Mechnisms underlying different facilitation forms at the lobster neuromuscular synapse

Area of Science:

• Neuroscience
• Cell Biology
• Synaptic Physiology

Background:

• Synaptic facilitation enhances neurotransmission during repetitive stimulation.
• Plateau facilitation (F(plateau)) is frequency-dependent, but its underlying mechanism remains unclear.

Purpose of the Study:

• To investigate the role of intracellular calcium (Ca(2+)) and the readily releasable pool (RRP) of synaptic vesicles in plateau facilitation at the crustacean neuromuscular junction.

Main Methods:

• Manipulated intracellular Ca(2+) using chelators (BAPTA-AM, EGTA-AM), altered extracellular Ca(2+) concentration, and enhanced Ca(2+) influx with 4-aminopyridin.
• Measured changes in plateau facilitation (F(plateau)) and facilitation growth (F(growth)) under various Ca(2+) conditions.
• Assessed the size of the readily releasable pool (RRP) using hypertonic solutions (HS) before and after inducing plateau facilitation.

Main Results:

• Plateau facilitation (F(plateau)) remained unaffected by manipulations of intracellular or extracellular Ca(2+) levels.
• Facilitation growth (F(growth)) was sensitive to Ca(2+) changes.
• The readily releasable pool (RRP) size significantly increased after plateau facilitation was induced.
• These findings indicate that F(plateau) is not caused by intracellular Ca(2+) accumulation.

Conclusions:

• Plateau facilitation is mediated by two mechanisms: residual Ca(2+) increase and RRP expansion.
• The primary driver of frequency-dependent plateau facilitation is the increase in the readily releasable pool (RRP) of synaptic vesicles.