Synaptic Transmission - m-learning.zju.edu.cnm-learning.zju.edu.cn/G2S/eWebEditor/uploadfile/20141126183232244.pdf · Solid evidence given in 1921 by Otto Loewi; 2. B. Katz et al

Synaptic Transmission

Jianhong Luo, Ph.D.

Department of Neurobiology

Zhejiang University School of Medicine

Main Reference: Neuroscience Exploring the Brain, 3rd Ed.

By M.F. Bear, B.W. Connors, and M.A. Paradiso

Introduction

Types of synapses

Electrical synapses

Chemical synapses

Principles of chemical synaptic transmission

- Neurotransmitters (NT)

- Synthesis and storage

- Release

- Receptors and effectors

- Recovery and degradation

- Neuropharmacology

Principles of synaptic integration

The integration of EPSPs

The contribution of dendritic properties

Inhibition

Modulation

Introduction

By the end of 19th century, it was recognized that this transfer of information

from one neuron to another occurs at specialized sites of contact.

（例：疼痛的反应）

Synapse

(1897 Charles Sherrington)

Synaptic transmission – two hypothesis

Argued for a century on its physical nature.

Electrical synapse

(Proven in 1959 by E. Furshpan and D. Potter in crayfish)

Chemical synapse

1. Solid evidence given in 1921 by Otto Loewi;

2. B. Katz et al. demonstrated fast transmission at NMJ

chemically mediated.

3. By 1951, John Eccles studied the synaptic transmission

in the mammalian CNS using the glass microelectrode

4. During the last decade, new methods of studying the

molecules revealed that synapses are far more complex.

A large and fascinating topic

Introduction

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Introduction

Otto Loewi (1873-1961), a German born

pharmacologist, discoverer of acetylcholine,

nobel prize laureate in Physiology or Medicine

in 1936.

His most famous experiment came from his dream in the night

of Easter Sunday, 1921 and found ―vagusstoff‖, turned out to be

acetylcholine, showing that synaptic signaling used chemical

messengers.

//upload.wikimedia.org/wikipedia/commons/9/99/Otto_Loewi_nobel.jpg

//upload.wikimedia.org/wikipedia/commons/c/c6/Vagusstoff2.png

Types of synapses

A synapse is the specialized junction where one part of a neuron

contacts and communicates with another neuron or cell type

(such as a muscle or glandular cell). Information tends to flow in

one direction, from a neuron to its target cell. The first is said to

be presynaptic and the target is said to be postsynaptic.

Electrical synapses (电突触)

Six connexins form a channel (connexon), and two connexons

(one from each cell) form a gap junction channel.

The pore of channels is relatively large with diameter 1–2 nm,

enough for all the major cellular ions, and many small organic

molecules, to pass through directly from the cytoplasm of one cell

to the other’s.

Types of synapses

Neurites of two cells connected by a gap junction.

Six connexin subunits form one connexon, two

connexons form one gap junction channel, and

many channels comprise one gap junction.

Types of synapses

Functional properties of electrical synapses:

Equally pass in both direction

Electrically coupled

Very fast, and if the synapse is large, fail-safe. Thus, an AP in

the presynaptic neuron can produce, almost instantaneously,

an AP in the postsynaptic neuron.

In invertebrate species, such as the crayfish, electrical synapses

are sometimes found between sensory and motor neurons in

neural pathways mediating escape reflexes.

Types of synapses

Electrical synapses also occur in the vertebrate brain.

Are common in every part of the mammalian CNS

Among electrically coupled neurons, AP in the presynaptic

neuron can cause a small amount of ionic current to flow

across the gap junction channels into the other neuron,

producing postsynaptic potential (PSP).

The PSP generated by a single electrical synapse in the

mammalian brain is usually small—about 1 mV or less at its

peak—and may not, by itself, be large enough to trigger an

AP in the postsynaptic cell.

The precise roles of electrical synapses vary from one brain

region to another (synchronize; developmental coordination;

in non-neuron cells).

Box 5-2 by Michael V. L. Bennett

Types of synapses

Electrical synapses. (a) A gap junction coupling the dendrites of two

neurons constitutes an electrical synapse. (b) An AP generated in

one neuron causes a small amount of ionic current to flow through

gap junction channels into a second neuron, inducing an PSP.

