Class 10 - Neurotransmitters
Neurotransmitters
Where we left off
Chemical transmission involves packaging NTs into SVs
The transporter present on SV determines what can be packaged
Synaptic vesicle release is
Ca2+-dependent
is regulated
SV are recovered and refilled
Termed synaptic vesicle cycle
Synaptic vesicle recycling
molecular mechanisms regulate synaptic transmission which are critical for function
Need to be able to respond to sustained signals
To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle
Will study this in detail in next Unit
We do not make a new SV everytime from the soma
Neurotransmitters
Are chemical transducers
Signal that one neuron gives to another
released by electrical impulse into the synaptic cleft from presynaptic membrane from synaptic vesicles
they then diffuse to the postsynaptic membrane
react and activate the receptors present leading to initiation of new electrical signals.
Properties of neurotransmitters
Synthesized in the presynaptic neuron
Localized to vesicles in the presynaptic neuron
Released from the presynaptic neuron under physiological conditions
Rapidly removed from the synaptic cleft by uptake or degradation
Presence of receptor on the post-synaptic neuron
Binding to the receptor elicits a biological response
Neurotransmitter criteria
A CLASSICAL NEUROTRANSMITTER
Fate of neurotransmitters
consumed (broken down or used up) at postsynaptic membrane leading to action potential generation.
Degraded by enzymes present in synaptic cleft.
NMJ
Reuptake mechanism (reutilization)
this is the most common fate.
Chemical Classification of neurotransmitters
Can also be classified into categories based on size
1. Small molecule transmitters (Classical)
Ester
Amines
Amino acids
2. Neuroactive peptides
3. Gaseous neurotransmitters
4. ATP
5. Endocannabinoids
Peptides = non classical neurotransmitters
Other
Gas neurotransmitters
ATP
Endocannabinoids
Gases and endocannabinoids cannot be packed into SV because they can diffuse
But they elicit change in another neuron and have receptors
Classical Neurotransmitters
small molecule transmitters
synthesized in cytoplasm from compounds of intermediary metabolism
Ester (Acetylcholine)
Biogenic Amines (dopamine, norepinephrine, epinephrine, serotonin, histamine)
Amino Acids (gamma-amino butyric acid = GABA, glycine, glutamate)
Neuroactive Peptides
synthesized on rough ER and processed for secretion by the Golgi apparatus
1. Hypothalamic-releasing Hormones
2. Neurohypophyseal Hormones
Vasopressin, oxytocin
3. Adenohypophyseal Hormones
Growth hormone, endorphins, melanocyte-stimulating hormone
4. Gastrointestinal Peptides
Gastrin, substance P, insulin, glucagon, vasoactive intestinal peptide
5. Others
angiotensin, bradykinin, sleep peptides
Debatable whether these are neurotransmitters or a class by themselves: hormones
do not fit all the criteria of neurotransmitters
their actions are far-reaching, rather than just synaptic
Atypical Neurotransmitters
In addition to the conventional neurotransmitters, some unusual molecules are used for signaling between neurons and their targets.
can be considered as neurotransmitters because of their roles in interneuronal signaling and because their release from neurons is regulated by Ca2+.
are unconventional in comparison with typical neurotransmitters because
they are not stored in synaptic vesicles
not released from presynaptic terminals via exocytotic mechanisms.
are often associated with retrograde signaling
i.e. from postsynaptic cells back to presynaptic terminals
(back to the soma)
Include gaseous transmitters and endocannabinoids and ATP
ATP
Virtually most, and possibly all, types of secretory vesicles found in cells contain ATP, which often accumulates at high concentrations and, commonly, in conjunction with different types of neurotransmitters.
Thus synaptic vesicles released by the terminal membrane of a nerve contain ATP as well as other neurotransmitters
BUT ATP can only function as a neurotransmitter if the post-synaptic terminal membrane contains ATP receptors (purigenic receptors)
Activity-dependent release of ATP from synapses, axons and glia activates purinergic membrane receptors that modulate intracellular calcium and cyclic AMP.
