Synapsin I has been proposed to be involved in the modulation of neurotransmitter release by controlling the availability of synaptic vesicles for exocytosis. To further understand the role of synapsin I in the function of adult nerve terminals, we studied synapsin I-deficient mice generated by homologous recombination. The organizaition of synaptic vesicles at presynaptic terminals of synapsin I-deficient mice was markedly altered: densely packed vesicles were only present in a narrow rim at active zones, whereas the majority of vesicles were dispersed throughout the terminal area. A great deal of evidence has implicated the synapsins in the regulation of synaptogenesis and in the modulation of neurotransmitter release from adult nerve terminals (1-4). To further assess the possible roles of synapsin I in the regulation of these processes, we have generated synapsin I-deficient mice by homologous recombination. In an accompanying paper (5), we report that axonal outgrowth and synaptogenesis are severely impaired in these mutant mice. Herein we present evidence that synapses of the adult synapsin I mutant mice manifest a variety of structural and physiological abnormalities. MATERIALS AND METHODSSynapsin I-Deficient Mice. Synapsin I mutant mice were generated by homologous recombination (5). Littermates of wild-type and synapsin I mutant mice were used in all of the analyses. Only male mice were used to avoid any variation caused by the estrous cycle of female mice. Except for glutamate release assays, all analyses were carried out by investigators without any knowledge of the genotype of the animal.Electron Microscopy. Wild-type (n = 2) and synapsin I-deficient mice (n = 3) were anesthetized with pentobarbital (40 mg/kg; i.p.) and perfused transcardially with Tyrode's solution followed by 3% (vol/vol) glutaraldehyde/0.5% paraformaldehyde in 0.1 M sodium phosphate-buffered saline (pH 7.4). Spinal cords and brains were dissected and postfixed in the same fixative for 4 h. Segments L4 and L5 of the spinal cord were cut into 60-to 100-,um transverse sections and hippocampi were cut into longitudinal sections. The sections were postfixed in 1% osmium tetroxide, dehydrated in alcohol, and embedded in Durcupan. Semithin (1 ,um) and ultrathin ("silver") sections were cut from the tissue blocks on an ultratome. The semithin sections were mounted on glass slides and counterstained with cresyl violet for light microscopic analysis. The ultrathin sections were mounted on Formvarcoated copper grids, counterstained with uranyl acetate and lead citrate, and examined in a Philips CM12 electron microscope. To compare the structural organization of synapses between the two groups of animals, we focused on one