Types of synapses

Chemical synapses

General description:

synaptic cleft (20–50 nm ), filled with a matrix of fibrous

extracellular protein. One function of this matrix is to make the pre-

and postsynaptic membranes adhere to each other.

presynaptic element, is usually an axon terminal.

synaptic vesicles (50 nm in diameter), store neurotransmitter

used to communicate with the postsynaptic neuron.

secretory granules (larger vesicles, about 100 nm diameter)

contain soluble protein (dark in EM, large dense-core vesicles)

Membrane differentiations on either side of the synaptic cleft

Active zone looks like pyramid, the sites of NT release

postsynaptic density contains receptors converting signal from

intercellular to intracellular

The components of a chemical synapse.

Types of synapses

Presynaptic axon

Postsynaptic spine

Postsynaptic density

Active zone

Astrocyte

Coated vesicle

Dense-core vesicle

Double-walled vesicle

Endo. Reticulum

Mitochondrion

Punctum adhaerens

Synaptic cleft

Synaptic vesicle

Types of synapses

Different types in of synapse in the CNS (CNS synapses)

The sizes and shapes of CNS synapses also vary widely

Axodendritic, Axosomatic, axoaxonic, dendrodendritic synapses.

Types of synapses

Chemical synapses as seen with EM

(left) A fast excitatorysynapse in the CNS

(right) A synapse in the PNS, with numerous dense-core vesicles

DCV

AZ

Mt

Presynaptic

Postsynaptic

V

Synaptic arrangements in the CNS. (a) An axodendritic synapse.

(b) An axosomatic synapse. (c) An axoaxonic synapse.

Types of synapses

Various sizes of CNS synapses.

Notice that larger synapses

have more active zones.

Types of synapses

Two categories of CNS synaptic membrane differentiations.

(a) A Gray’s type I synapse is asymmetrical and usually excitatory.

(b) A Gray’s type II synapse is symmetrical and usually inhibitory.

Types of synapses

Synaptic junctions also exist outside the central nervous system.

Axons of the autonomic nervous system innervate glands,

smooth muscle, and the heart.

Neuromuscular junctions occur between the axons of motor

neurons of the spinal cord and skeletal muscle. NMJ has many

of the structural features of chemical synapses in the CNS.

Neuromuscular synaptic transmission is fast and reliable. An

AP in the motor axon always causes an AP in the muscle cell it

innervates (What structural features for this reliability?)

Most knowledge from the research on NMJ transmission.

Types of synapses

The neuromuscular junction. The postsynaptic membrane,

known as the motor endplate, contains junctional folds with

numerous neurotransmitter receptors.

Types of synapses

//upload.wikimedia.org/wikipedia/commons/a/aa/Electron_micrograph_of_neuromuscular_junction_%28cross-section%29.jpg


There basic requirements for chemical synaptic transmission:

Synthesis and package into vesicles of neurotransmitter (NT);

Release of vesicle NT to cleft in response to a presynaptic AP;

Induction of an electrical or biochemical response to NT in the

postsynaptic neuron

Clearance of NT from the synaptic cleft

And, occur very rapidly to be useful for sensation, perception, and

the control of movement.


Neurotransmitters （three chemical categories）

g-氨基丁酸乙酰胆碱胆囊收缩素

谷氨酸多巴胺强腓肽

甘氨酸肾上腺素脑啡肽

组胺 N-乙酰门冬氨酰谷氨酸

去甲肾上腺素神经肽Y

5-羟色胺生长抑素

P物质

促甲状腺素释放激素

血管活性肠肽

Neurotransmitters

Three chemical categories

Amine, amino acid, peptide

Secretory granules and synaptic vesicles

Often co-exist in the same axon terminals

amine + peptide

amino acid + peptide

Different neurons release different neurotransmitters

Fast transmission; NMJ; Slow transmission


Representative neurotransmitters

(a) glutamate, GABA, and glycine.

(b) acetylcholine and norepinephrine.

(c) substance P.



Neurotransmitter Synthesis and Storage

Amine and amino acid neurotransmitters:

➀ Enzymes are transported to the axon terminal and convert

precursor molecules into neurotransmitter molecules in the cytosol.

➁ Transporter proteins load the neurotransmitter into synaptic

vesicles in the terminal, where they are stored.

Glu, Gly vs GABA, the amines

Peptides:

➀ A precursor peptide (a long peptide) synthesis in the rough

ER in cell body.

➁ Then split in the Golgi apparatus to yield the active one.

➂ Secretory vesicles with the peptide bud off from the Golgi

apparatus.

➃ The secretory granules are transported (axoplasmic) down

the axon to the terminal where the peptide is stored.


Transporters, proteins in the vesicle membrane, take up and

concentrate the amino acid and amaine neurotransmitters inside

the vesicle.


