Enriched environment and visual stimuli protect the retinal pigment epithelium and photoreceptors in a mouse model of non-exudative age-related macular degeneration

Non-exudative age-related macular degeneration (NE-AMD), the main cause of blindness in people above 50 years old, lacks effective treatments at the moment. We have developed a new NE-AMD model through unilateral superior cervical ganglionectomy (SCGx), which elicits the disease main features in C57Bl/6J mice. The involvement of oxidative stress in the damage induced by NE-AMD to the retinal pigment epithelium (RPE) and outer retina has been strongly supported by evidence. We analysed the effect of enriched environment (EE) and visual stimulation (VS) in the RPE/outer retina damage within experimental NE-AMD. Exposure to EE starting 48 h post-SCGx, which had no effect on the choriocapillaris ubiquitous thickness increase, protected visual functions, prevented the thickness increase of the Bruch’s membrane, and the loss of the melanin of the RPE, number of melanosomes, and retinoid isomerohydrolase (RPE65) immunoreactivity, as well as the ultrastructural damage of the RPE and photoreceptors, exclusively circumscribed to the central temporal (but not nasal) region, induced by experimental NE-AMD. EE also prevented the increase in outer retina/RPE oxidative stress markers and decrease in mitochondrial mass at 6 weeks post-SCGx. Moreover, EE increased RPE and retinal brain-derived neurotrophic factor (BDNF) levels, particularly in Müller cells. When EE exposure was delayed (dEE), starting at 4 weeks post-SCGx, it restored visual functions, reversed the RPE melanin content and RPE65-immunoreactivity decrease. Exposing animals to VS protected visual functions and prevented the decrease in RPE melanin content and RPE65 immunoreactivity. These findings suggest that EE housing and VS could become an NE-AMD promising therapeutic strategy.

Melatonin protects the retina from experimental nonexudative age-related macular degeneration in mice

Nonexudative age-related macular degeneration (NE-AMD) represents the leading cause of blindness in the elderly. Currently, there are no available treatments for NE-AMD. We have developed a NE-AMD model induced by superior cervical ganglionectomy (SCGx) in C57BL/6J mice, which reproduces the disease hallmarks. Several lines of evidence strongly support the involvement of oxidative stress in NE-AMD-induced retinal pigment epithelium (RPE) and outer retina damage. Melatonin is a proven and safe antioxidant. Our aim was analysing the effect of melatonin in the RPE/outer retina damage within experimental NE-AMD. The treatment with melatonin starting 48 h after SCGx, which had no effect on the ubiquitous choriocapillaris widening, protected visual functions and avoided Bruch´s membrane thickening, RPE melanin content, melanosome number loss, retinoid isomerohydrolase (RPE65)-immunoreactivity decrease, and RPE and hotoreceptor ultrastructural damage induced within experimental NE-AMD exclusively located at the central temporal (but not nasal) region. Melatonin also prevented the increase in outer retina/RPE oxidative stress markers and a decrease in mitochondrial mass at 6 weeks post-SCGx. Moreover, when the treatment with melatonin started at 4 weeks post-SCGx, it restored visual functions and reversed the decrease in RPE melanin content and RPE65-immunoreactivity. These findings suggest that melatonin could become a promising safe therapeutic strategy for NE-AMD.

Therapeutic benefit of environmental enrichment on optic neuritis

Optic neuritis (ON) is an inflammatory, demyelinating, neurodegenerative, and presently untreatable condition of the optic nerve which might induce blindness. We analyzed the effect of environmental enrichment (EE) on visual pathway damage provoked by experimental ON induced by a microinjection of bacterial lipopolysaccharide (LPS) into the optic nerve. For this purpose, LPS was microinjected into the optic nerve from male Wistar rats. After injection, one group of animals was submitted to EE, and another group remained in standard environment (SE) for 21 days. EE prevented the decrease in pupil light reflex (PLR), visual evoked potentials, retinal anterograde transport, phosphorylated neurofilament immunoreactivity, myelination (luxol fast blue staining), and axon (toluidine blue staining) and retinal ganglion cell (Brn3a-immunoreactivity) number. EE also prevented microglial/macrophage reactivity (Iba-1- and ED1-immunoreactivity), and astrocytosis (glial fibrillary acidic protein-immunostaining) induced by experimental ON. LPS-injected optic nerves displayed oxidative damage and increased inducible nitric oxide synthase, cyclooxygenase-2, and interleukin-1β and TNFα mRNA levels which were prevented by EE. EE increased optic nerve brain-derived neurotrophic factor levels. When EE started at 4 (but not 7) days post-injection of LPS, a preservation of the PLR was observed at 21 days post-LPS, which was blocked by the daily administration of ANA-12 from day 4 to day 7 post-LPS. Moreover, EE from day 4 to day 7 post-LPS significantly preserved the PLR at 21 days post-injection. Taken together, our data suggest that EE preserved visual functions and reduced neuroinflammation of the optic nerve. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Involvement of microglia in early axoglial alterations of the optic nerve induced by experimental glaucoma

