Transcript LTP

Synaptic Plasticity I:
Long-Term Potentiation
1.
2.
3.
4.
5.
6.
7.
8.
9.
The mystery of memory
The Hebbian synapse & memory
Long-term potentiation
Synapse specificity & associativity of LTP
Phases of LTP
Pre- versus post-synaptic changes in LTP
Changes in the shape dendritic spines in LTP
The connection between LTP & memory
Can memory be enhanced?
The Mystery of Memory
- Long-term memory is an exotic
biological phenomenon…it is
encoded immediately, yet can
last for 100 years or more.
- Presumably, there are physical
changes to neurons that underlie
memory formation…however,
the proteins and lipids that
comprise neurons are turned over
every few hours or days.
- How can stable memories be stored
by such unstable components?
Synapses & Memory Capacity
- The legendary Spanish neuroanatomist
Santiago Ramon y Cajal, who showed
that the brain is not a syncytium
but is in fact made up of discrete cells,
postulated that synaptic connections
between neurons were probably
the sites of memory storage.
- There are 10 billion neurons in the
human brain, with an average of 10,000
synaptic connections each…thus, if whole neurons must change
properties to encode memories, the information storage capacity
of the human brain is roughly 10 billion bits, whereas if memories
are encoded by changes to synapses, then the storage capacity
increases 4 orders of magnitude to 100 trillion bits.
Hebbian Synapses
- In 1949, Canadian psychologist Donald Hebb
published “The Organization of Behavior”,
an influential book in which he described
his vision for how synaptic plasticity
might give rise to memory formation.
- Hebb’s Postulate: “When an axon of cell A
is near enough to excite cell B or repeatedly
or consistently takes part in firing it,
some growth or metabolic change takes place in one or both cells
such that A’s efficiency, as one of the cells firing B, is increased.”
weak synapse
A
B
A + B are active
at the same time
strong synapse
A
B
To put it another way: “Cells that fire together, wire together.”
Long-Term Potentiation
- There are many different forms of synaptic plasticity, which
probably underlie different types of memory, but the most
intensively-studied example of synaptic plasticity over
the past 40 years has been long-term potentiation (LTP),
which can be observed in certain regions of the hippocampus
and cerebral cortex.
- A lot is now known about LTP,
so to help make sense of this massive
amount of information, we will discuss
work on LTP chronologically, using
the history of classic experiments
performed in this field as an organizing
principle to understand what is
presently known versus unknown.
The Discovery of LTP
- Terje Lomo & Tim Bliss first described the LTP phenomenon
in a 1973 paper that described experiments where they
recorded from the dentate gyrus (a part of the hippocampus)
in anethestized rabbits.
- Lomo & Bliss found that when they
delivered a strong tetanic stimulation
(for 10 seconds) to the perforant path
(the connection between the entorhinal
cortex & dentate gyrus), the responses
to test pulses of the perforant path
were much stronger following the
tetanic stimulation…this effect lasted
for at least 3 hours.
(from Bliss & Lomo,
J. Physiol., 1973)
Hippocampal Slices
- Following the report from
Bliss and Lomo, there were
only a handful of papers
about LTP over the next
few years, mainly because
it was very difficult to
record from intact animals.
- By the mid-1970’s, there
were great improvements
in understanding how to
make brain slices and keep
them alive for hours in vitro,
which allowed work on LTP to proceed
on hippocampal slice preparations.
Synapse Specificity of LTP
- Lynch & colleagues demonstrated
that LTP in region CA1 of the
hippocampus is specific to only
the set of synapses receiving
the tetanic stimulation.
- This demonstrated that
LTP does in fact represent
“synaptic plasticity” rather
than simply reflecting
a change in the global
excitability of the
neurons under study.
(based on Dunwiddie & Lynch,
J. Physiol., 1978)
Associativity of LTP
- The synapse-specificity of LTP
has now been studied in many labs,
and it has been widely found that
pairing a weak stimulation of one set
of inputs (insufficient on its own to
create LTP) with a concurrent strong
stimulation of another set of inputs
can result in LTP of both pathways.
