Neuro- Dr Lin

Question 1

Who demonstrated using two isolated frog hearts that nerves release a chemical which slows the heartbeat? briefly describe the experiment

Answer 1

• Otto Loewi
  1. stimulated the vagus nerve
  2. Heart rate slowed
  3. transfer fluid from donor to recipient
  4. Heart rate also slowed

Question 2

Common features of chemical synapses

Answer 2

• Presynaptic cell
• Many mitochondria to produce energy- as energy intensive
• Postsynaptic cell- receives signals
• Synaptic cleft: 20-50nm wide, matrix of fibrous extracellular protein

Question 3

Neuron to neuron chemical synapse

Answer 3

Neuron to neuron

• Within the CNS
• Extremely varied
• Different neurotransmitters
• Different sizes and morphologies

Question 4

Principles of chemical synapses

Answer 4

1. neurotransmitter molecules are synthesised and packaged into vesicles
2. an action potential arrives at presynaptic terminal
3. voltage gated calcium channels open, calcium enters
4. a rise in calcium triggers fusion of synaptic vesicles with the presynaptic membrane
5. transmitter molecules diffuse across the synaptic cleft and bind to specific receptors activate the postsynaptic cell
6. bound receptors activate the postsynaptic cell
7. a neurotransmitter breakdown, is taken up by the presynaptic terminal or other cells, or diffuses away from the synapse

Question 5

Why do we need synapses?

Answer 5

Simple transference of a signal
Synapses allow information processing that is:
- Complex
- Subtle
- Flexible
Defective neurotransmission = many neurological and psychiatric disorders

Question 6

The Neuromuscular junction

Answer 6

• Fast and reliable neurotransmission- we want muscle to contract, if not reliable we cannot move efficiently
• Motor neuron action potentials always cause muscle cell action potentials
• Uses the neurotransmitter ACh
• One of the largest synapses in the body

Question 7

Specialisations of the Neuromuscular Junction

Answer 7

Presynaptic:
- Large number of active zones
Postsynaptic (motor endplate):
- Contains junctional folds, densely filled with neurotransmitter receptors
Precise alignment of active zones and junctional folds

Junctional fold where receptors are clustered

Question 8

CNS synapses

Answer 8

• 86 billion neurons in the human brain
• How many in the CNS? 1000 x 86 billion

Depolarisation of post synaptic neuron by pre-synaptic neuron is integrated at the initial segment at axon

Question 9

Variability of CNS chemical synapses

Answer 9

• Synapses can be different sizes, can grow, can shrink
• Synapses can release transmitter from more than one active zone in a terminal
• Larger synapses usually have more active zones
• Synapse arrangement and structure relates to their function
• Further classification based on the appearance of the pre and post-synaptic membrane features
• Asymmetrical synapses are usually excitatory
• Symmetrical synapses are usually inhibitory

Question 10

Types of neurotransmitters

Answer 10

Most neurotransmitters fall into 3 chemical categories:

• Amino acids: Glutamine, glycine, GABA
• Amines: acetylcholine, dopamine
• Peptides

Question 11

What are the two types of vesicles?

Answer 11

Synaptic vesicles: 
-	Amino acid and amine neurotransmitters 
-	40-50 nm diameter 
Dense-core secretory vesicles: 
-	Peptide neurotransmitters 
-	100-200 nm diameter

Question 12

Synaptic vesicles for neurotransmitter synthesis and storage

Answer 12

• synthesised in soma
• filled at the presynaptic terminal
• requires ATP to load neurotransmitter into vesicles

Question 13

Dense-core secretory granules for Neurotransmitter synthesis and storage

Answer 13

• synthesised in ER, often as precursors
• bud from the Golgi apparatus in soma
• transported along microtubules

Question 14

Neurotransmitter release: docking

Answer 14

Some vesicles are already “docked” at active zones within the presynaptic neuronal membrane

physically go to location when ready to fuse with membrane docked using special proteins

Question 15

Neurotransmitter release: the trigger

Answer 15

Arrival of AP opens voltage-gated Ca2+ channels
Ca2+ moves into the presynaptic terminal as E(Ca2+) is ~123 mV
Triggers vesicle fusion and release – exocytosis

calcium is a trigger
moves from extracellular space into presynaptic terminal
strong driving force for calcium to move into cell