Glaucoma is a leading cause of blindness, characterized by retinal ganglion cell (RGC) loss and optic nerve (ON) damage. Cumulative evidence suggests glial cell involvement in the degeneration of the ON and RGCs. We analyzed the contribution of microglial reactivity to early axoglial alterations of the ON in an induced model of ocular hypertension. For this purpose, vehicle or chondroitin sulfate (CS) were weekly injected into the eye anterior chamber from Wistar rats for different intervals. The amount of Brn3a(+) RGC significantly decreased in CS-injected eyes for 10 and 15 (but not 6) weeks. A reduction in anterograde transport of β-subunit cholera toxin was observed in the superior colliculus and the lateral geniculate nucleus contralateral to CS-injected eyes for 6 and 15 weeks. A disruption of cholera toxin β-subunit transport was observed at the proximal myelinated ON. A significant decrease in phosphorylated neurofilament heavy chain immunoreactivity, an increase in ionized calcium-binding adaptor molecule 1(+), ED1(+) (microglial markers), and glial fibrillary acidic protein (astrocytes) (+) area, and decreased luxol fast blue staining were observed in the ON at 6 and 15 weeks of ocular hypertension. Microglial reactivity involvement was examined through a daily treatment with minocycline (30 mg/kg, i.p.) for 2 weeks, after 4 weeks of ocular hypertension. Minocycline prevented the increase in ionized calcium-binding adaptor molecule 1(+), ED-1(+), and glial fibrillary acidic protein(+) area, the decrease in phosphorylated neurofilament heavy-chain immunoreactivity and luxol fast blue staining, and the deficit in anterograde transport induced by 6 weeks of ocular hypertension. Thus, targeting microglial reactivity might prevent early axoglial alterations in the glaucomatous ON. Cover Image for this issue: doi: 10.1111/jnc.13807.

Melatonin as a Therapeutic Resource for Inflammatory Visual Diseases

BACKGROUND

Uveitis and optic neuritis are prevalent ocular inflammatory diseases, and highly damaging ocular conditions. Both diseases are currently treated with corticosteroids, but they do not have adequate efficacy and are often associated with severe side effects. Thus, uveitis and optic neuritis remain a challenging field to ophthalmologists and a significant public health concern.

OBJECTIVE

This review summarizes findings showing the benefits of a treatment with melatonin in experimental models of these inflammatory ocular diseases.

RESULTS

Oxidative and nitrosative damage, tumor necrosis factor, and prostaglandin production have been involved in the pathogeny of uveitis and optic neuritis. Melatonin is an efficient antioxidant and antinitridergic, and has the ability to reduce prostaglandin and tumor necrosis factor levels both in the retina and optic nerve. Moreover, melatonin not only prevents functional and structural consequences of experimental uveitis and optic neuritis, but it is also capable of suppressing the actively ongoing ocular inflammatory response.

CONCLUSIONS

Since melatonin protects ocular tissues against inflammation, it could be a potentially useful anti-inflammatory therapy in ophthalmology.

