Response: Raising the speed limit

Abstract

We agree with the criticisms raised by Dr Helmchen and with his balanced and critical review of the topic. The problem he raises about our assumed slow diffusion constant of Calcium Green™-1, a number that came out of our fluorescence-recovery after photobleaching (FRAP) measurements [1.xMechanisms of calcium decay kinetics in hippocampal spines: role of calcium pumps and calcium diffusion through the spine neck in biochemical compartmentalization. Majewska, A. et al. J. Neurosci. 2000; 20: 1722–1734PubMedSee all References, 2.xCalcium dynamics of spines depend on their dendritic location. Holthoff, K. et al. Neuron. 2002; 33: 425–437Abstract | Full Text | Full Text PDF | PubMed | Scopus (88)See all References], is important and has also worried us. We would also like to praise the detailed measurements of Sabatini et al. and their ingenious use of the variance method to estimate diffusional equilibration [3xThe life-cycle of Ca2+ ions in dendritic spines. Sabatini, B.L. et al. Neuron. 2002; 33: 439–452Abstract | Full Text | Full Text PDF | PubMed | Scopus (400)See all References[3]. At the same time, we would like to remark that the overall difference between our interpretation and that of Sabatini et al. is not as large as it seems – both papers essentially agree on the importance of Ca2+ pumps in controlling spine Ca2+ dynamics. Indeed, our kinetic predictions for the zero-exogenous-buffer condition match well with those of Sabatini et al., and we even explicitly propose that the pumps will be stronger than diffusional pathways in this case. In fact, the agreement between these two sets of results, using very different methods and analyses, is remarkable.Ca2+ extrusion by spine pumps, first described by Majewska et al. [1xMechanisms of calcium decay kinetics in hippocampal spines: role of calcium pumps and calcium diffusion through the spine neck in biochemical compartmentalization. Majewska, A. et al. J. Neurosci. 2000; 20: 1722–1734PubMedSee all References[1], appears in our data to vary among different spines. Based on the large heterogeneity in spine structure and function, at this point we are cautious not to extrapolate to a simple scenario for all spines. Indeed, one of the major lessons learned from our past work is that of spine heterogeneity. As Dr Helmchen remarks, the importance of diffusional pathways is likely to be major in stubby spines (those with no neck), and we provide direct evidence for this in our Fig. 5e. Also, long duration Ca2+ accumulations produced by trains of action potentials, excitatory postsynaptic potentials (EPSPs), or the long clearing times present in thick dendrites, could allow diffusional pathways to shape considerably Ca2+ kinetics in spines. More measurements under physiological conditions are clearly necessary to understand the roles of Ca2+ extrusion and diffusion in vivo.Finally, we would also like to point out a few typographic errors present in our manuscript (although not in the code for the model and, therefore, not affecting our conclusions). Specifically, the extrusion rate coefficients used in the model were 30 μl/(cm2*s) and 7.5 μl/(cm2*s) for dendrite and spine, respectively. Also, the factor of (1 + Km + Kf) should be substituted by 1 in Equation 12, and D should be marked Deff in Equation 21.