Considerable differences between auditory medulla, auditory midbrain, and hippocampal synapses during sustained high-frequency stimulation: Exceptional vesicle replenishment restricted to sound localization circuit

Abstract

Reliable synaptic transmission is essential for interneuronal communication. Synaptic inputs to auditory brainstem neurons, particularly those involved in sound localization, are characterized by resilience during sustained activity and temporal precision in the sub-millisecond range. Both features are obtained by synchronous release of a high number of synaptic vesicles following a single action potential. Here, we compare transmission behavior of three heterogeneous types of inputs in the auditory midbrain and medulla. The first terminate in the central inferior colliculus (ICc) and are glutamatergic (activated from the lateral lemniscus, LL). The medullary inputs terminate in the lateral superior olive (LSO) and are glutamatergic (from the cochlear nuclear complex, CN) or glycinergic (from the medial nucleus of the trapezoid body, MNTB). LSO neurons are the first to integrate binaural information and compute interaural level differences, whereas ICc neurons receive information from almost all auditory brainstem nuclei and construct an initial auditory image used for reflexive behavior. We hypothesized that CN-LSO and MNTB-LSO inputs are more resilient to synaptic fatigue during sustained stimulation than LL-ICc inputs. To test the hypothesis, we performed whole-cell patch-clamp recordings in acute brainstem slices of juvenile mice. We investigated the synaptic performance during prolonged periods of high-frequency stimulation (60 s, up to 200 Hz) and assessed several features, e.g. depression, recovery, latency, temporal precision, quantal size and content, readily releasable pool size, release probability, and replenishment rate. Overall, LL-ICc inputs performed less robustly and temporally precisely than CN-LSO and MNTB-LSO inputs. When stimulated at ≥50 Hz, the former depressed completely within a few seconds. In contrast, CN-LSO and MNTB-LSO inputs transmitted faithfully up to 200 Hz, indicative of very efficient replenishment mechanisms. LSO inputs also displayed considerably lower latency jitter than LL-ICc inputs. The latter behaved similarly to two types of input in the hippocampus for which we performed a meta-analysis. Mechanistically, the high-fidelity behavior of LSO inputs, particularly MNTB-LSO synapses, is based on exceptional release properties not present at auditory midbrain or hippocampal inputs. We conclude that robustness and temporal precision are hallmarks of auditory synapses in the medullary brainstem. These key features are less eminent at higher stations, such as the ICc, and they are also absent outside the central auditory system, namely the hippocampal formation.