Neuroprotective effect of melatonin in experimental optic neuritis in rats

Optic neuritis (ON) is an inflammatory, demyelinating, and neurodegenerative condition of the optic nerve, which might induce permanent vision loss. Currently, there are no effective therapies for this disorder. We have developed an experimental model of primary ON in rats through a single microinjection of 4.5 μg of bacterial lipopolysaccharide (LPS) into the optic nerve. Since melatonin acts as a pleiotropic therapeutic agent in various neurodegenerative diseases, we analyzed the effect of melatonin on LPS-induced ON. For this purpose, LPS or vehicle were injected into the optic nerve from adult male Wistar rats. One group of animals received a subcutaneous pellet of 20 mg melatonin at 24 hr before vehicle or LPS injection, and another group was submitted to a sham procedure. Melatonin completely prevented the decrease in visual evoked potentials (VEPs), and pupil light reflex (PLR), and preserved anterograde transport of cholera toxin β-subunit from the retina to the superior colliculus. Moreover, melatonin prevented microglial reactivity (ED1-immunoreactivity, P < 0.01), astrocytosis (glial fibrillary acid protein-immunostaining, P < 0.05), demyelination (luxol fast blue staining, P < 0.01), and axon (toluidine blue staining, P < 0.01) and retinal ganglion cell (Brn3a-immunoreactivity, P < 0.01) loss, induced by LPS. Melatonin completely prevented the increase in nitric oxide synthase 2, cyclooxygenase-2 levels (Western blot) and TNFα levels, and partly prevented lipid peroxidation induced by experimental ON. When the pellet of melatonin was implanted at 4 days postinjection of LPS, it completely reversed the decrease in VEPs and PLR. These data suggest that melatonin could be a promising candidate for ON treatment.

Treatment with melatonin after onset of experimental uveitis attenuates ocular inflammation

BACKGROUND AND PURPOSE

Uveitis is a prevalent intraocular inflammatory disease and one of the most damaging ocular conditions. Pretreatment with melatonin prevented ocular inflammation induced by an intravitreal injection of bacterial LPS in the Syrian hamster. Here, we have assessed the anti-inflammatory effects of melatonin administered after the onset of ocular inflammation.

EXPERIMENTAL APPROACH

The eyes of male Syrian hamsters were intravitreally injected with vehicle or LPS. Melatonin was injected i.p. every 24 h, starting 12 or 24 h after the LPS injection. A clinical evaluation (with a score index based on clinical symptoms), the number of infiltrating cells, protein concentration and PGE2 and PGF2α levels in the aqueous humour, as well as retinal NOS activity, lipid peroxidation and TNF-α levels were assessed. Retinal function was assessed by scotopic electroretinography, and light microscopy and immunohistochemistry were used to evaluate the state of the retinal structure.

KEY RESULTS

Both treatment regimens with melatonin decreased clinical symptoms, reduced the leakage of cells and proteins, and decreased PG levels in aqueous humour from eyes injected with LPS. In addition, melatonin treatment blocked the decrease in scotopic electroretinogram a- and b-wave amplitude, protected the retinal structure and reduced the increase in NOS activity, lipid peroxidation and TNF-α levels, induced by LPS.

CONCLUSIONS AND IMPLICATIONS

These results indicate that treatment with melatonin, starting after the onset of uveitis, attenuated ocular inflammation induced by LPS in the Syrian hamster and support the use of melatonin as a therapeutic resource for uveitis treatment.

Experimental optic neuritis induced by the microinjection of lipopolysaccharide into the optic nerve

Optic neuritis (ON) is a condition involving primary inflammation, demyelination, and axonal injury in the optic nerve which leads to retinal ganglion cell (RGC) loss, and visual dysfunction. We investigated the ability of a single microinjection of bacterial lipopolysaccharide (LPS) directly into the optic nerve to induce functional and structural alterations compatible with ON. For this purpose, optic nerves from male Wistar rats remained intact or were injected with vehicle or LPS. The effect of LPS was evaluated at several time points post-injection in terms of: i) visual pathway and retinal function (visual evoked potentials (VEPs) and electroretinograms, (ERGs), respectively), ii) anterograde transport from the retina to its projection areas, iii) consensual pupil light reflex (PLR), iv) optic nerve histology, v) microglia/macrophage reactivity (by Iba-1- and ED1-immunostaining), vi) astrocyte reactivity (by glial fibrillary acid protein-immunostaining), vii) axon number (by toluidine blue staining), vii) demyelination (by myelin basic protein immunoreactivity and luxol fast blue staining), viii) optic nerve ultrastructure, and ix) RGC number (by Brn3a immunoreactivity). LPS induced a significant and persistent decrease in VEP amplitude and PLR, without changes in the ERG. In addition, LPS induced a deficit in anterograde transport, and an early inflammatory response consisting in an increased cellularity, and Iba-1 and ED1-immunoreactivity in the optic nerve, which were followed by changes in axonal density, astrocytosis, demyelination, and axon and RGC loss. These results suggest that the microinjection of LPS into the optic nerve may serve as a new experimental model of primary ON.

