Advances in Molecular Neurobiology: Neural Stem Cells and … · 2019. 10. 18. · Molecular...
Transcript of Advances in Molecular Neurobiology: Neural Stem Cells and … · 2019. 10. 18. · Molecular...
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Week 4
Molecular mechanisms of synaptic plasticity
Advances in Molecular Neurobiology:
Neural Stem Cells and Neuroregenerative Approaches
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SYNAPTIC TRANSMISSION-I(Chapter 10)
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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
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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......
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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
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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”.
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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
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• 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
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Upon arrival of an action potential at a synapse:
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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.
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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.
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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.
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•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
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• 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
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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
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Neurotransmitter synthesis in the axon terminals – Acetylcholine synthesis
acetylcholine
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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).
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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.
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G-protein coupled receptor (GPCR) signaling
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g-amino butyric acid
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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.
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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.
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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.
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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.
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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
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Noradrenaline (norepinephrine) receptors are G-protein coupled
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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
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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
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Modulation of Synaptic
Transmission
(Chapter 13)
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1. DIRECT GATING (Ionotropic Receptor) 2. INDIRECT GATING
a) G protein-coupled receptor
b) Receptor Tyrosine kinase
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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
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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).
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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).
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Secondary Messengers - cAMP
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Secondary messengers “relay” the incoming signal to the nucleus or cytoplasm
and generate a response in the cell.....
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Secondary Messengers – Ca++
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SYNAPTIC INTEGRATION
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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).
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Temporal summation
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Spatial summation
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Shunting inhibition
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Drug Action
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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
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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.
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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.
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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
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(NIH – NIDA)