Synaptic transmission: communication between neurons

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Synaptic transmission communication between
neurons
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Two principal kinds of synapses electrical and
chemical
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Gap junctions are formed where hexameric pores
called connexons connect with one between cells
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Electrical synapses are built for speed
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Contrast with chemical synapse
Delay of about 1 ms
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Electrical coupling is a way to synchronize
neurons with one another
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Electrical synapses are not presently considered
to be the primary means of communication between
neurons in the mammalian nervous system, but they
may prove to be more important than presently
recognized
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Rectification and uni-directionality of
electrical synapses not just simple
bidirectional bridges between cells Conductance
through gap junctions may be sensitive to the
junctional potential (i.e the voltage drop
between the two coupled cells), or sensitive to
the membrane potential of either of the coupled
cells
Glial cells can also be connected by gap
junctions, which allows synchronous oscillations
of intracellular calcium http//users.umassmed.e
du/michael.sanderson/mjslab/MOVIE.HTM
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Chemical synapses the predominant means of
communication between neurons
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An early experiment to support the
neurotransmitter hypothesis
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• Criteria that define a neurotransmitter
• Must be present at presynaptic terminal
• Must be released by depolarization,
  Ca-dependent
• Specific receptors must be present

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Neurotransmitters may be either small molecules
or peptides Mechanisms and sites of synthesis
are different
Peptides, or neuropeptides are synthesized in the
endoplasmic reticulum and transported to the
synapse, sometimes they are processed along the
way. Neuropeptides are packaged in large
dense-core vesicles
Small molecule transmitters are synthesized at
terminals, packaged into small clear-core
vesicles (often referred to as synaptic vesicles
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Neurotransmitter is released in discrete
packages, or quanta
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Failure analysis reveals that neurons release
many quanta of neurotransmitter when stimulated,
that all contribute to the response
Quantal content The number of quanta released by
stimulation of the neuron
Quantal size How size of the individual quanta
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Quanta correspond to release of individual
synaptic vesicles EM images and biochemistry
suggest that a MEPP could be caused by a single
vesicle EM studies revealed correlation between
fusion of vesicles with plasma membrane and size
of postsynaptic response
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4-AP was used to vary the efficiency of release
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Calcium influx is necessary for neurotransmitter
release
Voltage-gated calcium channels
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Calcium influx is sufficient for neurotransmitter
release
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• Synaptic release II
• The synaptic vesicle release cycle
• Tools and Pools
• Molecular biology and biochemistry of vesicle
  release
• Docking
• Priming
• Fusion
• Recovery and recycling of synaptic vesicles

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The synaptic vesicle cycle
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How do we study vesicle dynamics? Morphological
techniques Electron microscopy to obtain static
pictures of vesicle distribution TIRFM (total
internal reflection fluorescence microscopy) to
visualize movement of vesicles close to the
membrane
Physiological studies Chromaffin
cells Neuroendocrine cells derived from adrenal
medulla with large dense-core vesicles. Can
measure membrane fusion (capacitance
measurements), or direct release of catecholamine
transmitters using carbon fiber electrodes
(amperometry) Neurons Measure release of
neurotransmitter from a presynaptic cell by
quantifying the response of a postsynaptic cell
Genetics Delete or overexpress proteins in mice,
worms, or flies, and analyze phenotype using the
above techniques
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• Synaptic vesicle release consists of three
  principal steps
• Docking
• Docked vesicles lie close to plasma membrane
  (within 30 nm)
• Priming
• Primed vesicles can be induced to fuse with the
  plasma membrane by sustained depolarization, high
  K, elevated Ca, hypertonic sucrose treatment
• Fusion
• Vesicles fuse with the plasma membrane to
  release transmitter. Physiologically this occurs
  near calcium channels, but can be induced
  experimentally over larger area (see priming).
  The active zone is the site of physiological
  release, and can sometimes be recognized as an
  electron-dense structure.
• .

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Synaptic vesicles exist in multiple pools within
the nerve terminal
(Release stimulated by flash-photolysis of caged
calcium)
(reserve pool)
Becherer, U, Rettig, J. Cell Tissue Res (2006)
326393
Morphologically, vesicles are classified as
docked or undocked. Docked vesicles are further
subdivided into primed and unprimed pools
depending on whether they are competent to fuse
when cells are treated with high K, elevated
Ca, sustained depolarization, or hypertonic
sucrose treatment.
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In CNS neurons, vesicles are divided into
Reserve pool (80-95) Recycling pool
(5-20) Readily-releasable pool (0.1-2 5-10
synapses per active zone) Rizzoli, Betz (2005).
Nature Reviews Neuroscience 657-69)
A small fraction of vesicles (the recycling pool)
replenishes the RRP upon mild stimulation.
Strong stimulation causes the reserve pool to
mobilize and be released
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Vesicle release requires many proteins on vesicle
and plasma membrane
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• Docking
• UNC-18 (or munc-18) is necessary for vesicle
  docking
• (Weimer et al. 2003, Nature Neuroscience 61023)
• unc-18 mutant C. elegans have neurotransmitter
  release defect
• unc-18 mutant C. elegans have reduction of docked
  vesicles

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Unc-18 mutants are defective for evoked and
spontaneous release
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Unc-18 mutants are defective for
calcium-independent release
primed vesicles occasionally fuse in the absence
of calcium a calcium-independent fusion defect
suggests a lack of primed vesicles
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UNC-18 (munc18) is required for docking unc-18
mutants have fewer docked vesicles
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Summary Unc-18 mutants are unable to dock
vesicles efficiently. Impaired docking leads to
fewer primed vesicles fewer primed vesicles
leads to reduced overall neurotransmitter release.
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Priming Vesicles in the reserve pool undergo
priming to enter the readily-releasable pool At
a molecular level, priming corresponds to the
assembly of the SNARE complex
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The SNARE complex
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UNC-13 is a critical priming factor Richmond and
Jorgensen (1999) Nature Neuroscience 2959
unc-13 mutants have higher levels of synaptic
vesicles than normal
normal
unc-13 mutants
No docking defect was observed
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unc-13 mutants have evoked release defect
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Calcium-indepenent release is also defective,
indicating that the defect is in priming
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Munc-13 function in priming
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Inhibitory domain, folds back on itself open
syntaxin doesnt fold properly
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unc-13 defect can be bypassed by providing an
open form of syntaxin
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Model for unc-13, unc-18, syntaxin interaction in
priming
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Synaptotagmin functions as a calcium sensor,
promoting vesicle fusion
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Synaptic vesicles recycle post-fusion
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Modern methods to track recycling membrane
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Endocytosis retrieves synaptic vesicle membrane
and protein from the plasma membrane following
fusion
The ATP-ase NSF disassembles the SNARE complex
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