Underpinning heterogeneity in synaptic transmission by presynaptic ensembles of distinct morphological modules

Synaptic heterogeneity is widely observed but its underpinnings remain elusive. We addressed this issue using mature calyx of Held synapses whose numbers of bouton-like swellings on stalks of the nerve terminals inversely correlate with release probability (Pr). We examined presynaptic Ca²⁺ currents and transients, topology of fluorescently tagged knock-in Ca²⁺ channels, and Ca²⁺ channel-synaptic vesicle (SV) coupling distance using Ca²⁺ chelator and inhibitor of septin cytomatrix in morphologically diverse synapses. We found that larger clusters of Ca²⁺ channels with tighter coupling distance to SVs elevate Pr in stalks, while smaller clusters with looser coupling distance lower Pr in swellings. Septin is a molecular determinant of the differences in coupling distance. Supported by numerical simulations, we propose that varying the ensemble of two morphological modules containing distinct Ca²⁺ channel-SV topographies diversifies Pr in the terminal, thereby establishing a morpho-functional continuum that expands the coding capacity within a single synapse population.