Coordination of presynaptic and postsynaptic maturation in a zebra finch forebrain motor control nucleus during song learning

While some species of birds retain the ability to learn new songs as adults, many species can only learn during a restricted period when young. Previous studies have suggested that one potential mechanism of such a limited learning period, an alteration in the composition of postsynaptic NMDA receptors, does not competely block further song learning. Here, we examined whether presynaptic function could play a role in the regulation of learning capacity. We first showed that the participation of NMDA receptor NR2B subunits in synaptic currents in the robust nucleus of the arcopallium (RA), a critical location for integration of signals during song learning by young birds, decreases from young birds to adults. Using release-dependent block of postsynaptic NMDA receptors by an open-channel antagonist to assay presynaptic function, we showed that transmitter release at RA synapses from both HVC and the lateral magnocellular nucleus of the anterior nidopallium systematically decreases during the period of song learning, and in adults is about half that of juveniles. Further, activation of postsynaptic NMDA receptors could induce an acute depression of transmitter release, while lack of exposure to a normal learning environment could delay the developmental reduction in transmitter release. These results suggest that regulation of learning capacity may occur in part by coordination of presynaptic and postsynaptic function.