Injury-specific functional alteration of N-type voltage-gated calcium channels in synaptic transmission of primary afferent C-fibers in the rat spinal superficial dorsal horn

We investigated functional alterations of voltage-gated calcium channels (VGCCs) in excitatory synaptic transmission from primary afferent A- and C-fibers after peripheral nerve injury. Patch-clamp recordings were performed on substantia gelatinosa (SG) neurons of spinal cord slices with an attached dorsal root, prepared from L5 spinal nerve-ligated (SNL) rats. The effects of neuronal VGCC blockers, omega-conotoxin GVIA (omega-CgTX) for N-type channels and omega-agatoxin IVA (omega-AgaIVA) for P/Q-type channels, on evoked excitatory postsynaptic currents (eEPSCs) by stimulation of A- or C-fibers were studied. Besides, electrophysiological assay using dorsal root ganglion (DRG) and immunohistochemistry were done. In na ve rats, omega-CgTX (0.1-1 mu M) reduced more effectively A-fiber eEPSCs than C-fiber ones. After nerve injury, omega-CgTX produced great inhibition of C-fiber eEPSCs in slices with the injured L5 dorsal root of SNL model rats, as compared to sham-operated rats. By contrast, in slices with the non-injured L4 one, inhibitory effects of omega-CgTX were not changed. This occurred concurrently with increased expression of N-type VGCCs in L5 spinal dorsal horn and with enhanced Ca2+ currents through N-type VGCCs in small-sized (C-type) L5 DRG. In terms of A-fiber eEPSCs, omega-CgTX elicited similar inhibition in nerve-injured and sham-operated rats. omega-AgaIVA (0.1 ttM) had less effect on A- or C-fiber eEPSCs. These results indicate that N-type, but not P/Q-type, VGCCs mainly contribute to excitatory synaptic transmission from A- and C-fibers in the spinal dorsal horn. More importantly, following nerve injury, the functional contribution of N-type VGCCs to nociceptive transmission is increased in the pre-synaptic terminals of injured C-fibers. (C) 2016 Elsevier B.V. All rights reserved.