Neurovascular coupling is not mediated by potassium siphoning from glial cells

Abstract

Neuronal activity evokes localized changes in blood flow, a response termed neurovascular coupling. One widely recognized hypothesis of neurovascular coupling holds that glial cell depolarization evoked by neuronal activity leads to the release of K+ onto blood vessels (K+ siphoning) and to vessel relaxation. We now present two direct tests of this glial cell-K+ siphoning hypothesis of neurovascular coupling. Potassium efflux was evoked from glial cells in the rat retina by applying depolarizing current pulses to individual cells. Glial depolarizations as large as 100 mV produced no change in the diameter of adjacent arterioles. We also monitored light-evoked vascular responses in Kir4.1 knock-out mice, where functional Kir K+ channels are absent from retinal glial cells. The magnitude of light-evoked vasodilations was identical in Kir4.1 knock-out and wild-type animals. Contrary to the hypothesis, the results demonstrate that glial K+ siphoning in the retina does not contribute significantly to neurovascular coupling.

Figure 1.

Light stimulation and K⁺ increase, but not glial cell depolarization, evokes vasodilation. A, B, Micrographs showing a whole-cell-patched astrocyte contacting an arteriole. The patch pipette is seen on the left. A, IR-DIC image. B, Fluorescence image showing the Lucifer-filled astrocyte contacting the arteriole. Additional astrocytes coupled to the patched cell are also seen. Scale bar: (in B) A, B, 10 μm. C–E, Experiments performed on the arteriole and astrocyte in A and B. C, Light stimulation evokes vasodilation. D, The astrocyte is depolarized by injection of 1 nA current. The arteriole diameter does not change during astrocyte depolarization. E, Ejection of 9 mm K⁺ in the region shown in A evokes vasodilation. F, Ejection of 5 mm K⁺ solution but not control solution (3 mm) evokes vasodilation in a series of experiments. Asterisks indicate significant change, p < 0.05. Error bars indicate SEM.

Figure 2.

Light-evoked vasodilation is not reduced in Kir4.1 knock-out mice. A–D, IR-DIC micrographs of arterioles before (A, C) and during (B, D) light stimulation. A, B, Wild-type mouse. C, D, Kir4.1 knock-out mouse. Scale bar: (in D) A–D, 10 μm. Arrowheads indicate luminal diameter of arterioles. E–F, Light stimulation evokes vasodilation in wild-type and Kir4.1 knock-out mice. G, The amplitude of light-evoked vasodilation is identical in wild-type and Kir4.1 knock-out mice. H, Ba²⁺-sensitive current–voltage relationships of Müller cells in Kir4.1 WT and KO mice. Ba²⁺-sensitive inward current is completely absent in the KO cell. Error bars indicate SEM.