Functional and morphological effects of β-estradiol in eyes with N-methyl-D-Aspartate-induced retinal neurotoxicity in rats

Sex-related differences in retinal function in Wistar rats: implications for toxicity and safety studies

Introduction: Wistar Han rats are a preferred strain of rodents for general toxicology and safety pharmacology studies in drug development. In some of these studies, visual functional tests that assess for retinal toxicity are included as an additional endpoint. Although the influence of gender on human retinal function has been documented for more than 6 decades, preclinically it is still uncertain if there are differences in retinal function between naïve male and female Wistar Han rats. Methods: In this study, sex-related differences in the retinal function were quantified by analyzing electroretinography (ERG) in 7-9-week-old (n = 52 males and 51 females) and 21-23-week-old Wistar Han rats (n = 48 males and 51 females). Optokinetic tracking response, brainstem auditory evoked potential, ultrasonic vocalization and histology were tested and evaluated in a subset of animals to investigate the potential compensation mechanisms of spontaneous blindness. Results/Discussion: Absence of scotopic and photopic ERG responses was found in 13% of 7-9-week-old (7/52) and 19% of 21-23-week-old males (9/48), but none of female rats (0/51). The averaged amplitudes of rod- and cone-mediated ERG b-wave responses obtained from males were significantly smaller than the amplitudes of the same responses from age-matched females (-43% and -26%, respectively) at 7-9 weeks of age. There was no difference in the retinal and brain morphology, brainstem auditory responses, or ultrasonic vocalizations between the animals with normal and abnormal ERGs at 21-23 weeks of age. In summary, male Wistar Han rats had altered retinal responses, including a complete lack of responses to test flash stimuli (i.e., blindness), when compared with female rats at 7-9 and 21-23 weeks of age. Therefore, sex differences should be considered when using Wistar Han rats in toxicity and safety pharmacology studies with regards to data interpretation of retinal functional assessments.

Girl Power in Glaucoma: The Role of Estrogen in Primary Open Angle Glaucoma

Estrogen is essential in maintaining various physiological features in women, and a decline in estrogen levels are known to give rise to numerous unfortunate symptoms associated with menopause. To alleviate these symptoms hormone replacement therapy with estrogen is often used, and has been shown to be fruitful in improving quality of life in women suffering from postmenopausal discomforts. An often forgotten condition associated with menopause is the optic nerve disorder, glaucoma. Thus, estrogen may also have an impact in maintaining the retinal ganglion cells (RGCs), which make up the optic nerve, thereby preventing glaucomatous neurodegeneration. This review aims to provide an overview of possible associations of estrogen and the glaucoma subtype, primary open-angle glaucoma (POAG), by evaluating the current literature through a PubMed-based literature search. Multiple in vitro and in vivo studies of RGC protection, as well as clinical and epidemiological data concerning the well-defined retinal neurodegenerative disorder POAG have been reviewed. Over all, deficiencies in retinal estrogen may potentially instigate RGC loss, visual disability, and eventual blindness. Estrogen replacement therapy may therefore be a beneficial future treatment. However, more studies are needed to confirm the relevance of estrogen in glaucoma prevention.

The lack of Irs2 induces changes in the immunocytochemical expression of aromatase in the mouse retina

Insulin receptor substrate (Irs) belongs to a family of proteins that mediate the intracellular signaling of insulin and IGF-1. Insulin receptor substrate 2 (Irs2) is necessary for retinal function, since its failure in Irs2-deficient mice in hyperglycemic situation promotes photoreceptor degeneration and visual dysfunction, like in diabetic retinopathy. The expression of P450 aromatase, which catalyzes androgen aromatization to form 17ß-estradiol, increases in some neurodegenerative diseases thus promoting the local synthesis of neuroestrogens that exert relevant neuroprotective functions. Aromatase is also expressed in neurons and glial cells of the central nervous system (CNS), including the retina. To further understand the role of Irs2 at the retinal level, we performed an immunocytochemical study in adult normoglycemic Irs2-deficient mice. For this aim, the retinal immunoexpression of neuromodulators, such as aromatase, glutamine synthetase (GS), and tyrosine hydroxylase (TH) was analyzed, joint to a morphometric and planimetric study of the retinal layers. Comparing with wild-type (WT) control mice, the Irs2-knockout (Irs2-KO) animals showed a significant increase in the immunopositivity to aromatase in almost all of the retinal layers. Besides, Irs2-KO mice exhibited a decreased immunopositive reaction for GS and TH, in Müller and amacrine cells, respectively; morphological variations were also found in these retinal cell types. Furthermore, the retina of Irs2-KO mice displayed alterations in the structural organization, and a generalized decrease in the retinal thickness was observed in each of the layers, except for the inner nuclear layer. Our findings suggest that the absence of Irs2 induces retinal neurodegenerative changes in Müller and amacrine cells that are unrelated to hyperglycemia. Accordingly, in the Irs2-KO mice, the increased retinal immunocytochemical reactivity of aromatase could be associated with an attempt to repair such neural retina injuries by promoting local neuroprotective mediators.