Question 16

Neurotransmitter release: the SNAREs examples of Synaptotagmin and Botulinum toxin

Answer 16

The SNARE proteins are crucial for exocytosis
Synaptotagmin binds to calcium and cause vesicle fusion
Botulinum toxin (or BOTOX) is a bacterial toxin that selectively destroys some SNARE proteins – this can block synaptic neurotransmission

When calcium goes into cell and binds to Synaptotagmin, it undergoes conformational change. Two proteins, syntoxin and synaptobrevin then form a zipper structure, helped by SNAP-25 which is tethered to plasma membrane

conformational change force vesicles over energy barrier to fuse with the membrane

Question 17

The synaptic cleft has a very small volume, what does this mean?

Answer 17

, so neurotransmitter concentration can rise to the mM range

Question 18

What are the two receptors which Neurotransmitters bind to

Answer 18

Specific receptors are embedded in the postsynaptic density
Some neurotransmitter molecules will bind to these receptors
There are two main types of receptor:
- Ligand-gated ion channels (ionotropic)
- G-protein coupled receptors (metabotropic)

Question 19

How are Neurotransmitters cleared away

Answer 19

Neurotransmitters must be cleared rapidly from the synaptic cleft
There are 3 main ways to achieve this:
1. Simple diffusion out of the synaptic cleft
2. Actively reuptake into the presynaptic membrane (or glia) by specific transporters, for recycling
3. Enzymatic destruction within the synaptic cleft- localised enzymes

eg acetylcholineasterase

Question 20

What happens to the vesicle the neurotransmitter was in?

Answer 20

Initially it is added to the membrane
Then it is recovered by endocytosis
Vesicles can be recycled and filled with new neurotransmitter

Question 21

Quantal release

Answer 21

Each synaptic vesicle can cause a mini response at the postsynaptic cell
The effect of one vesicle being released is known as the quantal size
Quantal content is the number of quanta (or vesicles) released per presynaptic event/ per action potential

Question 22

Receptor-dependant action- what are the receptors, give example

Answer 22

Specific receptors are embedded in the postsynaptic density
Some neurotransmitter molecules will bind to these receptors

Ligand gated ion channel Permeable to Na+ e.g. skeletal muscle contraction
-nicotinic

G-coupled receptor Activating K+ channel e.g. heart slows down
-muscarinic

Question 23

What does transmitter release at a ‘fast’ EXCITATORY chemical synapse generate?

Answer 23

an EXCITATORY POST-SYNAPTIC POTENTIAL (EPSP)
Excitatory will generate excitatory post synaptic potential, pre-synaptic which fires an action potential releases an excitatory neurotransmitter – this will stimulate ionotropic receptors which will open and then have a ‘bump’ where membrane potential goes up a bit (this is the EPSP)

Very short, not usually enough to stimulate the neuron to fire.
However, if neuron receives many EPSPs quickly in short succession they will sum up to reach firing threshold

Question 24

What does transmitter release at a ‘fast’ INHIBITORY chemical synapse generates

Answer 24

an INHIBITORY POST-SYNAPTIC POTENTIAL (IPSP)

Chloride permeable channel, when GABA bind to receptor it opens chloride channel
High chloride outside cell, low chloride in the cell- so chloride will go into cell
This is an negative ion going into the cell- tends to hyperpolarise a neurone, causing an downward deflection in the voltage (an IPSP)

Question 25

G-coupled receptors (metabotropic)

Answer 25

Synaptic transmission is slower and more complex than transmission via ligand-gated ion channels Signal amplification occurs Multiple channels may be affected

Question 26

Axodendritic synaptic arrangement

Answer 26

pre-synaptic cell sends it axon onto dendrite of opposing synaptic neuron

Question 27

what two types of receptors can Ionotropic receptors be?

Answer 27

excitatory or inhibitory receptors

Question 28

Dendro-dendritic synaptic arrangement

Answer 28

Dendrodendritic synapses are connections between the dendrites of two different neurons cells input compartment can also provide outputs to other neurons

Question 29

Axoaxonic synaptic arrangement

Answer 29

use to regulate transmission from one neuron to another.

Question 30

Axosomatic synaptic arrangement

Answer 30

send information onto the soma of post-synaptic neuron

Question 31

Neurotransmitter need mechanisms for

Answer 31

* synthesis and/or storage * release * transmitter action i.e. specific receptors * transmitter removal

Question 32

A neurotransmitter should be...

