Constant illumination causes spatially discrete dopamine depletion in the normal and degenerate retina

Burning the candle at both ends: Intraretinal signaling of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are photoreceptors located in the ganglion cell layer. They project to brain regions involved in predominately non-image-forming functions including entrainment of circadian rhythms, control of the pupil light reflex, and modulation of mood and behavior. In addition to possessing intrinsic photosensitivity via the photopigment melanopsin, these cells receive inputs originating in rods and cones. While most research in the last two decades has focused on the downstream influence of ipRGC signaling, recent studies have shown that ipRGCs also act retrogradely within the retina itself as intraretinal signaling neurons. In this article, we review studies examining intraretinal and, in addition, intraocular signaling pathways of ipRGCs. Through these pathways, ipRGCs regulate inner and outer retinal circuitry through both chemical and electrical synapses, modulate the outputs of ganglion cells (both ipRGCs and non-ipRGCs), and influence arrangement of the correct retinal circuitry and vasculature during development. These data suggest that ipRGC function plays a significant role in the processing of image-forming vision at its earliest stage, positioning these photoreceptors to exert a vital role in perceptual vision. This research will have important implications for lighting design to optimize the best chromatic lighting environments for humans, both in adults and potentially even during fetal and postnatal development. Further studies into these unique ipRGC signaling pathways could also lead to a better understanding of the development of ocular dysfunctions such as myopia.

Light and myopia: from epidemiological studies to neurobiological mechanisms

Myopia is far beyond its inconvenience and represents a true, highly prevalent, sight-threatening ocular condition, especially in Asia. Without adequate interventions, the current epidemic of myopia is projected to affect 50% of the world population by 2050, becoming the leading cause of irreversible blindness. Although blurred vision, the predominant symptom of myopia, can be improved by contact lenses, glasses or refractive surgery, corrected myopia, particularly high myopia, still carries the risk of secondary blinding complications such as glaucoma, myopic maculopathy and retinal detachment, prompting the need for prevention. Epidemiological studies have reported an association between outdoor time and myopia prevention in children. The protective effect of time spent outdoors could be due to the unique characteristics (intensity, spectral distribution, temporal pattern, etc.) of sunlight that are lacking in artificial lighting. Concomitantly, studies in animal models have highlighted the efficacy of light and its components in delaying or even stopping the development of myopia and endeavoured to elucidate possible mechanisms involved in this process. In this narrative review, we (1) summarize the current knowledge concerning light modulation of ocular growth and refractive error development based on studies in human and animal models, (2) summarize potential neurobiological mechanisms involved in the effects of light on ocular growth and emmetropization and (3) highlight a potential pathway for the translational development of noninvasive light-therapy strategies for myopia prevention in children.

Crosstalk: The diversity of melanopsin ganglion cell types has begun to challenge the canonical divide between image-forming and non-image-forming vision

Melanopsin ganglion cells have defied convention since their discovery almost 20 years ago. In the years following, many types of these intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged. In the mouse retina, there are currently six known types (M1-M6) of melanopsin ganglion cells, each with unique morphology, mosaics, connections, physiology, projections, and functions. While melanopsin-expressing cells are usually associated with behaviors like circadian photoentrainment and the pupillary light reflex, the characterization of multiple types has demonstrated a reach that may extend far beyond non-image-forming vision. In fact, studies have shown that individual types of melanopsin ganglion cells have the potential to impact image-forming functions like contrast sensitivity and color opponency. Thus, the goal of this review is to summarize the morphological and functional aspects of the six known types of melanopsin ganglion cells in the mouse retina and to highlight their respective roles in non-image-forming and image-forming vision. Although many melanopsin ganglion cell types do project to image-forming brain targets, it is important to note that this is only the first step in determining their influence on image-forming vision. Even so, the visual system has canonically been divided into these two functional realms and melanopsin ganglion cells have begun to challenge the boundary between them, providing an overlap of visual information that is complementary rather than redundant. Further studies on these ganglion cell photoreceptors will no doubt continue to illustrate an ever-expanding role for melanopsin ganglion cells in image-forming vision.

