Communication by receptors

Question 1

What is in the presynaptic terminal?(4)

Answer 1

Neurotransmitter synthesis
Synaptic vesicle transporters
Reuptake transporters
Enzymes to metabolise neurotransmitters

Question 2

What is in the postsynaptic dentrite? (5)

Answer 2

Transmitter-gated ion channels (ionotropic receptors)
G-protein-coupled receptors (metabotropic receptors)
G-proteins
G-protein-gated ion channels
Second messenger cascades

Question 3

What do receptors do and what can they be connected to (3)?

Answer 3

Neurotransmitters communicate with the post synaptic cell via RECEPTORS
Not all receptors are the same, there are diff types with diff fctions, allows to have more effects on cells

Receptors can be connected to
1. ion channels (ionotropic receptors)
2. G-proteins (G-protein coupled receptors and second messenger cascades)
3. G-protein coupled ion channels

Question 4

What is a ligand?

Answer 4

Ligand = neurotransmitter or agonist docks to the channel to open it

Question 5

What is an ionotropic receptor?

Answer 5

Ion-gated channels (receptors) are composed of subunits (a, b, d, g)
Usually 5 subunits for each receptor e.g a, a, b, d, g
Each subunit
an the membrane
-Polypeptide transmembrane spanning domains
Binding sites on the subunits allow neurotransmitters to attach to the receptor to open the ion channel
Binding of a ligand (such as ACh, Glutamate or GABA) is required to open the ion channel
Binding of the ligand changes the conformation of the channel molecule so that ions may pass through

Question 6

What are metabotropic receptors (RCPG)?

Answer 6

A metabotropic receptor is a single polypeptide
- it does not have subunits like ionotropic receptors
- it has SEVEN membrane spanning domains
- each is an a-helix (coil)
Neurotransmitters bind to the a-helices.
Instead of directly opening ion channels, the receptor couples/binds to a G-protein
Neurotransmitters bind to the a-helices.
Instead of directly opening ion channels, the receptor couples/binds to a G-protein
When inactive (before binding), the a subunit of the G-protein binds GDP
Binding Activates the G-Protein
GTP replaces GDP to activate the G-protein
When GTP is bound to the G-protein, the subunits split
The split subunits of the G-protein are available to activate Effector Proteins. Effector proteins are linked to “second messengers”
Second messenger cascades are activated. GTP
This is how information is spread within the neuron
The G-protein subunits may also bind to ION CHANNELS

Question 7

What is a second messenger system?

Answer 7

There are many second messenger systems. Binding of a neurotransmitter can lead to downstream activation of enzymes important in neuronal function additional mechanisms and fctions after the binding occurred
Second messenger cascades allow regulation of systems within the neuron to respond to information that the neuron has received from outside transmitters a GTP
Activation of one receptor can lead to the activation of several downstream mechanisms
1 receptor, several fctions –> more versatility

Question 8

Why have a second messenger system?

Answer 8

Ligand-gated ion channels are very FAST - they are involved in quick changes in the polarity of the neuron
G-protein coupled ion channels are also relatively fast as second messenger cascades aren’t used (G-protein binds directly to channel)
G-protein coupled receptors linked to second messenger systems are SLOW - what is the advantage?
requires energy and more reactions
1. More sophisticated transfer of information as many different systems can be involved
2. AMPLIFICATION of the signal - one receptor can effect many downstream systems (eg intracellular calcium release)
=> Slow but EFFICIENT

Question 9

What are inhibitory G proteins?

Answer 9

The binding of a neurotransmitter to a G-protein coupled receptor does not always lead to the activation of second messenger systems
Many receptors are linked to inhibitory G-proteins which prevent the second messenger cascade (Gi proteins)

Question 10

What is receptor feedback?

Answer 10

Receptors are also located PRE-synaptically
This provides feedback to the presynaptic neuron to stop releasing neurotransmitter

Question 11

What are agonists and antagonists?

Answer 11

A receptor AGONIST binds to a receptor and has an effect on the neuron they affect neuronal signaling and can can provide feedback to the presynaptic neuron to stop releasing neurotransmitter

A receptor ANTAGONIST binds to a receptor and does not have an effect on the neuron
Neurotransmitters can’t have an effect when the antagonist binds to the binding site of the receptor - neural signalling is blocked