Turner Lectures (4-6) Flashcards
Engram/Memory Trace
The hypothetical physical manifestation of a memory, associated with the brain regions involved in memory systems.
Aplysia Californica (basics, why it’s useful)
AKA Sea Hare
Mollusc, 15-30cm long, feeds on kelp.
Useful for studying because: withdrawal reflexes can be altered through experience, neural circuits underlying this are relatively well understood, neurons are easily accessible for intracellular recording
CNS: Relatively few (20,000) but large neurons. Organised into series of ganglia which communicate via pathways called CONNECTIVES. Made it possible to relate individual neurons to behaviours and create wiring diagrams.
Gill and siphon reflex
Aplysia breathes through the gill on its back.
Gill is covered by overhanging mantle shelf.
Parapodia protect the gill by wrapping over dorsal surface of the animal.
Breathing occurs by water being drawn across the gill from the front (between the parapodia) then being ejected through the rear-facing siphon.
Withdrawal reflex of the gill and siphon is adaptive - protects the respiratory organs from harm.
Habituation vs sensitization
Habituation: repeated weak tactile stimulation of siphon (non-noxious) –> gradual loss of reflex
Sensitization: strong stimulation (noxious) of tail i.e. electric shock –> enhancement of reflex
Both = NON-ASSOCIATIVE
Can learn to associate the two (i.e. electric shock = US, gill withdrawal = UR, weak siphon stim. = CS)
Aplysia training (3 groups)
1) Paired (received CS followed by US)
2) Unpaired (same but with large interval)
3) US alone (sensitization control)
All received 30 training trials with 5 min intervals, then tested with CS alone.
(1) Responded more to CS alone than (2) or (3)
Learning lasted for 4 days.
(2) Had lowest response.
Closer temporal pairing –> larger response to tactile stim.
Duration of memories also increases as the number and spacing of trials increases (distributed learning)
CS-US interval
Optimal learning occurs if CS precedes US by 0.5 secs (=forward pairing, positive CS-US interval).
Learning doesn’t occur with backward pairing (negative CS-US interval).
Aplysia NS (+wiring diagram. How many MNs etc?)
Bilateral system of 10 ganglia (each ~2000 neurons)
E.g. buccal ganglion, cerebral ganglion etc (see diagram)
Gill withdrawal mediated via “abdominal ganglion”. Individual neurons have been mapped in this ganglion, with specific neurons related to specific functions.
6 MNs control the gill. Input from siphon to gill via these MNs. Also input to these MNs from 40 SNs.
Simplified:
(SN)
I
Siphon —-> MN —-> gill
(Think about it as population representation - see diagram for other inputs)
Where do changes occur for habituation? Pathway?
3 possible places: the sensory nerve endings (at the siphon), the ganglion synapse (onto the MN) or the NMJ (onto the gill)
Recordings from cells tell us sensitivity to sensory stimulus is UNCHANGED (same no. of APs)
EPSP of MN is decreased - due to decreased sensitivity or decreased NT release?
Test activation of MNs with exogenous NTs e.g. glutamate –> sensitivity unchanged. Must be decreased release from presynaptic terminal onto MN (homosynaptic pathway)
Smaller EPSPs = less likely to generate APs in MN
Overall number of vesicles remains unchanged, they are just less likely to become docked/release L-glutamate. Originally: due to down-reg of Ca2+ channels (so less Ca2+ entering presynaptic). Now: caused by “silencing of release” - direct switching off of release machinery (mechanism unclear, involves GTP-binding protein Arf).
Changes in circuit during (SHORT-TERM) sensitization
No direct modification of synapse used in habituation. Modulatory IN receiving sensory input from the tail –> indirect modification of the synapse output (heterosynaptic pathway)
Leads to increased NT release onto MN
Modulatory INs
Modulatory INs that underlie sensitization = serotonergic. Increase in release due to modification of calcium and potassium channel function:
Activation of IN –> 5HT release –> activation of GPCRs (Gs and G0/q-linked)
Gs –> activation of adenylyl cyclase –> increase cAMP –> activation of PKA
Go –> activation of PLC (phospholipaseC) –> increase DAG –> activation of PKC
PKA phosphorylates K+ channels, decreasing K+ conductance and broadening AP
PKA and PKC phosphorylate Ca2+ channels, increasing Ca2+ conductance –> increased Ca2+ –> increase NT release
Sensitization blocked by…
PKA inhibitors e.g. Rp-cAMPs
PKC inhibitors e.g. H7
Mechanism underlying associative learning
Sensory neuron stim –> activates Ca2+ channels. Ca2+ –> activation of calmodulin –> increased activity of adenylyl cyclase –> increased cAMP level –> PKA –> more transmitter released
Difference between association and sensitization: increase transmitter release occurs DURING INDUCTION as well as after (in sensitization, this is only seen after a tail shock, once tactile stimulation of siphon is resumed). Therefore see larger EPSPs and more postsynaptic depol. during pairing-induction –> activation of L-glu receptors that are voltage dependent (NMDARs) during induction.
Forward pairing leads to calmodulin more effectively priming adenylyl cyclase (than backwards pairing)
Effect of blocking NMDARs
Blocked with APV or AP5. Prevents conditioning. EPSP of MN will not increase.
NMDARs are blocked by Mg2+ at negative membrane potentials. Depolarisation –> unblocked, cations can flow through (Ca2+/Na+). Initial depolarisation provided by AMPA (another glutamate R which mediates the EPSPs)
Cellular changes in conditioning
Presynaptic changes: same as sensitization (involved in early stages of conditioning)
Also get postsynaptic changes: increased EPSP is detected and fed back to presynaptic (tells it changes occuring should be maintained). Involves NMDA receptors –> increased calcium –> feedback
2x effects on presynaptic adenylate cyclase (CS–>increased calcium–>increased activated calmodulin–>activates AC) (US–>AC activation via 5HT)
LTM (massed vs distributed learning paradigms for habituation and sensitization)
Prolonged biochemical up/down reg. = inefficient (could become saturated, meaning learning would cease - memory full)
Extent of LTM determined by whether learning occurs by “massed learning” (repeated exposure in single learning session) or “distributed learning” (repeated exposure over several sessions) - latter = higher retention.
E.g. habituation: tactile stim. over 40 trials. 10 trials on each of 4 days. Retention tested after 1, 7 and 21 days. Habituation still present after 21. If 40 trials given in 1 session, retention is far less.
Sensitization: 1s electric shock given as single shocks (30-120 mins apart) or as trains every 3 secs (30 mins apart). Distributed = 4 trains of 4 shocks (1 train per day for 4 days). Massed = 4 trains of 4 shocks in 1 day.
Or 4 single shocks in 1 day.
In order, where first paradigm has best retention, last has worst (tested 1, 4 and 7 days after training with tactile stim.)
LTM of sensitization requires repeated training. Applying 5ht to onto SN-MN synapse has same effect. (one application –> memory lasts 15 minutes, 5 applications spaced over 90 minutes –> lasts 24hrs)
Therefore 5ht involved in STM AND LTM of sensitizaton.