Circadian organization of the mammalian retina: from gene regulation to physiology and diseases

The retinal circadian system represents a unique structure. It contains a complete circadian system and thus the retina represents an ideal model to study fundamental questions of how neural circadian systems are organized and what signaling pathways are used to maintain synchrony of the different structures in the system. In addition, several studies have shown that multiple sites within the retina are capable of generating circadian oscillations. The strength of circadian clock gene expression and the emphasis of rhythmic expression are divergent across vertebrate retinas, with photoreceptors as the primary locus of rhythm generation in amphibians, while in mammals clock activity is most robust in the inner nuclear layer. Melatonin and dopamine serve as signaling molecules to entrain circadian rhythms in the retina and also in other ocular structures. Recent studies have also suggested GABA as an important component of the system that regulates retinal circadian rhythms. These transmitter-driven influences on clock molecules apparently reinforce the autonomous transcription-translation cycling of clock genes. The molecular organization of the retinal clock is similar to what has been reported for the SCN although inter-neural communication among retinal neurons that form the circadian network is apparently weaker than those present in the SCN, and it is more sensitive to genetic disruption than the central brain clock. The melatonin-dopamine system is the signaling pathway that allows the retinal circadian clock to reconfigure retinal circuits to enhance light-adapted cone-mediated visual function during the day and dark-adapted rod-mediated visual signaling at night. Additionally, the retinal circadian clock also controls circadian rhythms in disk shedding and phagocytosis, and possibly intraocular pressure. Emerging experimental data also indicate that circadian clock is also implicated in the pathogenesis of eye disease and compelling experimental data indicate that dysfunction of the retinal circadian system negatively impacts the retina and possibly the cornea and the lens.

The absence of melanopsin alters retinal clock function and dopamine regulation by light

The retinal circadian clock is crucial for optimal regulation of retinal physiology and function, yet its cellular location in mammals is still controversial. We used laser microdissection to investigate the circadian profiles and phase relations of clock gene expression and Period gene induction by light in the isolated outer (rods/cones) and inner (inner nuclear and ganglion cell layers) regions in wild-type and melanopsin-knockout (Opn 4 (-/-) ) mouse retinas. In the wild-type mouse, all clock genes are rhythmically expressed in the photoreceptor layer but not in the inner retina. For clock genes that are rhythmic in both retinal compartments, the circadian profiles are out of phase. These results are consistent with the view that photoreceptors are a potential site of circadian rhythm generation. In mice lacking melanopsin, we found an unexpected loss of clock gene rhythms and of the photic induction of Per1-Per2 mRNAs only in the outer retina. Since melanopsin ganglion cells are known to provide a feed-back signalling pathway for photic information to dopaminergic cells, we further examined dopamine (DA) synthesis in Opn 4 (-/-) mice. The lack of melanopsin prevented the light-dependent increase of tyrosine hydroxylase (TH) mRNA and of DA and, in constant darkness, led to comparatively high levels of both components. These results suggest that melanopsin is required for molecular clock function and DA regulation in the retina, and that Period gene induction by light is mediated by a melanopsin-dependent, DA-driven signal acting on retinal photoreceptors.

Functional integrity and modification of retinal dopaminergic neurons in the rd1 mutant mouse: roles of melanopsin and GABA

The progressive loss of rod and cone photoreceptors in human subjects with retinitis pigmentosa causes a gradual decline in vision and can result in blindness. Current treatment strategies for the disease rely on the integrity of inner retinal neurons, such as amacrine cells, that are postsynaptic to photoreceptors. Previous work has demonstrated that a specialized subclass of retinal amacrine cell that synthesizes and releases the key neurotransmitter dopamine remains morphologically intact during the disease; however, the pathophysiological function of these neurons remains poorly understood. Here we examined spontaneous and light-evoked spike activity of genetically labeled dopamine neurons from the retinas of retinal degeneration 1 (rd1) mice. Our results indicated that rd1 dopamine neurons remained functionally intact with preserved spontaneous spiking activity and light-evoked responses. The light responses were mediated exclusively by melanopsin phototransduction, not by surviving cones. Our data also suggested that dopamine neurons were altered during photoreceptor loss, as evidenced by less spontaneous bursting activity and increased light-evoked responses with age. Further evidence showed that these alterations were attributed to enhanced GABA/melanopsin signaling to dopamine neurons during disease progression. Taken together, our studies provide valuable information regarding the preservation and functional modification of the retinal dopamine neuronal system in rd1; this information should be considered when designing treatment strategies for retinitis pigmentosa.