- This “associative” property of LTP
has received much attention because it seems
to many neuroscientists to fulfill the prediction of
Hebb’s Postulate (“cells that fire together, wire together”).
Phases of LTP
- LTP can be divided into three discrete phases, which have distinct
mechanistic underpinnings: induction, expression & stabilization:
Expression Stabilization
Induction (“early LTP”) (“late LTP”)
Stable for many
hours, days, weeks
Induction of LTP Requires Postsynaptic Ca2+
- Lynch & colleagues showed
that injection of calcium
chelators such as EGTA
into the postsynaptic neuron
completely blocked the
induction of LTP.
EGTA
(based on Lynch et al.,
Nature, 1983)
- This finding was soon replicated in many labs and considered very
surprising, since most concurrent work on other types of synaptic
plasticity (i.e., Kandel’s work on Aplysia, to be discussed in the
next lecture) was focused on presynaptic changes…indeed, some
labs of this era had already claimed to have found increased
glutamate release with LTP, although these findings could not
be consistently replicated in other labs.
Induction of LTP Requires NMDA Receptors
- Collingridge & colleagues
showed that treatment of
hippocampal slices with
NMDA receptor antagonists
(such as AP5) blocked the
induction of LTP.
AP5
(based on Collingridge et al.,
J. Physiol., 1983)
- It was soon discovered that NMDA receptors are permeable to
calcium and are also voltage-dependent, and thus the mechanisms
underlying LTP induction became quite clear: postsynaptic
depolarization facilitated glutamate activation of NMDA
receptors, leading to postsynaptic calcium influx and the
generation of LTP.
Expression of LTP Requires AMPA Receptors
- Lynch & colleagues
showed that treatment of
hippocampal slices with
AMPA receptor antagonists
(such as CNQX) masked the
expression of LTP.
- Since NMDA-R responses are
the same before & after LTP,
this is a further problem for the
idea that LTP results from
increased release of glutamate…
recent work has shown an
increased number of AMPA
receptors at potentiated synapses.
CNQX
(based on Muller, Joly & Lynch,
Science, 1988)
Control
NMDA receptors
AMPA receptors
Potentiated
LTP Requires Calcium-Activated Enzymes
- Calcium influx through NMDA receptors activates a variety of
different enzymes that are found in the post-synaptic density…
the three most intensively-studied enzymes with regard to LTP
are the kinases CaMKII & PKC and the protease calpain.
- CaMKII and PKC directly
phosphorylate AMPA-R’s
to alter their activity and
trafficking, and also
phosphorylate NMDA-R’s
and other PSD targets…
calpains cleave away at
the cytoskeleton & other
PSD components, resulting
in changes to spine shape.
NMDA
AMPA
Ca2+
CaMKII
PKC
Calpain
Glutamate
Dendritic
spine
The Mystery of Memory
- Long-term memory is a very exotic
biological phenomenon…it is
encoded immediately, yet can
last for 100 years or more.
- Presumably, there are physical
changes to neurons that underlie
memory formation…however,
the proteins and lipids that
comprise neurons are turned over
every few hours or days.
- How can stable memories be stored
by such unstable components?
Changes in Spine Shape Accompany LTP
- Stabilization of LTP is accompanied by changes in the shape
and size of dendritic spines…this was first shown by Lynch
and colleagues in 1980,
and subsequently studied
in exquisite detail by
Kristen Harris and
colleagues in analyses
of 3-dimensional serial
reconstructions of spines.
- Changes to the shape of dendritic spines can radically alter the
calcium dynamics within the spine, and can also help to make
room for more AMPA receptors…this can strengthen the
synapse in a stable manner, since the change in shape can be
preserved even as the individual components are turned over.
Changes in Gene Expression with LTP?
- Kandel: “Behavior is the result
of the interaction between
genes and the environment.”
