Lecture 14: Long Lasting Potentiation – Induction and Expression Flashcards
How did Donald Hebb view the brain?
as a set of interconnected cell assemblies (neural circuits) through which there are flows of activity – repeated flows of activity in one set of neurons would strengthen connections (synapses) between them
What is Hebb’s postulate?
when axon of cell A is near enough to excite cell B, and repeatedly and persistently takes part in firing it, some growth process or metabolic changes takes place in one or both cells such that A’s efficiency in firing B is increased
What happens if you increase the efficiency (strength) of synapse that has been active during a particular flow of activity?
that flow will happen more easily in the future
Why does enhancing the strength of a particular synapse in a cell assembly that is active during a particular event (ie. memory) produce memory?
if part of the assembly is activated in the future, the strengthened synapses will enable efficient transmission/replay of the same pattern of activity, even if it is a weaker triggering stimulus
What are the 2 key model systems for understanding long-term synaptic plasticity?
- vertebrate hippocampus
- Aplysia nervous system
Where in the brain would you even look for a memory ‘trace’?
- removal of medial temporal lobes (including hippocampus) caused inability to form new memories
- knowing the hippocampus is involved in memory does not explain how it forms those memories
What were the roadblocks to being able to link Hebb’s ideas to real brain structure?
- knowing what to look for – what is encoded in a memory, and how sparsely it would be spread
- being able to look – hippocampus is in middle of CNS, and it is very seizure prone
What type of circuit does the hippocampus form?
trisynaptic circuit – pass information from one sub-region to the next, then sending it back to the rest of the brain
How many neurons and synapses are there in the hippocampus?
millions of small neurons, thousands of individual synapses each
What are the advantages of Aplysia’s nervous system for doing physiology and molecular biology experiments, compared to vertebrate hippocampus?
- accessible – no skull or BBB
- small – 10,000s of neurons
- identifiable – cells with known function, easily distinguished
- large – individual neurons up to 1mm
Aplysia exhibit simple learning behaviours relating to repeated/salient stimuli.
- habituation: animal’s reaction to siphon touch (gill retraction) diminishes in intensity with repeated trials
- sensitization: after 13 trials with mild touch stimulus to one body part, the same mild stimulus is delivered at the same time the tail is given an electric shock (this is not strong enough to damage the animal, but it alarms it) – gill retraction response dishabituated (becomes larger again)
Strength of synapses from sensory to motor neurons mirror…
behavioural changes in habituation
METHOD: Possible to use nervous system of Aplysia after removing it from the rest of the animal (a preparation that produces fictive behaviour) to study the changes in the gill withdrawal circuit – electrical stimulation of mechanosensory neuron replaces the actual touch stimulus, and activity of motor neuron represents response size.
- fictive habituation: triggered by repeated electrical stipulation of siphon sensory neuron every 10s (1 AP every ten seconds)
- observation: habituation is mediated by a form of short- to moderate-term synaptic depression at synapse between sensory neuron and gill motor neuron
how it’s expressed – quantal content analysis
Synaptic strength is increased during sensitization, and changes last for tens of minutes.
- fictive sensitization: this stimulation mimics the events that would happen in a behaving animal undergoing sensitization training (siphon nerve stimulation is paired with strong stimuli to the tail nerve)
- observation: single (sensitizing) tail nerve shock induces enhancement of strength of the sensory neuron → motor neuron synapse that can last for tens of minutes after it is triggered
- by the rules for changes in synaptic strength (L13), this plasticity should be called a potentiation – however, Kandel & co. have always referred to it as facilitation, even though it lasts longer than we usually think of facilitation lasting
Long-lasting Sensitization/Facilitation in Aplysia
What is sensitization of sensory motor synapses induced by?
third factor neuromodulator
hypothesis 1: serotonin released onto siphon sensory neuron via axoaxonic synapse is the key stimulus for inducing synaptic plasticity that underlies sensitization
Results of testing the hypothesis that serotonin induces synaptic sensitization.
- exogenous serotonin mimics the synaptic enhancement created by a tail shock stimulus
- controls (not shown here) found that other neurotransmitters are not capable of mimicking tail shock
- depleting serotonin from Aplysia nervous system prevents sensitization from occurring
What is sensitization expressed by?
changes in presynaptic (sensory neuron) axon terminal
- serotonergic interneuron only made axoaxonic synapses
Hypothesis 2: locus of expression will be presynaptic because serotonin acts directly on sensory neuron axon terminal
What is sensitization expressed by?
changes in presynaptic (sensory neuron) axon terminal
- serotonergic interneuron only made axoaxonic synapses
Hypothesis 2: locus of expression will be presynaptic because serotonin acts directly on sensory neuron axon terminal
Results from testing the hypothesis that synaptic sensitization is expressed presynaptically.
