Vitamin C Enhances Analgesic Effects Of Gabapentin In Neuropathic Pain In Rats And Its Mechanism

Objective:Neuropathic pain is associated with reactive oxygen species(ROS), and ROS scavenger can attenuate neuropathic pain. Although vitamin C(VitC) is considered as a kind of ROS scavenger, Its effects on neuropathic pain is still not clear. Gabapentin can relieve neuropathic pain effectively, however, as an antiepileptic drug, gabapentin has side effects such as drowsiness, dizziness and so on, especially in high dose. We aim to investigate whether vitamin C enhance analgesic effect of gabapentin in chronic constriction injury (CCI) neuropathic pain rats and its possible mechanisms.Methods:SD rats were randomly assigned to groups(n=10):Naive group, Sham group, CCI group, VitC treatment group, gabapentin(Gap) treatment groups, and combinaton of VitC+Gap treatment groups. Except Naive and Sham groups, all the neuropathic pain rats received chronic constriction injury on left sciatic nerve. Rats of treatment groups received vitamin C500mg/kg, gabapentin10,30,100mg/kg or combination of vitamin C500mg/kg and gabapentin10,30,100mg/kg respectively, all drugs were injected intraperitoneally twice a day on7consecutive days from seventh day after surgery. The mechanical paw withdrwal threashold(PWT) and thermal paw withdrwal latency(PWL) were tested to assess painbehavior before surgery and on day post operative3,7, and14. The blood sample, L4-6dorsal root ganglion(DRG) and lumbar spinal cord were harvested on day post operative14. Then, Serum was isolated, while spinal cord was made into tissue homogenate. Total superoxide dismutase(T-SOD) activity and malondialdehyde (MDA) content of serum and spinal cord tissue homogenate were estimated to assess oxidative stress level. Furthermore, Quantitative real-ime PCR and western blotting were used to detect the expression level of soudium dependent vitamin C transporter2(SVCT2), glucose transpoter3(GLUT3) and Cav3.2T-type calcium channel(Cav3.2), which are related to vitamin C transportation, DHA transportation and spontaneous excitatory synaptic transmission respectively. Moreover, The location of SVCT2and GLUT3proteins expression was confirmed by immunohistochemistry.Results:Gabapentin100mg/kg relieved the decrease of PWT and PWL induced by CCI significantly, and there was also effectively analgesic effects of gabapentin in a dose of 30mg/kg when combined with vitamin C. But there are no significant effects of VitC or gabapentin30mg/kg treatment alone on either PWT or PWL in CCI rats. For CCI rats, The oxidative stress level was increased together with SVCT2expression decreased in DRG, as could be reverseed by VitC or combination of VitC and gabapentin. Gabapentin or combination with VitC reversed the increase of GLUT3mRNA and protein expression. VitC, gabapentin or combination of two drugs reversed upregulation of Cav3.2mRNA and protein in CCI rats. Moreover, SVCT2and GLUT3proteins were mainly expressed is in neuron, not in satellitellite cells in DRGConclusions:Vitamin C enhances analgesic effects of gabapentin, thus could reduce the dose and decrease the side-effects of gabapentin, possible mechanism involved that vitamin C increase SVCT2protein expression and itself transmembrane transport in DRG neurons. There may be a synergistic effect of svitamin C and gabapentin in reduceing oxidative stress induced by nerve injury in cell and neuronal excitability.