Dark adaptation is faster in pigmented than albino rats

Rat superior colliculus encodes the transition between static and dynamic vision modes

The visual continuity illusion involves a shift in visual perception from static to dynamic vision modes when the stimuli arrive at high temporal frequency, and is critical for recognizing objects moving in the environment. However, how this illusion is encoded across the visual pathway remains poorly understood, with disparate frequency thresholds at retinal, cortical, and behavioural levels suggesting the involvement of other brain areas. Here, we employ a multimodal approach encompassing behaviour, whole-brain functional MRI, and electrophysiological measurements, for investigating the encoding of the continuity illusion in rats. Behavioural experiments report a frequency threshold of 18±2 Hz. Functional MRI reveal that superior colliculus signals transition from positive to negative at the behaviourally-driven threshold, unlike thalamic and cortical areas. Electrophysiological recordings indicate that these transitions are underpinned by neural activation/suppression. Lesions in the primary visual cortex reveal this effect to be intrinsic to the superior colliculus (under a cortical gain effect). Our findings highlight the superior colliculus' crucial involvement in encoding temporal frequency shifts, especially the change from static to dynamic vision modes.

Levodopa Rescues Retinal Function in the Transgenic A53T Alpha-Synuclein Model of Parkinson's Disease

BACKGROUND

Loss of substantia nigra dopaminergic cells and alpha-synuclein (α-syn)-rich intraneuronal deposits within the central nervous system are key hallmarks of Parkinson's disease (PD). Levodopa (L-DOPA) is the current gold-standard treatment for PD. This study aimed to evaluate in vivo retinal changes in a transgenic PD model of α-syn overexpression and the effect of acute levodopa (L-DOPA) treatment.

METHODS

Anaesthetised 6-month-old mice expressing human A53T alpha-synuclein (HOM) and wildtype (WT) control littermates were intraperitoneally given 20 mg/kg L-DOPA (50 mg levodopa, 2.5 mg benserazide) or vehicle saline (n = 11-18 per group). In vivo retinal function (dark-adapted full-field ERG) and structure (optical coherence tomography, OCT) were recorded before and after drug treatment for 30 min. Ex vivo immunohistochemistry (IHC) on flat-mounted retina was conducted to assess tyrosine hydroxylase (TH) positive cell counts (n = 7-8 per group).

RESULTS

We found that photoreceptor (a-wave) and bipolar cell (b-wave) ERG responses (p < 0.01) in A53T HOM mice treated with L-DOPA grew in amplitude more (47 ± 9%) than WT mice (16 ± 9%) treated with L-DOPA, which was similar to the vehicle group (A53T HOM 25 ± 9%; WT 19 ± 7%). While outer retinal thinning (outer nuclear layer, ONL, and outer plexiform layer, OPL) was confirmed in A53T HOM mice (p < 0.01), L-DOPA did not have an ameliorative effect on retinal layer thickness. These findings were observed in the absence of changes to the number of TH-positive amacrine cells across experiment groups. Acute L-DOPA treatment transiently improves visual dysfunction caused by abnormal alpha-synuclein accumulation.

CONCLUSIONS

These findings deepen our understanding of dopamine and alpha-synuclein interactions in the retina and provide a high-throughput preclinical framework, primed for translation, through which novel therapeutic compounds can be objectively screened and assessed for fast-tracking PD drug discovery.

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.

Retinal alpha-synuclein accumulation correlates with retinal dysfunction and structural thinning in the A53T mouse model of Parkinson's disease

Abnormal alpha-synuclein (α-SYN) protein deposition has long been recognized as one of the pathological hallmarks of Parkinson's disease's (PD). This study considers the potential utility of PD retinal biomarkers by investigating retinal changes in a well characterized PD model of α-SYN overexpression and how these correspond to the presence of retinal α-SYN. Transgenic A53T homozygous (HOM) mice overexpressing human α-SYN and wildtype (WT) control littermates were assessed at 4, 6, and 14  months of age (male and female, n = 15-29 per group). In vivo retinal function (electroretinography, ERG) and structure (optical coherence tomography, OCT) were recorded, and retinal immunohistochemistry and western blot assays were performed to examine retinal α-SYN and tyrosine hydroxylase. Compared to WT controls, A53T mice exhibited reduced light-adapted (cone photoreceptor and bipolar cell amplitude, p < 0.0001) ERG responses and outer retinal thinning (outer plexiform layer, outer nuclear layer, p < 0.0001) which correlated with elevated levels of α-SYN. These retinal signatures provide a high throughput means to study α-SYN induced neurodegeneration and may be useful in vivo endpoints for PD drug discovery.

