Displaced retinal ganglion cells in albino and pigmented rats

Neuroprotection by α2-Adrenergic Receptor Stimulation after Excitotoxic Retinal Injury: A Study of the Total Population of Retinal Ganglion Cells and Their Distribution in the Chicken Retina

We have studied the effect of α2-adrenergic receptor stimulation on the total excitotoxically injured chicken retinal ganglion cell population. N-methyl-D-aspartate (NMDA) was intraocularly injected at embryonic day 18 and Brn3a positive retinal ganglion cells (Brn3a+ RGCs) were counted in flat-mounted retinas using automated routines. The number and distribution of the Brn3a+ RGCs were analyzed in series of normal retinas from embryonic day 8 to post-hatch day 11 retinas and in retinas 7 or 14 days post NMDA lesion. The total number of Brn3a+ RGCs in the post-hatch retina was approximately 1.9x10⁶ with a density of approximately 9.2x10³ cells/mm². The isodensity maps of normal retina showed that the density decreased with age as the retinal size increased. In contrast to previous studies, we did not find any specific region with increased RGC density, rather the Brn3a+ RGCs were homogeneously distributed over the central retina with decreasing density in the periphery and in the region of the pecten oculli. Injection of 5–10 μg NMDA caused 30–50% loss of Brn3a+ cells and the loss was more severe in the dorsal than in the ventral retina. Pretreatment with brimonidine reduced the loss of Brn3a+ cells both 7 and 14 days post lesion and the protective effect was higher in the dorsal than in the ventral retina. We conclude that α2-adrenergic receptor stimulation reduced the impact of the excitotoxic injury in chicken similarly to what has been shown in mammals. Furthermore, the data show that the RGCs are evenly distributed over in the retina, which challenges previous results that indicate the presence of specific high RGC-density regions of the chicken retina.

Understanding Glaucomatous Optic Neuropathy: The Synergy Between Clinical Observation and Investigation

Glaucoma is a complex disorder of aging defined by the death of retinal ganglion cells and remodeling of connective tissues at the optic nerve head. Intraocular pressure-induced axonal injury at the optic nerve head leads to apoptosis. Loss of retinal ganglion cells follows a slowly progressive sequence. Clinical features of the disease have suggested and corroborated pathological events. The death of retinal ganglion cells causes secondary loss of neurons in the brain, but only as a by-product of injury to the retinal ganglion cells. Although therapy to lower intraocular pressure is moderately effective, new treatments are being developed to alter the remodeling of ocular connective tissue, to interrupt the injury signal from axon to soma, and to upregulate a variety of survival mechanisms.

Non-image Forming Light Detection by Melanopsin, Rhodopsin, and Long-Middlewave (L/W) Cone Opsin in the Subterranean Blind Mole Rat, Spalax Ehrenbergi: Immunohistochemical Characterization, Distribution, and Connectivity

The blind mole rat, Spalax ehrenbergi, can, despite severely degenerated eyes covered by fur, entrain to the daily light/dark cycle and adapt to seasonal changes due to an intact circadian timing system. The present study demonstrates that the Spalax retina contains a photoreceptor layer, an outer nuclear layer (ONL), an outer plexiform layer (OPL), an inner nuclear layer (INL), an inner plexiform layer (IPL), and a ganglion cell layer (GCL). By immunohistochemistry, the number of melanopsin (mRGCs) and non-melanopsin bearing retinal ganglion cells was analyzed in detail. Using the ganglion cell marker RNA-binding protein with multiple splicing (RBPMS) it was shown that the Spalax eye contains 890 ± 62 RGCs. Of these, 87% (752 ± 40) contain melanopsin (cell density 788 melanopsin RGCs/mm²). The remaining RGCs were shown to co-store Brn3a and calretinin. The melanopsin cells were located mainly in the GCL with projections forming two dendritic plexuses located in the inner part of the IPL and in the OPL. Few melanopsin dendrites were also found in the ONL. The Spalax retina is rich in rhodopsin and long/middle wave (L/M) cone opsin bearing photoreceptor cells. By using Ctbp2 as a marker for ribbon synapses, both rods and L/M cone ribbons containing pedicles in the OPL were found in close apposition with melanopsin dendrites in the outer plexus suggesting direct synaptic contact. A subset of cone bipolar cells and all photoreceptor cells contain recoverin while a subset of bipolar and amacrine cells contain calretinin. The calretinin expressing amacrine cells seemed to form synaptic contacts with rhodopsin containing photoreceptor cells in the OPL and contacts with melanopsin cell bodies and dendrites in the IPL. The study demonstrates the complex retinal circuitry used by the Spalax to detect light, and provides evidence for both melanopsin and non-melanopsin projecting pathways to the brain.

