Optical approaches to synaptic plasticity: Optical approaches to synaptic plasticity: From unitary events to learning rules From unitary events to learning rules

Sam WangPrinceton University

Optical physiology andOptical physiology and synaptic plasticity:synaptic plasticity:

Shari GelberShari GelberBernd KuhnBernd KuhnDaniel O’ConnorDaniel O’ConnorIlker OzdenIlker OzdenDmitry SarkisovDmitry SarkisovShy ShohamShy ShohamMegan SullivanMegan SullivanGayle WittenbergGayle Wittenberg

Brain scaling andBrain scaling and evolution:evolution:

Mark BurishMark BurishDamon ClarkDamon ClarkKim Hatch HarrisonKim Hatch HarrisonAline JohnsonAline JohnsonJennifer ShultzJennifer ShultzPat TanapatPat TanapatMatt WagersMatt WagersKrysta WyattKrysta Wyatt

Laboratory of Sam WangLaboratory of Sam Wang

• Learning rules: from single synapses to STDP

• The hippocampal CA3-CA1 synapse

• Multiphoton optical approaches: imaging and uncaging

CA3-CA1 synapse of hippocampus

Rich history of extracellular and single-cell recordingThe cell biology and plasticity literature is vastOne synapse per connectionHas AMPA, NMDA, mGluR,…

Plasticity is usually measured across many synapses…

30,000 synapses

…what are its properties at single synapses?

Delivery of glutamate receptors

Calmodulin-dependent protein kinase II: a molecular switch

J. Lisman, H. Schulman, and H. Cline (2002)Nature Rev. Neurosci. 3:175

Plasticity events are single steps occurring at distributed times

Plasticity in single steps

Simulated plasticity

events

Realplasticity

data

Potentiation and depression are symmetrically-sized events

The distribution of unitary events can account for the time course of plasticity

Unitary properties of plasticity at CA3-CA1 synapses

All-or-none

Single steps up and down

Steps are of similar size

Learning Rules

A

Presynaptic Neuron Postsynaptic Neuron

B

Spike Times from Neuron A + Spike Times from Neuron B+ Learning Rule

= Expected Synaptic Plasticity

Different Synapses, Different Learning Rules

Abbott and NelsonNat. Neurosci. 2000

CA3-CA1 synaptic plasticity: a vast literature

Any new learning rule measured here should be consistent with this large body of previous work.

• Can we separate bidirectional learning rules into specific requirements for potentiation and depression?

• Can we resolve STDP with previous whole-cell and field recording studies at this synapse?

• How might the learning rule map to behavior?

Questions at the CA3-CA1 synapse

Plasticity at CA3-CA1: a separable process?

LTD and LTP are separable

In cultured neurons:Causal pairings lead to LTPBi and Poo 199960 total pairings at 1 Hz

In CA3-CA1 brain slice: Causal pairings:

Pike et al. 199940 total pairings at 5 Hz

Christie et al. 1996900 total pairings at 3 Hz

STDP studies at CA3-CA1 have yielded conflicting results

…no change.

…LTD.

Causal pairing fails to induce LTP

Single EPSPs before single APs

70-200 pairings with t=0-10 ms (pre before post)

Pairing frequency 0.1-5 Hz

If causal parings lead to LTD, could this explain why low-frequency stimulation induces LTD?

Low frequency stimulation either causes cell to spike or doesn’t

- If cell is held at -70 mV, LFS does not result in LTD- If cell doesn’t spike, LFS does not result in LTD (Christie and Johnston 96)

Spike timings during an LTD protocol are causal

STDP rule I:Pairing single pre- and post-synaptic spikes

What are the conditions for LTP?

Patterned presynaptic stimulation studies:

Pre APs0.2 sec

100 Hz

LTP

Pre APs2 sec

100 Hz

No Plasticity

Pre APs

100 Hz

No Plasticity

Field recording studies suggest bursts and theta frequency

Rose and Dunwiddie 1986, Larson and Lynch 1986, 1989

LTP induction requires postsynaptic bursting

Pairings of EPSP with postsynaptic bursts of two APs spaced 10 ms apart

100 pairings at 5 Hz

EPSP 0-10 ms before first AP

Spike timing dependent LTPis frequency-dependent

t

•Potentiation requires postsynaptic bursts

•Pairings are effective at 5 Hz (near theta)

…can LTD be induced by reversing the pairing order?

LTD occurs when pairing order is reversed

Pairings of EPSP with postsynaptic bursts of two APs spaced 10 ms apart

100 pairings at 5 Hz

EPSP 10-20 ms after first AP

10 ms

n=6

STDP rule II:With postsynaptic bursts

Can we find conditions that lead to an LTP-only rule?

LTP requires fewer pairings than LTD

Reduce number of pairings to 30. LTP is observed; LTD is not.

30 vs. 100 pairings

30 pairings (black circles)100 pairings (gray circles)

Causal: EPSP 2-20 ms before 1st AP

Anti-Causal: EPSP 2-20 ms after second AP

… by decreasing number of pairings we should observe an LTP-only rule.

30 100 30 100

The Potentiation Rule

The Depression Rule

-Is narrow in time-Requires causal pairings-Requires postsynaptic bursts-Requires high frequency pairings-Few pairings still lead to LTP

-Is broad in its timing dependence-Requires many pairings-Has no strong requirement for frequency or bursting

Bidirectional plasticity

In regions of parameter spacethat satisfy criteria for bothLTP and LTD,a bidirectional rule will be measured.

However, this is not a unique rule for the synapse.

STDP is just one slice taken through a high-dimensional parameter space.

What do multiple STDP rules mean for the animal?

LTP rule appears well matched to in vivo recordings

What about LTD? Can LTD be induced with the number of action potentials fired during one pass through a place field?

Is the concept of “one-shot learning” specific to LTP?

What about other behavioral states like various phases of sleep?

Why not continue to explore parameter space exhaustively?

…because it’s exhausting!

What’s Next?

Manipulating biochemistry with caged compounds

Furuta, Wang et al. (1999) PNAS 96:1193Pettit, Wang et al. (1997) Neuron 19:465

Shy Shoham

Uncaging in spatial patterns

Resolution: <1 micron30,000 locations per second

Simulating presynaptic stimulation with glutamate uncaging

Enables us to further dissect learning rules into:

- Plasticity at individual synapses

- To have greater control teasing apart spatial effects

- Can study many synapses at once which should speed exploration of the many parameters of learning rules.

Potassium Cesium

Cs+

K+

pairings given at 5 Hz

Requirement Theta Bursting

LTP yes yes

LTD no no

Two learning rules in one synapse

G.M. Wittenberg

Observations on the LTP learning rule

• Potentiation requires postsynaptic bursts

• Pairings are effective at 5 Hz (near theta)