Memory Mechanisms in Aplysia- 1

Question 1

What is the purpose of the neurobiological approach to learning and memory?

Answer 1

To determine: the specific brain/CNS regions that support memory storage and recall; How synapses and cellular activity are involved in memory in these regions are affected functionally during these activities; The molecules/protein that mediate these changes at the synaptic level

Question 2

Define engram

Answer 2

The hypothetical physical manifestation of memory that is associated with those brain areas involved in memory systems

Question 3

Outline Aplysia and its use for LMC research

Answer 3

Reddish brown mollusc 15-30cm long
Useful bc: defensive withdrawal reflexes alterable with experience; neural circuits for these well understood; neurones easily accessible for IC recording

Question 4

Describe the basic anatomy and function of the gill and siphon in Aplysia

Answer 4

Aplysia breathe through delicate (dorsal gill) covered by mantle shelf
Parapodia protect the gill be wrapping over the animal’s dorsal surface
Breathing in Aplysia- draw water across gill from front then eject through siphon

Question 5

Nature of the withdrawal reflex?

Answer 5

The reflex withdrawal of the gill and siphon into the mantle cavity is adaptive in that it is performed to protect the respiratory organs from potential harm

Question 6

Habituation in Aplysia

Answer 6

Progressive loss of gill reflex responsiveness to repeated weak tactile stimulation - gentle touch of the siphon (non-noxious).

Question 7

Sensitisation in Aplysia

Answer 7

Enhancement of gill reflex responsiveness following strong simulation - electrical shock to the tail (noxious).

Question 8

What are habituation and sensitisation both examples of?

Answer 8

Non-associative learning

Question 9

Outline conditioning in Aplysia (very brief)

Answer 9

(shock) US –> (withdrawal)
weak tactile (CS)
CS + US –> CR (irt CS only)

Question 10

Basic experimental paradigm for conditioning in Aplysia

Answer 10

Paired: 0.5s siphon stim (CS) + 1.5s tail shock (US)
Unpaired: Large CS-US interval
US alone: sensitisation control
Aplysia received 30 training trials with a 5 min. interval. Then tested with the CS alone

Question 11

Results of basic experimental paradigm for conditioning in Aplysia

How long did this type of learning last?

Answer 11

Paired group responded more to CS alone than did US or unpaired groups
This type of learning lasted for 4 days after a single training session (2.5 hours).

Question 12

CS-US interval and the relationship to conditioning

Answer 12

Optimal learning: CS precedes US by 0.5s, does not occur if pairing is backward (general rule)

Question 13

Minimum number of training sessions/trials to produce conditioning?

Answer 13

1 session/ trial

Question 14

How many training sessions/trials are necessary for improved learning?

Answer 14

5+ sessions/trials

Question 15

Outline the Aplysia nervous system

Answer 15

Standard invertebrate nervous system; CNS c.20,000 large neurones organised into ganglia that communicate via connectives

Question 16

Benefits of the neural architecture of Aplysia to LMC research

Answer 16

Architecture has allowed the relation of individual neurones to discrete behaviours and consequently the production of wiring diagrams for many behavioural responses

Question 17

Ganglia in Aplysia (posterior to anterior)

Answer 17

Buccal
Cerebral
Pleural
Pedal
Abdominal
(learn diagram for wiring)

Question 18

Habituation circuit in Aplysia

Answer 18

DIAGRAM

Question 19

Electrophysiology of habituation in Aplysia

Answer 19

Underlying neural change is decreased EPSP at SN-MN synapses

Question 20

At which point is the reflex pathway for habituation (short term) affected in Aplysia?

Answer 20

Locus of change appears to be the presynaptic SN terminal bc: similar numbers of APs are seen irt stimulus in SNs post habituation; postsynaptic MNs show similar sensitivity to exogenous NT (no change in postsynaptic Glu-Rs)

Question 21

What is the mechanism of short term habituation in Aplysia?

Answer 21

Depression of transmission due to decrease in no. of synaptic vesicles released / AP
No change in overall no. of synaptic vesicles just less likely to be dock and release

Question 22

Original VS current view on the cause of decreased docking probability of vesicles in short term habituation?

Answer 22

Original: thought to be caused by downregulation of VGCCs
Current: direct switching off of release machinery thus preventing vesicle fusion (mechanism unclear but could be Ca2+ inhibition of Arf)

Question 23

Sensitisation circuit in Aplysia

Answer 23

DIAGRAM

Question 24

Electrophysiology of sensitisation in Aplysia

Answer 24

The neural change underlying sensitisation is an increase in EPSP at SM-MN synapses

Question 25

At which point is the reflex pathway for sensitisation (short term) affected in Aplysia?

Answer 25

Locus of change appears to be the presynaptic SN terminal bc: similar numbers of APs still seen in response to stimulus in SNs; no change in no. of postsynaptic Glu-Rs

Question 26

What is the mechanism of short term sensitisation in Aplysia?

Answer 26

1. Activation of interneurone
2. Release of 5HT
3. Activates Gs & Go/q linked G-protein coupled receptors
4. Gs –> activates adenylyl cyclase –> ↑ [cAMP] –> activation of PKA
5. Go–> activates PLC–>↑ DAG–> activation of PKC
6. K+ channels phosphorylated (PKA) –>↓ K+ conductance—> broadens AP
7. Ca2+ channels phosphorylated (PKA & PKC)–> ↑ Ca2+ conductance
   6/7. ↑ Ca2+ entry via Ca2+ channels, ↑ presynaptic [Ca2+]
8. ↑ NT release
9. ↑ MN activity - more/longer receptor activation