Week 4

Molecular mechanisms of synaptic plasticity

Advances in Molecular Neurobiology:

Neural Stem Cells and Neuroregenerative Approaches

SYNAPTIC TRANSMISSION-I(Chapter 10)

Type of

Synapse

Distance btw

membranes

Cytoplasmic

continuity

Ultrastructural Agents of

transmission

Synaptic

delay

Direction of

transmission

Electrical 3.5 nm Yes Gap-junctions İon current None Bidirectional

Chemical 20 - 40 nm No Presynaptic

vesicles; Post-

synaptic

receptors

Chemical

transmitters

~0.3 msec

to 5 msec

unidirectional

PROPERTIES OF ELECTRICAL AND CHEMICAL SYNAPSES

Electrical transmission through gap junctions

Actual physical continuity between the presynaptic and postsynaptic cells.

The pre- and post-synaptic neurons are joined by gap junctions which are

formed by hexameric protein subunits known as connexons (also found

embryos during development, as well as in heart and epithelial cells).

Each connexon is made up of 6 identical protein subunits called connexins.

They form very large channels -- 1 to 2 nm -- between the connecting cells, so

that not only ions but various molecules with MW 's up to about 1500 can pass

through -- big enough so that substances like cAMP or ion current itself can

pass through ...

Amacrine cells in retina, for instance, pass along glycine to photoreceptors

• Speed

• Synchrony among cells

• Metabolic coupling

Electrical transmission is “graded”, and occurs even when the currents in the “presynaptic cell” are below

the threshold for action potential....

Most electrical synapses will transmit both depolarizing and hyperpolarizing currents.

openclosed

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Most gap junctions close in response to :

1- lowered cytoplasmic pH; or

2- elevated cytoplasmic Ca++

These properties are necessary to separate or “decouple” damaged cells from other cells....

At some specialized gap junctions –

Voltage-dependent gates that permit them to conduct depolarizing current only from the

presynaptic to post-synaptic neuron.....

(rectifying synapses)

Gap junctions can be found between glial cells as well as neurons......

The synapse consists of:

1. The presynaptic terminal at the end of an axon. This contains tiny vesicles which contain neurotransmitters - the small

molecules which carry the nerve impulse from the sending neuron to the receiving neuron.

2. The synaptic cleft - a gap between the two neurons across which the neurotransmitters migrate.

3. The postsynaptic terminal usually in the dendrites of receiving neurons. This contains receiving sites for the

neurotransmitters.

Chemical transmission

Ion signaling at nerve endings

electrical chemical chemical electrical

Action potentials at the presynaptic axon termini cause

opening of voltage-gated calcium channels....

Ca++ influx causes neurotransmitter-containing vesicles to

fuse with the cell membrane and release their contents.

Released neurotransmitters diffuse across the synaptic

cleft and bind receptors on the membrane of the post-

synaptic neuron. The receptors cause ion channels to

open or close, depending on the “instruction”.

Frog neuromuscular junction as a relatively simple model of

a chemical synapse.

A. Ultrastructure:

Chemical synapses are more complex than gap junctions

and show a great deal of structural specialization both on the

presynaptic side of things and on the post-synaptic side of

things. At the neuromuscular junction:

The incoming motor nerve loses its myelin sheath and gives

off branches that run in shallow grooves in the surface of the

muscle. (Although the Schwann cell still hovers protectively

around the nerve terminal.)

In between the nerve terminal and the muscle is the

synaptic cleft which is a gap about 30 nm wide. So there's a

significant space between the presynaptic nerve terminal's

plasma membrane and the post-synaptic muscle's plasma

membrane.

In the cleft is the basal lamina, a network of connective

tissue that follows the contours of the muscle membrane

(where the AChE is found)

and on the muscle side are grooves and channels known as

the post-junctional folds. The acetylcholine receptors are

found at the outer lips of the post-junctional folds. This whole

region of the muscle in apposition with the nerve terminal is

known as the motor endplate.

Back in the nerve terminal, are mitochondria, which may be

important in buffering the free calcium concentration under

certain conditions and synaptic vesicles.

