Factors Controlling the Input-Output Relationship of Spherical Bushy Cells in the Gerbil Cochlear Nucleus

Despite the presence of large endbulb inputs, the spherical bushy cells (SBCs) of the rostral anteroventral cochlear nucleus do not function as simple auditory relays. We used the good signal-to-noise ratio of juxtacellular recordings to dissect the intrinsic and network mechanisms controlling the input-output relationship of SBCs in anesthetized gerbils. The SBCs generally operated close to action potential (AP) threshold and showed no evidence for synaptic depression, suggesting that the endbulbs of Held have low release probability in vivo. Analysis of the complex waveforms suggested that in the absence of auditory stimulation, postsynaptic spike depression and stochastic fluctuations in EPSP size were the main factors determining jitter and reliability of the endbulb synapse. During auditory stimulation, progressively larger EPSPs were needed to trigger APs at increasing sound intensities. Simulations suggested hyperpolarizing inhibition could explain the observed decrease in EPSP efficacy. Synaptic inhibition showed a delayed onset and generally had a higher threshold than excitatory inputs, but otherwise inhibition and excitation showed mostly overlapping frequency-response areas. The recruitment of synaptic inhibition caused postsynaptic spikes to be preferentially triggered by well-timed, large EPSPs, resulting in improved phase locking despite more variable EPSP-AP latencies. Our results suggest that the lack of synaptic depression, caused by low release probability, and the apparent absence of sound-evoked synaptic inhibition at low sound intensity maximize sensitivity of SBCs. At higher sound intensities, the recruitment of synaptic inhibition constrains their firing rate and optimizes their temporal precision.