Answer 32

* present in presynaptic terminals * released in response to stimulation * able to interact with postsynaptic receptors * rapidly removed from the synapse

Question 33

What is AMPA? what does it mediate?

Answer 33

AMPA is the main work-force transmitter: mediate fast excitatory activation of post synaptic neuron

Question 34

What does Glutamate bind to? what does this trigger?

Answer 34

Glutamate binding to AMPA receptors triggers Na+ and K+ currents resulting in an EPSP

Question 35

Amino acid and amine neurotransmitters are:

Answer 35

Small molecules Stored and released from synaptic vesicles Capable of many bindings to and activating both • ligand-gated channel receptors • G-protein coupled receptors • Act on both ionotropic and metabotropic receptors • vesicles recycled at terminal • re-formed vesicles

Question 36

The peptide neurotransmitters are:

Answer 36

* large molecules * stored in secretory granules * only activate G-protein coupled receptors * once vesicle is released that’s it, cannot be recycled * no ligand channels for peptides

Question 37

Dale’s principle- why is it not always the case?

Answer 37

‘A neuron only has one neurotransmitter’ Dale classified neurons into mutually exclusive groups by the neurotransmitter they released HOWEVER, …Many peptide-containing neurons have both peptide transmitter AND an amino acid or amine NT (or cotransmitters e.g. ATP) …Some neurons also possess two types of amino acid NT e.g. GABA and glycine THEREFORE…Dale’s principle is violated in these cases

Question 38

Properties of Glutamate neurotransmitter

Answer 38

* Most common excitatory transmitter in CNS * Amino acid, therefore, found in all neurons * 3 glutamate receptor subtypes based on the drugs which act as selective agonists * Action is terminated by selective uptake into presynaptic terminals and glia Three main types of glutamate receptors: AMPA, NMDA, Kainate and named based on drugs that activate them

Question 39

What glutamate receptor co-exists with AMPA? How does it work?

Answer 39

* NMDA receptors often co-exist with AMPA receptors * NMDA receptors have a voltage dependent Mg2+ block * So, NMDA receptors need to be indirectly activated by another transmitter * NMDA receptors are permeable to Ca2+ as well as Na+ and K+ * Therefore, their activation can have more widespread, lasting changes in the postsynaptic cell NMDA is the second type of glutamate receptor

Question 40

Properties of GABA (γ- amino butyric acid) neurotransmitter. What does its synthesis require? How is its action terminated?

Answer 40

inhibitory neurotransmitter Not an amino acid used to synthesise proteins Precursor is glutamate Synthesis requires the enzyme glutamic acid decarboxylase Action is terminated by selective uptake into presynaptic terminals and glia Most common inhibitory transmitter in the CNS

Question 41

What does GABA (γ- amino butyric acid) produce?

Answer 41

Produces IPSPs via GABA-gated chloride channels Found throughout the CNS especially in cortex and striatum The right amount of inhibition via GABA is critical: • Too much ⇒ coma or loss of consciousness • Too little ⇒ seizures Fast inhibition occurs through GABA gated chlorine channels

Question 42

What is GABA (γ- amino butyric acid) ?

Answer 42

- GABA is a neurotransmitter, Fast inhibition occurs through GABA gated chlorine channels. - GABA receptor is not a ion channel and is a metabotropic receptor which can have different effects - There are two classes of GABA receptors: GABAA and GABAB

Question 43

Presynaptic inhibition

Answer 43

One neuron suppresses the action of another Or autoinhibition (go to notes for more info)

Question 44

Presynaptic Disinhibition

Answer 44

Important Crucial for inhibiting inhibition (go to notes for more info)

Question 45

Modulation of GABAA Receptors

Answer 45

* Other chemicals can bind to the GABAA receptor and modulate the response to GABA binding * These chemicals have no effects without GABA binding * Ethanol has behavioural effects, addictive * Benzodiazepines e.g. diazepam, used to treat anxiety * Barbiturates are sedatives and anti-convulsants * Neurosteroids are metabolites of steroid hormones e.g. progesterone

