Neurotransmitters

Question 1

Recall the structure of neurons

Answer 1

1. Dendrites detect the input (covered in dendritic spines).
2. Information propagates down the dendrite and is integrated in the soma.
3. The AP is generated at the axon hillock.

▪ The synapse is an area of high resistance.
▪ The synaptic cleft is about 20-100nm wide.
▪ There are lots of mitochondria in the axon terminal because energy is needed for NT release.
▪ It takes ~2ms for the AP to get from one cell to the next.

Question 2

Describe neurotransmission

Answer 2

Information transfer across the synapse requires release of neurotransmitters and their interaction with postsynaptic receptors

Question 3

Describe the two types of synaptic transmission

Answer 3

Chemical (neurotransmitter required)

Electrical (cytoplasmic connection between the cells- seen in astrocytes and cardio myocytes).

Question 4

Explain the diversity of synapses

Answer 4

Different genes encoding different receptors.

Question 5

Describe synaptic plasticity

Answer 5

Synapses can modify their response over time (structural changes, changes in the number of synapses).

Question 6

How do we detect synapses

Answer 6

Fluorescent antibodies that bind to synaptic proteins.

Question 7

What are spines

Answer 7

Protrusions that stick out from synapses

Question 8

What happens when the post-synaptic neurone receives the impulse

Answer 8

it integrates the action potential and generates a new action potential rapidly

Question 9

Why is it important that synapses involve chemical transfer rather than electrical transfer

Answer 9

Chemicals can transfer lots of information in a small amount of space, whereas electrical conduction would need a large surface area

Question 10

How long does it take to transmit an action potential from one cell to the next

Answer 10

2ms

Question 11

Describe the structure of a synapse

Answer 11

Presynaptic nerve ending /terminal
GAP ~ 20 - 100 nm
Postsynaptic region (dendrite or cell soma)

Question 12

Why do synapses require lots of mitochondria

Answer 12

Due to their high energy demand

Question 13

What is meant by an asymmetric synapse

Answer 13

Asymmetric refers to the direction of travel of the information.

Question 14

What are the three stages of synaptic transmission

Answer 14

Biosynthesis, packaging and 
    release of neurotransmitter (T).
II. Receptor 
         action
III. Inactivation

Question 15

Describe the diversity of neurotransmitters

Answer 15

Provide enormous diversity in variety of transmitters and their receptors.
Amino acids (e.g. glutamate, gamma amino butyric acid [GABA], glycine [gly]), amines (e.g. noradrenaline [NA] and dopamine [DA] ) and neuropeptides (e.g. opioid peptides).
May mediate rapid (µs - ms) or slower effects (ms)
Vary in abundance from mM to nM CNS tissue concentrations.
Neurones receive multiple transmitter influences which are integrated to produce diverse functional responses

Question 16

Describe the amino acid neurotransmitters

Answer 16

Glutamate, GABA, glycine. Large prevalence and a large number of effects in the CNS. GABA is the major inhibitory neurotransmitter in the CNS. Glutamate is the major stimulatory neurotransmitter in the CNS, it is also highly abundant in the brain and is the most potent neurotransmitter (only small quantities are needed to transmit the signal). Other neurotransmitters tend to modulate or augment glutamate transmission.
Glycine is found in the brain and spinal cord.

Question 17

What are the essential components for synaptic transmission

Answer 17

Restricted to specialised structures - the SYNAPSE
Fast ~ within ms (200 s)
Calcium is essential - transmitter release requires an increase in intracellular Ca2+ (200 M)
Synaptic vesicles (SVs) provide the source of neurotransmitter (4,000-10,000 molecules per SV)

Question 18

What does the effect of the neurotransmitter depend on

Answer 18

The type of receptor present at its synapse. One neurotransmitter can have different effects throughout the nervous system, depending on the receptors it acts upon.

Question 19

How do the effects of amines and neuropeptides compare to that of amino acids

Answer 19

They have slower effects.