ATP is now recognized as a co-transmitter in all nerves in the peripheral and central nervous systems.
In addition to vesicular release there are also non-vesicular mechanisms for ATP release from neurons which are likely to have a broader range of functions than synaptic release
Voltage- and Ca2+ o-gated CALHM channels constitute a novel ion channel family
Voltage gated Calhm1/3 channels can release ATP I taste receptor cells
Receptor…P2X receptor
Gaseous transmitters
NO and CO are small gaseous molecules that can be synthesized de novo in neuronal tissue and can diffuse readily through the plasma membrane.
Not characterized as neurotransmitters until the 1980s
Biosynthetic enzymes localized to specific neuronal populations and inhibitors of the enzyme block neurotransmission in certain systems
Somewhat controversial as they cannot be stored in synaptic vesicles, released by exocytosis, or act at receptor proteins on cell membranes
Endocannabinoids
a family of related endogenous signals that interact with cannabinoid receptors
anandamide and 2-arachidonoylglycerol (2-AG) have best evidence
At least two types of cannabinoid receptors have been identified
CB1 and CB2
GPCRs
most actions of endocannabinoids in the CNS mediated by the CB1 type
participate in several forms of synaptic regulation.
best evidence so far is inhibition of communication between presynaptic inputs and their postsynaptic target cells.
Can have neurotransmitters we consider released on the postsynaptic cells and signaling back to the presynaptic terminal
Original Dale’s Law (1950s)
“A mature neuron makes use of the same transmitter substance at all of its synapses”
Discovery of peptide transmitters led to modification
Modified Dale’s Law = “A mature neuron makes use of the same combination of chemical transmitters at all of its synapses.”
when co- secretion occurs, it usually involves a small molecule transmitter and one or more peptides
SVs vs DCVs
Small, clear synaptic vesicles (SVs) generally contain low–molecular weight neurotransmitters, whereas neuropeptides are packaged in larger, dense-core vesicles (DCVs)
Can regulate their endocytosis differently
Packaging different classes of molecules into SVs and DCVs provides for an effective means of regulating their exocytosis independently
DCVs often contain multiple neuropeptides, which are released together during DCV exocytosis
Small molecule transmitter: Synthesis, transport, and release
1. Enzymes for synthesis made in rER
2. Transported to Golgi
3. Modified in Golgi (e.g. glycosylated)
4. Transported down axons
Soluble enzymes – slow anterograde axonal transport
Other enzymes – fast axonal transport
5. Precursors taken up by transporters and used by enzymes to synthesize transmitter in terminal
6. Synthesized neurotransmitter loaded into vesicles by vesicular transport proteins
Typically into clear-core vesicles (synaptic vesicles)
Exceptions: biogenic amines often in both SVs and DCVs
Serotonin and norepinephrine in dense-core vesicles
7. Stimulus results in their release
Released transmitter is deactivated by enzymes or re-uptake
Neuropeptide transmitters: Synthesis, transport, and release
1. Polypeptides (pre-propeptides: larger than final peptide) are synthesized in rER and converted into a pro-peptide. Enzymes for cleavage are also synthesized in rER
2. Both transported to Golgi and packaged into vesicles
3. Pro-peptide and enzyme filled vesicles moved down axons by fast axonal transport
4. Enzymes cleave propeptide to produce peptide neurotransmitter
Remains in dense-core vesicle
5. Released into synaptic cleft by exocytosis
6. After release, peptide neurotransmitters diffuse away and are degraded by proteolytic enzymes
No reuptake like small-molecule neurotransmitters
A number of different receptors
Agnost
Binds to a receptor and elicits the same response as a ligand
Antagonist
Binds to receptor and prevents response
Seven processes in neurotransmitter action
Anything not packaged quickly gets destroyed by enzymes
Packaged into vesicles protects neurotransmitters from enzymes
Drugs
Neurotransmitters found in the nervous system
* metabotropic receptors are modulatory
The neurotransmitter when it binds to the receptor signals
Early in development GABA is excitatory
Ester: Acetylcholine (ACh)
The first transmitter to be discovered.