The microphthalmia-associated transcription factor (Mitf) gene and its role in regulating eye function

Mutations in the microphthalmia-associated transcription factor (Mitf) gene can cause retinal pigment epithelium (RPE) and retinal dysfunction and degeneration. We examined retinal and RPE structure and function in 3 month old mice homo- or heterozygous or compound heterozygous for different Mitf mutations (Mitf^mi-vga9/+, Mitf^{mi-enu22(398)}/Mitf^{mi-enu22(398)}, Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi) which all have normal eye size with apparently normal eye pigmentation. Here we show that their vision and retinal structures are differentially affected. Hypopigmentation was evident in all the mutants while bright-field fundus images showed yellow spots with non-pigmented areas in the Mitf^mi-vga9/+ mice. Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice showed large non-pigmented areas. Fluorescent angiography (FA) of all mutants except Mitf^mi-vga9/+ mice showed hyperfluorescent areas, whereas FA from both Mitf^-Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice showed reduced capillary network as well as hyperfluorescent areas. Electroretinogram (ERG) recordings show that Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice are severely impaired functionally whereas the scotopic and photopic ERG responses of Mitf^mi-vga9/+ and Mitf^{mi-enu22(398)}/Mitf^{mi-enu22(398)} mice were not significantly different from wild type mice. Histological sections demonstrated that the outer retinal layers were absent from the Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi blind mutants. Our results show that Mitf mutations affect eye function, even in the heterozygous condition and that the alleles studied can be arranged in an allelic series in this respect.

The neuroprotective effects of novel estrogen receptor GPER1 in mouse retinal ganglion cell degeneration

PURPOSE

To investigate the potential protective effect of novel G protein coupled estrogen receptor (GPER1) against the neurotoxicity induced by NMDA in the mouse retina.

METHODS

We induce retinal ganglion cells (RGCs) toxic injury through intravitreal injection of NMDA or acute ocular hypertension (AOH) induced by anterior chamber infusion with saline. Endogenous ligand 17-β-estradiol (E2), GPER1 agonist (G-1), and E2 with GPER1 antagonist (G-15) or classic estrogen receptor α and β (ERα and ERβ) antagonist tamoxifen (TAM) were subcutaneous administered before NMDA to identify the possible involved receptors. Immunofluorescence staining was performed to explore the survival of RGCs and Müller cell gliosis. TUNEL staining was used to evaluate the RGC apoptosis. The involved molecular pathway was detected via antibody array expression profiling.

RESULTS

Activation of estrogen receptor by E2 or G-1 could significantly rescue the RGCs injury in NMDA administration. The protective effect was carried exclusively by GPER1 activation. E2 application can still mimicked the protective function when estrogen receptor α and β (ERα and ERβ) blocked by tamoxifen (TAM), while the effects was blocked by GPER1 antagonist G-15. Moreover, the TUNEL positive RGCs and GFAP expression level were both attenuated in G-1 application and the effects could be reversed by G-15. In addition, application of the PI3K/Akt antagonist LY294002 counteracted the effect of G-1. And a number of apoptosis regulatory factors decreased dramatically in the G-1 group, including Bad, Caspase 3, Caspase 7, Smad2, P-53 and TAK1. Also, similar protective effect of G-1 was spotted in acute ocular hypertension (AOH) model.

CONCLUSION

Estrogen played a protective role via a novel estrogen receptor, GPER1, instead of classical receptors ERα or ERβ. Activation of GPER1 attenuated RGCs apoptosis and Müller cells gliosis, indicating GPER1 as a potential treatment target in RGCs degeneration diseases.

Isoflurane and ketamine:xylazine differentially affect intraocular pressure-associated scotopic threshold responses in Sprague-Dawley rats

PURPOSE

Amplitudes of electroretinograms (ERG) are enhanced during acute, moderate elevation of intraocular pressure (IOP) in rats anaesthetised with isoflurane. As anaesthetics alone are known to affect ERG amplitudes, the present study compares the effects of inhalant isoflurane and injected ketamine:xylazine on the scotopic threshold response (STR) in rats with moderate IOP elevation.

