Synaptic Mechanisms Underlying Temporally Precise Information Processing in the VNLL, an Auditory Brainstem Nucleus

Abstract

Large glutamatergic, somatic synapses mediate temporally precise information transfer. In the ventral nucleus of the lateral lemniscus, an auditory brainstem nucleus, the signal of an excitatory large somatic synapse is sign inverted to generate rapid feedforward inhibition with high temporal acuity at sound onsets, a mechanism involved in the suppression of spurious frequency information. The mechanisms of the synaptically driven input-output functions in the ventral nucleus of the lateral lemniscus are not fully resolved. Here, we show in Mongolian gerbils of both sexes that, for stimulation frequencies up to 200 Hz, the EPSC kinetics together with short-term plasticity allow for faithful transmission with only a small increase in latency. Glutamatergic currents are exclusively mediated by AMPARs and NMDARs. Short-term plasticity is frequency-dependent and composed of an initial facilitation followed by depression. Physiologically relevant output generation is limited by the decrease in synaptic conductance through short-term plasticity (STP). At this endbulb synapse, STP acts as a low pass filter and increases the dynamic range of the conductance dependent inputoutput relation, while NMDAR signaling slightly increases the sensitivity of the input-output function. Our computational model shows that STP-mediated filtering limits the intensity dependence of the spike output, thus maintaining selectivity to sound transients. Our results highlight the interaction of cellular features that together give rise to the computations in the circuit.

Neuronal morphology and synaptic input patterns of neurons in the intermediate nucleus of the lateral lemniscus of gerbils

“…Again only in the DNLL and VNLL, the functional input patterns have been investigated. Globular VNLL neurons receive a large excitatory input 6 , 16 , 22 , 23 while other VNLL neurons receive many small excitatory inputs 6 , and inhibitory currents are similar in size and kinetics throughout the VNLL 6 . In the DNLL, excitation is based on AMPA and NMDA receptor mediated currents 4 , 24 – 27 and inhibition is largely GABA A receptor mediated 28 , 29 .…”