Synaptic Transmission

Question 1

What is synaptic transmission?

Answer 1

Neurotransmitter release - how signals are sent between neurons. Postsynaptic signalling machinery (e.g. receptors, intracellular signaling molecules) - how signals are received within a neuron.

Question 2

Who created the term ‘synapse’?

Answer 2

Cajal (1980) - made sketches of neurons. Characterised some dendritic properties.
Term ‘synapse’ coined by Sherrington. Demonstrated unidirectional transmission.

Question 3

What is chemical transmission?

Answer 3

First demonstrated by Loewi in the 1920s. Take a heart and stimulate the vagus nerve. Showed that stuff was capable of slowing down heart rate, showing neurotransmitter is important for slowing down heart rate.

Question 4

What is a chemical synapse?

Answer 4

Communication via transmitter release. Each neuron has 1000 synaptic connections. There are around 100 billion neurons in the human brain. Synapse is important because it is the site for most psychoactive/recreational drugs.
Can stain neurons for their morphology using a gogi stain - can look at which spines contain lots of postsynaptic receptors.

Question 5

What is an electrical synapse?

Answer 5

A minority of synapses are not chemical, but electrical. Don’t find this much in adults. Neurons can directly influence each other in a very fast way - not a gap between them.

Question 6

What are the three main types of synapses?

Answer 6

(1) Axodendritic - most important/common. Axon on a dendrite, influencing activity there.
(2) Axosomatic - sometimes on GABA synapses
(3) Axoaxonic - can be gabaergic: prevent electrical impulse from going through, messes with neurotransmitter release.

Question 7

What are the key steps behind fast synaptic transmission?

Answer 7

1. Neurotransmitter synthesis - synthesised in the presynaptic terminal
2. Transport
3. Storage - stored in the vesicles
4. Release - released because calcium flow in because of AP. Once released neurotransmitters diffuse and bind.
5. Receptor binding - bind and exert their effects
6. Inactivation - don’t want too much transmission going on.

Question 8

NT synthesis (1 and 2)
What are classical neurotransmitters?

Answer 8

Amino acids - glutamate (major excitatory NT) and GABA (major inhibitory NT).
Monoamines - single amine group, modified amino acids. Dopamine, Norepinephrine, serotonin (5-GT). Monoamines involved in mood regulation and reward.
Acetycholine - controls muscles, but in brain controls things like cognition.

Question 9

What are non-classical neurotransmitters?

Answer 9

Neuropeptides - made on amino acids, much heavier. Endorphins, corticotropin-releasing factor (CRF) etc., control pain.
Lipids - fatty based NT. Anandamide - involved in reward.
Gases - nitric oxide (NO).

Question 10

What do synthesis and transport depend on?

Answer 10

Synthesis and transport depends on type of neurotransmitter - monoamines made in terminals, peptides are made in the cell body.

Question 11

How are classical neurotransmitters synthesised?

Answer 11

Need a good diet (e.g. tyrosine, tryptophan) to cross BBB. Amino acids will cross the BBB either passively or through active transfer. Once in neuron it can be made with enzymes and packaged into small vesicles (20nm radius). Classical neurotransmitters remain
in vesicles, don’t readily cross membranes. When used, they are rapidly resynthesises and replenished (need to make them again). They are all over the brain controlling neurons.

Question 12

How are neuropeptide transmitters (non-classical) synthesised in the cell body?

Answer 12

They are protein synthesis dependent. Made in the cell body, all starts in the nucleus. It is transcribed, mRNA sent for translation, needs to be moved through the axon (heavy, active transfer), goes into the terminal and then can be released. Main difference to classical NTs is that they are large.

Question 13

3. How are neurotransmitters stored?

Answer 13

NTs are actively transported into vesicles, via proteins called ‘vesicular transports’. Different types are vesicular monoamine transporter and vesicular glutamate transporter. Transporter is important for determining what that NT is going to become - its phenotype. Can what what NT it is by vesicle.

Question 14

3. What are the 3 R’s of neurotransmitter storage?

Answer 14

1. Readily releasable pool - ready to be released at any time in less than a second, e.g. response of a postsynaptic neuron (high frequency stimulation).
2. Recycling pool - further away. Mobilised by moderate stimulation, fairly rapid release (a few seconds)
3. Reserve pool - furthest away. Vast majority, mobilised by intense stimulation, slow release (tens of seconds of minutes).

Question 15

4. What is excitation-secretion coupling (release)?

Answer 15

AP causes an explosive change in membrane potential, travels to presynaptic terminal and leads to NT release. AP comes down the terminal, activates a voltage-gated calcium channel, activates another protein called CaM Kinase II (this is calcium sensitive, turns on more proteins), leads to phosphorylation of proteins which leads to movements of vesicles going toward the release site, then fuse there to release NT.

Question 16

What helps dock and release the proteins?

Answer 16

Vesicle fusion with the cell membrane is mediated by SNARE proteins. SNARE proteins help dock the vesicle. There is a priming stage to ensure they are docked properly. Once depolarisation happens and calcium kicks in, this is when fusion happens and NT are released.