Retinal neurodegeneration in experimental glaucoma

In rats and mice, limbar tissues of the left eye were laser-photocoagulated (LP) and ocular hypertension (OHT) effects were investigated 1 week to 6 months later. To investigate the innermost layers, retinas were examined in wholemounts using tracing from the superior colliculi to identify retinal ganglion cells (RGCs) with intact retrograde axonal transport, melanopsin immunodetection to identify intrinsically photosensitive RGCs (m(+)RGC), Brn3a immunodetection to identify most RGCs but not m(+)RGCs, RECA1 immunodetection to examine the inner retinal vessels, and DAPI staining to detect all nuclei in the GC layer. The outer retinal layers (ORLs) were examined in cross sections analyzed morphometrically or in wholemounts to study S- and L-cones. Innervation of the superior colliculi was examined 10 days to 14 weeks after LP with orthogradely transported cholera toxin subunit B. By 2 weeks, OHT resulted in pie-shaped sectors devoid of FG(+)RGCs or Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. Brn3a(+)RGCs were significantly greater than FG(+)RGCs, indicating the survival of large numbers of RGCs with their axonal transport impaired. The inner retinal vasculature showed no abnormalities that could account for the sectorial loss of RGCs. m(+)RGCs decreased to approximately 50-51% in a diffuse loss across the retina. Cross sections showed focal areas of degeneration in the ORLs. RGC loss at 1m diminished to 20-25% and did not progress further with time, whereas the S- and L-cone populations diminished progressively up to 6m. The retinotectal projection was reduced by 10 days and did not progress further. LP-induced OHT results in retrograde degeneration of RGCs and m(+)RGCs, severe damage to the ORL, and loss of retinotectal terminals.

Effects of ocular hypertension in the visual system of pigmented mice

To study the effects of ocular hypertension (OHT) on the visual system of C57BL/6 pigmented mice, the limbal and episcleral veins of the left eye were laser photocoagulated (LP). LP increased the intraocular pressure during the first five days (d), reaching basal values at 7d. To investigate the effect of OHT on the retinal ganglion cell (RGC) retrograde axonal transport, hydroxistilbamidine methanesulfonate (OHSt) was applied to both superior colliculi (SCi) and the retinas were dissected 2 or 4 weeks after LP. To determine RGC survival, these same retinas were immunoreacted against Brn3a (general RGC population) and melanopsin (intrinsically photosensitive RGCs, m⁺RGCs). To study whether OHT affected non-RGC neurons in the ganglion cell layer (GCL), RGCs were immunodetected with Brn3a and all GCL nuclei counterstained with DAPI in a group of animals examined 4 weeks post-LP. Innervation of the SCi was examined at 10 days, 8 or 14 weeks after LP with the orthogradely transported cholera toxin subunit-B. OHT resulted in diffuse and sectorial loss of OHSt+RGCs (50% at 2 weeks and 62% at 4 weeks) and in a comparable loss of Brn3a+RGCs at the same time intervals. m+RGCs decreased to 59% at 2 weeks and to 46% at 4 weeks, such loss was diffuse, did not parallel the sectorial loss of the general RGC population and was more severe in the superior-temporal retina. In the GCL, cell loss is selective for RGCs and does not affect other non-RGC neurons. The retinotectal innervation appeared significantly reduced at 10 days (55.7%) and did not progress further up to 14 weeks (46.6%). Thus, LP-induced OHT results in retrograde degeneration of RGCs and m+RGCs, as well as in the loss of CTB-labelled retinotectal terminals.

Retino-retinal projection in juvenile and young adult rats and mice

Identification of retino-retinal projecting RGCs (ret-ret RGCs) has been accomplished by tracing RGCs in one retina after intravitreal injection of different tracers in the other eye. In mammals, rabbit and rat, ret-ret RGCs are scarce and more abundant in newborn than in adult animals. To our knowledge, ret-ret RGCs have not been studied in mice. Here we purpose to revisit the presence of ret-ret RGCs in juvenile and young adult rats and mice by using retrograde tracers applied to the contralateral optic nerve instead of intravitreally. In P20 (juvenile) and P60 (young adult) animals, the left optic nerve was intraorbitally transected and Fluorogold (rats) or its analogue OHSt (mice) were applied onto its distal stump. P20 animals were sacrificed 3 (mice) or 5 (rats) days later and adult animals at 5 (mice) or 7 (rats) days. Right retinas were dissected as flat-mounts and double immunodetected for Brn3a and melanopsin. Ret-ret RGCs were those with tracer accumulation in their somas. Out of them some expressed Brn3a and/or melanopsin, while other were negative for both markers. In young adult rats, we found 2 ret-ret RGCs displaced to the inner nuclear layer. In both species, ret-ret RGCs are quite scarce and found predominantly in the nasal retina. In juvenile animals there are significantly more ret-ret RGCs (9 ± 3, rats, 13 ± 3 mice) than in young adult ones (5 ± 6 rats, 7 ± 3 mice). Finally, juvenile and young adult mice have more ret-ret RGCs than rats.