The impact of early-life environment on absence epilepsy and neuropsychiatric comorbidities

This review discusses the long-term effects of early-life environment on epileptogenesis, epilepsy, and neuropsychiatric comorbidities with an emphasis on the absence epilepsy. The WAG/Rij rat strain is a well-validated genetic model of absence epilepsy with mild depression-like (dysthymia) comorbidity. Although pathologic phenotype in WAG/Rij rats is genetically determined, convincing evidence presented in this review suggests that the absence epilepsy and depression-like comorbidity in WAG/Rij rats may be governed by early-life events, such as prenatal drug exposure, early-life stress, neonatal maternal separation, neonatal handling, maternal care, environmental enrichment, neonatal sensory impairments, neonatal tactile stimulation, and maternal diet. The data, as presented here, indicate that some early environmental events can promote and accelerate the development of absence seizures and their neuropsychiatric comorbidities, while others may exert anti-epileptogenic and disease-modifying effects. The early environment can lead to phenotypic alterations in offspring due to epigenetic modifications of gene expression, which may have maladaptive consequences or represent a therapeutic value. Targeting DNA methylation with a maternal methyl-enriched diet during the perinatal period appears to be a new preventive epigenetic anti-absence therapy. A number of caveats related to the maternal methyl-enriched diet and prospects for future research are discussed.

Characterization of retinal function and structure in the MPTP murine model of Parkinson’s disease

In addition to well characterized motor symptoms, visual disturbances are increasingly recognized as an early manifestation in Parkinson’s disease (PD). A better understanding of the mechanisms underlying these changes would facilitate the development of vision tests which can be used as preclinical biomarkers to support the development of novel therapeutics for PD. This study aims to characterize the retinal phenotype of a mouse model of dopaminergic dysfunction and to examine whether these changes are reversible with levodopa treatment. We use a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD to characterize the neurotoxic effects of MPTP on in vivo retinal function (electroretinography, ERG), retinal structure (optical coherence tomography, OCT) and retinal dopaminergic cell number (tyrosine hydroxylase immunohistochemistry, IHC) at two time points (21 and 45 days) post MPTP model induction. We also investigate the effect of levodopa (L-DOPA) as a proof-of-principle chronic intervention against MPTP-induced changes in the retina. We show that MPTP decreases dopaminergic amacrine cell number (9%, p < 0.05) and that a component of the ERG that involves these cells, in particular oscillatory potential (OP) peak timing, was significantly delayed at Day 45 (7–13%, p < 0.01). This functional deficit was paralleled by outer plexiform layer (OPL) thinning (p < 0.05). L-DOPA treatment ameliorated oscillatory potential deficits (7–13%, p < 0.001) in MPTP animals. Our data suggest that the MPTP toxin slows the timing of inner retinal feedback circuits related to retinal dopaminergic pathways which mirrors findings from humans with PD. It also indicates that the MPTP model causes structural thinning of the outer retinal layer on OCT imaging that is not ameliorated with L-DOPA treatment. Together, these non-invasive measures serve as effective biomarkers for PD diagnosis as well as for quantifying the effect of therapy.

Evidences Suggesting that Distinct Immunological and Cellular Responses to Light Damage Distinguishes Juvenile and Adult Rat Retinas

To unravel the mechanisms behind the higher resistance to light damage of juvenile (JR) versus adult (AR) rats, Sprague Dawley rats were exposed to a bright luminous environment of 10, 000 lux. The light-induced retinopathy (LIR) was assessed with histology, electroretinography and immunohistochemistry (IHC). In JR, 2 days of exposure induced the typical LIR, while >3 days added little LIR. IHC revealed a subtle migration of microglia (Iba1 marker) from the inner to the outer retina after 3 days of exposure in JR contrasting with the stronger reaction seen after 1 day in AR. Similarly, in JR, the Müller cells expressed less intense GFAP, CNTF and FGF2 staining compared to AR. Our results suggest that in JR the degree of retinal damage is not proportional to the duration of light exposure (i.e., dose-independent retinopathy), contrasting with the dose-dependent LIR reported in AR. The immature immune system in JR may explain the delayed and/or weaker inflammatory response compared to AR, a finding that would also point to the devastating contribution of the immune system in generating the LIR phenotype, a claim also advanced to explain the pathophysiology of other retinal degenerative disorders such as Age-related Macular Degeneration, Diabetic Retinopathy and Retinitis Pigmentosa.