First identified by Henry Dale (1915) and confirmed as a neurotransmitter by Otto Loewi, 1921
The primary neurotransmitter secreted by the efferent axons (motor neurons) of the central nervous system.
All muscular movement is accomplished by the release of acetylcholine.
Appears to be involved in regulating voluntary movement, REM sleep, perceptual learning, and memory.
Too much acetylcholine is associated with depression, and too little in the hippocampus has been associated with dementia
ACh synthesis
In the cholinergic neurons acetylcholine is synthesized from choline.
Choline is a water-soluble essential nutrient (get it from our diet)
Dietary source
This reaction is activated by choline acetyltransferase (ChAT)
In CNS
ACh removal
acetylcholinesterase (AChE) is a serine protease that hydrolyzes the neurotransmitter acetylcholine.
↑ [AChE] present in synaptic cleft
AChE synthesized in rER in soma and major dendrites and transported to presynaptic terminal
Hydrolyses ACh in the synaptic cleft
Liberated choline is taken back up by terminal and reused for synthesis of ACh
Very few places where we actually make ACh in the CNS…but a lot of projections of cholinergic neurons
ACh Neuropharmacology
Disesases ACh implicated in
Myasthenia Gravis
an autoimmune disease which results in muscle fatigability and weakness throughout the day.
Antibodies attack acetylcholine (ACh) receptors at the post-synaptic neuromuscular junction (NMJ)
cause ACh receptor blockade
Alzheimer's
ACh+ neurons in nucleus of Meynert degenerated
Sleep disorders
ACh is involved in controlling REM sleep as well as arousal
In patients with decreased ACh (like aging and Alzheimer's) REM sleep, total sleep time, and delta sleep are decreased
Drugs and toxins that influence ACh
Donepezil (Aricept)
AChE inhibitor
Used for mild to moderate patients
Botulinum toxin
An Ach antagonist
prevents release by terminal buttons.
Black widow spider venom
triggers the release of ACh.
Neostigmine
inhibits AChE
Used to improve muscle tone in people with myasthenia gravis
Hemicholinium
A drug that inhibits the uptake of choline.
has no clinical use
Glutamate
The carboxylate anions and salts of glutamic acid are known as glutamate
Is a non-essential amino acid
Does not cross BBB
Must be synthesized locally
We have to make it…cannot get it from diet
is the most abundant excitatory neurotransmitter in the vertebrate nervous system.
Nearly all excitatory synapses in CNS are glutamtergic
Over ½ of all synapses release glutamate
Elevated concentrations are toxic
Often occurs after injury
Have to regulate the release
Does have some other functions
Synthesis of proteins and peptides
Important for Learning and memory
Glutamate synthesis
a nonessential amino acid that does not cross the blood-brain barrier and must be synthesized in neurons from local precursors
Synthesized in CNS by 2 processes
1. Synthesized from glutamate recycled from synaptic cleft (see diagram)
the most prevalent glutamate precursor in synaptic terminals is glutamine
2. some of the glucose metabolized by neurons can also be used for glutamate synthesis.
Glucose enters neuron by facilitated diffusion and is metabolized
synthesized by transamination of 2-oxoglutarate, an intermediate of the tricarboxylic acid (TCA) cycle
VGluTs – package Glutamate into SVs
EAATS – excitatory amino acid transporters
Remove glutamate from synaptic cleft
Glutamate Removal
the action of glutamate released into the synaptic cleft is terminated by uptake into neurons and surrounding glial cells via specific transporters.
