Physiologic diversity and development of intrinsically photosensitive retinal ganglion cells

Immunotoxin-induced ablation of melanopsin retinal ganglion cells in a non-murine mammalian model

In mammals, non-image-forming visual functions, including circadian photoentrainment and the pupillary light reflex, are thought to be mediated by the combination of rods, cones, and the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). Although several genetic models have been developed to clarify the individual roles of the rod, cone, and ipRGC systems in mediating non-image visual function, assessing the in vivo role(s) of the ipRGCs has been complicated by the possibility of ontogenetic issues in these genetically modified animal models. In the present study, we describe the development and validation of an immunotoxin that specifically targets the ipRGC population in the mature mammalian retina. This ipRGC immunotoxin, consisting of saporin conjugated to a melanopsin polyclonal antibody, was evaluated with respect to its effectiveness and specificity in depleting the ipRGC population in the fully developed rat retina. The results showed that the ipRGC toxin rapidly and permanently depleted approximately 70% of the ipRGC population, without inducing appreciable changes in the cell number or morphology of any of the non-melanopsin-containing retinal cell populations investigated. These findings suggest that the newly developed ipRGC immunotoxin provides a potent method for achieving relatively rapid, permanent, and selective depletion of the ipRGC population in a non-murine model system. The development of this ipRGC-ablation method is the next step in elucidating the role of ipRGCs in mediating non-visual and circadian light-resetting responses in a wide range of non-murine mammalian models.

Ectopic expression of multiple microbial rhodopsins restores ON and OFF light responses in retinas with photoreceptor degeneration

By expressing channel rhodopsin-2 (ChR2) in inner retinal neurons, previous studies have demonstrated restoration of ON responses in the retina after the death of rod and cone photoreceptors. In this study, we report that the expression of halorhodopsin (HaloR), a light-driven chloride pump, can effectively restore OFF responses in inner retinal neurons of mice with retinal degeneration. We show that HaloR-expressing retinal ganglion cells respond to light with rapid hyperpolarization and suppression of spike activity. After termination of the light stimulus, their membrane potential exhibits a rapid rebound overshoot with robust sustained or transient spike firing. Furthermore, we show that coexpression of ChR2/HaloR in retinal ganglion cells can produce ON, OFF, and even ON-OFF responses, depending on the wavelength of the light stimulus. Our results suggest that the expression of multiple microbial rhodopsins such as ChR2 and HaloR is a possible strategy to restore both ON and OFF light responses in the retina after the death of rod and cone photoreceptors.

ON inputs to the OFF layer: bipolar cells that break the stratification rules of the retina

The vertebrate retina is a distinctly laminar structure. Functionally, the inner plexiform layer, in which bipolar cells synapse onto amacrine and ganglion cells, is subdivided into two sublaminae. Cells that depolarize at light offset ramify in sublamina a; those that depolarize at light onset ramify in sublamina b. The separation of ON and OFF pathways appears to be a fundamental principle of retinal organization that is reflected throughout the entire visual system. We show three clear exceptions to this rule, in which the axons of calbindin-positive ON cone bipolar cells make ribbon synapses as they pass through the OFF layers with three separate cell types: (1) dopaminergic amacrine cells, (2) intrinsically photosensitive ganglion cells, and (3) bistratified diving ganglion cells. The postsynaptic location of the AMPA receptor GluR4 at these sites suggests that ON bipolar cells can make functional synapses as their axons pass through the OFF layers of the inner plexiform layer. These findings resolve a long-standing question regarding the anomalous ON inputs to dopaminergic amacrine cells and suggest that certain ON bipolar cell axons can break the stratification rules of the inner plexiform layer by providing significant synaptic output before their terminal specializations. These outputs are not only to dopaminergic amacrine cells but also to at least two ON ganglion cell types that have dendrites that arborize in sublamina a.

