Can presynaptic depolarization release transmitter without calcium influx?

Area of Science:

• Neuroscience
• Cellular Biology
• Synaptic Transmission

Background:

• Previous studies suggested presynaptic depolarization could release neurotransmitters without calcium influx.
• The role of calcium influx in neurotransmitter release under varying depolarization levels requires clarification.

Purpose of the Study:

• To re-examine experimental evidence on presynaptic depolarization and neurotransmitter release.
• To investigate the relationship between depolarization magnitude, calcium influx, and transmitter release at the squid giant synapse.
• To clarify the mechanisms underlying synaptic facilitation at neuromuscular junctions.

Main Methods:

• Voltage clamp recordings of presynaptic calcium currents and postsynaptic responses at the squid giant synapse.
• Variable depolarization of presynaptic terminals.
• Analysis of neurotransmitter release in response to different depolarization levels and pulse trains.

Main Results:

• Larger depolarizations, despite similar macroscopic calcium currents, evoked greater transmitter release than smaller ones.
• This effect is attributed to increased overlap of calcium concentration domains around open calcium channels at higher depolarizations.
• Synaptic facilitation at neuromuscular junctions is primarily due to post-tetanic supernormal excitability causing action potentials, not a direct voltage effect.

Conclusions:

• Neurotransmitter release is voltage-dependent, influenced by calcium channel domain overlap.
• Synaptic facilitation is an indirect consequence of action potential generation, modulated by prior activity and calcium levels.
• The interplay between depolarization, calcium dynamics, and synaptic output is complex and context-dependent.