Synapses and transmitters

Synapses


Fox, part of chapter 7 (part of chapter 12) (a figure from chapter 8)

Major point

Cell theory (cells being separated) implies that cells must communicate with each other through an extracellular connections and most communication is through chemical messages
Golgi developed a special staining that highlighted individual cells among the zillions in the CNS but did not believe that cells were separate; Ramon y Cajal used this technique a lot and did develop a cell theory; both won the 1906 Nobel prize

Fig. 7.23
Synaptic terminal has synaptic vesicles that contain and release chemical transmitter.
Calcium coming in through channel is involved in transmitter release into synaptic cleft.

Role of synapses in reflexes

Fig. 12.28
Fig. 12.29
Fig. 12-30
reflexes
knee-jerk reflex - tap patellar ligament, spindle (stretch receptor), dorsal root ganglion, monosynaptic, alpha motoneuron to muscle
too much stretch monitored by Golgi tendon organ, act through inhibitory interneuron (two synapses, less excitation)

->1906 Sir Charles S. Sherrington (England) Integrative Action of the Nervous System 1932 Nobel "functions of neurons" coined "synapse" = "clasp"
spinal reflex - Spinal motoneuron - Sherrington - final common pathway (for integrative action of the nervous system)

Fig. 12.31
integrated reflexes, reciprocal reflexes, extensor activated then flexor inhibited
integrated reflex, crossed-extensor reflex, if flexor activated, opposite extensor will be activated to compensate for weight support

History

1926 Otto Loewi (Austria) 1936 Nobel Prize
Reportedly, he thought of this experiment in a dream
vagus-stuff slows heart (10th cranial nerve, parasympathetic)
take substance and show that it slows a heart in another dish, vagus substance = acetylcholine (ACh) a monamine transmitter


Inhibition, excitation and integration

Fig. 7.34
Sir John C. Eccles 1963 Nobel (with Hodgkin & Huxley) EPSP & IPSP
Postsynaptic potentials (Eccles, using spinal motor neurons)
EPSP - depolarize
increase sodium and potassium conductance
IPSP - hyperpolarize
increase potassium and chloride conductance
Excitatory and Inhibitory integrate before axon hillock "decides" to fire.

Structure

Here are some classic pictures, work by Heuser and Hueser and Reese, of vesicle release at the neuromuscular junction, a transmission electon micrograph and a freeze fracture electron micrograph

Chemical synapses
Presynaptic membrane, cleft, Postsynaptic membrane (intracellular density seen in EM [electron microscopy]), vesicle
vesicles and T-shaped ribbons in Drosophila

Here is a transmission electron micrograph of a synapse

Vertebrate - inputs to cell or dendrite (spine)
Invertebrate - cell body is usually away from the action, and cells surround processes where connections are made, "neuropil(e)"

Vesicle release

Fig. 7.23
again

General

Vesicles are interesting.
Transmitter is very concentrated, there are pumps to move transmitter "up hill" (against gradient) into vesicle.
Sometimes part of transmitter synthesis is in vesicle.
Ca2+ in through voltage gated Ca2+ channel
Note that figure shows that Ca2+ activates calmodulin which activates protein kinase and that "kinase phosphorylates synapsin proteins"
There are vesicle membrane proteins, target (presynaptic) membrane proteins, and cytoplasmic proteins

A lot of detail

Vesicle proteins:
Synaptotagmin - binds calcium
Synaptobrevin / VAMP (vesicle-associated membrane protein) = v-SNARE (SNAP receptor)
Botulinum and Tetanus* toxin (clostridial toxins) are proteases which cleave synaptobrevin
Botulism (Clostridium botulinum) improper canning - block release
BoTox
When I ws 10, in the Cold War, we discussed, at the dining room table, how 1 teaspoon in the reservoir would kill the city. Now. 45 years later, people take it (injected) to get rid of face wrinkles.

