Neurons, membranes synaptic vesicle fusion with

Fig. 1 Structure of the neuronal SNAREs. Upper panel domain structure of the three neuronal SNARE proteins involved in synaptic vesicle fusion. Syntaxin 1A and SNAP-25 (contains two SNARE motifs) are associated with the presynaptic membrane, whereas synaptobrevin 2 is synaptic vesicle associated. The SNARE motifs form a stable complex (core complex) whose crystal structure has been analyzed (lower panel). In the complex, each of the SNARE motifs adopts an alpha-helical structure, and the four alpha-helices are aligned in parallel forming a twisted bundle (modified from Sutton et al. 1998). Stability of the complex is mediated by layers of interaction (—7 to +8) in which amino acids from each of the four alpha-helices participate (see text).

The process of information flow between neurons is termed synaptic transmission, and in its most basic form it is characterized by unidirectional communication from the presynaptic to postsynaptic neuron. The process begins with the initiation of an electrical impulse in the axon of the presynaptic neuron. This electrical signal—the action potential—propagates to the axon terminal, which thereby stimulates the fusion of a transmitter-fllled synaptic vesicle with the presynaptic terminal membrane. The process of synaptic vesicle fusion is highly regulated and involves numerous biochemical reactions it culminates in the release of chemical neurotransmitter into the synaptic cleft. The released neurotransmitter diffuses across the cleft and binds to and activates receptors on the postsynaptic site, which thereby completes the process of synaptic transmission. 

Neuronal action potentials (APs), when they arrive to the nerve terminal, stimulate the fusion of the synaptic vesicles with the cellular membrane, and NE is released into the synaptic space. Certain compounds, such as methamphetamines or bupropion, increase NE release via an AP-independent release mechanism (Dong and Blier 2001 Piacentini et al., 2003). 

In neurons, the SNARE complex consists of three main proteins the v-SNARE synaptobrevin or VAMP (vesicle-associated membrane protein), and two t-SNAREs, syntaxin and SNAP-25 (synaptosomal associated protein of 25 kD). Synaptobrevins traverse the synaptic vesicle membrane in an asymmetric manner a few amino acids are found inside the vesicle, but most of the molecule lies outside the vesicle, within the cytoplasm. Synaptobrevin makes contact with another protein anchored to the plasma membrane of the presynaptic neuron, syntaxin, which is associated with SNAP-25. Via these interactions, the SNARE proteins play a role in the docking and fusion of synaptic vesicles to the active zone. 

The exocytosis of neurotransmitters from synaptic vesicles Involves targeting and fusion events similar to those that lead to release of secreted proteins In the secretory pathway. However, several unique features permit the very rapid release of neurotransmitters In response to arrival of an action potential at the presynaptic axon terminal. For example. In resting neurons some neurotransmitter-fllled synaptic vesicles are docked at the plasma membrane others are In reserve In the active zone near the plasma membrane at the synaptic cleft. In addition, the membrane of synaptic vesicles contains a specialized Ca -blndlng protein that senses the rise In cytosolic Ca " after arrival of an action potential, triggering rapid fusion of docked vesicles with the presynaptic membrane. 

In addition, cholesterol is required for the fusion of synaptic vesicles with the presynap-tic membrane. Consistently, reducing cholesterol levels in cultured hippocampal neurons impaired vesicles exocytosis, and the effect was reversed by cholesterol reloading. The... 

Mechanism of action. The cellular actions of bot-ulinum toxin at the neuromuscular junction have recently been clarified.84 This toxin is attracted to glycoproteins located on the surface of the presynaptic terminal at the skeletal neuromuscular junction.33 Once attached to the membrane, the toxin enters the presynaptic terminal and inhibits proteins that are needed for acetylcholine release (Figure 13-4).84 Normally, certain proteins help fuse presynaptic vesicles with the inner surface of the presynaptic terminal, thereby allowing the vesicles to release acetylcholine via exocytosis. Botulinum toxin cleaves and destroys these fusion proteins, thus making it impossible for the neuron to release acetylcholine into the synaptic cleft.32,84 Local injection of botulinum toxin into specific muscles will therefore decrease muscle excitation by disrupting synaptic transmission at the neuromuscular junction. The affected muscle will invariably undergo some degree of paresis and subsequent... 

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