Arteriole dilation to synaptic activation that is sub-threshold to astrocyte endfoot Ca2+ transients.

Ca(2+)-dependent pathways in neurons and astrocyte endfeet initiate changes in arteriole diameter to regulate local brain blood flow. Whether there exists a threshold of synaptic activity in which arteriole diameter is controlled independent of astrocyte endfeet Ca(2+) remains unclear. We used two-photon fluorescence microscopy to examine synaptically evoked synthetic or genetic Ca(2+) indicator signals around penetrating arterioles in acute slices of the rat neocortex. We discovered a threshold below which vasodilation occurred in the absence of endfeet Ca(2+) signals but with consistent neuronal Ca(2+) transients, suggesting endfoot Ca(2+) is not necessary for activity-dependent vasodilation under subtle degrees of brain activation.