Two distinct inputs to an avian song nucleus activate different glutamate receptor subtypes on individual neurons

Although neural circuits mediating various simple behaviors have been delineated, those generating more complex behaviors are less well described. The discrete structure of avian song control nuclei promises that circuits controlling complex behaviors, such as birdsong, can also be understood. To this end, we developed an in vitro brain slice preparation containing the robust nucleus of the archistriatum (RA), a forebrain song control nucleus, and its inputs from two other song nuclei, the caudal nucleus of the ventral hyperstriatum (HVc) and the lateral part of the magnocellular nucleus of the anterior neostriatum (L-MAN). Using intracellular recordings, we examined the pharmacological properties of the synapses made on RA neurons by L-MAN and HVc axons. Electrical stimulation of the L-MAN and the HVc fiber tracts evoked excitatory postsynaptic potentials (EPSPs) from >70% of RA neurons when slices were prepared from male birds of 40-90 days of age, suggesting that many individual RA neurons receive excitatory input from L-MAN and HVc axons. The "L-MAN" EPSPs were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (D-APV) as well as the broad-spectrum glutamate receptor antagonist kynurenic acid but were relatively unaffected by the non-NMDA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, "HVc" EP-SPs were relatively insensitive to D-APV but almost completely abolished by CNQX. These experiments suggest that L-MAN and HVc axons provide pharmacologically distinct types of excitatory input to many of the same RA neurons.