My name is Andre Fenton.
I'm a professor of neural science at New York University.
I also co-direct the Neural Systems and Behavior Course
at Marine Biology Laboratories in Woods Hole.
And I founded a company called Bio-Signal Group,
and we make neurological devices that help
patients and physicians manage neurological emergencies.
This is Part 2 of a series of three lectures
that I will talk about how we think about memory
and understand the neurobiology
that underlies this feature of the human mind.
I want to remind you that the work I'm going to talk about,
which focuses on research that has been done in my laboratory,
has been done by a very talented team of people,
not only people in my laboratory,
but people in other laboratories,
and this work extends across decades.
In Part 1 of this series,
I talked about how brains encode information
in the activity of the individual neuron.
And there are approximately 100 billion or so neurons
in the human brain
and each of those neurons is able to communicate with approximately
10,000 or so other neurons.
So, that's trillions of communications
and potential for trillions of interactions
within the human brain,
within the neural network there.
And so that's not something that we're used to thinking about.
What I want to offer you is a conceptual framework
for thinking about how information might be communicated
and flow through the networks
that make up your brain, the networks of neurons.
And the internet provides
a useful conceptual framework.
In fact, this is an image of the activity in the internet.
And what you can see here...
when you think about the internet,
it's just a bunch of computers
that operate under a set of protocols or rules,
and they exchange information.
And in this image, you can see that some of the nodes in the internet,
some of the computers or the addresses in the internet
are very bright and have a dense number of connections,
and that means that many, many computers
are trying to communicate or effectively communicating
with those bright nodes.
We think of them as hubs.
These are the Googles and the Yahoos of the world,
and the other companies like Akamai
that we usually don't hear about
that process a lot of the information traffic in the internet.
So, if you think about that,
what's interesting about the internet is it's self-organized.
It's just a bunch of computers following some rules,
and based on usage some of those computers or addresses
become more important than others,
and they compete with each other for communication, if you will.
And as more computers get used,
more resources go to those computers
or to those addresses,
so that they can store more information
and process more information.
And they can outcompete
other computer networks or other hubs.
And so the computers and their interactions
are dynamic on the internet.
It's never static in time and it evolves.
And in many ways, this is a useful way
to think about how the activity in your brain
evolves and is organized.
With use, certain neurons
change and collect more resources,
outcompete other neurons,
so that the functional architecture of a brain
changes as that brain is used.
So, if you think about that,
a natural question emerges:
What guides and controls the information in the brain?
Well, I ask you, what guides and controls the flow of traffic in the internet?
You might be more familiar with traffic on the street
and the analogy applies.
We can ask the same question:
What guides and controls the flow of traffic on the streets?
And this is analogous to the information communication
within networks of neurons.
So, as traffic flows,
you can imagine these as the action potentials or the discharge of neurons,
single neurons,
coursing through a network, not of streets,
but a network of neurons in the brain.
And what should be apparent is that
this flow is not random;
it's not random here in the traffic either.
It's controlled in large part by the junctions,
by how the different intersections
are coordinated and their coordination...
we don't see it here, but their coordination is accomplished,
typically, by traffic lights.
And if you think about those traffic lights,
what they're really doing is making it easier or harder
for traffic to flow in a particular direction.
And we will talk about the junctions between neurons, called synapses,
which can be modified to make it
easier or harder for one neuron
to communicate with another,
and to make pathways that facilitate and impede
the flow of neural information through the network or neurons.
These synapses, cartooned here,
are the junctions of one neuron
communicating with another neuron.
Now, remember, a single neuron, on average, in the brain,
would have 10,000 or so of these junctions.
One neuron would connect to approximately
10,000 or so other neurons at each of these junctions.
What's important about the junctions is that they
allow an exchange of chemicals, shown here as the little green balls...
these are neurotransmitters that can be released
from what we call the presynaptic neuron
onto the postsynaptic neuron.
