Munc13-1 is essential for fusion competence of glutamatergic synoptic vesicles

Neurotransmitter release at synapses between nerve cells is mediated by cal cium-triggered exocytotic fusion of synaptic vesicles'. Before fusion, vesi cles dock at the presynaptic release site where they mature to a fusion-com petent state(1,2). Here we identify Munc13-1, a brain-specific presynaptic phorbol ester receptor(3,4), as an essential protein for synaptic vesicle m aturation. We show that glutamatergic hippocampal neurons from mice lacking Munc13-1 form ultrastructurally normal synapses whose synaptic-vesicle cyc le is arrested at the maturation step. Transmitter release from mutant syna pses cannot be triggered by action potentials, calcium-ionophores or hypert onic sucrose solution. In contrast, release evoked by alpha-latrotolrin is indistinguishable from wild-type controls, indicating that the toxin can by pass Munc13-1-mediated vesicle maturation. A small subpopulation of synapse s of any given glutamatergic neuron as well as all synapses of GABA (gamma- aminobutyric acid)-containing neurons are unaffected by Munc13-1 loss, demo nstrating the existence of multiple and transmitter-specific synaptic vesic le maturation processes in synapses.