Noradrenaline contracts rat retinal arterioles via stimulation of α(1A)- and α(1D)-adrenoceptors

α1D Adrenergic Receptor Antagonism Protects Against High Glucose-Induced Mitochondrial Dysfunction and Blood Retinal Barrier Breakdown in ARPE-19 Cells

Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus and a leading cause of blindness in the working-age population. Current pharmacological treatments counteract DR's later stages without targeting the earlier disease phases. Using computational approaches, our group previously identified the α1D and α2C adrenoceptors (α1DR and α2CR) as new putative drug targets for DR. Therefore, the aim of this work was to validate the role of these receptors in an in vitro model of DR, i.e., retinal pigmented epithelial cells (ARPE-19) challenged with high glucose (HG, 50 mM). We examined the effects of selective α1DR and α2CR agonists and antagonists on hyperglycemia-induced mitochondrial dysfunction and blood retinal barrier breakdown. Seahorse XFe was employed to assess the oxygen consumption rate and extracellular acidification rate. The integrity of the ARPE-19 barrier was evaluated through transepithelial electrical resistance measurements and a sodium fluorescein permeability test. α1DR pharmacological modulation through the α1DR antagonist BMY 7378 (0.1-1 µM, 24 h), but not α2CR, significantly attenuated HG-induced mitochondrial dysfunction. BMY 7378 (0.1-1 µM, 48 h) also prevented HG-mediated damage to retinal epithelial integrity. In contrast, the α1DR agonist phenylephrine (1-10 μM, 24 h) further reduced ARPE-19 mitochondrial activity compared to HG, indicating that α1D activation is directly implicated in DR-mediated mitochondrial dysfunction. In conclusion, the current in vitro study validated α1DR as a pharmacological target for DR.

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.

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.

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.

Relationship between plasma levels of vasoactive mediators and optic nerve head circulation shown by laser speckle flowgraphy

PROPOSE

To evaluate relationships between optic nerve head (ONH) circulation by laser speckle flowgraphy (LSFG), and secretion levels of plasma norepinephrine (NE), plasma renin activity (PRA), plasma aldosterone concentration (PAC), and plasma cortisol.

METHOD

Forty subjects were included in the study. The mean blur rates (MBR) throughout the ONH (MBR-all), in the tissue (MBR-tissue), and in vessels (MBR-vessel) were analyzed. In 29 subjects the following parameters were evaluated: plasma NE and ONH circulation in the evening and morning (Δ plasma NE, Δ MBR-all, -tissue, -vessel, and Δ area ratio of blood stream).

RESULTS

Plasma NE was significantly correlated with MBR-all (r = 0.39, P = 0.01) and MBR-vessel (r = 0.51, P = 0.0008). Plasma cortisol was significantly correlated with MBR-vessel (r =0.35, P = 0.03). The PRA (r = 0.31, P = 0.05) and PAC (r = 0.31, P = 0.05) tended to correlate with MBR-vessel. The multiple regression analysis revealed that plasma NE, PAC, and ocular perfusion pressure (OPP) were identified as factors contributing independently to the MBR-vessel (plasma NE: standard regression = 0.48, t-value = 3.12, P = 0.004; PAC: 0.43, 3.10, 0.04; OPP: 0.58, 2.34, 0.03, r = 0.71). Positive correlations between Δ plasma NE and Δ MBR-all (r = 0.46, P = 0.01), Δ MBR-tissue (0.38, 0.04), Δ MBR-vessel (0.41, 0.03), and Δ area ratio of blood stream (0.38, 0.04) were observed.

CONCLUSION

Our results clarified that the measurements of ONH circulation by LSFG is reflecting the plasma secretion levels of vasoactive mediators.

Simultaneous activation of the α1A-, α1B- and α1D-adrenoceptor subtypes in the nucleus accumbens reduces accumbal dopamine efflux in freely moving rats

Intra-accumbal infusion of the α1-adrenergic agonist methoxamine, which has comparable affinity for α1A-, α1B- and α1D-adrenoceptor subtypes, fails to alter noradrenaline efflux but reduces dopamine efflux in the nucleus accumbens of rats. In-vivo microdialysis experiments were carried out to analyse the putative contribution of α1A-, α1B- and α1D-adrenoceptor subtypes to the methoxamine-induced decrease in accumbal dopamine efflux in freely moving rats. The drugs used were dissolved in the infusion medium and administered locally through a dialysis membrane. Intra-accumbal infusions of the α1A-adrenoceptor antagonist 5-methylurapidil (6 pmol), the α1B-adrenoceptor antagonist cyclazosin (0.6 and 6 pmol) and the α1D-adrenoceptor antagonist BMY 7378 (0.6 pmol) did not alter accumbal efflux of noradrenaline or dopamine: pretreatment with each of these α1-adrenoceptor subtype-selective antagonists counteracted the methoxamine (24 pmol)-induced decrease in accumbal dopamine efflux. Doses indicated are the total amount of drug administered over a 60-min infusion period. These results clearly suggest that the α1A-, α1B- and α1D-adrenoceptor subtypes in the nucleus accumbens mediate the α1-adrenergic agonist methoxamine-induced decrease in accumbal dopamine efflux. The present study also provides in-vivo neurochemical evidence indicating that concomitant, but not separate, activation of the α1A-, α1B- and α1D-adrenoceptors in the nucleus accumbens is required for α1-adrenergic inhibition of accumbal dopaminergic activity.

