L10-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick Lecture 10. Release of Transmitter Summary: At the pre-synaptic terminal, the release of neurotransmitter is very carefully regulated by a number of processes. The ultimate trigger for release is an increase in intracellular calcium, usually via the opening of voltage-dependent calcium channels. Release is in quantal packets, as a result of fusion of synaptic vesicles with the plasma membrane and release of their contents to the synaptic cleft. Release involves a number of calcium-dependent events, including vesicle transport and docking at the membrane, priming and formation of the SNARE complex, and a final fusion step probably mediated by calcium binding to the protein synaptotagmin. There is considerable plasticity in the amount of transmitter released per action potential. A terminal can briefly remember a previous release process such that the next spike can cause increased (facilitation) or decreased (depression) release. Further examples of synaptic plasticity will be considered in the next lecture. Learning Objectives 1. To understand how calcium is the ultimate trigger for release of transmitter. 2. To see that synaptic vesicles filled with neurotransmitter are the quanta for the quantal release of transmitter, and that the EPSP arises from the nearly simultaneous fusion of many quanta and the exocytosis of their transmitter contents into the synaptic cleft. 3. To begin to understand the many molecules that are involved in the regulation and control of exocytosis, with each protein involved at a different stage in the multi-step pathway leading up to release of transmitter 4. To start to understand the neural mechanisms for plasticity in transmitter release, in particular understanding how synaptic facilitation and depression arise from events occurring during a previous action potential. Reading Assignment Purves, et al., Chapter 5, especially pp. 94-107. Purves et al., Chapter 8, especially pp. 177-180. Lecture Outline A. Ionic requirements for Release In past lectures, we have explored the post-synaptic actions of neurotransmitters, and seen how the post-synaptic cell integrates its many inputs. In this lecture, we explore the mechanisms for release of chemical transmitters from the pre- synaptic terminal. This is important, as the amount of transmitter released by an action potential is highly variable, and this variability underlies interesting changes in behavior.
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BioNB222 Spring 2010 Cornell University Ronald Harris-Warrick A first question regards the actual trigger for release. The action potential propagates to the nerve terminal, with sodium rushing in to depolarize the cell and potassium rushing out to terminate the spike. Neither of these ionic currents is directly related to release. Instead, it is the depolarization of the action potential that is responsible for the next step: it opens voltage-sensitive calcium channels. Calcium flows down a very steep electrochemical gradient into the
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L10-2010 - BioNB222 Cornell University Spring 2010 Ronald...

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