*tetany is term for sustained muscle contraction based on twitches adding up
tetanus toxin cleaves synaptobrevin in inhibitory interneurons
the disease is contracted in deep (because it is an anaerobic bacterium) dirty puncture wounds
you would die with muscles contracted, called "lock-jaw"
there is a vaccine and boosters every 10 years are suggested

Target membrane proteins:
Syntaxin = t-SNARE = unc-18 (uncoordinated C. elegans roundworm mutant)
Neurexin - black widow spider venom (alpha Latrotoxin) causes too much release
Neuroexins bind to synaptotagmin

Cytoplasmic:
NSF - N-ethylmaleimide sensitive factor (ATPase activity when complex dispersed)
SNAP - soluable NSF associated protein
Rab3 (like ras, small GTP binding protein) (lots of rab's, specific for transport)
rabphillin

vesicle membranes are recycled

endocytosis is via coated pit, coated with protein called clathrin
coated pit from my work another (not related to synapses)
dynamin is protein required for pinching off coated pit
dynamin is product of temperature sensitive shibire mutant with paralysis at restrictive temperature
from my work, coated pits that fail to pinch off to coated vesicles in shibire

Neurotransmitters and neuromodulators

Fig. 7.24
Reminder how nerve works

Pharmacology was pivotal in discussing chemical transmission.
agonist - a drug that mimics the neurotransmitter
antagonist - a drug that blocks the neurotransmitter

Acetylcholine

Fig. 7.26
(shown before in the membrane lecture)
Nicotinic ACh receptor
Receptors in this figure are channels and this kind of transmission is called ionotropic.
For Acetylcholine (cholinergic transmission), the nicotinic receptor is an example.
Nicotine is an agonist (though it has some properties of an antagonist).
Channel opens and both K+ and Na+ conductance increase

Fig. 7.28
There is another kind of receptor, the G-protein-coupled receptor
For cholinergic transmission, the muscarinic receptor is an example

Aceylcholine metabolism
Dale 1936 Nobel "cholinergic" ("-ergic" used universally)
Dietary choline -reuptake or uptake (transporter is Na+ dependent) -> intraneural choline
-Choline-O-acetyltransferase-> H3-CO-O-CH2-N+-(CH3)3
Acetyl Co-A is acetate donor
Acetylcholinesterase blocked by malathion and neostigmine
organophosphates, nerve gas, etc

Catecholamines

Fig. 7.30
Axelrod
Adrenergic
(1970 Nobel ) Julius Axelrod, Noradrenalin: Fate and control of its biosynthesis, Science 173, 598-606, 1971. Science publishes Nobel Prize papers. Reflection, I saw Axelrod (twice) and he gave great talks, in an easy-going manner, said everything that was known.
tyrosine hydroxylase - rate limiting and regulated by end-product inhibition
calcium activates
it is DOPA quinones which polymerase to make melanin
substantia nigra is pale in Parkinson's disease => synthesis overlap
DOPA decarboxylase - gets rid of l vs. d
in insects, dopamine quinones "tan the hide"
dopamine beta hydroxylase - adds optical asymetry back again
interestingly, within vesicle
ATP is released with NE, ATPase turns to adenosine
Important agonists and antagonists and other drugs
PNMT (phentolamine N-methyltraansferase)
interestingly, in cytosol, necessitating transport out then in vesicle
Table 6.1 (31, this is part of the table, note mistake for ACh, no this projected version but not in the book)
Most removal is by transporters, but there is breakdown
MAO - monamine oxidase intracellular, inhibitors (MAOI's) are antidepressants
on outer mitochondrial membrane
COMT - catechol O-methyltransferase extracellular, but there are no inhibitors)
but reuptake most important

Parkinson's

(mentioned here because of dopamine)

Fig. 8.21
degeneration of substantia nigra
1817 Shaky palsey
Degenerate dopaminergic input to striatum from substantia nigra
Aflicted have bradykinesia, akinesia, rigitystilted gait, tremors, walk in shuffle, stone (expressionless) face, loss of affect.
1% of people over 50 years old
Lateral hypothalamic lesions make thin rat and some motivational defects, dopamine in medial forebrain bundle toward basal ganglia.
Dopaminergic neurons degenerate, animal model - 6-OHDA uptake makes peroxide, cells die.
Cannot give dopamine because it coes not cross the blood brain barrier.
Give l-DOPA (in large doses because l-AAAdeCOOHase is everywhere); give decarboxylase inhibitor carbidopa. Jill Smith in Dr. Fisher's lab (Bio, SLU) works on this.
Extrapyramidal motor syndrome also comes from long term administration of antipsychotic phenothiozines such as chlorpromazine (brand name Thorazine).
(Chronic use of these drugs also cause a corioretinopathy.)
There was a bad batch of street drugs with an impurity called MPTP which gave its users a Parkinson's like disease.
There had been some experimental cell transplant therapies - controversal.
Arvid Carlsson made contributions here and shared 2000 Nobel Prize in Physiology and Medicine.
Several famous people have Parkinson's - Pope, Mohammad Ali, Michael J Fox.
Mostly it is "sporatic" (not genetic), but familial cases have been interesting.
Alpha-synuclein, Parkin and DJ-1.

