Astrocytes play an important role in the coupling between neuronal activity and brain blood flow, via their capacity to "sense" neuronal activity and transmit that information to parenchymal arterioles. Here we show another role for astrocytes in neurovascular coupling—the ability to act as a signaling conduit for the vitally important process of upstream vasodilation (represented by pial arterioles) during both excessive (seizure) and physiologic (sciatic nerve stimulation) increases in cerebral cortical neuronal activity. The predominance of an astrocytic rather than a vascular route was indicated by data showing that pial arteriolar dilating responses to neuronal activation were completely blocked following selective disruption of the superficial glia limitans, while interference with inter-endothelial signaling was without effect. Results also revealed contributions from connexin-43, implying a role for gap junctions and/or hemichannels in the signaling process; and that signaling from the glia limitans to pial arterioles may involve a diffusible mediator.