Neurotransmitters and Receptors

Question 1

What is a classical neurotransmitter?

Answer 1

Act quickly- produce fast EPSPs and IPSPs which last a few milliseconds

Question 2

What are modulators?

Answer 2

Act slowly- modify the actions of classical transmitters- produce slow EPSPs and IPSPs which can last for several hundred milliseconds or even longer

Question 3

What do modulators often act through?

Answer 3

G-protein coupled receptors

Question 4

What are some examples of classical neurotransmitters?

Answer 4

Acetylcholine

Amino acid transmitters

Question 5

What are the two groups classical neurotransmitters can be separated into according to their actions?

Answer 5

Excitatory transmitters- e.g. L-glutamte

Inhibitory transmitters- e.g. GABA (γ-aminobutyric acid)

Question 6

What are some examples of modulators?

Answer 6

Monoamines (noradrenaline, dopamine, serotonin) and neuropeptides (e.g. substance P, enkephalin) are examples of modulators.

Question 7

What is the criteria in order to determine if a substance is a neurotransmitter or modulator?

Answer 7

It must be synthesised in the presynaptic neuron - high concentrations of the
substance must also be present in axon terminals of the presynaptic neuron
It must be released from the presynaptic terminal
It must have an action on the postsynaptic membrane, i.e. it must activate
receptors
It must be inactivated

Question 8

Where are transmitters stored?

Answer 8

In vesicles in the presynaptic membrane

Question 9

Where are small molecule transmitters synthesised?

Answer 9

In axon terminals

Question 10

How are transmitters formed?

Answer 10

Precursor molecules (e.g. choline is a precursor for acetylcholine) are converted to the transmitter by enzymatic action (in the case of acetylcholine the enzyme is choline acetyltransferase)

Question 11

Where are neuropeptides synthesised?

Answer 11

In the cell body

Question 12

How are neuropeptides formed?

Answer 12

Initially produced as large polypeptides and are enzymatically split into smaller peptide molecules
These are transported to the nerve terminals by anterograde axonal transport

Question 13

What is essential for transmitter release?

Answer 13

Calcium ions

Question 14

What happens when the presynaptic membrane depolarises?

Answer 14

Precedes release and voltage-gated calcium channels open

Question 15

What do calcium ions do when they enter the presynaptic membrane?

Answer 15

Enter and initiate transmitter release

Question 16

What happens if other cations are used e.g. magnesium ions?

Answer 16

Release is blocked

Question 17

What are miniature endplate potentials (MEPPs)?

Answer 17

Tiny spontaneous potenitals

Can be recorded with micropipettes at the postsynaptic membrane of the neuromusclar junction

Question 18

What happens to MEPP is one packet of transmitter is released?

Answer 18

A unitary MEPP (which always has the same amplitude) is observed

Question 19

What happens if two packets of transmitter is released?

Answer 19

MEPP is twice the amplitude of the unitary MEPP …… and so on for three and four packets.

Question 20

What does the evidence of packets of transmitter and it effect on MEPP show?

Answer 20

Release of transmitter is quantal, i.e. transmitters are released as uniform packets.

Question 21

What does the contents of a single vesicle correspond to?

Answer 21

A quantum of transmitter

Question 22

Where are autoreceptors?

Answer 22

Present at the presynaptic membrane

Question 23

What are autoreceptors activated by?

Answer 23

The transmitter released from the same terminal

Question 24

What can autoreceptors influence?

Answer 24

Release by regulating transmitter production

Question 25

Where do transmitters activate receptors?

Answer 25

The postsynaptic membrane

Question 26

What are the receptors on the postsynaptic membrane?

Answer 26

Protein molecules embedded in the membrane and have active sites in the extracellular space

Question 27

What do neurotransmitters specifically bind to?

Answer 27

Active sites on receptors

Question 28

What are the two classes of receptor?

Answer 28

Ionotropic

metabotropic (G-protein coupled)

Question 29

What are ionotropic receptors?

Answer 29

Ion channels with active sites for neurotransmitters on them

Question 30

What happens when a neurotransmitter binds to an active site on an ionotropic receptor?

Answer 30

The channel opens and ions pass through

Question 31

What happens when the transmitter dissociates from the receptor on an ionotropic receptor?

Answer 31

The channel closes

Question 32

How do ionotropic receptors act?

Answer 32

Quickly and are associated with fast transmission

E.g. nicotinic aceytlcholine receptor

Question 33

What are metabotropic receptors?

Answer 33

Complex and not directly linked to ion channels

Question 34

What do metabotropic receptors use?

Answer 34

“second messengers” (E.g. cyclic-AMP)

Question 35

What do second messengers activate in metabotropic receptors?

Answer 35

Activates an effector system which opens the ion channel

Question 36

What kind of receptors are metabotropic receptors?

Answer 36

Operate slowly, hence the slow action of neuromodulators

e.g. muscarinic acetylcholine receptor

Question 37

What can second messengers trigger for metabotropic receptors?

Answer 37

Other events in addition to opening ion channels
These events are known as cascades and often result in long-term changes
Cascades are important in learning and memory

Question 38

Why is precise timing important for synaptic action?

Answer 38

Transmitter action must be stopped quickly

Question 39

What are the 3 mechanisms of inactivation?

Answer 39

Specific enzymes, e.g. acetylcholine is broken down by acetylcholine esterase
Specific uptake by neurons or glial cells, e.g. monoamines are taken up by neurons and recycled
Diffusion, e.g. peptides can diffuse for considerable distances