The postsynaptic neuron has proteins,
what we would call receptors, that, upon receiving the neurotransmitter,
change the activity in the postsynaptic cell
and can make that cell more or less likely to move,
to use the analogy of traffic,
and specifically more or less likely to generate
an electrical discharge.
So, what's important to recognize is that
these different interactions can be regulated.
What I'm going to now describe for you
is some experiments that we've done in our laboratory
that show that the experience,
in particular cognitive experience,
the manipulation of knowledge,
leaves a substantial, detectable imprint, if you will,
or set of changes in the neural system
that we'll study,
within the synaptic organization of the brain,
and we'll focus on the hippocampus,
which is a hub for storing memories.
And this is the work that's been done in my laboratory,
as well as my collaborator Todd Sacktor,
and in particular really, truly heroic work
by Dr. Panos Tsokas.
So, the hippocampus...
this might be a part of the brain that you've heard about
because it's crucial... in the analogy,
it's a hub for not only forming and storing,
but processing memory.
And so here's an animation
that will help you orient as to where the hippocampus is.
This is a mouse, you can imagine slicing through the mouse's brain,
and this structure in the middle that's delineated by the dark cells there,
on one side and the other,
that's the hippocampus.  Okay?
And that's the part of the brain that we're going to study
and we're going to study and talk about the neurons and the synapses
in this part, this region of the brain.
We're going to need the animals, or our subjects,
to actually form memories and communicate that they form those memories to us.
And so this is a task that we're going to use
called the active place avoidance task.
You can see, here, a mouse on a grid floor
and the mouse, you'll notice,
is staying down near, you know, 6 o'clock or so of this,
if it were a clock face,
and it's staying away from the area that's been highlighted red.
Now, the area that's been highlighted red has been highlighted red
for you and me.
The mouse doesn't see red.
What the mouse can do is look around
and see where it is in the room
and recognize that if it goes into the area that's been indicated
in red for you and me, the floor will become electrified,
and that's why the mouse
is spending all of its time away from that area.
So, it's a rather simple task for the animal
to learn to stay out of that red zone
and it does so by being active and walking away,
and that's why we call it the active place avoidance task.
And while the mouse can't speak to us and say,
oh yeah, I remember I was shocked over there,
what it can show us through its behavior is that it's,
for some reason, staying away from that area,
and the natural inference would be because it has remembered
that it was shocked in that area.
So, we can use this task to assess and to quantify and measure
the extent to which the mouse remembers where it was shocked.
And while doing this task,
what we can do is record neuronal activity,
shown here in cartoon form...
imagine 15 cells
and each tick corresponding to the discharge,
the electrical discharge, of a single neuron in the hippocampus.
What's shown down below in green
is a slice through the hippocampus of a mouse
after it has learned this task,
and this mouse has been genetically engineered such that the neurons
that were active in a strong manner
while the animal was learning this task are now green.
And so we can recognize them,
and there are a couple of things to notice.
Not all cells are green; only about 30 or maybe even fewer
percent of the cells are green.
And the allocation of the green cells
is not homogeneous in the structure.
We won't talk about the anatomical confines,
but you can notice that in some places
the green cells are more dense than others,
so different regions of the hippocampus,
as a neural circuit,
activate and allocate different numbers of cells
to form the memories that this mouse is forming
and going to use in order to
organize where it goes in space,
and presumably remember where the shock was.
But we might now ask... that was the easy question to answer, okay?
We might now ask,
how do the neurons remember whether
or not they should be active?
And in order to study that,
what we understand is that the synapses,
the connection between the neurons,
these are the things that actually differ
and they can change with experience.  That's the dominant hypothesis for answering the question,
how do the neurons remember whether or not to be active
for one memory or another memory?
And now let's talk a little bit about the neurobiology
and some of the findings that support that point of view.
The story begins with an important paper
by my colleague, Todd Sacktor,
where he identified a particular molecule
called protein kinase Mζ, PKMζ, and what he was able to show is that
it's necessary and sufficient for something called LTP maintenance.