Question 17

How does Botulinum Toxin (Botox) work?

Answer 17

Botox affects SNARE proteins. Prevents fusion, and therefore NT cannot be released - if inject into muscles, won’t get proper acetycholine release and muscles won’t work properly.

Question 18

5. Diffusion - how does the NT get across?

Answer 18

Diffuse across synaptic cleft, binding to any receptors they reach (including post-synaptic receptors on adjacent cells). Some is spilled over - may not make it to postsynaptic site, so may spill to neighbouring synapse and can modulate the signal.

Question 19

How is NT release regulated?

Answer 19

Can regulate at many different levels:

1. Rate of Paps (neuronal firing) - the more Paps you have, the more NT you can release.
2. Probability of transmitter release - calcium influx does not always guarantee NT release. Some variability between neurons - have more NT release than others.
3. Autoreceptors - sometimes on the terminals. Typically g-protein coupled receptors, can regulate NT release if theres binding to it, causes molecular cascades which can slow down NT release.
4. Somatodendritic - on soma and dendrites. NT in some cases can be released from dendrites, and these receptors detect this and slow down neuronal firing.

Question 20

What are autoreceptors vs. postsynaptic receptors?

Answer 20

Autoreceptors are receptors on the same neurons thats releasing the NT. ITs job is to modulate activity of the cell (e.g. release, firing, etc.), which is dependent on the location.

Question 21

6. How is synaptic transmission terminated?

Answer 21

Reuptake transporter - protein located on the terminal, and job is to remove excess NT on the synaptic cleft back into the terminal. Uses enzyme degradation - helps break down NT, so they cannot function anymore. Two main mechanisms of termination are reuptake and breakdown.

Question 22

What are steps of fast synaptic transmission?

Answer 22

1. Synthesis, transport & storage
2. Depolarization
3. Open voltage-gated Ca2+ channel
4. Ca2+ influx
   4a. Activate CamKII & phosphorylate proteins
5. Movement and docking of vesicles
6. Exocytosis-diffusion
   7,8. Interact with receptors
   Inactivation

Question 23

How is neurotransmission a fast and transient process?

Answer 23

A presynaptic AP triggers a calcium current, triggering exocytosis (NT being released), which results in an evoked postsynaptic current (EPSC), which creates a postsynaptic actin potential. Not a perfect system - AP doesn’t always equal NT release?

Question 24

What are potential mechanisms which drugs can alter synaptic transmission?

Answer 24

Alcohol can affect the GABA receptor. Cocaine can block reuptake of receptor (e.g. dopamine).

Question 25

How do synaptic transmitters exert their effects?

Answer 25

AP in bouton generated by opening of sodium channels. Depolarisation opens calcium channels. Calcium elevation occurs in micro domain. Calcium binds to synaptotagmin causing opening of fusion pore. Glutamate passes through fusion pore and diffuses in the cleft.

Question 26

What are receptors?

Answer 26

Most receptors on which NTs, and other ligands, such as drugs of abuse, therapeutics, act are embedded in the cell membrane. Transmitters bind to a receptor, changes the conformation and elicit intercellular chances (creates a cascade).

Question 27

What are the two major classes of receptors?

Answer 27

1. Ionotropic - ligand-gated ion channels - fast, allow ions to pass. 2. Metabotropic - G-protein-coupled receptors - slow, use second messengers.

Question 28

What are ligand-gated channels?

Answer 28

Ionotropic receptors. Comprised of multiple subunits bound together to form an ion channel. Has a pore which allows certain ions to go in and out. Ions flow down electrochemical gradient, depends on concentration and membrane voltage. Effects are super fast and can be rapidly reversed.

Question 29

What is an example of a ligand-gated channel?

Answer 29

Nicotinic acetylcholine receptor. Nicotine binds to this, when it binds sodium flows in and it excites the neuron (depolarisation). Receptors have agonists and antagonists. For nicotinic acetylcholine receptor, agonist is nicotine and antagonist is curare. Curare can be used as muscle relaxant in low doses, in high doses it is poison. It interferes with the acetycholine receptor, so less sodium coming in, and therefore muscles start to get relaxed, so much pretty much paralysed.

Question 30

What is an example of a ligand-gated channel?

Answer 30

GABA-A receptor. Ionotropic receptor, when gaba binds chloride can come in and it inhibits the neuron. To access gaba receptor fast = alcohol, diazepam.

Question 31

What are G-protein-coupled receptors (GPCRs)?

Answer 31

Metabotropic receptors. Also called serpentine receptors. Single proteins with seven transmembrane domains. Indirectly influence cellular activity. Receptor coupled to an intracellular effector - G protein. Gs = stimulatory, Gi = inhibitory. There are lots of the receptors in the human body - more than 367. Over 90% expressed in the brain. Vast majority are in your brain – important neuronal target for transmitters and drugs. Comes in many forms e.g. monamines and GABA.

