CBNS 120 Synaptic Transmission III: Synaptic Mechanisms & Quantal Release

The squid giant synapse (stellate ganglion) revealed that:

Presynaptic depolarization triggers transmitter release; Ca2+ entry is required and sufficient.

At the squid giant synapse, TTX and TEA block:

Na+ and K+ currents, isolating presynaptic Ca2+ current.

Injecting caged Ca2+ (Nitrophen) and photolyzing it with UV light:

Triggers transmitter release without presynaptic depolarization (Ca2+ is sufficient).

BAPTA (fast Ca2+ chelator) blocks release; EGTA (slow Ca2+ chelator) has less effect, indicating:

Ca2+ entry sites are within 100 nm of release sites (microdomains).

Quantal release refers to:

Neurotransmitter release in small packets (quanta) of several thousand molecules via synaptic vesicle exocytosis.

Spontaneous miniature end-plate potentials (MEPPs) are:

~1 mV depolarizations caused by release of a single quantum of ACh.

Fatt & Katz (1952) discovered MEPPs and concluded:

MEPPs are caused by quantal ACh release; they are not due to single ACh molecules.

Lowering extracellular Ca2+ causes:

Reduced evoked EPP amplitude and stepwise fluctuations (0, 1, 2, or 3 times MEPP amplitude) due to reduced quantal release probability.

The Poisson distribution applies to quantal release because:

Release is probabilistic; each quantum has a low probability of being released (deCastillo & Katz, 1954).

Freeze-fracture EM (Heuser et al., 1979) captured:

Synaptic vesicles in the act of fusing with the presynaptic membrane (using the "slammer" cryopress).

4-aminopyridine (4-AP, K+ channel blocker) increases quantal release by:

Broadening the presynaptic AP.

Kuffler & Yoshikami (1975) estimated that one quantum contains approximately:

7,000 ACh molecules (by ionophoretic pulse mimicking a MEPP).

One quantum (MEPP) activates approximately:

1,300 AChR channels (corresponding to 2,600 ACh molecules).

The SNARE complex (vesicle fusion) consists of:

V-SNARE: synaptobrevin, synaptotagmin (Ca2+ sensor); T-SNARE: syntaxin, SNAP-25.

Ca2+ binding to synaptotagmin triggers fusion by:

Causing it to bind to the SNARE complex and membrane phospholipids.

Vesicle recycling occurs via:

Endocytosis (clathrin triskelions form latices; dynamin pinches off the vesicle).

Ribbon synapses (retinal photoreceptors, bipolar cells, hair cells) exhibit:

Continuous glutamate release modulated by graded potentials (not APs).