Excitatory and Inhibitory Neurons Utilize Different Ca Sensors and Sources to Regulate Spontaneous Release.

Spontaneous neurotransmitter release (mini) is an important form of Ca-dependent synaptic transmission that occurs in the absence of action potentials. A molecular understanding of this process requires an identification of the underlying Ca sensors. Here, we address the roles of the relatively low- and high-affinity Ca sensors, synapotagmin-1 (syt1) and Doc2α/β, respectively. We found that both syt1 and Doc2 regulate minis, but, surprisingly, their relative contributions depend on whether release was from excitatory or inhibitory neurons. Doc2α promoted glutamatergic minis, while Doc2β and syt1 both regulated GABAergic minis. We identified Ca ligand mutations in Doc2 that either disrupted or constitutively activated the regulation of minis. Finally, Ca entry via voltage-gated Ca channels triggered miniature GABA release by activating syt1, but had no effect on Doc2-driven minis. This work reveals an unexpected divergence in the regulation of spontaneous excitatory and inhibitory transmission in terms of both Ca sensors and sources.