Oxygen modulation of neurovascular coupling in the retina

Neurovascular coupling is a process through which neuronal activity leads to local increases in blood flow in the central nervous system. In brain slices, 100% O-2 has been shown to alter neurovascular coupling, suppressing activity-dependent vasodilation. However, in vivo, hyperoxia reportedly has no effect on blood flow. Resolving these conflicting findings is important, given that hyperoxia is often used in the clinic in the treatment of both adults and neonates, and a reduction in neurovascular coupling could deprive active neurons of adequate nutrients. Here we address this issue by examining neurovascular coupling in both ex vivo and in vivo rat retina preparations. In the ex vivo retina, 100% O-2 reduced light-evoked arteriole vasodilations by 3.9-fold and increased vasoconstrictions by 2.6-fold. In vivo, however, hyperoxia had no effect on light-evoked arteriole dilations or blood velocity. Oxygen electrode measurements showed that 100% O-2 raised pO(2) in the ex vivo retina from 34 to 548 mm Hg, whereas hyperoxia has been reported to increase retinal pO(2) in vivo to only similar to 53 mm Hg [Yu DY, Cringle SJ, Alder VA, Su EN (1994) Am J Physiol 267:H2498-H2507]. Replicating the hyperoxic in vivo pO(2) of 53 mm Hg in the ex vivo retina did not alter vasomotor responses, indicating that although O-2 can modulate neurovascular coupling when raised sufficiently high, the hyperoxia-induced rise in retinal pO(2) in vivo is not sufficient to produce a modulatory effect. Our findings demonstrate that hyperoxia does not alter neurovascular coupling in vivo, ensuring that active neurons receive an adequate supply of nutrients.