Signal transduction

TRANSPARENCY (from intro book)
peptide and small molecule hormones (and neurotransmitters) have membrane receptors
(steroids, covered later in the semester, bind intracellular receptor after crossing membrane)

TRANSPARENCY (from intro book)
Epinephrine activates a G protein signal transduction cascade

Fig. 7.31
1971 Nobel Earl W. Sutherland, Jr. (US) "mechanisms of actions of hormones"
Metabotropic neurotransmitter action, adrenergic, muscarinic cholinergic
G-protein-coupled receptor to heterotrimeric G protein
Alpha binds GTP (hence the name "G protein") activates adenylate cyclase converts ATP to make cAMP and that activates protein kinase A (PKA).
Alpha subunit eventually breaks down GTP to GDP and P ("inorganic" phosphate, i.e. PO4)
A kinase phosphorylates proteins.
Phosphodiesterase inactivates cAMP and caffeine inhibits PDE

More

There is so much more that could be said about neurotransmitters! The chapter does have more. Maybe we will get back to them. But we will stop with Adrenergic and Cholinergic for now to pave the way for the lecture outline on the Autonomic nervous system, so important in Physiology.

Exam questions from 2004 - 2011 relevant to this outline

"Hodgkin and Huxley won a Nobel Prize for telling us about Na+ and K+ conductances and how they mediated the action potential." What additional type of channel becomes critical when the action potential arrives at the axon terminal where the presynaptic membrane is located?

one for Ca2+

There is another kind of receptor for acetylcholine, other than nicotinic. Answer one of the following (1) Describe the structure of this other receptor. (2) What is it called (a pharmacological name)? or (3) What is a famous drug that blocks this other receptor?

(1) crosses membrane 7 times (2) muscarinic

We would get an EPSP with cholinergic activation of a nicotinic receptor. Conductance to what two ions is increased?

sodium and potassium

Why would it be ineffective to feed dopamine to a patient with Parkinson's disease?

it does not cross the blood brain barrier

Why doesn't cAMP keep activating PKA forever?

a phosphodiesterase turns it to AMP

Although Ramon y Cajal used Golgi's technique, and the two shared the Nobel Prize in 1906, only Ramon y Cajal's viewpoint required the existence of synaptic connections such as Nobelist Sherrington proposed. In what way did Ramon y Cajal's vs Golgi's viewpoints differ regarding this issue?

only Ramon y Cajal thought cells were separate entities that would need to signel to eachother

"There are no inhibitory neuromuscular junctions (where the spinal motor neuron makes connection to the muscle cell)." Then how is the flexor inhibited in the knee-jerk reflex?

via an inhibitory interneuron in the spinal cord's gray matter

"While you are sitting quietly in physiology lecture, unstressed, your heart is beating slower than it would automatically." Say something about (1) the transmitter. or (2) the nerve.

1 acetylcholine 2 vagus 10th cranial nerve, parasympathetic

Among the chemical intermediates in the synthesis of the neurotransmitter norepinephrine from tyrosine, there is a chemical that was eventually shown to be a neurotransmitter in its own right. What is this chemical?

dopamine

"Acetylcholine's action is terminated by acetylcholinesterase." By contrast, what entirely different mechanism is predominant in the inactivation of norepinephrine?

reutake

For clostridial toxins, botulinum and tetanus toxin, answer one of the following. (1) Why would you squeeze blood out of a deep dirty puncture wound in the finger? (2) What is the name of the protein which these proteases cleave? or (3) Which specific membrane houses this protein?

