Receptor dependence of BDNF actions in superficial dorsal horn: relation to central sensitization and actions of macrophage colony stimulating factor 1.

Peripheral nerve injury elicits an enduring increase in the excitability of the spinal dorsal horn. This change, which contributes to the development of neuropathic pain, is a consequence of release and prolonged exposure of dorsal horn neurons to various neurotrophins and cytokines. We have shown in rats that nerve injury increases excitatory synaptic drive to excitatory neurons but decreases drive to inhibitory neurons. Both effects, which contribute to an increase in dorsal horn excitability, appear to be mediated by microglia-derived BDNF. We have used multiphoton Ca imaging and whole cell recording of spontaneous excitatory postsynaptic currents in defined-medium organotypic cultures of GAD67-GFP mice spinal cord to determine the receptor dependence of these opposing actions of BDNF. In mice, as in rats, BDNF enhances excitatory transmission onto excitatory neurons. This is mediated via presynaptic TrkB and p75 neurotrophin receptors and exclusively by postsynaptic TrkB. By contrast with findings from rats, in mice BDNF does not decrease excitation of inhibitory neurons. The cytokine macrophage colony-stimulating factor 1 (CSF-1) has also been implicated in the onset of neuropathic pain. Nerve injury provokes its de novo synthesis in primary afferents, its release in spinal cord, and activation of microglia. We now show that CSF-1 increases excitatory drive to excitatory neurons via a BDNF-dependent mechanism and decreases excitatory drive to inhibitory neurons via BDNF-independent processes. Our findings complete missing steps in the cascade of events whereby peripheral nerve injury instigates increased dorsal horn excitability in the context of central sensitization and the onset of neuropathic pain. Nerve injury provokes synthesis of macrophage colony-stimulating factor 1 (CSF-1) in primary afferents and its release in the dorsal horn. We show that CSF-1 increases excitatory drive to excitatory dorsal horn neurons via BDNF activation of postsynaptic TrkB and presynaptic TrkB and p75 neurotrophin receptors. CSF-1 decreases excitatory drive to inhibitory neurons via a BDNF-independent processes. This completes missing steps in understanding how peripheral injury instigates central sensitization and the onset of neuropathic pain.