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Neurotransmitter Systems

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

Studying Neurotransmitter Systems Localization of transmitters and synthesizing enzymes

Studying transmitter release / synaptic mimicry / receptors

Neurotransmitter Chemistry Cholinergic/Catecholaminergic/Serotonergic/mino acidergic neurons

Other neurotransmitter candidates and intercellular messengers

Transmitter-gated Channels The basic structure of transmitter-gated channels

Amini acid-gated channels

G-Protein-Coupled Receptors And Effectors The basic structure of G-protein-coupled receptors

The ubiquitous G-protein

G-protein-coupled effector systems

Divergence and Convergence in Neurotransmitter Systems

Neurotransmitters (amino acids, amines, and peptides)

Plus

The molecular machinery (for transmitter synthesis, vesicular

packaging, reuptake and degradation, and transmitter action)

Acetylcholine (ACh), the first NT, identified in the 1920s.

The neurons producing and releasing Ach given the term

cholinergic by British pharmacologist Henry Dale (shared the

1936 Nobel Prize with Loewi).

The suffix –ergic: noradrenergic, glutamatergic, GABAergic,

peptidergic, and so on, for the various synapses, neurons and

neurotransmitter systems.

Introduction

Introduction

Elements of neurotransmitter systems

Studying Neurotransmitter Systems

Criteria to identify a neurotransmitter:

1. The molecule must be synthesized and stored in the

presynaptic neuron.

2. The molecule must be released by the presynaptic axon

terminal upon stimulation.

3. The molecule, when experimentally applied, must produce a

response in the postsynaptic cell that mimics the response

produced by the release of neurotransmitter from the

presynaptic neuron.

Localization of Transmitters and their Synthesis Enzymes

Whatever the inspiration, the first step in confirming the

hypothesis on a new neurotransmitter is to show that the molecule

is, in fact, localized in, and synthesized by, particular neurons.

Many methods have been used to satisfy this criterion for

different neurotransmitters. Two of the most important techniques

used today are immunocytochemistry（免疫细胞化学）and in

situ hybridization （原位杂交）.


Immunocytochemistry. This method uses labeled antibodies

to identify the location of molecules within cells.


Immunocytochemistry can be used to localize any molecule

for which a specific antibody can be generated, including the

neurotransmitters themselves and the synthesizing enzymes

for transmitter candidates.

Immunocytochemical localization of

a peptide neurotransmitter in neurons


Strands of mRNA consist of

nucleotides arranged in a specific

sequence. Each nucleotide will

stick to one other complementary

nucleotide. In the method of in situ

hybridization, a synthetic probe is

constructed containing a sequence

of complementary nucleotides that

will allow it to stick to the mRNA.

If the probe is labeled, the location

of cells containing the mRNA will

be revealed.

In situ hybridization


In situ hybridization of the mRNA for a neuropeptide

transmitter in hippocampus of mice (A. wild type and B. the

peptide Knockout). Only neurons with the proper mRNA are

labeled, with clusters of white dots.


Studying Transmitter Release

To show that a neurotransmitter candidate is actually released

upon stimulation.

Axon stimulation → test biological activity → chemical analysis

(as Loewi and Dale did in identification of ACh as a transmitter at

many peripheral synapses)

A diverse mixture synapses in CNS makes it impossible to

stimulate a single population of synapses. Researchers have to

collect and measure all the chemical mixture (e.g. using brain

slices in a solution containing a high K+ concentration).

Also have to show Ca2+ dependency of release, and from the

presynaptic axon terminal upon


Studying Synaptic Mimicry

To meet the third criterion, microionophoresis (离子微电泳) is

often use to assess the postsynaptic actions of a transmitter

candidate.

The candidates in solutions in a glass pipette is ejected on in very

small amounts by passing electrical current through the surface of

neurons, and the membrane potential can be measured.

If it mimics the effects of transmitter released at the synapse, and if

the other criteria of localization, synthesis, and release have been

met, then the molecule and the transmitter usually are considered to

be the same chemical.


