Synaptic transmission communication between neurons Two principal kinds

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 terminal 2. Must be released by depolarization, Ca++-dependent 3. 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. Docking 2. Priming 3. Fusion 3. Recovery and recycling of synaptic vesicles

The synaptic vesicle cycle

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

Synaptic vesicle release consists of three principal steps: 1. Docking 2. Docked vesicles lie close to plasma membrane (within 30 nm) 3. Priming 1. Primed vesicles can be induced to fuse with the plasma membrane by sustained depolarization, high K+, elevated Ca++, hypertonic sucrose treatment 4. Fusion 5. 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. 6. .

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) 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.

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 (munc 18) 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 readilyreleasable pool At a molecular level, priming corresponds to the assembly of the SNARE complex

The SNARE complex

UNC-13 is a critical priming factor Richmond and Jorgensen (1999) Nature Neuroscience 2: 959 unc-13 mutants have higher levels of synaptic vesicles than normal unc-13 mutants No docking defect was observed