Bi/CNS 150 Lecture 20 Friday November 15, 2014 Learning & Memory 1. Synaptic plasticity Bruce Cohen Kandel,Chap. 12: pp , Chap. 67 1

What is synaptic plasticity? Synaptic strength is a measure of the size of the postsynaptic depolarization produced by a given presynaptic stimulus Synaptic plasticity is the use-dependent modification of synaptic strength Memories are encoded by use-dependent changes in synaptic strength Without synaptic plasticity, there would be no memories The cartoon emphasizes the importance of the hippocampus in memory formation 2

Types of synaptic plasticity Short term (lasts minutes to hours) –Post-tetanic potentiation (facilitation) –Synaptic depression Long term (lasts hours to days) –Long-term potentiation (LTP) –Long-term depression (LTD) – Spike-timing dependent plasticity (STDP ) Homosynaptic and heterosynaptic 3

First demonstration of synaptic plasticity was in autonomic nervous system Post-tetanic facilitation in the cat stellate ganglion was first reported example of synaptic plasticity Preganglionic fibers stimulated with bipolar electrode (Trace A, orthodromic stimulation) Compound action potential (CAP) of the postsynaptic fibers was recorded Repetitive stimulation (15/s for 10 s) increased postsynaptic CAP size for 2-3 min Antidromic stimulation of postsynaptic fibers using the same protocol did not (Trace B) Absence of antidromic facilitation shows that it is caused by synaptic activity rather than a change in postsynaptic excitability 4 Orthodromic Antidromic tetanus

I. The size of synaptic potentials can be modulated: A.by regulating the number of number of vesicles (quanta) released. B.by regulating the size of the current generated by a released quantum at the postsynaptic membrane. II. Short -term plasticity (ms - min) A.The mechanisms of these forms of modulation are almost always presynaptic. B.Paired-pulse facilitation (~10 to 100 ms) C.Synaptic depression (50 ms to min) D.Post-tetanic potentiation (min) III.Long-term plasticity A.The mechanisms of these forms of modulation are usually both pre- and postsynaptic B.LTP (30 min to yr) C.LTD (30 min to yr) Presynaptic vs. postsynaptic mechanisms of plasticity 5

The Hippocampus—a Key Region for Memory and Learning 6

Post-tetanic and long-term potentiation (LTP) in a hippocampal slice PTP believed to be caused by a large accumulation of Ca 2+ in the terminal caused by a high frequency tetanic stimulation. 7

Long-term potentiation in a hippocampal slice is measured from the slope of the rising phase of the field potential Stimulation frequencies that produce LTP usually range from ~50 to 200 Hz. 8 Why the slope? Extracellular voltage (field potential) is proportional to synaptic current Peak current = slope X time to peak Assuming time to peak is constant Slope  peak synaptic current

Mechanisms of LTP induction 9 Transmitter released from CA3 terminals (Schaffer collaterals) is glutamate (A) Glutamate activates AMPA and NMDA receptors in the spine of the postsyanptic CA3 neuron (A) High-frequency tetanus causes strong depolarization, relieves Mg 2+ block of NMDA receptors, and allows Ca 2+ to enter CA3 spine Ca 2+ binds to the cytoplasmic protein calmodulin Ca 2+ /calmodulin activates calcium/calmodulin- dependent kinase (CAMKII), protein kinase C (PKC), and tyrosine kinase FYN Activation of these cascades has two important effects 1.Phosphorylation of AMPA receptors increases their probability of opening by glutamate and their Ca 2+ permeability 2.More AMPA receptors are inserted in the plasma membrane and they have a higher single-channel conductance (more GluR1) Retrograde signals also enhance transmitter release

Early and late LTP 10 A single tetanic stimulus induces early LTP which lasts a couple of hours and does not require protein synthesis Multiple tetanic stimuli (4) delivered over a short time (10 min apart induces late LTP which lasts >8 h Late LTP requires new protein synthesis Injecting a protein synthesis inhibitor such as anisomycin before applying the tetanic stimulus blocks late, but not early, LTP Late LTP induces the formation of new dendritic spines which are the sites of excitatory synaptic input Formation of these spines involves both local translation and the activation of a genomic cascade Local translation is carried out by ribosomes in the dendrites Genomic cascade involves the phosphorylation of the transciption factor CREB and gene translation

