Molecular and organizational diversity intersect to generate functional synaptic heterogeneity within and between excitatory neuronal subtypes

Synaptic heterogeneity is a hallmark of complex nervous systems that enables reliable and responsive communication in neural circuits. In this study, we investigated the contributions of voltage-gated calcium channels (VGCCs) to synaptic heterogeneity at two closely related Drosophila glutamatergic motor neurons, one low- and one high-P(r). We find that VGCC levels are highly predictive of heterogeneous release probability among individual active zones (AZs) of low- or high-P(r) inputs, but not between neuronal subtypes. Underlying organizational differences in the AZ cytomatrix, VGCC composition, and a more compact arrangement of VGCCs alter the relationship between VGCC levels and P(r) at AZs of low- vs. high -P(r) inputs, explaining this apparent paradox. We further find that the CAST/ELKS AZ scaffolding protein Bruchpilot differentially regulates VGCC levels at low- and high-P(r) AZs following acute glutamate receptor inhibition, indicating that synapse-specific organization also impacts adaptive plasticity. These findings reveal intersecting levels of molecular and spatial diversity with context-specific effects on heterogeneity in synaptic strength and plasticity.