I: Signal Transduction

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I Signal Transduction

• Regulation of cell permeability by ligand-gated
  channels the nicotinic receptor and other
  ionotropic receptors

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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 waythe different
  subtypes of muscarinic receptors are all
  metabotropic receptors in which
  messenger-receptor binding works by initiating a
  2nd message within the target cell.

3
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 .
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Inhibitory Postsynaptic Potentials (IPSPs) All
inhibitory mechanisms oppose depolarization Simple
st 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.
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Skeletal Muscle Synapse Nicotinic ACh receptors
are the classic example of ligand-gated channels
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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.

7
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 11 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.

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The nicotonic ACh receptor is a pentameric channel
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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.

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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 Cl- more dominant in determining
  the resting potential.

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The GABAA receptor and its binding sites for
drugs and modulatorsBenzodiazepines are
antidepressants barbiturates are depressants
steroids exert an antidepressant effect
pentobarbitol is a local anesthetic.
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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.
• Huntingtons 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.

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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 cells
  response to the 1st message.

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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 message- generally
  this is the outcome of a multistage signal
  cascade.

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Second messenger amplification increases the
ligands effect, but this takes time
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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.
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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

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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 G-proteins.
• Ligands for the GPCR range from photons through
  small transmitters to protein signaling molecules
  of the immune system.
• GPCRs are targets for 40-50 of medicinal drugs

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A generalized picture of the G-Protein-linked or
G-Protein-coupled receptor
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G Proteins aß? alpha, beta, gamma subunits

• G Proteins are molecular switches whose on or off
  state depends on whether GDT or GTP is bound to
  the a 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 a dissociates from ß? (which are
  permanently linked). Both pieces of the G
  protein can interact with messenger systems,
  although in many cases the ß? subunits roles are
  not known.
• When Ga locates its target, the process of
  activating the enzyme causes hydrolysis of GTP,
  leaving GDP, and then the aß? subunits must
  reunite. This terminates the active response to
  the ligand.

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G-Protein Activation
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G Proteins One possible target of G protein
signal cascades is adenyl cyclase, the enzyme
that catalyzes the formation of cyclic AMP
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Formation of cyclic AMP the cyclic AMP is
destroyed by phosphodiesterase, yielding AMP
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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.

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Summary Comparison of Ionotropic and
Metabotropic receptor activity metabotropic
pathways can either open or close channels
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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.

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All muscarinic receptors are coupled to one of
two main 2nd messenger systems
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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
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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

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Gi coupled
Gs coupled
Gq coupled