(Ch. 62 of Principles)
- Changes in gene expression
have been found following
high-frequency stimulation
…however, it is uncertain
if these changes play a role
in the stabilization of LTP,
as it is unclear how changes
in transcription in the nucleus
could give rise to synapsespecific functional changes.
CREB
“synaptic tagging?”
LTP & Memory
- Does the formation of long-term memories depend on LTP in vivo?
…there are several lines of evidence that indicate a connection:
1. LTP fits Hebb’s Postulate and is synapse-specific, which
seems a pre-requisite for a biological process that would
have a high capacity for information storage.
2. LTP is observed in regions of the brain known to be involved
in the formation of long-term memories (hippocampus, cortex)
and is either not observed at all, or has very different properties,
in other regions of the brain (such as brainstem).
3. LTP is induced by brief stimulation and can be very stable,
which are properties shared with the formation of long-term
memories.
LTP & Memory (con’t)
4. LTP is optimally induced by brain rhythms associated with learning.
- several-second bursts of 100 Hz activity
do not occur naturally, except during
seizures…Larson & Lynch (Science,
1986) studied the ability of more
physiological rhythms to induce LTP,
and found that separation of brief
bursts by 200 msec (a 5 Hz rhythm)
is optimal for LTP induction…
interestingly, 5 Hz corresponds
to the EEG “theta rhythm” observed
in the hippocampus and cortex
during learning (and REM sleep).
Beta
(15-30 Hz)
(arousal, alertness, anxiety)
Alpha
(8-14 Hz)
(relaxation, meditation, pre-sleep)
Theta
(5-6 Hz)
(learning, novelty, REM sleep)
Delta
(1-4 Hz)
(deep sleep, unconsciousness)
LTP & Memory (con’t)
5. Drugs or genetic manipulations that block LTP also impair memory.
Control
rat
Morris water maze
(Morris et al., Nature, 1986)
Control rats rapidly
learn how find the
hidden platform.
AP5treated
rat
(and later,
various
KO mice)
Treatment of rats with the
NMDAR antagonist AP5
blocks hippocampal LTP
and also blocks learning
of the platform location.
LTP & Memory (con’t)
6. Drugs or genetic manipulations that enhance LTP also enhance
certain types of memory.
- Joe Tsien & colleagues (Nature, 1999)
created a line of transgenic mice that
over-express the NMDA receptor
subunit NR2B in their forebrains…
these mice exhibit enhanced LTP
in various hippocampal and cortical
regions, and also exhibit significantly
enhanced performance on a number of different memory tasks,
including the Morris water maze…thus, the official name for
this line of smart mice is “Doogie”, derived from the brilliant
lead character in the TV show “Doogie Howser, M.D.”
Qui ckTime™ and a TIFF (Uncompressed) decompr essor ar e needed to see this picture.
Can Memory Be Enhanced By Drugs?
- Cognition-enhancing drugs are common
in our culture…caffeine, for example,
blocks inhibitory adenosine signaling
in the brain, and hundreds of papers have
shown that caffeine enhances LTP and
improves learning on certain tasks…
furthermore, drugs like methylphenidate
(Ritalin) and amphetamine (Adderall)
enhance the release of dopamine,
norepinephrine & serotonin, leading to
increased NMDA & AMPA receptor
function, enhanced LTP, and improved
performance of certain memory tasks…
problems: cardiovascular side effects,
potential for addiction.
The Future of Memory-Enhancing Drugs
- Many companies, including some founded by top LTP researchers,
are pursuing the development of memory-enhancing therapeutics:
Cortex Pharmaceuticals
Memory Pharmaceuticals
Irvine, CA
New York
Founder:
Gary
Lynch
Founder:
Eric
Kandel
Concept: Use AMPA receptor positive
allosteric modulators (“AMPAkines”)
to specifically enhance LTP & memory.
Currently in Phase II clinical trials.
Concept: Use phosphodiesterase inhibitors
to slow the breakdown of cAMP, enhance
CREB activation, and improve memory.
Currently in Phase II clinical trials.
PDE inhibitors
PDE
PKA
cAMP
AMPAkines