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

BACKGROUND AND PURPOSE

The α₁-adrenoceptor family plays a critical role in regulating ocular perfusion by mediating responses to catecholamines. The purpose of the present study was to determine the contribution of individual α₁-adrenoceptor subtypes to adrenergic vasoconstriction of retinal arterioles using gene-targeted mice deficient in one of the three adrenoceptor subtypes (α₁A-AR(-/-), α₁B-AR(-/-) and α₁D-AR(-/-) respectively).

EXPERIMENTAL APPROACH

Using real-time PCR, mRNA expression for individual α₁-adrenoceptor subtypes was determined in murine retinal arterioles. To assess the functional relevance of the three α₁-adrenoceptor subtypes for mediating vascular responses, retinal vascular preparations from wild-type mice and mice deficient in individual α₁-adrenoceptor subtypes were studied in vitro using video microscopy.

KEY RESULTS

Retinal arterioles expressed mRNA for all three α₁-adrenoceptor subtypes. In functional studies, arterioles from wild-type mice with intact endothelium responded only negligibly to the α₁-adrenoceptor agonist phenylephrine. In endothelium-damaged arterioles from wild-type mice, phenylephrine evoked concentration-dependent constriction that was attenuated by the α₁-adrenoceptor blocker prazosin. Strikingly, phenylephrine only minimally constricted endothelium-damaged retinal arterioles from α₁B-AR(-/-) mice, whereas arterioles from α₁A -AR(-/-) and α₁D-AR(-/-) mice constricted similarly to arterioles from wild-type mice. Constriction to U46619 was similar in endothelium-damaged retinal arterioles from all four mouse genotypes.

CONCLUSIONS AND IMPLICATIONS

The present study is the first to demonstrate that α₁-adrenoceptor-mediated vasoconstriction in murine retinal arterioles is buffered by the endothelium. When the endothelium is damaged, a vasoconstricting role of the α₁B-adrenoceptor subtype is unveiled. Hence, the α₁B-adrenoceptor may represent a target to selectively modulate retinal blood flow in ocular diseases associated with endothelial dysfunction.

Constriction of porcine retinal arterioles induced by endothelin-1 and the thromboxane analogue U46619 in vitro decreases with increasing vascular branching level

PURPOSE

The retinal blood flow depends on the diameter of retinal arterioles, but diameter changes in these vessels have hitherto only been assessed in vessels larger than approximately 100 μm. Therefore, a new method was developed for studying diameter changes along the vascular tree of arterioles in whole perfused segments of porcine retinas, and the effect of known vasoconstrictors on the diameter of retinal arterioles at different branching levels were studied.

METHODS

Thirty-four whole-mounted porcine retinas were placed in a specially designed tissue chamber. On the basis of video recordings through an inverted microscope, the diameter of retinal arterioles was measured at five different branching levels before and after addition of a high potassium concentration, or increasing concentrations of endothelin-1, the prostaglandin analogue U46619, noradrenaline or none (time controls).

RESULTS

The baseline diameter ranged from 136 μm (95% CI 132-140 μm) for 1st order arterioles to 33 μm (95% CI 21-44 μm) for 5th order arterioles. In 1st order arterioles, endothelin produced 56.6% (95% CI 47.6-64.0) and U46619 14.6% (95% CI 5.7-22.6) relative constriction compared with baseline, which for both compounds decreased significantly with increasing branching level (p<0.0001 and p<0.0001, respectively). The change in diameter during addition of noradrenaline did not differ significantly from the time controls (p=0.07).

CONCLUSIONS

The effect of retinal vasoconstrictors differs among larger and smaller arterioles. The study highlights the need for investigating diameter regulation in smaller retinal arterioles as a basis for understanding normal and pathological changes in retinal blood flow.

[Elucidation of dysfunctional mechanisms of retinal circulation in the rat models of glaucoma and exploration of novel therapeutic drugs]

In recent times, glaucoma has become the leading cause of acquired blindness among the Japanese. As visual disorders markedly decrease the quality of life (QOL), it is important to develop new strategies for preventing the onset of and delaying the progression of glaucoma. Glaucoma has long since been recognized as a serious disease caused by increased intraocular pressure and subsequent injury and death of the neuronal retinal cells. Therefore, numerous studies have focused on the mechanisms that damage neuronal cells and on the drugs that possess protective effects in reversing this damage. However, injury to the retinal vasculature has been recently shown in animal models of glaucoma. Hence, thus far, only few papers have been published on retinal circulation in glaucoma. These study results have indicated that retinal circulation is altered in glaucoma and that this vascular abnormality may be the cause of and/or may accelerate retinal degeneration. In this report, we have attempted to elucidate the mechanisms of retinal circulation and explore novel drugs for the treatment of retinal circulation disorders. We have also introduced here our previous research results on retinal circulation. We reported that the drugs that improved retinal circulation, by intravitreal injection, in the rat model of glaucoma also inhibited retinal nerve injury, thereby representing possibilities that they might be novel candidate drugs for glaucoma prevention and treatment.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Neurovascular interactions in the retina: physiological and pathological roles

Increasing evidence suggests that the complex interactions among multiple cell types including neuronal, glial, and vascular cells, are critical for maintaining adequate cerebral blood flow that is necessary for normal brain function and survival. The disturbance of these interactions contributes to the pathogenesis of central nervous system disorders such as stroke and Alzheimer's disease. The retina is part of the central nervous system, and the properties of vasculature in the retina are similar to those in the brain. The interactions among multiple cell types in the retina also play an important role in the maintenance of tissue homeostasis, and the impairment of interactions can contribute to the onset and/or progression of retinal diseases. In this review, we describe the neurovascular interactions in the retina and alternations of interactions in pathological conditions such as diabetic retinopathy and glaucoma.