Microionophoresis


Studying Receptors

As a rule, no two neurotransmitters bind to the same receptor;

however, one neurotransmitter can bind to many different receptors,

receptor subtype. e.g. two different cholinergic receptor subtypes.

Three approaches to study the different receptor subtypes:

Neuropharmacological Analysis.

For instance, cholinergic receptor subtypes respond differently to

various drugs.

Nicotine (烟碱), a receptor agonist in skeletal muscle, nicotinic ACh

receptors (channels). Curare (筒剑毒) is its selective antagonist.

Muscarine (毒蕈碱), a receptor agonist in the heart, muscarinic

receptors (GPCR). Atropine is its selective antagonist

Nicotinic and muscarinic receptors also exist in the brain.


Three subtypes of glutamate receptors at the synaptic excitation in

the CNS: AMPA receptors, NMDA receptors, and kainate receptors,

each named for a different chemical agonist.

The neurotransmitter glutamate activates all the subtypes, but

AMPA acts only at the AMPA receptor…..

Two subtypesthes of NE receptors, α and β, and of GABA receptors,

GABAA and GABAB.

Thus, selective drugs have been extremely useful for categorizing

receptor subclasses. In addition, neuropharmacological analysis has

been invaluable for assessing the contributions of neurotransmitter

systems to brain function.


The neuropharmacology of cholinergic synaptic transmission.

Sites on transmitter receptors can bind either the transmitter itself

(ACh), an agonist that mimics the transmitter, or an antagonist

that blocks the effects of the transmitter and agonists.


The neuropharmacology of glutamatergic synaptic transmission

Three subtypes of glutamate receptors, each of which binds glutamate,

and each of which is activated selectively by a different agonist.


Ligand-Binding Methods.

Selective drugs provide an opportunity to analyze

receptors directly, even before the neurotransmitter

itself had been identified.

Story of discovery of Opiate receptors

Solomon Snyder (1938-) at Johns Hopkins University

Opiates effects on the brain

Hypothesis: opiates are agonists for specific receptors in CNS.

Radioactively labeled opiate compounds

labeled specific sites on some neurons in the brain.

Led to the discovery of opiate receptors, and identification of

endogenous opiates, or endorphins (内啡肽), e.g. enkephalin

Opiate neurotransmitter systems eventually proved .


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Opiate receptor binding to a slice of rat brain. Special film was

exposed to a brain section that had radioactive opiate receptor

ligands bound to it. The dark regions contain more receptors.


Molecular Analysis

Enable us to divide the neurotransmitter receptor proteins into

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

coupled (metabotropic) receptors

The structure of receptor subunits by molecular analysis

presented a broad extent of the diversity in subunit composition

e.g. Each GABA receptor channel requires five subunits, from

five major classes, α, β, γ, δ and ρ, α 1-6 isoforms, β1-4, γ1-4.

Theoretically, there are 151,887 possible combinations and

arrangements of subunits.

What this means?


Evolution is conservative and opportunistic, and it often puts

common and familiar things to new uses.

Amino acids are essential to life. Most of the known

neurotransmitter molecules are either (1) amino acids, (2)

amines derived from amino acids, or (3) peptides constructed

from amino acids.

ACh is an exception; but it is derived from acetyl CoA, a

ubiquitous product of cellular respiration in mitochondria, and

choline, important for fat metabolism.

Amino acid and amine transmitters are generally each stored

in and released by separate sets of neurons, so-called Dale’s

principle. However, many peptide-containing neurons violate

Dale’s principle. Co-transmitters: peptide + amino acid or

peptide + amine

Neurotransmitter Chemistry

Cholinergic Neurons Acetylcholine (ACh)

The neurotransmitter at NMJ, synthesized by all the motor

neurons in the spinal cord and brain stem. Other cholinergic

cells contribute to the functions of specific circuits in the PNS

and CNS.

ACh synthesis needs an enzyme, choline acetyltransferase

(ChAT). Only cholinergic neurons contain ChAT, so this

enzyme is a good marker, e.g. antibody-ICC. ChAT transfers

an acetyl group from acetyl CoA to choline .