Effects of Sodium Lighting On Circadian Rhythms in Rats

Rodent studies often must be conducted during an animal's active phase (that is, in darkness) yet also during a typical day shift for staff. Low-pressure sodium lighting (LPSL), to which human retinas are more sensitive than rodents' at low intensity, has been used to facilitate study conduct in dark phase. The assumption was that LPSL would be equivalent to total darkness due to low rodent retinal sensitivity but provide enough lighting for safe technical manipulations due to higher human retinal sensitivity. Unlike other light sources, LPSL has been tested for effects on circadian rhythm specific to locomotive activities in albino mice. Whether LPSL affects circadian rhythms in rats is unknown. In this study, circadian endpoints were derived from body temperature and locomotor activity via telemeters in 8 adult male Wistar rats. When moved from a 12:12-h white-light (that is, cold white fluorescent light):dark (LD) cycle to a 12:12-h white-light:sodium-light cycle, rats demonstrated free-running and disrupted circadian rhythms (that is, lengthened circadian period and reduced circadian robustness and amplitude). Body temperature and locomotor activity were significantly lower in the LPSL phase as compared with dark phase under the baseline condition. When exposed to a 12:12 h sodium-light:dark (SD) cycle, rats entrained with a circadian period similar to 12:12-h white-light:dark (LD), but significantly different from the period under constant darkness (DD). Circadian onset and acrophase were delayed under SD compared with LD. When illuminated with a LPSL pulse under DD, rats showed phase shifts similar to white-light pulse effects, consistent with the phase response curve. To determine whether the image-forming photoreceptors are involved in this process, we used electroretinography. Compared with white light, 589-nm light generated during electroretinography elicited rod photoreceptors responses with longer latency and cone photoreceptor responses with lower amplitude. These results indicate that LPSL is a weaker zeitgeber than white light and may alter the circadian system in rats. Furthermore, because LPSL appeared to be visible to rats, it may not be an appropriate substitute for actual darkness.

Methods for In Vivo CRISPR/Cas Editing of the Adult Murine Retina

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein (Cas) is used by some bacteria and most archaea to protect against viral phage intrusion and has recently been adapted to allow for efficient editing of the mammalian genome. Whilst CRISPR/Cas-based technology has been used to modify genes in mammalian cells in vitro, delivery of CRISPR/Cas system into mammalian tissue and/or organs is more difficult and often requires additional vectors. With the use of adeno-associated virus (AAV) gene delivery system, active CRISPR/Cas enzyme can be maintained for an extended period of time and enable efficient editing of genome in the retina in vivo. Herein we outline the method to edit the genome in mouse retina using a dual AAV vector -mediated CRISPR/Cas9 system.

Characterization of the Circumlimbal Suture Model of Chronic IOP Elevation in Mice and Assessment of Changes in Gene Expression of Stretch Sensitive Channels

To consider whether a circumlimbal suture can be used to chronically elevate intraocular pressure (IOP) in mice and to assess its effect on retinal structure, function and gene expression of stretch sensitive channels. Anesthetized adult C57BL6/J mice had a circumlimbal suture (10/0) applied around the equator of one eye. In treated eyes (n = 23) the suture was left in place for 12 weeks whilst in sham control eyes the suture was removed at day two (n = 17). Contralateral eyes served as untreated controls. IOP was measured after surgery and once a week thereafter. After 12 weeks, electroretinography (ERG) was performed to assess photoreceptor, bipolar cell and retinal ganglion cell (RGC) function. Retinal structure was evaluated using optical coherence tomography. Retinae were processed for counts of ganglion cell density or for quantitative RT-PCR to quantify purinergic (P2x7, Adora3, Entpd1) or stretch sensitive channel (Panx1, Trpv4) gene expression. Immediately after suture application, IOP spiked to 33 ± 3 mmHg. After 1 day, IOP had recovered to 27 ± 3 mmHg. Between weeks 2 and 12, IOP remained elevated above baseline (control 14 ± 1 mmHg, ocular hypertensive 19 ± 1 mmHg). Suture removal at day 2 (Sham) restored IOP to baseline levels, where it remained through to week 12. ERG analysis showed that 12 weeks of IOP elevation reduced photoreceptor (−15 ± 4%), bipolar cell (−15 ± 4%) and ganglion cell responses (−19 ± 6%) compared to sham controls and respective contralateral eyes (untreated). The retinal nerve fiber layer was thinned in the presence of normal total retinal thickness. Ganglion cell density was reduced across all quadrants (superior −12 ± 5%; temporal, −7% ± 2%; inferior −9 ± 4%; nasal −8 ± 5%). Quantitative RT-PCR revealed a significant increase in Entpd1 gene expression (+11 ± 4%), whilst other genes were not significantly altered (P2x7, Adora3, Trpv4, Panx1). Our results show that circumlimbal ligation produces mild chronic ocular hypertension and retinal dysfunction in mice. Consistent with a sustained change to purinergic signaling we found an up-regulation of Entpd1.