Active zone

Active zone : docking and release sites for vesicles

http://www.williams.edu/williams-only/Neuroscience/courses/Biol304/00lec1/lec7d.gifhttp://www.williams.edu/williams-only/Neuroscience/courses/Biol304/00lec1/lec7d.gif

• Fast, electrical response to arrival of

presynaptic action potential: EPSP, IPSP

• Use amino acids and amines as

neurotransmitters

• Slow, modulatory chemical synapses

• Slow response to arrival of presynaptic action

potential, activate second messenger system,

not always a direct electrical effect

• May use amino acids, amines, or peptides

Upon arrival of an action potential at a synapse:

Upon arrival of an action potential at a

synapse:

1. Opening of voltage-gated sodium and

calcium channels

2. Influx of calcium results in the docking of

synaptic vesicles at the presynaptic

membrane

3. Vesicles and membrane fuse, transmitter

substance is released into synaptic cleft

transmitter molecules open ligand-gated

sodium channels in the postsynatic

membrane.

4. Small changes of potential occur locally -

these events are called e.g. excitatory

postsynaptic potentials (epsps).

Epsps summate (add together) when there

are enough synapses near together.

This is called spatial summation.

5. When the postsynatic membrane is

depolarized in rapid succession, epsps also

add to provide temporal summation.

6. Spatial or temporal summation can

produce an action potential in the next

neuron.

Neurotransmitters are additive –

if the net effect of all the excitory neurotransmitters minus all the inhibitory ones achieves this threshold

=

then an action potential will be initiated.

Question: What happens if a large EPSP is "summed" with an equally large IPSP?

If a neurotransmitter binds to an ion channel, that channel may open up.

Let's say two different neurotransmitters are floating around in the synapse, and one of them causes a

Na+ channel to open and Na+ is drawn into the cell by both electrical and concentration forces.

The other neurotransmitter affects a different ion channel, let's say it causes a Cl- ion channel to open,

and Cl-is drawn into the cell by concentration forces.

Na+ entering the cell causes the cell to become more positive (or less polarized since the resting

membrane potential is negative) which is refered to as a depolarization. Cl- entering the cell would

cause an already negative membrane potential to become even more negative, this is refered to as a

hyperpolarization.

Back to the original question, if these events were to happen simultaneously (Na+ moves into the

cell, while Cl- does the same) the net effect is no change in membrane potential, and thus no action

potential will occur.

The important point here is that the generation of an action potential does not depend on single,

independent inputs on the dendrites. Rather, the generation of an action potential depends on the

summation of multiple inputs distributed over the network of dendrites.

Answer: They cancel one another out.

•Small molecule transmitters - amino acids and amines

•Examples of amino acid neurotransmitters - gamma-amino butyric acid (GABA),

glutamate (Glu), glycine (Gly)

•Examples of amine neurotransmitters - acetylcholine (ACh), dopamine (DA),

epinephrine, histamine, norepinephrine (NE), serotonin (5-HT)

•Synthesis occurs in axon terminal

•Precursor molecule is transformed by synthetic enzyme into neurotransmitter molecule

•Neurotransmitter molecules are gathered by transporter molecules and packaged in

synaptic vesicles

glycine

dopamine

serotonin

acetylcholine

epinephrine

http://webvision.med.utah.edu/imageswv/GLU1.jpeghttp://webvision.med.utah.edu/imageswv/GLU1.jpeghttp://www.psychiatry.co.uk/doxepine/index.htmlhttp://www.psychiatry.co.uk/doxepine/index.htmlhttp://www.psychiatry.co.uk/doxepine/index.htmlhttp://www.psychiatry.co.uk/doxepine/index.htmlhttp://www.psychopharmacology.net/sertraline/index.htmlhttp://www.psychopharmacology.net/sertraline/index.html

• Large molecule transmitters - peptides Examples - substance P,

somatostatin, leu-enkephalin, met- enkephalin, vasoactive

intestinal polypeptide (VIP), bombesin

• Occurs in soma, secretory vesicles transported down axon by

orthograde axonal transport

• Peptide synthesized in rough endoplasmic reticulum

• Packaged in Golgi apparatus

• Transported down axon to presynaptic ending of axon terminal

Neuropeptide

neurotransmitters

Corticotropin releasing

hormone

Corticotropin (ACTH)