Question 46

What are opioids and opiates

Answer 46

Opiates are drugs derived from the opioid poppy • E.g. heroin, morphine ``` Opioids are a broad class of natural and synthetic compounds • E.g. opiates and the endogenous opioids, endorphins • Elicit effect by activating opioid receptors ``` Endorphins are naturally occurring small proteins or peptides, including: • endorphin, enkephalin, dynorphin

Question 47

Synthesis of opioids and opiates

Answer 47

As they are peptides, they are: formed in rough ER packaged into secretory granules by Golgi apparatus

Question 48

Distribution of Opioid receptors

Answer 48

Opioid receptors: • are widely distributed in the CNS but concentrated in nociceptive areas (areas that deal with pain) • have at least 3 main types • Include mu (µ), kappa (κ), sigma (σ) (functions in different parts of brain can be quite varied) • Spinal - block pain signal (analgaesia) naturally occurring endorphin will have a analgesic effect • Periaqueductal grey - regulates sensation of "pain” experience of pain • Amygdala - regulates emotional component to pain (this is horrible, why wont this end.. etc) • Frontal cortex - cognitive aspects of pain • Brain stem (medulla) - depress respiration and cough reflex (may induce vomiting) breathing, when people overdoes on heroin, they stop breathing, why opiates are used to treat coughs When under extreme stress, endorphins can supress experience of pain

Question 49

Opiate receptors

Answer 49

Receptors coupled to inhibitory G-proteins Act as modulators, decreasing the excitability of the cell Opiate receptors are G-protein coupled receptors, which will do stuff to various downstream signalling molecules/enzymes to change the excitability to the post-synaptic neuron and they will generally decrease excitability of the neuron Ie supress pain signalling

Question 50

Therapeutic uses of opiates

Answer 50

Analgesia: reduces perception of and emotional response to pain reduce symptoms of pain, opiates receptors are in higher emotion and cognitive regulatory regions of the brain Intestinal disorders: reduces diarrhoea; decreases dehydration Antitussive: cough suppressant (codeine) Can also have side effects

Question 51

Problems restrict therapeutic use- the serious side effects

Answer 51

• Respiratory depression • Sedation • Constipation Tolerance develops – reduced clinical effect Dependence develops – leads to withdrawal symptoms Relieve dull visceral pain better than sharp pain

Question 52

Diffuse modulatory systems of the brain

Answer 52

• Have effects over large areas of brain • Perform regulatory functions rather than specific tasks e.g. – Falling asleep/waking up/becoming attentive – Changing mood • Common features: – Core of each system has a small set of neurons – Neurons arise from the central core of the brain (often in the brain stem) – Each neuron may contact more than 100,000 postsynaptic neurons spread widely across the brain – Neurons release neurotransmitter into the extracellular fluid to allow diffusion to many neurons

Question 53

Neurons release neurotransmitters into the extracellular fluid to allow diffusion to many neurons- which transmitters are these?

Answer 53

* ACh * Catecholamines (e.g. dopamine, noradrenaline) * Serotonin (5-hydroxytryptamine (5-HT))

Question 54

Acetylcholine metabolism

Answer 54

Synthesis: Acetyl CoA + Choline + choline acetyltransferase = acetylcholine + CoA Degredation: acetylcholine + acetylcholinesterase = Acetic acid + Choline * ChAT = good marker for cholinergic neurons * Acetyl CoA produced by cellular respiration in mitochondria * Choline is taken up from the extracellular solution

Question 55

Interfering with the life cycle of ACh by preventing release

Answer 55

1.Prevent release • Botulinum toxin • Black widow spider

Question 56

Interfering with the life cycle of ACh by AChE inhibitors

Answer 56

2. AChE inhibitors: • nerve gas • insecticides • Alzheimer’s treatments

Question 57

Interfering with the life cycle of ACh by block receptors

Answer 57

``` 3. Block receptors • Nicotinic - Curare - -bungarotoxin • Muscarinic - Atropine ```

Question 58

ACh- 2 cholinergic complexes

Answer 58

Basal forebrain complex: • Amongst first neurons to die in Alzheimer’s disease • Regulate brain excitability during arousal and sleep/wake cycles • Possible role in learning and memory Pontomesencephalotegmental complex (brainstem)

Question 59

Synthesis of Catecholamines

Answer 59