Retinal cross talk in the mammalian visual system

The existence and functional relevance of efferent optic nerve fibers in mammals have long been debated. While anatomical evidence for cortico-retinal and retino-retinal projections is substantial, physiological evidence is lacking, as efferent fibers are few in number and are severed in studies of excised retinal tissue. Here we show that interocular connections contribute to retinal bioelectrical activity in adult mammals. Full-field flash electroretinograms (ERGs) were recorded from one or both eyes of Brown-Norway rats under dark-adapted (n = 16) and light-adapted (n = 11) conditions. Flashes were confined to each eye by an opaque tube that blocked stray light. Monocular flashes evoked a small (5-15 μV) signal in the nonilluminated eye, which was named "crossed ERG" (xERG). The xERG began under dark-adapted conditions with a positive (xP1) wave that peaked at 70-90 ms and ended with slower negative (xN1) and positive (xP2) waves from 200 to 400 ms. xN1 was absent under light-adapted conditions. Injection of tetrodotoxin in either eye (n = 15) eliminated the xERG. Intraocular pressure elevation of the illuminated eye (n = 6) had the same effect. The treatments also altered the ERG b-wave in both eyes, and the alterations correlated with xERG disappearance. Optic nerve stimulation (n = 3) elicited a biphasic compound action potential in the nonstimulated nerve with 10- to 13-ms latency, implying that the xERG comes from slow-conducting (W type) fibers. Monocular dye application (n = 7) confirmed the presence of retino-retinal ganglion cells in adult rats. We conclude that mammalian eyes communicate directly with each other via a handful of optic nerve fibers. The cross talk alters retinal activity in rats, and perhaps other animals.

Depth-resolved rhodopsin molecular contrast imaging for functional assessment of photoreceptors

Rhodopsin, the light-sensing molecule in the outer segments of rod photoreceptors, is responsible for converting light into neuronal signals in a process known as phototransduction. Rhodopsin is thus a functional biomarker for rod photoreceptors. Here we report a novel technology based on visible-light optical coherence tomography (VIS-OCT) for in vivo molecular imaging of rhodopsin. The depth resolution of OCT allows the visualization of the location where the change of optical absorption occurs and provides a potentially accurate assessment of rhodopsin content by segmentation of the image at the location. Rhodopsin OCT can be used to quantitatively image rhodopsin distribution and thus assess the distribution of functional rod photoreceptors in the retina. Rhodopsin OCT can bring significant impact into ophthalmic clinics by providing a tool for the diagnosis and severity assessment of a variety of retinal conditions.

Melatonin modulates M4-type ganglion-cell photoreceptors

In the retina, melatonin is secreted at night by rod/cone photoreceptors and serves as a dark-adaptive signal. Melatonin receptors have been found in many retinal neurons including melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting it could modulate the physiology of these inner retinal photoreceptors. Here, we investigated whether melatonin modulates the alpha-like M4-type ipRGCs, which are believed to mediate image-forming vision as well as non-image-forming photoresponses. Applying melatonin during daytime (when endogenous melatonin secretion is low) caused whole-cell-recorded M4 cells' rod/cone-driven depolarizing photoresponses to become broader and larger, whereas the associated elevation in spike rate was reduced. Melanopsin-based light responses were not affected significantly. Nighttime application of the melatonin receptor antagonist luzindole also altered M4 cells' rod/cone-driven light responses but in the opposite ways: the duration and amplitude of the graded depolarization were reduced, whereas the accompanying spiking increase was enhanced. These luzindole-induced changes confirmed that M4 cells are modulated by endogenous melatonin. Melatonin could induce the above effects by acting directly on M4 cells because immunohistochemistry detected MT1 receptors in these cells, although it could also act presynaptically. Interestingly, the daytime and nighttime recordings showed significant differences in resting membrane potential, spontaneous spike rate and rod/cone-driven light responses, suggesting that M4 cells are under circadian control. This is the first report of a circadian variation in ipRGCs' resting properties and synaptic input, and of melatoninergic modulation of ipRGCs.

