High-resolution analysis of bound Ca in neurons and synapses.

Calcium (Ca) is a well-known second messenger in all cells, and is especially relevant for neuronal activity. Neuronal Ca is found in different forms, with a minority being freely soluble in the cell and more than 99% being bound to proteins. Free Ca has received much attention over the last few decades, but protein-bound Ca has been difficult to analyze. Here, we introduce correlative fluorescence and nanoscale secondary ion mass spectrometry imaging as a tool to describe bound Ca As expected, bound Ca is ubiquitous. It does not correlate to free Ca dynamics at the whole-neuron level, but does correlate significantly to the intensity of markers for GABAergic pre-synapse and glutamatergic post-synapses. In contrast, a negative correlation to pre-synaptic activity was observed, with lower levels of bound Ca observed in the more active synapses. We conclude that bound Ca may regulate neuronal activity and should receive more attention in the future.