Transport of choline into the neuron is the rate-limiting step

in ACh synthesis.

Acetylcholinesterase (AChE) secreted from Cholinergic and

noncholinergic neurons to degrades ACh. Inhibition of AChE

disrupts transmission at cholinergic synapses on skeletal

muscle and heart muscle.



The life cycle of ACh

low micromolar

concentrations

rate-limiting step

with the fastest catalytic

rate. the target of nerve

gases and insecticides


Acetylcholine. (a) ACh synthesis. (b) ACh degradation.


Catecholaminergic Neurons

The amino acid tyrosine is the precursor for three different

amine neurotransmitters that contain a chemical structure

catechol, collectively called catecholamines (儿茶酚胺).

Include dopamine (DA), norepinephrine (NE), and

epinephrine. Catecholaminergic neurons are found in regions of

the nervous system for regulation of movement, mood,

attention, and visceral function.

Catechol

group

Dopamine norepinephrine epinephrine

(nonadrenaline) adrenaline


All such neurons contain tyrosine hydroxylase (TH), which

catalyzes the first step in catecholamine synthesis, the

conversion of tyrosine to a compound called dopa (L-

dihydroxyphenylalanine 二羟苯基丙氨酸).

The activity of TH is rate limiting for catecholamine

synthesis, regulated by various signals in the cytosol of the

axon terminal (end-product inhibition, increase when [Ca2+]i

elevated by a high rate release).

Dopa is converted into the neurotransmitter dopamine by the

enzyme dopa decarboxylase. Parkinson’s disease and dopa

supplement therapy.


酪氨酸

二羟苯基丙氨酸（多巴）

多巴胺

去甲肾上腺素

肾上腺素

酪氨酸羟化酶多巴脱羧基酶多巴胺β羟化酶苯基乙醇胺- N-甲基转移酶

rate limiting enzyme

located within the

synaptic vesicles

located in the

cytosol


Neurons that use NE as a neurotransmitter contain, in

addition to TH and dopa decarboxylase, the enzyme

dopamine β-hydroxylase (DBH), which converts dopamine

to norepinephrine. DA is transported from the cytosol to the

synaptic vesicles, and there it is made into NE.

The last in the line of catecholamine neurotransmitters is

epinephrine (adrenaline). Adrenergic neurons contain the

enzyme phentolamine Nmethyltransferase (PNMT), which

converts NE to epinephrine.

In addition to serving as a neurotransmitter in the brain,

epinephrine is released by the adrenal gland into the

bloodstream. Circulating epinephrine acts at receptors

throughout the body to coordinate visceral response.


The actions of catecholamines in the synaptic cleft are

terminated by selective uptake of the neurotransmitters back

into the axon terminal via Na+-dependent transporters.

This step is sensitive to a number of different drugs. For

example, amphetamine and cocaine block catecholamine

uptake.

Once inside, the axon terminal, the catecholamines may be

reloaded into synaptic vesicles for reuse, or they may be

enzymatically destroyed by the action of monoamine oxidase

(MAO), an enzyme found on the outer membrane of

mitochondria.


Serotonergic Neurons

The amine neurotransmitter serotonin, also called 5-

hydroxytryptamine and abbreviated 5-HT, is derived from the

amino acid tryptophan. Serotonergic neurons are relatively few

in number, but they appear to play an important role in the brain

systems that regulate mood, emotional behavior, and sleep.

Serotonin synthesis appears to be limited by the availability of

tryptophan in the extracellular fluid bathing neurons. The source

of brain tryptophan is the blood, and the source of blood

tryptophan is the diet.

5-HT is removed from the synaptic cleft by the action of a

specific transporter, which is sensitive to a number of different

drugs. e.g. antidepressant drugs like fluoxetine. Once back in the

cytosol, 5-HT is either reloaded to SVs or degraded by MAO.


The synthesis of serotonin from tryptophan.

