Functional hyperemia and mechanisms of neurovascular coupling in the retinal vasculature

Abstract

The retinal vasculature supplies cells of the inner and middle layers of the retina with oxygen and nutrients. Photic stimulation dilates retinal arterioles producing blood flow increases, a response termed functional hyperemia. Despite recent advances, the neurovascular coupling mechanisms mediating the functional hyperemia response in the retina remain unclear. In this review, the retinal functional hyperemia response is described, and the cellular mechanisms that may mediate the response are assessed. These neurovascular coupling mechanisms include neuronal stimulation of glial cells, leading to the release of vasoactive arachidonic acid metabolites onto blood vessels, release of potassium from glial cells onto vessels, and production and release of nitric oxide (NO), lactate, and adenosine from neurons and glia. The modulation of neurovascular coupling by oxygen and NO are described, and changes in functional hyperemia that occur with aging and in diabetic retinopathy, glaucoma, and other pathologies, are reviewed. Finally, outstanding questions concerning retinal blood flow in health and disease are discussed.

Figure 1

Anatomy of the ocular circulation. (A) Cut away drawing of the human eye along the superior–inferior axis through the optic nerve, showing the vascular supply to the retina and choroid. The retinal vessels are supplied by the central retinal artery. a, artery; v, vein; n, nerve. (B) Drawing showing the vasculature of the retina and choroid. Retinal arterioles and venules lie on the vitreal surface of the retina while capillary plexi lie in just beneath the surface and in the inner nuclear layer. Drawings by Dave Schumick from Anand-Apte and Hollyfield.

Figure 2

The functional hyperemia response in the retina. (A) Mean increase in primary arterial diameter to flicker stimulation in humans measured with the retinal vessel analyzer (RVA). Light onset is at time=0. ⁺P<0.001. From Nagel and Vilser. (B) Blood flow increase to flicker stimulation (black bar) measured at the rim of the optic disc in cats with laser Doppler flowmetry. From Riva et al. (C, D) Increase in primary arteriole diameter to flicker stimulation in a single trial in the rat measured with confocal microscopy. From Mishra and Newman. Shown are segments of a confocal line scan image before, during, and after stimulation (C) and the diameter of the arteriole (D). Scale bars in panel C, 25 μm and 100 ms.

Figure 3

Cytoplasmic Ca²⁺ increases in Müller glial cells of the rat retina. (A) Calcium increases in eight Müller cells. The frequency of Ca²⁺ transients in individual cells increases during flicker stimulation. (B) Mean Müller cell Ca²⁺ increase to flicker stimulation. The time course of the flicker stimulus is shown at the bottom in A and B, with the dashed lines indicating zero intensity. From Newman.

Figure 4

Calcium increases in glial cells evokes arteriole dilation in the rat retina. (A–C) Fluorescence images showing Ca²⁺ in glial cells. Photolysis of caged-Ca²⁺ within a glial cell (yellow dot in panel A) evokes an increase in glial Ca²⁺ that propagates through several glial cells adjacent to the blood vessels. (D–F) Infrared differential interference contrast images showing a higher magnification view of the small arteriole indicated by the yellow box in panel A. Each frame was acquired 0.5 s after the corresponding image above. Glial stimulation evokes arteriole dilation. Yellow arrowheads indicate luminal diameter of vessel. (G) Time course of the glial Ca²⁺ increase and vessel dilation. From Metea and Newman.

Figure 5

Summary of the arachidonic acid metabolite-mediated neurovascular coupling mechanism. ATP released from active neurons stimulates P2Y receptors on glial cells, resulting in the production of inositol 1,4,5 trisphosphate (IP₃) and the release of Ca²⁺ from internal stores. Glial Ca²⁺ activates phospholipase A₂ (PLA₂) resulting in the production of arachidonic acid (AA) from membrane phospholipids (MPL). Increased AA levels lead to the production of its metabolites, including the vasodilators epoxyeicosatrienoic acids (EETs) and prostaglandin E₂ (PGE₂) and the vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE). Under physiologic conditions, the vasodilators have a stronger influence on vessels than the vasoconstrictor, leading to vessel dilation and increased blood flow. Drawing by Anusha Mishra, unpublished.

Figure 6

Oxygen depresses flicker-induced vasodilation in the ex vivo whole-mount retina but not in vivo. (A) Under normoxic conditions, when the ex vivo rat retina is superfused with saline equilibrated with air, flicker stimulation dilates arterioles. (B) When exposed to 100% O₂, vasodilations are depressed, unmasking a flicker-induced vasoconstriction. (C) Summary of results in the ex vivo preparation. In 21% O₂, flicker-induced vasodilations (gray bars) and much larger than vasoconstrictions (black bars). In 100% O₂, vasodilations are depressed and vasoconstrictions enhanced. *P<0.005. (D, E) In the in vivo preparation, hyperoxia, induced by breathing 100% O₂, does not depress flicker-induced arteriole dilation (D) nor increase blood velocity (E). From Mishra et al.

Figure 7

Nitric oxide (NO) depresses light-induced vasodilation and unmasks vasoconstriction in the retina. (A) Flicker stimulation evokes arteriole dilation in the ex vivo whole-mount rat retina. When NO levels are raised by the addition of an NO donor (S-nitroso-N-acetylpenicillamine (SNAP)), the vasodilation is depressed, revealing a flicker-induced vasoconstriction. (B) When NO concentration is low, flicker evokes vasodilations but not vasoconstrictions. As NO levels are raised by an NO donor, the fraction of vessels displaying vasoconstrictions increases. From Metea and Newman.

Figure 8

Flicker-induced vasodilation is depressed in diabetic retinopathy. (A) Flicker-induced vasodilation of primary arterioles and venules is depressed in patients with type 1 diabetes. *P<0.02. From Pemp et al. (B, C) The depression of flicker-induced vasodilation in diabetic rats is reversed by the inducible nitric oxide synthase inhibitor aminoguanidine (AG). (B) Both acute AG administration (AG-IV) and chronic administration in drinking water (AG-H₂O) reverses the loss of flicker-induced vasodilation in diabetic animals in vivo. (C) Summary of results. *P<0.001. From Mishra and Newman.