Modeling Synaptic Ca2+ Dynamics in

Hippocampal Dendritic Spines  

 

 Pablo d’Alcantara, PhD

Department of NeurophysiologyNational Institute for Medical Research

Columbus, 26 January 2004 London, United Kingdom

• Introduction of Hippocampus and Plasticity

• Methods and Results

• Modeling Ca2+ Dynamics

• Discussion

• Introduction of Hippocampus and Plasticity

• Methods and Results

• Modeling Ca2+ Dynamics

• Discussion

The Hippocampus

• Implication in Memory

• Change of [Ca2+] in dendritic spines induces synaptic plasticity

Selective plasticity induction in CA1

by specific patterns of Ca2+ elevation

via flash photolysis of caged Ca2+

Yang et al., J. Neurophysiol., 1999

Model describing the regulation of

synaptic plasticity by postsynaptic

Ca2+

d’Alcantara et al., Eur. J. Neurosci. 2003

While the mechanisms induced by [Ca2+]

changes are being elucidated, how the Ca2+

dynamics is governed remains mysterious.

• Introduction of Hippocampus and Plasticity

• Methods and Results

• Modeling Ca2+ Dynamics

• Discussion

10 µm

1 µm

Confocal Laser Scanning Microscopy of

CA1 Hippocampal Dendritic Spines in

rat organotypic slices

Line Scan

x

y

t

y

Dendritic Ca2+

transient triggered

by

Backpropagating

Action PotentialBAP

AP

t

y

Confocal fluorescence imaging allows us to measure

properties of Ca2+ channels and Ca2+ buffering:

Ca2+ Saturation of Dye Measure of [Ca2+](Maravall et al., Biophys. J. 2000)

Optical Fluctuation Analysis Measure of VSCC N and po

(Sabatini and Svoboda, Nature 2000)

Experiments with Different [Dye] Ca2+ Buffer Capacity(Neher and Augustine, J. Physiol. 1992)

Saturation of the

intracellular Ca2+

indicator by

membrane

depolarisation

Patch Clamp

Depolarisation

Dye Saturation

Calculation of

[Ca2+] with

Normalised Fluorescence

[Ca2+]

[Ca2+] = 215nM - 3.7M [Ca2+]0 = 81 ± 2 nM

[Ca2+] = Kd . (F - Fmin) / (Fmax - F)

Analysis of Ca2+ channels in spines by

Optical Fluctuation Analysis

50 x

Single Spine BAP-induced Fluorescence

Mean BAP-induced Fluorescence

Variance on BAP-induced Fluorescence

CVF,[Ca] CVopen

N . po = (1- po) / CV2F,[Ca]

and

Failure Probability P(0) = (1- po)N

System of 2 Equations with N and po

Calculation of N and po from the

optical fluctuation analysis with

Coefficients of Variation of [Ca2+] and

Nopen

Experiments with Different Dye

Concentration:15 M OGB1

= 20 ms

30 M OGB1

= 36 ms

Calculation of Dye

Buffering Capacity D:

Measurement of Ca2+ Buffer Capacity E

Fit with equation of

in function of E and

D:

• Introduction of Hippocampus and Plasticity

• Methods and Results

• Modeling Ca2+ Dynamics

• Discussion

IVSCC = iVSCC . N . po . (Vm - ECa2+)

JPump = ( [Ca2+]0 - [Ca2+] ) / Ca2+

[Ca2+] + [Buffer]free [Buffer]Bound

Behaviour of Ca2+ current, Ca2+ pump

and Ca2+ buffering

[Buffer] = 100 M

Kd Buffer =0.5 M

[Ca2+]0 = 60 nM

[Ca2+]ext = 2 mM

Spine Volume = 0.1 fl

(Ca2+)0 = 20 ms

iVSCC = 15 pS

N = 12

po = 60%

Parameters used during modeling

According to our

data and

previously

published data,

simulation of

Ca2+ transients

predicts a huge

concentration

increase

Simulation of [Ca2+] during a BAP

Simulation a BAP

• Introduction of Hippocampus and Plasticity

• Methods and Results

• Modeling Ca2+ Dynamics

• Discussion

Additional Ca2+ buffering could

explain lower [Ca2+] increases

Simulation of [Ca2+] during a BAP with 10x more buffer

Simulation of Ca2+ dynamics in a spine

with spatiotemporal diffusion shows a

high local increase of [Ca2+]

Max (M)

Min (M)

Sharp et al., 1993Simulation made in Virtual Cell

In this work we’ve shown how to measure

[Ca2+], NVSCC, po,VSCC and E to build a

physiology-based model of Ca2+ dynamics

in dendritic spines.

We find that additional Ca2+ buffer and

spatiotemporal diffusion of Ca2+ can

explain experimental observations.

Conclusions

NIMR, London• Dr. Donald Dixon (Experimentalist) • Dr. Reza Zamani (Experimentalist)

• Thomas Stackhouse (Technician)

Acknowledgeme

ntsDalhousie University, Halifax• Prof. Alan Fine (Supervisor)

NRCAM, Connecticut• Prof. Leslie Loew (Virtual Cell) • Dr. Jim Schaff (Virtual Cell)

• Dr. Boris Slepchenko (Virtual Cell)

Emory University, Atlanta• Prof. George Hentschel (Modeling)