Photoreceptor organisation and phenotypic characterization in retinas of two diurnal rodent species: Potential use as experimental animal models for human vision research

The Mongolian gerbil as an advanced model to study cone system physiology

In this work, we introduce a diurnal rodent, the Mongolian gerbil (Meriones unguiculatus) (MG) as an alternative to study retinal cone system physiology and pathophysiology in mice. The cone system is of particular importance, as it provides high-acuity and color vision and its impairment in retinal disorders is thus especially disabling. Despite their nocturnal lifestyle, mice are currently the most popular animals to study cone-related diseases due to the high availability of genetically modified models. However, the potential for successful translation of any cone-related results is limited due to the substantial differences in retinal organization between mice and humans. Alternatively, there are diurnal rodents such as the MG with a higher retinal proportion of cones and a macula-like specialized region for improved visual resolution, the visual streak. The focus of this work was the evaluation of the MG's cone system functionality using full-field electroretinography (ERG), together with a morphological assessment of its retinal/visual streak organization via angiography, optical coherence tomography (OCT), and photoreceptor immunohistochemistry. We found that rod system responses in MGs were comparable or slightly inferior to mice, while in contrast, cone system responses were much larger, more sensitive, and also faster than those in the murine counterparts, and in addition, it was possible to record sizeable ON and OFF ERG components. Morphologically, MG cone photoreceptor opsins were evenly distributed throughout the retina, while mice show a dorsoventral M- and S-opsin gradient. Additionally, each cone expressed a single opsin, in contrast to the typical co-expression of opsins in mice. Particular attention was given to the visual streak region, featuring a higher density of cones, elongated cone and rod outer segments (OSs), and an increased thickness of the inner and outer retinal layers in comparison to peripheral regions. In summary, our data render the MG a supreme model to investigate cone system physiology, pathophysiology, and to validate potential therapeutic strategies in that context.

Affinity-controlled release of rod-derived cone viability factor enhances cone photoreceptor survival

Retinitis pigmentosa (RP) is a group of genetic diseases that results in rod photoreceptor cell degeneration, which subsequently leads to cone photoreceptor cell death, impaired vision and eventual blindness. Rod-derived cone viability factor (RdCVF) is a protein which has two isoforms: a short form (RdCVF) and a long form (RdCVFL) which act on cone photoreceptors in the retina. RdCVFL protects photoreceptors by reducing hyperoxia in the retina; however, sustained delivery of RdCVFL remains challenging. We developed an affinity-controlled release strategy for RdCVFL. An injectable physical blend of hyaluronan and methylcellulose (HAMC) was covalently modified with a peptide binding partner of the Src homology 3 (SH3) domain. This domain was expressed as a fusion protein with RdCVFL, thereby enabling its controlled release from HAMC-binding peptide. Sustained release of RdCVFL was demonstrated for the first time as RdCVFL-SH3 from HAMC-binding peptide for 7 d in vitro. To assess bioactivity, chick retinal dissociates were harvested and treated with the affinity-released recombinant protein from the HAMC-binding peptide vehicle. After 6 d in culture, cone cell viability was greater when cultured with released RdCVFL-SH3 relative to controls. We utilized computational fluid dynamics to model release of RdCVFL-SH3 from our delivery vehicle in the vitreous of the human eye. We demonstrate that our delivery vehicle can prolong the bioavailability of RdCVFL-SH3 in the retina, potentially enhancing its therapeutic effects. Our affinity-based system constitutes a versatile delivery platform for ultimate intraocular injection in the treatment of retinal degenerative diseases. STATEMENT OF SIGNIFICANCE: Retinitis pigmentosa (RP) is the leading cause of inherited blindness in the world. Rod-derived cone viability factor (RdCVF), a novel protein paracrine factor, is effective in preclinical models of RP. To extend its therapeutic effects, we developed an affinity-controlled release strategy for the long form of RdCVF, RdCVFL. We expressed RdCVFL as a fusion protein with an Src homology 3 domain (SH3). We then utilized a hydrogel composed of hyaluronan and methylcellulose (HAMC) and modified it with SH3 binding peptides to investigate its release in vitro. Furthermore, we designed a mathematical model of the human eye to investigate delivery of the protein from the delivery vehicle. This work paves the way for future investigation of controlled release RdCVF.

