Astrocyte-induced cortical vasodilation is mediated by D -serine and endothelial nitric oxide synthase

Abstract

Astrocytes play a critical role in neurovascular coupling by providing a physical linkage from synapses to arterioles and releasing vaso-active gliotransmitters. We identified a gliotransmitter pathway by which astrocytes influence arteriole lumen diameter. Astrocytes synthesize and release NMDA receptor coagonist, D-serine, in response to neurotransmitter input. Mouse cortical slice astrocyte activation by metabotropic glutamate receptors or photolysis of caged Ca 2+ produced dilation of penetrating arterioles in a manner attenuated by scavenging D-serine with D-amino acid oxidase, deleting the enzyme responsible for D-serine synthesis (serine racemase) or blocking NMDA receptor glycine coagonist sites with 5,7-dichlorokynurenic acid. We also found that dilatory responses were dramatically reduced by inhibition or elimination of endothelial nitric oxide synthase and that the vasodilatory effect of endothelial nitric oxide synthase is likely mediated by suppressing levels of the vasoconstrictor arachidonic acid metabolite, 20-hydroxy arachidonic acid. Our results provide evidence that D-serine coactivation of NMDA receptors and endothelial nitric oxide synthase is involved in astrocyte-mediated neurovascular coupling.two-photon | functional hyperemia | Ca 2+ uncaging C erebral blood flow is regulated by autoregulation, which maintains constant flow during changes in systemic blood pressure, and functional hyperemia, which refers to matched increases in blood flow to brain areas with high neuronal energy demand. Intracerebral arterioles and capillaries account for 30-40% of total cerebrovascular blood flow resistance (1), and therefore, changes to the diameter of small penetrating cortical vessels result in significant changes in local cerebral blood flow. Astrocytes have endfeet directly apposed to these resistance vessels and are critical regulators of arteriole lumen diameter. Astrocyte endfeet express Ca 2+ -activated K + channels that gate vasodilatory K + efflux in response to glutamatergic input (2, 3). Glutamate neurotransmission also causes Ca 2+ -dependent arachidonic acid (AA) metabolism and release of vasodilatory AA metabolites from astrocytes, including prostaglandin E 2 (PGE 2 ), produced by cyclooxygenase (COX), and epoxyeicosatrienoic acids (EETs), produced by cytochrome P450 epoxygenase (4-8). Astrocytederived AA can also be metabolized by cytochrome P450 ω-hydroxylase to the vasoconstrictor, 20-hydroxyeicosatetranoic acid (20-HETE) (4, 6-8). Ambient tissue oxygen levels dictate whether astrocytes produce AA-dependent vasodilation or vasoconstriction in brain slices and isolated retina (6, 7). Production of 20-HETE is preferred at high pO 2 (95% O 2 in solution) (6-8), whereas pO 2 closer to physiologic levels (20% O 2 ) inhibits prostaglandin-lactate transporter activity, producing high extracellular PGE 2 levels (7) and reduced 20-HETE synthesis (6).Application of glutamate or NMDA directly to the brain surface dilates pial arteries (9, 10) by a mechanism mediated by NMDA receptors (9-12) and ...

IGF‐1 deficiency impairs neurovascular coupling in mice: implications for cerebromicrovascular aging

“…Microvascular endothelial cells are directly exposed to circulating IGF‐1 and are known to abundantly express IGF‐1 receptors (Ungvari & Csiszar, 2012). Importantly, there is growing experimental evidence that NO production by cerebromicrovascular endothelial cells has an important role in functional hyperemia (Girouard et al ., 2007; Longden & Nelson, 2011; Stobart et al ., 2013). This concept is supported by our observation that inhibition of NO synthesis significantly reduces neurovascular coupling in control animals (Fig.…”