Neurotransmitter Release

An action potential in the axon terminal → depolarization of

the terminal membrane → voltage-gated calcium channels in

the active zones to open （[Ca2+]i 0.0002 mM → ˃0.1 mM）→ vesicles release（exocytosis）→ the contents to spill out

into the synaptic cleft

The exocytosis occurs very rapidly within 0.2 msec of the

Ca2+ influx into the terminal. Why?

The mechanism by which [Ca2+] i stimulates exocytosis:

Reserve pool of vesicles bound to the cytoskeleton

Docking of vesicles to active zone

SNARE protein complex, conformation altered by ↑[Ca2+]i

Endocytosis

Recycled vesicle refilled with neurotransmitter


The Release of Neurotransmitter by Exocytosis


(a) This is a view of the extracellular

surface of the active zone of a

neuromuscular junction in a frog.

The particles are believed to be

calcium channels.

(b) In this view, the presynaptic terminal

had been stimulated to release

neurotransmitter. The exocytotic

fusion pores are where synaptic

vesicles have fused with the

presynaptic membrane and released

their contents.

A “receptor’s eye” view of neurotransmitter release


SNAREs and vesicle fusion

(Box 5.3)

SNARE: SNAP Receptor

SNAP: Soluble NSF Attach Protein

NSF: N-ethylmaleimide-sensitive factor

(N-乙基马来酰亚胺敏感的融合因子)


Secretory granules also release peptide neurotransmitters by

exocytosis：

in a calcium-dependent fashion

typically not at the active zones

requires high-frequency trains of AP and more calcium influx.

a leisurely process to taking 50 msec or more.


Neurotransmitter Receptors and Effectors

binding to specific receptor proteins in the postsynaptic

density.

key in a lock, induce conformational changes in the

receptor and lead to different functions.

More than 100 different receptors can be classified into

two types: transmitter-gated ion channels and G-protein-

coupled receptors.

Receptors Ion channels

Receptor

channels

or Ionotropic receptors,

or Ligand-gated ion channels

G-protein Coupled Receptors

Enzyme linked receptors

Nuclear receptors

Voltage-gated

Mechanically-gated

Non-gated


Transmitter-gated ion channels


Transmitter-Gated Ion Channels

Membrane-spanning proteins consisting of four or five

subunits to form a pore.

Closed to open, neurotransmitter, binds to specific sites,

induces a conformational change

The functional consequence depends on which ions.

The structure of an ACh-gated ion channel

Principles of chemical synaptic transmission Ion selectivity of transmitter-gated channels

and postsynaptic potential

Channels permeable to Na+, Depolarization, to be excitatory

Excitatory postsynaptic potential (EPSP)

Ach- or Glutamate-gated channels


Channels permeable to Cl-, Hyperpolarization, to be inhibitory

Inhibitory postsynaptic potential (IPSP)

BABA- or Glycine-gated channels


G-Protein-Coupled Receptors (GPCR)

Fast chemical synaptic transmission is mediated by amino acid

and amine neurotransmitters acting on transmitter-gated ion

channels. However, all three types of neurotransmitter, acting on

GPCR, can also have slower, longer-lasting, and much more

diverse postsynaptic actions.

This type of transmitter action involves three steps:

① Transmitters bind to receptors in the postsynaptic membrane.

② The receptors activate G-proteins, free to move along the

intracellular face of the postsynaptic membrane.

③ The activated G-proteins activate “effector” proteins.


Effector proteins

G-protein-gated ion channels in the membrane (left)

Enzymes that synthesize second messengers (right)

Second messengers can activate additional enzymes in the

cytosol that can regulate ion channel function and alter cellular

metabolism. GPCR often referred to as metabotropic receptors.


In the heart, a metabotropic ACh receptor is coupled by a

G-protein to a potassium channel. It slows the rhythmic

contractions of the heart by causing a slow

hyperpolarization of the cardiac muscle cells.

In skeletal muscle, the receptor is an ACh-gated ion channel,

permeable to Na+. ACh induces contraction by causing a

rapid depolarization of the muscle fibers.

The same neurotransmitter can have different postsynaptic

actions, depending on what receptors it binds to.

The shortcut pathway.

(a) G-proteins in heart

muscle are activated by

ACh binding to

muscarinic receptors.

(b) The activated G

subunit directly gates a

potassium channel.



Neurotransmitter receptors are also commonly found in the

membrane of the presynaptic axon terminal.

Sensitive to the neurotransmitter, called autoreceptors.

Typically, autoreceptors are GPCR

The common consequences of activating autoreceptors is

inhibition of neurotransmitter release. This allows a

presynaptic terminal to regulate itself

Autoreceptors

Neurotransmitter Recovery and Degradation


Neurotransmitter in the synaptic cleft must be cleared to allow

another round of synaptic transmission.