And what's cartooned here is
if you were able to make a measurement of the electrical activity
of a single one of these neurons
in the hippocampus,
what you could see and measure is,
how easy is it for that cell to generate an electrical current?
And what we see here, shown on the x axis is time
and on the y axis is shown the amount of current
that could be measured from across a pair of these cells,
and what you can see is that, in one of them,
starting at time 0, the current is going up and then it stabilizes
-- it takes about 10 minutes to go up and then it stabilizes.
And the trace that shows the increasing current,
what the folks in Todd's lab did
was they delivered into the cell protein kinase Mζ,
so it was sufficient to increase
the electrical communication between neurons.
Another experiments, I'll show you one now,
that I did with Todd,
shows that it's necessary to maintain this enhanced electrical activity.
So, here's an experiment, a truly heroic experiment
done by Eva Pastalkova when she was in my laboratory...
what we did was... what Eva did
was she gave a high-frequency stimulation at time 0
and that caused neurons
-- many, many neurons, thousands of them --
to discharge together, and that active discharge together
caused them to subsequently
increase their response to a simple stimulation.
So, they became elevated, if you will, or enhanced,
and what we say is that their response was potentiated
and it was potentiated for a long period of time.
In fact, in Eva's experiment,
the potentiation lasted at least 24 hours
and would have probably gone on,
except that what Eva did was she injected an inhibitor, okay,
of this molecule protein kinase Mζ.
And you can see that that's...
the ZIP stands for zeta-inhibitory peptide,
and what you can see is that when ZIP was injected,
that the potentiated response
went back down to the baseline.
And so that's quite remarkable.
There are no other known molecules
that one can say are
both sufficient as well as necessary
for maintaining the activity-dependent enhancement
of communication between neurons.
And so we decided to study whether or not
PKMζ was also important, whether or not it was crucial,
for maintaining memories.
The first thing to know is if you give an animal,
like a mouse or a rat,
the training in the place-avoidance task that I showed you,
you can actually find that if you look one day later or one month later
by grinding up the brain and measuring the protein level of PKMζ,
what's shown here in the Western blots,
or some control protein, actin,
what you can see is after one day or even after one month after the training,
the levels of PKMζ have elevated.
And that's really quite remarkable
because, again, I'm unaware of any other molecule that,
based simply on experience,
will have an elevated expression like that,
certainly no other memory molecule,
again, I'm aware of.
And so that's quite enticing.
Experience itself is sufficient to increase the elevated...
the activity.
And it does so in a persistent way, lasting at least one month.
And so where does PKMζ reside?
PKMζ resides on the postsynaptic side of the synapse,
and it's a molecule that
engages and modifies other molecules
that change the expression of the electrical response
when one cell is active and the presynaptic cell is active
and releases its neurotransmitter to the postsynaptic cell.
And if we were to look and to make slices,
instead of grinding up the brain,
if we were to make slices
and stain the brain so that we can find out,
where is the PKMζ?,
we can see that, after one month
in an animal that received this training,
so one month later we can sacrifice the animal
and look for PKMζ,
and we see the hotter colors, if you will,
on the right side in the trained animal,
compared to the control animal.
And the control animal had exactly the same experience,
the shock was simply never turned on.
Okay, so PKMζ seems to form
some kind of a long-lasting trace, if you will,
of the experience of the animal,
and we would imagine and work under the hypothesis
that that's a trace of the experience,
the kind of thing that we call a long-term memory,
and how it's maintained in the brain.
So, to actually test that idea,
it's not sufficient simply to look and see that the molecule is there,
we actually want to make a manipulation.
And so this is a video I'll show you of an experiment that we did,
and what we did in this case was we trained the..
. in this experiment it was a rat...
we trained rats on this rotating area to walk around,
first of all, right?
In this case, the shock isn't turned on,
the rat is walking around,
the rotation is about 5x faster,
otherwise it would be very boring to watch.
You'll notice that the rat goes everywhere and we can track where the animal goes,
and the computer can decide, based on where the rat is,
whether or not to electrify the floor.