Clocks Ticking in the Dark: A Review of Biological Rhythms in Subterranean African Mole-Rats

Biological rhythms are rhythmic fluctuations of biological functions that occur in almost all organisms and on several time scales. These rhythms are generated endogenously and entail the coordination of physiological and behavioural processes to predictable, external environmental rhythms. The light-dark cycle is usually the most prominent environmental cue to which animals synchronise their rhythms. Biological rhythms are believed to provide an adaptive advantage to organisms. In the present review, we will examine the occurrence of circadian and seasonal rhythms in African mole-rats (family Bathyergidae). African mole-rats are strictly subterranean, they very rarely emerge aboveground and therefore, do not have regular access to environmental light. A key adaptation to their specialised habitat is a reduction in the visual system. Mole-rats exhibit both daily and seasonal rhythmicity in a range of behaviours and physiological variables, albeit to different degrees and with large variability. We review previous research on the entire circadian system of African mole-rats and discuss output rhythms in detail. Laboratory experiments imply that light remains the strongest zeitgeber for entrainment but in the absence of light, animals can entrain to ambient temperature rhythms. Field studies report that rhythmic daily and seasonal behaviour is displayed in their natural habitat. We suggest that ambient temperature and rainfall play an important role in the timing of rhythmic behaviour in mole-rats, and that they likely respond directly to these zeitgebers in the field rather than exhibit robust endogenous rhythms. In the light of climate change, these subterranean animals are buffered from the direct and immediate effects of changes in temperature and rainfall, partly because they do not have robust circadian rhythms, however, on a longer term they are vulnerable to changes in their food sources and dispersal abilities.

Multilocus phylogeny of African striped grass mice (Lemniscomys): Stripe pattern only partly reflects evolutionary relationships

Murine rodents are one of the most evolutionary successful groups of extant mammals. They are also important for human as vectors and reservoirs of zoonoses and agricultural pests. Unfortunately, their fast and relatively recent diversification impedes our understanding of phylogenetic relationships and species limits of many murine taxa, including those with very conspicuous phenotype that has been frequently used for taxonomic purposes. One of such groups are the striped grass mice (genus Lemniscomys), distributed across sub-Saharan Africa in 11 currently recognized species. These are traditionally classified into three morphological groups according to different pelage colouration on the back: (a) L. barbarus group (three species) with several continuous pale longitudinal stripes; (b) L. striatus group (four species) with pale stripes diffused into short lines or dots; and (c) L. griselda group (four species) with a single mid-dorsal black stripe. Here we reconstructed the most comprehensive molecular phylogeny of the genus Lemniscomys to date, using the largest currently available multi-locus genetic dataset of all but two species. The results show four main lineages (=species complexes) with the distribution corresponding to the major biogeographical regions of Africa. Surprisingly, the four phylogenetic lineages are only in partial agreement with the morphological classification, suggesting that the single-stripe and/or multi-striped phenotypes evolved independently in multiple lineages. Divergence dating showed the split of Lemniscomys and Arvicanthis genera at the beginning of Pleistocene; most of subsequent speciation processes within Lemniscomys were affected by Pleistocene climate oscillations, with predominantly allopatric diversification in fragmented savanna biome. We propose taxonomic suggestions and directions for future research of this striking group of African rodents.

Diurnal rodents as pertinent animal models of human retinal physiology and pathology

This presentation will survey the retinal architecture, advantages, and limitations of several lesser-known rodent species that provide a useful diurnal complement to rats and mice. These diurnal rodents also possess unusually cone-rich photoreceptor mosaics that facilitate the study of cone cells and pathways. Species to be presented include principally the Sudanian Unstriped Grass Rat and Nile Rat (Arvicanthis spp.), the Fat Sand Rat (Psammomys obesus), the degu (Octodon degus) and the 13-lined ground squirrel (Ictidomys tridecemlineatus). The retina and optic nerve in several of these species demonstrate unusual resilience in the face of neuronal injury, itself an interesting phenomenon with potential translational value.

Characterization of Retinal Functionality at Different Eccentricities in a Diurnal Rodent

Although the properties of the neurons of the visual system that process central and peripheral regions of the visual field have been widely researched in the visual cortex and the LGN, they have scarcely been documented for the retina. The retina is the first step in integrating optical signals, and despite considerable efforts to functionally characterize the different types of retinal ganglion cells (RGCs), a clear account of the particular functionality of cells with central vs. peripheral fields is still wanting. Here, we use electrophysiological recordings, gathered from retinas of the diurnal rodent Octodon degus, to show that RGCs with peripheral receptive fields (RF) are larger, faster, and have shorter transient responses. This translates into higher sensitivity at high temporal frequencies and a full frequency bandwidth when compared to RGCs with more central RF. We also observed that imbalances between ON and OFF cell populations are preserved with eccentricity. Finally, the high diversity of functional types of RGCs highlights the complexity of the computational strategies implemented in the early stages of visual processing, which could inspire the development of bio-inspired artificial systems.

