Microglia, the main immunocompetent cells of the brain, regulate neuronal function, but their contribution to cerebral blood flow (CBF) regulation has remained elusive. Here, we identify microglia as important modulators of CBF both under physiological conditions and during hypoperfusion. Microglia establish direct, dynamic purinergic contacts with cells in the neurovascular unit that shape CBF in both mice and humans. Surprisingly, the absence of microglia or blockade of microglial P2Y12 receptor (P2Y12R) substantially impairs neurovascular coupling in mice, which is reiterated by chemogenetically induced microglial dysfunction associated with impaired ATP sensitivity. Hypercapnia induces rapid microglial calcium changes, P2Y12R-mediated formation of perivascular phylopodia, and microglial adenosine production, while depletion of microglia reduces brain pH and impairs hypercapnia-induced vasodilation. Microglial actions modulate vascular cyclic GMP levels but are partially independent of nitric oxide. Finally, microglial dysfunction markedly impairs P2Y12R-mediated cerebrovascular adaptation to common carotid artery occlusion resulting in hypoperfusion. Thus, our data reveal a previously unrecognized role for microglia in CBF regulation, with broad implications for common neurological diseases.

We compared the effects of two different high-caloric diets administered to 4-week-old rats for 12 weeks: a diet rich in sugar (30% sucrose) and a cafeteria diet rich in sugar and high-fat foods. We focused on the hippocampus, particularly on the gamma-aminobutyric acid (GABA)ergic system, including the Ca²⁺-binding proteins parvalbumin (PV), calretinin (CR), calbindin (CB), and the neuropeptides somatostatin (SST) and neuropeptide Y (NPY). We also analyzed the density of cholinergic varicosities, brain-derived neurotrophic factor (BDNF), reelin (RELN), and cyclin-dependent kinase-5 (CDK-5) mRNA levels, and glial fibrillary acidic protein (GFAP) expression. The cafeteria diet reduced PV-positive neurons in the granular layer, hilus, and CA1, as well as NPY-positive neurons in the hilus, without altering other GABAergic populations or overall GABA levels. The high-sugar diet induced a decrease in the number of PV-positive cells in CA3 and an increase in CB-positive cells in the hilus and CA1. No alterations were observed in the cholinergic varicosities. The cafeteria diet also reduced the relative mRNA expression of RELN without significant changes in BDNF and CDK5 levels. The cafeteria diet increased the number but reduced the length of the astrocyte processes. These data highlight the significance of determining the mechanisms mediating the observed effects of these diets and imply that the cognitive impairments previously found might be related to both the neuroinflammation process and the reduction in PV, NPY, and RELN expression in the hippocampal formation.

Microglia closely associate with the vasculature, but their roles in leukocyte trafficking and cerebral blood flow (CBF) during systemic inflammation are unclear. We show that sustained systemic inflammation leads to altered microglia-vascular interactions, leukocyte recruitment to cerebral blood vessels and reduced hypercapnia-induced vasodilation. We find that microglia and border-associated macrophages (BAMs) play distinct roles in these processes: while BAM depletion with clodronate increases leukocyte recruitment, microglia/BAM depletion with PLX5622 and the absence of microglia, but not BAMs in CSF1R^{ΔFIRE/ΔFIRE} mice, result in markedly reduced leukocyte recruitment. Supporting this, conditional microglial IL-1 deletion inhibits leukocyte recruitment. Unlike BAM depletion, the absence of microglia impairs hypercapnia-induced vasodilation, while in vivo two-photon imaging reveals microglial IL-1α-dependent interactions between leukocytes and penetrating arteries. Accordingly, reduced hypercapnia-induced vasodilation involves endothelial IL-1R1 and microglial IL-1α. Thus, distinct brain macrophage populations exert opposite effects on leukocyte recruitment and CBF through IL-1 dependent mechanisms in given vascular beds in male mice.