The retino-retinal projection: Tracing retinal ganglion cells projecting to the contralateral retina

We investigated the presence of a direct retino-retinal (R-R) projection between the two eyes via the optic chiasm of retinal ganglion cells (RGCs) in adult Long-Evans rats. We also explored the presence of collateral projections originating from these cells to the brain. In the first group of animals, right optic nerves (ONs) were orbitally transected approximately 2mm behind the globe followed by application of fluorochrome (2% Fluorogold [FG]) to the optic nerve stump to retrogradely label the R-R projection RGCs (R-RGCs) on the contralateral side. Animals were then sacrificed after 3, 5, 7, or 21 days. Contralateral retinas were fixed, whole-mounted, and imaged for R-RGCs. In a second group of animals, RGCs were retrogradely labeled with 15% rhodamine-β-isothiocynate (RITC) at the superior colliculi, where approximately 96% of rat RGCs synapse. Seven days later, the right ONs were transected and 2% FG applied to the proximal and distal ON stumps. Animals were then sacrificed after 5 days. Contralateral retinas were examined for co-labeled (RITC/FG) RGCs. Control rats underwent the same procedures excluding fluorescent tracer application. In the first group of animals, the number of R-RGCs in the contralateral eye ranged from 3 to 25 and did not depend on survival time. The second group of animals revealed evidence of co-labeled contralateral RGCs. Results suggest that a greater number of R-RGCs persist into adulthood than previously reported [M. Müller, H. Holländer, 1988]. Furthermore, the presence of co-labeled RGCs in the contralateral eye indicates that in adult rodents some R-R projections have a collateral projection to the brain, whereas previous reports had only found collateral projections in newborns.

Laser-induced ocular hypertension in adult rats does not affect non-RGC neurons in the ganglion cell layer but results in protracted severe loss of cone-photoreceptors

To investigate the long-term effects of laser-photocoagulation (LP)-induced ocular hypertension (OHT) in the innermost and outermost (outer-nuclear and outer segment)-retinal layers (ORL). OHT was induced in the left eye of adult rats. To investigate the ganglion cell layer (GCL) wholemounts were examined at 1, 3 or 6 months using Brn3a-immunodetection to identify retinal ganglion cells (RGCs) and DAPI-staining to detect all nuclei in this layer. To study the effects of LP on the ORL up to 6 months, retinas were: i) fresh extracted to quantify the levels of rod-, S- and L-opsin; ii) cut in cross-sections for morphometric analysis, or; iii) prepared as wholemounts to quantify and study retinal distributions of entire populations of RGCs (retrogradely labeled with fluorogold, FG), S- and L-cones (immunolabeled). OHT resulted in wedge-like sectors with their apex on the optic disc devoid of Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. The levels of all opsins diminished by 2 weeks and further decreased to 20% of basal-levels by 3 months. Cross-sections revealed focal areas of ORL degeneration. RGC survival at 15 days represented approximately 28% and did not change with time, whereas the S- and L-cone populations diminished to 65% and 80%, or to 20 and 35% at 1 or 6 months, respectively. In conclusion, LP induces in the GCL selective RGCs loss that does not progress after 1 month, and S- and L-cone loss that progresses for up to 6 months. Thus, OHT results in severe damage to both the innermost and the ORL.

A novel in vivo model of focal light emitting diode-induced cone-photoreceptor phototoxicity: neuroprotection afforded by brimonidine, BDNF, PEDF or bFGF