METHODS

Isoflurane-anaesthetised (n = 9) and ketamine:xylazine-anaesthetised (n = 6) rats underwent acute unilateral IOP elevation using a vascular loop anterior to the equator of the right eye. STRs to a luminance series (subthreshold to -3.04 log scotopic cd s/m²) were recorded from each eye of Sprague-Dawley rats before, during, and after IOP elevation.

RESULTS

Positive STR (pSTR) amplitudes for all conditions were significantly smaller (p = 0.0001) for isoflurane- than for ketamine:xylazine-anaesthetised rats. In addition, ketamine:xylazine was associated with a progressive increase in pSTR amplitudes over time (p = 0.0028). IOP elevation was associated with an increase in pSTR amplitude (both anaesthetics p < 0.0001). The absolute interocular differences in IOP-associated enhancement of pSTR amplitudes for ketamine:xylazine and isoflurane were similar (66.3 ± 35.5 vs. 54.2 ± 24.1 µV, respectively). However, the fold increase in amplitude during IOP elevation was significantly higher in the isoflurane- than in the ketamine:xylazine-anaesthetised rats (16.8 ± 29.7x vs. 2.1 ± 2.7x, respectively, p = 0.0004).

CONCLUSIONS

The anaesthetics differentially affect the STRs in the rat model with markedly reduced amplitudes with isoflurane compared to ketamine:xylazine. However, the IOP-associated enhancement is of similar absolute magnitude for the two anaesthetics, suggesting that IOP stress and anaesthetic effects operate on separate retinal mechanisms.

Estradiol mitigates ischemia reperfusion-induced acute renal failure through NMDA receptor antagonism in rats

In the present study, we investigated possible involvement of N-methyl-D-aspartate receptors (NMDAR) in estradiol mediated protection against ischemia reperfusion (I/R)-induced acute renal failure (ARF) in rats. Bilateral renal ischemia of 40 min followed by reperfusion for 24 h induced ARF in male wistar rats. Quantification of serum creatinine, creatinine clearance (CrCl), blood urea nitrogen (BUN), uric acid, potassium, fractional excretion of sodium (Fe_Na), and urinary microproteins was done to assess I/R-induced renal damage in rats. Oxidative stress in kidneys was measured in terms of myeloperoxidase activity, thiobarbituric acid reactive substances, superoxide anion generation, and reduced glutathione levels. Hematoxylin & eosin and periodic acid Schiff stains were used to reveal structural changes in renal tissues. Estradiol benzoate (0.5 and 1.0 mg/kg, intraperitoneally, i.p.) was administered 1 h prior to I/R in rats. In separate groups, rats were treated with NMDAR agonists, glutamic acid (200 mg/kg, i.p.), and spermidine (20 mg/kg, i.p.) before administration of estradiol. Marked increase in serum creatinine, BUN, uric acid, serum potassium, Fe_Na, microproteinuria, and reduction in CrCl demonstrated I/R-induced ARF in rats. Treatment with estradiol mitigated I/R-induced changes in serum/urine parameters. Moreover, estrogen attenuated oxidative stress and structural changes in renal tissues. Prior administration of glutamic acid and spermidine abolished estradiol mediated renoprotection in rats. These results indicate the involvement of NMDAR in estradiol mediated renoprotective effect. In conclusion, we suggest that NMDAR antagonism serves as one of the mechanisms in estradiol-mediated protection against I/R-induced ARF in rats.

Allogeneic Transplantation of Müller-Derived Retinal Ganglion Cells Improves Retinal Function in a Feline Model of Ganglion Cell Depletion