色氨酸

5-羟色氨酸

5-羟色胺

色氨酸羟化酶

5-羟基色氨酸脱羧酶


Amino Acidergic Neurons

glutamate (Glu) Glycine (Gly) GABA

Amino acid neurotransmitters Glu, Gly, and GABA serve as

neurotransmitters at most CNS synapses

Glutamate and glycine are synthesized from glucose and other

precursors by enzymes existing in all cells. Differences among

neurons are quantitative rather than qualitative.

e.g. the glutamatergic terminals have about 20 mM Glu, only

2-3 times higher than nonglutamatergic cells. Importantly, in

glutamatergic terminalss, but not in other’s, the glutamate

transporter concentrates Glu in SVs to reach about 50 mM.


GABA is not one of the 20 amino acids used to construct

proteins, it is synthesized in large quantities only by the neurons

that use it as a neurotransmitter.

The precursor for GABA is glutamate. The key synthesizing

enzyme is glutamic acid decarboxylase (GAD), a good marker

for GABAergic neurons. One chemical step to convert the major

excitatory into the major inhibitory neurotransmitter in the brain!

The synaptic actions of GABA are terminated by selective

uptake into the terminals and glia via specific Na+-dependent

transporters. Inside the cytosol, GABA is metabolized by the

enzyme GABA transaminase.

GAD

Other Neurotransmitter Candidates and Intercellular

Messengers

ATP is concentrated in vesicles at many synapses in the CNS

and PNS, released into the cleft by presynaptic spikes in a Ca2+-

dependent manner.

ATP is often packaged in vesicles along with another classic

transmitter (e.g. catecholamine) which means they are probably

co-transmitters.

ATP directly excites some neurons by gating a cation channel.

ATP binds to purinergic receptors, both transmitter-gated ion

channels and a large class of G-protein coupled purinergic

receptors.


The interesting discovery in the past few

years is that small lipid molecules,

endocannabinoids (大麻酚,endogenous

cannabinoids), can be released from

postsynaptic neurons and act on

presynaptic terminals, called retrograde

signaling; thus, endocannabinoids are

retrograde messengers, a kind of

feedback regulation.

Vigorous firing in the postsynaptic

neuron → Ca2+ influx throughvoltage-

gated calcium channels of postsynaptic

neurons→ [Ca2+ ]i increase→ stimulates

the synthesis of endocannabinoid

molecules from membrane lipids.


Retrograde signaling

with endocannabinoids

There are several unusual qualities about endocannabinoids:

1. They are not packaged in vesicles like other neurotransmitters;

instead, they are produced rapidly and on-demand.

2. They are small and membrane permeable; once synthesized,

they can diffuse rapidly across the membrane to contact

neighboring cells.

3. They bind selectively to the CB1 type of cannabinoid

receptor, mainly located on certain presynaptic terminals.


Cannabis

Marijuana

大麻

THC (Δ9-tetrahydrocannabinol)

Short-term effects on brain?

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CB1 receptors are GPCRs, and their main effect is often to

reduce the opening of presynaptic calcium channels, and

inhibit release of its neurotransmitter.

Gaseous molecule, nitric oxide (NO). Carbon monoxide (CO)

has also been suggested as a messenger, being extensively

studied and hotly debated.

Many of the chemicals we call neurotransmitters may also be

present and function in non-neural parts of the body. (Amino

acids, ATP, Nitric oxide, Ach, serotonin)

Transmitter-gated Channels

The transmitter-gated ion channels are magnificent tiny

machines.

A single channel can be a sensitive detector of chemicals and

voltage, it can regulate the flow of surprisingly large currents

with great precision, it can sift and select between very

similar ions, and it can be regulated by other receptor systems.

Yet each channel is only about 11 nm long, just barely

visible with the best computer-enhanced electron microscopic

methods.


The Basic Structure of Transmitter-Gated Channels

The most thoroughly studied transmitter-gated ion channel is

the nicotinic ACh receptor at NMJ. Five subunits arrange like a

barrel to form a single pore. Four different subunits α, β, γ, δ

are used. There is one ACh binding site on each of the α

subunits.

The nicotinic ACh receptor on neurons is also a pentamer, but,

unlike the muscle receptor, most of them are comprised of α

and β subunits only.