The effect of intraocular and intracranial pressure on retinal structure and function in rats

An increasing number of studies indicate that the optic nerve head of the eye is sensitive not only to changes in intraocular pressure (IOP), but also to intracranial pressure (ICP). This study examines changes to optic nerve and retinal structure in a rat model in response to a range of IOP and ICP levels using optical coherence tomography. Furthermore, we examine the functional sequelae of these structural changes by quantifying the effect of pressure changes on the electroretinogram. IOP elevation (10–90 mmHg) induces progressive deformation of the optic nerve head and retinal surface (P < 0.05), compression of the retina (P < 0.05) and bipolar cell (b-wave), and retinal ganglion cell (scotopic threshold response) dysfunction (P < 0.05). Simultaneously altering ICP (−5 to 30 mmHg) modifies these IOP-induced responses, with lower ICP (−5 mmHg) exacerbating and higher ICP (15–30 mmHg) ameliorating structural and functional deficits. Thus, the balance between IOP and ICP (optic nerve pressure gradient, ONPG = IOP − ICP) plays an important role in optic nerve integrity. Structural and functional parameters exhibit a two-phase relationship to ONPG, with structural changes being more sensitive to ONPG modification (threshold = −0.6 to 11.3 mmHg) compared with functional changes (threshold = 49.7–54.6 mmHg). These findings have implications for diseases including glaucoma, intracranial hypertension, and long-term exposure to microgravity.

Using the Electroretinogram to Assess Function in the Rodent Retina and the Protective Effects of Remote Limb Ischemic Preconditioning

The ERG is the sum of all retinal activity. The ERG is usually recorded from the cornea, which acts as an antenna that collects and sums signals from the retina. The ERG is a sensitive measure of changes in retinal function that are pan-retinal, but is less effective for detecting damage confined to a small area of retina. In the present work we describe how to record the 'flash' ERG, which is the potential generated when the retina is exposed to a brief light flash. We describe methods of anaesthesia, mydriasis and corneal management during recording; how to keep the retina dark adapted; electrode materials and placement; the range and calibration of stimulus energy; recording parameters and the extraction of data. We also describe a method of inducing ischemia in one limb, and how to use the ERG to assess the effects of this remote-from-the-retina ischemia on retinal function after light damage. A two-flash protocol is described which allows isolation of the cone-driven component of the dark-adapted ERG, and thereby the separation of the rod and cone components. Because it can be recorded with techniques that are minimally invasive, the ERG has been widely used in studies of the physiology, pharmacology and toxicology of the retina. We describe one example of this usefulness, in which the ERG is used to assess the function of the light-damaged retina, with and without a neuroprotective intervention; preconditioning by remote ischemia.

In vivo imaging rhodopsin distribution in the photoreceptors with nano-second pulsed scanning laser ophthalmoscopy

BACKGROUND

Rhodopsin is a biomarker for the function of rod photoreceptors, the dysfunction of which is related to many blinding diseases like retinitis pigmentosa and age-related macular degeneration. Imaging rhodopsin quantitatively may provide a powerful clinical tool for diagnosis of these diseases. To map rhodopsin distribution accurately in the retina, absorption by rhodopsin intermediates need to be minimized.

METHODS AND MATERIALS

We developed nano-second pulsed scanning laser ophthalmoscopy (SLO) to image rhodopsin distribution in the retina. The system takes advantage of the light-induced shift of rhodopsin absorption spectra, which in turn affects the fundus spectral reflection before and after photo-bleaching. By imaging the retina twice, one in the dark-adapted state and the other one in the light-adapted state, the rhodopsin absorption change can be calculated from the differential image, which is a function of the rhodopsin concentration in the rod photoreceptors.

RESULTS

The system was successfully applied to in vivo imaging of rat retina in different bleaching conditions to verify its feasibility. Our studies showed that the differential image between the dark- and light-adapted states represents rhodopsin distribution in the retina. We also conducted a dynamic bleaching experiment to prove the importance of reducing light absorption of rhodopsin intermediates.

CONCLUSIONS

The preliminary results showed that our nano-second pulsed-light SLO is promising in imaging the functional biomarker of the rod photoreceptors. By using nanosecond pulsed laser, in which one laser pulse generates one pixel of the image, the absorption of rhodopsin intermediates can be reduced.

