Jeffrey A. M. Coull1,2,7, Simon Beggs3,7, Dominic Boudreau1, Dominick Boivin1, Makoto Tsuda3,4, Kazuhide Inoue4, Claude Gravel5,6, Michael W. Salter3 and Yves De Koninck1,2,6

Neuropathic pain that occurs after peripheral nerve injury depends on the hyperexcitability of neurons in the dorsal horn of the spinal cord1, 2, 3. Spinal microglia stimulated by ATP contribute to tactile allodynia, a highly debilitating symptom of pain induced by nerve injury4. Signalling between microglia and neurons is therefore an essential link in neuropathic pain transmission, but how this signalling occurs is unknown. Here we show that ATP-stimulated microglia cause a depolarizing shift in the anion reversal potential (Eanion) in spinal lamina I neurons. This shift inverts the polarity of currents activated by GABA (-amino butyric acid), as has been shown to occur after peripheral nerve injury5. Applying brain-derived neurotrophic factor (BDNF) mimics the alteration in Eanion. Blocking signalling between BDNF and the receptor TrkB reverses the allodynia and the Eanion shift that follows both nerve injury and administration of ATP-stimulated microglia. ATP stimulation evokes the release of BDNF from microglia. Preventing BDNF release from microglia by pretreating them with interfering RNA directed against BDNF before ATP stimulation also inhibits the effects of these cells on the withdrawal threshold and Eanion. Our results show that ATP-stimulated microglia signal to lamina I neurons, causing a collapse of their transmembrane anion gradient, and that BDNF is a crucial signalling molecule between microglia and neurons. Blocking this microglia-neuron signalling pathway may represent a therapeutic strategy for treating neuropathic pain.