I: Signal Transduction
Regulation of cell permeability by
ligand-gated channels: the
nicotinic receptor and other
ionotropic receptors
Ionotropic vs metabotropic receptors
• Opening a channel directly, as is seen in the
nicotinic ACh receptor, provides for very rapid
transduction of chemical detection into an
electrical signal – this is an example of an
ionotropic receptor.
• As we will see, the muscarinic ACh receptor
works in a completely different way…the
different subtypes of muscarinic receptors are
all metabotropic receptors – in which
messenger-receptor binding works by initiating
a 2nd message within the target cell.
What makes a given synapse
excitatory or inhibitory for the
postsynaptic cell?
It depends on what happens after
transmitter binds receptor – not on
some intrinsic property of the
transmitter chemical itself .
Inhibitory Postsynaptic Potentials
(IPSPs)
All inhibitory mechanisms oppose
depolarization
Simplest mechanism: increased K+
permeability – which would lead to
hyperpolarization
Less simple: increased Cl- permeability
– sometimes called silent inhibition –
tends to stabilize membrane potential at
the rest value.
Skeletal Muscle Synapse: Nicotinic ACh receptors are
the classic example of ligand-gated channels
Points from preceding slide:
• Accumulation of ACh into the vesicles is driven by a H+
pump.
• ACh synthesis occurs in the cytoplasm of the terminal.
• The channel that is opened by ACh is called “ligand
gated”. Ligand-gated channels are opened or closed by
lock-and-key binding with a chemical.
• The synaptic potential (end plate potential) is above
threshold for an action potential.
• The action potential in muscle cells is similar to that in
nerve cell axons: Na+ and K+ voltage-gated channels.
• The quick recovery from the ACh binding is the result of
acetylcholinesterase, which terminates the
neurotransmitter effect.
Neuromuscular synaptic transmission: effect of
ACh on nicotinic receptors of skeletal muscle
• 1. A single action potential releases so many vesicles
that the depolarization of the muscle cell membrane
reaches the threshold for an action potential –
transmission is 1:1 – this is a unique synapse!
• The nicotinic acetylcholine receptor protein is a
ligand-gated cation channel: it is a channel that
allows passage of Na+ and K+ when ACh binds.
• Because the muscle resting potential is near the K+
equilibrium potential, Na+ is the dominant ion that
flows through the channel, exerting a depolarizing
effect on the muscle cell membrane.
• Patch-clamping reveals that each channel remains
open for 2-3 msec., allowing 15,000 – 30,000 Na+ to
flow through.
The nicotonic ACh receptor is a pentameric channel
Toxins that target the ACh receptor have been
tools for research
• The receptor gets its name from the fact that
nicotine mimics the effect of ACh.
• a-bungarotoxin is produced by the snake called
the banded Krait. Scientists in Taiwan showed
that paralysis was the result of binding to the
ACh receptor.
• Curare is a mixture of plant toxins (purified:
tubocarine) used by S.A. Indians for arrowhead
poison and in surgery to block muscle reflexes.
It blocks the receptor and prevents ACh binding.
More examples of ionotropic receptors
• The fast synaptic response seen upon
activation of nicotinic ACh receptors is similar to
the synaptic depolarization initiated by the 2
glutamate ionotropic receptors: both Na+ and
K+ can move through the channels.
• GABA (gamma amino butyric acid) and glycine
receptors are Cl- channels. Their inhibitory
effect is to either hyperpolarize the postsynaptic
cell by increasing the Cl- permeability (if it is not
at equilibrium) and/or to “clamp” the membrane
potential at the resting state by making Clmore dominant in determining the resting
potential.
The GABAA receptor
and its binding sites
for drugs and
modulators
Benzodiazepines are
antidepressants;
barbiturates are
depressants; steroids
exert an
antidepressant effect;
pentobarbitol is a
‘local anesthetic’.
GABAA receptor: another ligand-gated channel
• The GABAA receptor has a hyperpolarizing effect
on its target cell which is called an inhibitory
postsynaptic potential = IPSP
• The effect of benzodiazapene is to increase the
Cl- movement through the open channels; this
inhibition has a calming effect, as its action is
particularly important in brain regions associated
with emotional behavior.
• Huntington’s chorea is a degenerative disease in
which GABA-ergic (GABA-releasing) neurons
are lost and the result is uncontrolled
movements. The GABA-releasing neurons are
thought to die off due to an inherited excessive
activity by glutamate-releasing cells, one
example of excitotoxicity.
Metabotropic receptors
• What is a 2nd messenger? Any substance
that is released inside the cell or
synthesized there in response to
messenger-receptor binding at the cell
surface, and that effects the target cell’s
response to the 1st message.
