Adenosine enhances functional activation of blood flow in cat optic nerve head during photic stimulation independently from nitric oxide

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Cerebrovascular reactivity during visual stimulation: Does hypnotizability matter?

Hypnotizability is a trait associated with several physiological correlates including cardiovascular control. The present study aimed to investigate the posterior cerebral artery flow velocity (PCAv) in basal closed eyes (B) and during visual stimulation (VS) conditions in med-highs and med-lows. Twenty-four healthy volunteers were submitted to the hypnotic assessment through the Stanford Hypnotic Susceptibility Scale, form A which classified 13 low-to-medium (med-lows) and 10 high-to-medium (med-highs) hypnotizable participants. One subject scoring 6 out of 12 was excluded from the comparisons between groups. Arterial blood pressure, heart rate, and partial pressure of end-tidal CO₂ were monitored during both B and VS conditions. Simultaneously, PCAv was assessed by transcranial Doppler. Cerebrovascular Reactivity (CVR) was computed as a percentage of the PCAv change occurring during VS with respect to B (ΔPCAv). During VS both groups increased their PCAv (mean ± SD: 7.9 ± 5.2 %) significantly with no significant group difference. However, among med-highs, CVR was negatively correlated with hypnotizability scores. Thus, higher hypnotizability may be associated with lower metabolic demand in response to VS only within med-highs hypnotizable participants.

Retinal Neurovascular Coupling in Diabetes

Neurovascular coupling, also termed functional hyperemia, is one of the physiological key mechanisms to adjust blood flow in a neural tissue in response to functional activity. In the retina, increased neural activity, such as that induced by visual stimulation, leads to the dilatation of retinal arterioles, which is accompanied by an immediate increase in retinal and optic nerve head blood flow. According to the current scientific view, functional hyperemia ensures the adequate supply of nutrients and metabolites in response to the increased metabolic demand of the neural tissue. Although the molecular mechanisms behind neurovascular coupling are not yet fully elucidated, there is compelling evidence that this regulation is impaired in a wide variety of neurodegenerative and vascular diseases. In particular, it has been shown that the breakdown of the functional hyperemic response is an early event in patients with diabetes. There is compelling evidence that alterations in neurovascular coupling precede visible signs of diabetic retinopathy. Based on these observations, it has been hypothesized that a breakdown of functional hyperemia may contribute to the retinal complications of diabetes such as diabetic retinopathy or macular edema. The present review summarizes the current evidence of impaired neurovascular coupling in patients with diabetes. In this context, the molecular mechanisms of functional hyperemia in health and disease will be covered. Finally, we will also discuss how neurovascular coupling may in future be used to monitor disease progression or risk stratification.

The Neurovascular Unit in Glaucomatous Neurodegeneration

Glaucoma is a neurodegenerative disease of the visual system and leading cause of blindness worldwide. The disease is associated with sensitivity to intraocular pressure (IOP), which over a large range of magnitudes stresses retinal ganglion cell (RGC) axons as they pass through the optic nerve head in forming the optic projection to the brain. Despite clinical efforts to lower IOP, which is the only modifiable risk factor for glaucoma, RGC degeneration and ensuing loss of vision often persist. A major contributor to failure of hypotensive regimens is the multifactorial nature of how IOP-dependent stress influences RGC physiology and structure. This stress is conveyed to the RGC axon through interactions with structural, glial, and vascular components in the nerve head and retina. These interactions promote pro-degenerative pathways involving biomechanical, metabolic, oxidative, inflammatory, immunological and vascular challenges to the microenvironment of the ganglion cell and its axon. Here, we focus on the contribution of vascular dysfunction and breakdown of neurovascular coupling in glaucoma. The vascular networks of the retina and optic nerve head have evolved complex mechanisms that help to maintain a continuous blood flow and supply of metabolites despite fluctuations in ocular perfusion pressure. In healthy tissue, autoregulation and neurovascular coupling enable blood flow to stay tightly controlled. In glaucoma patients evidence suggests these pathways are dysfunctional, thus highlighting a potential role for pathways involved in vascular dysfunction in progression and as targets for novel therapeutic intervention.

