High-resolution analysis of bound Ca(2+) in neurons and synapses

Calcium (Ca(2+)) is a well-known second messenger in all cells, and is especially relevant for neuronal activity. Neuronal Ca(2+) is found in different forms, with a minority being freely soluble in the cell and more than 99% being bound to proteins. Free Ca(2+) has received much attention over the last few decades, but protein-bound Ca(2+) has been difficult to analyze. Here, we introduce correlative fluorescence and nanoscale secondary ion mass spectrometry imaging as a tool to describe bound Ca(2+). As expected, bound Ca(2+) is ubiquitous. It does not correlate to free Ca(2+) 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(2+) observed in the more active synapses. We conclude that bound Ca(2+) may regulate neuronal activity and should receive more attention in the future.