- shocking the tail nerve led to broadening of presynaptic AP (this was shown elsewhere to be due to reduction in VG-K currents)
- serotonin mimics effect of tail nerve shock on presynaptic AP
- blocking cAMP breakdown or adding cAMP to the terminal also mimics effect of tail nerve shock on presynaptic AP
Which Gα subunit type is serotonin likely working through?
Gαs (stimulatory) – cAMP is increasing
What is the conclusion of Kandel and co’s experiments on synaptic enhancement in Aplysia?
synaptic enhancement in Aplysia is presynaptic and serotonin-dependent
- induction of sensitization: occurs via activation of facilitatory interneuron
- expression of sensitization: occurs by spike broadening which enhances Ca2+ influx through presynaptic neuron
Short bursts of high frequency stimulation (HFS) cause in stimulated mammalian hippocampal synapses?
shows a form of long-term potentiation that is maintained for hours
After a single tetanus (HFS) to a subset of presynaptic axons, how is the postsynaptic response changed?
postsynaptic response (to single test pulses) is strengthened for at least an hour in axons that received HFS, but control synapses (no presynaptic HFS) are unchanged
Why do most LTP researchers actually report slope of EPSP in its rising phase, NOT amplitude of the peak?
because it’s less affected by noise
LTP (long-lasting enhancement of synaptic strength) triggered by a high frequency tetanus can be observed where?
in all three of the major synapse types in hippocampal trisynaptic circuit
but LTP can have different characteristics depending on the synapse, and be harder or easier to work with, depending on the preparation
What is post-tetanic potentiation?
delayed increase in EPP amplitude that occurs after a series of high frequency stimuli
seen in many different synapses, but usually lasts only ~5-10 minutes so it is considered to be a separate form of plasticity from LTP
When you use a tetanus to induce LTP, how many pulses is it?
usually a longer train of pulses (usually at least 100 pulses)
What does tetanus refer to?
refers to the fact that when you give this type of stimulation to an intact non-curarized muscle cell, it contracts so strongly it will not relax (which also occurs in the disease tetanus)
What does HFS stimulation lead to?
- many APs in presynaptic neuron
- lots of neurotransmitter release
- and therefore strong depolarization (and probably many APs) in postsynaptic neuron
How many of the above are actually important for LTP induction?
What does LTP depend on?
depends on coincidence of presynaptic and postsynaptic activity, NOT stimulation frequency
What is not necessary for induction of LTP?
HFS of presynaptic pathway
Low-frequency (0.1 Hz) presynaptic stimulation, which during the pairing phase is set to always occur milliseconds before postsynaptic depolarization (so that synapse’s EPSP and its general depolarization coincided)
Does LTP occur?
yes
Received 0.1 Hz LFS out of synch with CA1 depolarizations during the pairing phase (so that synapse’s EPSP and its general depolarization never coincided)
Does LTP occur?
no
How does induction of LTP occur?
postsynaptic cells are detecting the coincidence of strong activity between presynaptic and postsynaptic cells, inducing changes in only the synapses that are part of the coincidence
What are the two odd properties of NMDA receptors?
gating: NMDARs are voltage-sensitive (in presence of physiological levels of Mg2+)
selectivity: NMDARs are permeable to Ca2+, K+, and Na+
Blocking NMDA receptors had a specific effect on LTP experiments at CA3-CA1 synapse.
NMDA blockers block LTP induction, but not basal synaptic transmission
- blocking NMDARs with APV had no effect on baseline synaptic transmission (1)
- but APV could completely prevent LTP being induced with a normal HFS stimulus (2)
- APV did not affect expression of LTP induced before (4) or after (3) was present
What do the unusual properties of NMDAR allow it to function as?
molecular coincidence detector that induces changes to synaptic strength
How does NMDAR act as a coincidence detector?
because they have dual gating, NMDARs only pass current when both presynaptic and postsynaptic cells are highly active at the same time (ie. during HFS)
- presynaptic activity = glutamate in synaptic cleft
- postsynaptic activity = depolarization
How does NMDAR act as a signal transducer?
because NMDARs allow Ca2+ (second messenger) into the cell, their activation can trigger biochemical transduction cascades that alter other cell properties
calcium = biochemistry happening
What do the dendritic spines of hippocampal neurons allow?
helps to maintain synaptic specificity during LTP induction
What are the dendritic spines of hippocampal neurons?
postsynaptic sites for the vast majority of glutamatergic inputs to hippocampal neurons – help keep the contents of individual synapses (such as signal transduction pathway enzymes) biochemically isolated from other synapses
What is CaMKII? What pathway is it involved in?
