BDNF-induced facilitation of afferent evoked responses in lamina II neurons is reduced following neonatal spinal cord contusion injury

We previously reported that brain derived neurotrophic factor (BDNF), a pronociceptive neurotransmitter, induces synaptic facilitation of EPSC in lamina II neurons of neonatal rats up to P14 in an NMDA-receptor dependent manner (Garraway et al. 2003). Here we used the patch-clamp technique to study synaptic and NMDA-evoked responses in transverse spinal slices in the lumbar enlargement as well as the ability of BDNF to modify these responses from 1 day to 6 weeks after neonatal contusion. In older uninjured animals (>P14), BDNF continued to evoke synaptic facilitation although superfusion of NMDA (in TTX) induced inward current of significantly smaller amplitude than that observed in younger rats. Following contusion injury, BDNF was unable to facilitate dorsal root-evoked EPSCs in lamina II neurons despite the finding that NMDA-evoked currents were only slightly smaller than those observed in age-matched uninjured animals. These findings suggest that although BDNF-induced facilitation of the AMPA/kainate receptor-mediated response to dorsal root stimulation is maintained in the mature dorsal horn from intact rats, BDNF may no longer elicit these pronociceptive actions after neonatal contusion injury. The lack of change in NMDA-evoked currents in contused cords suggests that diminished NMDA receptor function is not the major cause of the decline in BDNF action after contusion. It seems more likely that diminished trkB expression and enhanced expression of truncated trkB receptors in the contused cord (Liebl et al. 2001) play a significant role in determining the reduced effect of BDNF under these conditions.