The α₁B -adrenoceptor subtype mediates adrenergic vasoconstriction in mouse retinal arterioles with damaged endothelium

Adrenoceptors in the Eye - Physiological and Pathophysiological Relevance

The autonomic nervous system plays a crucial role in the innervation of the eye. Consequently, it comes as no surprise that catecholamines and their corresponding receptors have been extensively studied and characterized in numerous ocular structures, including the cornea, conjunctiva, lacrimal gland, trabecular meshwork, uvea, and retina. These investigations have unveiled substantial clinical implications, particularly in the context of treating glaucoma, a progressive neurodegenerative disorder responsible for irreversible vision loss on a global scale. The primary therapeutic approaches for glaucoma frequently involve the modulation of α1-, α2-, and β-adrenoceptors, making them pivotal targets. In this chapter, we offer a comprehensive overview of the expression, distribution, and functional roles of adrenoceptors within various components of the eye and its associated structures. Additionally, we delve into the pivotal role of adrenoceptors in the pathophysiology of glaucoma. Furthermore, we provide a concise historical perspective on adrenoceptor research, examine the distinct contributions of individual adrenoceptor subtypes to the treatment of various ocular conditions, and propose potential future avenues of exploration in this field.

Fetoplacental vascular effects of maternal adrenergic antihypertensive and cardioprotective medications in pregnancy

Maternal cardiovascular diseases, including hypertension and cardiac conditions, are associated with poor fetal outcomes. A range of adrenergic antihypertensive and cardioprotective medications are often prescribed to pregnant women to reduce major maternal complications during pregnancy. Although these treatments are not considered teratogenic, they may have detrimental effects on fetal growth and development, as they cross the fetoplacental barrier, and may contribute to placental vascular dysregulation. Medication risk assessment sheets do not include specific advice to clinicians and women regarding the safety of these therapies for use in pregnancy and the potential off-target effects of adrenergic medications on fetal growth have not been rigorously conducted. Little is known of their effects on the fetoplacental vasculature. There is also a dearth of knowledge on adrenergic receptor activation and signalling within the endothelium and vascular smooth muscle cells of the human placenta, a vital organ in the maintenance of adequate blood flow to satisfy fetal growth and development. The fetoplacental circulation, absent of sympathetic innervation, and unique in its reliance on endocrine, paracrine and autocrine influence in the regulation of vascular tone, appears vulnerable to dysregulation by adrenergic antihypertensive and cardioprotective medications compared with the adult peripheral circulation. This semi-systematic review focuses on fetoplacental vascular expression of adrenergic receptors, associated cell signalling mechanisms and predictive consequences of receptor activation/deactivation by antihypertensive and cardioprotective medications.

The short-term effects of blood donation on the ocular parameters including blood flow of the retina and choroid in healthy people using OCT- angiography

BACKGROUND

To investigate the short-term effects of blood donation on the morphology and blood flow of the retina and choroid in healthy people using optical coherence tomography angiography (OCTA).

METHODS

Twenty-eight healthy blood donors (56 eyes) who participated in the 200 ml voluntary blood donation between March 2, 2021 and January 20, 2022 were included. The best corrected visual acuity (BCVA), systolic (SBP) and diastolic blood pressure (DBP), intraocular pressure (IOP), subfoveal choroid thickness (SFCT), retinal thickness (RT), retinal superficial vascular density (SVD), deep vascular density (DVD) and foveal avascular were a (FAZ) were measured and statistically analysed 10 min before, 30 min and 24 h after the blood donation.

RESULTS

The 200 ml blood donation could cause significant IOP reduction at 24 h (P = 0.006), which was negatively correlated with SBP (r = -0.268, P = 0.046), while SBP, DBP, or ocular perfusion pressure were not affected (> 0.05). Moreover, no significant difference existed in the OCT and OCTA indexes, including SFCT, RT, SVD, DVD, and FAZ, before and after the 200 ml blood donation (P > 0.05). The visual acuity was not affected either (P > 0.05).

CONCLUSIONS

The 200 ml blood donation was noted to be associated with statistically significant IOP reduction at 24 h, while SBP, DBP, or OPP was not affected. The blood flow of the retina and choroid or the visual acuity did not change significantly after the blood donation. Larger studies with different volumes of blood donation were needed to further analysis the effect of blood donation on ocular parameters.