EAATS
Important to protect against excitotoxicity
Red = membrane transporter
Blue = vesicular transporter
Glutamate: neuropharmacology
Diseases glutamate implicated in
Excitotoxicity (excessive stimulation of neurotransmitters kills neurons) has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases
Cerebral ischemia
traumatic brain injury (TBI)
prolonged seizure activity
ALS
excitotoxicity
Alzheimers
Excitotoxicity
Drugs that influence Glutamate
Memantine (Namenda)
NMDA receptor antagonist
mitigates excitotoxicity and slows down Alzheimer's symptoms
GABA
γ-Aminobutyric acid
Although chemically it is an amino acid, it is not an alpha amino acid, and is never incorporated into a protein.
chief inhibitory neurotransmitter in the mammalian CNS
binding causes the opening of ion channels to allow the flow of either Cl- ions into the cell or K+ out of the cell.
causes a negative change in the transmembrane potential, usually causing hyperpolarization.
~1/3 of CNS synapses use GABA
Regulates neuronal excitability
also directly responsible for the regulation of muscle tone
GABA synthesis
Synthesized from glutamate (therefore glucose primary precursor)
Vesicular GABA transporters are the vGATS
GAD – glutamic acid decarboxylase
Exclusively in GABAergic neurons
Requires pyridoxal phosphate for activity
Derived from VitaminB6
Thus B6 deficiency can lead to ↓ GABA synthesis
Removal by transporters, GATs
In terminals and glia
Degradative enzymes are in mitochondria
GABA: neuropharmacology
Disease GABA implicated in
Epilepsy
Infant seizures
Vitamin B6 deficiency can impair GABA synthesis
Huntington's
Parkinson's
Dementia
Alzheimer's
Schizophrenia
Drugs that influence GABA
Valproic acid
Anticonvulsant
Inhibits GABA transminase
Barbiturates
Agonists or modulators of GABA receptors
Used to treat epilepsy
γ-hyydroxybuturate
GABA Derivative (CNS depressant)
Date rape drug (also called cherry meth, liquid X, fantasy, organic quaalude, GBH, salty water, Georgia home boy, scoop, sleep-500, soap, liquid E, somatomaz, liquid ecstasy, and
vita-G.Euphoria, memory deficits, unconsciousness
Treatment for narcolepsy
Increase GABA to terminate seizures
Seizures facilitated by lack of neuronal inhibition
GABA is a major source of the drugs used to treat various things
Biogenic amines
regulate many CNS functions
Homeostasis to attention
Also active in PNS
Defects implicated in most psychiatric disorders
Pharmacology of the amine synapse critically important for psychotherapy
Drugs affecting synthesis, receptor binding or catabolism amongst the most important in modern pharmacology
5 well established biogenic amine neurotransmitters
Catecholamines (synthesized from Tyrosine)
Domapine
Norepinephrine (noradrenaline)
Ephinephrine (adrenaline)
Indolamines (synthesized from Tryptophan)
Serotonin
Histamine
Catecholamines in CNS
Widespread projections…not that many neurons
Catecholamine Synthesis
Biogenic amines regulate many CNS functions
Homeostasis to attention
Also active in PNS
Defects implicated in most psychiatric disorders
Which enzymes you traffic to the terminal determines how far donw the pathway you go
Dopamine
a simple organic chemical in the catecholamine family
As a chemical messenger, dopamine is similar to adrenaline.
plays a major role in the brain system that is responsible for reward-driven learning
a variety of highly addictive drugs, including stimulants such as cocaine and methamphetamine, act directly on the dopamine system.
affects brain processes that control movement, emotional response, and ability to experience pleasure and pain.