Beta-endorphin

Substance P

Neurotensin

Somatostatin

Bradykinin

Vasopressin

Angiotensin II

the arginine vasopressin (AVP)

polypeptide that is comprised of

9 amino acids

Small molecule neurotransmitters

Type Neurotransmitter Postsynaptic effect

Acetylcholine Excitatory

Amino acids Gamma aminobutyric acid (GABA) Inhibitory

Glycine Inhibitory

Glutamate Excitatory

Aspartate Excitatory

Biogenic amines Dopamine Excitatory

Noradrenaline Excitatory

Serotonin Excitatory

Histamine Excitatory

Neurotransmitter synthesis in the axon terminals – Acetylcholine synthesis

acetylcholine

Schematic diagram of the nAChR showing the arrangement of

subunits and a cross-sectional representation of the protein

Acetylcholine and glutamate are examples of excitatory

neurotransmitters…

The ligand-gated ion channels differ from voltage-gated

ion channels in that they are activated by binding of

neurotransmitters.

In addition, they also differ in terms of selective ion

permeability.

Upon activation and channel opening, both Na+ and K+

can flow through the channel.

At first this might seem to exert no net effect on a

neuron, due to the counterbalancing effects of Na+

influx and K+ efflux.

Based on the resting membrane potential (near -70

mV) and the equilibrium potentials for each ion

however, we can see that there is a tremendous

potential difference for Na+, yet a very small differential

for K+.

As a result, at rest, acetylcholine triggers a rapid influx

of Na+. This influx of Na+ leads to an excitatory

postsynaptic potential (epsp).

Muscarinic acetylcholine receptor

Muscarinic receptors can exert multiple actions upon a neuron,

depending upon the subtypes of receptor, G proteins, and target

proteins.

For example, in the CNS, activation of M1 receptors, generally

postsynaptic, leads to an increase in phospholipase C activity via Gaq.

This in turn leads to the formation of 1,2-diacylglycerol and inositol

triphosphate. 1,2-Diacylglycerol stimulates protein kinase C activity,

which leads to phosphorlyation of intracellular proteins.

In addition, IP3 generation leads to mobilization of Ca2+ from

intracellular stores.

Increases in Ca2+ can enhance the release of neurotransmitters and

activate calmodulin.

Activation of M2 receptors, which have been found on presynaptic

neurons, leads to the opening of potassium channels, which tends to

hyperpolarize the cell and prevent neurotransmitter release.

G-protein coupled receptor (GPCR) signaling

g-amino butyric acid

Neurotransmitter reuptake

represents an important

mechanism for inactivating

neurotransmitters.

Specific transporter proteins

have been identified for several

neurotransmitters, including

GABA, glycine, dopamine, and

serotonin.

A number of neuroactive drugs

exert their action through

inhibition of neurotransmitter

reuptake.

For example, cocaine strongly

inhibits the dopamine reuptake

mechanism, while fluoxetine

(Prozac®) blocks the uptake of

serotonin, thereby exerting an

antidepressant effect.

Glutamate is the major excitatory neurotransmitter in central

nervous system (CNS) and as such the glutamate receptors

play a vital role in the mediation of excitatory synaptic

transmission (see animation). This process is the means by

which cells in the brain (neurons) communicate with each other.

An electrical impulse in one cell causes an influx of calcium ions

and the subsequent release of a chemical neurotransmitter (e.g.

glutamate). The transmitter diffuses across a small gap between

two cells (the synaptic cleft) and stimulates (or inhibits) the next

cell in the chain by interacting with receptor proteins. The

specialised structure that performs this vital function is the

synapse and it is in the synapse that the ionotropic glutamate

receptors are generally found.

The ionotropic receptors themselves are ligand gated ion

channels, ie on binding glutamate that has been released from

a companion cell, charged ions such as Na+ and Ca2+ pass

through a channel in the centre of the receptor complex. This

flow of ions results in a depolarisation of the plasma membrane

and the generation of an electrical current that is propagated

down the processes (dendrites and axons) of the neuron to the

next in line.

The NMDA receptor (NMDAR) is an ionotropic receptor for glutamate (NMDA (N-methyl d-aspartate) is a

name of its selective specific agonist). Activation of NMDA receptors results in the opening of an ion

channel which is nonselective to cations. This allows flow of Na+ and K+ ions, and small amounts of Ca2+ .