The subunits of GABA- and Glycine-gated channels have a

similar primary structure to the nicotinic ACh receptor, with

four hydrophobic segments to span the membrane, also thought

to be pentameric complexes.


The subunit arrangement of the nicotinic ACh receptor (a) Side view showing how the four α-helices of each subunit packed together.

(b) Top view showing the location of the two ACh binding sites.


Similarities in subunit structure for different transmitter-gated ion channels

(a) They have in common the four regions called M1–M4, which are segments

where the polypeptides will coil into alpha helices to span the membrane. Kainate

receptors are subtypes of glutamate receptors. (b) M1–M4 regions of the ACh

subunit, as they are believed to be threaded through the membrane.

?


The glutamate-gated channels are most likely tetramers

structure. The M2 region does not span the membrane, but

instead forms a hairpin that both enters and exits from the

inside of the membrane, resembling potassium channels.

The purinergic (ATP) receptors also have an unusual structure.

Each subunit has only two membrane-spanning segments, and

the number of subunits of a complete receptor is not known.

Different transmitter binding sites let one channel respond to

distinct transmitters; certain amino acids around the narrow ion

pore allow only Na+ and K+ to flow through some channels,

Ca2+ through others, and only Cl- through yet others.


Amino Acid-Gated Channels

Amino acid-gated channels mediate most of the fast synaptic

transmission in the CNS.

Several properties of these channels distinguish them from

one another and define their functions within the brain.

All these properties are a direct result of the molecular

structure of the channels.

pharmacology

kinetics

selectivity

conductance


Glutamate-Gated Channels.

Three glutamate receptor subtypes: AMPA, NMDA, and

kainate. The AMPA- and NMDA-gated channels mediate the

bulk of fast excitatory synaptic transmission in the brain.

AMPA-gated channels are permeable to both Na+ and K +, and

most of them are not permeable to Ca2+. The net effect is to

admit Na+ ions into the cell, causing a rapid and large

depolarization.

NMDA-gated channels differ from AMPA receptors in two

very important ways: (1) NMDA-gated channels are permeable

to Ca2+, and (2) inward ionic current through NMDA-gated

channels is voltage dependent.


The coexistence of NMDA and AMPA receptors in the postsynaptic membrane

of a CNS synapse. (a) An impulse arriving in the presynaptic terminal causes the

release of glutamate. (b) Glutamate binds to AMPA-gated and NMDA-gated

channels in the postsynaptic membrane. (c) The entry of Na through the AMPA

channels, and Na and Ca2 through the NMDA channels, causes an EPSP.


Inward ionic current through the

NMDA-gated channel.

(a) Glutamate alone causes the channel

to open, but at the resting membrane

potential, the pore becomes blocked by

Mg2+ ions.

(b) Depolarization of the membrane

relieves the Mg2+ block and allows Na+

and Ca2+ to enter.


It is hard to overstate the importance of intracellular Ca2+ to cell

functions. presynaptical and postsynaptical; physiological and

pathological. Thus, activation of NMDA receptors can, in

principle, cause widespread and lasting changes in the

postsynaptic neuron.

The magnitude of this inward Ca2+ and Na+ through NMDA-

gated channels depends on the postsynaptic membrane potential

in an unusual way, for an unusual reason. “magnesium block”,

voltage dependent release. Both glutamate and depolarization

must coincide before the channel will pass current. This

property has a significant impact on synaptic integration at

many locations in the CNS.


GABA-Gated and Glycine-Gated Channels.

GABA mediates most synaptic inhibition in the CNS, and

glycine mediates most of the rest. Both are chloride channels.

Synaptic inhibition must be tightly regulated in the brain. Too

much causes a loss of consciousness and coma; too little leads to

a seizure.

It is why the GABAA receptor has several other sites where

chemicals can dramatically modulate its function. e.g.

Benzodiazepines（苯二氮卓）and barbiturates （巴比妥）.

When GABA is present, increase the frequency, or the duration

of channel openings, respectively, thus, more Cl- current, stronger

IPSPs, enhanced behavior inhibition. And selective for GABAA

receptor, no effect on glycine receptor.


