The large calyx of Held nerve terminal is a pivotal element in the circuitry that computes sound source localization in the mammalian auditory brainstem. Precise timing of action potential output from this synapse is thought to be central for this task. However, the synaptic mechanisms that modulate and preserve action potential timing during high frequency firing are not well understood in mature animals because of the difficultly of recording and visualizing neurons in the heavily myelinated adult brainstem. Our lab has pioneered patch clamp recordings in brainstem slices from more mature and adult-like stages of mouse development, when they fully acquire their fine-tuned ability to hear and localize sound. The first hypothesis to be tested is that the large amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs) observed in mature calyx of Held synapses is due to a highly synchronous form of multiquantal release. We suggest that these large and multiquantal mEPSCs are a natural consequence of the progressively tighter coupling of Ca2+ channels to docked vesicles as the synapse matures. We will reconstruct the active zones of mature calyces at a resolution of 3-5 nm and correlate this information with our quantal analysis at the single vesicle exocytosis level. Novel insights into the structure and function relationship of auditory synapses will thus be revealed. The second hypothesis to be tested is that a fast Ca2+-dependent recruitment of vesicles from a reserve pool, triggered by the opening of presynaptic Ca2+ stores, produces a transient EPSC enhancement, or a late-tetanic rebound, during a stimulus train in more mature synapses. We will test this hypothesis by using pharmacological approaches that either block increases in Ca2+ concentration or block the Ca2+ stores from releasing Ca2+. The third hypothesis to be tested is that postsynaptic MNTB principal cell dendrites play a major role in shortening the excitatory postsynaptic potential (EPSP) decay. We hypothesize that this dendritic speeding of the evoked EPSP enhances the precision of postsynaptic spikes triggered by afferent fiber stimulation at high frequencies. Finally, to study the passive and active properties of the adult-like MNTB principal cells and their dendrites we will use patch-clamp and Ca2+ imaging techniques. This will allow us to build realistic computer models of how the MNTB cell integrates synaptic inputs and fires action potentials to preserve the timing of incoming sound signals.
Several fundamental insights on how auditory synapses and neurons operate in the adult mammalian brainstem will be obtained from the studies proposed here. This will probably transfer to a better understanding of healthy adult human hearing, and lays the basic groundwork for future treatment strategies of CNS hearing disorders. Eventually, we hope these studies will lead to an improved understanding of older animals that become hard of hearing with age, and thus our studies may lead to procedures that help prevent hearing loss in our progressively aging population.
