Transcript Long term memory

MESTRADO INTEGRADO EM CIÊNCIAS FARMACÊUTICAS
BIOQUÍMICA II
2008-09
AULA 22
Cecília M. P. Rodrigues
Sumário
Parte III: Organização e Funcionamento Subcelular
Bases bioquímicas do funcionamento da célula nervosa
Sinapses e transmissão do impulso
Neurotransmissores e receptores de neurotransmissores
Reciclagem de vesículas sinápticas
Transdução sensorial
Aprendizagem e memória
Synapses and impulse transmission
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Synapses are the junctions where neurons pass signals to target cells,
which may be other neurons, muscle cells, or gland cells;
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In most nerve-to-nerve signaling and all known nerve-to-muscle and
nerve-to-gland signaling, the neuron releases chemical
neurotransmitters at the chemical synapse that act on the target cell;
•
Much rarer, but simpler in function, are electric synapses in which the
action potential is transmitted directly and very rapidly from the
presynaptic to the postsynaptic cell.
Synapses and impulse transmission
Transmission of action potentials across electric and chemical synapses
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Signals are transmitted across an
electric synapse within a few
microseconds because ions flow
directly from the presynaptic cell to the
postsynaptic cell through gap
junctions.
Signal transmission across a chemical
synapse is delayed about 0.5 ms —
the time required for secretion and
diffusion of neurotransmitter and the
response of the postsynaptic cell to it.
Lodish, Molecular Cell Biology
Chemical synapses
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Chemical synapses can be fast or slow, excitatory or inhibitory, and
can exhibit signal amplification and computation (integrated function of
all incoming signals).
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In excitatory and inhibitory synapses, the action of a neurotransmitter
tends to promote or inhibit the generation of an action potential in the
postsynaptic cell, by binding of the neurotransmitter to an excitatory or
inhibitory receptor, respectively.
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Excitatory: acetylcholine for nicotinic receptor, glutamate for NMDA and nonNMDA receptors, serotonin for 5HT3 receptors
Inhibitory: GABBA for A-class receptors, glycine
In fast synapses, binding of the neurotransmitter causes an immediate
conformational change in neurotransmitter receptors, which are ligandgated ion channels.
In slow synapses, the neurotransmitter receptors are coupled to G
proteins.
The same neurotransmitter binds many types of receptors.
Structures of several small molecules that function as
neurotransmitters
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Except for acetylcholine,
the neurotransmitters
shown here are amino
acids (glycine and
glutamate) or derived
from the indicated amino
acids (tyrosine,
tryptophan, histidine).
The 3 transmitters
synthesized from
tyrosine, which contain
the catechol moiety (blue
highlight), are referred to
as catecholamines.
Lodish, Molecular Cell Biology
(a) Acetylcholine (or nicotine) in frog skeletal
muscle produces a rapid postsynaptic
depolarization of about 10 mV, for 20 ms.
The nicotinic acetylcholine receptors
are ligand-gated cation channels;
binding of acetylcholine opens the
channel, admitting both Na+ and K+.
(b) Acetylcholine (or muscarine) in frog heart
muscle produces, after a lag period of
about 40 ms (not visible in graph), a
hyperpolarization of 2-3 mV, for several
seconds.
These cells contain muscarinic
acetylcholine receptors, which are
coupled via a G protein to K+
channels. Activation of the receptor
leads to channel opening.
Lodish, Molecular Cell Biology
Excitatory and inhibitory responses in postsynaptic
cells stimulated by acetylcholine
Release of neurotransmitters
Recycling of synaptic vesicles
1. Vesicles import
neurotransmitters (red
circles) from the cytosol
using a H+/neurotransmitter
antiporter.
The low intravesicular pH,
generated by a V-type
ATPase in the vesicle
membrane, powers
neurotransmitter import.
1. The vesicles then move to
the active zone near the
plasma membrane.
2. Vesicles “dock” at defined
membrane sites by
interacting with specific
proteins.
Lodish, Molecular Cell Biology
Release of neurotransmitters and the recycling of
synaptic vesicles
4. A rise in cytosolic Ca2+
triggers fusion of the docked
vesicles and release of
neurotransmitters into the
synaptic cleft.
5. Synaptic-vesicle membrane
proteins are then specifically
recovered by endocytosis,
usually in clathrin-coated
vesicles. The clathrin coat is
depolymerized, yielding
vesicles that are the same
size as synaptic vesicles.
