Spontaneous single synapse activity predicts evoked neurotransmission by using overlapping machinery

Synaptic transmission relies on presynaptic neurotransmitter (NT) release from synaptic vesicles (SVs), and on NT detection by postsynaptic receptors. Two principal modes exist: action-potential (AP) evoked and AP-independent "spontaneous" transmission. Though universal to all synapses and essential for neural development and function, regulation of spontaneous transmission remains enigmatic. Mechanisms divergent from AP-evoked transmission were described, but are difficult to reconcile with its established function in adjusting AP-evoked transmission. By studying neurotransmission at individual synapses of Drosophila larval neuromuscular junctions (NMJs), we show a clear interdependence of transmission modes: Components of the AP-evoked NT-release machinery (Unc13, Syntaxin-1 and BRP) also predicted spontaneous transmission. Both modes were reduced when blocking voltage-gated calcium channels and engaged an overlapping pool of SVs and NT-receptors. While a small subset (~21%) of spontaneously active synapses appeared limited to this mode, most also mediated AP-evoked transmission and activity was highly correlated. Thus, by engaging overlapping molecular machinery, spontaneous transmission predicts AP-evoked transmission at single synapses.