LECTURE 11: SYNAPSES IV: TRANSMITTER SYNTHESIS AND RELEASEREQUIRED READING: Kandel text, Chapters 14, 15

Giant synapse of squid used in classical experiments to determinethe mechanism of chemical synaptic transmission

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Voltage Recording

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Current Injecting

StimulatingElectrode

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

CommandVm

Ipipet

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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 bycalcium/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 DURINGAXONAL 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.

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.

MODULATION OF PRESYNAPTIC INTRACELLULAR CALCIUM DURINGAXONAL ACTION POTENTIAL MODIFIES NEUROTRANSMITTER EXOCYTOSIS AND EPSP

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

postsynapticstimulation 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 PEPTIDASESACTING 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

NEUROTRANSMITTER TYPEPROPERTY SMALL-MOLECULE PEPTIDE

Site of synthesis Cytoplasm Intravesicular

Mechanism of vesicle loading Pre-synaptic transporter Budding from Golgi

Vesicle type Small clear (usually) Large dense-core

Vesicle localization Pre-synaptic Diffuse

Exocytic mechanism Fast Slow

Vesicle recycling Yes No

Neurotransmitter recycling Yes No