Mechanisms underlying sustained synaptic release of glutamate

Sammanfattning: Fast communication between neurons in the brain is mediated mainly by amino acidtransmitters, such as glutamate, GABA and glycine. These transmitters are stored in smallsynaptic vesicles which undergo exocytosis at synaptic release sites (active zones) in responseto action potentials. Synaptic transmission often occurs at very high rates, thus making highdemands on the efficiency of the release machinery. This thesis explores two mechanisms which are of importance in sustaining transmitterrelease at glutamatergic synapses. The first concerns the organization of synaptic vesicles atrelease sites. The major pool of synaptic vesicles is shown to depend on synapsin for itsorganization, while a small pool located in the vicinity of the presynaptic membrane isindependent of synapsin. Disruption of the synapsin-dependent vesicle pool by presynapticantibody injection in lamprey or gene knockout in mice, was found to impair sustainedglutamate release, whereas release at low rates was unaffected. The second mechanism concerns the supply of neurotransmitter to synaptic vesicles.The synthesis and turnover of transmitter glutamate is shown to be linked to the pattern ofrelease activity. Quantitative immunogold analysis of lamprey and crayfish synapses showedthat the axoplasmic matrix and mitochondria in nerve terminals adapted to high levels of activity(tonic synapses) contain a higher glutamate concentration than in those adapted to low levels(phasic synapses). This will ensure a rapid refilling of the synaptic vesicles during periods ofsustained release. The concentration of transmitter glutamate in synaptic vesicles was estimatedto be more than 60 mM. In addition, the uptake of glutamate analogs into perisynaptic glialprocesses was found to be more efficient in tonic than in phasic synapses and the level of amajor glutamate precursor, glutamine, was also found to be higher in tonic synapses.Key words: synaptic transmission, synaptic vesicle proteins, synapsin, glutamate,glutamine, glutamate uptake, glia, spinal cord, hippocampus, neuromuscular junction,lamprey, crayfish, mouse.ISBN 91-628-2150-4Repro Print ABStockholm 1996