Crizotinib reduces the rate of dark adaptation in the rat retina independent of ALK inhibition

Crizotinib (Xalkori) is a tyrosine kinase inhibitor of both anaplastic lymphoma kinase (ALK) and mesenchymal-epithelial transition factor (c-Met). Though not predicted from standard nonclinical toxicological evaluation, visual disturbance became a frequently observed adverse event in humans. To understand the possible mechanism of this vision effect, an in vivo electroretinogram (ERG) study was conducted to assess retinal functional changes following oral administration of crizotinib. Immunohistochemical (IHC) staining of ALK and c-Met in the neural retinas of human, non-human primate, dog, rat, and mouse was used to aid in the animal model selection. ALK IHC staining was identified predominantly in the ganglion cell and inner nuclear layers of most species evaluated, in the inner plexiform layer in human and rodent, and in the nerve fiber layer in human and rat only. There was no apparent staining of any layer of the neural retina for c-Met in any of the species evaluated. ERG measurements identified a significant reduction in b-wave amplitude during the initial phase of dark adaptation in the crizotinib-treated rats. ERGs were also taken following oral administration of PF-06463922 (an ALK-selective inhibitor), for an understanding of potential kinase involvement. ERG effects were not observed in PF-06463922-treated animals when comparable exposures in the vitreous humor were achieved. Collectively, our results suggest that the ERG b-wave amplitude decreases during dark adaption following crizotinib administration may be related to signaling changes within the retina in rats, likely independent of ALK inhibition.

Layer-specific manganese-enhanced MRI of the retina in light and dark adaptation

PURPOSE

To employ functional manganese-enhanced MRI (MEMRI) to image layer-specific changes in calcium-dependent activities in the rat retina during light versus dark adaptation.

METHODS

Functional MEMRI at 20 × 20 × 700 μm was used to study light and dark adaptation in the same animals (N = 10) in which one eye was covered and the fellow eye was not. The activity encoding of the light and dark adaptation was achieved in awake conditions and imaged under anesthesia. T(1)-weighted MRI at 11.7 tesla (T) was performed using two identical radiofrequency transceiver coils to allow interleaved MRI acquisitions of the two eyes. An intravascular contrast agent was also used to verify layer assignments.

RESULTS

MEMRI detected contrasts among the inner retina, outer retina, and choroid. Independent confirmation of the vascular layers and boundaries between layers was documented with an intravascular contrast agent. The retinal layer thicknesses agreed with published data. The outer retina had lower MEMRI activity in light compared with dark adaption (P < 0.001), consistent with the increased metabolic demand associated with the "dark current." The inner retina had higher MEMRI activity in light compared with dark adaption (P < 0.05). The choroid MEMRI activity was not statistically different between light and dark adaptation (P > 0.05).

CONCLUSIONS

This study demonstrated a high-resolution MEMRI protocol to image functional activities among different layers of the retinas in awake animals during light and dark adaptation. This approach could have potential applications in animal models of retinal dysfunction.

AAV-mediated tyrosinase gene transfer restores melanogenesis and retinal function in a model of oculo-cutaneous albinism type I (OCA1)

Oculo-cutaneous albinism type 1 (OCA1) is characterized by congenital hypopigmentation and is due to mutations in the TYROSINASE gene (TYR). In this study, we have characterized the morpho-functional consequences of the lack of tyrosinase activity in the spontaneous null mouse model of OCA1 (Tyr(c-2j)). Here, we show that adult Tyr(c-2j) mice have several retinal functional anomalies associated with photoreceptor loss. To test whether these anomalies are reversible upon TYR complementation, we performed intraocular administration of an adeno-associated virus (AAV)-based vector, encoding the human TYR gene, in adult Tyr(c-2j) mice. This resulted in melanosome biogenesis and ex novo synthesis of melanin in both neuroectodermally derived retinal pigment epithelium (RPE) and in neural crest-derived choroid and iris melanocytes. Ocular melanin accumulation prevented progressive photoreceptor degeneration and resulted in restoration of retinal function. Our results reveal novel properties of pigment cells and show that the developmental anomalies of albino mice are associated with defects occurring in postnatal life, adding novel insights on OCA1 disease pathogenesis. In addition, we provide proof-of-principle of an effective gene-based strategy relevant for future application in albino patients.

Electroretinographic study of spontaneously diabetic Torii rats

Spontaneously diabetic Torii (SDT) rats are an inbred strain of rats with a non-obese type 2 diabetes mellitus that were isolated from an outbred colony of Sprague-Dawley (SD) rats. Electroretinograms (ERGs) were recorded from SDT and SD (controls) rats at 10- and 44-weeks-of-age to determine their retinal function. The amplitudes and implicit times of the ERGs of the right and left eyes were not significantly different indicating that the intra-individual variation was small. Both amplitudes and implicit times of the ERGs in the SDT rats were not significantly different from those of SD rats at 10-weeks-of-age. At 44-weeks-of-age, however, the a- and b-waves and the oscillatory potentials were significantly reduced with prolonged implicit times in the SDT rats compared to SD rats. These depressed ERGs may reflect vascular and neuronal damage throughout the retina as are seen in the advanced stages of human diabetic retinopathy. Thus, the SDT rat can be used to study the retinal physiology of diabetic retinopathy.

