Sodium Channel-Dependent and -Independent Mechanisms Underlying Axonal Afterdepolarization at Mouse Hippocampal Mossy Fibers

Abstract

Action potentials propagating along axons are often followed by prolonged afterdepolarization (ADP) lasting for several tens of milliseconds. Axonal ADP is thought to be an important factor in modulating the fidelity of spike propagation during repetitive firings. However, the mechanism as well as the functional significance of axonal ADP remain unclear, partly due to inaccessibility to small structures of axon for direct electrophysiological recordings. Here, we examined the ionic and electrical mechanisms underlying axonal ADP using whole-bouton recording from mossy fiber terminals in mice hippocampal slices. ADP following axonal action potentials was strongly enhanced by focal application of veratridine, an inhibitor of Na⁺ channel inactivation. On the contrary, tetrodotoxin (TTX) partly suppressed ADP, suggesting that a Na⁺ channel-dependent component is involved in axonal ADP. The remaining TTX-resistant Na⁺ channel-independent component represents slow capacitive discharge reflecting the shape and electrical properties of the axonal membrane. We also addressed the functional impact of axonal ADP on presynaptic function. In paired-pulse stimuli, we found that axonal ADP minimally affected the peak height of subsequent action potentials, although the rising phase of action potentials was slightly slowed, possibly due to steady-state inactivation of Na⁺ channels by prolonged depolarization. Voltage clamp analysis of Ca²⁺ current elicited by action potential waveform commands revealed that axonal ADP assists short-term facilitation of Ca²⁺ entry into the presynaptic terminals. Taken together, axonal ADP maintains reliable firing during repetitive stimuli and plays important roles in the fine-tuning of short-term plasticity of transmitter release by modulating Ca²⁺ entry into presynaptic terminals.

Significance Statement Axonal action potentials are often followed by depolarizing or hyperpolarizing afterpotentials. This study illuminated the mechanisms of ADP in the hippocampal mossy fibers, where morphologic as well as biophysical data were accumulated. We found that slow activating Na⁺ channels are partly involved in ADP. Capacitive components also substantially contribute to ADP, suggesting that axonal shape and electrical properties are optimized for high-fidelity propagation during repetitive stimuli. We also tested the roles of ADP in the activity-dependent tuning of the presynaptic Ca²⁺ current. Action potential-driven Ca²⁺ entry into the axon terminals was facilitated by paired stimuli, possibly due to Ca²⁺ current facilitation by ADP. Therefore, ADP contributes to fine-tuning of transmitter release and ensures high-fidelity spiking of axons.