These new synaptic vesicles
then are filled with
neurotransmitters (step 1),
completing the cycle, which
typically takes about 60 sec.
Removal of the neurotransmitter from the synapse
is essential to ensure its repeated functioning
3D-structure of the nicotinic acetylcholine receptor
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Pentameric receptor (the β
subunit is not shown).
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The M2 α helix (red) in each
subunit is part of the lining of the
ion channel. Aspartate and
glutamate side chains at both
ends of each M2 helix form two
rings of negative charges that
help exclude anions from and
attract cations to the channel.
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The gate, which is opened by
binding of acetylcholine, lies
within the pore.
Lodish, Molecular Cell Biology
Acetylcholine-induced opening of gated ion channels
Neuromuscular junction
1. Arrival of an action potential at the terminus
of a presynaptic motor neuron induces
opening of voltage-gated Ca2+ channels
and subsequent release of acetylcholine.
2. Acetylcholine triggers opening of the
ligand-gated nicotinic receptors in the
muscle plasma membrane.
3. The resulting influx of Na+ produces a
localized depolarization of the membrane,
leading to opening of voltage-gated Na+
channels and generation of an action
potential.
4. Spreading depolarization triggers opening
of voltage-gated Ca2+-release channels
and release of Ca2+ from the sarcoplasmic
reticulum into the cytosol.
The rise in cytosolic Ca2+ causes muscle
contraction.
Lodish, Molecular Cell Biology
Cardiac muscarinic
acetylcholine receptors
activate a G protein and
open K+ channels
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Binding of acetylcholine by muscarinic acetylcholine receptors triggers
activation of a transducing G protein by catalyzing exchange of GDP for GTP on
the α subunit. The released Gβγ subunit then binds to and opens a K+ channel.
The increase in K+ permeability hyperpolarizes the membrane, which reduces the
frequency of heart muscle contraction. The activation is terminated when the GTP
bound to Gα is hydrolyzed to GDP and Gα·GDP recombines with Gβγ.
Lodish, Molecular Cell Biology
Acetylcholine-induced opening of K+ channels
Heart muscle plasma membrane
Action of a serotonin modulatory synapse
Serotonin receptor modulates K+ channel function via activation
of adenylate cyclase
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Serotonin secreted by an
activated facilitator neuron binds
to the G protein-coupled
serotonin receptors, leading to
activation of adenylate cyclase
and an increase in cAMP in the
sensory neuron.
Phosphorylation of the voltagegated K+ channel protein or a
channel-binding protein prevents
the K+ channels from opening,
leading to prolonged
depolarization.
This leads to enhanced secretion
of the neurotransmitter glutamate,
which stimulates the motor
neuron.
Lodish, Molecular Cell Biology
Sensory transduction systems
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The nervous system receives input from a large number of sensory receptors;
Photoreceptors in the eye, taste receptors on the tounge, odorant receptors in
the nose, touch receptors on the skin monitor various aspects of the outside
environment.
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Each receptor must convert, or transduce, its sensory input into an electric
signal.
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How does a sensory cell, usually a specialized epithelial cell, transduce its
input into an electric signal?
Often, the connection between a sensory receptor protein and the ion channel
is indirect; the sensory receptor activates a G protein that, in turn, directly or
indirectly induces the opening or closing of ion channels.
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Ex: The light receptors in the rod cells in mammalian retina
The olfactory receptors in the nose
Visual system
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The human retina contains 2 types of photoreceptors, rods (stimulated by
weak light) and cones (involved in color vision).
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Rod cells form synapses with neurons that, in turn, synapse with others
that transmit impulses to the brain.
Lodish, Molecular Cell Biology
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In the dark, the membrane potential of a rod cell is -30mV; rod cells
constantly secrete neurotransmitters.
A brief pulse of light causes a transient hyperpolarization of the rod cell
membrane and decreases neurotransmitter release.
Light triggered closing of sodium channels hyperpolarizes rod cells.
How is the signal transduced into the closing of sodium channels?
Rhodopsin
Lodish, Molecular Cell Biology
Rhodopsin, the photoreceptor in rod cells, is formed from
11-cis-retinal and opsin, a transmembrane protein
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Absorption of light causes rapid photoisomerization of the cis-retinal to the
trans isomer, forming the unstable intermediate meta-rhodopsin II, or
activated opsin.
