Functional neuroimaging, such as fMRI, is based on coupling neuronal activity and accompanying changes in cerebral blood flow (CBF) and metabolism. However, the relationship between CBF and events at the level of the penetrating arterioles and capillaries is not well established. Recent findings suggest an active role of capillaries in CBF control, and pericytes on capillaries may be major regulators of CBF and initiators of functional imaging signals. Here, using two-photon microscopy of brains in living mice, we demonstrate that stimulation-evoked increases in synaptic activity in the mouse somatosensory cortex evokes capillary dilation starting mostly at the first- or second-order capillary, propagating upstream and downstream at 5-20 µm/s. Therefore, our data support an active role of pericytes in cerebrovascular control. The gliotransmitter ATP applied to first- and second-order capillaries by micropipette puffing induced dilation, followed by constriction, which also propagated at 5-20 µm/s. ATP-induced capillary constriction was blocked by purinergic P2 receptors. Thus, conducted vascular responses in capillaries may be a previously unidentified modulator of cerebrovascular function and functional neuroimaging signals.

Three new chromophores containing an electron‐donating dithiafulvene (DTF) unit attached to derivatives of retinal, diethynylethene (DEE), and dihydroazulene (DHA), respectively, are synthesized and characterized by using X‐ray crystallography as well as absorption and fluorescence spectroscopy. In general, the DTF unit induces a significant red‐shift in the lowest‐energy absorption in all investigated chromophores. The influence of the DTF unit on light‐induced cis–trans isomerization and on retro‐electrocyclization is investigated. Although the retinylidene derivative decomposes when subjected to light, the DTF‐functionalized DEE and DHA compounds undergo reversible isomerization. Computations show that the highest occupied molecular orbital (HOMO) of DTF–DHA is spread over most of the π‐conjugated system, whereas it is more localized in the ring‐opened product. This difference is reflected in the redox properties.