Cellular Mechanisms of Learning and Memory

Question 1

What is the basic principle of Hebb’s rule for associative learning?

Answer 1

Cells that fire together wire together. This is due to some growth or metabolic change in one or both cells.

Question 2

What is associative learning and how is it achieved?

Answer 2

Learning by classical conditioning: the pairing of a conditioned stimulus, which gives a neutral response, to an unconditioned stimulus, which gives a strong response.

Question 3

Explain how classical conditioning is done in Aplysia sea slugs.

Answer 3

1. A reduced preparation is made.
2. The tail is stimulated with electrodes. This is the unconditioned stimulus.
3. The siphon is stimulated with a tapping device. This is the conditioned stimulus.
4. The contraction of the siphon is detected by a movement transducer.
5. The activity of the sensory and motor neurons are recorded by electrodes in the abdominal ganglia.
6. This is repeated in spaced trials in order to increase the response from the conditioned stimulus.

Question 4

What does the reduced preparation in Aplysia sea slugs include, and why?

Answer 4

• The siphon connected to the abdominal ganglia, which is connected to a collection of ganglia which then leads to the tail.
• The siphon and tail is where the 2 stimuli are delivered (unconditioned to the tail, conditioned to the siphon).
• The abdominal ganglia contains the sensory motor synapse which is responsible for the withdrawal reflex.
• The collection of ganglia include: cerebral, pleural and pedal. These contain the majority of the slugs’ neurons. The pedal ganglia is connected to the tail via nerves.

Question 5

What is the behavioural outcome from classical conditioning on touching the siphon?

Answer 5

The behavioural response is greatly increased when the siphon is touched.

Question 6

What is the cellular outcome from classical conditioning on touching the siphon?

Answer 6

A complex EPSP (excitatory post-synaptic potential) is seen in the LFS motor neuron (controls muscles of the siphon).

Question 7

How was monosynaptic activity measured and why was this done?

Answer 7

This was done as touching the siphon activates the whole neural circuit, so doesn’t give information about the sensory neuron - motor neuron synapse.
It was measured by selectively firing an action potential into the electrode inserted into the sensory neuron.

Question 8

What is the monosynaptic outcome from classical conditioning?

Answer 8

The EPSP is only enhanced in the paired environment, and it is no longer complex as it is smooth and from a single input, not multiple.

Question 9

What is co-incidence detection, and what are the molecular mechanisms for this in both Aplysia and mammals?

Answer 9

Co-incidence detection molecular mechanisms allow for the unconditioned stimulus to be detected with the conditioned stimulus in order to prepare neurons for plasticity.
Aplysia - Serotonin
Mammals - NMDA type gultamate receptors

Question 10

What are the properties of the NMDA glutamate receptors?

Answer 10

• Excitatory post-synaptic receptors in the CNS
• Ionotropic receptors permeable to Na+, K+ and Ca2+
• They are blocked by Mg2+ ions at negative membrane potentials.

Question 11

When is the Mg2+ block removed in these NMDARs?

Answer 11

• When both depolarisation occurs and glutamate is present pre-synaptically.
• When depolarisation occurs, Mg2+ is flung out.

This is coincidence detection.

Question 12

Describe coincidence detection by NMDARs

Answer 12

1. There is some strong depolarisation pre-synaptically.
2. Post-synaptic cell is receiving depolarisation from another input.
3. Mg2+ is repulsed.
4. Glutamate binds to the NMDAR.
5. Calcium enters the post-synaptic cell.

Question 13

Why is calcium entry through NMDARs important?

Answer 13

• Charge allows for depolarisation.
• Starts intracellular signalling cascades, including activation of kinases which lead to gene expression changing synaptic strength.

Question 14

Why is the hippocampus important?

Answer 14

It is key for learning and memory.

Question 15

Describe the structure of the hippocampus

Answer 15

It is arranged in a tri-synaptic circuit.

1. Dentate gyrus.
   - Has granule cells.
   - Receives input from the cortex.
   - Sends information to the CA3 region.
2. CA3
   - Has pyramidal cells.
   - Receives input from the dentate gyrus.
   - Sends information to the CA1 region.
3. CA1
   - Has pyramidal cells.
   - Receives input from CA3 cells via interacting their dendrites with CA3 axons.

Question 16

What is the Schaffer collateral?

Answer 16

The pathway of the CA3 neurons to CA1 neurons.

