Metformin inhibits paclitaxel-induced mechanical allodynia by activating opioidergic pathways and reducing cytokines production in the dorsal root ganglia and thalamus.

It has been shown that AMP-activated protein kinase (AMPK) is involved in the nociceptive processing. This observation has prompted us to investigate the effects of the AMPK activator metformin on the paclitaxel-induced mechanical allodynia, a well-established model of neuropathic pain. Mechanical allodynia was induced by four intraperitoneal (i.p) injections of paclitaxel (2 mg/kg.day) in mice. Metformin was administered per os (p.o.). Naltrexoneandglibenclamide were used to investigate mechanisms mediating metformin activity. Concentrations of cytokines in the dorsal root ganglia (DRG) and thalamus were determined. After a single p.o. administration, the two highest doses of metformin (500 and 1000 mg/kg) attenuated the mechanical allodynia. This response was attenuated by all doses of metformin (250, 500 and 1000 mg/kg) when two administrations, 2 h apart, were carried out. Naltrexone (5 and 10 mg/kg, i.p.), but not glibenclamide (20 and 40 mg/kg, p.o.), attenuated metformin activity. Concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1ß and CXCL-1 in the DRG were increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentrations of TNF-α, IL-1ß and CXCL-1 in the DRG. Concentration of IL-6, but not TNF-α, in the thalamus was increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentration of IL-6 in the thalamus. In summary, metformin exhibits activity in the model of neuropathic pain induced by paclitaxel. This activity may be mediated by activation of opioidergic pathways and reduced production of TNF-α, IL-1ß and CXCL-1 in the DRG and IL-6 in the thalamus.

Thiamine, riboflavin, and nicotinamide inhibit paclitaxel-induced allodynia by reducing TNF-α and CXCL-1 in dorsal root ganglia and thalamus and activating ATP-sensitive potassium channels.

Some B vitamins exhibit activities in models of nociceptive pain, inflammatory pain, and neuropathic pain induced by nerve lesions and also in certain painful conditions in humans. In the present study, we investigated the effects of thiamine, riboflavin, and nicotinamide in a neuropathic pain model induced by the chemotherapeutic paclitaxel in mice. Four intraperitoneal (i.p.) administrations of paclitaxel (2 mg/kg day, cumulative dose 8 mg/kg) induced a long-lasting mechanical allodynia. Per os (p.o.) administration of two doses of thiamine (150, 300 and 600 mg/kg), nicotinamide (250, 500 and 1000 mg/kg) or riboflavin (125, 250 and 500 mg/kg), on the seventh day after the first administration of paclitaxel, the mechanical allodynia was attenuated. The antinociceptive activity of all B vitamins was attenuated by glibenclamide (20 and 10 mg/kg, p.o.). Naltrexone (5 and 10 mg/kg, i.p.) attenuated the antinociceptive activity of thiamine. Thiamine, riboflavin, and nicotinamide also reduced the concentrations of tumor necrosis factor-α (TNF-α) and CXCL-1 in dorsal root ganglia (DRG) and thalamus. In conclusion, thiamine, riboflavin, and nicotinamide exhibit antinociceptive activity in the neuropathic pain model induced by paclitaxel. Inhibition of TNF-α and CXCL-1 production in DRG and thalamus, as well as activation of ATP-sensitive potassium channels, underly their antinociceptive activity.