmission at dorsal horn nociceptive synapses during inflammatory pain in the rat

Glutamate receptor response properties of nociceptive synapses on neurokinin I receptor positive (NK1R(+)) lamina I neurons were determined 3 days after induction of chronic peripheral inflammation with Freund's Complete Adjuvant (CFA). A significant increase in the AMPAR/NMDAR ratio was found during inflammation, which was associated with a significant reduction in the quantal amplitude of NMDAR-mediated synaptic currents. A significant shortening of the quantal AMPA current decay, a greater inward rectification of the AMPAR-mediated eEPSC amplitude and an increased sensitivity to the Ca2+-permeable AMPAR channel blocker 1-naphthylacetyl spermine (NAS) was also observed, indicating an increase in the contribution of Ca2+-permeable AMPARs at this synapse during inflammation. Furthermore the reduced effectiveness of the NR2B-specific antagonist CP-101,606 on NMDAR-mediated eEPSCs together with a decrease in Mg2+ sensitivity suggests a down regulation of the highly Mg2+-sensitive and high conductance NR2B subunit at this synapse. These changes in glutamatergic receptor function during inflammation support the selective effectiveness of Ca2+-permeable AMPAR antagonists in inflammatory pain models and may underlie the reported ineffectiveness of NR2B antagonists in spinal antinociception.