The topography of rods, cones and intrinsically photosensitive retinal ganglion cells in the retinas of a nocturnal (Micaelamys namaquensis) and a diurnal (Rhabdomys pumilio) rodent

We used immunocytochemistry to determine the presence and topographical density distributions of rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) in the four-striped field mouse (Rhabdomys pumilio) and the Namaqua rock mouse (Micaelamys namaquensis). Both species possessed duplex retinas that were rod dominated. In R. pumilio, the density of both cones and rods were high (cone to rod ratio: 1:1.23) and reflected the species' fundamentally diurnal, but largely crepuscular lifestyle. Similarly, the ratio of cones to rods in M. namaquensis (1:12.4) reflected its nocturnal lifestyle. Similar rod density peaks were observed (R. pumilio: ~84467/mm2; M. namaquensis: ~81088/mm2), but a density gradient yielded higher values in the central (~56618/mm2) rather than in the peripheral retinal region (~32689/mm2) in R. pumilio. Two separate cone types (S-cones and M/L-cones) were identified implying dichromatic color vision in the study species. In M. namaquensis, both cone populations showed a centro-peripheral density gradient and a consistent S- to M/L-cone ratio (~1:7.8). In R. pumilio, S cones showed a centro-peripheral gradient (S- to M/L-cone ratio; central: 1:7.8; peripheral: 1:6.8) which appeared to form a visual streak, and a specialized area of M/L-cones (S- to M/L-cone ratio: 1:15) was observed inferior to the optic nerve. The number of photoreceptors per linear degree of visual angle, estimated from peak photoreceptor densities and eye size, were four cones and 15 rods per degree in M. namaquensis and 11 cones and 12 rods per degree in R. pumilio. Thus, in nocturnal M. namaquensis rods provide much finer image sampling than cones, whereas in diurnal/crepuscular R. pumilio both photoreceptor types provide fine image sampling. IpRGCs were comparably sparse in R. pumilio (total = 1012) and M. namaquensis (total = 862), but were homogeneously distributed in M. namaquensis and densest in the dorso-nasal quadrant in R. pumilio. The adaptive significance of the latter needs further investigation.

Animal modelling for inherited central vision loss

Disease-causing variants of a large number of genes trigger inherited retinal degeneration leading to photoreceptor loss. Because cones are essential for daylight and central vision such as reading, mobility, and face recognition, this review focuses on a variety of animal models for cone diseases. The pertinence of using these models to reveal genotype/phenotype correlations and to evaluate new therapeutic strategies is discussed. Interestingly, several large animal models recapitulate human diseases and can serve as a strong base from which to study the biology of disease and to assess the scale-up of new therapies. Examples of innovative approaches will be presented such as lentiviral-based transgenesis in pigs and adeno-associated virus (AAV)-gene transfer into the monkey eye to investigate the neural circuitry plasticity of the visual system. The models reported herein permit the exploration of common mechanisms that exist between different species and the identification and highlighting of pathways that may be specific to primates, including humans.

Human organotypic retinal cultures (HORCs) as a chronic experimental model for investigation of retinal ganglion cell degeneration

There is a growing need for models of human diseases that utilise native, donated human tissue in order to model disease processes and develop novel therapeutic strategies. In this paper we assessed the suitability of adult human retinal explants as a potential model of chronic retinal ganglion cell (RGC) degeneration. Our results confirmed that RGC markers commonly used in rodent studies (NeuN, βIII Tubulin and Thy-1) were appropriate for labelling human RGCs and followed the expected differential expression patterns across, as well as throughout, the macular and para-macular regions of the retina. Furthermore, we showed that neither donor age nor post-mortem time (within 24 h) significantly affected the initial expression levels of RGC markers. In addition, the feasibility of using human post mortem donor tissue as a long-term model of RGC degeneration was determined with RGC protein being detectable up to 4 weeks in culture with an associated decline in RGC mRNA and significant, progressive, apoptotic labelling of NeuN(+) cells. Differences in RGC apoptosis might have been influenced by medium compositions indicating that media constituents could play a role in supporting axotomised RGCs. We propose that using ex vivo human explants may prove to be a useful model for testing the effectiveness of neuroprotective strategies.

