The role of GABA in the central processing of complex auditory signals is not fully understood. We have studied the involvement of GABA A-mediated inhibition in the processing of birdsong, a learned vocal communication signal requiring intact hearing for its development and maintenance. We focused on caudomedial nidopallium (NCM), an area analogous to parts of the mammalian auditory cortex with selective responses to birdsong. We present evidence that GABA A-mediated inhibition plays a pronounced role in NCM's auditory processing of birdsong. Using immunocytochemistry, we show that approximately half of NCM's neurons are GABAergic. Whole cell patch-clamp recordings in a slice preparation demonstrate that, at rest, spontaneously active GABAergic synapses inhibit excitatory inputs onto NCM neurons via GABA A receptors. Multi-electrode electrophysiological recordings in awake birds show that local blockade of GABA A-mediated inhibition in NCM markedly affects the temporal pattern of song-evoked responses in NCM without modifications in frequency tuning. Surprisingly, this blockade increases the phasic and largely suppresses the tonic response component, reflecting dynamic relationships of inhibitory networks that could include disinhibition. Thus processing of learned natural communication sounds in songbirds, and possibly other vocal learners, may depend on complex interactions of inhibitory networks.

The underlying mechanisms that promote precise spiking in upper motor neurons controlling fine motor skills are not well understood. Here we report that projection neurons in the adult zebra finch song nucleus RA display robust high-frequency firing, ultra-narrow spike waveforms, superfast Na+ current inactivation kinetics, and large resurgent Na+ currents (INaR). These properties of songbird pallial motor neurons closely resemble those of specialized large pyramidal neurons in mammalian primary motor cortex. They emerge during the early phases of song development in males, but not females, coinciding with a complete switch of Na+ channel subunit expression from Navβ3 to Navβ4. Dynamic clamping and dialysis of Navβ4’s C-terminal peptide into juvenile RA neurons provide evidence that Navβ4, and its associated INaR, promote neuronal excitability. We thus propose that INaR modulates the excitability of upper motor neurons that are required for the execution of fine motor skills.

GluA2-lacking Ca 2ϩ -permeable AMPARs (CP-AMPARs) play integral roles in synaptic plasticity and can mediate excitotoxic cellular signaling at glutamatergic synapses. However, the developmental profile of functional CP-AMPARs at the auditory brainstem remains poorly understood. Through a combination of electrophysiological and live-cell Ca 2ϩ imaging from mice of either sex, we show that the synaptic release of glutamate from the calyx of Held nerve terminal activates CP-AMPARs in the principal cells of the medial nucleus of the trapezoid body in the brainstem. This leads to significant Ca 2ϩ influx through these receptors before the onset of hearing at postnatal day 12 (P12). Using a selective open channel blocker of CP-AMPARs, IEM-1460, we estimate that ϳ80% of the AMPAR population are permeable to Ca 2ϩ at immature P4 -P5 synapses. However, after the onset of hearing, Ca 2ϩ influx through these receptors was greatly reduced. We estimate that CP-AMPARs comprise approximately 40% and 33% of the AMPAR population at P18 -P22 and P30 -P34, respectively. By quantifying the rate of EPSC block by IEM-1460, we found an increased heterogeneity in glutamate release probability for adult-like calyces (P30 -P34). Using tetraethylammonium (TEA), a presynaptic potassium channel blocker, we show that the apparent reduction of CP-AMPARs in more mature synapses is not a consequence of presynaptic action potential (AP) speeding. Finally, through postsynaptic AP recordings, we show that inhibition of CP-AMPARs reduces spike fidelity in juvenile synapses, but not in more mature synapses. We conclude that the expression of functional CP-AMPARs declines over early postnatal development in the calyx of Held synapse.The calyx of Held synapse is pivotal to the circuitry that computes sound localization. Postsynaptic Ca 2ϩ influx via AMPARs may be critical for signaling the maturation of this brainstem synapse. The GluA4 subunit may dominate the AMPAR complex at mature synapses because of its fast gating kinetics and large unitary conductance. The expectation is that AMPARs dominated by GluA4 subunits should be highly Ca 2ϩ permeable. However, we find that Ca 2ϩ -permeable AMPAR expression declines during postnatal development. Using the rate of EPSC block by IEM-1460, an open channel blocker of Ca 2ϩ -permeable AMPARs, we propose a novel method to determine glutamate release probability and uncover an increased heterogeneity in release probability for more mature calyces of Held nerve terminals.