1 flush out the bacteria 2 synaptobrevin=VAMP=vSNARE 3 vesicle membrane

Why doesn't the alpha subunit of the G protein keep activating adenylate cyclase forever?

when it catalyses GTP conversion to GDP plus phosphate, the alpha subunit reassociates with beta-gamma

For stretch of the extensor to mediate contraction of the extensor, there is a monosynaptic reflex arc. What needs to be added to this circuit for there to be a corresponding inhibition of the flexor?

an inhibitory interneuron

"Vagus-stuff slows the heart." Answer one of the following. (1) What major subdivision of the autonomic nervous system does vagus-stuff come from? (2) What chemical is vagus-stuff?

parasympathetic, acetylcholine

After calcium ions come into the synaptic terminal, what do they do to assist in vesicle release?

binds to synaptotagmin, a vesicle protein, fugure also shows calcium activating calmodulin

For EITHER the bacterium that causes tetanus or the bacterium that causes botulism, what is the significance of that bacterium being anaerobic?

would thrive in improperly canned goods or in deep puncture wounds

What molecule does malathion INHIBIT (leading the heart to stop)?

acetylcholinesterase

Chemically, what is the RELATIONSHIP of the dark pigmentation of the substantia nigra and the neurotransmitter made by the substantia nigra?

melanin and dopamine have l-DOPA as a common precursor


What is different in the gating of Ca2+ channels involved in release of synaptic vesicles vs. when IP3 interacts with the smooth endoplasmic reticulum that acts as a Ca2+-sequestering cistern?
 
The voltage of the action potential gates the channels for vesicle release, whereas the inositol trisphosphate channel is a ligand-gated channel
 
"Muscarine is an agonist for a G protein coupled receptor for acetylcholine." Name the famous agonist for the acetylcholine receptor that is a channel.
 
nicotine
 
When norepinephrine is the "first messenger" activating the G protein coupled receptor, cyclic AMP has been called the "second messenger." Answer either: (1) What is the precursor of cAMP? Or (2) What is the enzyme that turned that precursor to cAMP?
 
ATP, adenylate cyclase

What does a kinase do to a protein?

phosphorylates it

Suppose you are stimulating the nerve to the gastrocnemius muscle. What would BoTox do to the response?

decrease it

Why might a physician give a patient a monamine oxidase inhibitor?

to relieve depression

Caffeine inhibits the enzyme that breaks down what substance?

it inhibits cAMP's PDE

What kind of post-synaptic potential (PSP) is usually a hyperpolarization?

IPSP

The nicotinic receptor is a cation channel for what two ions?

K+ and Na+

What is the calcium binding protein of the synaptic vesicle?

synaptotagmin

The alpha subunit of the heterotrimeric G protein binds what molecule?

GTP (also adenylate cyclase, phospholipase C, etc)

Insecticides like malation, nerve gas, and Loewi's "vagus-stuff" would all do what to the heart?

slow it

Why might a physician give a patient l-DOPA?

to relieve Parkinson's disease, precursor of dopamine

Without a booster shot, a deep, dirty puncture wound might cause death because of the toxin of an anaerobic bacterium that cleaves synaptobrevin in inhibitory interneurons. The
disease?

tetanus

What chemical is in low supply in the brains of Parkinson's patients?

dopamine

What would anti-acetylcholinesterase poisoning do to heart rate?

slow or stop

The garden tomatoes were not cooked enough before being "canned" in mason jars. What would happen at the cellular or subcellular level if you eat them?

vesicles would not be released

After the alpha subunit of the heterotrimeric G protein binds GTP, what does it do to GTP?

breaks it to GDP (and P)

Although dopamine is a transmitter itself, it can be converted into what other neurotransmitter by the enzyme dopamine beta-hydroxylase?

norepinephrine

Muscarinic receptors are for what neurotransmitter molecule?

acetylcholine

What second messenger activates protein kinase when the beta-adrenergic receptor of a liver cell binds epinephrine?

cAMP

For the monosynaptic knee-jerk reflex, what cell does the sensory neuron of the spindle's stretch receptor synapse onto?

spinal motor neuron

Conductance to what anion increases when the postsynaptic membrane hyperpolarizes?