Arvicanthis ansorgei, a Novel Model for the Study of Sleep and Waking in Diurnal Rodents

STUDY OBJECTIVES

Sleep neurobiology studies use nocturnal species, mainly rats and mice. However, because their daily sleep/wake organization is inverted as compared to humans, a diurnal model for sleep studies is needed. To fill this gap, we phenotyped sleep and waking in Arvicanthis ansorgei, a diurnal rodent widely used for the study of circadian rhythms.

DESIGN

Video-electroencephalogram (EEG), electromyogram (EMG), and electrooculogram (EOG) recordings.

SETTING

Rodent sleep laboratory.

PARTICIPANTS

Fourteen male Arvicanthis ansorgei, aged 3 mo.

INTERVENTIONS

12 h light (L):12 h dark (D) baseline condition, 24-h constant darkness, 6-h sleep deprivation.

MEASUREMENTS AND RESULTS

Wake and rapid eye movement (REM) sleep showed similar electrophysiological characteristics as nocturnal rodents. On average, animals spent 12.9 h ± 0.4 awake per 24-h cycle, of which 6.88 h ± 0.3 was during the light period. NREM sleep accounted for 9.63 h ± 0.4, which of 5.13 h ± 0.2 during dark period, and REM sleep for 89.9 min ± 6.7, which of 52.8 min ± 4.4 during dark period. The time-course of sleep and waking across the 12 h light:12 h dark was overall inverted to that observed in rats or mice, though with larger amounts of crepuscular activity at light and dark transitions. A dominant crepuscular regulation of sleep and waking persisted under constant darkness, showing the lack of a strong circadian drive in the absence of clock reinforcement by external cues, such as a running wheel. Conservation of the homeostatic regulation was confirmed with the observation of higher delta power following sustained waking periods and a 6-h sleep deprivation, with subsequent decrease during recovery sleep.

CONCLUSIONS

Arvicanthis ansorgei is a valid diurnal rodent model for studying the regulatory mechanisms of sleep and so represents a valuable tool for further understanding the nocturnality/diurnality switch.

Localization of diacylglycerol lipase alpha and monoacylglycerol lipase during postnatal development of the rat retina

In recent decades, there has been increased interest in the physiological roles of the endocannabinoid (eCB) system and its receptors, the cannabinoid receptor types 1 (CB1R) and 2 (CB2R). Exposure to cannabinoids during development results in neurofunctional alterations, which implies that the eCB system is involved in the developmental processes of the brain. Because of their lipophilic nature, eCBs are synthesized on demand and are not stored in vesicles. Consequently, the enzymes responsible for their synthesis and degradation are key regulators of their physiological actions. Therefore, knowing the localization of these enzymes during development is crucial for a better understanding of the role played by eCBs during the formation of the central nervous system. In this study, we investigated the developmental protein localization of the synthesizing and catabolic enzymes of the principal eCB, 2-arachidonoylglycerol (2-AG) in the retinas of young and adult rats. The distribution of the enzymes responsible for the synthesis (DAGLα) and the degradation (MAGL) of 2-AG was determined for every retinal cell type from birth to adulthood. Our results indicate that DAGLα is present early in postnatal development. It is highly expressed in photoreceptor, horizontal, amacrine, and ganglion cells. MAGL appears later during the development of the retina and its presence is limited to amacrine and Müller cells. Overall, these results suggest that 2-AG is strongly present in early retinal development and might be involved in the regulation of the structural and functional maturation of the retina.

The circadian gene Clock oscillates in the suprachiasmatic nuclei of the diurnal rodent Barbary striped grass mouse, Lemniscomys barbarus: a general feature of diurnality?