``` Tyrosine = amino acid Tyrosine hydroxylase - present in all catecholaminergic neurons - rate limiting factor Dopamine β-hydroxylase - found in synaptic vesicles PNMT - found in the cytosol ``` • Catecholamines (dopamine, noradrenaline and adrenaline) are produced in a series of enzymatic conversions Fig 6-13, 4th Ed, Neuroscience: Exploring the brain

Question 60

Removal and degradation of catecholamines

Answer 60

• Majority undergoes reuptake into presynaptic terminals • Metabolised by: – Catechol-O-methyltransferase (COMT) - mainly in cytoplasm – Monoamine oxidase (MAO)-on outer mitochondrial membrane * MAO-A mainly NA + 5-HT * MAO-B - mainly dopamine

Question 61

Name two Catecholamine systems

Answer 61

Dopaminergic and noradrenergic system Catecholamines (dopamine, noradrenaline and adrenaline) are produced in a series of enzymatic conversions

Question 62

Nigrostriatal pathway

Answer 62

• Neurons found in the substantia nigra of the midbrain • Axons project to the striatum • Pathway facilitates the initiation of voluntary movements • Degeneration of this pathway leads to Parkinson’s disease - Characterised by motor dysfunction e.g. tremor, rigidity

Question 63

Modulation of dopaminergic neurotransmission

Answer 63

* As Parkinson’s is characterized by a decrease in dopamine, treatments aim to increase it: * Addition of L-Dopa removes the rate limiting step of TH, so increases dopamine levels * MAO-B inhibitors reduce the breakdown of dopamine, increasing levels

Question 64

Mesocorticolimbic pathway

Answer 64

* Neurons found in the ventral tegmental area of the midbrain * Axons project to the frontal cortex and limbic system * Assigned many functions * Involved in a ‘reward’ system i.e. pleasure * We are motivated to perform behaviours that stimulate dopamine release * Behaviours associated with the delivery of drugs which result in dopamine release are reinforced = addiction

Question 65

Noradrenergic system

Answer 65

* Arises from locus coeruleus * ~25,000 neurons * Innervates nearly all of the brain * 1 neuron can make 250,000 synapses * Involved in regulating attention, arousal, sleep-wake cycles, learning and memory, anxiety and pain, mood * Most strongly activated by new, unexpected, non painful sensory stimuli

Question 66

Serotonergic system

Answer 66

* Arises from Raphe nuclei * Each nucleus projects to a different area * Similar diffuse innervation of brain to noradrenergic system * Modulates pain-related sensory signals, sleep-wake cycles, mood and emotional behaviour * Most strongly activated during wakefulness

Question 67

5-HT life cycle

Answer 67

• Tryptophan: – Obtained from our diet e.g. grains, meat, dairy, chocolate – Moves from gut to blood to extracellular fluid – Rate limiting factor in synthesis • 5HT receptors: – 7 subtypes – All but one are GPCRs – Excitatory or inhibitory • Termination of activity : – Removed from synaptic cleft by a specific transporter – Reloaded into vesicles or degraded by MAO-A Fig 6.14, 4th Ed, Neuroscience: Exploring the brain

Question 68

Treatment of affective disorders

Answer 68

• Tricyclic compounds: – block uptake of 5-HT and noradrenaline – Serotonin-selective reuptake inhibitors (SSRIs) – Selectively prevent 5HT uptake – e.g. fluoxetine (Prozac) • MAO-A inhibitors - reduce enzymatic degradation of 5HT and noradrenaline Mechanism of action is unknown Increase in noradrenaline and 5HT levels is immediate but therapeutic effects take weeks to develop

Question 69

The cheese effect:

Answer 69

Tyramine is an amine found in high quantities in cheese It has sympathomimetic effects by increasing noradrenalin release MAO normally breaks down tyramine MAO-A inhibitors lead to a hypertensive crisis.

Question 70

ATP as neurotransmitter

Answer 70

• ATP – Often packaged in vesicles as a co-transmitter – Binds to purinergic receptors - P2X = ligand gated ion channels - P2Y = G-protein coupled receptors

Question 71

Endocannabinoids as a neurotransmitter

Answer 71

– Endogenous forms of cannabis – Small lipid molecules that do not require synaptic vesicles – Binds to cannabinoid receptors that are G-protein coupled

Question 72

Nitric oxide as a neurotransmitter

Answer 72

– Gasotransmitter that is small and membrane permeable | – Rapidly broken down