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Lecture

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School
Wilfrid Laurier University
Department
Psychology
Course
PS271
Professor
Leanne Hagarty
Semester
Fall

Description
Principles of Chemical Synaptic Transmission  Neurotransmitters o Most neurotransmitters fall into one of three chemical categories:  Amino acids  Amines  Peptides o The amino acid and amine neurotransmitters are all small organic molecules containing at least one nitrogen atom, and they are stored in and released from synaptic vesicles. o Peptide neurotransmitters are large molecules stored in and released from secretory granules  Secretory granules and synaptic vesicles are frequently observed in the same axon terminals o Different neurons in the brain release different neurotransmitters  Fast synaptic transmission at most CNS synapses is mediated by the amino acids glutamate (Glu), gamma-aminobutyric acid (GABA) and glycine (Gly).  The amine acetylcholine (Ach) mediates fast synaptic transimission at all neuromuscular junctions. Slower forms of synaptic transmission in the CNS and in the periphery are mediated by transmitters from all three chemical categories.  Neurotransmitter Synthesis and Storage o Chemical synaptic transmission requires that neurotransmitters be synthesized and ready for release. o The synthesizing enzymes for both amino acid and amine neurotransmitters are transported to the axon terminal, where they locally and rapidly direct transmitter synthesis. o Once synthesized in the cytosol of the axon terminal, the amino acid and amin neurotransmitters must be taken up by the synaptic vesicles.  Concentrating these neurotransmitters is the job of transporters o Different mechanisms are used to synthesize and store peptides in secretory granules  This occurs in the rough ER  Generally, a long peptide synthesized in the rough ER is split in the Golgi apparatus, and of the smaller peptide fragments is the active neurotransmitter.  Secretory granules containing the peptide neurotransmitter bud off the Golgi apparatus and are carried to the axon terminal by axoplasmic transport  Neurotransmitter Release o Neurotransmitter release is triggered by the arrival of an action potential in the axon terminal o The depolarization of the terminal membrane causes voltage-gated calcium channels in the active zones to open.  These membrane channels are very s2+ilar to the sodium channels o There is a large inward driving force on Ca o The resulting elevation in [Ca ]Iis the signal that causes neurotransmitter to be released from synaptic vesicles o The vesicle releases their contents by a process called exocytosis. o The membrane of the synaptic vesicle fuses to the presynaptic membrane at the active zone, allowing the contents of the vesicle to spill out into the synaptic cleft (See Fig. 5.11.). o Exocytosis is quick because Ca 2+enters at the active zone, precisely where synaptic vesicles are ready and waiting2+o release their contents o The precise mechanism by which [Ca ] stimIlates exocytosis is poor understood o The speed of neurotransmitter release suggests that the vesicles involved are those at ready “docked” at the active zones.  Docking is believed to involved interactions between proteins in the synaptic vesicle membrane and the active zone  In the presence of high [Ca ], Ihese proteins alter their conformation so that the lipid bilayers of the vesicle and presynaptic membranes fuse, forming a pore that allows the neurotransmitter to escape into the cleft.  The mouth of this exocytotic fusion pore continues to expand until the membrane of the vesicle is fully incorporated into the presynaptic membrane  The vesicle membrane is later recovered by the process of endocytosis and the recycle vesicle is refilled with neurotransmitter o During periods of prolonged stimulation, vesicles are mobilized from a “reserve pool” that is bound to the cytoskeletons of the axon terminal  The release of these vesicles from the cytoskeleton, an2+their docking to the active zone, is also triggered by elevations of [Ca ].i o Secretory Granules also release peptide neurotransmitters by exocytosis, in a calcium –dependent fashion, but not at the active zones  Because the sites of granule exocytosis occur at a distance from the sites 2+ of Ca entry, peptide neurotransmitters are usually not released in response to every action potential invading the terminal.  Release of peptides generally requires high-frequency trains of action potentials, so that the [Ca ] throughout the terminal can I build to the level required to trigger release away from the active zones  Neurotransmitter Receptors and Effectors o Transmitter-Gated Ion Channels  Receptors known as transmitter-gated ion channels are membrane- spanning proteins consisting of four or five subunits that come together to form a pore between them.  In the absence of neurotransmitter, the pore is usually closed.  When neurotransmitter binds to specific sites on the extracellular region of the channel, it induces a conformational change which opens the pore.  Transmitter-gated channels generally do not show the same degree of ion selectivity as do voltage-gated channels (e.g. Ach-gated ion channels are permeable to both Na ions and K ions).  As a rule, if the open channels are permeable to Na , the net effect will be to depolarize the po
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