CHAPTER 8 – Release of Neurotransmitters

This chapter describes the release of neurotransmitters. Chemical synapses permit one neuron to excite or inhibit the activity of another cell. A diversity of transmitters and receptors guarantees a multiplicity of postsynaptic responses. The opportunity for presynaptic and postsynaptic interactions among inputs provides complex computational capabilities. The packaging of transmitter into vesicles and its release in quanta enable a single action potential to secrete hundreds of thousands of molecules of transmitter almost instantaneously at a synapse onto another cell. Ca 2+ acts as an intracellular messenger tying the electrical signal of presynaptic depolarization to the act of neurosecretion. The release of slow transmitters depends linearly on (Ca 2+ ) and may be governed by a Ca 2+ -sensitive rate-limiting step different from that triggering the exocytosis of docked vesicles at fast synapses. The life of the synaptic vesicle involves much more than just the Ca 2+ -dependent fusion of a vesicle with the plasma membrane. It is a cyclical progression that includes endocytosis, transmitter loading, docking, and priming steps as well. The interaction of the vesicular and plasma membrane proteins of the SNARE complex—VAMP/synaptobrevin, syntaxin, and synaptosomal-associated protein (SNAP)-25—is an essential, late step in fusion. The short-term synaptic plasticity allows synaptic strength to be modulated as a function of prior activity. A frequency-dependent increase in synaptic efficacy is due to the effects of residual presynaptic Ca 2+ acting to modulate the release process.