Retinal electrophysiology for toxicology studies: applications and limits of ERG in animals and ex vivo recordings

Assessing retinal drug toxicity is becoming increasingly important as different molecules are now developed for the treatment of neurodegenerative diseases and vascular disorders. In pharmacology and toxicology, the electroretinogram (ERG) and the multielectrode array (MEA) recording techniques can be used to quantify the possible side effects of retino-active xenobiotics. Toxicity testing requires the use of rodent as well as non-rodent models for extrapolation to the human model when determining risk and safety. Animal species differ in their retinal anatomo-physiology: most rodents used in toxicology studies are essentially nocturnal species, whereas the non-rodent laboratory species normally used (e.g. dogs, pigs and monkeys) are diurnal. The ratio between the photoreceptor populations which varies from species to species, should be considered when designing the experiment protocol and the interpretation. The described ERG procedures are designed to comply with all applicable good laboratory practice standards. Use of these procedures should yield an acceptable level of intra- and inter-subject variability for compiling a historical database, and for detecting possible retinal toxicity in animal studies. They could therefore be used as specific and standardized tools for screening of potential retinotoxic molecules in drug discovery and development in order to compare methods and results with those obtained in human electrophysiological assessments. Recording of ganglion cell light responses on ex vivo retina with the MEA technique can further demonstrate how retino-active xenobiotics affect retinal visual information processing by eliminating potential obstacles related to bioavailability and blood barrier permeability.

Comparing the retinal structures and functions in two species of gulls (Larus delawarensis and Larus modestus) with significant nocturnal behaviours

Ring-billed gulls (Larus delawarensis) and gray gulls (Larus modestus) are two species active both by day and night. We have investigated the retinal adaptations that allow the diurnal and nocturnal behaviours of these two species. Electroretinograms and histological analyses show that both species have a duplex retina in which cones outnumber rods, but the number of rods appears sufficient to provide vision at night. Their retinas respond over the same scotopic dynamic range of 3.4logcdm(-2), which encompasses all of the light levels occurring at night in their photic environment. The amplitudes of the scotopic saturated a- and b-wave responses as well as the photopic saturated b-wave response and the photopic sensitivity parameter S are however higher in ring-billed gulls than in gray gulls. Moreover, the process of dark adaptation is about 30min faster in gray gulls than in ring-billed gulls. Our results suggest that both species have acquired in the course of their evolution functional adaptations that can be related to their specific photic environment.

Amelioration of both functional and morphological abnormalities in the retina of a mouse model of ocular albinism following AAV-mediated gene transfer

X-linked recessive ocular albinism type I (OA1) is due to mutations in the OA1 gene (approved gene symbol GPR143), which is expressed in the retinal pigment epithelium (RPE). The Oa1 (Gpr143) knockout mouse (Oa1(-/-)) model recapitulates many of the OA1 retinal morphological anomalies, including a lower number of melanosomes of increased size in the RPE. The Oa1(-/-) mouse also displays some of the retinal developmental abnormalities observed in albino patients such as misrouting of the optic tracts. Here, we show that these anomalies are associated with retinal electrophysiological abnormalities, including significant decrease in a- and b-wave amplitude and delayed recovery of b-wave amplitude from photoreceptor desensitization following bright light exposure. This suggests that lack of Oa1 in the RPE impacts on photoreceptor activity. More interestingly, adeno-associated viral vector-mediated Oa1 gene transfer to the retina of the Oa1(-/-) mouse model results in significant recovery of its retinal functional abnormalities. In addition, Oa1 retinal gene transfer increases the number of melanosomes in the Oa1(-/-) mouse RPE. Our data show that gene transfer to the adult retina unexpectedly rescues both functional and morphological abnormalities in a retinal developmental disorder, opening novel potential therapeutic perspectives for this and other forms of albinism.

Changes of retinal functions following the induction of ocular hypertension in rats using argon laser photocoagulation

BACKGROUND

Electroretinography (ERG) provides a longitudinal monitoring of pathological changes in retina. Scotopic threshold response (STR) of ERG was shown to reflect inner retinal activity and is particularly useful in the evaluation of inner retinal changes in ocular hypertension models. Recently, STR was demonstrated to be attenuated after the induction of ocular hypertension using injection of hypertonic saline into episcleral veins which indicates an impairment of retinal function. However, little is known on the changes of retinal function in an ocular hypertension model induced by laser photocoagulation at episcleral veins and limbal veins.