β-Adrenoreceptors as Therapeutic Targets for Ocular Tumors and Other Eye Diseases-Historical Aspects and Nowadays Understanding

β-adrenoreceptors (ARs) are members of the superfamily of G-protein-coupled receptors (GPCRs), and are activated by catecholamines, such as epinephrine and norepinephrine. Three subtypes of β-ARs (β₁, β₂, and β₃) have been identified with different distributions among ocular tissues. Importantly, β-ARs are an established target in the treatment of glaucoma. Moreover, β-adrenergic signaling has been associated with the development and progression of various tumor types. Hence, β-ARs are a potential therapeutic target for ocular neoplasms, such as ocular hemangioma and uveal melanoma. This review aims to discuss the expression and function of individual β-AR subtypes in ocular structures, as well as their role in the treatment of ocular diseases, including ocular tumors.

Methods to measure blood flow and vascular reactivity in the retina

Disturbances of retinal perfusion are involved in the onset and maintenance of several ocular diseases, including diabetic retinopathy, glaucoma, and retinal vascular occlusion. Hence, knowledge on ocular vascular anatomy and function is highly relevant for basic research studies and for clinical judgment and treatment. The retinal vasculature is composed of the superficial, intermediate, and deep vascular layer. Detection of changes in blood flow and vascular diameter especially in smaller vessels is essential to understand and to analyze vascular diseases. Several methods to evaluate blood flow regulation in the retina have been described so far, but no gold standard has been established. For highly reliable assessment of retinal blood flow, exact determination of vessel diameter is necessary. Several measurement methods have already been reported in humans. But for further analysis of retinal vascular diseases, studies in laboratory animals, including genetically modified mice, are important. As for mice, the small vessel size is challenging requiring devices with high optic resolution. In this review, we recapitulate different methods for retinal blood flow and vessel diameter measurement. Moreover, studies in humans and in experimental animals are described.

Delayed retinal vein recovery responses indicate both non-adaptation to stress as well as increased risk for stroke: the SABPA study

OBJECTIVES

Low or high sympatho-adrenal-medullary axis (SAM) and hypothalamic-pituitary-adrenal axis (HPA) dysregulation reflect chronic stress. Retinal vessel dynamics may relate to SAM, HPA activity and stroke risk. Our objectives were therefore to assess the relationships between retinal vessel, SAM and HPA responses, and to determine stroke risk.

METHODS

A prospective bi-ethnic gender cohort (n = 275, 45 ± 9 years) was included. Urine/serum/saliva samples for SAM [norepinephrine:creatinine ratio (u-NE)] and HPA [adrenocorticotrophic hormone (ACTH), cortisol] were obtained at baseline, three-year follow up and upon flicker light-induced provocation. Diastolic ocular perfusion pressure was measured as a marker of hypo-perfusion. Retinal arterial narrowing and venous widening calibres were quantified from digital images in the mydriatic eye. A validated stress and stroke risk score was applied.

RESULTS

An interaction term was fitted for venous dilation in u-NE tertiles (p ≤ 0.05) and not in u-NE median/quartiles/quintiles. Independent of race or gender, tertile 1 (low u-NE) had a 112% increase in u-NE, decreases in cortisol, and no changes in ACTH over three years (positive feedback). Tertile 3 (high u-NE) contradictorily had decreases in u-NE and cortisol, and increases in ACTH (negative feedback). In tertile 1, reduced arterial dilation, and faster arterial vasoconstriction and narrowing were related to higher SAM activity and hypo-perfusion (p ≤ 0.05), whereas delayed venous dilation, recovery and widening were related to cortisol hypo-secretion (p ≤ 0.05). In tertile 1, delayed venous recovery responses predicted stress and stroke risk [odds ratio 4.8 (1.2-19.6); p = 0.03]. These associations were not found in u-NE tertiles 2 and 3.

CONCLUSIONS

In response to low norepinephrine, a reflex increase in SAM activity occurred, enhancing arterial vasoconstriction and hypo-perfusion. Concomitant HPA dysregulation attenuated retinal vein vasoactivity and tone, reflecting delayed vein recovery responses and non-adaptation to stress. These constrained vein recovery responses are indicative of increased chronic stress and stroke risk.

