Brain function requires precisely orchestrated connectivity between neurons. Establishment of these connections is believed to require signals secreted from outgrowing axons, followed by synapse formation between selected neurons. Deletion of a single protein, Munc18-1, in mice leads to a complete loss of neurotransmitter secretion from synaptic vesicles throughout development. However, this does not prevent normal brain assembly, including formation of layered structures, fiber pathways, and morphologically defined synapses. After assembly is completed, neurons undergo apoptosis, leading to widespread neurodegeneration. Thus, synaptic connectivity does not depend on neurotransmitter secretion, but its maintenance does. Neurotransmitter secretion probably functions to validate already established synaptic connections.

SYNAPTIC vesicles are coated by synapsins, phosphoproteins that account for 9% of the vesicle protein^1-3. To analyse the functions of these proteins, we have studied knockout mice lacking either synapsin I, synapsin II, or both. Mice lacking synapsins are viable and fertile with no gross anatomical abnormalities, but experience seizures with a frequency proportional to the number of mutant alleles. Synapsin-II and double knockouts, but not synapsin-I knockouts, exhibit decreased post-tetanic potentiation and severe synaptic depression upon repetitive stimulation. Intrinsic synaptic-vesicle membrane proteins, but not peripheral membrane proteins or other synaptic proteins, are slightly decreased in individual knockouts and more severely reduced in double knockouts, as is the number of synaptic vesicles. Thus synapsins are not required for neurite outgrowth, synaptogenesis or the basic mechanics of synaptic vesicle traffic, but are essential for accelerating this traffic during repetitive stimulation. The phenotype of the synapsin knockouts could be explained either by deficient recruitment of synaptic vesicles to the active zone, or by impaired maturation of vesicles at the active zone, both of which could lead to a secondary destabilization of synaptic vesicles.