Neurochemistry

1. Synaptic Plasticity
 The term synaptic plasticity is described as the variability of the
strength of a signal constituted through a synapse. Synaptic
plasticity is thought to be be neurochemical fundations of memory
and learning. It was postulated by Donald Hebb in 1949. Hebb
formulated his principle on purely theoretical grounds. He realized
that such a mechanism would help to stabilize specific neuronal
activity patterns in the brain.
 When an axon of cell A is near enough to excite cell B or repeatedly
or persistently takes part in firing it, some growth process or
metabolic change takes place in one or both cells such that A's
efficiency, as one of the cells firing B, is increased. The change at
synapse wij depends on the state of the presynaptic neuron j and the
postsynaptic neuron i and the present efficacy wij, but not on the
state of other neurons k.
j i
A B
Wij
A
B
j i
Wij
j
i
Wij
k k
Fig A
Fig B

2. Long-term potentiation
Definition
 In neuroscience, long-term potentiation (LTP) is the
strengthening (or potentiation) of the connection
between two nerve cells.
 This connection lasts for an extended period of time
(minutes to hours in vitro and hours to days and months
in vivo).
 LTP can be induced experimentally by applying a
sequence of short, high-frequency stimulations to nerve
cell synapses.
 The phenomenon was discovered in the mammalian
hippocampus by Terje Lømo in 1966 and is commonly
regarded as the cellular basis of memory.

3. History
 Santiago Ramón y Cajal proposed that memories
might be stored in the connections between nerve
cells.
 By the turn of the 19th century, neurobiologists had
good reason to believe that memories were generally
not the product of new nerve cell growth. Scientists
generally believed that the number of neurons in the
adult brain (roughly 1011) did not increase
significantly with age.
 With this realization came the need to explain how
memories were created in the absence of new cell
growth.

4. History
 Among the first neuroscientists to suggest that
learning was not the product of new cell growth was
the Spanish anatomist Santiago Ramón y Cajal.
 In 1894 Santiago Ramón y Cajal proposed that
memories might be formed by strengthening the
connections between existing neurons to improve the
effectiveness of their communication.
 Hebbian theory, introduced by Donald Hebb in 1949,
echoed Ramón y Cajal's ideas, and further proposed
that cells may grow new connections between each
other to enhance their ability to communicate:

5. History
 When an axon of cell A is near enough to excite a cell B and
repeatedly or persistently takes part in firing it, some growth process
or metabolic change takes place in one or both cells such that A's
efficiency, as one of the cells firing B, is increased. (Hebb, The
organization of behavior)
 Similarly, memories may be forgotten through the weakening or loss
of connections. For example, a man might be startled by the sound of
a car alarm outside. Sensory cells in the ear record the sound and send
it to the brain where it activates neurons that control the man's
muscles. But as the blaring alarm continues, those connections are
weakened so that the alarm no longer causes the man to be startled.
 These theories about memory formation were unfortunately
foresighted. Neuroscientists were simply not yet equipped with the
neurophysiological techniques necessary for elucidating the biological
underpinnings of learning in animals. These skills would not come
until the latter half of the 20th century, at about the same time as the
discovery of long-term potentiation.

6. Discovery of long-term potentiation
1. LTP was first discovered in the rabbit hippocampus. In
humans the hippocampus is located in the medial temporal
lobe.
2. LTP was first observed by Terje Lømo in 1966 in the Oslo,
Norway, laboratory of Per Andersen (12740104). There, Lømo
conducted a series of neurophysiological experiments
exploring the role of the hippocampus in the rabbit short-term
memory. Targeting the synapses between granule cells of the
perforant pathway and those of the dentate gyrus, Lømo
elicited excitatory postsynaptic potentials (EPSPs) from
dentate gyrus cells by stimulating the perforant pathway. He
observed that a high-frequency train of stimulation produced
larger, prolonged EPSPs compared to the responses evoked by
a single stimulation. This phenomenon was soon dubbed
"long-term potentiation".
3. Timothy Bliss, who joined the Andersen laboratory in 1968,
collaborated with Lømo in 1973 to publish the first
characterization of LTP in rabbit hippocampus.

7. Types of LTP
 Since its original discovery in the rabbit hippocampus, LTP
has been observed in a variety of other neural structures,
including the cerebral cortex, cerebellum, amygdala, and
many others.
 The underlying mechanisms of LTP are generally conserved
across these different regions, but there are subtle differences
in LTP's precise molecular machinery between sites.
 Very broadly, there are two types of LTP:
1. Associative and
2. Nonassociative.
 At rest, the NMDA receptor is blocked by magnesium,
preventing the flow of calcium into the postsynaptic cell.