Ethanol, another popular drug, strongly enhances GABAA

receptor function in a way of dependence on the receptor

specific structure. Ethanol has also complex effects on

NMDA, glycine, nicotinic ACh, and serotonin receptors.

What is the endogenous ligands for these drug binding sites?

They may serve as regulators of inhibition. Substantial

evidence indicates that natural benzodiazepine-like ligands

exist. Other good candidates as natural modulators of

GABAA receptors are the neurosteroids (神经甾体), natural

metabolites of steroid hormones, but also in glial cells of the

brain. Some neurosteroids enhance inhibitory function while

others suppress it, and they seem to do so by binding to their

own site on the GABAA receptor.


The binding of drugs to the GABAA receptor

The drugs by themselves do not open the channel, but change the

effect when GABA binds to the channel at the same time as the drug.

G-Protein Coupled Receptors and Effectors

Transmission at GPCRs involves three steps: (1) binding of the

neurotransmitter to the receptor protein, (2) activation of G-

proteins, and (3) activation of effector systems.

The Basic Structure of GPCRs

A family members (about 100), a single polypeptide, seven

membrane-spanning α-helices, two extracellular loops

(binding sites). Two intracellular loops (binding to and

activating G-proteins)

Structural variations at these two sites determine which

agonist binding and which G-proteins and effector systems

activated in response to transmitter binding.


The basic structure of a G-protein-coupled receptor.

Most metabotropic receptors have seven membrane-spanning α-

helices, a transmitter binding site on the extracellular side, and a G-

protein binding site on the intracellular side.


The Ubiquitous G-Proteins

G-proteins are the common link signaling pathways; GTP

binding protein, about 20 family members; Less than

transmitter receptors.

The same basic mode of operation:

1. Each has three subunits, α, β, and γ. In the resting state, GDP

is bound to the Gα.

2. If this G-protein hits the proper receptor with a transmitter

bound , then releases its GDP and exchanges it for a GTP.

3. The activated G-protein splits into 2 parts: the Gα plus GTP,

and the Gβγ complex. Both can influence various effectors.

4. The Gα is itself an enzyme that eventually breaks down GTP

into GDP, and terminates its own activity.

5. The Gα and Gβγ subunits come back together, allowing the

cycle to begin again.

The basic mode of operation of G-proteins

(a) In its inactive state, the α subunit of the G-protein binds GDP. (b) When

activated by a G-protein-coupled receptor, the GDP is exchanged for GTP. (c) The

activated G-protein splits, and both the Gα (GTP) subunit and the Gβγ subunit

become available to activate effector proteins. (d) The G subunit slowly removes

phosphate (PO4) from GTP, converting GTP to GDP and terminating its own activity.



G-Protein-Coupled Effector Systems

Activated G-proteins exert their effects by binding to either of:

G-protein-gated ion channels and G-protein-activated enzymes.

The Shortcut Pathway:

A variety of neurotransmitters use the shortcut pathway, from

receptor to G-protein to ion channel. e.g. ① the muscarinic

receptors in the heart to explain why ACh slows the heart rate.

② neuronal GABAB receptors.

The fastest of the G-protein-coupled systems (30–100 msec)

since no intermediary between receptor and channel. And also

very localized since it is within the membrane and cannot

move very far.

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.


Second Messenger Cascades.:

G-proteins can also directly activate certain enzymes and the

laters trigger an elaborate series of biochemical reactions, a

cascade that often ends in the activation of other “downstream”

enzymes that alter neuronal function.

Between the first enzyme and the last are several second

messengers. The whole process that couples the

neurotransmitter, via multiple steps, to activation of a

downstream enzyme is called a second messenger cascade.

the cAMP second messenger cascade initiated by the

activation of the NE β receptor.


The components of a second messenger cascade


Many biochemical processes are regulated with a push-pull

method, one to stimulate them and one to inhibit them, and

cAMP production is no exception.

The activation of NE α2 receptor leads to the activation of Gi,

which suppresses the activity of adenylyl cyclase.