And so we have training trials where the floor is electrified
when the animal is detected in a particular part of the space,
a particular part of the room.
And it can know where it is by looking at the cues
that are on the curtain and such
that aren't necessarily visible to you.
The red mark on the arena is there
just so that we can see what the computer has decided to shock.
So, now we can do the experiment.
We can do a control injection just of saline
into the hippocampus a day later,
and put the animal back and watch...
does it express memory
by staying away from that place that it's shocked.
And if it does, as in this case, you'll see that it runs away...
it just enters and runs away.
We didn't turn the shock on --
presumably it ran away because it remembered
the location of shock.
But what if we inject an inhibitor of PKMζ?
What we see instead is the animal doesn't avoid any longer...
it seems to walk around as if, in fact,
it was naive for being in this arena
and, in fact, as if it was naive, certainly,
that the shock was ever delivered
in the upper part of the arena.
In fact, if we compare that same rate
its first time in the area
with the time after the ZIP injection,
it's very difficult to tell the difference.
And this makes it look like it's possible, for the first time,
to erase a memory,
because what we did in subsequent experiments is show that after this erasure,
if you will,
the animal had no difficulty to form a new memory
-- it could form short-term memories and it could even form long-term memories
after the ZIP had been metabolized a couple of hours later --
so we hadn't damaged the brain in any way.
And because it looked like it was now possible to edit,
if you will, to erase memories whether or not specific,
this got the attention of quite a number of people
in the public media as well as our friends in the scientific world.
In fact, that inspired some other researchers
to test that idea, and they had a very interesting and reasonable approach.
They said, well, if PKMζ is so important,
if it's crucial for memory,
then it should be possible to genetically delete PKMζ in a mouse
and, if we did that, then the mouse should not show normal learning,
it should not be able to acquire memories,
it should not have the ability to remember,
and it should not have the ability to make LTP
-- this long-term potentiation of the synaptic transmission
between neurons.
But, as you can see from the titles of the papers,
right, deleting or knocking out the gene for PKMζ...
and the mice look pretty normal
and they remembered rather well.
So, that led these authors to conclude that
PKMζ was not crucial for long-term memory
or LTP and that, in fact,
our inhibitor must have been acting on something else.
So, this is the nature of science
and what is interesting about science
is that it breeds controversy,
and with controversy it breeds further investigation.
Very recently, we explained how it is possible that, by deleting the gene,
you could still observe that there would be
normal learning and memory, when, in fact, that gene
is indeed crucial for learning and memory.
And I'll explain that now in the next few slides.
The answer was quite simple.
In Part 1, I talked about how we need to think about
complex systems like the brain
and complex processes like memory
at multiple levels of biological organization.
And the answer was simple:
there was compensation.
When the gene for PKMζ was deleted,
an alternative gene, another related,
in fact, very closely related gene,
itself became expressed.
And, otherwise, a normal or in a wild type animal
its expression tends to be low or suppressed,
presumably by the presence and actions of PKMζ in memory.
And so what you can see here is that this other protein,
called PKCι/λ,
another part of the same family of proteins called protein kinase Cs,
and if you look on the left you can see that,
in a knockout mouse,
when you take the halfbrain,
the hemibrain of that mouse
and you measure how much PKCι/λ is there,
you can see that it's about the same.
But if you look in the specific part of the brain,
the hippocampus, that's crucial for memories,
there's an overexpression,
by almost 60%,
of that particular molecule.
And this just spontaneously.
And, in fact, if you look at the active form,
which is a phosphorylated form of the molecule,
you see that the increase is over 400%,
so there was a massive increase of this other molecule,
PKCι/λ.
But that doesn't prove there's compensation;
that just proves there's a suspect.
So we have to do more work.
What we... the logic is,
if PKMζ is crucial for memory
in wild type animals but is compensated
and therefore the mechanism for memory
and the mechanism for LTP is different in
the PKMζ knockout mice,
then we should be able to selectively inhibit
the new synthesis of PKMζ to prevent LTP
and, I'll show you subsequently, memory,
but only in the wild type mice.