Human Müller glia with stem cell characteristics (hMGSCs) have been shown to improve retinal function upon transplantation into rat models of retinal ganglion cell (RGC) depletion. However, their translational potential may depend upon successful engraftment and improvement of retinal function in experimental models with anatomical and functional features resembling those of the human eye. We investigated the effect of allogeneic transplantation of feline Müller glia with the ability to differentiate into cells expressing RGC markers, following ablation of RGCs by N-methyl-d-aspartate (NMDA). Unlike previous observations in the rat, transplantation of hMGSC-derived RGCs into the feline vitreous formed aggregates and elicited a severe inflammatory response without improving visual function. In contrast, allogeneic transplantation of feline MGSC (fMGSC)-derived RGCs into the vitrectomized eye improved the scotopic threshold response (STR) of the electroretinogram (ERG). Despite causing functional improvement, the cells did not attach onto the retina and formed aggregates on peripheral vitreous remnants, suggesting that vitreous may constitute a barrier for cell attachment onto the retina. This was confirmed by observations that cellular scaffolds of compressed collagen and enriched preparations of fMGSC-derived RGCs facilitated cell attachment. Although cells did not migrate into the RGC layer or the optic nerve, they significantly improved the STR and the photopic negative response of the ERG, indicative of increased RGC function. These results suggest that MGSCs have a neuroprotective ability that promotes partial recovery of impaired RGC function and indicate that cell attachment onto the retina may be necessary for transplanted cells to confer neuroprotection to the retina. Significance: Müller glia with stem cell characteristics are present in the adult human retina, but they do not have regenerative ability. These cells, however, have potential for development of cell therapies to treat retinal disease. Using a feline model of retinal ganglion cell (RGC) depletion, cell grafting methods to improve RGC function have been developed. Using cellular scaffolds, allogeneic transplantation of Müller glia-derived RGC promoted cell attachment onto the retina and enhanced retinal function, as judged by improvement of the photopic negative and scotopic threshold responses of the electroretinogram. The results suggest that the improvement of RGC function observed may be ascribed to the neuroprotective ability of these cells and indicate that attachment of the transplanted cells onto the retina is required to promote effective neuroprotection.

Neuroprotective Effect of Tauroursodeoxycholic Acid on N-Methyl-D-Aspartate-Induced Retinal Ganglion Cell Degeneration

Retinal ganglion cell degeneration underlies the pathophysiology of diseases affecting the retina and optic nerve. Several studies have previously evidenced the anti-apoptotic properties of the bile constituent, tauroursodeoxycholic acid, in diverse models of photoreceptor degeneration. The aim of this study was to investigate the effects of systemic administration of tauroursodeoxycholic acid on N-methyl-D-aspartate (NMDA)-induced damage in the rat retina using a functional and morphological approach. Tauroursodeoxycholic acid was administered intraperitoneally before and after intravitreal injection of NMDA. Three days after insult, full-field electroretinograms showed reductions in the amplitudes of the positive and negative-scotopic threshold responses, scotopic a- and b-waves and oscillatory potentials. Quantitative morphological evaluation of whole-mount retinas demonstrated a reduction in the density of retinal ganglion cells. Systemic administration of tauroursodeoxycholic acid attenuated the functional impairment induced by NMDA, which correlated with a higher retinal ganglion cell density. Our findings sustain the efficacy of tauroursodeoxycholic acid administration in vivo, suggesting it would be a good candidate for the pharmacological treatment of degenerative diseases coursing with retinal ganglion cell loss.

Differences in Retinal Structure and Function between Aging Male and Female Sprague-Dawley Rats are Strongly Influenced by the Estrus Cycle

Purpose

Biological sex and age are considered as two important factors that may influence the function and structure of the retina, an effect that might be governed by sexual hormones such as estrogen. The purpose of this study was to delineate the influence that biological sex and age exert on the retinal function and structure of rodents and also clarify the effect that the estrus cycle might exert on the retinal function of female rats.

Method

The retinal function of 50 normal male and female albino Sprague-Dawley (SD) rats was investigated with the electroretinogram (ERG) at postnatal day (P) 30, 60, 100, 200, and 300 (n = 5–6 male and female rats/age). Following the ERG recording sessions, retinal histology was performed in both sexes. In parallel, the retinal function of premenopausal and menopausal female rats aged P540 were also compared.

Results

Sex and age-related changes in retinal structure and function were observed in our animal model. However, irrespective of age, no significant difference was observed in ERG and retinal histology obtained from male and female rats. Notwithstanding the above we did however notice that between P60 and P200 there was a gradual increase in ERG amplitudes of female rats compared to males. Furthermore, the ERG of premenopausal female rats aged 18 months old (P540) was larger compared to age-matched menopausal female rats as well as that of male rats.

Conclusion

Our results showed that biological sex and age can influence the retinal function and structure of albino SD rats. Furthermore, we showed that cycled female rats have better retinal function compared to the menopausal female rats suggesting a beneficial effect of the estrus cycle on the retinal function.

