E. NeherBrainsignals, Synaptic Transmission and

Short-term Plasticity

.... and about Young Investigator Groups in Europe

Ideas of the ERC and Excellence in Research

Tallinn

May 29th, 2007

The Origin of ‚Bioelectricity‘:

1780 - 1800

Galvani

Volta

Lichtenberg

Ramon y Cajal1904First, second and third layer of the cerebrum of a child of 1 month age

Our brain is a network of 1012 neurons, each of which is connected with thousands of other cells by synapses

Ramon y Cajal already predicted the direction of signal propagation

- and was mostly right -

Each neuron receives input at its dendrites from thousands

of other neurons

These signals are being integrated (added up) and a

new nerve impulse is generated, when a certain threshold is surpassed.

Axon

Dendrite

Cell Body

Brain:- 100 000 000 000 nerve cells- 1 000 000 000 000 000 connections- 1000 computational cycles per sec

Computer:- 1 000 000 transistors- 1 000 000 000 cycles per second

Moore‘s Law:The number of transistors, which can be packed onto a chip, doubles every 18 months----> the number of transistors in a computer could reach the number of brain nerve cells in 25 years

Important Difference: The connections within the brain are ‚plastic‘Information processing in the brain is highly parallel

Axon

Dendrite

Cell Body

Synaptic Plasticity

• Neuroscientists use the term ‚Plasticity‘ to describe the observation that synaptic strength changes constantly, depending upon use of a synapse

• Plasticity of synaptic connections underlies the complex information processing of the CNS

• Plasticity occurs on time scales of milliseconds to years

• Nature uses all possible mechanisms, to achieve a finely tuned regulation of synaptic transmission

• When we study synaptic transmission today, we not only want to understand the process of transmission per se, but also why synaptic strength changes in a use-dependent manner

Short Term Plasticity is specific for individual synapses and may be different for two branches of the same axon

Short term depression is a key mechanism for a number of network properties

•Sensory Adaptation (Chung et al., 2002. Neuron 34, 437-446)

•Cortical Gain Control (Abbott et al.,1997. Science 275, 220-224)

•Rhythm Generation (Senn et al., 1996. Neural Networks 9 ,575-588)

•Network Resonance (Houweling et al., 2002. J. Physiol 542, 599-617)

•Temporal Filtering (Fortune and Rose, 2001. TINS 24, 381-385)

Synaptic Transmission

Short term plastic changes may have many causes:

• Presynaptic- action potential waveform

- modulation of Ca++-currents

- Ca++ buffers

- Depletion of release-ready

vesicles

• Postsynaptic

- Desensitization

- Block by Polycations

In general, presynaptic terminals are very small, such that the study of neurotransmitter release is

difficult....However, it has been known for more than 100 years, that there are giant nerve terminals in certain regions of the brain

Cajal: ‚Calices de Held, para refutar antineuronistas‘

The calyx of Held synapse in the auditory brainstem pathway

A

B

C

Calyx pioneers: Hans Held (1893) Arch. Anat. u. Physiol 17, 201 - 248Ian D. Forsythe (1994) J. Physiol 479,381 -387Gerard J. Borst et al. (1995) J. Physiol 489,825 -840

....the postsynaptic current elicited by afferent nerve stimulation

Synaptic depresssion and facilitation can be expressed at the same synapse

(reponses to 100Hz trains of stimuli)

5 ms

1 nA

4 nA

2 mM Ca2+

0.5 mM Ca2+

...both pre- and postsynaptic changes contribute to depression

ICa

p(Ca)

Presynaptic parameters influencing short-term plasticity

Readily-releasable pool

of vesicles, N

Local intracellularCalcium concentration

What is the microdomain [Ca2+] needed for presynaptic vesicle fusion?

p(Ca)Ca -DMN photoproducts + Ca2+FlashICa

p (Ca)

Transmitter release evoked by presynapticCa2+ uncaging

10 µm

post pre

A6

4

2

0

20 ms

5 nA

200 pA

6.4 µM [Ca]i(flash 1)

6.3 µM [Ca]i(flash 7)

B FLASH

20 pA

C

Conclusions I

• Ca++ uncaging allows one to establish a ‚dose-response-curve‘- release-rate versus [Ca2+] -

• During an action potential [Ca2+] is postulated to rise to a peak of ≈ 20µM and 0.5msec width at the release site

• Such high Ca++ concentrations are only obtained in microdomains around open Ca++ channels, which rapidly collapse, when channels close.

... recent measurements by Bollmann and Sakmann Nat Neurosci. (2005), 8, 426-34, in which short [Ca2+] -transients were produced by uncaging, show that only such short transients produce responses, which are similar to action potential-induced ones

Facilitation: Proposed Mechanisms

Question:

Does Ca++ sensitivity change during paired-pulse facilitation

•Residual Calcium (Katz and Miledi)•Extra Calcium Senor•Unblock of Polycations (postsynaptic)

A B

A form of synaptic facilitation downstream of Ca2+ entry studied by presynaptic voltage clamp

cont

rol

faci

litat

ed

Next: Replace this test pulse by a flash

Is the Ca2+ sensitivity of vesicle fusion increased during synaptic facilitation?

Δ[Ca]i

56789

1

2

3

4

56789

10

91

2 3 4 5 6 7 8 910

Peak [Ca]i (µM)

1

10

100

1000

91

2 3 4 5 6 7 8 910

Peak [Ca]i (µM)

control facilitated

Unchanged relation between transmitter release andintracellular Ca2+ during synaptic facilitation

A B

.....short term facilitation is not an increase in the sensitivity of the release apparatus, but rather an increase in the effectiveness of Ca++ influx.

• Ca++ uncaging allows to establish a ‚dose-response-curve‘- release-rate versus [Ca2+]

• during an action potential [Ca2+] rises to a peak of ≈ 20uM

• the Ca++ sensitivity of the release apparatus does not change during short term facilitation

Conclusions from Ca++ uncaging :

‚Top-Down‘ and ‚Bottom-Up‘ Approach to Neuroscience

ENI-Net:A European network, dedicated to the

promotion of Young Investigators

Its Goals:

• Promote the independent research of Young Investigators (Career Development)• Intensify Collaboration and trainig• Stimulate Joint Activities, including applications to other programmes of the EU• Contribute to the establishment of the ‘European Research Area’

ENI-Net:A European network

- the Alicante Meeting -

January, 2004

The participating Institutions commit themselves to provide laboratory space and infrastructure for at least 2 Young Investigator Groups

Independence of Young Investigators is monitored by a ‘Steering committee’

Plans for:Yearly meetings, workshops, and exchange stimulating intensive collaboration

Steering Committee of ENI-Net:Dr. David Attwell, LondonDr. Carlos Belmonte, AlicanteDr. Christoph Mulle, BordeauxDr. Erwin Neher, Göttingen (Chair)Dr. Eva Sykova, Prague

ENI-Net:A European network, dedicated to the

promotion of Young Investigators

Application to Brussels for a Coordination Action in the Neurosciences (Sep. to Nov. 2004) :

Twelve Institutes in 2004, now 18

48 Young Investigators approved by the Steering Committee

1.2 Mio € granted through a CA for yearly meetings, workshops, and exchange

......as of Dec, 2006

2007: Waiting for ERC-groups to join