8. Associative LTP
Associative LTP is the molecular analog of associative learning
(e.g. classical conditioning). It is the strengthening of the
connection between two neurons that have been
simultaneously active. To detect the simultaneous activity of
the pre- and postsynaptic cells, associative LTP requires a so-
called coincidence detector.
In many parts of the brain where associative LTP is observed,
the NMDA receptor (NMDAR) fills the role of coincidence
detector.
At rest, the NMDAR's calcium channel is blocked by
magnesium; the blockade is relieved only after strong
postsynaptic depolarization. The calcium channel is also
ligand-gated, so that it only opens when presynaptically-
released glutamate binds the receptor. When the NMDAR
opens, calcium floods the postsynaptic cell triggering
associative LTP.

9. Associative LTP
 NMDAR-dependent LTP has been demonstrated in the
hippocampus, particularly in the Schaffer collaterals and
perforant pathway, and several other brain regions
including parts of the amygdala (9403688) and cerebral
cortex (2446147).
 There are several types of associative LTP that do not
depend on NMDA receptors.
 NMDAR-independent LTP has been observed, for
example, in the amygdala, where it depends instead on
voltage-gated calcium channels.

10. Nonassociative LTP
 Nonassociative LTP is brought about by the repeated
application of one stimulus (whereas in associative
LTP there are at least two stimuli).
 At nonassociative synapses, such as those involved in
habituation and sensitization, persistent stimulation of
the synapse triggers an influx of calcium into the
postsynaptic cell.
 As in associative LTP, calcium signals the beginning
of long-term potentiation, but the precise mechanisms
of nonassociative LTP are still unknown.

11. Properties of LTP
 NMDA receptor-dependent LTP classically exhibits four
main properties:
Rapid induction,
Cooperativity,
Associativity, and
Input specificity:

12. Properties of LTP
 Induction: LTP can be rapidly induced by applying one or more brief
tetanic stimuli to a presynaptic cell. (A tetanic stimulus is a high-
frequency sequence of individual stimulati.)
 Cooperativity: LTP can be induced either by strong tetanic
stimulation of a single pathway, or cooperatively via the weaker
stimulation of many. It is explained by the presence of a stimulus
threshold that must be reached in order to induce LTP.
 Associativity: refers to the observation that when weak stimulation
of a single pathway is insufficient for the induction of LTP,
simultaneous strong stimulation of another pathway will induce LTP
at both pathways. There is some evidence that associativity and
cooperativity share the same underlying cellular mechanism
(Synaptic tagging).
 Input specific: Once induced, LTP at one synapse is not propagated
to adjacent synapses; rather LTP is input specific.

13. Properties of LTP
Rapid induction: LTP can be rapidly
induced by applying one or more brief
tetanic stimuli to a presynaptic cell. (A
tetanic stimulus is a high-frequency
sequence of individual stimuli.)
A neuron is impaled by an intracellular
electrode to record the membrane potential
while presynaptic fibers are stimulated by
means of a second extracellular electrode.
Small pulses are applied to the presynaptic
fibers in order measure the strength of the
postsynaptic response. The amplitude of
the test pulse is chosen so that the
stimulation evokes a postsynaptic potential,
but no action potentials.
No action potential

14. Properties of LTP
In a second step, the
input fibers are
strongly stimulated
by a sequence of high
frequency pulses so
as to evoke
postsynaptic firing
(Fig. B).
Rapid induction:
LTP

15. Properties of LTP
After that the strength
of the postsynaptic
response to small
pulses is tested again
and a significantly
increased amplitude of
postsynaptic potentials
is found (Fig. C). This
change in the synaptic
strength persists over
many hours and is thus
called long-term
potentiation.
Rapid induction:
LTP

16. Properties of LTP
Cooperativity: LTP can be induced either by of a single pathway,
or cooperatively via the weaker stimulation of many. It is explained
by the presence of a stimulus threshold that must be reached in
order to induce LTP.
Strong tetanic stimulation
LTP

17. Properties of LTP
Cooperativity: LTP can be induced either by of a single pathway,
or cooperatively via the weaker stimulation of many.
It is explained by the presence of a stimulus threshold that must be
reached in order to induce LTP.
Many weaker stimulation
LTP

18. Properties of LTP
Associativity: refers to the observation that when weak stimulation
of a single pathway is insufficient for the induction of LTP,
simultaneous strong stimulation of another pathway will induce LTP
at both pathways. There is some evidence that associativity and
cooperativity share the same underlying cellular mechanism
(Synaptic tagging).
Weak stimulation
Simultaneous strong stimulation
LTP

