Acute modulation of synaptic transmission to motoneurons by BDNF in the neonatal rat spinal cord

We investigated the acute effects of bath applied BDNF on synaptic input to motoneurons in the hemisected spinal cord of the neonatal rat. Motoneurons were recorded intracellularly, and BDNF-induced modulation of the synaptic response to stimulation of the homologous dorsal root (DR) and the ventrolateral funiculus (VLF) was examined. All motoneurons exhibited long-lasting (up to several hours) depression of the DR-activated monosynaptic AMPA/kainate-receptor mediated EPSP in response to BDNF but in about half of the motoneurons this was preceded by facilitation. VLF-evoked AMPA/kainate EPSPs in the same motoneurons were unaffected. BDNF effects were blocked by K252a and were not observed in neonates older than 1 week. Bath applied NMDA antagonists APV and MK-801 abolished both facilitatory and inhibitory actions of BDNF on the AMPA/kainate responses indicating the requirement for functional NMDA receptors. The pharmacologically isolated, DR-evoked, NMDA receptor-mediated response exhibited the same pattern of changes after BDNF superfusion. When introduced into the motoneuron through the recording microelectrode, MK-801 selectively blocked the facilitatory action of BDNF. Furthermore, BDNF enhanced NMDA-induced depolarization of the motoneuron in the presence of tetrodotoxin (TTX), thus, confirming its facilitatory effect on motoneuron NMDA receptors. Bath application of either BDNF or NMDA depressed the monosynaptic EPSP after selective blockade of postsynaptic NMDA receptors indicating a role for presynaptic NMDA receptors in BDNF-induced inhibitory action. Thus, BDNF-induced facilitation of monosynaptic EPSPs in neonatal rats is mediated by direct effects on postsynaptic NMDA receptors, while its inhibitory action occurs presynaptically.