Morphine hyperalgesia gated through microglia-mediated disruption of neuronal Cl- homeostasis

A major unresolved issue in treating pain is the paradoxical hyperalgesia produced by the gold-standard analgesic morphine and other opiates. We found that hyperalgesia-inducing treatment with morphine resulted in downregulation of the K+-Cl- co-transporter KCC2, impairing Cl- homeostasis in rat spinal lamina I neurons. Restoring the anion equilibrium potential reversed the morphine-induced hyperalgesia without affecting tolerance. The hyperalgesia was also reversed by ablating spinal microglia. Morphine hyperalgesia, but not tolerance, required mu opioid receptor-dependent expression of P2X4 receptors (P2X4Rs) in microglia and mu-independent gating of the release of brain-derived neurotrophic factor (BDNF) by P2X4Rs. Blocking BDNF-TrkB signaling preserved Cl- homeostasis and reversed the hyperalgesia. Gene-targeted mice in which Bdnf was deleted from microglia did not develop hyperalgesia to morphine. However, neither morphine antinociception nor tolerance was affected in these mice. Our findings dissociate morphine-induced hyperalgesia from tolerance and suggest the microglia-to-neuron P2X4-BDNF-KCC2 pathway as a therapeutic target for preventing hyperalgesia without affecting morphine analgesia.