A Critical Role for Astrocytes in Hypercapnic Vasodilation in Brain.
Howarth C., Sutherland B., Choi HB., Martin C., Lind BL., Khennouf L., LeDue JM., Pakan JMP., Ko RWY., Ellis-Davies G., Lauritzen M., Sibson NR., Buchan AM., MacVicar BA.
Cerebral blood flow (CBF) is controlled by arterial blood pressure, arterial CO2, arterial O2, and brain activity and is largely constant in the awake state. Although small changes in arterial CO2are particularly potent to change CBF (1 mmHg variation in arterial CO2changes CBF by 3%-4%), the coupling mechanism is incompletely understood. We tested the hypothesis that astrocytic prostaglandin E2(PgE2) plays a key role for cerebrovascular CO2reactivity, and that preserved synthesis of glutathione is essential for the full development of this response. We combined two-photon imaging microscopy in brain slices within vivowork in rats and C57BL/6J mice to examine the hemodynamic responses to CO2and somatosensory stimulation before and after inhibition of astrocytic glutathione and PgE2synthesis. We demonstrate that hypercapnia (increased CO2) evokes an increase in astrocyte [Ca2+]iand stimulates COX-1 activity. The enzyme downstream of COX-1 that synthesizes PgE2(microsomal prostaglandin E synthase-1) depends critically for its vasodilator activity on the level of glutathione in the brain. We show that, when glutathione levels are reduced, astrocyte calcium-evoked release of PgE2is decreased and vasodilation triggered by increased astrocyte [Ca2+]iin vitroand by hypercapniain vivois inhibited. Astrocyte synthetic pathways, dependent on glutathione, are involved in cerebrovascular reactivity to CO2Reductions in glutathione levels in aging, stroke, or schizophrenia could lead to dysfunctional regulation of CBF and subsequent neuronal damage.SIGNIFICANCE STATEMENTNeuronal activity leads to the generation of CO2, which has previously been shown to evoke cerebral blood flow (CBF) increases via the release of the vasodilator PgE2We demonstrate that hypercapnia (increased CO2) evokes increases in astrocyte calcium signaling, which in turn stimulates COX-1 activity and generates downstream PgE2production. We demonstrate that astrocyte calcium-evoked production of the vasodilator PgE2is critically dependent on brain levels of the antioxidant glutathione. These data suggest a novel role for astrocytes in the regulation of CO2-evoked CBF responses. Furthermore, these results suggest that depleted glutathione levels, which occur in aging and stroke, will give rise to dysfunctional CBF regulation and may result in subsequent neuronal damage.