The latter dissociates spontaneously to give opsin and all-trans-retinal, which
is converted back to the cis isomer by enzymes in rod cells.
cGMP is a key transducing molecule
Lodish, Molecular Cell Biology
Coupling of light absorption by rhodopsin to activation of cGMP
phosphodiesterase in rod cells
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In dark-adapted rod cells, a high level of cGMP acts to keep nucleotide-gated
nonselective cation channels open and the membrane depolarized compared with
the resting potential of other cell types.
Light absorption leads to activation of opsin (O*) and conversion of inactive
transducin (Gt) with bound GDP to the active state with bound GTP accompanied
by dissociation of Gβγ (not shown).
cGMP is a key transducing molecule
Lodish, Molecular Cell Biology
Coupling of light absorption by rhodopsin to activation of cGMP
phosphodiesterase in rod cells
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The free Gtα·GTP thus generated then activates cGMP phosphodiesterase (PDE)
by binding to and dissociating its two inhibitory  subunits; as a result, the released
catalytic α and β subunits of activated PDE (PDE*) can convert cGMP to GMP.
The resultant decrease in cGMP causes dissociation of cGMP from the nucleotidegated channels in the plasma membrane; the channels then close and the
membrane becomes transiently hyperpolarized.
Role of opsin phosphorylation in adaptation of rod cells
to changes in ambient light levels
Lodish, Molecular Cell Biology
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Light-activated opsin (opsin*), but not dark-adapted rhodopsin, is a substrate
for rhodopsin kinase.
The extent of opsin* phosphorylation is directly proportional to the ambient
light level, and the ability of an opsin* molecule to catalyze activation of
transducin is inversely proportional to the number of sites phosphorylated.
Thus the higher the ambient light level, the larger the increase in light level
needed to activate the same number of transducin molecules. At very high
light levels, arrestin binds to the completely phosphorylated opsin, forming a
complex that cannot activate transducin at all.
Lodish, Molecular Cell Biology
The absorption spectra of the three human opsins
responsible for color vision
Individual cone cells express one of the three cone opsins.
The spectra were determined by measuring in a microspectrophotometer the light
absorbed by individual cone cells obtained from cadavers.
Memory and neurotransmitters
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Learning is a process by which we modify our behavior as a
result of experience or as a result of aquisition of
information about the environment.
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Memory is the process by which this information is stored
and retrieved.
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Depending upon how long it persits, memory can be short
term (minutes to hours) and long term (days to years).
Memory and neurotransmitters
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Memory results from changes in the structure or function of
particular synapses.
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Long term memory involves the formation or elimination
of specific synapses in the brain and the synthesis of new
mRNAs and proteins.
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In contrast, short term memory is very rapid and involves
changes in the release and function of neurotransmitters
at particular synapses
Glutamate receptors in long-term potentiation
• The hyppocampus is associated with many types of shortterm memory.
• Long-term potentiation is a type of short-term memory.
• In long-term potentiation, continual stimulation of a
postsynaptic neuron makes it more responsive to
subsequent stimulation by presynaptic neurons.
• Two types of glutamate receptors in the postsynaptic
neuron combine to generate long-term potentiation, NMDA
and non-NMDA receptors.
NMDA, N-methyl-D aspartate
(nonnatural amino acid)
Two types of glutamate receptors in long-term potentiation
• The ion channel in the NMDA
receptor (green) is normally
blocked by Mg2+, and thus the
glutamate released by firing of
presynaptic neurons leads, at first,
to opening of only the non-NMDA
glutamate receptors (pink).
• The resultant influx of Na+ partially
depolarizes the membrane.
Lodish, Molecular Cell Biology
Two types of glutamate receptors in long-term potentiation
• If many presynaptic neurons
(here 2 are shown) fire in
synchrony, the membrane of
the postsynaptic cell becomes
sufficiently depolarized so that
the Mg2+ blocking the NMDA
receptors are removed; then
both the NMDA and the nonNMDA glutamate receptors open
in response to glutamate.
• Ca2+ as well as Na+ enter
through the open NMDA
receptors, causing an enhanced
response in the postsynaptic
cells.
• The synapse “learns” to have an
enhanced response to the
electric signals in the presynaptic
cells.
Lodish, Molecular Cell Biology
Bom estudo!!!
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