Cl- is the biologically important anion

What is the role of the voltage gated (activated) Ca2+ channel in the presynaptic membrane?

triggers cascade for vesicle release

What would repeated overstimulation of the vagus (10th cranial nerve) do to the heart rate?

slow it or stop it

Where is the cell body of the sensory nerve mediating the knee jerk reglex?

just outside the spinal cord (dorsal root ganglion)

Why would the toxin from Clostridium botulinum be given as a cosmetic for the face by a dermatologist?

prevent facial expression that forms wrinkles

What is the pharmacological name for the type of cholinergic receptor that is a G protein coupled receptor for the parasympathetic nervous system?

muscarinic

What neurotransmitter does the substantia nigra make for the control of motor movements?

dopamine

What would happen to levels of acetylcholine as a result of poisoning by malathion?

they would go up

What neurotransmitter is made from dopamine by the action of dopamine beta hydroxylase?

norepinephrine

When acetylcholine binds the nicotinic receptor, conductances to what ions are increased?

K+ and Na+

The alpha subunit of the heterotrimeric G protein activates adenylate cyclase to make what out of ATP?

cAMP

How is the hormone from the adrenal medulla related to the neurotransmitter from the postganglionic neuron of the sympathetic nervous system?

epinephrine (adrenalin, hormone) is made from norepinephrine (noradrenalin, neurotransmitter) with one additional enzymatic step

The nicotinic receptor is a ligand-gated channel with low cation specificity. What is the ligand?

acetylcholine

One SNARE is on the presynaptic membrane. Where is the other?

on the vesicle

What happens to the voltage when a neurotransmitter causes K+ and Cl- channels to open?

hyperpolarize

For acetylcholine, there are ionotropic (channel) and metabotropic (G protein-coupled) receptors. What is the situation for norepinephrine?

Only metabotropic

For what kind of patient would you feed l-DOPA, and why can't you feed dopamine instead?

Parkinson's, does not cross blood brain barrier

What is it that causes the calcium channels in the axon's presynaptic terminal to open?

they are voltage gated and report the arrival of the action potential

If the extensor is activated by stretch in the knee jerk reflex, what is the mechanism by which the flexor is inhibited?

an inhibitory interneuron in the spinal cord

What does "vagus-stuff" (acetylcholine) from the parasympathetic nervous system's 10th cranial nerve do to the heart rate?

slows it

Transporters to take norepinephrine back into the synaptic terminal from the synaptic cleft constitute the main mechanism to terminate the action of this neurotransmitter. By contrast, how is acetylcholine eliminated?

acetylcholinesterase breaks it down

Given that tetanus toxin is a Clostridial toxin that cleaves synaptobrevin to inhibit vesicle release, how come muscles are overactivated in lock-jaw?

it is an inhibitory path that is blocked

How does caffeine have its stimulatory effect?

blocks cAMP phosphodiesterase

What happens with botulism toxin? (Your answer can be molecular, cellular, or physiological.)

cleave synaptobrevin, block vesicle release, muscles blocked

The conclusion from Loewi's work was that there must be a substance involved. How did his classic experiment show this?

juice from chamber where vagus stimulation slowed heart slows heart in another chamber

Describe the geometry of the input to the spinal cord of the muscle stretch receptor.

cell body is in ganglion as axon goes into dorsal root

Why do they call neurotransmitter vesicles in the process of release "omega figures?

in EM they look like the Greek letter

Ionotropic vs. metatotropic is a way to distinguish neurotransmitter receptors. If muscarinic is metabotropic, what is the corresponding ionotropic receptor for cholinergic transmission?

nicotinic

27. What is the immediate precursor for dopamine and why is it especially useful as a treatment for patients?

l-DOPA can be given to Parkinson's patients because it crosses the blood brain barrier

What binds to adenylate cyclase to activate so that it makes cAMP out of ATP?

alpha subunit of heterotrimeric G protein

What does protein kinase A (PKA = A kinase) do to the proteins it affects?

phosphorylates them

Once it is activated, what keeps the alpha subunit of the heterotrimeric G protein from running amuck and continuing to activate the next molecule in line in the cascade?

it has GTPase activity then recombines with beta gamma

Where (specifically) is synaptobrevin and what (molecularly) does botulinum toxin do to it?

on vesicle, cleaves (prevents vesicle release)

Reuptake is the predominant mechanism to terminate norepinephrine action. By contrast, how is acetylcholine action ended?

breakdown by AChE

By what mechanism would an injection of BoTox prevent the formation of wrinkles in your face?