Differential expression of melanopsin isoforms Opn4L and Opn4S during postnatal development of the mouse retina

Photosensitive retinal ganglion cells (pRGCs) respond to light from birth and represent the earliest known light detection system to develop in the mouse retina. A number of morphologically and functionally distinct subtypes of pRGCs have been described in the adult retina, and have been linked to different physiological roles. We have previously identified two distinct isoforms of mouse melanopsin, Opn4L and Opn4S, which are generated by alternate splicing of the Opn4 locus. These isoforms are differentially expressed in pRGC subtypes of the adult mouse retina, with both Opn4L and Opn4S detected in M1 type pRGCs, and only Opn4L detected in M2 type pRGCs. Here we investigate the developmental expression of Opn4L and Opn4S and show a differential profile of expression during postnatal development. Opn4S mRNA is detected at relatively constant levels throughout postnatal development, with levels of Opn4S protein showing a marked increase between P0 and P3, and then increasing progressively over time until adult levels are reached by P10. By contrast, levels of Opn4L mRNA and protein are low at birth and show a marked increase at P14 and P30 compared to earlier time points. We suggest that these differing profiles of expression are associated with the functional maturation of M1 and M2 subtypes of pRGCs. Based upon our data, Opn4S expressing M1 type pRGCs mature first and are the dominant pRGC subtype in the neonate retina, whereas increased expression of Opn4L and the maturation of M2 type pRGCs occurs later, between P10 and P14, at a similar time to the maturation of rod and cone photoreceptors. We suggest that the distinct functions associated with these cell types will develop at different times during postnatal development.

Intrinsically photosensitive retinal ganglion cells

Life on earth is subject to alternating cycles of day and night imposed by the rotation of the earth. Consequently, living things have evolved photodetective systems to synchronize their physiology and behavior with the external light-dark cycle. This form of photodetection is unlike the familiar "image vision," in that the basic information is light or darkness over time, independent of spatial patterns. "Nonimage" vision is probably far more ancient than image vision and is widespread in living species. For mammals, it has long been assumed that the photoreceptors for nonimage vision are also the textbook rods and cones. However, recent years have witnessed the discovery of a small population of retinal ganglion cells in the mammalian eye that express a unique visual pigment called melanopsin. These ganglion cells are intrinsically photosensitive and drive a variety of nonimage visual functions. In addition to being photoreceptors themselves, they also constitute the major conduit for rod and cone signals to the brain for nonimage visual functions such as circadian photoentrainment and the pupillary light reflex. Here we review what is known about these novel mammalian photoreceptors.

Intraretinal signaling by ganglion cell photoreceptors to dopaminergic amacrine neurons

Retinal dopaminergic amacrine neurons (DA neurons) play a central role in reconfiguring retinal function according to prevailing illumination conditions, yet the mechanisms by which light regulates their activity are poorly understood. We investigated the means by which sustained light responses are evoked in DA neurons. Sustained light responses were driven by cationic currents and persisted in vitro and in vivo in the presence of L-AP4, a blocker of retinal ON-bipolar cells. Several characteristics of these L-AP4-resistant light responses suggested that they were driven by melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), including long latencies, marked poststimulus persistence, and a peak spectral sensitivity of 478 nm. Furthermore, sustained DA neuron light responses, but not transient DA neuron responses, persisted in rod/cone degenerate retinas, in which ipRGCs account for virtually all remaining retinal phototransduction. Thus, ganglion-cell photoreceptors provide excitatory drive to DA neurons, most likely by way of the coramification of their dendrites and the processes of DA neurons in the inner plexiform layer. This unprecedented centrifugal outflow of ganglion-cell signals within the retina provides a novel basis for the restructuring of retinal circuits by light.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.