1. Calcium ion flows through the activated NMDA receptor. Role of CaMKII in LTP 2. One of its targets is calcium/calmodulin-regulated Protein Kinase II (CaMKII). 3. CaMKII can phosphorylate the subunits of the AMPA receptor. A.The phosphorylated AMPA receptor has a higher probability of opening and phosphorylation increases single-channel conductance by increasing the number of GluR1-containing receptors B.This is likely one mechanism of relatively short LTP (30 min or so). 5. Helps regulate processes that re-arrange and enlarge the cytoskeleton. 4. CaMKII initiates a process that results in addition of new AMPA receptors to the synapse A.This process may be developmentally important B.It likely also contributes to longer lasting LTP. 11

Mechanism of Activation of CaMKII, and “Autophosphorylation” Autophosphorylation of CaMKII can prolong its activation by calcium. CaMKII is activated by the calcium-binding protein calmodulin “Ca 2+/ calmodulin-dependent protein kinase” 12

Recording of LTD in the Hippocampus Stimulation frequencies usually range from 1 to 10 Hz. 13

Role of Calcineurin in LTD 1.Calcium ion flows through the activated NMDA receptor. 2.One of its targets is calcineurin (or protein phosphatase 2B), a Ca 2+ /CaM-dependent protein phosphatase. 3.Calcineurin regulates an inhibitor (Inhibitor 1) of a more general protein phosphatase called phosphatase 1. A.Inhibition of calcineurin blocks induction of LTD B.LTD results from removal of AMPA receptors by endocytosis. C.A popular hypothesis is that the direction of long-term changes in synaptic strength depends on the relative levels of activation of CaMKII and calcineurin. 14

Two cellular processes underlie the major changes during LTP and LTD 1.Insertion of AMPA receptors into the postsynaptic membrane (LTP) or their removal from the postsynaptic membrane (LTD). 2. Growth or shrinkage of the spine via reshaping of the actin cytoskeleton. 15

Spike-timing Dependent Synaptic Plasticity (STDP) From Bi and Poo J. Neurosci. 18, (1998) These recordings were made on cultured neurons “anti-Hebbian” “Hebbian” Pre- fires 5-30 ms before post → LTP (right side below) Pre- fires 5-30 ms after post → LTD (left side below) 16

I.Frequency-dependent Long-term Potentiation (LTP) A.This term actually represents many mechanisms, all of which result in strengthening of the synapse for varying periods of time following tetanic stimulation. B.The mechanisms for LTP lasting 30 min to a few hr do not require new protein synthesis C.The mechanisms for LTP lasting longer than a few hr do require protein synthesis. II.Frequency-dependent Long-term Depression (LTD) A.This term also represents many mechanisms B.LTD, like LTP is thought to be used for sculpting circuits to store information. III.Spike-timing dependent synaptic plasticity (STDP) may arise from the similar molecular mechanisms to LTP and LTD. Long-term Synaptic Plasticity 17

Postsynaptic Calcium Levels and Synaptic Plasticity 1. Level and timing of Ca 2+ rise in spine determines LTD or LTP. 2.Low frequency synaptic firing (~5 Hz) produces LTD; high frequency synaptic firing (~50 to 100 Hz) produces LTP. 3. The same Ca 2+ rules may underlie spike-timing-dependent synaptic plasticity (STDP). 18

Properties of LTP at Schaffer collateral-CA1 synapse Figure Cooperativity: LTP requires simultaneous activation of multiple inputs to unblock NMDA receptors Associativity: pairing of a weak and a strong input will be sufficient to relieve NMDA block Synapse specificity: only active synapses will be potentiated, not inactive ones

LTP and fear conditioning in the amygdala 20 A, mouse hears tone and evoked field potential in lateral nucleus of amygdala is measured B, electrical shock is paired with presentation of tone After pairing, mouse freezes in response to tone alone and evoked field potential in amygdala is increased Increase in field potential requires NMDA receptors

End of lecture Bruce Cohen’s”office” hours 1:15 – 2:15 Friday 328 Kerckhoff