Figure 8.

Pauses in interneuron firing can shape HVC_(RA) neuron output. A, Stimulation of the HVC-RA fiber tract (50 μA) recruits disynaptic inhibition in the HVC_(RA) neuron network. Left, Simultaneous recording of an HVC_(RA) neuron (average trace = red) and an interneuron (average trace = blue). The peak of monosynaptic excitation evoked in the interneuron (dashed blue line) occurs before the onset of the disynaptic inhibitory current in the HVC_(RA) neuron. Right, An HVC_(RA) neuron in a similar configuration. The disynaptic inhibitory current is abolished at the chloride reversal potential (bottom trace), revealing a small EPSC (stimulation current: 70 μA). B, Population data from a set of HVC_(RA) neurons recorded in this fashion, with peak IPSC and EPSC strengths quantified at −46 mV. 3 of 9 cases exhibited evidence for an excitatory current (average EPSC amplitude: 15.7 ± 9.0 pA). C, D, In vitro recording of an interneuron (C) and an HVC_(RA) neuron (D) while stimulating the HVC-RA fiber tract with an extracellular electrode at 60 μA (stimulation artifacts clipped for clarity) to evoke interneuron spiking similar to the patterns seen during singing. Gaps in firing are indicated by gray shaded regions. Mean IPSP strength in the HVC_(RA) neuron (measured at location of red star) was 7.8 ± 2.3 mV (n = 11 cells). E–G, Configuration as above showing spiking responses in the HVC_(RA) neuron to a 35 ms somatic current pulse during an inhibitory gap (E) or ongoing inhibition (F, G). H, I, Quantification of the HVC_(RA) spiking responses of individual neurons to various current pulses during an inhibitory gap (H) and ongoing inhibition (I). Each line represents the spiking response of a single neuron to a current pulse normalized to the maximal response from that cell across conditions. All-or-none bursting was only observed during pauses of inhibition. The L-type calcium agonist BAY K8644 was bath-applied to facilitate HVC_(RA) neuron burst firing from somatic current injection in C–I.