The Role of Adrenoceptors in the Retina

The retina is a part of the central nervous system, a thin multilayer with neuronal lamination, responsible for detecting, preprocessing, and sending visual information to the brain. Many retinal diseases are characterized by hemodynamic perturbations and neurodegeneration leading to vision loss and reduced quality of life. Since catecholamines and respective bindings sites have been characterized in the retina, we systematically reviewed the literature with regard to retinal expression, distribution and function of alpha₁ (α₁)-, alpha₂ (α₂)-, and beta (β)-adrenoceptors (ARs). Moreover, we discuss the role of the individual adrenoceptors as targets for the treatment of retinal diseases.

Elevated Intraocular Pressure Causes Abnormal Reactivity of Mouse Retinal Arterioles

Objective

Glaucoma is a leading cause of severe visual impairment and blindness. Although high intraocular pressure (IOP) is an established risk factor for the disease, the role of abnormal ocular vessel function in the pathophysiology of glaucoma gains more and more attention. We tested the hypothesis that elevated intraocular pressure (IOP) causes vascular dysfunction in the retina.

Methods

High IOP was induced in one group of mice by unilateral cauterization of three episcleral veins. The other group received sham surgery only. Two weeks later, retinal vascular preparations were studied by video microscopy in vitro. Reactive oxygen species (ROS) levels and expression of hypoxia markers and of prooxidant and antioxidant redox genes as well as of inflammatory cytokines were determined.

Results

Strikingly, responses of retinal arterioles to stepwise elevation of perfusion pressure were impaired in the high-IOP group. Moreover, vasodilation responses to the endothelium-dependent vasodilator, acetylcholine, were markedly reduced in mice with elevated IOP, while no differences were seen in response to the endothelium-independent nitric oxide donor, sodium nitroprusside. Remarkably, ROS levels were increased in the retinal ganglion cell layer including blood vessels. Expression of the NADPH oxidase isoform, NOX2, and of the inflammatory cytokine, TNF-α, was increased at the mRNA level in retinal explants. Expression of NOX2, but not of the hypoxic markers, HIF-1α and VEGF-A, was increased in the retinal ganglion cell layer and in retinal blood vessels at the protein level.

Conclusion

Our data provide first-time evidence that IOP elevation impairs autoregulation and induces endothelial dysfunction in mouse retinal arterioles. Oxidative stress and inflammation, but not hypoxia, appear to be involved in this process.

Neural activity regulates autoimmune diseases through the gateway reflex

The brain, spinal cord and retina are protected from blood-borne compounds by the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB) and blood-retina barrier (BRB) respectively, which create a physical interface that tightly controls molecular and cellular transport. The mechanical and functional integrity of these unique structures between blood vessels and nervous tissues is critical for maintaining organ homeostasis. To preserve the stability of these barriers, interplay between constituent barrier cells, such as vascular endothelial cells, pericytes, glial cells and neurons, is required. When any of these cells are defective, the barrier can fail, allowing blood-borne compounds to encroach neural tissues and cause neuropathologies. Autoimmune diseases of the central nervous system (CNS) and retina are characterized by barrier disruption and the infiltration of activated immune cells. Here we review our recent findings on the role of neural activity in the regulation of these barriers at the vascular endothelial cell level in the promotion of or protection against the development of autoimmune diseases. We suggest nervous system reflexes, which we named gateway reflexes, are fundamentally involved in these diseases. Although their reflex arcs are not completely understood, we identified the activation of specific sensory neurons or receptor cells to which barrier endothelial cells respond as effectors that regulate gateways for immune cells to enter the nervous tissue. We explain this novel mechanism and describe its role in neuroinflammatory conditions, including models of multiple sclerosis and posterior autoimmune uveitis.