A major challenge in the field of circadian rhythms is to understand the neural mechanisms controlling the oppositely phased temporal organization of physiology and behaviour between night- and day-active animals. Most identified components of the master clock in the suprachiasmatic nuclei (SCN), called circadian genes, display similar oscillations according to the time of day, independent of the temporal niche. This has led to the predominant view that the switch between night- and day-active animals occurs downstream of the master clock, likely also involving differential feedback of behavioral cues onto the SCN. The Barbary striped grass mouse, Lemniscomys barbarus is known as a day-active Muridae. Here we show that this rodent, when housed in constant darkness, displays a temporal rhythmicity of metabolism matching its diurnal behaviour (i.e., high levels of plasma leptin and hepatic glycogen during subjective midday and dusk, respectively). Regarding clockwork in their SCN, these mice show peaks in the mRNA profiles of the circadian gene Period1 (Per1) and the clock-controlled gene Vasopressin (Avp), which occur during the middle and late subjective day, respectively, in accordance with many observations in both diurnal and nocturnal species. Strikingly, expression of the circadian gene Clock in the SCN of the Barbary striped grass mouse was not constitutive as in nocturnal rodents, but it was rhythmic. As this is also the case for the other diurnal species investigated in the literature (sheep, marmoset, and quail), a hypothesis is that the transcriptional control of Clock within the SCN participates in the mechanisms underlying diurnality and nocturnality.

Structural and physiological responses to prolonged constant lighting in the cone-rich retina of Arvicanthis ansorgei

Cone photoreceptor death is a leading cause of blindness in industrialised countries. Despite this, there are few mammalian models available to study cone pathophysiology. The diurnal rodent Arvicanthis possesses a high cone percentage and ease of maintenance. We recently described the effect of ambient light conditions on cyclic disc shedding, and observed that 24 h of constant illumination ("LL") completely disrupted the normal rhythmic process and increased cone shedding fourfold. The current study was undertaken to see whether protracted constant illumination (7 days LL) would further perturb cone (and rod) turnover, and possibly lead to photoreceptor degeneration. Whereas control (cyclic lighting) retinas exhibited a typical early morning burst in phagosomes, LL retinas exhibited only low uniform numbers of rod and cone phagosomes across 24 h, with no peak of shedding at any time. Morphometric and immunohistochemical analyses of Arvicanthis retinas after 7 days LL (300 lux) showed no structural changes compared to control retinas.

Novel rodent models for macular research

Background

Many disabling human retinal disorders involve the central retina, particularly the macula. However, the commonly used rodent models in research, mouse and rat, do not possess a macula. The purpose of this study was to identify small laboratory rodents with a significant central region as potential new models for macular research.

Methodology/Principal Findings

Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli, laboratory rodents less commonly used in retinal research, were subjected to confocal scanning laser ophthalmoscopy (cSLO), fluorescein and indocyanine green angiography, and spectral-domain optical coherence tomography (SD-OCT) using standard equipment (Heidelberg Engineering HRA1 and Spectralis™) adapted to small rodent eyes. The existence of a visual streak-like pattern was assessed on the basis of vascular topography, retinal thickness, and the topography of retinal ganglion cells and cone photoreceptors. All three species examined showed evidence of a significant horizontal streak-like specialization. cSLO angiography and retinal wholemounts revealed that superficial retinal blood vessels typically ramify and narrow into a sparse capillary net at the border of the respective area located dorsal to the optic nerve. Similar to the macular region, there was an absence of larger blood vessels in the streak region. Furthermore, the thickness of the photoreceptor layer and the population density of neurons in the ganglion cell layer were markedly increased in the visual streak region.

Conclusions/Significance

The retinal specializations of Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli resemble features of the primate macula. Hence, the rodents reported here may serve to study aspects of macular development and diseases like age-related macular degeneration and diabetic macular edema, and the preclinical assessment of therapeutic strategies.

Cone loss is delayed relative to rod loss during induced retinal degeneration in the diurnal cone-rich rodent Arvicanthis ansorgei

Cone photoreceptor breakdown underlies functional vision loss in many blinding diseases. Cone loss is often secondary to that of rods, but little experimental data are available on the relationship between the two populations. Because of its high cone numbers, we used the diurnal rodent Arvicanthis ansorgei to explore changes in rod and cone survival and function during chemically-induced retinal degeneration. Adult animals received intraperitoneal injections of N-methyl-N-nitrosourea (MNU), and changes in retinal fundus appearance, histology, phenotype, apoptosis (TUNEL staining) and functionality (scotopic and photopic electroretinography) were monitored as a function of post-treatment time and retinal topography. Relative to control animals injected with vehicle only, MNU-injected animals showed time-, region- and population-specific changes as measured by morphological and immunochemical criteria. Histological (gradual thinning of photoreceptor layer) and phenotypical (reduced immunostaining of rhodopsin and rod transducin, and mid wavelength cone opsin and cone arrestin) modifications were first observed in superior central retina at 11 days post-injection. These degenerative changes spread into the superior peripheral and inferior hemisphere during the following 10 days. Rod loss preceded that of cones as visualized by differential immunolabelling and presence of apoptotic cells in rod but not cone cells. By 3 months post-injection, degeneration of the photoreceptor layer was complete in the superior hemisphere, but only partial in the inferior hemisphere. Despite the persistence of cone photoreceptors, scotopic and photopic electroretinography performed at 90 days post-treatment showed that both rod and cone function were severely compromised. In conclusion, MNU-induced retinal degeneration in Arvicanthis follows a predictable spatial and temporal pattern allowing clear separation of rod- and cone-specific pathogenic mechanisms. Compared to other rodents in which MNU has been used, Arvicanthis ansorgei demonstrates pronounced resistance to photoreceptor cell loss.