Melanopsin bistability: a fly's eye technology in the human retina

In addition to rods and cones, the human retina contains light-sensitive ganglion cells that express melanopsin, a photopigment with signal transduction mechanisms similar to that of invertebrate rhabdomeric photopigments (IRP). Like fly rhodopsins, melanopsin acts as a dual-state photosensitive flip-flop in which light drives both phototransduction responses and chromophore photoregeneration that bestows independence from the retinoid cycle required by rods and cones to regenerate photoresponsiveness following bleaching by light. To explore the hypothesis that melanopsin in humans expresses the properties of a bistable photopigment in vivo we used the pupillary light reflex (PLR) as a tool but with methods designed to study invertebrate photoreceptors. We show that the pupil only attains a fully stabilized state of constriction after several minutes of light exposure, a feature that is consistent with typical IRP photoequilibrium spectra. We further demonstrate that previous exposure to long wavelength light increases, while short wavelength light decreases the amplitude of pupil constriction, a fundamental property of IRP difference spectra. Modelling these responses to invertebrate photopigment templates yields two putative spectra for the underlying R and M photopigment states with peaks at 481 nm and 587 nm respectively. Furthermore, this bistable mechanism may confer a novel form of "photic memory" since information of prior light conditions is retained and shapes subsequent responses to light. These results suggest that the human retina exploits fly-like photoreceptive mechanisms that are potentially important for the modulation of non-visual responses to light and highlights the ubiquitous nature of photoswitchable photosensors across living organisms.

Daily rhythm of melanopsin-expressing cells in the mouse retina

In addition to some other functions, melanopsin-expressing retinal ganglion cells (RGCs) constitute the principal mediators of the circadian photoentrainment, a process by which the suprachiasmatic nucleus (the central clock of mammals), adjusts daily to the external day/night cycle. In the present study these RGCs were immunohistochemically labelled using a specific polyclonal antiserum raised against mouse melanopsin. A daily oscillation in the number of immunostained cells was detected in mice kept under a light / dark (LD) cycle. One hour before the lights were on (i.e., the end of the night period) the highest number of immunopositive cells was detected while the lowest was seen 4 h later (i.e., within the first hours of the light period). This finding suggests that some of the melanopsin-expressing RGCs “turn on” and “off” during the day/night cycle. We have also detected that these daily variations already occur in the early postnatal development, when the rod/cone photoreceptor system is not yet functional. Two main melanopsin-expressing cell subpopulations could be found within the retina: M1 cells showed robust dendritic arborization within the OFF sublamina of the inner plexiform layer (IPL), whilst M2 cells had fine dendritic processes within the ON sublamina of the IPL. These two cell subpopulations also showed different daily oscillations throughout the LD cycle. In order to find out whether or not the melanopsin rhythm was endogenous, other mice were maintained in constant darkness for 6 days. Under these conditions, no defined rhythm was detected, which suggests that the daily oscillation detected either is light-dependent or is gradually lost under constant conditions. This is the first study to analyze immunohistochemically the daily oscillation of the number of melanopsin-expressing cells in the mouse retina.

Photon capture and signalling by melanopsin retinal ganglion cells

A subset of retinal ganglion cells has recently been discovered to be intrinsically photosensitive, with melanopsin as the pigment. These cells project primarily to brain centres for non-image-forming visual functions such as the pupillary light reflex and circadian photoentrainment. How well they signal intrinsic light absorption to drive behaviour remains unclear. Here we report fundamental parameters governing their intrinsic light responses and associated spike generation. The membrane density of melanopsin is 10(4)-fold lower than that of rod and cone pigments, resulting in a very low photon catch and a phototransducing role only in relatively bright light. Nonetheless, each captured photon elicits a large and extraordinarily prolonged response, with a unique shape among known photoreceptors. Notably, like rods, these cells are capable of signalling single-photon absorption. A flash causing a few hundred isomerized melanopsin molecules in a retina is sufficient for reaching threshold for the pupillary light reflex.

Functional and morphological differences among intrinsically photosensitive retinal ganglion cells

A subset of ganglion cells in the mammalian retina express the photopigment melanopsin and are intrinsically photosensitive (ipRGCs). These cells are implicated in non-image-forming visual responses to environmental light, such as the pupillary light reflex, seasonal adaptations in physiology, photic inhibition of nocturnal melatonin release, and modulation of sleep, alertness, and activity. Morphological studies have confirmed the existence of at least three distinct subpopulations of ipRGCs, but studies of the physiology of ipRGCs at the single cell level have focused mainly on M1 cells, the dendrites of which stratify solely in sublamina a (OFF sublamina) of the retinal inner plexiform layer (IPL). Little work has been done to compare the functional properties of M1 cells to those of M2 cells, the dendrites of which stratify solely in sublamina b (ON sublamina) of the IPL. The goal of the current study was to compare the morphology, intrinsic light response, and intrinsic membrane properties of M1 and M2 cells in the mouse retina. Here we demonstrate additional morphological differences between M1 and M2 cells as well as distinct physiological characteristics of both the intrinsic light responses and intrinsic membrane properties. M2 cells displayed a more complex dendritic arborization and higher input resistance, yet showed lower light sensitivity and lower maximal light responses than M1 cells. These data indicate morphological and functional heterogeneity among ipRGCs.