Muscles that made you wrinkle your face would be paralyzed (b/c no transmitter release)

"There are no channel receptors for norepinephrine." Then how are these norepinephrine receptor proteins characterized?

GPCR

How is the color of the substantia nigra related to its role in distributing a transmitter through the brain?

Melanin is a polymer of DOPA, dopamine is a product of DOPA

A cell might depolarize or hyperpolarize even when a metabotropic receptor that is not an ion channel is activated. How can this be?

Through inhibitory vs excitatory G proteins (alpha subunits)

How is it that the calcium (Ca2+) ions involved in transmitter release come into the terminal only when they're supposed to?

they come in through a voltage gated calcium channel

How is it that the flexor is inhibited in the knee-jerk reflex when all God's children know that the neuromuscular junction is only excitatory?

through an inhibitory interneuron in the gray matter of the spinal cord

A caveat to cell theory is the functional syncitium, like electrical (and cytoplasmic) continuities between adjacent myocardial (heart muscle) cells. What membrane specialization allows this intimate connection?

gap junction

A few EPSPs (excitatory postsynaptic potentials) are present simultaneously in a spinal motor neuron (Sherrington's "final common pathway" in the "integrative action of the nervous system"). Why might they not lead to an action potential in the axon hillock?

decremental conduction means they might not reach threshold at the hillock, or maybe an IPSP heads them off at the pass

The nicotinic channel is called ligand-gated to differentiate it from (what kind of channel?) involved in the action potential.

voltage gated

The synthesis of norepinephrine, an amine, from the amino acid tyrosine has long been known. Eventually, an intermediate in this biosynthesis was acknowledged as a transmitter itself. What was this molecule?

dopmine

You get a deep dirty puncture wound in your finger. Why do you try to make that cut bleed?

see if you can get the bacteria (that cause tetanus and are anaerobic) out (of that anaerobic location)

"Black widow spider venom binds neurexin." And thus it affects what process?

vesicle release (it speeds up release)

There is an enzyme that makes cAMP. How is it that the enzyme is activated to make cAMP only when called upon?

when it binds GTP, the alpha subunit of the G protein interacts with adenylate cyclase

"Nicotinic" is a term applied to the postganglionic cell in the autonomic nervous system. Which part of this cell?

the postsynaptic membrane

Sherrington said that the spinal motor neuron was the final common pathway of the nervous system. How does this relate to excitation and inhibition in the spinal motor neuron vs. the muscle cell?
 
b/c there is only excitation on the end plate, the neuron is the last chance to integrate IPSPs and EPSPs
 
"Botulinum and tetanus toxins cleave synaptobrevin." In terms of nerve function, why would they kill you?
 
Prevent vesicle release
 
What would be the cause of death if you were poisoned by malathion, an inhibitor of acetylcholinesterase?
 
Heart would stop
 
At one time, the precursor of norepinephrine was considered to be just that, a step in the pathway to norepinephrine synthesis. Now it is considered an important transmitter. Answer either (1) What is it called? (2) Where does it come from in the brain? Or (3) Its deficiency is the cause of what disorder?
 
1 dopamine 2 substantia nigra 3 Parkinsons
 
"The alpha subunit stimulates adenylate cyclase." Answer either (1)The alpha subunit of what heterotrimeric molecule? (2) What stimulated this heterotrimeric molecule? Or (3) What does the adenylate cyclase make when it is stimulated?
 
1 G protein 2 G protein coupled receopto 3 cAMP
 
Caffeine: Answer either (1) What does it do chemically? Or (2) What neurotransmitters action is potentiated by its action?
 
1 block the PDE that inactivates cAMP 2 NE

Choline-O-acetyltransferase: Answer either (1) What is it used for? or (2) Where is it located (specific cell compartment)?

make acetylcholine, in synaptic terminal

Return to Syllabus

Return to Stark Home Page

this page was last revised 6/21/12