Removal of NMDA Receptor Mg2+ Block Extends the Action of NT-3 on Synaptic Transmission in Neonatal Rat Motoneurons

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Abstract
NT-3 has previously been reported to enhance AMPA/kainate receptor-mediated synaptic responses in motoneurons via an effect on the N-methyl-d-aspartate (NMDA) receptor. To investigate neurotrophin-3 (NT-3) action further, we measured the NMDA receptor (NMDAR)-mediated synaptic response directly by intracellular recording in motoneurons after blocking AMPA/kainate, GABAA, GABAB and glycine receptor-mediated responses pharmacologically. Two pathways were stimulated, the segmental dorsal root (DR) and the descending ventrolateral fasciculus (VLF). The DR-evoked NMDAR-mediated response in motoneurons of rats younger than 1 wk has two components, the initial one of which is generated monosynaptically. NT-3 strongly potentiated both NMDA components in a rapidly reversible manner. No NMDAR-mediated responses were present at VLF connections and at DR connections in older (1- to 2-wk-old) neonates. Bath-applied NT-3–induced potentiation of the AMPA/kainate receptor-mediated response occurred only at connections that exhibit a synaptic NMDA receptor-mediated response. Reducing Mg2+concentration in the bathing solution restored the NMDAR-mediated response elicited by DR stimulation in older neonates and by VLF throughout the neonatal period (0–2 wk). In low-Mg2+, NT-3 enhanced AMPA/kainate receptor-mediated responses elicited by inputs normally not influenced by NT-3. Thus a major reason for the loss of NT-3 action on AMPA/kainate synaptic responses is the reduced activity of the NMDA receptor due to developing Mg2+ block of NMDA receptor-channel complex as the animal matures, and both can be re-established by reducing Mg2+ concentration in fluid bathing the spinal cord.