The development of melanopsin-containing retinal ganglion cells in mice with early retinal degeneration

In mammals, the neuronal pathways by which rod and cone photoreceptors mediate vision have been well documented. The roles that classical photoreceptors play in photoentrainment, however, have been less clear. In mammals, intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin project directly to the suprachiasmatic nucleus of the hypothalamus, the site of the circadian clock, and thereby contribute to non-image-forming responses to light. Classical photoreceptors are not necessary for photoentrainment as loss of rods and cones does not eliminate light entrainment. Conflicting evidence arose, however, when attenuated phase-shifting responses were observed in the retinal-degenerate CBA/J mouse. In this study, we examined the time course of retinal degeneration in CBA/J mice and used these animals to determine if maturation of the outer retina regulates the morphology, number and distribution of ipRGCs. We also examined whether degeneration during the early development of the outer retina can alter the function of the adult circadian system. We report that dendritic stratification and distribution of ipRGCs was unaltered in mice with early retinal degeneration, suggesting that normal development of the outer retina was not necessary for these processes. We found, however, that adult CBA/J mice have greater numbers of ipRGCs than controls, implicating a role for the outer retinal photoreceptors in regulating developmental cell death of ipRGCs.

Absence of long-wavelength photic potentiation of murine intrinsically photosensitive retinal ganglion cell firing in vitro

Melanopsin is an opsin-family photopigment required for photosensitivity of the intrinsically photosensitive retinal ganglion cells (ipRGCs), which subserve photic entrainment of circadian rhythms in mammals. The melanopsin photocycle is presently unknown but is independent of the enzymatic photocycle employed by rhodopsin and cone opsins. Recent experiments have demonstrated that red-light exposure potentiates circadian phase-shifting responses to blue-light stimuli, consistent with the hypothesis that melanopsin functions as a bistable photopigment. To further test this hypothesis, we analyzed ipRGC firing activity in response to 480-nm blue light with or without intervening long-wavelength 620-nm red-light stimulation, using in vitro multielectrode array recording of postnatal day 8 to 10 murine retina. Cell-firing responses to 480-nm light were highly reproducible. No significant potentiating or bleaching effect of intervening subthreshold 620-nm light on ipRGC firing to 480-nm light could be discerned. Further physiologic and biochemical analysis of the ipRGC photoreception is required to reconcile the presence of long-wavelength potentiation at the level of the SCN with its absence in light-induced ipRGC firing.

Restoration of visual function in retinal degeneration mice by ectopic expression of melanopsin

The rod and cone cells of the mammalian retina are the principal photoreceptors for image-forming vision. They transmit information by means of a chain of intermediate cells to the retinal ganglion cells, which in turn send signals from the retina to the brain. Loss of photoreceptor cells, as happens in a number of human diseases, leads to irreversible blindness. In a mouse model (rd/rd) of photoreceptor degeneration, we used a viral vector to express in a large number of retinal ganglion cells the light sensitive protein melanopsin, normally present in only a specialized subset of the cells. Whole-cell patch-clamp recording showed photoresponses in these cells even after degeneration of the photoreceptors and additional pharmacological or Cd(2+) block of synaptic function. Interestingly, similar responses were observed across a wide variety of diverse types of ganglion cell of the retina. The newly melanopsin-expressing ganglion cells provided an enhancement of visual function in rd/rd mice: the pupillary light reflex (PLR) returned almost to normal; the mice showed behavioral avoidance of light in an open-field test, and they could discriminate a light stimulus from a dark one in a two-choice visual discrimination alley. Recovery of the PLR was stable for at least 11 months. It has recently been shown that ectopic retinal expression of a light sensitive bacterial protein, channelrhodopsin-2, can restore neuronal responsiveness and simple visual abilities in rd/rd mice. For therapy in human photodegenerations, channelrhodopsin-2 and melanopsin have different advantages and disadvantages; both proteins (or modifications of them) should be candidates.

