420
UNIT 3
Regulation and Integration of the Body
11
Channel-Linked Receptors: Mechanism of Action
Channel-linked receptors
(
ionotropic receptors
) are ligand-gated
ion channels that mediate direct neurotransmitter action. Tey
are composed of several protein subunits in a “rosette” around
a central pore. As the ligand binds to one (or more) receptor
subunits, the proteins change shape. Tis event opens the central
channel and allows ions to pass
(Figure 11.20)
. As a result, the
membrane potential of the target cell changes.
Channel-linked receptors are always located precisely op-
posite sites of neurotransmitter release, and their ion channels
open instantly upon ligand binding and remain open 1 ms or
less while the ligand is bound. At excitatory receptor sites (nico-
tinic ACh channels and receptors for glutamate, aspartate, and
A±P), the channel-linked receptors are cation channels that al-
low small cations (Na
1
, K
1
, Ca
2
1
) to pass, but Na
1
entry con-
tributes most to membrane depolarization. Channel-linked
receptors that respond to GABA and glycine, and allow Cl
2
to
pass, mediate fast inhibition (hyperpolarization).
G Protein–Linked Receptors: Mechanism of Action
Unlike responses to neurotransmitter binding at channel-linked
receptors, which are immediate, simple, and brief, the activity
mediated by
G protein–linked receptors
is indirect, complex,
slow (hundreds of milliseconds or more), and oFen prolonged—
ideal as a basis for some types of learning. Receptors in this class
Neurotransmitter Receptors
In Chapter 3, we introduced the various receptors involved in cell
signaling. Now we are ready to pick up that thread again as we ex-
amine the action of receptors that bind neurotransmitters. ²or the
most part, neurotransmitter receptors are either channel-linked
receptors, which mediate fast synaptic transmission, or G protein–
linked receptors, which oversee slow synaptic responses.
Ion flow blocked
Ions flow
Closed ion
channel
Ligand
Open ion
channel
Figure 11.20
Direct neurotransmitter receptor mechanism:
Channel-linked receptors.
These chemically gated ion channels
promote rapid synaptic transmission. In this case, ligand binding
directly opens the channel.
Ligand (1st
messenger)
Receptor
G protein
Enzyme
2nd
messenger
1
2
Neurotransmitter
(1st
messenger) binds
and activates receptor.
Receptor
activates G
protein.
3
G protein
activates
adenylate
cyclase.
4
Adenylate
cyclase converts
ATP to cAMP
(2nd messenger).
5a
cAMP changes membrane
permeability by opening or
closing ion channels.
5b
cAMP activates
enzymes.
5c
cAMP activates
specific genes.
Receptor
G protein
Active enzyme
Closed ion channel
Adenylate cyclase
Open ion channel
Nucleus
ATP
GTP
GDP
cAMP
GTP
GTP
Recall from Chapter 3 that G protein
signaling mechanisms are like a
molecular relay race.
Figure 11.21
Indirect neurotransmitter receptor mechanism: G protein–linked
receptors.
These receptors cause the formation of intracellular second messengers—cyclic
AMP (cAMP) in this example—that indirectly bring about the cell’s response.
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