METHODS

Ocular hypertension was induced unilaterally using laser photocoagulation at episcleral veins and limbal veins in adult Sprague-Dawley rats. Intraocular pressure was monitored and the number of retinal ganglion cell loss was counted across the 8-week experimental period. The animals were dark adapted overnight and flash ERGs were measured before the laser treatment, 5 and 8 weeks after treatment. The changes of STR, scotopic negative response, a-wave and b-wave were analysed.

RESULTS

Approximately 1.6-fold elevation of intraocular pressure was induced in the experimental eyes and 3% retinal ganglion cell loss per week was found. The amplitude of STR was significantly attenuated which indicates an impairment of inner retinal activities. There was also a reduction of scotopic negative response, a-wave and b-wave after the induction of ocular hypertension.

CONCLUSION

The laser-induced ocular hypertension model in the present study produced a substantial reduction of retinal functions. Understanding the characteristic of pathological changes is crucial for further study using this model.

Photoreceptor adaptation in intrinsically photosensitive retinal ganglion cells

A rare type of mammalian retinal ganglion cell (RGC) expresses the photopigment melanopsin and is a photoreceptor. These intrinsically photosensitive RGCs (ipRGCs) drive circadian-clock resetting, pupillary constriction, and other non-image-forming photic responses. Both the light responses of ipRGCs and the behaviors they drive are remarkably sustained, raising the possibility that, unlike rods and cones, ipRGCs do not adjust their sensitivity according to lighting conditions ("adaptation"). We found, to the contrary, that ipRGC sensitivity is plastic, strongly influenced by lighting history. When exposed to a constant, bright background, the background-evoked response decayed, and responses to superimposed flashes grew in amplitude, indicating light adaptation. After extinction of a light-adapting background, sensitivity recovered progressively in darkness, indicating dark adaptation. Because these adjustments in sensitivity persisted when synapses were blocked, they constitute "photoreceptor adaptation" rather than "network adaptation." Implications for the mechanisms generating various non-image-forming visual responses are discussed.

The relationship between ambient lighting conditions, absolute dark-adapted thresholds, and rhodopsin in black and hypopigmented mice

Significant variation in absolute dark-adapted thresholds is observed both within and between strains of mice with differing ocular pigmentation levels. Differences in threshold within a single strain are related to the Williams' photostasis effect, that is, photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. To examine threshold differences among strains, we equalized rhodopsin levels by maintaining albino mice (c2J/c2J) at 2 × 10−4 cd/m2 (dim light) and black mice at 2 × 102 cd/m2 (bright light). This resulted in ocular rhodopsin levels for albino mice (albino—dim) of 494 ± 11 pmoles/eye and rhodopsin levels for black mice (black—bright) of 506 ± 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained in dim light are 586 ± 46 pmoles/eye and 217 ± 46 pmoles/eye in albino mice maintained in bright light. We found similar dark-adapted thresholds (6.38 log cd/m2vs. 6.47 log cd/m2)) in albino and black mice with equivalent rhodopsin determined with a water maze test. This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level of ocular melanin. The number of photoreceptors, photoreceptor layer thickness, and outer segment length did not differ significantly between albino (dark) and black mice (bright). These results demonstrate that the visual sensitivity defect found in hypopigmented animals is secondary to abnormal rhodopsin regulation and that hypopigmented animals have either an improper input to the photostasis mechanism or that the photostasis mechanism is defective.

Vigabatrin, the GABA-transaminase inhibitor, damages cone photoreceptors in rats

Epileptic patients experienced an irreversible loss of their peripheral visual field upon treatment with vigabatrin (gamma-vinyl GABA), an inhibitor of the GABA degrading enzyme, GABA transaminase. Subsequently, central visual function was reported to also be irreversibly altered. This visual loss is associated with a decrease in the electroretinogram measurement localizing the deficit to the retina. To investigate its cellular origin, we treated rats daily with vigabatrin for 45 days. Two days after arresting this treatment, rats exhibited an irreversible decrease in the photopic electroretinogram, the flicker response, and the oscillatory potentials. These functional alterations were associated with a peripheral disorganization of the outer retina. However, photoreceptor damage was not limited to these disorganized areas, but cone inner and outer segments were severely injured in more central areas and their numbers were irreversibly decreased by 17 to 20%. Ultrastructural examination of the retina confirmed the presence of major photoreceptor damages, which were further supported by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) and caspase-3 activation both indicative of photoreceptor apoptosis. This study suggests that the visual field loss in vigabatrin-treated epileptic patients may result from a sequence of events starting from cone cell injury to a more severe disorganization of the photoreceptor layer.