Environmental enrichment protects the retina from early diabetic damage in adult rats

Diabetic retinopathy is a leading cause of reduced visual acuity and acquired blindness. Available treatments are not completely effective. We analyzed the effect of environmental enrichment on retinal damage induced by experimental diabetes in adult Wistar rats. Diabetes was induced by an intraperitoneal injection of streptozotocin. Three days after vehicle or streptozotocin injection, animals were housed in enriched environment or remained in a standard environment. Retinal function (electroretinogram, and oscillatory potentials), retinal morphology, blood-retinal barrier integrity, synaptophysin, astrocyte and Müller cell glial fibrillary acidic protein, vascular endothelial growth factor, tumor necrosis factor-α, and brain-derived neurotrophic factor levels, as well as lipid peroxidation were assessed in retina from diabetic animals housed in standard or enriched environment. Environmental enrichment preserved scotopic electroretinogram a-wave, b-wave and oscillatory potential amplitude, avoided albumin-Evan's blue leakage, prevented the decrease in retinal synaptophysin and astrocyte glial fibrillary acidic protein levels, the increase in Müller cell glial fibrillary acidic protein, vascular endothelial growth factor and tumor necrosis factor-α levels, as well as oxidative stress induced by diabetes. In addition, enriched environment prevented the decrease in retinal brain-derived neurotrophic factor levels induced by experimental diabetes. When environmental enrichment started 7 weeks after diabetes onset, retinal function was significantly preserved. These results indicate that enriched environment could attenuate the early diabetic damage in the retina from adult rats.

Therapeutic benefit of radial optic neurotomy in a rat model of glaucoma

Radial optic neurotomy (RON) has been proposed as a surgical treatment to alleviate the neurovascular compression and to improve the venous outflow in patients with central retinal vein occlusion. Glaucoma is characterized by specific visual field defects due to the loss of retinal ganglion cells and damage to the optic nerve head (ONH). One of the clinical hallmarks of glaucomatous neuropathy is the excavation of the ONH. The aim of this work was to analyze the effect of RON in an experimental model of glaucoma in rats induced by intracameral injections of chondroitin sulfate (CS). For this purpose, Wistar rats were bilaterally injected with vehicle or CS in the eye anterior chamber, once a week, for 10 weeks. At 3 or 6 weeks of a treatment with vehicle or CS, RON was performed by a single incision in the edge of the neuro-retinal ring at the nasal hemisphere of the optic disk in one eye, while the contralateral eye was submitted to a sham procedure. Electroretinograms (ERGs) were registered under scotopic conditions and visual evoked potentials (VEPs) were registered with skull-implanted electrodes. Retinal and optic nerve morphology was examined by optical microscopy. RON did not affect the ocular hypertension induced by CS. In eyes injected with CS, a significant decrease of retinal (ERG a- and b-wave amplitude) and visual pathway (VEP N2-P2 component amplitude) function was observed, whereas RON reduced these functional alterations in hypertensive eyes. Moreover, a significant loss of cells in the ganglion cell layer, and Thy-1-, NeuN- and Brn3a- positive cells was observed in eyes injected with CS, whereas RON significantly preserved these parameters. In addition, RON preserved the optic nerve structure in eyes with chronic ocular hypertension. These results indicate that RON reduces functional and histological alterations induced by experimental chronic ocular hypertension.