Calcium flux through NMDARs is thought to play a critical role in synaptic plasticity, a cellular mechanism

for learning and memory.

The NMDA receptor is interesting in that it is both ligand-gated and voltage-dependent.

Activation of NMDA receptors requires binding of both glutamate and the co-agonist glycine for the

efficient opening of the ion channel which is a part of this receptor.

D-serine has also been found to co-agonize the NMDA receptor with even greater potency than glycine.

D-serine is produced by serine racemase in astrocyte cells and is enriched in the same areas as NMDA

receptors. Removal of D-serine can block NMDA mediated excitatory neurotransmission in many areas.

Recently, it has been shown that D-serine is also synthesized in neurons, indicating a role for neuron-

derived D-serine in NMDA receptor regulation.

In addition, a third requirement is membrane depolarization. A positive change in transmembrane

potential will make it more likely that the ion channel in the NMDA receptor will open by expelling the

Mg2+ ion that blocks the channel from the outside.

Dopamine receptors are a class of metabotropic G protein-coupled

receptors that are prominent in the vertebrate central nervous system.

The neurotransmitter dopamineis the endogenous ligand for dopamine

receptors.

http://en.wikipedia.org/wiki/Dopamine

Noradrenaline (norepinephrine) receptors are G-protein coupled

Noradrenaline

Receptor TypeDistribution Postulated Roles

Alpha1 Brain, heart, smooth muscle Vasoconstriction, smooth muscle control

Alpha2 Brain, pancreas, smooth muscle Vasoconstriction, presynaptic effect in GI

(relaxant)

Beta1 Heart, brain Heart rate (increase)

Beta2 Lungs, brain, skeletal muscle Bronchial relaxation, vasodilatation

Beta3 Postsynaptic effector cells Stimulation of effector cells

Receptor

TypeDistribution Postulated Roles

5-HT1 Brain, instetinal nerves Neuronal inhibition, behavioural effects, cerebral

vasoconstriction

5-HT2 Brain, heart, lungs, smooth muscle control, GI

system, blood vessels, platelets

Neuronal excitation, vasoconstriction, behavioural

effects, depression, anxiety

5-HT3 Limbic system, ANS Nausea, anxiety

5-HT4 CNS, smooth muscle Neuronal excitation, GI

5-HT5, 6,

7

Brain Not known

Modulation of Synaptic

Transmission

(Chapter 13)

1. DIRECT GATING (Ionotropic Receptor) 2. INDIRECT GATING

a) G protein-coupled receptor

b) Receptor Tyrosine kinase

G proteins are so-called because they bind the guanine

nucleotides GDP and GTP.

They are heterotrimers (i.e., made of three different subunits)

associated with:

•the inner surface of the plasma membrane and

•transmembrane receptors of hormones, etc.

•These are called G protein-coupled receptors (GPCRs).

The three subunits are:

•Gα, which carries the binding site for the nucleotide.

(At least 20 different kinds of Gα molecules are

found in mammalian cells)

•Gβ

•Gγ

Protein Kinase A (cAMP-Dependent Protein Kinase) transfers Pi from ATP to the hydroxyl group of a

serine or threonine that is part of a particular 5-amino acid sequence. Protein Kinase A exists in the

resting state as a complex of:

•2 regulatory subunits (R)

•2 catalytic subunits (C)

Each regulatory subunit (R) of Protein Kinase A contains a pseudosubstrate sequence comparable to

the substrate domain of a target protein for Protein Kinase A, but with alanine substituting for the serine

or threonine. The pseudosubstrate domain of the regulatory subunit, which lacks a hydroxyl that can be

phosphorylated, binds to the active site of the catalytic subunit, blocking its activity.

When each regulatory subunit binds 2

cAMP, a conformational change

causes the regulatory subunits to

release the catalytic subunits. The

catalytic subunits (C) can then

catalyze phosphorylation of serine or

threonine residues on target proteins.

R2C2 + 4 cAMP R2cAMP4 + 2 C

PKIs, Protein Kinase Inhibitors,

modulate activity of the catalytic

subunits (C).

Some Types of Gα Subunits

Gαs

This type stimulates (s = "stimulatory") adenylyl cyclase. It is associated with the receptors for many

hormones such as adrenaline or adrenocorticotropic hormone (ACTH).

