Synaptic transmission: communication between neurons

Two principal kinds of synapses: electrical and chemical

Gap junctions are formed where hexameric pores called connexons connect with one between cells

Electrical synapses are built for speed

Contrast with chemical synapse

Delay of about 1 ms

Electrical coupling is a way to synchronize neurons with one another

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

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.edu/michael.sanderson/mjslab/MOVIE.HTM

Chemical synapses: the predominant means of communication between neurons

An early experiment to support the neurotransmitter hypothesis

Criteria that define a neurotransmitter:1. Must be present at presynaptic terminal2. Must be released by depolarization, Ca++-dependent3. Specific receptors must be present

Neurotransmitters may be either small molecules or peptides

Mechanisms and sites of synthesis are different

Small molecule transmitters are synthesized at terminals, packaged into small clear-core vesicles (often referred to as ‘synaptic vesicles’

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

Neurotransmitter is released in discrete packages, or quanta

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

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

4-AP was used to vary the efficiency of release

Calcium influx is necessary for neurotransmitter release

Voltage-gated calcium channels

Calcium influx is sufficient for neurotransmitter release

Synaptic release II

The synaptic vesicle release cycle

1. Tools and Pools

2. Molecular biology and biochemistry of vesicle release:1. Docking2. Priming3. Fusion

3. Recovery and recycling of synaptic vesicles

The synaptic vesicle cycle

How do we study vesicle dynamics?

Morphological techniquesElectron microscopy to obtain static pictures of vesicle distribution; TIRFM (total internal reflection fluorescence microscopy) to visualize movement of vesicles close to the membranePhysiological studies Chromaffin cellsNeuroendocrine 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)

NeuronsMeasure release of neurotransmitter from a presynaptic cell by quantifying the response of a postsynaptic cell

GeneticsDelete or overexpress proteins in mice, worms, or flies, and analyze phenotype using the above techniques

Synaptic vesicle release consists of three principal steps:

1. DockingDocked vesicles lie close to plasma membrane (within 30 nm)

1. PrimingPrimed vesicles can be induced to fuse with the plasma membrane by sustained depolarization, high K+, elevated Ca++, hypertonic sucrose treatment

2. FusionVesicles 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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Becherer, U, Rettig, J. Cell Tissue Res (2006) 326:393

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.

Synaptic vesicles exist in multiple pools within the nerve terminal

(reserve pool)

(Release stimulated by flash-photolysis of caged calcium)

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 6:57-69)

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

Vesicle release requires many proteins on vesicle and plasma membrane

Docking:

UNC-18 (or munc-18) is necessary for vesicle docking(Weimer et al. 2003, Nature Neuroscience 6:1023)

1. unc-18 mutant C. elegans have neurotransmitter release defect

2. unc-18 mutant C. elegans have reduction of docked vesicles

Unc-18 mutants are defective for evoked and spontaneous release

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

UNC-18 (munc18) is required for docking: unc-18 mutants have fewer docked vesicles

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.

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

The SNARE complex

UNC-13 is a critical priming factorRichmond and Jorgensen (1999) Nature Neuroscience 2:959

normal unc-13 mutantsunc-13 mutants have higher levels of synaptic vesicles than normal

No docking defect was observed

unc-13 mutants have evoked release defect

Calcium-indepenent release is also defective, indicating that the defect is in priming

Munc-13 function in priming

Inhibitory domain, folds back on itself“open” syntaxin doesn’t fold properly

unc-13 defect can be bypassed by providing an “open” form of syntaxin

Model for unc-13, unc-18, syntaxin interaction in priming

Synaptotagmin functions as a calcium sensor, promoting vesicle fusion

Synaptic vesicles recycle post-fusion

Modern methods to track recycling membrane

The ATP-ase NSF disassembles the SNARE complex

Endocytosis retrieves synaptic vesicle membrane and protein from the plasma membrane following fusion