Hyperglycemia Enhances Constriction of Retinal Venules via Activation of the Reverse-Mode Sodium-Calcium Exchanger

Diabetes is associated with hyperglycemia and impairment of retinal microvascular function. However, the impact of hyperglycemia on retinal venular constriction remains unknown. We examined retinal venular responsiveness to endogenous vasoconstrictors and the contribution of the reverse-mode sodium-calcium exchanger (NCX) to these responses during hyperglycemia. Retinal venules were isolated from pigs with streptozocin-induced diabetes (2 weeks, in vivo hyperglycemia) and age-matched control pigs for vasoreactivity and molecular studies. For in vitro hyperglycemia, vessels from euglycemic pigs were exposed to high glucose (25 mmol/L) for 2 h, and 5 mmol/L glucose served as the control. Constrictions of venules from euglycemic pigs to endothelin-1 (ET-1), thromboxane analog U46619, and norepinephrine were mediated by ET_A, thromboxane, and α₂-adrenergic receptors, respectively, and were insensitive to reverse-mode NCX blockade (KB-R7943). In vivo hyperglycemia enhanced these vasoconstrictions without altering respective receptor mRNA expression. Similarly, in vitro hyperglycemia augmented venular constrictions. Enhanced vasoconstrictions during hyperglycemia were prevented by KB-R7943, while mRNA expression of venular NCX isoforms was unaltered. In vivo hyperglycemia increased vitreous levels of ET-1 but not thromboxane B₂ In conclusion, both in vitro and in vivo hyperglycemia enhance retinal venular responses to endogenous vasoconstrictors by activating reverse-mode NCX. Therapies targeting this vascular molecule may alleviate retinal complications during diabetes.

Photopic light-mediated down-regulation of local α1A-adrenergic signaling protects blood-retina barrier in experimental autoimmune uveoretinitis

We have reported the gateway reflex, which describes specific neural activations that regulate immune cell gateways at specific blood vessels in the central nervous system (CNS). Four types of gateway reflexes exist, all of which induce alterations in endothelial cells at specific vessels of the blood-brain barrier followed by inflammation in the CNS in the presence of CNS-autoreactive T cells. Here we report a new gateway reflex that suppresses the development of retinal inflammation by using an autoreactive T cell-mediated ocular inflammation model. Exposure to photopic light down-regulated the adrenoceptor pathway to attenuate ocular inflammation by suppressing breaching of the blood-retina barrier. Mechanistic analysis showed that exposure to photopic light down-regulates the expression of α_1A-adrenoceptor (α_1AAR) due to high levels of norepinephrine and epinephrine, subsequently suppressing inflammation. Surgical ablation of the superior cervical ganglion (SCG) did not negate the protective effect of photopic light, suggesting the involvement of retinal noradrenergic neurons rather than sympathetic neurons from the SCG. Blockade of α_1AAR signaling under mesopic light recapitulated the protective effect of photopic light. Thus, targeting regional adrenoceptor signaling might represent a novel therapeutic strategy for autoimmune diseases including those that affect organs separated by barriers such as the CNS and eyes.

Retinal arteriole reactivity in mice lacking the endothelial nitric oxide synthase (eNOS) gene

Dysfunctional vascular endothelial nitric oxide synthase (eNOS) has been proposed to play a main pathophysiological role in various ocular diseases. The aim of the present study was to test the hypothesis that the chronic lack of eNOS impairs endothelium-dependent vasodilation in retinal arterioles. The relevance of eNOS for mediating vascular responses was studied in retinal vascular preparations from eNOS-deficient mice (eNOS-/-) and wild-type controls in vitro. Changes in luminal diameter in response to vasoactive agents were measured by videomicroscopy. The thromboxane mimetic, U46619, induced similar concentration-dependent constriction of retinal arterioles in eNOS-/- and wild-type mice. Responses to the endothelium-independent vasodilator, nitroprusside, did not differ between both mouse genotypes, either. In contrast, responses to the endothelium-dependent vasodilator, acetylcholine, were blunted in eNOS-/- mice. Non-isoform-selective blockade of either nitric oxide synthase (NOS) or cyclooxygenase (COX) alone did not affect responses to acetylcholine. However, combined blockade of both enzyme families markedly attenuated cholinergic vasodilation. Also, combined blockade of COX and neuronal NOS (nNOS) blunted acetylcholine-induced vasodilation, while combined COX and inducible NOS (iNOS) inhibition had no effect. Simultaneous NOS and COX-1 blockade did not affect cholinergic vasodilation, whereas combined NOS and COX-2 inhibition markedly reduced vasodilation to acetylcholine. These findings are the first to demonstrate that the chronic lack of eNOS is associated with moderate endothelial dysfunction in retinal arterioles. However, eNOS-deficiency is partially compensated by nNOS and COX-2 metabolites, which are reciprocally regulated.

