Figure 7.

Blocking GABA_A-mediated inhibition in HVC could unmask additional excitatory and inhibitory synaptic pathways from HVC_RA to HVC_X neurons. A, In a PTX-treated brain slice, antidromic stimulation of HVC_RA neurons could evoke repetitive bursting (early burst: 3 spikes, 200 Hz mean burst rate; later burst: 2 spikes, 59 Hz burst rate) and slow hyperpolarizing responses, which were blocked by the bath application of the ionotropic glutamate receptor antagonists NBQX and d-APV. B, Before PTX treatment, antidromic stimulation of the HVC_RA fibers evoked a fast IPSP in an HVC_X neuron (control); subsequent PTX treatment blocked the early, fast IPSP and unmasked a multiphasic IPSP that included a slow component (PTX early). At later times during the treatment (PTX late), the same stimulation evoked an initial excitatory response, followed by a prolonged, biphasic hyperpolarization. The subthreshold depolarization that occurs during the middle of the slow, biphasic hyperpolarization is believed to be similar to the event associated with the longer-latency bursting behavior in A.