Dendritic diameters affect the spatial variability of intracellular calcium dynamics in computer models.

Haroon Anwar,Weiliang Chen,Bernd Kuhn,Erik De Schutter,Christopher J Roome,Hermina Nedelescu

doi:10.3389/fncel.2014.00168

Abstract

There is growing interest in understanding calcium dynamics in dendrites, both experimentally and computationally. Many processes influence these dynamics, but in dendrites there is a strong contribution of morphology because the peak calcium levels are strongly determined by the surface to volume ratio (SVR) of each branch, which is inversely related to branch diameter. In this study we explore the predicted variance of dendritic calcium concentrations due to local changes in dendrite diameter and how this is affected by the modeling approach used. We investigate this in a model of dendritic calcium spiking in different reconstructions of cerebellar Purkinje cells and in morphological analysis of neocortical and hippocampal pyramidal neurons. We report that many published models neglect diameter-dependent effects on calcium concentration and show how to implement this correctly in the NEURON simulator, both for phenomenological pool based models and for implementations using radial 1D diffusion. More detailed modeling requires simulation of 3D diffusion and we demonstrate that this does not dissipate the local concentration variance due to changes of dendritic diameter. In many cases 1D diffusion of models of calcium buffering give a good approximation provided an increased morphological resolution is implemented.

Highlights

Intracellular Ca2+ has a central role in the information processing capabilities of neuronal dendrites
In this study we explore the effect of dendrite diameter on Ca2+ dynamics in models of different complexity
Using this approach no gradients of Ca2+ concentration are predicted within the dendrite; this result is unlikely to be physiological considering the large variation in surface to volume ratio (SVR) across the dendrite

Summary

Introduction

Intracellular Ca2+ has a central role in the information processing capabilities of neuronal dendrites. The effects of dendritic geometry on Ca2+ transients are often quantified in terms of the surface to volume ratio (SVR). This is because Ca2+ influx scales with membrane surface while the change in Ca2+ concentration due to diffusion and buffering strongly depends on the volume. This results in larger amplitude transients expected in small diameter dendrites because they have a large SVR. Even in the absence of intracellular Ca2+ mechanisms (endogenous buffers, internal Ca2+ stores) and diffusion, changes in dendritic diameter across the dendrite will result in spatially variable Ca2+ levels.

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Conclusion