Cerebral blood flow is coupled to local neuronal demands. However, this coupling would be compromised without upstream dilation in pial arterioles to permit more blood to reach dilated downstream vessels. The process that permits vasodilating signals, originating in neurons, to reach the pial arterioles, which are isolated from parenchymal neurons by the glia limitans (GL), is unclear. The central hypothesis is that the signaling mechanisms involved will vary as the intensity of the neuronal activation increases. To that end, we will compare pial arteriolar responses during seizure (topical bicuculline) and sciatic nerve stimulation (SNS) in the presence of a variety of pharmacologic and molecular interventions. The following specific hypotheses will be tested: 1) Neuronal activation induces pial arteriolar dilation (PAD) via a signaling process involving astrocytes (and the GL) and vascular endothelium. These studies will use validated models for selective injury to the GL or endothelium. 2) Neuronal activation-induced PAD involves ATP efflux-related signal propagation within astrocytic networks and gap junctions and/or hemichannels. Both pharmacologic blockade and siRNA-linked knockdown will be used to target specific ATP receptors and connexin-43. 3) Paracrine factors arising from the GL act on pial vessels to elicit relaxation. The leading candidates are K+ AND breakdown products of ATP hydrolysis (adenosine;ADP), formed via ectonucleotidase (EN) action. The effects of specific pharmacologic blockers or siRNA-linked knockdown of K+ release channels (BKCa and Kir-4.1) and ENs, on seizure- vs SNS-induced PAD will be tested. 4) The released K+ and adenosine, respectively, stimulate Kir-2.1 channels and adenosine A2A receptors, perhaps interactively, on pial arteriolar smooth muscle. As above, specific pharmacologic and siRNA-based interventions will be applied. The results will provide vital new insights into how neurons, during periods of enhanced activity, signal specific cerebral vessels to dilate. The resulting increase in nutrient delivery to activated neurons acts to ensure normal brain function and provides protection in pathologic states. How vasodilating signals, originating in the neurons, reach the pial arterioles remain unclear. The core objective of this project is to identify mechanisms through which increased neuronal activity signals pial arterioles to dilate. The central hypothesis is that astrocytes and the glia limitans represent a vital signaling conduit in the pial arteriolar dilation arising from neural activation.