Question 20

Describe the mechanism of neurotransmission

Answer 20

1. Action potential arrives at the terminal bouton and the depolarisation opens VGCCs in the presynaptic terminal.
2. Calcium ions enter the terminal bouton. Ca2+ influx results in the phosphorylation and alteration of several pre-synaptic calcium-binding proteins. This process liberates vesicles from their presynaptic actin network.
3. The vesicle membrane then fuses with the pre-synaptic membrane and the contents are released into the synaptic cleft. The vesicle membrane is then invaginated back into the presynaptic terminal and recycled to form more vesicles which are filled with transmitter for reuse. This process is used by viruses to get into the interior of the cell (poliovirus or herpes virus).
4. The transmitter diffuses across the cleft to postsynaptic receptors and, in some systems, to presynaptic receptors to regulate transmitter release. Once the transmitter is bound to the receptor it causes a change in postsynaptic membrane potential (excitatory or inhibitory).
5. The effect of the chemical transmitter is terminated by one or more of the following mechanisms:
   Enzymatic destruction of the NT
   Re-uptake of the neurotransmitter in the terminal bouton
   Uptake of neurotransmitter into glial cells
   Diffusion out of cleft.
   Sodium-Potassium pumps then restore membrane potential

Question 21

Where are the vesicles found in the pre-synaptic membrane

Answer 21

1. Vesicles are either: i. Docked in the active zone at the site of synapse. ii. Floating in the terminal region.

Question 22

Describe how vesicles release their contents

Answer 22

2. There is an interaction between the presynaptic membrane and the vesicle proteins, allowing the vesicle to be docked stably.
3. There are alpha helical structures which interact together to form a super helix.
4. This forms a stable complex of the vesicle at the synapse, full of NT. The vesicle then awaits the Ca2+ signal.
5. At these sites of docking, a large concentration of VGCCs exist and Ca2+ can enter which causes a calcium dependant change in a calcium sensor protein on the vesicle.
6. The complex undergoes a conformational change and this drives the release of transmitter into the synaptic cleft (pore opened)

Question 23

Describe how the vesicular proteins are targets for neurotoxins

Answer 23

Tetanus – Spastic Paralysis – Zinc-dependant endopeptidases that inhibit transmitter release. C. Tetani
Botulinum – Flaccid Paralysis. C.Botulinum
Binds to protein at site of release and prevents the vesicle closing down and recycling, the NT is released to complete depletion.

Question 24

What does neurotransmitter release require

Answer 24

Transmitter containing vesicles to be docked on the presynaptic membrane
Protein complex formation between vesicle, membrane and cytoplasmic proteins to enable both vesicle docking and a rapid response to Ca2+ entry leading to membrane fusion and exocytosis.
ATP and vesicle recycling

Question 25

Describe the structure and functional units of ionotropic receptors

Answer 25

Structure- transmembrane ion channel composed of five subunits, binding sites for ligand and modulators outside cell Functional Units- Each subunit has 4 transmembrane domains and subunits create a charge field to attract other cations or anions.

Question 26

Describe metabotropic receptors

Answer 26

Structure- Single transmembrane protein with sites for interaction with ligand outside cell and interaction with G=Protein inside cell Functional Units- Seven transmembrane domains with specific amino acid residues with domains important for ligand binding.

Question 27

Describe the roles of ion channel receptors

Answer 27

FAST msecs | Mediate all fast excitatory and inhibitory transmission

Question 28

Describe the roles of G-protein coupled receptors

Answer 28

SLOW secs/mins | Effectors may be enzymes (adenyl cyclase, phospholipase C, cGMP-PDE) or channels (e.g. Ca2+ or K+)

Question 29

Give some examples of ion channel receptors

Answer 29

CNS: Glutamate, Gamma amino butyric acid (GABA) NMJ: Acetylcholine (ACh) at nicotinic receptors Nicotinic cholinergic receptors (nAChR), glutamate (GLUR), GABA (GABAR), Glycine (GlyR) receptors.

