BDNF sensitizes the response of lamina II neurons to high threshold primary afferent inputs

Brain-derived neurotrophic factor (BDNF) is up-regulated and released in the dorsal horn following peripheral inflammation and has therefore been implicated in spinal mechanisms of sensitization. Despite these observations, the mechanisms associated with such a role for BDNF are not yet fully determined. Here, we investigate the effect of BDNF on dorsal root-evoked synaptic transmission in lamina II neurons. In a transverse spinal cord slice preparation from neonatal rats (P1-15), the whole cell patch-clamp technique was used to record from these neurons. Brief application of BDNF (50-200 ng/mL) facilitated the evoked synaptic currents; they remained enhanced even after BDNF was washed out. A significant minority of cells was minimally affected by BDNF and consistent with this, not all neurons in lamina II were immunoreactive for the tyrosine kinase (trk) B receptor. No facilitation was elicited when N-methyl-D-aspartate (NMDA) receptors were blocked with D-APV, when the postsynaptic NMDA receptors were selectively blocked with intracellular MK-801, or when postsynaptic neurons were loaded with BAPTA. Additionally, inhibiting phospholipase C (PLC) or protein kinase C (PKC) prior to BDNF application completely blocked facilitation. However, once synaptic current underwent BDNF-induced facilitation, the PKC inhibitors failed to reverse the effect, suggesting that PKC is needed for initiation, but not maintenance of BDNF-induced facilitation. These results demonstrate that BDNF functions at the spinal level to enhance synaptic efficacy in an NMDA receptor-dependent manner and requires the action of the PLC/PKC pathway. This action of BDNF may contribute to central sensitization and exaggerated pain states.