Retinal tissue oxygen tension and consumption during light flicker stimulation in rat

Light flicker stimulation has been shown to increase inner retinal oxygen metabolism and supply. The purpose of the study was to test the hypothesis that sustained light flicker stimulation of various durations alters the depth profile metrics of oxygen partial pressure in the retinal tissue (tPO₂) but not the outer retinal oxygen consumption rate (QO₂). In 17 rats, tPO₂ depth profiles were derived by phosphorescence lifetime imaging after intravitreal injection of an oxyphor. tPO₂ profile metrics, including mean inner retinal tPO₂, maximum outer retinal tPO₂ and minimum outer retinal tPO₂ were determined. QO₂ was calculated using a one-dimensional oxygen diffusion model. Data were acquired at baseline (constant light illumination) and during light flicker stimulation at 10 Hz under the same mean illumination levels, and differences between values obtained during flicker and baseline were calculated. None of the tPO₂ profile metrics or QO₂ differences depended on the duration of light flicker stimulation (R² ≤ 0.03). No significant change in any of the tPO₂ profile metrics was detected with light flicker compared with constant light (P ≥ 0.08). Light flicker decreased QO₂ from 0.53 ± 0.29 to 0.38 ± 0.30 mL O₂/(min*100 gm), a reduction of 28% (P = 0.02). The retinal compensatory responses to the physiologic challenge of light flicker stimulation were effective in maintaining the levels of oxygen at or near baseline in the inner retina. Oxygen availability to the inner retina during light flicker may also have been enhanced by the decrease in QO₂.

Impact of combined hormonal contraceptives on vessels functionality

PURPOSE

To evaluate the dynamic and static retinal vascular functionality in young females using combined hormonal contraceptive (CHC).

METHODS

Thirty-eight consecutive young female subjects were enrolled in this study between January 2015 and December 2015. Subjects were divided in two groups: CHC group, defined as CHC use for ≥6 months, and control group, defined as no current and prior CHC use. Participants underwent a dynamic and static retinal vessel analysis using the Dynamic Vessel Analyzer (DVA, Imedos, Jena, Germany).

RESULTS

Seventeen subjects continuously took CHC for 54.6 ± 29.3 months, while 21 subjects belonged to control group. No difference was found between the CHC and control groups for age (p = 0.1), smoking status (p = 0.6), and systolic (p = 0.3) and diastolic (p = 0.1) blood pressure. With regard to dynamic analysis, women taking CHC exhibited a marked significant vasoconstriction following flicker stimulation in comparison with control group (-2.43 ± 2.5 vs 0.63 ± 2.1, respectively; p = 0.0002). No significant difference was observed between groups for mean arterial (p = 0.2) and venous dilatations (p = 0.3), arteriovenous ratio (p = 0.09), central retinal artery equivalent (p = 0.4), and central retinal venous equivalent (p = 0.5).

CONCLUSIONS

CHC may affect vessel reactivity to flicker light by increasing arteries constriction. This may reflect systemic changes in vascular functionality in subjects using CHC. Moreover, CHC should be considered as a confounding bias in studies involving DVA.

Nitric oxide and the cardiovascular system

Nitric oxide (NO) generated by endothelial cells to relax vascular smooth muscle is one of the most intensely studied molecules in the past 25 years. Much of what is known about NO regulation of NO is based on blockade of its generation and analysis of changes in vascular regulation. This approach has been useful to demonstrate the importance of NO in large scale forms of regulation but provides less information on the nuances of NO regulation. However, there is a growing body of studies on multiple types of in vivo measurement of NO in normal and pathological conditions. This discussion will focus on in vivo studies and how they are reshaping the understanding of NO's role in vascular resistance regulation and the pathologies of hypertension and diabetes mellitus. The role of microelectrode measurements in the measurement of [NO] will be considered because much of the controversy about what NO does and at what concentration depends upon the measurement methodology. For those studies where the technology has been tested and found to be well founded, the concept evolving is that the stresses imposed on the vasculature in the form of flow-mediated stimulation, chemicals within the tissue, and oxygen tension can cause rapid and large changes in the NO concentration to affect vascular regulation. All these functions are compromised in both animal and human forms of hypertension and diabetes mellitus due to altered regulation of endothelial cells and formation of oxidants that both damage endothelial cells and change the regulation of endothelial nitric oxide synthase.

Response of inner retinal oxygen extraction fraction to light flicker under normoxia and hypoxia in rat

PURPOSE

Oxygen extraction fraction (OEF), defined by the ratio of oxygen metabolism (MO2) to delivery (DO2), determines the level of compensation of MO2 by DO2. In the current study, we tested the hypothesis that inner retinal OEF remains unchanged during light flicker under systemic normoxia and hypoxia in rats due to the matching of MO2 and DO2.

METHODS

Retinal vascular oxygen tension (PO2) measurements were obtained in 10 rats by phosphorescence lifetime imaging. Inner retinal OEF was derived from vascular PO2 based on Fick's principle. Measurements were obtained before and during light flicker under systemic normoxia and hypoxia. The effects of light flicker and systemic oxygenation on retinal vascular PO2 and OEF were determined by ANOVA.

