Characterization of developmental and molecular factors underlying release heterogeneity at <i>Drosophila</i> synapses.

Abstract

Neurons communicate through neurotransmitter release at specialized synaptic regions known as active zones (AZs). Using biosensors to visualize single synaptic vesicle fusion events at <i>Drosophila</i> neuromuscular junctions, we analyzed the developmental and molecular determinants of release probability (<i>P_r</i>) for a defined connection with ~300 AZs. <i>P_r</i> was heterogeneous but represented a stable feature of each AZ. <i>P_r</i> remained stable during high frequency stimulation and retained heterogeneity in mutants lacking the Ca²⁺ sensor Synaptotagmin 1. <i>P_r</i> correlated with both presynaptic Ca²⁺ channel abundance and Ca²⁺ influx at individual release sites. <i>P_r</i> heterogeneity also correlated with glutamate receptor abundance, with high <i>P_r</i> connections developing receptor subtype segregation. Intravital imaging throughout development revealed that AZs acquire high <i>P_r</i> during a multi-day maturation period, with <i>P_r</i> heterogeneity largely reflecting AZ age. The rate of synapse maturation was activity-dependent, as both increases and decreases in neuronal activity modulated glutamate receptor field size and segregation.