Modern questions in learning + memory

Question 1

What is the typical apparatus for training drosophila with conditioned + unconditioned stimuli?

Answer 1

Odour A paired with an electric shock
Flies sent down ‘elevator’ and pick between odour A and B
Pick B as A gives electric shock - which they remembered

Question 2

What are Kenyon cells?

Answer 2

type of neuron found in the mushroom body of the insect brain- crucial role in olfactory processing and associative learning

Question 3

What are some properties of Kenyon cells which aid olfactory input and processing?

Answer 3

• Receive signals from various olfactory sensory neurons that are activated when the fly detects different odors
• signals can come from multiple neurons at the same time, allowing the Kenyon cells to integrate information from different sources
• by receiving inputs from multiple neurons simultaneously, they can effectively sample specific combinations of odors in the brain

Question 4

Why is it good that Kenyon cells respond sparsely to odours?

Answer 4

-respond selectively to specific combinations of odors
- allows them to distinguish between different odors more effectively
- Also reduced the overlap of what response is paired with what odour

Question 5

How can the Kenyon cells distinguish between odours?

Answer 5

• associative memory, which associates specific odors with certain outcomes or experiences, is stored in the patterns of activity within Kenyon cells

Question 6

How does synaptic plasticity occur within kenyon cells?

Answer 6

• When dopaminergic and olfactory fire at the same time, it causes synaptic plasticity
• Means that odour is now associated with a positive outcome

Question 7

Explain the Gal4/UAS System

Answer 7

• Gal4 is transcription factor from yeast, not native to flies
• Can add Gal4 next to a promoter/enhancer which will cause nearby genes to be expressed in particular tissues in a fly
• Gal4 binds to the UAS which stands for the upstream activating sequence , can bind to reporters etc.
• When the two transgenic genes are put together, RNA transcription occurs
• You can mix and match any Gal4 transgene with any UAS trans gene by just breeding the flies together

Question 8

What is the split Gal4 system?

Answer 8

• involves splitting the Gal4 protein into two separate fragments, typically an N-terminal fragment and a C-terminal
• Each fragment is expressed in different cell population
• populations come into close proximity or interact, the split Gal4 fragments reconstitute to form an active Gal4 transcription factor
• Then the typical Gal4/UAS system occurs

Question 9

What is the mushroom body and how are axons arranged in it?

Answer 9

• Structure in insects where kenyon cells are found
• Axons are sent down structures and split into two lobes where they form parallel bundles

Question 10

What are MBONs?

Answer 10

• Mushroom Body Output Neurons
• Innervation of MBONs causes parallel axons to be subdivided into compartments
• Each MBON goes to a different compartment (found using split Gal4 system)

Question 11

What neurons also subdivide kenyon cell axons into compartments?

Answer 11

Dopaminergic neurons

Question 12

Explain an experiment scientists use to look at the function of MBONs

Answer 12

• Channel rhodopsin is a light-sensitive protein. - Can be genetically engineered to eb expressed in certain MBONs
• Flies that have undergone this are put in a chamber that can have red light WHICH DOESNT ACTIVATE THE CHANNEL RHODOPSIN
• By turning the red light on and off they can study attractant and avoidance behaviour in the flies depending on whether the MBON is activated or not
• Helps them understand the MBON neural circuitry

Question 13

How can you implant ‘fake’ olfactory memories in to flies and what is it useful for?

Answer 13

-optogenetics
- give fly odour and express channelrhodopsin in punishment encoding dopaminergic neurons to give ‘fake impression of pain’ so they then avoid odour in future experiments
- Can be used to investigate neural circuits and the formation of memories

Question 14

How does learning in flies happen by the weakening of synapses?

Answer 14

• In the form of LTD
• If forward-pairing behaviour i.e. approach paired with an electric shock, the synapses involved in this may undergo LTD
• This is so that the likelihood of the fly approaching or responding to that odour in the future decreases

Question 15

What is forward and backward pairing?

Answer 15

Forward pairing: Conditioned stimulus followed by UC stimulus
Backward pairing: UC stimulus followed by conditioned stimulus

Question 16

What did the kenyon cell/dopamine activation experiment show about forward and backward pairing?

Answer 16

The experiment used an electrode to stimulate either the dopamine receptor or the Kenyon cell first to see the effect on behaviour

1. Kenyon cell then dopamine (forward pairing):
   - Led to depression in the avoidance KC-MBON synapse as the odour predicts good dopamine so they are less likely to avoid
2. Dopamine then Kenyon cell (backward pairing)
   - Led to potentiation in the KC-MBON synapse as the odour predicts loss of dopamine and so the fly is more likely to avoid the odour

Question 17

Explain the function of the two dopaminergic receptors in the Kenyon cells

Answer 17

DopR1:
- Activates adenylate cyclase producing cAMP
- Involved in learning forward conditioning (odour predicts reward/shock)
- Pushes depression

DopR2:
- Involved in ‘forgetting’ which is an active biochemical process
- Signals through GQ
- activates PLC which makes IP3 which makes the IP3 receptor release Ca2+ from the endoplasmic reticulum
- This pushes potentiation because flies are weird

Point is there is two separate signalling pathways for behaviour

Question 18

What is the role of GCamp?

Answer 18

measures calcium in endoplasmic retiiculum, when the signal goes down it means calcium is released in the ER

Question 19

How is it that the ER releasing Ca2+ can depend on the order of dopamine and Kenyon cell activity?

Answer 19

SPECULATION
- IP3 receptor is also itself sensitive to Ca2+ despite the fact it releases it
- Activity causes Ca2+ to enter the cell, Kenyon cell activity
- If Ca2+ binds first from Kenyon cell preceding dopamine, it locks the cell preventing IP3 to bind
- If there is no Ca2+ first then the cell isn’t locked and IP3 can bind and then Ca2+ can bind-no locking

NOT PROVEN

Question 20

What is the mushroom body structurally similar to?

Answer 20

• The vertebrate cerebellum
• Have the same suppressing approach mechanism
• Same sparse layout, climbing fibres, granule cells etc etc

Question 21

How does the electro sensory organ work in fish?

Answer 21

• can detect electrosensing from prey, the key feature of this is when they emit a pulse from the tail they need to suppress their pulse from their organ
• this is fired as a teaching signal, so the electrosensory organ knows its them and not the prey, depression is used to ‘cancel out’ the detection of its own signal