Gαq

This activates phospholipase C (PLC) which generates the second messengers inositol trisphosphate (IP3)

and diacylglycerol (DAG).

Gαq is found in G proteins coupled to receptors for vasopressin and angiotensin.

Gαi

This inhibits (i = "inhibitory") adenylyl cyclase lowering the level of cAMP in the cell. Gai is activated by

the receptor for somatostatin.

Gαt

The "t" is for transducin, the molecule responsible for generating a signal in the rods of the retina in

response to light. Gαt triggers the breakdown of cyclic GMP (cGMP).

Secondary Messengers - cAMP

Secondary messengers “relay” the incoming signal to the nucleus or cytoplasm

and generate a response in the cell.....

Secondary Messengers – Ca++

SYNAPTIC INTEGRATION

Nerve impulses are transmitted down to the presynaptic terminal in the axon of

one neuron and across the synaptic cleft to the postsynaptic terminal in the

dendrite of another neuron.

Synapses do not only join axons to dendrites (axodendritic synapses) –

they can also joins axons to other axons (axoaxonic synapses) –

or to the soma - the neuronal cell body - (axosomatic synapses).

Temporal summation

Spatial summation

Shunting inhibition

Drug Action

Nicotine action on synaptic transmission

Nicotine works by docking to a subset of receptors that bind the neurotransmitter acetylcholine.

Acetylcholine is the neurotransmitter that (depending on what region of the brain a neuron is in):

- Delivers signals from your brain to your muscles

- Controls basic functions like your energy level, the beating of your heart and how you breathe

- Acts as a "traffic cop" overseeing the flow of information in your brain

- Plays a role in learning and memory

One of the neurotransmitters playing a major role in addiction is dopamine. Many of the

concepts that apply to dopamine apply to other neurotransmitters as well.

As a chemical messenger, dopamine is similar to adrenaline. Dopamine affects brain

processes that control movement, emotional response, and ability to experience

pleasure and pain.

Some drugs are known as dopamine agonists. These drugs bind to

dopamine receptors in place of dopamine and directly stimulate those

receptors. Some dopamine agonists are currently used to treat Parkinson's

disease. These drugs can stimulate dopamine receptors even in someone

without dopamine neurons.

In contrast to dopamine agonists, dopamine antagonists are drugs that

bind but don't stimulate dopamine receptors. Antagonists can prevent or

reverse the actions of dopamine by keeping dopamine from attaching to

receptors.

One important aspect of drug addiction is how cells adapt to

previous drug exposure.

For example, long-term treatment with dopamine antagonists

increases the number of dopamine receptors. This happens as

the nervous system tries to make up for less stimulation of the

receptors by dopamine itself.

Likewise, the receptors themselves become more sensitive to

dopamine. Both are examples of the same process, called

sensitization.

An opposite effect occurs after dopamine or dopamine agonists

repeatedly stimulate dopamine receptors.

Here overstimulation decreases the number of receptors, and the

remaining receptors become less sensitive to dopamine. This

process is called desensitization.

Desensitization is better known as tolerance, where exposure to a

drug causes less response than previously caused.

Caffeine

Caffeine is an addictive drug. Among its many actions, it operates using the same mechanisms that

amphetamines, cocaine, and heroin use to stimulate the brain.

On a spectrum, caffeine's effects are more mild than amphetamines, cocaine and heroin, but it is

manipulating the same channels, and that is one of the things that gives caffeine its addictive qualities.

cocaine

Cocaine prevents dopamine reuptake by binding to proteins that normally transport dopamine. Not only

does cocaine "bully" dopamine out of the way – it also hangs on to the transport proteins much longer than

dopamine does.

As a result, more dopamine remains to stimulate neurons, which causes a prolonged feelings of pleasure

and excitement. Amphetamine also increases dopamine levels. Again, the result is over-stimulation of

these pleasure-pathway nerves in the brain.

http://www.erowid.org/chemicals/caffeine/images/archive/caffeine_3d.jpghttp://www.erowid.org/chemicals/caffeine/images/archive/caffeine_3d.jpghttp://en.wikipedia.org/wiki/Image:Cocaine.pnghttp://en.wikipedia.org/wiki/Image:Cocaine.png

(NIH – NIDA)