Synaptic Transmission

Neurotransmitter-containing vesicles in the presynaptic terminal are recycled
After fusing with the active zone membrane clathrin coated vesicles are endocytosed (1). Vesicles may be directly recycled to a readily releasable pool (2-3-4) or fuse with an endosome to allow breakdown of fusion apparatus protein. Vesicular transmembrane pumps fill vesicles with neurotransmitter (3, green) and vesicles are redocked and primed to allow Ca2+ (red) triggered exocytosis (6) and release of neurotransmitter.

 

Exocytotic release of a neurotransmitter is an essential building block of neuron to neuron communication. Neurotransmitter is stored in membrane bound vesicles and is released through exocytosis. Vesicular membrane is fused with the cell membrane during exocytosis is thought to be continuously recycled through endocytosis nearly as quickly (Betz and Angleson, 1998). The protein machinery which drives and regulates exocytosis in vertebrate neurons is remarkably conserved and shares much in common even with exocytotic machinery of yeast (Golding, 1994). Many proteins are involved neurotransmitter secretion, though whether they play an essential or a regulatory role is not always known. The best characterized of these comprise the 'core complex', the formation of which is necessary for fusion. The core complex, or soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), is a bundle of four a-helices approximately 65 amino acids in length. These a-helices are donated by 3 different proteins; a family member from each of the syntaxin and SNAP-25 families, both at the plasma membrane, and a VAMP (synaptobrevin) family member, located in the synaptic vesicular membrane. These three proteins form a sodium dodecyl-sulfate-resistant coiled-coil structure (SNARE; (Sutton et al., 1998)). This parallel a-helical structure is thought to bridge the synaptic vesicle and plasma membranes. The syntaxins bind to many regulatory proteins (Wu et al., 1999); perhaps the most notable is synaptotagmin, an important regulator of fusion. The SNAP-25 (synaptosomal-associated protein of 25 kDa) family of proteins is thought to associate with the plasma membrane via palmitoyl groups attached at the center of the protein and contributes two a-helices to the core complex. SNAP-25 also binds synaptotagmin (Gerona et al., 2000). The VAMP (synaptobrevin) family of proteins are 18-20 kDa integral membrane proteins and may exist in a bound state with other regulatory proteins before they participate in SNARE formation (Augustine et al., 1996). The core complex is sufficient to mediate fusion of lipid micelles in vitro (Weber et al., 1998) and fusion of the synaptic vesicle with the plasma membrane requires the interaction of syntaxin, SNAP-25 and VAMP. Synaptic vesicular fusion is inhibited by proteolytic cleavage of core complex proteins by clostridial neurotoxins (botulinum and tetanus toxins). However, once in the form of a core complex they are resistant to cleavage by these neurotoxins (Hayashi et al., 1994). SNAREs clearly figure prominently in the rapid orchestration of proteins associated with the synaptic vesicle and plasma membrane to cause fusion of the primed vesicle with the cell membrane of the terminal and release of transmitter (Sudhof, 1995; Augustine et al., 1996). While secretion is a ubiquitous cellular process, synaptic exocytosis has several unique properties. For synapses to signal rapidly (in µseconds) vesicles must be located very near to the point of fusion (active zone). This initial contact between the vesicular and plasma membranes is termed docking. Multi-step fusion reactions are ruled out due to the speed of release. Thus, it is believed that there is a pool of ready-to-fuse vesicles that have undergone a further maturation step termed priming. In neuroendocrine cells this requires ATP, submicromolar Ca2+ and alterations in membrane lipids by lipid transferases and kinases (Hay and Martin, 1992; Martin et al., 1995; Hay et al., 1995; Parsons et al., 1995; Vautrin et al., 1993). Electrophysiological studies in chromaffin cells support the priming hypothesis (Schneggenburger et al., 1999; Lukyanetz and Neher, 1999). Action potential invasion of the presynaptic terminal evokes neurotransmitter release. Depolarization opens voltage-operated Ca2+ channels of more than one subtype (Neher and Zucker, 1993; Huston et al., 1995), which may vary from synapse to synapse (Uchitel et al., 1992; Stanley, 1993). Ca2+ then binds to one or more closely associated Ca2+-binding proteins with a low affinity (Augustine et al., 1991; Llinas, 1991). The brain specific synaptotagmins are suspected to play a large role in Ca2+ sensing necessary to initiate evoked fusion due to their ability to bind syntaxin, SNAP-25, Ca2+ and phospholipids. Although mutagenesis studies in C. elegans and Drosophila support this idea, they also indicate that other proteins may act, along with synaptotagmins, as part of the Ca2+ sensor (DiAntonio et al., 1993; Nonet et al., 1993; Geppert et al., 1994).