Molecular Mechanisms of Learning and Memory

1. Molecular Mechanisms of Learning and Memory

2. Procedural Learning • Learning a motor response (procedure) in relation to a sensory input • Two types: • Nonassociative learning • Associative learning

3. Contrast to Declarative Memory • Declarative Memory: • Easily formed and easily forgotten • Created by small modifications of synapses • Widely distributed in the brain • Difficult to study • Procedural Memory: • Is robust (not easily lost) • Can be formed along simple reflex pathways • Easier to study

4. Nonassociative Learning • A change in behavior over time in response to a single type of stimulus • Two types: • Habituation • Learning to ignore a stimulus that lacks meaning • The response to a repeated stimulus decreases • Sensitization • A strong sensory stimulus can intensify your response to all stimuli • The response to a given stimulus increases

5. Associative Learning • Formation of associations between two events • Two Types: • Classical conditioning • associating an effective, response-evoking stimulus with a second, normally ineffective stimulus • Pavlov’s dogs • Instrumental conditioning • associating a motor action with a stimulus • pressing a lever produces a food pellet

6. Invertebrate Systems • Provide models to study learning & behavior: • Small nervous systems • perhaps 1000 neurons, 107 fewer than humans • Large neurons • easy to study electro-physiologically • Identifiable neurons • can be identified from animal to animal • Identifiable circuits • identifiable neurons make the same connections with one another from animal to animal • Simple genetics • small genomes and short life cycles

7. Aplysia as a Model for Learning • The sea slug Aplysis californica, is used for studies in neurobiology • Exhibits simple forms of learning, including habituation, sensitization, and classical conditioning

8. Aplysia & Nonassociative Learning • Gill withdrawal reflex • A jet of water squirted on a portion of the slug (the siphon) causes withdrawal of the siphon & the gill • Habituation • After repeated trials, effect is diminished

9. What Causes Habituation? • Motor neuron, L7, receives direct sensory input from the siphon & innervates muscles used for gill withdrawal • Showed that habituation occurs at the synapse between sensory & motor neuron • Progressive decrease in the size of excitatory postsynaptic potentials (EPSP's) • Mechanism: • less calcium enters presynaptic terminal • so fewer transmitter molecules are released • Therfore presynaptic modification

10. Neurons in Habituation

11. Gill Withdrawal Reflex Sensitization • Shock to head associated with stimulation of siphon increases gill withdrawal reflex = sensitization • How does this work? • Neuron from head (L29) synapses on the axon terminal of the sensory neuron • Releases serotonin • Causes molecular cascade that sensitizes sensory axon terminal

12. Neurons in Sensitization

13. Sensitization Cascade • Serotonin receptor on the sensory axon terminal is a G-protein coupled receptor • Binding activates adenylyl cyclase enzyme • Which produces cyclic AMP (2nd messenger) • Which activates protein kinase A (PKA) • Which phosphorylates a protein forming the potassium channel • Which causes it to close • Prolonging the presynaptic action potential • So more calcium enters • Thus more neurotransmitters are released

14. Associative Learning in Aplysia • Classical conditioning: • Unconditioned stimulus = shock to tail • Conditioned stimulus = siphon stimulation • If the 2 stimuli were paired, subsequent gill withdrawal response to siphon stimulation alone was greater • Uses same neuron as sensitization, through an interneuron

15. Molecular Mechanism • CS response (gill withdrawal) results from influx of calcium ions • US (tail shock) causes G-protein coupled activation of adenylyl cyclase • Elevated Ca++ causes adenylyl cyclase to make more cAMP • This increases total cascade, resulting in more neurotransmitter release • Learning occurs when presynaptic Ca++ release coincides with G-protein activation of adenylyl cyclase producing abundant cAMP • Memory occurs when K+ channels are phosporylated increasing transmittere release

16. Molecular Changes & Memory • One synapse affects another synapse. • Short term memory can be produced when a weak stimulus causes phosphorylation of ion channels, leading to release of an increased amount of transmitter. • Long term memory requires a stronger and more long-lasting stimulus causing increased cAMP, which causes further activation of protein kinases.

17. Visualizing Memory Changes • Short-term memory • thin arrows in the left lower part of the figure • Long-term memory • bold arrows

18. Lessons Learned • Learning and memory can result from modification of synaptic transmission • Synaptic modifications can be triggered by conversion of neural activity to 2nd messengers • Memories can result from alterations in existing synaptic proteins

19. Vertebrate Models of Learning • The cerebellum, because of its role in motor control, is a model system to study synaptic basis of learning in higher organisms • Site of motor learning • Place where corrections of movement are made

20. Anatomy of the Cerebellar Cortex • 2 layers of neuronal cell bodies: • Purkinje cell layer • Granule cell layer • Purkinje cells • modify the output of the cerebellum • Use GABA – so influence is inhibitory • Fibers: • Climbing fibers • innervate Purkinje cell from inferior olive • Mossy fibers • innervate granule cells from pons 1:1 • Parallel fibers from granule cells • innervate Purkinje cell 100,000:1

21. Layers of Cerebellar Cortex

22. Long Term Depression (LTD) • Occurs when climbing fibers and parallel fibers are active together • Molecular mechanism: • Climbing fiber activation causes surge of Ca++ into Purkinje cell • Glutamate from parallel fiber activates AMPA receptor (glutamate receptor that mediates excitatory transmission) • Na+ increases • But this process employs a second receptor . . .

