Synaptic vesicle endocytosis at a CNS nerve terminal: Faster kinetics at physiological temperatures and increased endocytotic capacity during maturation

Synaptic vesicle membrane must be quickly retrieved and recycled after copious exocytosis to limit the depletion of vesicle pools. The rate of endocytosis at the calyx of Held nerve terminal has been measured directly using membrane capacitance measurements from immature postnatal day P7-P10 rat pups at room temperature (RT: 23-24°C). This rate has an average time constant of tens of seconds and becomes slower when the amount of exocytosis (measured as capacitance jump) increases. Such slow rates seem paradoxical for a synapse that can operate continuously at high-input frequencies. Here we perform time-resolved membrane capacitance measurements from the mouse calyx of Held in brain stem slices at physiological temperature (PT: 35-37°C), and also from more mature calyces after the onset of hearing (P14-P18). Our results show that the rate of endocytosis is strongly temperature dependent, whereas the endocytotic capacity of a nerve terminal is dependent on developmental stage. At PT we find that endocytosis accelerates due to the addition of a kinetically fast component (time constant: τ = 1-2 s) immediately after exocytosis. Surprisingly, we find that at RT the rate of endocytosis triggered by short (1- to 5-ms) or long (≥10-ms) depolarizing pulses in P14-P18 mice are similar (τ ≈ 15 s). Furthermore, this rate is greatly accelerated at PT (τ ≈ 2 s). Thus endocytosis becomes faster and less saturable during synaptic maturation, making the calyceal terminal more capable of sustaining prolonged high-frequency transmitter release.