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LECTURE 11: SYNAPSES IV: TRANSMITTER SYNTHESIS AND RELEASE
REQUIRED READING: Kandel text, Chapters 14, 15
Giant synapse of squid used in classical experiments to determine
the mechanism of chemical synaptic transmission
Voltage Recording
Current Injecting
Pre
Post
Stimulating
Electrode
Voltage
Recording
EPSP REQUIRES THRESHOLD PRESYNAPTIC DEPOLARIZATION
EPSP DOES NOT SPECIFICALLY REQUIRE PRESYNAPTIC SODIUM OR POTASSIUM FLUXES
EPSP COMMENCES ALMOST COINCIDENT WITH PRESYNAPTIC CALCIUM INFLUX
VOLTAGE-GATED CALCIUM CHANNELS ARE CONCENTRATED AT PRESYNAPTIC TERMINAL
QUANTAL RELEASE OF NEUROTRANSMITTER AT NEUROMUSCULAR JUNCTION:
STIMULUS-EVOKED mEPSPs ARE MULTIPLES OF SPONTANEOUS MINIATURE mEPSPs
QUANTAL RELEASE OF NEUROTRANSMITTER:
PRESYNAPTIC STIMULUS INDUCES RAPID SYNAPTIC VESICLE EXOCYTOSIS
QUANTAL RELEASE OF NEUROTRANSMITTER:
CALCIUM INFLUX AND VESICLE EXOCYTOSIS ARE NEARLY SIMULTANEOUS
Membrane capacitance can be continuously sampled by sinusoidal voltage clamp
Command
Vm
Ipipet
Cm
QUANTAL RELEASE OF NEUROTRANSMITTER:
VESICLE EXOCYTOSIS AND NEUROTRANSMITTER RELEASE ARE NEARLY SIMULTANEOUS
CALCIUM-MEDIATED SYNAPTIC VESICLE FUSION:
REGULATION OF A BASAL VESICLE FUSION MACHINERY
SYNAPSIN AND SYNAPTOTAGMIN ARE MEDIATORS OF CALCIUM REGULATION
Synapsins restrain vesicles in a reserve pool. Synapsin phosphorylation by
calcium/CAM-dependent protein kinase releases synapsin from vesicles.
Synaptotagmin can bind to t-SNARE proteins SNAP25 and syntaxin,
and also binds phospholipids in a calcium-dependent manner.
MODULATION OF PRESYNAPTIC INTRACELLULAR CALCIUM DURING
AXONAL ACTION POTENTIAL MODIFIES NEUROTRANSMITTER EXOCYTOSIS AND EPSP
An action potential normally produces a transient increase in presynaptic
calcium, which is dissipated by diffusion and calcium buffers. A high-frequency
train of spike (tetanus) saturates the buffering capacity, creating a period of
“potentiation”, where each action potential releases more neurotransmitter.
Short-term potentiation, which does not require new protein synthesis lasts on the
order of minutes.
MODULATION OF PRESYNAPTIC INTRACELLULAR CALCIUM DURING
AXONAL ACTION POTENTIAL MODIFIES NEUROTRANSMITTER EXOCYTOSIS AND EPSP
Small changes in resting potential of presynaptic terminal dramatically affect
resting [Ca+2]in by altering number of L-type Ca channels open at rest.
These sub-threshold calcium levels combine with action potential-induced
cacium influx in determining amount of synaptic vesicle exocytosis.
L-type Ca channels regulated through axo-axonic serotonergic synapses.
Alternatively, axo-axonic synapses can regulate K+ channels to determine
duration of depolarization capable of activating calcium channels.
SMALL-MOLECULE NEUROTRANSMITTERS
Small molecule neurotransmitters are amino acids or metabolic products
(usually of amino acids) generated by neuron-specific enzymes
TRANSPORTERS PACKAGE SMALL-MOLECULE NEUROTRANSMITTERS INTO
SMALL CLEAR VESICLES AND MEDIATE REUPTAKE FROM SYNAPTIC CLEFT
Transporters are targets of many clinical drugs and
drugs of abuse.
Transporter-specific inhibitors (e.g., amphetamines)
mediate prolonged postsynaptic
stimulation by uncleared neurotransmitter.
“False transmitter” is transmitter analog packaged
into vesicles in place of
endogenous transmitter, but with reduced or no
ability to bind NT receptor.
SMALL PEPTIDE NEUROTRANSMITTERS ARE GENERATED BY PEPTIDASES
ACTING ON PACKAGED PROPEPTIDES (PROHORMONES)
Propeptide is inserted into lumen of endoplasmic reticulum by N-terminal signal sequence.
Peptidases act within Golgi to generate peptides, which are budded off
to form large dense-core vesicles.
Many propeptides can give each give rise to multiple peptide transmitters.
Peptidases cleave N-terminal to adjacent pairs of basic residues (arginine, lysine)
DIFFERENCES BETWEEN SMALL-MOLECULE AND PEPTIDE TRANSMITTERS
PROPERTY
Site of synthesis
Mechanism of vesicle loading
Vesicle type
Vesicle localization
NEUROTRANSMITTER TYPE
SMALL-MOLECULE
PEPTIDE
Cytoplasm
Pre-synaptic transporter
Small clear (usually)
Pre-synaptic
Intravesicular
Budding from Golgi
Large dense-core
Diffuse
Exocytic mechanism
Fast
Slow
Vesicle recycling
Yes
No
Neurotransmitter recycling
Yes
No