We have investigated the effects of light-emitting diode (LED)-induced phototoxicity (LIP) on cone-photoreceptors and their protection with brimonidine (BMD), brain-derived neurotrophic factor (BDNF), pigment epithelium-derived factor (PEDF), ciliary neurotrophic factor (CNTF) or basic fibroblast growth factor (bFGF). In anesthetized, dark adapted, adult albino rats a blue (400 nm) LED was placed perpendicular to the cornea (10 sec, 200 lux) and the effects were investigated using Spectral Domain Optical Coherence Tomography (SD-OCT) and/or analysing the retina in oriented cross-sections or wholemounts immune-labelled for L- and S-opsin and counterstained with the nuclear stain DAPI. The effects of topical BMD (1%) or, intravitreally injected BDNF (5 µg), PEDF (2 µg), CNTF (0.4 µg) or bFGF (1 µg) after LIP were examined on wholemounts at 7 days. SD-OCT showed damage in a circular region of the superotemporal retina, whose diameter varied from 1,842.4±84.5 µm (at 24 hours) to 1,407.7±52.8 µm (at 7 days). This region had a progressive thickness diminution from 183.4±5 µm (at 12 h) to 114.6±6 µm (at 7 d). Oriented cross-sections showed within the light-damaged region of the retina massive loss of rods and cone-photoreceptors. Wholemounts documented a circular region containing lower numbers of L- and S-cones. Within a circular area (1 mm or 1.3 mm radius, respectively) in the left and in its corresponding region of the contralateral-fellow-retina, total L- or S-cones were 7,118±842 or 661±125 for the LED exposed retinas (n = 7) and 14,040±1,860 or 2,255±193 for the fellow retinas (n = 7), respectively. BMD, BDNF, PEDF and bFGF but not CNTF showed significant neuroprotective effects on L- or S-cones. We conclude that LIP results in rod and cone-photoreceptor loss, and is a reliable, quantifiable model to study cone-photoreceptor degeneration. Intravitreal BDNF, PEDF or bFGF, or topical BMD afford significant cone neuroprotection in this model.

A role for the outer retina in development of the intrinsic pupillary light reflex in mice

Mice do not require the brain in order to maintain constricted pupils. However, little is known about this intrinsic pupillary light reflex (iPLR) beyond a requirement for melanopsin in the iris and an intact retinal ciliary marginal zone (CMZ). Here, we study the mouse iPLR in vitro and examine a potential role for outer retina (rods and cones) in this response. In wild-type mice the iPLR was absent at postnatal day 17 (P17), developing progressively from P21-P49. However, the iPLR only achieved ∼ 30% of the wild-type constriction in adult mice with severe outer retinal degeneration (rd and rdcl). Paradoxically, the iPLR increased significantly in retinal degenerate mice >1.5 years of age. This was accompanied by an increase in baseline pupil tone in the dark to levels indistinguishable from those in adult wild types. This rejuvenated iPLR response was slowed by atropine application, suggesting the involvement of cholinergic neurotransmission. We could find no evidence of an increase in melanopsin expression by quantitative PCR in the iris and ciliary body of aged retinal degenerates and a detailed anatomical analysis revealed a significant decline in melanopsin-positive intrinsically photosensitive retinal ganglion cells (ipRGCs) in rdcl mice >1.5 years. Adult mice lacking rod function (Gnat1(-/-)) also had a weak iPLR, while mice lacking functional cones (Cpfl5) maintained a robust response. We also identify an important role for pigmentation in the development of the mouse iPLR, with only a weak and transient response present in albino animals. Our results show that the iPLR in mice develops unexpectedly late and are consistent with a role for rods and pigmentation in the development of this response in mice. The enhancement of the iPLR in aged degenerate mice was extremely surprising but may have relevance to behavioral observations in mice and patients with retinitis pigmentosa.

Distribution of melanopsin positive neurons in pigmented and albino mice: evidence for melanopsin interneurons in the mouse retina

Here we have studied the population of intrinsically photosensitive retinal ganglion cells (ipRGCs) in adult pigmented and albino mice. Our data show that although pigmented (C57Bl/6) and albino (Swiss) mice have a similar total number of ipRGCs, their distribution is slightly different: while in pigmented mice ipRGCs are more abundant in the temporal retina, in albinos the ipRGCs are more abundant in superior retina. In both strains, ipRGCs are located in the retinal periphery, in the areas of lower Brn3a⁺RGC density. Both strains also contain displaced ipRGCs (d-ipRGCs) in the inner nuclear layer (INL) that account for 14% of total ipRGCs in pigmented mice and 5% in albinos. Tracing from both superior colliculli shows that 98% (pigmented) and 97% (albino) of the total ipRGCs, become retrogradely labeled, while double immunodetection of melanopsin and Brn3a confirms that few ipRGCs express this transcription factor in mice. Rather surprisingly, application of a retrograde tracer to the optic nerve (ON) labels all ipRGCs, except for a sub-population of the d-ipRGCs (14% in pigmented and 28% in albino, respectively) and melanopsin positive cells residing in the ciliary marginal zone (CMZ) of the retina. In the CMZ, between 20% (pigmented) and 24% (albino) of the melanopsin positive cells are unlabeled by the tracer and we suggest that this may be because they fail to send an axon into the ON. As such, this study provides the first evidence for a population of melanopsin interneurons in the mammalian retina.