people with extraverted (reward-seeking) personality types tend to show higher levels of dopamine activity than people with introverted personalities
Dopamine syntheiss
Produced by conversion of
1. tyrosine to DOPA by TH,
2. DOPA to dopamine by Dopa-decarboxylase
Vesicular DA transporters are the vesicular monoamine transporters (vMATs)
vMAT1 and vMAT2
Load all monoamine transporters into SVs
2 major enzymes involved in catabolism of DA
MAO (mitochondria)
COMT (cytoplasm)
Neurons contain both MAO and COMT
Reuptake by transporters in terminals
DAT (Dopamine transporter)
Na +/Cl − -dependent reuptake of extracellular dopamine (DA)
Can also be removed by NET (Norepinephrine transporter)
Remaining DA diffuse to circulatory system and destroyed in liver
Tyrosine
Amino acid used in protein synthesis
Can be synthesized in the body from phenylalanine
Dietary sources. Found in many high-protein food products
Dopamine: neuropharmacology
Diseases DA implicated in
Parkinsons
DA+ neurons degenerate in Substantia nigra
Addiction
Opioid and cannabinoid transmission instead of dopamine may modulate consummatory pleasure and food palatability
Drugs that influence DA
Neuroleptics (antipsychotics)
reduce dopamine activity
impair concentration
reduce motivation
cause anhedonia
Levodopa
Dopamine precursor
Used to treat Parkinson's disease and dopa-responsive dystonia
Carbidope (Sinemet)
Inhibitor of dopa-decarboxylase
Used to treat epilepsy
Norepinephrine (noradrenaline)
multiple roles including as a hormone and a neurotransmitter
PNS: Released from sympathetic neurons
CNS: in neurons of locus coeruleus (brain stem)
Influences sleep and wakefulness, attention, feeding
Acts as a stress hormone,
Affects amygdala, where attention and responses are controlled
Norepinephrine synthesis
1. Tyrosine (Tyr) is converted to DOPA by tyrosine hydroxylase (rate-limiting step for NE synthesis).
2. DOPA is converted to dopamine (DA) by DOPA decarboxylase.
3. Dopamine is transported into vesicles then converted to norepinephrine (NE) by dopamine β-hydroxylase (DBH)
transport into the vesicle can by blocked by the drug reserpine.
4. When NE released DBH is released along with the NE.
5. Removed via Norepinephrine transporter (NET) transporters
1. In terminals
2. Can also remove dopamine
Epinephrine (adrenaline)
a hormone and a neurotransmitter
PNS: increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system
CNS
lower levels and fewer neurons than other catecholamines
Primarily in lateral tegmental system and medulla (brainstem)
Function unknown
Also made in adrenal medulla
Release controlled by sympathetic nervous system
Epinephrine synthesis
PNMT only found in epinephrin-secreting neurons
Loaded into SVs by transporter, VMAT
have not identified specific transporter yet
NET capable of uptake
Extra enzymes…can’t be a dopaminergic neuron…will turn to norepinephrine….
Metabolism
NE and epinephrine are metabolized by catechol-O-methytransferase (COMT) and monoamine oxidase (MAO).
The final product of these pathways is vanillylmandelic acid (VMA).
Histamine and Serotonin in CNS
Serotonin
5-hydroxytryptamine (5-HT)
a monoamine neurotransmitter
in humans, serotonin levels are affected by diet.
An increase in the ratio of tryptophan to phenylalanine and leucine will increase serotonin levels.
Involved in regulation of mood, appetite, and sleep.
has some cognitive functions, including memory and learning.
Modulation of serotonin at synapses is thought to be a major action of several classes of pharmacological antidepressants.
thought to be a contributor to feelings of well-being and happiness
Found in gastrointestinal (GI) tract, platelets, and CNS
CNS: primarily in neurons of Raphe nucleus and PONS and upper brainstem
Serotonin synthesis
Biochemically derived from tryptophan
Amino acid used in protein synthesis
Essential amino acid
Dietary sources
routine constituent of most protein-based foods or dietary proteins
When food is ingested that contains tryptophan, the molecule is extracted during metabolic processes that take place in the small intestine, and absorbed into circulation.
There, it travels through the body, crosses the blood–brain barrier, and enters neurons, where it gets metabolized into indolamine neurotransmitters (classification of monoamine neurotransmitters including serotonin and melatonin), as well as niacin.
