Metformin reduces inflammatory nociception in mice through a serotonin-dependent mechanism

The antidiabetic drug metformin has demonstrated antinociceptive efficacy in different pain models, and these effects are usually attributed to activation of the AMP-dependent protein kinase (AMPK). However, the downstream targets that contribute to inhibition of nociception following AMPK activation have been only partially elucidated. Here, we examined the contribution of serotonergic mechanisms in mediating metformin's antinociceptive effects, seeing as AMPK activators (including metformin) have been shown to modulate serotonergic neurotransmission. The formalin test in mice was used as an inflammatory pain model. First, we examined metformin's effects following systemic (intraperitoneal) and local peripheral (intraplantar) administration. In the second part, we examined the roles of the AMPK and serotonin system in mediating metformin's antinociceptive effects by (locally and/or systemically) pretreating animals with the AMPK inhibitor (dorsomorphin), antagonists of serotonin 5-HT_1A (WAY100635) and 5-HT_1B/1D receptors (GR127935) or the tryptophan-hydroxylase inhibitor (PCPA). Metformin significantly reduced second phase nociceptive behavior following systemic and local application. In inhibitor/antagonist studies systemic application of dorsomorphin, WAY100635 or GR127935 significantly inhibited metformin's antinociceptive effects. Local application of dorsomorphin did not change metformin's antinociceptive effects, however locally administered serotonin receptor antagonists significantly reduced them. Finally, four-day pretreatment with PCPA (which depleted brainstem and spinal cord serotonin content) led to a significant reduction of metformin's antinociceptive effects. In conclusion, metformin produces serotonin-dependent antinociceptive effects against inflammatory pain via peripheral, and possibly central, serotonin 5-HT_1A and 5-HT_1B/1D receptors. The serotonin-mediated mechanism appears to be dependent on serotonin release, seeing as depletion of endogenous serotonin content attenuated metformin's antinociceptive effects.