Oscillatory potentials in electroretinogram as an early marker of visual abnormalities in vitamin A deficiency

Vitamin A deficiency (VAD) caused by malnutrition and certain intestinal diseases induces visual impairments, including night blindness and photoreceptor cell dysfunction as indicated by reduced a‑ and b‑waves in an electroretinogram (ERG). The effects of VAD on the inner retinal layer cells, including amacrine and ganglion cells, remain to be elucidated. The functions of these cells are reflected in oscillatory potentials (OPs), another component of the ERG. The present study investigated inner retinal layer cell function in VAD rats by analyzing OPs. In the present study, VAD was induced by feeding Brown Norway rats a vitamin A deficient diet for 10 weeks. A reduced body weight and peri‑papillary opacification indicative of papilledema without histopathological alterations were observed, which are considered early symptoms of VAD. At this stage, the ERG revealed reduced OPs as well as a‑ and b‑waves at various intensities of light stimulation. Further analysis indicated that the ratio of the alterations in OPs was more significant than those of a‑ and b‑waves. After 5 weeks of recovery, these changes returned to control levels. These results suggest that OPs are the most sensitive and early marker of VAD‑associated visual impairment in the ERG.

Oestrogen, ocular function and low-level vision: a review

Over the past 10 years, a literature has emerged concerning the sex steroid hormone oestrogen and its role in human vision. Herein, we review evidence that oestrogen (oestradiol) levels may significantly affect ocular function and low-level vision, particularly in older females. In doing so, we have examined a number of vision-related disorders including dry eye, cataract, increased intraocular pressure, glaucoma, age-related macular degeneration and Leber's hereditary optic neuropathy. In each case, we have found oestrogen, or lack thereof, to have a role. We have also included discussion of how oestrogen-related pharmacological treatments for menopause and breast cancer can impact the pathology of the eye and a number of psychophysical aspects of vision. Finally, we have reviewed oestrogen's pharmacology and suggest potential mechanisms underlying its beneficial effects, with particular emphasis on anti-apoptotic and vascular effects.

Neuroprotective role of superoxide dismutase 1 in retinal ganglion cells and inner nuclear layer cells against N-methyl-d-aspartate-induced cytotoxicity

The N-methyl-d-aspartate (NMDA) receptor-induced apoptosis is implicated in the pathological mechanisms of neural tissues, increasing the release of reactive oxygen species (ROS), resulting in a type of apoptotic cell death called excitotoxicity. Although intrinsic mechanisms to remove ROS, such as antioxidant enzymes, are provided by the tissue, the association between NMDA-induced excitotoxicity and antioxidative enzymes is not well understood. In this study, we focused on superoxide dismutase 1 (SOD1), an antioxidant enzyme, and investigated the role of SOD1 in the NMDA-induced neuronal cell death in the retina. NMDA was intravitreally injected into wild-type (WT) and SOD1 total knock-out (SOD1-deficient) mice. The number of TUNEL-positive cells in the retinal ganglion cell layer (GCL) and inner nuclear layer (INL) counted in the retinal sections and flatmount retinas were significantly higher in the SOD1-deficient mice than the WT mice after NMDA injection. Visual function assessed by dark-adapted electroretinogram (ERG) showed that the amplitudes of a-wave, b-wave, and oscillatory potential 2 were significantly reduced in the NMDA-injected SOD1-deficient mice. The level of ROS in the GCL and INL, measured using dihydroethidium, and the number of positive cells for γ-H2AX, a marker for DNA double strand breaks, and 8-OHdG, a marker for DNA oxidation, in the GCL were significantly increased in the SOD1-deficient mice after NMDA injection. We also measured mRNA and protein levels of SOD1 and SOD2 in the retina of WT mice, to find that mRNA and protein levels of SOD1, but not SOD2, were significantly reduced after NMDA injection. SOD1 deficiency exacerbated NMDA-induced damage to the inner retinal neurons, and NMDA reduced SOD1 levels in the retina of WT mice. Therefore, SOD1 protected retinal neurons against NMDA-induced retinal neurotoxicity, and NMDA-induced SOD1 reduction may be involved in neuronal vulnerability to excitotoxicity.

Distribution and protective function of pituitary adenylate cyclase-activating polypeptide in the retina

Pituitary adenylate cyclase-activating polypeptide (PACAP), which is found in 27- or 38-amino acid forms, belongs to the VIP/glucagon/secretin family. PACAP and its three receptor subtypes are expressed in neural tissues, with PACAP known to exert a protective effect against several types of neural damage. The retina is considered to be part of the central nervous system, and retinopathy is a common cause of profound and intractable loss of vision. This review will examine the expression and morphological distribution of PACAP and its receptors in the retina, and will summarize the current state of knowledge regarding the protective effect of PACAP against different kinds of retinal damage, such as that identified in association with diabetes, ultraviolet light, hypoxia, optic nerve transection, and toxins. This article will also address PACAP-mediated protective pathways involving retinal glial cells.