Therapeutic benefit of melatonin in experimental feline uveitis

Uveitis is a frequent ophthalmic disorder which constitutes one of the main causes of blindness in domestic cats. The aim of this report was to analyze the effect of melatonin on experimentally induced uveitis in cats. Bacterial lipopolysaccharide (LPS) was injected intravitreally into one eye from intact cats, while the contralateral eye was injected with vehicle. Melatonin was orally administered every 24 hr to a group of ten cats, from 24 hr before until 45 days after intravitreal injections. Eyes were evaluated by means of clinical evaluation, intraocular pressure (IOP), blood-ocular barrier integrity (via measurement of protein concentration and cell content in samples of aqueous humor [AH]), electroretinogram (ERG), and histological examination of the retinas. In LPS-treated eyes, several clinical signs were observed until day 45 postinjection. The treatment with melatonin significantly decreased clinical signs and prevented the reduction in IOP induced by LPS. In LPS-injected eyes, melatonin significantly preserved the blood-ocular barrier integrity, as shown by a decrease in the number of infiltrating cells and protein concentration in the AH. Mean amplitudes of scotopic ERG a- and b-waves were significantly reduced in eyes injected with LPS, whereas melatonin significantly prevented the effect of LPS. At 45 days after injection, LPS induced alterations in photoreceptors and at the middle portion of the retina, whereas melatonin preserved the retinal structure. These results indicate that melatonin prevented clinical, biochemical, functional, and histological alterations induced by LPS injection. Thus, melatonin might constitute a useful tool for the treatment of feline uveitis.

Induction of ischemic tolerance protects the retina from diabetic retinopathy

Diabetic retinopathy is a leading cause of acquired blindness. Available treatments are not very effective. We investigated the effect of a weekly application of retinal ischemia pulses (ischemic conditioning) on retinal damage induced by experimental diabetes. Diabetes was induced by an intraperitoneal injection of streptozotocin. Retinal ischemia was induced by increasing intraocular pressure to 120 mmHg for 5 minutes; this maneuver started 3 days after streptozotocin injection and was weekly repeated in one eye, whereas the contralateral eye was submitted to a sham procedure. Diabetic retinopathy was evaluated in terms of i) retinal function (electroretinogram and oscillatory potentials), ii) integrity of blood-retinal barrier (by albumin-Evans blue complex leakage and astrocyte glial fibrillary acidic protein IHC), iii) optical and electron microscopy histopathologic studies, and iv) vascular endothelial growth factor levels (using Western blot analysis and IHC). Brief ischemia pulses significantly preserved electroretinogram a- and b-wave and oscillatory potentials, avoided albumin-Evans blue leakage, prevented the decrease in astrocyte glial fibrillary acidic protein levels, reduced the appearance of retinal edemas, and prevented the increase in vascular endothelial growth factor levels induced by experimental diabetes. When the application of ischemia pulses started 6 weeks after diabetes onset, retinal function was significantly preserved. These results indicate that induction of ischemic tolerance could constitute a fertile avenue for the development of new therapeutic strategies for diabetic retinopathy treatment.

Ischemic tolerance protects the rat retina from glaucomatous damage

Glaucoma is a leading cause of acquired blindness which may involve an ischemic-like insult to retinal ganglion cells and optic nerve head. We investigated the effect of a weekly application of brief ischemia pulses (ischemic conditioning) on the rat retinal damage induced by experimental glaucoma. Glaucoma was induced by weekly injections of chondroitin sulfate (CS) in the rat eye anterior chamber. Retinal ischemia was induced by increasing intraocular pressure to 120 mmHg for 5 min; this maneuver started after 6 weekly injections of vehicle or CS and was weekly repeated in one eye, while the contralateral eye was submitted to a sham procedure. Glaucoma was evaluated in terms of: i) intraocular pressure (IOP), ii) retinal function (electroretinogram (ERG)), iii) visual pathway function (visual evoked potentials, (VEPs)) iv) histology of the retina and optic nerve head. Retinal thiobarbituric acid substances levels were assessed as an index of lipid peroxidation. Ischemic conditioning significantly preserved ERG, VEPs, as well as retinal and optic nerve head structure from glaucomatous damage, without changes in IOP. Moreover, ischemia pulses abrogated the increase in lipid peroxidation induced by experimental glaucoma. These results indicate that induction of ischemic tolerance could constitute a fertile avenue for the development of new therapeutic strategies in glaucoma treatment.