Role of α1-adrenoceptor subtypes on corneal epithelial thickness and cell proliferation in mice

Adrenergic stimuli are important for corneal epithelial structure and healing. The purpose of the present study was to examine the hypothesis that the lack of a single α₁-adrenoceptor (α₁-AR) subtype affects corneal epithelial thickness and cell proliferation. Expression levels of α₁-AR mRNA were determined in mouse cornea using real-time PCR. In mice devoid of one of the three α₁-AR subtypes (α_1A-AR^-/-, α_1B-AR^-/-, α_1D-AR^-/-) and in wild-type controls, thickness of individual corneal layers, the number of epithelial cell layers, and average epithelial cell size were determined in cryosections. Endothelial cell density and morphology were calculated in corneal explants, and epithelial cell proliferation rate was determined with immunofluorescence microscopy. Moreover, the ultrastructure of the corneal epithelium was examined by transmission electron microscopy. Messenger RNA for all three α₁-AR subtypes was expressed in whole cornea and in corneal epithelium from wild-type mice with a rank order of abundance of α_1A ≥ α_1B > α_1D. In contrast, no α₁-AR mRNA was detected in the stroma, and only α_1B-AR mRNA was found in the Descemet endothelial complex. Remarkably, corneal epithelial thickness and mean epithelial cell size were reduced in α_1A-AR^-/- mice. Our findings suggest that the α_1A-AR exerts growth effects in mouse corneal epithelial cells.

Preparation Steps for Measurement of Reactivity in Mouse Retinal Arterioles Ex Vivo

Vascular insufficiency and alterations in normal retinal perfusion are among the major factors for the pathogenesis of various sight-threatening ocular diseases, such as diabetic retinopathy, hypertensive retinopathy, and possibly glaucoma. Therefore, retinal microvascular preparations are pivotal tools for physiological and pharmacological studies to delineate the underlying pathophysiological mechanisms and to design therapies for the diseases. Despite the wide use of mouse models in ophthalmic research, studies on retinal vascular reactivity are scarce in this species. A major reason for this discrepancy is the challenging isolation procedures owing to the small size of these retinal blood vessels, which is ~ ≤ 30 µm in luminal diameter. To circumvent the problem of direct isolation of these retinal microvessels for functional studies, we established an isolation and preparation technique that enables ex vivo studies of mouse retinal vasoactivity under near-physiological conditions. Although the present experimental preparations will specifically refer to the mouse retinal arterioles, this methodology can readily be employed to microvessels from rats.

Are blood vessels a target to treat lower urinary tract dysfunction?

Bladder dysfunction is common in the general population (Stewart et al. 2010) and even more so among patients seeing a physician for any reason (Goepel et al. 2002). It often manifests as lower urinary tract symptoms (LUTS), a term originally coined to describe voiding and storage symptoms in men with benign prostatic hyperplasia (BPH) but now more universally used to describe any type of voiding and storage symptoms in both sexes. Studies into possible causes of urinary bladder dysfunction have long focused on detrusor smooth muscle cells (Turner and Brading 1999). More recently, it became clear that several other types of cells and organs contribute to regulating detrusor smooth muscle function. These include the urothelium (Andersson and McCloskey 2014; Michel 2015), afferent nerves (Michel and Igawa 2015; Yoshimura et al. 2014b), and the central and autonomic nervous systems (Fowler and Griffiths 2010; Yoshimura et al. 2014a). Alterations in any of these may at least partly be responsible for detrusor dysfunction and, accordingly, be potential targets for the treatment of bladder dysfunction. As highlighted by an article in this issue of Naunyn-Schmiedeberg's Archives of Pharmacology (Bayrak et al. 2015), there is an additional suspect, the bladder vasculature. This article will discuss the currently available experimental and clinical evidence for a role of the vasculature in causing bladder dysfunction, and how existing and emerging treatments may modulate bladder function by acting on blood vessels. Due to a similarity in concept, data on prostate perfusion will also be discussed to some extent.