So, you can look here at this trace...
along the x axis is time
and on the y axis is a measure of
the synaptic strength or the synaptic response
to stimulation of presynaptic cells
and how the postsynaptic cells are responding.
And at time 0 a strong stimulation
is given to induce this long-term potentiation.
And if you look at the wild type mice, in green,
that receive a scrambled version of this inhibiting molecule
-- we used an antisense oligodeoxynucleotide,
which is something that will bind to the DNA
that's designed to interfere with the manufacture or expression
of a new PKMζ molecule --
when we inject that, what we can see...
or, when we put that in the bath...
we're measuring LTP...
what we can see is, in the green trace,
when that oligodeoxynucleotide is a scrambled, ineffective form,
we see normal LTP,
but in the red you can see that the LTP is able to be induced
but it doesn't sustain.  Okay?
And it seems to have no effect in the null.
So, that's the first, if you will,
piece of evidence that the mechanism for LTP is
PKMζ-dependent in wild type mice,
but different in the knockout mice.
The second is the complementary experiment.
You can now try to inhibit the PKCι/λ
and there was a specific inhibitor for that called ICAP,
and you can see, there, if you give ICAP,
in the upper trace, to establish LTP,
when you add the ICAP the LTP goes back toward baseline,
it depotentiates,
but only in the PKMζ-null mouse
that's not making the PKMζ protein.
Whereas in the wild type mouse,
which can make PKCι/λ as well as PKMζ,
this inhibitor has no effect on the maintenance --
that's the black trace.
If you look at the open circles on the bottom,
you can see that second arrow at time 270 minutes,
that's a second simulation that we can give,
and what you can see is that it becomes
difficult to establish the LTP.
So, PKCι/λ
has been known to be important
for the initiation of LTP and memories,
but it seems to not be crucial for the maintenance of it,
at least in wild type animals.
But it seems to act as a backup mechanism
when you get rid of PKMζ
through the genetic deletion.
What does this do for long-term memory?
So, what we can do is we can repeat these experiments.
We can use the antisense, okay,
and we can deliver it before every memory trial.
And what's shown here in these circles are the traces of
where the mouse has gone after PKMζ inhibitor,
the antisense, has been injected,
and we can see in the pretraining the animals walk around everywhere,
in the training sessions they walk around
and they can learn and acquire this information
during the 10 minutes.
Remember, PKMζ is crucial for the maintenance,
the maintaining of the memory, not its acquisition.
And we can test retention, in this case, one day later,
and you can see in the wild type traces, the first two,
the scrambled or ineffective peptide
doesn't seem to impair the animals' ability to remember.
In the wild type, the antisense
seems to create that kind of impairment.
Now, the animal enters the shock zone very quickly,
the trace is very short before it enters the shock zone.
The scrambled peptide or the antisense
seems to not have an effect
-- you see a large trace, there.
The animal spends a lot of time before it first enters the shock zone
in the PKMζ knockout or null mouse.
And this can be quantified in these summary data.
You can see that after delivering the scrambled peptide
to the wild type mice, in the green bar,
that the retention -- the time it takes to first enter the shock zone --
is long.
However, not after delivering the antisense
to prevent the synthesis of PKMζ.
And in the PKMζ-null mice,
these things don't have an effect.
Those gray bars are quite tall,
indicating good memory, as I showed you in the pictures in the previous panel.
What about the PKCι/λ inhibitor, ICAP?
So, now what we can do is train the animals
to establish a long-lasting memory,
both the wild type and the PKMζ-null mice...
if you look at training trial 3,
they both established a memory,
they're staying away from the shock zone,
and now, one day later, before putting them back in the arena,
we can deliver the PKCι/λ inhibitor and it seems to have
no effect in the wild type mouse.
It continues to stay away from the shock zone.
