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•Next theme: ion channel modulation
(or “indirect” synaptic transmission)
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Reminder: The postsynaptic membrane
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Direct and indirect synaptic transmission:
where do they happen?
Direct:
•neuromuscular junction (ACh)
•central excitatory synapses (glutamate)
•central inhibitory synapses (GABA, glycine)
Indirect (today’s theme)
•parasympathetic nerve action on heart and other organs (ACh)
•sympathetic nerve action on heart and other organs (adrenaline,
noradrenaline)
•central excitatory and inhibitory synapses (glutamate, GABA,
peptide transmitters)
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Indirect transmission can be complex
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G protein coupled receptors
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G protein coupled receptors
7 transmembrane helices
G protein binding domain
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G protein coupled receptors
•7 transmembrane helices
•G protein binding domain
•These characteristics define a very large receptor
family: 7TM/GPCR family
•About 300 known human sequences for receptors in
this family...
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The 7TM/GPCR receptor family
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The 7TM/GPCR receptor family
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The 7TM/GPCR receptor family
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The 7TM/GPCR receptor family
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The 7TM/GPCR receptor family
...plus about 300 members of the family which are
olfactory receptors
•Many others are taste receptors
•Rhodopsin and its relatives (the light sensitive proteins
from the eye) are also members of this family
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The 7TM/GPCR receptor family
Different binding
sites for different
types of agonists
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G proteins
•All these receptors bind to and activate G proteins
•G proteins are GTP-binding proteins
•They “pass on” the message of receptor activation to
further processes within the cell
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G proteins
•Activation of the receptor (star) causes G protein (αβγ)
to split
•Either the α or the βγ parts can activate various further
processes
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G proteins
•Let’s look at the steps in detail
Adenylate cyclase
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G proteins
•Let’s look at the steps in detail
•Transmitter binds
•Exposes binding site for G protein
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G proteins
•Let’s look at the steps in detail
•Transmitter
binds
•G protein binds
to receptor
•Exposes
binding
site for G protein
•GTP replaces
GDP
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G proteins
•Let’s look at the steps in detail
•Transmitter
binds
•G subunit
•α
protein of
binds
G protein
to receptor
separates from βγ subunits
•Exposes
siteactivate
for G protein
•GTP replaces
•Both
partsbinding
canGDP
now
various effectors
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G proteins
•Let’s look at the steps in detail
•Transmitter
•G subunit
•α
protein binds
of binds
G protein
to effector
receptor
separates
(in thisfrom
caseβγ
adenylate
subunits
•Exposes
binding
siteactivate
for
•GTP replaces
•Both
cyclase)
parts
and
can
activates
GDP
now
it G protein
various effectors
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G proteins
•Let’s look at the steps in detail
•Transmitter
•G subunit
•α
•Hydrolysis
protein binds
ofofbinds
GGTP
protein
to effector
receptor
to GDP
separates
and
leaves
activates
from
the αβγsubunit
itsubunits
unable
•Exposes
siteactivate
for G protein
•GTP
•Both
to
activate
replaces
partsbinding
the
caneffector
GDP
now
various effectors
•It rejoins with the βγ subunits
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G proteins
•Let’s look at the steps in detail
•Transmitter
binds
•G subunit
•α
•Hydrolysis
•As
protein
long asbinds
ofof
transmitter
GGTP
protein
to effector
receptor
to GDP
separates
remains
and
leaves
activates
bound
from
the αβγ
tosubunit
itreceptor
subunits
unable
the
•Exposes
binding
siteactivate
for G protein
•GTP
•Both
to
G
protein
activate
replaces
parts
can
the
can
be
effector
GDP
now
reactivated
various effectors
•It rejoins with
•Unbinding
of transmitter
the βγ subunits
ends the sequence
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Example: making the heart beat stronger
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Noradrenaline increases force of contraction
•Noradrenaline
(norepinephrine): transmitter
released from sympathetic
nerves
•Increases heart rate and
force of contraction
•Force increases due to
increased Ca2+ entry
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How does noradrenaline work?
Ca2+
channel
NA
•NA applied
outside the cell
•Ca2+ channel
recorded in cell
attached patch
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How does noradrenaline work?
•NA increases Ca2+ channel activity
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How does noradrenaline work?
Ca2+
channel
NA
•How can NA
outside the cell
activate a Ca2+
channel in the
patch?
•There can’t be a
direct connection
•There must be a
mobile “second
messenger” inside
the cell
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How does noradrenaline work?
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Evidence for role of phosphorylation
•Directly apply activated protein kinase A + ATP
•Same effect as that of noradrenaline
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Noradrenaline effect on the heart
•Noradrenaline activates β-adrenergic receptor
- This activates G protein (Gs)
- Gs activates adenylyl cyclase to synthesise cyclic AMP
- cAMP activates protein kinase A
- Protein kinase A phophorylates calcium channel
- Phosphorylation activates the calcium channel
•The response is switched off in two ways:
- Phosphatases remove the phosphate groups from the
ion channels
- Phosphodiesterases break down cAMP
•All adrenergic receptors are members of the 7TM/GPCR
family
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Example 2: how ACh slows the heart
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Example 2: how ACh slows the heart
(This was the Vagusstoff effect that Loewi studied)
Slow inward
current :
“pacemaker”
depolarisation
Reduced
pacemaker
depolarisation:
reduced inward
current OR
increased
outward current
It turned out to
be an increased
outward current,
i.e. opening of a
K+ channel
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Two types of ACh receptors
Nicotinic:
activated by
nicotine as well
as ACh.
Neuromuscular
junction and
CNS
Muscarinic:
activated by muscarine
as well as ACh.
Parasympathetic nervous
system and CNS
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Two types of ACh receptors
Amanita
muscaria:
source of
muscarine
Muscarinic:
activated by muscarine
as well as ACh.
Parasympathetic nervous
system and CNS
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Two types of ACh receptors
Muscarine
Amanita
muscaria:
source of
muscarine
ACh
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Is a soluble second messenger involved?
K+
channel
ACh
•ACh applied
outside the cell
•K+ channel
recorded in cellattached patch
•No effect!
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No effect of ACh in bath
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How does ACh affect K+ channels?
K+
channel
ACh
•ACh applied in
the pipette
•K+ channel now
activated by ACh
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ACh in pipette activates K+ channel
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How does ACh work?
•Membrane-delimited pathway
•The βγ subunits of the G protein
activate the K+ channel
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Evidence for role of Gβγ
•Inside-out patch
•ACh applied in pipette (extracellular)
•Gβγ applied in bath (intracellular)
•Effect of both is the same
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ACh effect on the heart
•“Membrane delimited” pathway, initially surprising but
many examples discovered since
•ACh activates muscarinic receptor
- This activates G protein (Gi)
- Gi splits into α and βγ subunits
- The βγ subunits directly activate the K+ channel
•The response is switched off by unbinding of the βγ
subunits
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Reading for this lecture:
•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 lecture
Synaptic integration, plasticity and myasthenia gravis
•Purves et al chapter 5 (pages 101 - 107); chapter 8 (pages 169-177);
box 6B (page 117)
•Nicholls et al chapter 12 (pages 232-238); chapter 22 (pages 449-452)
•Kandel et al chapter 12, chapter 63 (pages 1259-1272), chapter 16