19. Properties of LTP
Once induced, LTP at one synapse is not
propagated to adjacent synapses; rather LTP is
input specific.
Synapse input-specific LTP
Basal activity (no LTP)
LTP

20. Phases of LTP
 LTP is often divided into two phases:
1. An early protein synthesis-independent phase (E-LTP) that lasts between
one and five hours, and
2. A late protein synthesis-dependent phase (L-LTP) that lasts from days to
months.
Broadly, E-LTP produces short-lived synaptic facilitation by making
existing postsynaptic glutamate receptors (e.g. AMPA receptors) more
sensitive to glutamate.
 Conversely, L-LTP results in a pronounced facilitation of the postsynaptic
response largely through the synthesis of new proteins. These proteins
include glutamate receptors (e.g. AMPAR), transcription factors, and
structural proteins that enhance existing synapses and form new connections.
There is also considerable evidence that late LTP prompts the postsynaptic
synthesis of a retrograde messenger that diffuses to the presynaptic cell
increasing the probability of neurotransmitter vesicle release on subsequent
stimuli.

21. Glutamate
No LTP
LTP
Early LTP
Glutamate
NMDAR
Ca2+

22. Early LTP
 E-LTP can be induced experimentally by applying a few trains of
tetanic stimulation to the connection between two neurons.
Through normal synaptic transmission, this stimulation causes the
release of neurotransmitters, particularly glutamate, from the
presynaptic terminal onto the postsynaptic cell membrane, where
they bind to neurotransmitter receptors embedded in the
postsynaptic membrane. Though a single presentation of the
stimulus is usually not sufficient to induce LTP, repeated
presentations cause the postsynaptic cell to be progressively
depolarized.
 In NMDAR-dependent synapses, this progressive depolarization
relieves the magnesium blockade of the NMDA receptor. When
the next stimulus is applied, glutamate binds the NMDA receptor
and calcium floods the postsynaptic cell, rapidly increasing the
intracellular concentration of calcium. It is this rapid rise in
calcium concentration that induces E-LTP.

23. ELTP
 Beyond calcium's critical role in the induction of E-LTP,
 Few downstream molecular events leading to the expression and
maintenance of E-LTP are known with certainty.
 E-LTP induction depends upon the activity of several protein kinases,
including calcium/calmodulin-dependent protein kinase II (CaMKII),
PKC, PKA, MAPK, and tyrosine kinases.
 Postsynaptically, the ELTP is expressed primarily through the
enhancement of receptor/channel sensitivity.
 In NMDA-dependent LTP in the CA1 hippocampus, the endogenous
calcium chelator calmodulin rapidly binds calcium as a result of NMDAR
opening. The calcium-calmodulin complex directly activates CaMKII
which
1) phosphorylates voltage-gated potassium channels increasing their
excitability;
2) enhances the activity of existing AMPA receptors; and
3) phosphorylates intracellular AMPARs and activates Syn GAP (a
Ras GTPase activating protein) and the MAPK cascade, facilitating
the insertion of AMPARs into the postsynaptic membrane.

24. E-LTP
 PKA serves a role similar to that of CaMKII, but PKA's effects are
more broad. PKA's activity is enhanced during LTP induction by
elevated levels of cAMP as a result of calcium's activation of adenylyl
cyclase. Like CaMKII, PKA phosphorylates voltage-dependent
potassium channels and also calcium channels enhancing their
excitability to future stimuli. Additionally, PKA phosphorylates
intracellular AMPAR stores, facilitating their insertion
postsynaptically. PKA may also enhance AMPAR delivery via
activation of the MAPK cascade.
 While LTP is induced postsynaptically, it is partially expressed
presynaptically. One hypothesis of presynaptic facilitation is that
enhanced CaMKII activity during early LTP gives rise to CaMKII
autophosphorylation and constitutive activation. Persistent CaMKII
activity then stimulates NO synthase, leading to the enhanced production
of the putative retrograde messenger, NO. Since NO is a diffusable gas, it
freely diffuses across the synaptic cleft to the presynaptic cell leading to
a chain of molecular events that facilitate the presynaptic response to
subsequent stimuli.