In fact, in this picture, it doesn't enter at all.
Whereas in the PKMζ-knockout mouse, or the null,
it goes everywhere and is acting as if it is naive,
and that's also shown here in the summary data,
the dark bar in the wild type mouse
showing the memory, or the measure of how long
it took to enter the shock zone, is very high in the wild type mouse,
but in the null it's very short because it doesn't demonstrate memory.
Again, evidence that the mechanisms of maintaining memory
are distinct in the wild type and the PKMζ-null mouse.
In fact, what we can look at is the detailed behavior
in the PKMζ-null mouse and ask,
did they actually learn in a normal way?
If you look at the details of, where did these animals actually spend their time,
you can see in the wild type mouse early in training...
okay, if we give them a training protocol that's harder to acquire,
early in training they spend their time...
see the red activity
-- that's the density of where the animal is spending it's time...
is across from the shock zone, around 6 o'clock.
And it happens to be there right on day 1
and, in fact, it intensities with training.
But look in the PKMζ-null mouse.  Right?
That red area is closer, if you will,
to about 9 o'clock, and it only very gradually...
it takes 7-10 trials before it actually moves...
the animals decide to spend their time
in the safer part of the arena.
And if you look and quantify this,
you can see it takes the animals...
you can measure the ratio of time spent opposite the shock zone,
which is the safer part of the shock zone versus the...
the safer part of the arena versus the part adjacent to the shock zone,
and what you can see here is that when that ratio is declining
gradually in the wild type mouse,
and it's established already within the first day.
However, it takes some time for the PKMζ-null mice,
about 7 or so trials,
which means 3 days of training before that drops down.
And you can also see,
by measuring the strength of the memory
as the time it takes for the animals to first enter the shock zone,
it takes much longer for the open circles, the PKMζ-null mice...
they take much longer to get up to their asymptote,
to the level of performance,
and that level or performance is about half of that
of the wild type mouse.
So their memory seems to be abnormal
and their way of acquiring information also seems to be abnormal
in the PKMζ-null mouse.
Again, they are different than the wild type mice.
And we can look at a different memory task.
In this case, what we can do is
have animals walk around in three different boxes,
cartooned here as green, yellow, and blue,
but they just look different and smell different to the animals,
and what we can do is take advantage
of the ability or interest of mice in different objects,
and they respond to novelty.
So, the animals were made familiar
with different numbers of objects in these boxes, and in the one case what we can do is,
after they've been familiar for a day and stored,
presumably, a memory for those objects,
what we can do is exchange the objects, for example,
from Context B, those orange objects...
we can exchange them for a pair of the object in Context A
and put the animal back,
and look to see where the animals spend their time.
Do they investigate the novel objects?
And they do, just like the wild animals.
And we can also ask,
what happens if we just exchange a pair of the objects
that were already in the Context B?
So, you can see the circle has been exchanged
for the cube in the yellow case,
and this is a much more subtle distinction.
You can think of the first distinction as if,
for example, someone were to exchange
your toothbrush for the fork at the dinner table.
That would be something that might gather your attention.
But much more subtle is exchanging the fork, for example,
with the spoon.
And that subtle distinction the animals don't seem to remember
if they're missing the PKMζ molecule,
as in the case of the PKMζ-null mouse.
And if you simply exchange the locations of a pair of objects
they have, again, very strong memory
for that and no difficulty in recollecting that.
So, what I've shown you so far is that
PKMζ is crucial for the persistence of long-term memory.
And you might ask another question, right?
At least it's crucial for the persistence of long-term memory
in normal or wild type mice.
We might ask another question:
Does PKMζ actually increase in long-term memory?
If it's maintaining the memory, how does it do that?
A simple, the most parsimonious,
way to accomplish that would be to make more of it.
And so I'll show you some experiments.
We can use the same experimental task.
We can train animals to work a memory
and in this case we can train them to form
a one-day memory in the place-avoidance task, here.