23. Mechanism of LTD (cont.) • There is a second glutamate receptor postsynaptic to the parallel fibers: metabotropic glutamate receptor • G-protein-coupled to enzyme phospholipase C. (PLC) • Which catalyzes formation of a second messenger, diacylglycerol (DAG) • Which activates protein kinase C (PKC) • Analogous to what happens in classical conditioning in Aplysia

24. Molecular Changes in Learning & Memory • Learning occurs when the three things happen together: • Elevated Ca++ due to climbing fiber activation • Elevated Na+ due to AMPA receptor activation • Activated PKC due to metabotropic receptor activation • Memory results from changes in AMPA receptor due to PKC - decrease AMPA openings

25. Declarative Memory & the Hippocampus • Declarative memory relies on the neocortex and structures in the medial temporal lobe, including the hippocampus • Long-term potentiation (LTP) • Brief high-frequency electrical stimulation of a pathway to the hippocampus produces long lasting increase in strength of stimulated synapses • LTD also found in the hippocampus • LTP & LDP may be the basis of how declarative memories form in the brain

26. Anatomy of the Hippocampus • Two thin sheets of neurons folded on each other: • Dentate gyrus • Ammon’s horn • Has 4 divisions • CA3 & CA1 are important here

27. Connections in the Hippocampus • Entorhinal cortex connects to the hippocampus via axons called the perforant path • Mossy fibers from the dentate gyrus synapses on CA3 • CA3 cells synapse via Schaffer collateral on cells in CA1 region • Both CA3 and CA1 cells have output fibers to the fornix

28. Hippocampus Structure

29. Long Term Potentiation (LTP) • LTP occurs in CA1 when multiple synapses are active at the same time that the CA1 cell is depolarized • Recall that glutamate receptors are responsible for excitatory transmission in the hippocampus

30. Mechanism of LTP • Glutamate released from synapse • Na+ ions pass through the AMPA receptor causing EPSPs • CA1 neurons also have post synaptic N-methyl-D-aspartate (NMDA) receptors • These conduct Ca++ ions when cell is depolarized • Thus Ca++ entering the NMDA receptor indicates that presynaptic & postsynaptic elements are active at the same time

31. Induction of LTP • Rise in postsynaptic Ca++ linked to LTP • LTP induction is prevented if NMDA receptors are inhibited • Rise in Ca++ activates 2 protein kinases: • Protein kinase C • Clacium-calmodulin-dependent protein kinase II (CaMKII) • Inhibition of either of these blocks long term potentiation • Following LTP a single axon may form multiple new synapses on a single postsynaptic neuron

32. Long Term Depression (LTD) • LTD occurs in CA1 when it is only weakly depolarized by other inputs • Inward calcium levels are lower, activating a different enzymatic response • Thus, LTP and LTD are two responses of the same system

33. LTD, LTP, & Memory • LTP & LDP are mechanisms of synaptic plasticity • They may contribute to the formation of declarative memory • Recordings from inferotemporal cortex slices from humans shows the same kind of interplay of LTP and LTD • Rats with damage to the hippocampus show reduced learning in Morris water maze • Injecting an NMDA-blocker into rats produces the same reduction of learning

34. Molecular Basis of Long-term Memory • Molecular mechanisms all involve the phosphorylation of something • Phosphorylation is not permanent • phosphate groups get removed, erasing memory • Proteins themselves are not permanent, but get replaced

35. Persistently Active Protein Kinases • Maybe memory is a turned on protein kinase • For LTP in CA1 in the hippocampus, an enzyme activating CaMKII may autophosphorylate and then just stay on • Molecular switch hypothesis - autophosphorylating kinase could store information at the synapse

36. Protein Synthesis & Memory Consolidation • Inhibitors of protein synthesis block consolidation in experimental animals, both mammals and Aplysia • Suggests some new protein must arrive to make short-term changes permanent

37. CREB & Memory • (CREB)  = cAMP response element binding protein • CREB regulates gene expression on DNA • CREB regulated gene expression is essential for consolidation in the fruit fly • Similar results have been shown in Aplysia • CREB may be able to regulate the strength of a memory

38. Structural Plasticity & Memory • In Aplysia long-term learning involves the addition of synapses • forgetting is the deletion of synapses • Some indication that such changes occur in mammals, despite being past the critical period for developmental plasticity