Monitoring presynaptic calcium dynamics in projection fibers by in vivo loading of a novel calcium indicator

Fluorometric calcium measurements have revealed presynaptic residual calciu m (Ca-res) to be an important regulator of synaptic strength. However, in t he mammalian brain, it has not been possible to monitor Ca-res in fibers th at project from one brain region to another. Here, we label neuronal projec tions by injecting dextran-conjugated calcium indicators into brain nuclei in vivo. Currently available dextran conjugates distort Ca-res due to their high affinity for calcium. Therefore, we synthesized a low-affinity indica tor, fluo-4 dextran, that can more accurately measure the amplitude and tim e course of Ca-res. We then demonstrate the utility of fluo-4 dextran by me asuring Ca-res at climbing fiber presynaptic terminals. This method promise s to facilitate the study of many synapses in the mammalian CNS, both in br ain slices and in vivo.