Thiamine Suppresses Thermal Hyperalgesia, Inhibits Hyperexcitability, and Lessens Alterations of Sodium Currents in Injured, Dorsal Root Ganglion Neurons in Rats

Background B vitamins can effectively attenuate inflammatory, and neuropathic pain in experimental animals, while their efficacy ill treating clinical pain syndromes remains unclear. To understand possible mechanisms underlying B vitamin-induced analgesia and provide further evidence that may support the clinical utility of B vitamins in chronic pain treatment, this study investigated effects of thiamine (B1) on the excitability and Na+ currents of dorsal root ganglion (DRG) neurons that have been altered by nerve injury. Methods: Nerve injury v.-as mimicked by chronic compression of DRG in rats. Neuropathic pain was evidenced by the presence of thermal hyperalgesia. intracellular and patch-clamp recordings were made in vitro from intact and dissociated DRG neurons, respectively. Results: (1) In vitro intraperitoneal administration of B1 (66 mg/kg/day 10-14 doses) significantly inhibited DRG compression-induced neural hyperexcitabillity, in addition to suppressing thermal hyperalgesia. (2) In vitro perfusion of B1 (0.1, 1 and 10 mM) resulted in a dose-dependent inhibition of DRG neuron hyperexcitability. In addition. the DRG neurons exhibited size-dependent sensitivity to B1 treatment, Le., the small and the medium-sized neurons, compared to the large neurons, were significantly more sensitive. (3) Both in vitro (1 mM) and in vivo application of B1 significantly reversed DRG compression-induced down-regulation of tetrodotoxin-resistant but not tetrodotoxin-sensitive,e Na+ current density in the small neurons. B1 at 1 mM also reversed the compress ion-induced hyperpolarizing shift of the inactivation curve of the tetrodotoxin-resistant currents and the upregulated ramp currents in small DRG neurons. Conclusion: Thiamine can reduce hyperexcitabillity and lessen alterations of Na+ currents in injured DRG neurons, in addition to suppressing thermal hyperalgesia.