| Upregulation of voltage gated calcium channel (VGCC) α2δ1 subunits and thrombospondin 4 (TSP4) contributes to allodynia and hyperalgesia, two symptoms frequently observed in neuropathic pain. At neural circuit level, imbalance of excitatory and inhibitory synaptic transmission is associated with pain states. I, therefore, conducted multiple studies to determine the functional plasticity of α2δ1 and TSP4 proteins in the control of excitatory and inhibitory synaptic transmission in the spinal dorsal horn in spinal nerve ligation (SNL) injury model and genetically modified mice. Whole cell voltage clamp recordings of spontaneous and miniature glutamatergic excitatory postsynaptic currents (sEPSC and mEPSC) and spontaneous and miniature inhibitory postsynaptic currents (sIPSC and mIPSC, respectively) were performed in spinal cord slices. First, frequency, but not amplitude, of sEPSC and mEPSC in dorsal horn neurons was increased in SNL and α2δ1 overexpression transgenic (Tg) mice, which could be attenuated by gabapentin dose dependently. Intrathecal α2δ1 antisense oligodeoxynucleotides treatment reversed increased sEPSC/mEPSC frequency and behavioral hypersensitivity in the SNL mice. In contrast, neither sIPSC nor mIPSC in dorsal horn neurons was altered in SNL and α2δ1 Tg mice. Interestingly, I observed gabapentin-responsive enhancement of firing evoked by depolarizing stimuli in dorsal horn neurons of the α2δ1 Tg mice, but not in SNL mice. Second, chronic but not acute treatment of TSP4 proteins produced a similar degree of increase in mEPSC frequency in dorsal horn neurons as that in the SNL and α2δ1Tg mice. TSP4 knockout affected neither frequency nor amplitude of mEPSC. However, it inhibited mEPSC frequency enhancement by SNL. Furthermore, I studied synaptic mechanism of α2δ1 and TSP4 interaction using genetically modified mice. TSP4 knockout in mice did not alter behavioral sensitivity to mechanical and thermal stimuli, mEPSC, evoked EPSC. However, TSP4 knockout from the α2δ1 Tg mice reversed α2δ1-induced behavioral hypersensitivities to mechanical and thermal stimuli, enhanced mEPSC frequency and evoked EPSC in dorsal horn neurons. Our data suggest that both α2δ1 and TSP4 are contributing to enhanced excitatory presynaptic transmission, leading to neuropathic pain states, through their direct or indirect interactions at the dorsal spinal cord level. |
