Interferon-gamma potentiates NMDA receptor signaling in spinal dorsal horn neurons via microglia-neuron interaction

Background: Glia-neuron interactions play an important role in the development of neuropathic pain. Expression of the pro-inflammatory cytokne -> cytokine Interferon-gamma (IFN gamma) is upregulated in the dorsal horn after peripheral nerve injury, and intrathecal IFN gamma administration induces mechanical allodynia in rats. A growing body of evidence suggests that IFN gamma might be involved in the mechanisms of neuropathic pain, but its effects on the spinal dorsal horn are unclear. We performed blind whole-cell patch-clamp recording to investigate the effect of IFN gamma on postsynaptic glutamate-induced currents in the substantia gelatinosa neurons of spinal cord slices from adult male rats. Results: IFN gamma perfusion significantly enhanced the amplitude of NMDA-induced inward currents in substantia gelatinosa neurons, but did not affect AMPA-induced currents. The facilitation of NMDA-induced current by IFN gamma was inhibited by bath application of an IFN gamma receptor-selective antagonist. Adding the Janus activated kinase inhibitor tofacitinib to the pipette solution did not affect the IFN gamma-induced facilitation of NMDA-induced currents. However, the facilitatory effect of IFN gamma on NMDA-induced currents was inhibited by perfusion of the microglial inhibitor minocycline. These results suggest that IFN gamma binds the microglial IFN gamma receptor and enhances NMDA receptor activity in substantia gelatinosa neurons. Next, to identify the effector of signal transmission from microglia to dorsal horn neurons, we added an inhibitor of G proteins, GDP-beta-S, to the pipette solution. In a GDP-beta-S-containing pipette solution, IFN gamma-induced potentiation of the NMDA current was significantly suppressed after 30 min. In addition, IFN gamma-induced potentiation of NMDA currents was blocked by application of a selective antagonist of CCR2, and its ligand CCL2 increased NMDA-induced currents. Conclusion: Our findings suggest that IFN gamma enhance the amplitude of NMDA-induced inward currents in substantia gelatinosa neurons via microglial IFN gamma receptors and CCL2/CCR2 signaling. This mechanism might be partially responsible for the development of persistent neuropathic pain.