T-type Calcium Channel Blocker Potentiates Gabapentin’s Efficacy In Treating Painful Diabetic Neuropathy Possibly Through PKCγRegulation

IntroductionPainful diabetic neuropathy is one of the most prevalent chronic complications of Diabetes Mellitus. The first line treatment recommended is gabapentin, whose mechanism still remains unclear and antinociceptive effect unsatisfactory. Recent researches have revealed that gabapentin suppresses the expression of protein kinase Cy (PKCy) in neurons, whereas suppressing PKC via many mechanisms will lead to the augmentation of T-type calcium channel activity, and impaction on gabapentin’s analgesic effect. This research aims at evaluating the impact of T-type calcium channel blocker on gabapentin’s antinociceptive effect when used in combination to treat type-1diabetic rats for their neuropathic pain, and determining how gabapentin affect the expression of T-type calcium channel and PKCy in rat dorsal spinal horn and dorsal root ganglion(DRG).MethodThis experiment used streptozotocin to induce type-1diabetic neuropathy model in rats, and evaluated the rats’mechanical hyperalgesia (paw withdrawal threshold, PWT) via von Frey method and thermal hyperalgesia (paw withdrawal latency, PWL) via thermal radiation method. Then different dosages and combination of drugs were administered, and PWT and PWL changes were monitored over time to determine whether combination therapy could improve gabapentin’s antinociceptive effect. Finally the DRG and dorsal spinal horn was harvested and RT-PCR and Western Blot were performed to determine the mRNA and protein quantity of T-type calcium channel and PKCy.ResultsSTZ-induced type-1diabetic rats with diabetic neuropathy weighed significantly lower than control group, with their PWT and PWL significantly lowered. Intraperitoneal injection of10mg/kg mibefradil could significantly increase diabetic rats’PWT (8.28±1.35mN vs.6.15±1.50mN. p<0.05) and PWL (9.314±1.104s vs.6.980±0.994s, p< 0.01)60min after injection compared to natural saline (NS) group. Chronic intraperitoneal administration of100mg/(kg·d) gabapentin could significantly improve diabet rats’PWT(8.68±1.64mN vs.4.53±1.28mN, p<0.001). Combination therapy of mibefradil and different dosages of gabapentin could significantly improve gabapentin’s antinociceptive effect compared to monotherapy of corresponding dosage of gabapentin60min after the injection. Combination of lOmg/kg mibefradil and100mg/kg gabapentin could even maintain the PWT (13.31±2.44mN vs.10.07±1.26mN, p<0.01)and PWL (10.243±2.139s vs.8.417±1.781s, p<0.05) significantly higher than monotherapy at120min after the injection. Meanwhile,30mg/kg gabapentin monotherapy failed to generate a significant antinociceptive effect60min and120min after injection compared to NS group, but combination with mibefradil produced a significant improvement of PWT60min after injection, which had no significant difference from100mg/kg gabapentin monotherapy. Furthermore,100mg/kg gabapentin could significant lower the quantity of mRNA of T-type calcium channel and PKCγ in dorsal spinal horn and DRG, also the quantity of their proteins in DRG of diabetic rats, to a level significantly lower than non-diabetic control group.Conclusion1. Mibefradil could improve painful behavior in type-1diabetic rats with painful diabetic neuropathy.2. Combination therapy of mibefradil and gabapentin could increase gabapentin’s antinociceptive effect in treating type-1diabetic rats with painful diabetic neuropathy, decrease gabapentin’s necessary dosage and possible side effects, and prolong gabapentin’s effective antinociceptive duration.3. Gabapentin could decrease the expression of T-type calcium channel and PKC y in the dorsal spinal horn and the DRG of type-1diabetic rats.