Melatonin: a novel neuroprotectant for the treatment of glaucoma

Glaucoma is a leading cause of blindness. Although ocular hypertension is the most important risk factor, several concomitant factors such as elevation of glutamate and decrease in gamma-aminobutyric acid (GABA) levels, disorganized NO metabolism, and oxidative damage could significantly contribute to the neurodegeneration. The aim of this report was to analyze the effect of melatonin on retinal glutamate clearance, GABA concentrations, NO synthesis, and retinal redox status, as well as on functional and histological alterations provoked by chronic ocular hypertension induced by intracameral injections of hyaluronic acid (HA) in the rat eye. In normal retinas, melatonin increased glutamate uptake, glutamine synthase activity, GABA turnover rate, glutamic acid decarboxylase activity, superoxide dismutase activity, and reduced glutathione (GSH) levels, whereas it decreased NOS activity, L-arginine uptake, and lipid peroxidation. To assess the effect of melatonin on glaucomatous neuropathy, weekly injections of HA were performed in the eye anterior chamber. A pellet of melatonin was implanted subcutaneously 24 hr before the first injection or after six weekly injections of HA. Melatonin, which did not affect intraocular pressure (IOP), prevented and reversed the effect of ocular hypertension on retinal function (assessed by electroretinography) and diminished the vulnerability of retinal ganglion cells to the deleterious effects of ocular hypertension. These results indicate that melatonin could be a promissory resource in the management of glaucoma.

Circadian rhythm of intraocular pressure in cats

OBJECTIVE

To evaluate the rhythm of intraocular pressure (IOP) in healthy domestic cats with no evidence of ocular disease and to analyze the influence of photoperiod, age, gender and ocular diseases on diurnal-nocturnal variations of cat IOP.

ANIMALS

All animals were Domestic Short-haired cats; 30 were without systemic or ocular diseases, classified as follows: 12 male intact adult cats, five intact adult female, five adult spayed female, and eight male cats; the latter were less than 1 year of age. In addition, five adult cats with uveitis and three adult cats with secondary glaucoma were included.

PROCEDURE

IOP was assessed with a Tono-Pen XL at 3-h intervals over a 24-h period in 12 healthy adult male cats kept under a photoperiod of 12-h light/12-h darkness for 2 weeks. Eight animals from the same group were then kept under constant darkness for 48 h, and IOP was measured at 3-h intervals for the following 24 h. In addition, IOP was assessed at 3 p.m. and 9 p.m. in five intact females, five spayed females, and in eight young cats, as well as in five adult cats with uveitis and three glaucomatous cats.

RESULTS

Consistent, daily variations in IOP were observed in animals exposed to a light-dark cycle, with maximal values during the night. In cats exposed to constant darkness, maximal values of IOP were observed at subjective night. Differences of IOP values between 3 p.m. and 9 p.m. (diurnal-nocturnal variations) persisted in intact females, spayed females, and young animals, as well as in uveitic and glaucomatous eyes.

CONCLUSIONS

The present results indicate a daily rhythm of cat IOP, which appears to persist in constant darkness, suggesting some level of endogenous circadian control. In addition, daily variations of cat IOP seem to be independent of gender, age, or ocular diseases (particularly uveitis and glaucoma).

A new experimental model of glaucoma in rats through intracameral injections of hyaluronic acid

An experimental model of pressure-induced optic nerve damage would greatly facilitate the understanding of the cellular events leading to ganglion cell death, and how they are influenced by intraocular pressure and other risk factors associated to glaucoma. The aim of the present report was to study the effect of a long-term increase of intraocular pressure in rats induced by intracameral injections of hyaluronic acid with respect to electroretinographic activity and retinal and optic nerve histology. For this purpose, hyaluronic acid was injected weekly in the rat anterior chamber of one eye, whereas the contralateral eye was injected with saline solution. The results showed a significant decrease of oscillatory potentials and a- and b-wave amplitude of the scotopic electroretinogram after 3 or 6 weeks of hyaluronic acid administration, respectively. These parameters were further reduced after 10 weeks of treatment with hyaluronic acid. No significant changes in anterior chamber angle structures from hyaluronic acid- and vehicle-injected eyes were observed, whereas a significant loss of ganglion cell layer cells and of optic nerve axons were detected in animals that received hyaluronic acid for 10 weeks, as compared to eyes injected with saline solution. In summary, present results indicate that the chronic administration of hyaluronic acid induced a significant decrease in the electroretinographic activity and histological changes in the retina and optic nerve that seem consistent with some features of chronic open-angle glaucoma. Therefore, this could be an experimental model to study the cellular mechanisms by which elevated intraocular pressure damages the optic nerve and the retina.