Measuring the Readily Releasable Synaptic Vesicle Pool at the Larval Neuromuscular Junction.

Presynaptic homeostatic potentiation (PHP) is a type of homeostatic regulation that stabilizes synaptic output under conditions where postsynaptic receptor function is impaired. PHP manifests as a significant increase in presynaptic neurotransmitter release, compensating for decreased postsynaptic receptor activity and thus maintaining stable excitation levels in postsynaptic cells. Presynaptic neurotransmitter release is calcium-dependent, initiated by calcium influx through voltage-gated calcium channels localized at the presynaptic active zones. This calcium influx triggers the fusion of vesicles from the readily releasable vesicle pool (RRP) that are ready for immediate release. Two key presynaptic cellular mechanisms are essential for PHP's induction and maintenance. First, a compensatory rise in the abundance of presynaptic calcium channels (and consequently, an increase in calcium influx) occurs when postsynaptic glutamate receptors are suppressed. Second, the RRP size enlarges during PHP. PHP is disrupted if either of these processes is impaired. This protocol outlines the use of the two-electrode voltage-clamp technique for assessing the RRP during PHP, induced either pharmacologically or genetically, at the neuromuscular junction (NMJ). Electrophysiological recordings typically take place at the NMJ of muscle 6 in abdominal segments A2 and A3.