Cellular distribution of the neutral amino acid transporter subtype ASCT2 in mouse brain.

ASCT2 is an ASC (alanine-, serine-, cysteine-preferring) neutral amino acid exchanger that may regulate CNS function by transporting amino acid substrates including L-serine, L-cysteine, L-glutamine, L-glutamate and D-serine. Despite the potentially important role of ASCT2 in influencing metabolic and signaling functions of these amino acids in brain, there has been little description of its distribution in brain tissue. We employed a commercially available human ASCT2 antibody in immunohistochemistry studies in adult mouse brain and found a wide regional distribution for ASCT2 that was limited to dendrites labeled by anti-microtubule-associated protein-2 in cortex, hippocampus and striatum. No ASCT2 immunoreactivity was observed in areas labeled by antibodies against a neuronal cell body marker (NeuN), or either of the astrocyte markers, glial fibrillary acidic protein or S100beta. In cerebellum both Purkinje cell bodies and dendrites were positive for ASCT2 immunoreactivity. In support of a dendritic localization for ASCT2 in cortex, low affinity (K(T) > 1 mM), Na(+)-dependent D-serine and L-glutamine uptake characteristic of ASCT2-mediated transport was observed in P2 synaptosomal preparations. These results suggest that ASCT2 may be an important neuronal neutral amino acid transporter and highlight a discrepancy between findings of astrocyte ASCT2 function in tissue culture and brain in situ.

Coactivation of NMDA receptors by glutamate and D-serine induces dilation of isolated middle cerebral arteries.

N-methyl-D-aspartate (NMDA) receptors are glutamate-gated cation channels that mediate excitatory neurotransmission in the central nervous system. In addition to glutamate, NMDA receptors are also activated by coagonist binding of the gliotransmitter, D-serine. Neuronal NMDA receptors mediate activity-dependent blood flow regulation in the brain. Our objective was to determine whether NMDA receptors expressed by brain endothelial cells can induce vasodilation of isolated brain arteries. Adult mouse middle cerebral arteries (MCAs) were isolated, pressurized, and preconstricted with norepinephrine. N-methyl-D-aspartate receptor agonists, glutamate and NMDA, significantly dilated MCAs in a concentration-dependent manner in the presence of D-serine but not alone. Dilation was significantly inhibited by NMDA receptor antagonists, D-2-amino-5-phosphonopentanoate and 5,7-dichlorokynurenic acid, indicating a response dependent on NMDA receptor glutamate and D-serine binding sites, respectively. Vasodilation was inhibited by denuding the endothelium and by selective inhibition or genetic knockout of endothelial nitric oxide synthase (eNOS). We also found evidence for expression of the pan-NMDA receptor subunit, NR1, in mouse primary brain endothelial cells, and for the NMDA receptor subunit NR2C in cortical arteries in situ. Overall, we conclude that NMDA receptor coactivation by glutamate and D-serine increases lumen diameter in pressurized MCA in an endothelial and eNOS-dependent mechanism.

Vascular and cardiac effects of grape powder in the spontaneously hypertensive rat.

BACKGROUND: We previously reported that resveratrol, a polyphenol found in red grapes, attenuated changes in small artery geometry and stiffness, as well as cardiac hypertrophy and cardiac function in the spontaneously hypertensive rat (SHR). However, in addition to resveratrol, grapes contain a variety of bioactive polyphenols such as catechins, anthocyanins, and flavonoids. Therefore, we investigated the effects of grape consumption in SHR. METHODS: Wistar-Kyoto (WKY) rats and SHR were treated with freeze-dried grape powder for 10 weeks. Dilatory, geometry, and stiffness properties of mesenteric small arteries were assessed by pressurized myography. Left ventricular mass index and cardiac function were assessed by two-dimensional guided M-mode and pulse-wave Doppler echocardiography. RESULTS: Elevated blood pressure in SHR was associated with remodeling and impaired endothelium-dependent relaxation of small arteries. Augmented left ventricular mass index (reflecting hypertrophy) and diminished cardiac function were also evident in SHR. Although grape treatment failed to affect cardiac dysfunction, it elicited a significant reduction in blood pressure, improved arterial relaxation, increased vascular compliance, and attenuated cardiac hypertrophy. CONCLUSIONS: Treatment with whole grape powder conferred mild vascular and cardiac benefits in SHR. Therefore, dietary grape consumption may be a feasible and salutary adjunct to pharmacological treatment of human hypertension.

Custom astrocyte-mediated vasomotor responses to neuronal energy demand.

Astrocytes mediate either constriction or dilation of local brain arterioles in response to synaptic activity. Recent work indicates that the directionality of this response may be dictated by ambient oxygen levels.