RESULTS

During light flicker, retinal venous PO2 decreased (P < 0.01, N = 10), while inner retinal OEF increased (P = 0.02). Under hypoxia, retinal arterial and venous PO2 decreased (P < 0.01), while OEF increased (P < 0.01). The interaction effect was not significant on OEF (P = 0.52), indicating the responses of OEF to light flicker were similar under normoxia and hypoxia. During light flicker, OEF increased from 0.46 ± 0.13 to 0.50 ± 0.11 under normoxia, while under hypoxia, OEF increased from 0.67 ± 0.16 to 0.74 ± 0.14.

CONCLUSIONS

Inner retinal OEF increased during light flicker, indicating the relative change in DO2 is less than that in MO2 in rats under systemic normoxia and hypoxia. Inner retinal OEF is a potentially useful parameter for assessment of the relative changes of MO2 and DO2 under physiologic and pathologic conditions.

Functional hyperemia and mechanisms of neurovascular coupling in the retinal vasculature

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.

Is the real in vivo nitric oxide concentration pico or nano molar? Influence of electrode size on unstirred layers and NO consumption

OBJECTIVE

There is a debate if the [NO] required to influence vascular smooth muscle is below 50 nM or much higher. Electrodes with 30 μm and larger diameter report [NO] below 50 nM, whereas those with diameters of <10-12 μm report hundreds of nM. This study examined how size of electrodes influenced [NO] measurement due to NO consumption and unstirred layer issues.

METHODS

Electrodes were 2 mm disk, 30 μm × 2 mm carbon fiber, and single 7 μm diameter carbon fiber within open tip microelectrode, and exposed 7 μm carbon fiber of ~15 μm to 2 mm length.

RESULTS

All electrodes demonstrated linear calibrations with sufficient stirring. As stirring slowed, 30 μm and 2 mm electrodes reported much lower [NO] due to unstirred layers and high NO consumption. The three 7 μm microelectrodes had minor stirring issues. With limited stirring with NO present, 7 μm open tip microelectrodes advanced toward 30 μm and 2 mm electrodes experienced dramatically decreased current within 10-50 μm of the larger electrodes due to high NO consumption. None of the 7 μm microelectrodes interacted.

CONCLUSIONS

The data indicate large electrodes underestimate [NO] due to excessive NO consumption under conditions where unstirred layers are unavoidable and true microelectrodes are required for valid measurements.

Assessment of retinal and choroidal blood flow changes using laser Doppler flowmetry in rats

PURPOSE

A new noninvasive laser Doppler flowmetry (LDF) probe (one emitting fiber surrounded by a ring of eight collecting fibers, 1-mm interaxis distance) was tested for its sensitivity to assess the retinal/choroidal blood flow variations in response to hypercapnia, hyperoxia, diverse vasoactive agents and following retinal arteries photocoagulation in the rat.

MATERIALS AND METHODS

After pupil dilation, a LDF probe was placed in contact to the cornea of anesthetized rats in the optic axis. Hypercapnia and hyperoxia were induced by inhalation of CO(2) (8% in medical air) and O(2) (100%) while pharmacological agents were injected intravitreously. The relative contribution of the choroidal circulation to the LDF signal was estimated after retinal artery occlusion by photocoagulation.

RESULTS

Blood flow was significantly increased by hypercapnia (18%), adenosine (14%) and sodium nitroprusside (16%) as compared to baseline values while it was decreased by hyperoxia (-8%) and endothelin-1 (-11%). Photocoagulation of retinal arteries significantly decreased blood flow level (-45%).

CONCLUSIONS

Although choroidal circulation most likely contributes to the LDF signal in this setting, the results demonstrate that LDF represents a suitable in vivo noninvasive technique to monitor online relative reactivity of retinal perfusion to metabolic or pharmacological challenge. This technique could be used for repeatedly assessing blood flow reactivity in rodent models of ocular diseases.

Chorioretinal vascular oxygen tension changes in response to light flicker

PURPOSE

To investigate oxygen tension (P(O2)) changes in the retinal and choroidal vasculatures in response to visual stimulation by light flicker.

METHODS

A previously developed optical section phosphorescence imaging system was used to measure P(O2) separately in the retinal veins, arteries, and capillaries and in the choroid before and during light flicker. Imaging was performed in rats during light flicker at frequencies between 0 and 14 Hz. Light flicker-induced changes in the chorioretinal vasculature P(O2) and arteriovenous P(O2) differences were determined. Retinal arterial and venous P(O2) were measured along blood vessels as a function of the distance from the optic nerve head.