“…CBF responses to whisker stimulation were repeated in the presence of the following inhibitors administered topically onto the brain surface of separate groups of animals: HET0016 (inhibitor of 20‐hydroxyeicosatrienoic acid (20‐HETE) production, 10 −6  mol L −1 for 30 min; Cayman Chemicals, Ann Arbor, MI, USA) (Liu et al ., 2008), MS‐PPOH (inhibitor of EET production, 20 × 10 −6  mol L −1 for 30 min; Cayman Chemicals) (Shi et al ., 2008), L‐NAME (N ω ‐Nitro‐L‐arginine methyl ester, inhibitor of nitric oxide synthase, 10 −4  mol L −1 for 20 min; Sigma‐Aldrich, St. Louis, MO, U.S.A.), apocynin (inhibitor of NADPH oxidases, 3 × 10 −4  mol L −1 for 30 min; Cayman Chemicals), fluoroacetate sodium (inhibitor of the tricarboxylic acid cycle predominantly in glial cells, 10 −4  mol L −1 min; Sigma‐Aldrich, St. Louis, MO, U.S.A.) (Fonnum et al ., 1997; Lecrux et al ., 2012), indomethacin (cyclooxygenase inhibitor, 5 × 10 −4  mol L −1 ; Sigma‐Aldrich, St. Louis, MO, U.S.A.) (Kitaura et al ., 2007), MPEP (6‐Methyl‐2‐(phenylethynyl)pyridine hydrochloride, group I metabotropic glutamate receptors (mGluR) subtype 5 antagonist, 5 × 10 −5  mol L −1 ) (Zonta et al ., 2003), and the NMDA (N‐methyl‐D‐aspartate) receptor antagonist D‐APV (D‐2‐Amino‐5‐Phosphonovaleric acid, 5 × 10 −5  mol L −1 ; Cayman Chemicals) (Stobart et al ., 2013). In a separate series of experiments ( n  = 8 in each group), CBF responses to topical administration of L‐glutamate (500 μmol L −1 ) (Hall et al ., 2014) were determined in the absence and presence of MPEP (5 × 10 −5  mol L −1 ) and D‐APV (5 × 10 −5  mol L −1 ) (Stobart et al ., 2013). CBF responses to acetylcholine (ACh; 10 −5  mol L −1 ) were also obtained to assess maximal endothelial NO‐mediated responses.…”

“…Although these cell types abundantly express IGF‐1 receptors and are known targets of IGF‐1 (Sonntag et al ., 2013), the role of IGF‐1 in the regulation of functional hyperemia in response to neuronal activation is not well understood. The cellular mechanisms underlying neurovascular coupling include endothelial production of nitric oxide (NO) (Ma et al ., 1996; Stobart et al ., 2013; Chen et al ., 2014) as well as astrocytic production of vasodilator metabolites of arachidonic acid, including epoxygenase‐derived epoxyeicosatrienoic acids (EETs) and cyclooxygenase‐derived prostaglandins (Peng et al ., 2002; Zonta et al ., 2003; Takano et al ., 2006). Importantly, previous studies demonstrate that IGF‐1 deficiency leads to endothelial dysfunction and impaired bioavailability of NO in the peripheral circulation (reviewed in Ungvari & Csiszar (2012)).…”

Cerebromicrovascular dysfunction predicts cognitive decline and gait abnormalities in a mouse model of whole brain irradiation-induced accelerated brain senescence

“…Future studies should determine how DNA damage responses in astrocytes alter the synthesis/release of these mediators. There is also strong experimental evidence that endothelial NO production contributes to glio-vascular coupling (Chen et al 2014;Girouard et al 2007;Longden and Nelson 2011;Ma et al 1996;Stobart et al 2013; Toth e t a l . 2 0 1 5 b ) .…”

Demonstration of impaired neurovascular coupling responses in TG2576 mouse model of Alzheimer’s disease using functional laser speckle contrast imaging

“…In healthy subject during times of increased neural activity, a homeostatic mechanism termed neurovascular coupling (functional hyperemia) matches the localized demand for glucose and oxygen with increased blood supply to ensure normal brain function. Neurovascular coupling is a feed-forward mechanism which requires the coordinated cellular interaction between neurons, astrocytes, pericytes, vascular endothelial and smooth muscle cells (Petzold and Murthy 2011;Stobart et al 2013;Wells et al 2015;Chen et al 2014;Tarantini et al 2016). A large body of evidence derived from both clinical and experimental studies demonstrate that aging significantly impairs neurovascular coupling responses, which likely contribute to cognitive decline in the elderly (Balbi et al 2015;Fabiani et al 2013;Sorond et al 2013;Tong et al 2012;Toth et al 2014;Zaletel et al 2005;Park et al 2007).…”