Simple diffusion (For most of the amino acid and amine

neurotransmitters)

Reuptake occurs by the action of specific transporter proteins

located in the presynaptic membrane (once inside the cytosol,

enzymatically destroyed, or reloaded into synaptic vesicles)

Neurotransmitter transporters also exist in the membranes of

glia surrounding the synapse, which assist in such removal.

Enzymatic destruction in the cleft. Ach is removed at the NMJ

by enzyme acetylcholinesterase, deposited in the cleft.

Importance of removal: desensitization (脱敏); nerve gases

Neuropharmacology

Each of the steps of synaptic transmission is chemical, and

therefore can be affected by specific drugs and toxins.

Inhibitors: e.g. Nerve gases inhibite the enzyme AChE. Inhibitors

of neurotransmitter receptors, called receptor antagonists (e.g.

Curare, an arrow-tip poison, binds tightly to the ACh receptors)

Receptor agonists. e.g. nicotine, binds to, and activates, the ACh

receptors in skeletal muscle and CNS. nicotinic ACh receptors

(nAChR).

Wrong neurotransmission is the root cause of many

neurological and psychiatric disorders. Knowledge of

neuropharmacology of synaptic transmission will be helpful for

development of new and effective therapeutic drugs.


Principles of synaptic integration

The integration of EPSPs

The contribution of dendritic properties

Inhibition

Modulation

Principles Of Synaptic Integration


The postsynaptic neuron integrates thousands of synaptic inputs

(complex ionic and chemical signals) and gives rise to a simple

form of output: AP

The transformation constitutes a neural computation. The brain

performs billions of neural computations every second.

Synaptic integration is the process by which multiple synaptic

potentials combine within one postsynaptic neuron.


The Integration of EPSPs

The opening of a single transmitter-gated channel

A patch-clamp recording from a transmitter-gated ion channel.

Ionic current passes through the channels when the channels are

open. In the presence of neurotransmitter, they rapidly alternate

between open and closed states.

Patch Clamps Permit Measurement of

Ion Movements through Single Channel

(not only in a whole cell)

Patch Clamp (膜片钳)

Different configurations



Quantal Analysis (量子分析) of EPSPs: a method of

comparing the amplitudes of miniature and evoked

postsynaptic potentials.

The neurotransmitter content in a single synaptic vesicle.

Spontaneous release w/o AP, one vesicle → miniature EPSP

(miniEPSP, mEPSP)

Multiple vesicle release w AP (evoked) → EPSP (multiples of

mEPSP)

i.e. postsynaptic EPSPs at a given synapse are quantized; they

are multiples of an indivisible unit, the quantum, that reflects

the number of transmitter molecules in a single synaptic

vesicle and the number of postsynaptic receptors available at

the synapse.


There is a big difference between excitatory transmission at NMJ

and CNS synapses.

Most neurons in CNS perform more sophisticated computations,

requiring that many EPSPs add together to produce a significant

postsynaptic depolarization. This is what is meant by integration

of EPSPs.

EPSP summation is the simplest form of synaptic integration.

Spatial summation is the adding together of EPSPs

generated simultaneously at many different synapses on a dendrite.

Temporal summation is the adding together of EPSPs

generated at the same synapse if they occur in rapid succession,

within about 1–15 msec of one another.


(a) An AP triggers a small EPSP in a postsynaptic neuron.

(b) Spatial summation: When two or more presynaptic inputs are

active at the same time, their individual EPSPs add together.

(c) Temporal summation: When the same presynaptic fiber fires APs

in quick succession, the individual EPSPs add together.


The Contribution of Dendritic Properties to Synaptic

Integration

The current of synaptic contact must spread down the dendrite and

the soma, and cause the membrane of the spike-initiation zone to be

depolarized beyond threshold, before an AP can be generated.

The effectiveness of an excitatory synapse in triggering an AP,

therefore, depends on how far the synapse is from the spike-

initiation zone and on the properties of the dendritic membrane.


Dendritic Cable Properties:

To simplify, let’s assume that dendrites function as cylindrical

cables that are electrically passive; that is, lacking voltage-gated ion

channels (in contrast, of course, with axons).

Imagine that the current at a synapse is like turning on the water

that will flow down a leaky garden hose (the dendrite).

Similarly, two paths that synaptic current can take: One is down the

inside of the dendrite; the other is across the dendritic membrane.

At some distance from the site of current

influx, the EPSP amplitude may approach

zero because of the dissipation of the current

across the membrane.


To simplify the mathematics, we assume the dendrite is infinitely

long, unbranched, and uniform in diameter.