Why 2nd messages?
• 2nd messages can do more than just affect
the electrical responsiveness of the target
cells
• 2nd messages reach effectors within the
cell and can also affect gene expression
• 2nd messages amplify the 1st messagegenerally this is the outcome of a
multistage signal cascade.
Second messenger amplification increases
the ligand’s effect, but this takes time…
Opening ion
channels is only
one of the effects
that such second
messengers can
have. A few of the
other pathways are
included here.
Although not all are
present in any one
example, more than
one change is often
activated by binding
of the ligand.
The 3 major 2nd messenger
systems
• Cyclic nucleotides (cAMP, cGMP)
• Inositol trisphosphate (IP3)
• Ca++
– Interaction between these is typical – for
example both cyclic nucleotides and IP3 can
trigger release of Ca++
G-protein Coupled Receptors
• G-Protein coupled receptors,
also known as 7-transmembrane
or serpentine receptors
because 7 a-helices pass
through the membrane, are the
largest family of transducer
proteins, and the largest family
of proteins known. The human
genome codes for at least 90 Gproteins.
• Ligands for the GPCR range
from photons through small
transmitters to protein signaling
molecules of the immune
system.
• GPCR’s are targets for 40-50%
of medicinal drugs
A generalized picture of the G-Protein-linked or
G-Protein-coupled receptor
G Proteins: αβγ = alpha, beta, gamma subunits
• G Proteins are molecular switches whose on or off
state depends on whether GDT or GTP is bound to the
α subunit. (A smaller monomeric G protein is called Ras and
is associated with tyrosine kinase receptors that mediate cell
growth and movement.)
• The G protein moves away from the receptor when
GTP binds, and α dissociates from βγ (which are
permanently linked). Both pieces of the G protein can
interact with messenger systems, although in many
cases the βγ subunit’s roles are not known.
• When Gα locates its target, the process of activating
the enzyme causes hydrolysis of GTP, leaving GDP,
and then the αβγ subunits must reunite. This
terminates the active response to the ligand.
G-Protein Activation
One possible target of G protein signal cascades is adenyl cyclase, the enzyme
that catalyzes the formation of cyclic AMP
Formation of cyclic AMP: the cyclic AMP is
destroyed by phosphodiesterase, yielding AMP
Different G-protein families are coupled
to different 2nd messenger pathways
• Gi inhibits the operation of adenyl cyclase
• Gs stimulates the operation of the same
enzyme
• Gq stimulates phospholipase C, resulting in
formation of inositol trisphosphate (IP3)
and diacylglycerol (DAG) from a common
membrane phospholipid, inositol
bisphosphate.
Summary: Comparison of Ionotropic and Metabotropic receptor activity:
metabotropic pathways can either open or close channels
Now to the muscarinic receptors
• Muscarinic receptors are characteristically found
on targets for the parasympathetic division of the
autonomic nervous system
• 5 subtypes M1-M5 have been identified
genetically, but at present pharmacologists can
distinguish only 4 subtypes.
• The subtypes differ in their locations in the body,
their effects (excitatory vs inhibitory) and the
particular G-proteins and 2nd messenger
systems they are coupled to.
All muscarinic receptors are coupled to one
of two main 2nd messenger systems
M1
Salivary glands,
stomach, CNS
Gq coupled to phospholipase C: inositol
trisphosphate and diacylglycerol; increases
intracellular Ca++
M2
Heart, CNS
Gi coupled to inhibition of adenylyl cyclase,
decreased cAMP
M3
Blood vessels, eye,
visceral smooth
muscle, lung airway
Gq coupled to phospholipase C: inositol
trisphosphate and diacylglycerol
M4
CNS
Gi coupled to inhibition of adenylyl cyclase,
decreased cAMP
M5
?
Gq coupled to phospholipase C: inositol
trisphosphate and diacylglycerol
Muscarinic receptors are targets for many drugs
and toxins – only a few are shown here
• M1: carbachol, atropine, scopolamine,
mambatoxin MT7
• M2: methachol, carbachol, atropine
• M3:, carbachol, methachol, atropine
• M4: bethanechol, carbachol, atropine,
mambatoxin MT3
Agonists are shown in black; antagonists in red
Adrenergic receptors
• Adrenergic receptors are found on the targets of
the sympathetic division of the autonomic
nervous system.
• The transmitter released at sympathetic
synapses is norepinephrine (noradrenaline);
the sympathetic hormone released by the
adrenal medulla is epinephrine (adrenaline) –
these are catecholamines.
• There are 2 families of adrenergic receptors,
alpha and beta.
• There are 2 subgroups of alpha receptors and 3
of beta receptors
Gi
coupled
Gq
coupled
Gs
coupled