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What’s going on at the postsynaptic membrane? Ligand-gated ion channels:- ACh receptors (excitatory)- glutamate receptors (excitatory)- GABA receptors (inhibitory)- glycine receptors (inhibitory)

1

How does ACh depolarise the neuromuscular junction?

Perhaps ACh opens Na+ channels? How to test:•Check the reversal potential of the current - in this case it should be near ENa

•Two microelectrodes in muscle fibre

2

Real synaptic current

•Result: they are not Na+ channels - current reverses near 0 mV, far from ENa

•So which ion conducts the current?

3

•Which ion conducts the current? - i.e.•Which ion has equilibrium potential near 0 mV? - i.e.•Which ion is at equal concentration outside and inside?

•None of them! - so the ACh receptor channel must conduct more than one ion•In fact both Na+ and K+ are conducted equally well•Substantial Ca2+ permeability too: but little Ca2+ available•So Na+ and K+ carry most of the current

4

Fluxes are equal here

5

Synaptic current and synaptic potential

•Current flows during the rising phase of the EPP•Inward current causes the depolarisation (“upstroke”) of the EPSP•Repolarisation is passive return to resting potential

6

Location of ACh receptors at the neuromuscular junction

7

How does ACh trigger an AP?

8

Recording single ACh receptors

9

Effect of a “puff” of ACh

•How to apply ACh briefly?•We need “outside-out” patches•ACh can then be added/removed very fast

10

How to apply a “puff” of ACh

ACh

No ACh

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Effect of a “puff” of ACh

•Channels stay open as long as ACh is bound

•Unbinding of ACh is random: so channel open time is random

•Add single channel currents: we get the macroscopic endplate current

12

What does the ACh receptor look like?

•Very distant relative of Na+ channel•Hydropathy plot like this:

13

What does the ACh receptor look like?•Unlike the K+ channel - it has 5 subunits and not 4

14

How does the ACh receptor select cations?

Conserved negative charges in most subunits in the M2 helix

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The negative charges are all on one side of the M2 helix

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The M2 helices face inwards around the pore

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So we would have 3 negatively charged rings around the pore

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Mutating the rings alters ion conductance: so they are important in selecting ions to go through the pore

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Central excitatory synaptic transmission•Introducing a new neurotransmitter: glutamate•Glutamate receptors: two types

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•Introducing a new neurotransmitter: glutamate•Glutamate receptors: two types

•Non-NMDA receptors:- don’t respond to the glutamate analogue NMDA•Functionally like ACh receptor...but little sequence similarity•Opened by glutamate•Allow Na+ and K+ to pass

Central excitatory synaptic transmission

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•NMDA receptors:- DO respond to the glutamate analogue NMDA•Opened by glutamate•Allow Na+ and K+ to pass•Similar sequence to non-NMDA receptors BUT very different functionally:- modulated by many substances- important in synaptic plasticity (maybe memory/learning: more later)

Central excitatory synaptic transmission

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ACh receptor

AChR and GluR: functionally similar

Glutamate receptor

23

Synaptic inhibition

Inhibitory neurone

Excitatory neurone

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Synaptic inhibition

Inhibitory neurone

Excitatory neurone

•What makes an excitatory neurone excitatory? (or an inhibitory neurone inhibitory)?- the kind of transmitter released from its terminal- the kind of receptor on the postsynaptic membrane

•Let’s look at inhibition

25

•Transmitters involved: GABA, glycine•Time to look at the transmitters

Central inhibitory synaptic transmission

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•Transmitters involved: GABA, glycine•Receptor sequences closely related to ACh receptor


•...but functionally they are the opposite of the AChR!27

Central inhibitory synaptic transmissionGlutamate receptor GABA/glycine receptors

- Cell insidemore positivethan Ecl

- Inward Cl- flux- Equivalent to outward flow of positive charge

- Cell insidemore negativethan Ecl

- Outward Cl- flux- Equivalent to inward flow of positive charge

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GABA/glycine receptors

Conclusion:•GABA and glycine receptors conduct chloride ions•ECl is usually close to resting potential Er

•So inhibitory postsynaptic potential (IPSP) is small: often negative but sometimes zero•Even if it’s zero it is still inhibitory (we will see why later)

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Reading for this lecture:

•Purves et al chapter 5 (page 96 - 107); chapter 6 (up to page 125)•Nicholls et al chapters 3 & 9 - sections on ACh, glutamate, GABA and

glycine receptor channels; chapter 13 - pages 244-247•Kandel et al chapter 11, chapter 12 (pages 212-214)

Next lecture:Ion channel modulation by G proteins and second messengers:

slow synaptic transmission•Purves et al chapter 7 (up to page 153)

•Nicholls et al chapter 10 (especially pages 184-188)•Kandel et al chapter 13

(Note: All these readings go into a lot of depth, so read selectively based on examples in the lecture)

next theme: what’s going on at the postsynaptic membrane? ligand-gated ion channels: - ach...

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