The amount of depolarization falls off exponentially with increasing

distance:

Vx=V0/e x/λ

when x=λ, then Vx=V0/e. Put another way, Vλ=0.37 (V0).

This distance λ, where the depolarization is 37% of that at the

origin, is called the dendritic length constant.

(Remember that this analysis is an oversimplification) .


(a) The depolarization

measured at a distance

from the site of current

injection is smaller than

that measured right

under it.

(b) A plot of membrane

depolarization as a

function of distance

along the dendrite.

Decreasing depolarization as a function of

distance along a long dendritic cable


The length constant is an index of how far depolarization can

spread down a dendrite or axon. The longer the length constant, the

more likely it is that EPSPs generated at distant synapses will

depolarize the membrane at the axon hillock（轴丘）.

λ depends on two factors:

(1) the internal resistance (ri); and (2) the membrane resistance (rm).

ri depends only on the diameter of the dendrite and the electrical

properties of the cytoplasm (relatively constant in a mature neuron)

rm, depends on the number of open ion channels, which changes

from moment to moment depending on what other synapses are

active.

The dendritic length constant, therefore, is not constant at all!


Excitable Dendrites.

Assumption: The dendrite’s membrane is electrically passive.

The dendrites of spinal motor neurons are very close to passive.

However, many other neuronal dendrites are decidedly not

passive.

The voltage-gated channels in dendrites can act as important

amplifiers of small EPSPs generated far out on dendrites.

Paradoxically, in some cells dendritic sodium channels may also

serve to carry electrical signals in the other direction—from the

soma outward along dendrites.


A cortical pyramidal neuron with a long apical dendrite

that has voltage-gated ion channels


Inhibition

EPSP → AP output depends on:

the number of coactive excitatory synapses

the distance the synapse is from the spike-initiation zone

the properties of the dendritic membrane

Plus:

inhibitory synapses that take the membrane potential away from

action potential threshold, and exert a powerful control over a

neuron’s output .


IPSPs and Shunting Inhibition (分流抑制) The postsynaptic inhibitory receptors are GABA or glycine-

gated ion channels that they only allow Cl- to pass through their

channels.

Opening of the chloride channel brings the membrane potential

toward the chloride equilibrium potential, ECl-, about - 65 mV.

So, whether its activation causes a hyperpolarizing IPSP or not

depend on the resting membrane potential.

If there is no visible IPSP, is the neuron really inhibited? The

answer is yes.

Shunting inhibition (分流抑制). The actual physical basis of

shunting inhibition is the inward movement of negatively

charged chloride ions, which is formally equivalent to outward

positive current flow.

Thus, inhibitory synapses also contribute to synaptic integration


(a) Stimulation of the

excitatory input causes

inward postsynaptic

current that spreads to

the soma, where it can be

recorded as an EPSP.

(b) When the inhibitory and

excitatory inputs are

stimulated together, the

depolarizing current

leaks out before it

reaches the soma.

Shunting inhibition.


The Geometry of Excitatory and Inhibitory Synapses

Inhibitory synapses (GABA or glycine), Gray’s type II.

Excitatory synapses (glutamate), Gray’s type I

Inhibitory synapses on many neurons are found clustered on the

soma and near the axon hillock.


Modulation (调制)

In addition to synaptic transmitter-gated channels, there are many

synapses with G-protein-coupled neurotransmitter receptors that do

not directly evoke EPSPs and IPSPs, but instead modifies the

effectiveness of EPSPs generated by other synapses. This is called

modulation.

e.g. norepinephrine β receptor. The binding of norepinephrine (NE)

to the receptor triggers a cascade of biochemical events within the

cell to produce the second messager cAMP


Modulation by the NE receptor. ➀ The binding of NE to the receptor

activates a G-protein in the membrane. ➁ The G-protein activates the enzyme

adenylyl cyclase. ➂ Adenylyl cyclase converts ATP into the second messenger

cAMP. ➃ cAMP activates a protein kinase. ➄ The protein kinase causes a

potassium channel to close by attaching a phosphate group to it.


decreasing the K+ conductance increases the dendritic membrane

resistance and therefore increases the length constant λ.

Distant or weak excitatory synapses will become more effective in

depolarizing the spike-initiation zone beyond threshold. i.e. the

cell becomes more excitable.

It is why excitability of a neuron is increased when NE is released

presynaptically.

END

谢谢！

Documents

Synaptic Transmission - m-learning.zju.edu.cnm-learning.zju.edu.cn/G2S/eWebEditor/uploadfile/20141126183232244.pdf · Solid evidence given in 1921 by Otto Loewi; 2. B. Katz et al