25.  The late phase of LTP is dependent upon gene expression and protein
synthesis, mediated largely by CREB-1.
 Late LTP can be experimentally induced by a series of three or more
trains of tetanic stimulation spaced roughly 10 minutes apart. Unlike
early LTP, late LTP requires gene transcription and protein synthesis
(3401749), making it an attractive candidate for the molecular analog of
long-term memory.
 The synthesis of gene products is driven by kinases which in turn
activate transcription factors that mediate gene expression. camp
response element binding protein-1 (CREB-1) is thought to be the
primary transcription factor in the cascade of gene expression that leads
to prolonged structural changes to the synapse enhancing its strength.
CREB-1 is both necessary (2141668) and sufficient for late LTP. It is
active in its phosphorylated form and induces the transcription of so-
called immediate-early genes, including c-fos and c-jun. Ultimately, the
products of CREB-1-mediated transcription and protein synthesis give
rise to new building materials for the synaptic connection between pre-
and postsynaptic cell.
Late LTP

26.  During L-LTP, constitutively active CaMKII activates a related
kinase, CaMKIV. Additionally, enhanced Ca2+ levels during
late LTP increase cAMP synthesis via adenylyl cyclase-1,
further activating PKA and resulting in the phosphorylation and
activation of MAPK (10964936). Facilitated by cAMP, both
CaMKII and CaMKIV translocate to the cell nucleus along
with PKA and MAPK (mediated by PKA), where they
phosphorylate CREB-1.
 There is also some evidence that L-LTP is mediated in part by
NO. In particular, NO may activate guanylate cyclase, leading
to the production of cyclic GMP and activation of protein
kinase G (PKG), which phosphorylates CREB-1. PKG may
also cause the release of Ca2+ from ryanodine receptor-gated
intracellular stores, increasing the Ca2+ concentration which
activates other previously mentioned kinase cascades to further
activate CREB-1.
Late LTP

27. LTP and behavioral memory
 In neuroscience, the Morris water maze is a behavioral procedure designed to test
the spatial memory of rats and other small mammals. It was developed by
neuroscientist Richard Morris in 1981, and is commonly used today to explore the
role of the hippocampus in the formation of memories about space.
 In the typical paradigm, a rat is placed into a small pool of opaque water which
contains a escape platform hidden a few millimeters below the water surface. Visual
cues, such as colored shapes, are placed around the pool in plain sight of the rat.
 When released, the rat swims around the pool in search of an exit while various
parameters are recorded, including the time spent in each quadrant of the pool, the
time taken to reach the platform (latency), and total distance traveled. The rat's
escape from the water reinforces its desire to quickly find the platform, and on
subsequent trials (with the platform in the same position) the rat is able to locate the
platform more rapidly. This improvement in performance occurs because the rat has
learned where the hidden platform is located relative to the salient visual cues.

29. LTP and behavioral memory
 Richard Morris provided some of the first evidence that LTP
was indeed required for the formation of memories. He tested
the spatial memory of two groups of rats,
 One whose hippocampi were bathed in the NMDA receptor blocker
APV (APV (also called AP5) is a selective NMDA receptor (NMDAR)
antagonist that competitively inhibits the active site of NMDAR. Its
chemical name is 2-amino-5-phosphonovalerate), and
 The other acting as a control group.
 Both groups were then subjected to the Morris water maze, in which
rats were placed into a pool of water and tested on how quickly they
could locate a platform hidden beneath the water's surface.

30. LTP and behavioral memory
 Rats in the control group were able to locate the
platform and escape from the pool, whereas the
ability of APV-treated rats to complete the task was
significantly impaired.
Water maze
Water
Platform Platform Water

31. LTP and behavioral memory
Moreover, when slices of the hippocampus were taken from
both groups of rats, LTP was easily induced in controls, but
could not be induced in the brains of APV-treated rats. This
provided some evidence that the NMDA receptor — and thus
LTP — was somehow involved with at least some types of
learning and memory.
LTP
Control
APV-treated

32. LTP and behavioral memory
 Similarly, Susumu Tonegawa has demonstrated that a specific region
of the hippocampus, namely CA1, is crucial to the formation of
spatial memories. So-called place cells located in this region are
responsible for creating "place fields" of the rat's environment,
which may be roughly equated with maps of the rat's surroundings.
The accuracy of these maps determines how well a rat learns about
its environment, and thus how well it can navigate about it.
 Tonegawa found that by impairing the NMDA receptor, specifically
by genetically removing the NMDAR1 subunit in the CA1 region,
the place fields generated were substantially less specific than those
of controls. That is, rats produced faulty spatial maps when their
NMDA receptors were impaired. As expected, these rats performed
very poorly on spatial tasks compared to controls, providing more
support to the notion that LTP is the underlying mechanism of
spatial learning.

33. LTP and behavioral memory
 Enhanced NMDA receptor activity in the hippocampus has
also been shown to produce enhanced LTP and an overall
improvement in spatial learning. Trangenic mice with
enhanced NMDA receptor function by overexpressing the
NR2B subunit in the hippocampus.
 These mice, nicknamed "Doogie mice" after the precocious
doctor Doogie Howser, had larger long-term potentiation and
excelled at spatial learning tasks, once again suggesting LTP's
involvement in the formation of hippocampal-dependent
memories.