Some messenger cascades can branch. e.g. G-proteins →

PLC → PIP2 → DAG + IP3. DAG (within the membrane) →

PKC; IP3 (water-soluble) → IP3 receptor (IP3-gated calcium

channels) on the smooth ER and other organelles → discharge

of stored Ca2+. Elevation in cytosolic Ca2+ can trigger

widespread and long-lasting effects. One effect is activation

of CaMK, an kinase implicated in memory.


The stimulation and inhibition of adenylyl cyclase by different

G-proteins. (a) Binding of NE to the β receptor activates Gs, which

in turn activates adenylyl cyclase. Adenylyl cyclase generates cAMP,

which activates the downstream enzyme protein kinase A. (b)

Binding of NE to the α2 receptor activates Gi, which inhibits

adenylyl cyclase.


Second messengers generated by the breakdown of PIP2, a

membrane phospholipid. ➀ Activated G-proteins stimulate

the enzyme phospholipase C (PLC). ➁ PLC splits PIP2 into DAG

and IP3. ➂ DAG stimulates the downstream enzyme protein kinase

C (PKC). ➃ IP3 stimulates the release of Ca2+ from intracellular

stores. The Ca2+ can go on to stimulate various downstream enzymes.


Phosphorylation and Dephosphorylation:

key downstream enzymes in many of the second messenger

cascades are protein kinases (PKA, PKC, CaMK), that

transfer phosphate from ATP to proteins (phosphorylation), It

changes the protein’s conformation slightly, thereby changing

its biological activity. e.g. ion channels.

NE β receptors on cardiac muscle cells → rise in cAMP

activates PKA → phosphorylation of voltage-gated calcium

channels → more Ca2+ influx → heart beats more strongly.

By contrast, the β-adrenergic receptors in neurons inhibits

certain potassium channels. Reduced K+ conductance causes a

slight depolarization, making the neuron more excitable.



Protein phosphorylation and dephosphorylation.

Protein phosphatases act rapidly to remove phosphate groups.

The degree of channel phosphorylation depends on the

dynamic balance of phosphorylation by kinases and

dephosphorylation by phosphatases.


The Function of Signal Cascades.

Transmission using transmitter-gated channels is simple and

fast, while that involving GPCRs is complex and slow.

The advantages of having such long chains of command:

1. one is signal amplification: The activation of one GPCR can

lead to the activation of not one, but many, ion channels.

2. the use of small messengers that can diffuse quickly (such as

cAMP) allows signaling at a longer distance.

3. provide any sites for further regulation, as well as interaction

between cascades.

4. Finally, generate very long-lasting chemical changes in cells,

which may form the basis for a lifetime of memories.


Signal amplification by

G-protein coupled second

messenger cascades

When a transmitter activates

a GPCR, there can be

amplification of the

messengers at several stages

of the cascade, so that

ultimately many channels

are affected.

The ability of one transmitter to activate more than one subtype

of receptor, and cause more than one type of postsynaptic

response, is called divergence (幅散). e.g. Glutamate and

GABA. Divergence is the rule among neurotransmitter systems.

It even occurs at points beyond the receptor level, depending on

which G-proteins and which effector systems are activated.

Neurotransmitters can also exhibit convergence (会聚) of

effects. Multiple transmitters can converge to affect the same

effector systems at the level of the G-protein, the second

messenger cascade, or the type of ion channel.

Neurons integrate divergent and convergent signaling systems,

resulting in a complex map of chemical effects.



Divergence and convergence in

neurotransmitter signaling

systems.

(a) Divergence.

(b) Convergence.

(c) Integrated divergence and

convergence.

扩展阅读及章后思考题 Box 6.1 Pumping Ions and Transmitters

Box 6.2 This is your brain on Endocannabinoids.

Box 6.3 Deciphering the language of neurons.

Box 6.4 The Brain’s exciting Poisons

谢谢！

neurotransmitter systemsm-learning.zju.edu.cn/g2s/ewebeditor/uploadfile/... · 2014-11-25 ·...

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