RESULTS

Retinal arterial P(O2) and arteriovenous P(O2) differences increased with increasing light flicker at frequencies up to 10 Hz, after which no further increase was observed. Significant increases in retinal arterial P(O2) (P = 0.009; n =10) and in retinal capillary P(O2) (P = 0.04, n = 10) were measured in response to light flicker at 10 Hz. Retinal arteriovenous P(O2) differences during light flicker were significantly greater than differences before light flicker (P = 0.01; n = 10). Retinal arterial P(O2) decreased significantly with increased distance from the optic nerve head (P < or = 0.004), whereas retinal venous P(O2) remained relatively unchanged (P > or= 0.4).

CONCLUSIONS

Measurement of changes in the chorioretinal vasculature P(O2) can potentially advance the understanding of oxygen dynamics in challenged physiological states and in animal models of human retinal diseases.

Visually evoked hemodynamical response and assessment of neurovascular coupling in the optic nerve and retina

The retina and optic nerve are both optically accessible parts of the central nervous system. They represent, therefore, highly valuable tissues for studies of the intrinsic physiological mechanism postulated more than 100 years ago by Roy and Sherrington, by which neural activity is coupled to blood flow and metabolism. This article describes a series of animal and human studies that explored the changes in hemodynamics and oxygenation in the retina and optic nerve in response to increased neural activity, as well as the mechanisms underlying these changes. It starts with a brief review of techniques used to assess changes in neural activity, hemodynamics, metabolism and tissue concentration of various potential mediators and modulators of the coupling. We then review: (a) the characteristics of the flicker-induced hemodynamical response in different regions of the eye, starting with the optic nerve, the region predominantly studied; (b) the effect of varying the stimulus parameters, such as modulation depth, frequency, luminance, color ratio, area of stimulation, site of measurement and others, on this response; (c) data on activity-induced intrinsic reflectance and functional magnetic resonance imaging signals from the optic nerve and retina. The data undeniably demonstrate that visual stimulation is a powerful modulator of retinal and optic nerve blood flow. Exploring the relationship between vasoactivity and metabolic changes on one side and corresponding neural activity changes on the other confirms the existence of a neurovascular/neurometabolic coupling in the neural tissue of the eye fundus and reveals that the mechanism underlying this coupling is complex and multi-factorial. The importance of fully exploiting the potential of the activity-induced vascular changes in the assessment of the pathophysiology of ocular diseases motivated studies aimed at identifying potential mediators and modulators of the functional hyperemia, as well as conditions susceptible to alter this physiological response. Altered hemodynamical responses to flicker were indeed observed during a number of physiological and pharmacological interventions and in a number of clinical conditions, such as essential systemic hypertension, diabetes, ocular hypertension and early open-angle glaucoma. The article concludes with a discussion of key questions that remain to be elucidated to increase our understanding of the physiology of ocular functional hyperemia and establish the importance of assessing the neurovascular coupling in the diagnosis and management of optic nerve and retinal diseases.

Effects of adenosine on optic nerve head circulation in rabbits

This study was performed to determine whether intravitreal or intravenous adenosine can alter the microcirculation in the optic nerve head (ONH) of rabbits. Capillary blood flow in the ONH was measured serially with a laser speckle tissue analyser for 2 hr after the intravitreal (0.1, 1.0 and 10 nmol) or intravenous (0.2 and 0.6 mg kg(-1)min) injections of adenosine. In addition, the effect of specific adenosine A(1) and A(2a) antagonists and an adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel blockers on the adenosine-induced changes on the ONH blood flow was analysed. Intravitreal adenosine increased the capillary blood flow in the ONH in a dose-dependent manner, while intravenous adenosine had no effect. Co-administration of the specific adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nmol) significantly suppressed (P=0.006, ANOVA) the increase in the ONH blood flow induced by adenosine (10 nmol). The specific A(2a) receptor antagonist, 8-(3-chlorostyryl) caffeine (CSC, 10 nmol), had a weak effect in inhibiting the increase but the change was not significant (P=0.08, ANOVA). Both specific A(1) and A(2a) receptor agonists, N(6)-cyclopentyladenosine (CPA, 10 nmol) and 2-p-(2-carboxyethyl) phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS-21680, 10 nmol), increased the ONH tissue blood flow (P<0.01, ANOVA). Glibenclamide (10 nmol), a selective K(ATP) channels antagonist, suppressed the increase of ONH blood flow induced by 10 nmol adenosine significantly (P<0.001, ANOVA). On the other hand, 10 nmol of 8-Br-cAMP, a cAMP analog, failed to enhance the capillary blood flow in the ONH. These results indicate that adenosine increases the capillary blood flow in the ONH of rabbits, and it acts through A(1) and A(2a) receptors from the ablumenal side where pericytes are located. Activation of K(ATP) channels is strongly related to the mechanism of adenosine-induced increase in ONH blood flow, while the participation of adenylate cyclase is less likely.