Postsynaptic glutamate response downregulates within presynaptic exaggerated glutamate release by activating TRPV1 in the spinal dorsal horn

Activating primary afferent TRPV1-positive (TRPV1⁺) fibers in the spinal dorsal horn triggers exaggerated glutamate release and induces acute pain. However, whether the glutamate postsynaptic responses on dorsal horn neurons are regulated by excessive glutamate is unknown, largely due to intrinsic technical difficulties. In the present study, capsaicin, a specific TRPV1 agonist, was used to activate TRPV1⁺ fibers in the spinal dorsal horn. Combining three-dimensional (3-D) holographic photostimulation and whole-cell recordings on acute spinal cord slices from adult rodents, we found that postsynaptic glutamate responses were attenuated when activating TRPV1⁺ fibers with capsaicin. Electron microscopy and Western blot studies found that postsynaptic GluA1 (a subtype of ionotropic glutamate receptors) on the postsynaptic membrane was decreased by acute capsaicin treatment. Therefore, postsynaptic glutamate receptor occupancy and/or downmodulation may underlie this postsynaptic attenuation. Our data thus clarify a scenario in which postsynaptic glutamate responses are largely downregulated upon TRPV1⁺ activation, and this change may contribute to homeostasis in the dorsal horn circuit when "acute pain" occurs.