Question 17

What is LTP?

Answer 17

Long term potentiation - The strengthening of synapses on a long term basis. This is important in memory and learning.

Question 18

How is LTP acheived?

Answer 18

By delivering high frequency stimulations to Schaffer collaterals.

Question 19

How was it shown that NMDARs are vital for LTP?

Answer 19

The NMDARs were blocked with the drug APV and it was shown that the synaptic response went down to 0. This showed there is an NMDA current present in Schaffer collaterals.

Question 20

What other factors are important in LTP?

Answer 20

• Depolarisation: needed to remove the Mg block

- Calcium ions: needed to activate kinases leading to synapse changes.

Question 21

How can you test to see if NMDARs are important in LTP? Include examples

Answer 21

Use pharmacology - drugs which blocks the channel.

• APV is a competitive antagonist for glutamate binding.
• MK801 is a non-competitve antagonist, as it binds to prevent Ca2+ flux.

Question 22

In what other parts of the brain is NMDAR important for LTP?

Answer 22

• Cortex

- Striatum

Question 23

Do all forms of LTP involve NMDARs?

Answer 23

No

Question 24

How can we test if LTP is important in learning?

Answer 24

Test if NMDARs are involved in learning.

Question 25

What part of the brain is important for spatial learning?

Answer 25

The hippocampus

Question 26

How was it shown that NMDARs are involved in spatial memory in rodents?

Answer 26

Through the morris water maze experiment:

1. The rodent is put in water and it tries to escape via a platform in one quadrant.
2. There are visual cues set up to help the rodent remember where the platform is.
3. The NMDAR blocking drug APV is added.

It was seen that when NMDARs are blocked, the rodents don’t form spatial memory so can’t escape.

Question 27

What is inhibitory avoidance training in rodents?

Answer 27

This experiment conditions the animal to learn to go against their instinct.

1. A rat was put in a chamber with a dark part and a light part.
2. The rat tries to go into the dark part, and when they do, they receive an electric shock.
3. The rat is trained to avoid the dark room.
4. Memory and learning is tested by timing how long it takes for the rat to go into the dark room.

Question 28

How was inhibitory avoidance training shown to utilise NMDARs?

Answer 28

By using CPP which competitively blocks NMDAR.

- In these animals, it was seen that they did not have the memory and they quickly go into the dark room.

Question 29

How is the link between memory and LTP tested?

Answer 29

By measuring hippocampal synapses in vivo. This was done by:

1. Remove part of the rodents skull.
2. Insert a 8 electrode microelectrode array into the brain.
3. Lower the array into the hippocampal CA1 dendrite layer.

The electrodes are separated so each one gives a distinct spatial potential.

Question 30

How did the microelectrode array method give information about LTP?

Answer 30

After doing inhibitory avoidance training on the rodent, it was found that 2/8 populations of hippocampal synapses showed strengthening and therefore LTP.

Question 31

What did the occlusion study test for?

Answer 31

Tests if synapses can be saturated with potentiation; and if they have a capacity for plasticity.

Question 32

What was found from occlusion studies after inhibitory avoidance training?

Answer 32

Showed that learning saturates synapses with potentiation, and more stimulation does not induce more potentiation.

Question 33

Why are optogenetic studies useful?

Answer 33

They identify individual dentate gyrus neurons, and link these to a site of memory storage.

Question 34

What construct was made and injected into transgenic mice? What is the promoter in this construct? What is expressed and when?

Answer 34

Construct: tTR–TRE–>ChR2-EYFP–tTR
(–> is where transcription starts).

• tTA = This activates TRE and is supressed by the drug doxycycline
• TRE = A fragment of DNA which is activated by tTA. This allows for the expression ChR2
• EYFP = reporter. This produces yellow fluorescence

Cfos is the promoter. This is a gene which is activated by neuronal activity.
ChR2 is the channel which is being expressed. It is expressed when Cfos is activated (when there is neural activity) and the drug doxycycline is absent.

Question 35

In the transgenic mice, what is seen in terms of optogenetics and ChR2?

Answer 35

When ChR2 is expressed, yellow fluorescence is seen.

Question 36

How was optogenetics used to induce freezing?

Answer 36

1. The animal has a light guide implanted which flashes blue light to activate ChR2 channels.
   - Activation of ChR2 causes freezing.
   - Only the neurons which are expressing ChR2 will respond.