Question 32

How do these G proteins signal?

Answer 32

They have two ways to alter cellular functioning: 1. Some are directly coupled to an ion channel (i.e. the effector is an ion channel). 2. G-protein is coupled to a 2nd messenger system. Typically these are enzymes which create more chemicals (messenger systems). Second messengers activate protein kinases, which in turn activate other 2nd messengers via phosphorylation (causes widespread effects).

Question 33

What is an example of how G proteins signal?

Answer 33

GIRK channel – channel that opens up because GABA b receptor is activate, and allows potassium to go outside. Adenosine receptors – a site of caffeine (where caffeine binds). Influences adenylate cyclase activity, causing secondary changes.

Question 34

What are two second messengers?

Answer 34

Cyclic AMP and calcium.

Question 35

What is cAMP?

Answer 35

Gs unit is stimulatory – activates adenylyl cyclase, takes ATP and converts it into cyclic AMP. Once it does this it phosphorylates more proteins and causes many more responses. Opposite of Gs is Gi – lowers production of cyclic AMP.

Question 36

What is protein kinases?

Answer 36

Add phosphate group to proteins to alters its function. Exert function by adding phosphate group. If add this group can activate it, has many targets. Increase receptors, enzymes etc. Or SNAREs to facilitate neurotransmitter release.

Question 37

How do drugs of abuse differentially activate kinases compared to non-drugs of abuse?

Answer 37

Looks at nACC – involved in reward. Wanted to see how drugs of abuse compared to non-drugs of abuse activate the nACC via immunohistochemistry. Decreased level of activated ERK. Drugs of abuse more powerful in activating signal transduction cascades than non-drugs of abuse, which is why they can affect things like cognitive function.

Question 38

What is phosphatases?

Answer 38

Enzyme which removes phosphate group or 'dephosphorylate' the protein to alter function. Its and drugs can alter the functional balance of proteins. Complex regulatory interaction between kinases and phosphatases. Tug of war determines what happens downstream.

Question 39

What are gene regulatory pathways?

Answer 39

In nucleus got DNA which contains gene information, second messengers can mess with gene regulation by activating transcription factor. Transcription factor is what allows DNA to be read. Transcription results in mRNA production, which results in protein synthesis. Gene expression alteration can lead to long or short lasting changes e.g. more transcription factors, which are necessary for learning and memory. Different pathways can activate CREB and cause gene transcription, such as calcium pathway.

Question 40

What is glutamate?

Answer 40

Glutamate is the key excitatory NT in the brain. Activates immediate early genes. Would not be able to function without glutamate, it is what switches on neurons. Fast neuron – e.g. with AMPA receptors. Neurons that produce glutamate are in the cortex, the hippocampus, thalamus, and cerebellar cortex. Looking at cell bodies which are located in these areas. The cortex is what allows complex computations to happen, more glutamate producing cell bodies. Drugs of abuse can mess with this. Changes in glutamate signalling appear to be a major mechanism for learning and memory.

Question 41

Where is glutamate synthesised?

Answer 41

Synthesised in the terminals. Made from glutamine, and enzyme glutaminase transforms it into glutamate. Packaged into the vesicles by VGLUT.

Question 42

What are types of glutamate receptors?

Answer 42

Both ionotropic and metabotropic. Both types are found pre- and post-synaptically.

Question 43

What are the three types of ionotropic glutamate receptors?

Answer 43

AMPA, Kainate, and NMDA. AMPA involved in fast excitatory transmission. AMPA and NMDA key for learning and memory, NMDA much slower in terms of function. At rest AMPA can still function, sodium can come in, NMDA magnesium is bound and it is plugged – if glutamate binds and there is enough of it in neighbouring neurons, this magnesium block is removed. Different roles: AMPA fast, sodium coming in. NMDA sodium and calcium coming in. Means going to have different effect.

Question 44

Where are AMPA and NMDA receptors?

Answer 44

Majority found on the spines of the dendrites. Much excitation happening there.

Question 45

What are these things involved in (AMPA)?

Answer 45

Two types of IEGs – an effector. Arc – once expressed, can help internalise AMPA receptor. MAP – removes phosphase group, becomes inactivated. Transcription factors affect expression of other genes.

Question 46

What do drugs of abuse do?

Answer 46

Drugs of abuse can robustly induce IEG expression compared to mild arousing stimuli. See amphetamine produces more c-fos in areas like cortex and striatum. Drugs profoundly affect your brain and can profoundly alter activity as well as gene expression, causing more changes than naturally occurring stimuli. Drugs have a profound effect, but only minority of neurons are affected. Look at Fos + - more mRNA following cocaine. Gene expression effected, may cause long term changes in minority of neurons.

Question 47

What is a summary of the lecture?

Answer 47

Neurotransmitters activate receptors. Receptors transduce their signals through a complex network of 2nd messengers, protein kinases, which in turn alters activity or state of structural proteins, receptors, ion channels, transcription factors. Drugs interfere with the intricate process in some neurons more than others.