Fig. 1

Metformin suppressed morphine-induced microglial activation and attenuated morphine tolerance. The tail-flick test was performed to evaluate the effect of metformin on morphine tolerance. Data are shown as a percentage of maximal possible effect (%MPE). a Metformin had no effect on acute morphine analgesic effect. Mice were pretreated with metformin (200 mg/kg, i.g.) 12 h before morphine (10 μg/μL, i.t.) administration. MPE was measured every 30 min after morphine injection (n = 8). b Metformin suppressed chronic morphine tolerance. Mice were injected intrathecally with morphine (10 μg/μL, i.t.) once daily for 7 days, and the MPE was measured 30 min after the injection of each day. Mice were pretreated with metformin (200 mg/kg, i.g.) 12 h before morphine administration everyday. c Representative immunofluorescence images showed the effect of metformin on the morphine-induced activation of microglia in dorsal horn lamina I–III of the spinal cord (labeled with a white dotted line) (n = 3). d, e The phosphorylation of p38 and p65 in the spinal cord was evaluated at day 7 by western blot (n = 3). f The protein level of SOCS3 in the spinal cord was evaluated at day 7 by western blot (n = 3). g Mice were treated with metformin (200 mg/kg, i.g.) once daily for 7 days. Spinal cord samples were collected after the last administration of metformin. Representative confocal microscopy study of the expression of SOCS3 (green) in microglia (IBA-1, red), neurons (NeuN, red) and astrocytes (GFAP, red) of dorsal horn lamina I-III of the spinal cord (labeled with white dotted line) (n = 3). Data were analyzed by two-way ANOVA in a and b. Data were analyzed by one-way ANOVA in c–f. Data were analyzed by Student’s t test in g. **p < .01, ***p < .001 vs. saline; ^##p < 0.01, ^###p < .001 vs. morphine-treated group. Scale bars = 100 μm