Serotonin synthesis and reuptake
Loaded into SVs by vMAT
Reuptake by SERT
Present in terminals
Metabolism
Serotonin is degraded in two reactions
5HT: neuropharmacology
Diseases 5HT implicated in
Depression
Migraine
Drugsst ath influence
SSRIs (e.g. Prozac)
Inhibit 5HT uptake in presynaptic terminals
Increase extracellular levels
Sumatriptan (Imatrex)
5HT receptor agonist
Vasoconstrictor of intracranial arteries
Used to treat migraine
NT Transporters
EAATS – excitatory amino acid transporters
Transporters and Co-transmissions
DO NOT NEED TO KNOW FOR THE TEST
Purinergic transmission
The purines are another group of small molecule transmitters
adenosine triphosphate (ATP)
adenosine
Excitatory in both CNS and PNS
Released with other neurotransmitters
co-transmitters
Act principally as neuromodulators but can be primary mediators
Neuromodulator vs Neurotransmitters
Neurotransmitter
a messenger released from a neuron at an anatomically specialized junction, which diffuses across a narrow cleft to affect one or sometimes two postsynaptic neurons, a muscle cell, or another effector cell.
Neuromodulator
a messenger released from a neuron in the central nervous system, or in the periphery, that affects groups of neurons, or effector cells that have the appropriate receptors.
It may not be released at synaptic sites, it often acts through second messengers and can produce long-lasting effects.
The release may be local so that only nearby neurons or effectors are influenced, or may be more widespread, which means that the distinction with a neurohormone can become very blurred
Neurohormone
a messenger that is released by neurons into the extracellular fluid/blood and which may therefore exert its effects on distant peripheral targets.
It may differ only in degree from a neuromodulator in the extent of its action
Gaseous transmitters
Fairly newly recognized class of neurotransmitters. Include
Nitric oxide (NO)
carbon monoxide (CO)
Do not satisfy some of the criteria formulated for classic neurotransmitters
Not stored in vesicles (made when needed)
Not released by Ca2+-dependent exocytosis from a presynaptic terminal
Inactivation is passive (no active process to terminate action)
Do not act on receptors on target cells
Not confined to a pre- to post-synaptic direction
Depends on diffusion
Acts as retrograde messengers
Regulates function of axon terminals presynaptic to the neuron in which it was made
Nitric oxide (NO)
a gas produced by nitric oxide synthase (NOS)
NOS is an enzyme that converts arginine to citrulline, thereby releasing NO
Can be considered a second messenger, rather than a neurotransmitter
Activates guanylyl cyclase to produce cGMP
NO can permeate plasma membrane (lipid soluble)
therefore can influence nearby cells
Can diffuse a few 10s of µm from its site of production before degradation
Can coordinates activities of small network of neurons
Decays spontaneously by reacting with oxygen to produce inactive nitrogen oxides
NO signal only lasts a short time (seconds or less)
Regulates a variety of synapses which also employ conventional neurotransmitters
Presynaptic glutamatergic terminals best studied target for NO in CNS
* Able to signal back to another cell
Carbon monoxide (CO)
a neuronal form of heme oxygenase occurs in discrete neuronal populations in the CNS
CO formed from it may be involved in regulating levels of cyclic GMP
Also formed in cytolplasm
Rapidly diffuses through membrane and can effect local cells to stimulate productions on second messengers
Broken down rapidly (only exist for a few seconds)
Difficult to study!!
Endocannabinoids
a family of lipid molecules that act as key regulators of synaptic transmission and plasticity
are synthetized “on demand” following physiological and/or pathological stimuli
Canonical mechanism of action
Once released from postsynaptic neurons, eCBs typically act as retrograde messengers to activate presynaptic type 1 cannabinoid receptors (CB1) and induce short- or long-term depression of neurotransmitter release
Recent findings have revealed a number of less conventional mechanisms by which eCBs regulate neural activity and synaptic function, suggesting that eCB-mediated plasticity is mechanistically more diverse than anticipated.
mechanisms include non-retrograde signaling, signaling via astrocytes, participation in long-term potentiation, and the involvement of mitochondrial CB1.