You can see the control animals
-- the shock was never turned on,
they're just walking around,
whereas the trained animals, they stay away from the shock zone.
And when we measure how long it takes them to enter the shock zone,
it's elevated if you look at the dark bars a day later.
And a day later, we can see that,
where the asterisks are,
that the amount of PKMζ, studied by Western blot,
is enhanced within 1 day
if the animals have had spatial training,
whether or not they actually were tested for their memory.
The elevation already happens 2.5 hours after the training,
but it doesn't happen within 5 minutes,
if the animals, if you will, only establish a short-term memory.
So, it takes some time for PKMζ to get made
and to be elevated,
and then it can remain elevated persistently.
The control experiment, here,
is that a group of animals received an unavoidable shock,
so every time the trained animal was shocked,
this group of animals was also shocked,
and that doesn't itself increase the PKMζ.
It's really necessary to form a memory
for the PKMζ increase.
And that, in fact, if you were to look now
at just the data from the animals
that had the spatial training on day 1
and you were to measure how well they remember
-- shown on the y axis, their time to first enter --
and you looked at the amount of PKMζ
that was expressed in their hippocampus on the x axis,
you see that there's a strong relationship.
Those animals that express very low levels of PKMζ
in their memory retention show weak memory retention.
Those animals that express very strong levels of PKMζ
after memory retention
show strong memory retention.
So, it seems that PKMζ both increases with
and is correlated with the expression of long-term memory.
So, how long does this increase last?
We can repeat this experiment
and, instead of testing one day later,
we can test one month later.
What you can see here is, with this particular training,
the animals show very poor memory recall one month later
-- they're going everywhere --
and when you measure PKMζ levels
you can see that the PKMζ levels, in green,
are no different from baseline.
They're not increased one month later
when the animals show no memory one month later.
The bars on the graph show the control animals
who never formed a memory and the animals that were trained,
but one month later
also don't show memory expression.
And PKMζ also isn't increased by Western blot.
But if we trained the animals a little bit differently,
so that they form a one-month memory, as shown here,
you can see now the trained animals are staying
away from the shock zone in the retention test
given one month later.
The grey bar shows that it's elevated
-- they have an elevated time to first enter --
while you can also see that, in the green bars,
the trained animals have now an elevated PKMζ level.
And this is now one month after the initial training.
So, PKMζ seems to have a persistent increased expression
with long-term memory that lasts at least a month, okay?
And that's quite remarkable.
I remind you that when we looked one month later,
we could also see where PKMζ was distributed in the hippocampus
and that it's not everywhere in the brain,
it's not even everywhere in the hippocampus.
It's in particular subregions of the hippocampus,
the subregions, in fact,
that we can study the electrical activity
and the changes in how the conjunction of electrical activity
is able to manifest as increased synaptic potential
which is what we, remember,
hypothesized as a field as the
underlying expression of cellular mechanism
for the storage of memory.
So, to summarize,
what we've talked about is the molecular mechanisms
that underlie memory.
We don't know them yet;
there's active controversy that I just described,
but we have strong hypotheses
and we're well on our way to delineating
what the molecular neurobiological mechanisms are
that sustain memories.
PKMζ is a molecule that
seems to be crucial for the persistence of long-term memory,
remember it increases while animals
hold or form memories
and it seems to persist as long as long-term memories persist.
When the long-term memories decline,
you see that PKMζ levels decline back to baseline levels.
So, we would conclude that PKMζ is
at least the leading candidate component
of the molecular mechanism of memory maintenance,
which is actually saying something substantial
in the effort to understand the fundamental neurobiological underpinnings
of something as crucial and important to our existence
as memory.
And we've been able to trace, by understanding and studying the brain at the level of behavior,
at the level of neural circuits,
at the level of physiology,
and the level of gene expression,
the molecular constituents of
the mechanism for the storage and persistence of memory.
And I'd like to thank
the large number of people that have contributed to this work,
both in my laboratory as well in collaborating laboratories,
and of course the generous funding
that has made funding possible across the decades.