Question 30

Describe some examples of G-Protein couples receptors

Answer 30

CNS and PNS: ACh at muscarinic receptors, dopamine (DA), noradrenaline (NA), 5-hydroxytryptamine (5HT) and neuropeptides e.g. enkephalin.

Question 31

Where are GLUR receptors found

Answer 31

On dendritic spines

Question 32

Where are GABA receptors found

Answer 32

on the soma- can have downstream effects to reverse effects of glutamate receptors.

Question 33

Describe glycine receptors

Answer 33

They are inhibitory

Question 34

What are the different types of glutamate receptors

Answer 34

AMPA NMDA mGLUR- G-protein

Question 35

Describe AMPA

Answer 35

This is a glutamate-gated Na+ and K+ channel. Activation of AMPA receptors at normal negative membrane potentials permits passage of Na+ into the cell, permitting a rapid and large depolarisation. AMPA are the principal receptor through which neuronal transmission occurs. Majority of FAST excitatory synapses. Rapid onset, offset and desensitisation.

Question 36

Describe NMDA

Answer 36

NMDA receptors require certain conditions for activation: Presence of glutamate Removal of Mg2+ ion block in channel This occurs when the membrane potential of the neuron is raised (depolarisation) following AMPA activation. NMDA activation permits movement of large amounts of Na+ and Ca2+ Slow component of excitatory transmission Serve as coincidence detectors which underlie learning mechanisms.

Question 37

Describe the roles of Ca2+ that enters through the NMDA receptors

Answer 37

Involved with learning and memory Ca2+ modifies the AMPA receptor potentiating the AMPA receptor response and activates protein synthesis which modifies synapse formation

Question 38

How is glutamate formed

Answer 38

Glutamate is formed from intermediary metabolism (e.g. glycolysis and kreb’s cycle – formed from the transamination of alpha-ketoglutarate).

Question 39

Describe the reuptake of glutamate

Answer 39

▪ Transporters on the pre-synaptic membrane and on glial cells causes uptake of glutamate once it’s fulfilled its role. o The main transporter is EAAT2 (Excitatory AminoAcid Transporter 2) which is found on glial cells and on the pre-synaptic membrane. ▪ Once in glial cells or in the neurones, glutamate is then inactivated by glutamine synthetase to make glutamine (addition of an amino-acid). ▪ Abnormal cells firing leads to seizures associated with excess glutamate in the synapse.

Question 40

Describe epilepsy

Answer 40

Characterised by recurrent seizures due to abnormal neuronal excitability Despite advances in modulating seizure generation and propagation, the disease is disabling. 30% are refractory to treatment.

Question 41

How is GABA synthesised

Answer 41

▪ Both glutamate and GABA have very similar structures, removal of a carboxyl group in glutamate = GABA. ▪ GABA is synthesised by Glutamic Acid Decarboxylase (GAD) – Known as the Vitamin B6 enzyme

Question 42

What are the two types of GABA receptors

Answer 42

GABAa and GABAb

Question 43

How does GABA act as an inhibitory neurotransmitter

Answer 43

GABA inhibits synaptic function by permitting the movement of Cl- into the postsynaptic cell therefore hyperpolarising the cell and preventing depolarisation.

Question 44

Describe the reuptake of GABA

Answer 44

2. There are transporters on glial cells and on the presynaptic neurone which take up GABA (known as the GABA transporters – GAT). 3. Once GABA has been taken up, it is inactivated by GABA transaminase, giving Succinate semialdehyde (which feeds into the TCA cycle).

Question 45

Describe how we can exploit the GABA receptor

Answer 45

There is a binding site for: o Benzodiazepines (e.g. diazepam). o Barbiturates (e.g. for treatment of epilepsy – alters the frequency of channel opening Steroid, ethanol, Zn and convulsants can also bind. Drugs facilitating GABA transmission are: antiepileptic sedative muscle relaxant anxiolytic Upon binding to the allosteric sites, increases the frequency of GABA opening, enhancing its activation.