Ultrafast Action Potentials Mediate Kilohertz Signaling at a Central Synapse

Abstract

Fast synaptic transmission is important for rapid information processing. To explore the maximal rate of neuronal signaling and to analyze the presynaptic mechanisms, we focused on the input layer of the cerebellar cortex, where exceptionally high action potential (AP) frequencies have been reported in vivo. With paired recordings between presynaptic cerebellar mossy fiber boutons and postsynaptic granule cells, we demonstrate reliable neurotransmission up to ∼1 kHz. Presynaptic APs are ultrafast, with ∼100 μs half-duration. Both Kv1 and Kv3 potassium channels mediate the fast repolarization, rapidly inactivating sodium channels ensure metabolic efficiency, and little AP broadening occurs during bursts of up to 1.5 kHz. Presynaptic Cav2.1 (P/Q-type) calcium channels open efficiently during ultrafast APs. Furthermore, a subset of synaptic vesicles is tightly coupled to Ca(2+) channels, and vesicles are rapidly recruited to the release site. These data reveal mechanisms of presynaptic AP generation and transmitter release underlying neuronal kHz signaling.

Reduced endogenous Ca 2+ buffering speeds active zone Ca 2+ signaling

“…Ca 2+ currents displayed facilitation during 300-Hz bursts (Fig. 5 B ), consistent with P/Q-type voltage-gated Ca 2+ channels at cMFBs (8). Next, we used Ca 2+ imaging to measure the increased spatially averaged residual [Ca 2+ ] by 300-Hz train stimulations (Fig.…”

“…The model cMFB included the experimentally determined endogenous buffers (Figs. 2–5), Ca 2+ current amplitude (8), and Ca 2+ current facilitation (Fig. 5 B ).…”

“…Here, we combined direct patch-clamp recordings from en passant cMFBs (6, 8) (Fig. 1 A–C ) with quantitative two-photon Ca 2+ imaging.…”

“…In vivo, cMFBs fire bursts of APs with exceptionally high frequencies (6, 26), where vesicular transmitter release is remarkably synchronous (8, 27). To understand which mechanisms enable synchronous high-frequency release, we measured the buildup of Ca 2+ during high-frequency bursts (20 APs at 300 Hz).…”

“…The cerebellar mossy fiber bouton (cMFB) to granule cell synapse is ideally suited to analyze Ca 2+ signaling during repetitive synaptic transmission because of the synchronous neurotransmitter release at exceptionally high frequencies (6–8). Understanding rapid active zone Ca 2+ signaling requires knowledge about the Ca 2+ dynamics and the strength, mobility, and binding kinetics of endogenous Ca 2+ buffers.…”