Structure of cone photoreceptors

Although outnumbered more than 20:1 by rod photoreceptors, cone cells in the human retina mediate daylight vision and are critical for visual acuity and color discrimination. A variety of human diseases are characterized by a progressive loss of cone photoreceptors but the low abundance of cones and the absence of a macula in non-primate mammalian retinas have made it difficult to investigate cones directly. Conventional rodents (laboratory mice and rats) are nocturnal rod-dominated species with few cones in the retina, and studying other animals with cone-rich retinas presents various logistic and technical difficulties. Originating in the early 1900s, past research has begun to provide insights into cone ultrastructure but has yet to afford an overall perspective of cone cell organization. This review summarizes our past progress and focuses on the recent introduction of special mammalian models (transgenic mice and diurnal rats rich in cones) that together with new investigative techniques such as atomic force microscopy and cryo-electron tomography promise to reveal a more unified concept of cone photoreceptor organization and its role in retinal diseases.

From daily behavior to hormonal and neurotransmitters rhythms: comparison between diurnal and nocturnal rat species

Mammalian species can be defined as diurnal or nocturnal, depending on the temporal niche during which they are active. Even if general activity occurs during nighttime in nocturnal rodents, there is a patchwork of general activity patterns in diurnal rodents, including frequent bimodality (so-called crepuscular pattern, i.e., dawn and dusk peaks of activity) and a switch to a nocturnal pattern under certain circumstances. This raises the question of whether crepuscular species have a bimodal or diurnal - as opposed to nocturnal - physiology. To this end, we investigated several daily behavioral, hormonal and neurochemical rhythms in the diurnal Sudanian grass rat (Arvicanthis ansorgei) and the nocturnal Long-Evans rat (Rattus norvegicus). Daily rhythms of general activity, wheel-running activity and body temperature, with or without blocked wheel, were diurnal and bimodal for A. ansorgei, and nocturnal and unimodal for Long-Evans rats. Moreover, A. ansorgei and Long-Evans rats exposed to light-dark cycles were respectively more and less active, compared to conditions of constant darkness. In contrast to other diurnal rodents, wheel availability in A. ansorgei did not switch their general activity pattern. Daily, unimodal rhythm of plasma leptin was in phase-opposition between the two rodent species. In the hippocampus, a daily, unimodal rhythm of serotonin in A. ansorgei occurred 7 h earlier than that in Long-Evans rats, whereas a daily, unimodal rhythm of dopamine was unexpectedly concomitant in both species. Multiparameter analysis demonstrates that in spite of bimodal rhythms linked with locomotor activity, A. ansorgei have a diurnally oriented physiology.

Retinal anatomy and visual performance in a diurnal cone-rich laboratory rodent, the Nile grass rat (Arvicanthis niloticus)

Unlike laboratory rats and mice, muridae of the Arvicanthis family (A. ansorgei and A. niloticus) are adapted to functioning best in daylight. To date, they have been used as experimental models mainly in studies of circadian rhythms. However, recent work aimed at optimizing photoreceptor-directed gene delivery vectors (Khani et al. [2007] Invest Ophthalmol Vis Sci 48:3954-3961) suggests their potential usefulness for studying retinal pathologies and therapies. In the present study we analyzed the retinal anatomy and visual performance of the Nile grass rat (A. niloticus) using immunohistofluorescence and the optokinetic response (OKR). We found that approximately 35-40% of photoreceptors are cones; that many neural features of the inner retina are similar to those in other diurnal mammals; and that spatial acuity, measured by the OKR, is more than two times that of the usual laboratory rodents. These observations are consistent with the known diurnal habits of this animal, and further support its pertinence as a complementary model for studies of structure, function, and pathology in cone-rich mammalian retinae.