Presynaptic AMPA receptors modulate glutamate release in mouse spinal cord dorsal horn

Purpose of the study was the investigation of the role of AMPA receptors, expressed on nociceptive fiber terminals, in modulating glutamate release. We also intended to characterize the contribution of different AMPA receptor subunits to presynaptic modulation. We used the patch-clamp technique on dorsal horn neurons in mouse spinal cord slices. Excitatory postsynaptic currents (EPSCs), recorded from lamina II neurons, were evoked by stimulation of the attached dorsal root. Recordings were performed from both wild-type animals and transgenic mice carrying a nociceptor-specific deletion of GluA1 or GluA2 subunit. We observed that application of AMPA receptor agonists (AMPA or kainate) caused a significative depression of evoked glutamatergic EPSCs in wild type mice. The effect was reversible in wash. The inhibition of EPSCs was accompanied, in most neurons, by an increase of synaptic failures and a change in the coefficient of variation (CV) of EPSC peak amplitudes. Both effects are considered as indicators of presynaptic modulation. We observed similar effects, in the presence of AMPA, also in mice with nociceptor-specific deletion of the GluA2 subunit. However, in mice with deletion of the GluA1 subunit, presynaptic modulation exerted by AMPA (expressed as a change of CV) was significantly reduced. We showed that functional AMPA receptors are expressed on nociceptive primary afferents in mouse spinal cord. Their exogenous activation causes the depression of glutamate release. The decrease of presynaptic modulation observed in mice carrying a deletion of the GluA1 subunit, but not of GluA2, suggests that calcium-permeable AMPA receptors are particularly important in regulating glutamate release from primary afferents