Each presynaptic terminal of a chemical synapse is separated from the postsynaptic structure by a synaptic cleft that is 20–40 nm wide. Across the synaptic cleft are many neu rotransmitter receptors in the postsynaptic membrane, and usually a postsynaptic thickening called the postsynaptic density (Figures 1 and 2). The postsynaptic density is an ordered complex of specific receptors, binding proteins, and enzymes induced by postsynaptic effects.
Fig1. Electron micrograph of synaptic knob (S) ending on the shaft of a dendrite (D) in the central nervous system. P, postsynaptic density; M, mitochondrion (×56,000).
Fig2. Axodendritic, axoaxonal, and axosomatic synapses. Many presynaptic neurons terminate on dendritic spines, as shown at the top, but some also end directly on the shafts of dendrites. Note the presence of clear and granulated synaptic vesicles in endings and clustering of clear vesicles at active zones.
Inside the presynaptic terminal are many mitochondria, as well as many membrane-enclosed vesicles, which contain neurotransmitters. There are three kinds of synaptic vesicles: small, clear synaptic vesicles that contain acetylcholine, glycine, GABA, or glutamate; small vesicles with a dense core that contain catecholamines; and large vesicles with a dense core that contain neuropeptides. The vesicles and the proteins contained in their walls are synthesized in the neuronal cell body and transported along the axon to the endings by fast axoplasmic transport. The neuropeptides in the large dense-core vesicles must also be produced by the protein synthesizing machinery in the cell body. However, the small clear vesicles and the small dense-core vesicles recycle in the nerve ending. These vesicles fuse with the cell membrane and release transmitters through exocytosis and are then recovered by endocytosis to be refilled locally. In some instances, they enter endosomes and are budded off the endosome and refilled, starting the cycle over again. The steps involved are shown in Figure3. More commonly, however, the synaptic vesicle discharges its contents through a small hole in the cell membrane, then the opening reseals rapidly and the main vesicle stays inside the cell (kiss-and-run discharge). In this way, the full endocytotic process is short-circuited.
Fig3. Small synaptic vesicle cycle in presynaptic nerve terminals. Vesicles bud off the early endosome and then are filled with a neurotransmitter (NT). The vesicles then translocate toward release sites on the plasma membrane where they dock, and then become primed. Following arrival of an action potential at the nerve ending, Ca2+ influx triggers fusion and exocytosis of the vesicular contents into the synaptic cleft. The vesicle wall then is coated with clathrin, which likely facilitates endocytosis. Once back inside the cytoplasm, the vesicle becomes uncoated and fuses with the early endosome to start the cycle again.
The large dense-core vesicles are located throughout the presynaptic terminals that contain them and release their neuropeptide contents by exocytosis from all parts of the terminal. On the other hand, the small vesicles are located near the synaptic cleft and fuse to the membrane, discharging their contents very rapidly into the cleft at areas of membrane thickening called active zones (Figure 2). The active zones contain many proteins and rows of Ca2+ channels.
The Ca2+ that triggers exocytosis of transmitters enters the presynaptic neurons, and transmitter release starts within 200 μs. Therefore, it is not surprising that the voltage-gated Ca2+ channels are very close to the release sites at the active zones. In addition, the transmitter must be released close to the postsynaptic receptors to be effective on the postsynaptic neuron. This orderly organization of the synapse depends in part on neurexins, the proteins bound to the membrane of the presynaptic neuron that bind neurexin receptors in the membrane of the postsynaptic neuron. In many vertebrates, neurexins are produced by a single gene that codes for the α isoform. However, in mice and humans they are encoded by three genes, and both α and β isoforms are produced. Each of the genes has two regulatory regions and extensive alternative splicing of their mRNAs. In this way, over 1000 different neurexins are produced. This raises the possibility that the neurexins not only hold synapses together, but also provide a mechanism for the production of synaptic specificity.
As noted in Chapter 2, vesicle budding, fusion, and dis charge of contents with subsequent retrieval of vesicle mem brane are fundamental processes occurring in most, if not all, cells. Thus, neurotransmitter secretion at synapses and the accompanying membrane retrieval are specialized forms of the general processes of exocytosis and endocytosis. The fusion of synaptic vesicles with the cell membrane involves the action of several proteins, including the N-ethylmaleimide sensitive fusion protein (NSF), SNAPs (soluble NSF attachment proteins), and SNAREs (SNAP receptors; Figure 4). Synaptobrevin in the vesicle membrane locks with syntaxin and SNAPs in the cell membrane; GTPases regulate a multiprotein complex that includes Rab and Sec1/Munc18-like proteins as part of the fusion process. The one-way gate at the synapses is necessary for orderly neural function.
Fig4. Main proteins that interact to produce synaptic vesicle docking and fusion in nerve endings. Several proteins are involved in synaptic vesicle docking and fusion, including the N-ethylmaleimide–sensitive fusion protein (NSF), SNAPs (soluble NSF attachment proteins), and SNAREs (SNAP receptors). Synaptobrevin in the vesicle membrane links with syntaxin and SNAPs in the cell membrane; GTPases regulate a multiprotein complex that includes Rab and Sec1/Munc18-like proteins.
Several deadly toxins that block neurotransmitter release are zinc endopeptidases that cleave and hence inactivate proteins in the fusion-exocytosis complex. Clinical Box 1 describes how neurotoxins from bacteria called Clostridium tetani and Clostridium botulinum can disrupt neurotransmitter release in either the CNS or at the neuromuscular junction.
CLINICAL BOX 1