cellular kinase involved in transduction pathways triggered by NMDARs
- Ca2+ entering postsynaptic density (PSD) through NMDARs binds to transducer protein calmodulin (CaM) and then activates CaMKII (kinase present at very high levels in glutamatergic PSDs)
- active CaMKII phosphorylates a number of different post-synaptic proteins, triggering biochemical changes occurring in hippocampal LTP (and many other forms of LTP)
- CaMKII is locally (and transiently) activated by HFS targeting a single dendritic spine
Hippocampal LTP induction leads to increase in postsynaptic sensitivity to what neurotransmitter?
glutamate – in contrast to Aplysia data, hippocampal LTP experiments show changes in how potentiated synapses respond to exogenous glutamate
Sensitization (Aplysia): Is induction presynaptic or postsynaptic?
presynaptic – the inducing stimulus (serotonin) is acting on the presynaptic neuron in the synapse
Sensitization (Aplysia): Is expression presynaptic or postsynaptic?
presynaptic – serotonergic metabotropic receptor leading to generation of cAMP
LTP (Mammalian Hippocampus): Is induction presynaptic or postsynaptic?
postsynaptic – presynaptic neuron only has to give a small amount of glutamate to the synapse, and the postsynaptic neuron has to be depolarized
induction is occurring in LTP because NMDA receptors opened
LTP (Mammalian Hippocampus): Is expression presynaptic or postsynaptic?
postsynaptic – larger response after induction to the same amount of glutamate
How does LTP affect hippocampal neurons?
causes increase in size of dendritic spines, and enhances AMPAR currents
- LTP is correlated with increase in volume of dendritic spine heads for stimulated synapses
- LTP-inducing HFS enhances AMPAR-mediated PSPs, but has little to no effect on size of NMDAR-mediated PSPs
What happens to AMPARs when LTP is induced at a spine synapse?
more and more AMPARs are reaching the synaptic membrane (specifically the PSD)
What mediates the insertion of new AMPARs into the PSD?
Ca2+ influx and activation of CaMKII (induction) leads to insertion of extra AMPAR subunits into the postsynaptic densities where NMDARs were activated (expression)
- before LTP induction, these extra AMPARs are stored in endosomes located in the dendritic spines
- endosomes are then fused with plasma membrane after Ca2+ is elevated
What else does the Ca/CAMKII cascade contribute to?
reorganization of proteins in PSD and spine head (and therefore its volume)
What are some other mechanisms that contribute to LTP expression?
- CaMKII (and PKC) also directly phosphorylate existing AMPAR receptors in PSD, causing enhanced conductance magnitudes for a period of up to ~1 hr
- there is sometimes a contribution from retrograde signals (through NO gas) to presynaptic glutamate release enhancement through a variety of presynaptic proteins
What does the relative contribution of alternate mechanisms of LTP expression depend on?
depends on the particular synapse, age of animal, and the exact stimulation protocol (induction)
BUT for standard LTP protocols, AMPAR insertion is the dominant mechanism
Is the synaptic enhancement of LTP in the hippocampus predominantly presynaptic or postsynaptic? How long does it last?
postsynaptic (both induction and expression)
this mechanism can maintain strengthened potentials for at least several hours
What does induction of LTP occur via?
via activation of NMDARs through coincident activity in presynaptic and postsynaptic neurons
What does the expression of LTP occur via?
mainly occurs by insertion of extra AMPARs into postsynaptic density
When we consider the requirements of a learning rule that can turn synaptic alterations into meaningful learning and memory substrates, what are the two components to any proposed mechanism that we need to consider?
- mechanism by which selective synaptic change is INDUCED
- process by which the synaptic change is EXPRESSED
How are synapses mediating Aplysia withdrawal reflexes selectively enhanced for long periods of time?
- induction through the activity of a facilitatory interneuron, which releases serotonin on the sensory axon terminal
- expression by enhanced transmitter release
How does hippocampal LTP produce lasting enhancement of synaptic strength in circuits that are known to be necessary for episodic memory?
- induction through activation of postsynaptic coincidence detectors (NMDA channels)
- expression by increasing numbers of AMPARs in postsynaptic cell
What is long term depression (LTD) important for?
important to prevent neurons and neural circuits from becoming over-excitable, as would occur if synapses were only capable of strengthening
What does LTD induction rely on?
induction relies on NMDAR activity (like LTP)
What is LTD mediated by?
Ca2+ influx (like LTP)
What is LTD expression mediated by?
primarily by AMPAR internalization
How is calcium involved in both LTP and LTD?
magnitude and duration of Ca2+ elevation affect exactly which types of kinases (and phosphatases) get activated in a synaptic compartment
