Regulation of short-term plasticity at the endbulb of Held
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The strength of a synapse depends on three fundamental characteristics of transmitter release, the number of release sites n , quantal size q and probability of release P r . Each of these properties can be modulated by activity through multiple mechanisms of synaptic plasticity. I have investigated factors influencing q and P r at the endbulb of Held. The unitary quantal size is a basic determinant of synaptic strength. However, the factors that determine quantal size have proven difficult to study in central synapses. Spontaneous quantal events (sEPSCs) show large amplitude distributions, and are highly skewed towards large events at the endbulb of Held. I have investigated whether the large spontaneous quantal events are activity- and Ca 2+ -dependent. I have found that the spontaneous events are independent of action potentials. Furthermore, the large events seem to depend on a calcium-independent mechanism. The endbulb of Held has a high P r , and therefore is a depressing synapse. However, the factors regulating P r at the presynaptic terminal are not completely understood. Here, I have investigated a possible factor, neural activity, in setting P r . I have used different sensory input manipulations, either loud noise or ablation of the tympanic membrane, and investigated how these change P r at the endbulb of Held. I found that by rearing animals in loud noise, the endbulbs exhibited lower P r . Mice with ablated tympanic membranes seem to show higher P r .