Inducible ablation of melanopsin-expressing retinal ganglion cells reveals their central role in non-image forming visual responses

Rod/cone photoreceptors of the outer retina and the melanopsin-expressing retinal ganglion cells (mRGCs) of the inner retina mediate non-image forming visual responses including entrainment of the circadian clock to the ambient light, the pupillary light reflex (PLR), and light modulation of activity. Targeted deletion of the melanopsin gene attenuates these adaptive responses with no apparent change in the development and morphology of the mRGCs. Comprehensive identification of mRGCs and knowledge of their specific roles in image-forming and non-image forming photoresponses are currently lacking. We used a Cre-dependent GFP expression strategy in mice to genetically label the mRGCs. This revealed that only a subset of mRGCs express enough immunocytochemically detectable levels of melanopsin. We also used a Cre-inducible diphtheria toxin receptor (iDTR) expression approach to express the DTR in mRGCs. mRGCs develop normally, but can be acutely ablated upon diphtheria toxin administration. The mRGC-ablated mice exhibited normal outer retinal function. However, they completely lacked non-image forming visual responses such as circadian photoentrainment, light modulation of activity, and PLR. These results point to the mRGCs as the site of functional integration of the rod/cone and melanopsin phototransduction pathways and as the primary anatomical site for the divergence of image-forming and non-image forming photoresponses in mammals.

Targeted mutation of the calbindin D28K gene disrupts circadian rhythmicity and entrainment

The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker in mammals. A salient feature of the SCN is that cells of a particular phenotype are topographically organized; this organization defines functionally distinct subregions that interact to generate coherent rhythmicity. In Syrian hamsters (Mesocricetus auratus), a dense population of directly retinorecipient calbindin D(28K) (CalB) neurons in the caudal SCN marks a subregion critical for circadian rhythmicity. In mouse SCN, a dense cluster of CalB neurons occurs during early postnatal development, but in the adult CalB neurons are dispersed through the SCN. In the adult retina CalB colocalizes with melanopsin-expressing ganglion cells. In the present study, we explored the role of CalB in modulating circadian function and photic entrainment by investigating mice with a targeted mutation of the CalB gene (CalB-/- mice). In constant darkness (DD), CalB-/- animals either become arrhythmic (40%) or exhibit low-amplitude locomotor rhythms with marked activity during subjective day (60%). Rhythmic clock gene expression is blunted in these latter animals. Importantly, CalB-/- mice exhibit anomalies in entrainment revealed following transfer from a light : dark cycle to DD. Paradoxically, responses to acute light pulses measured by behavioral phase shifts, SCN FOS protein and Period1 mRNA expression are normal. Together, the developmental pattern of CalB expression in mouse SCN, the presence of CalB in photoresponsive ganglion cells and the abnormalities seen in CalB-/- mice suggest an important role for CalB in mouse circadian function.

Intrinsic and extrinsic light responses in melanopsin-expressing ganglion cells during mouse development

Melanopsin (Opn4) is a photopigment found in a subset of retinal ganglion cells (RGCs) that project to various brain areas. These neurons are intrinsically photosensitive (ipRGCs) and are implicated in nonimage-forming responses to environmental light such as the pupillary light reflex and circadian entrainment. Recent evidence indicates that ipRGCs respond to light at birth, but questions remain as to whether and when they undergo significant functional changes. We used bacterial artificial chromosome transgenesis to engineer a mouse line in which enhanced green fluorescent protein (EGFP) is expressed under the control of the melanopsin promoter. Double immunolabeling for EGFP and melanopsin demonstrates their colocalization in ganglion cells of mutant mouse retinas. Electrophysiological recordings of ipRGCs in neonatal mice (postnatal day 0 [P0] to P7) demonstrated that these cells responded to light with small and sluggish depolarization. However, starting at P11 we observed ipRGCs that responded to light with a larger and faster onset (<1 s) and offset (<1 s) depolarization. These faster, larger depolarizations were observed in most ipRGCs by early adult ages. However, on application of a cocktail of synaptic blockers, we found that all cells responded to light with slow onset (>2.5 s) and offset (>10 s) depolarization, revealing the intrinsic, melanopsin-mediated light responses. The extrinsic, cone/rod influence on ipRGCs correlates with their extensive dendritic stratification in the inner plexiform layer. Collectively, these results demonstrate that ipRGCs make use of melanopsin for phototransduction before eye opening and that these cells further integrate signals derived from the outer retina as the retina matures.

Photic sensitivity ranges of hamster pupillary and circadian phase responses do not overlap

Mammalian retinal photoreceptors form an irradiance detection system that drives many nonvisual responses to light such as pupil reflex and resetting of the circadian clock. To understand the role of pupil size in circadian light responses, pupil diameter was pharmacologically manipulated and the effect on behavioral phase shifts at different irradiance levels was studied in the Syrian hamster. Dose-response curves for steady-state pupil size and for behavioral phase shifts were constructed for 3 pupil conditions (dilated, constricted, and control). Retinal irradiance was calculated from corneal irradiance, pupil size, retinal surface area, and absorption of ocular media. The sensitivity of photic responses to retinal irradiance is approximately 1.5 log units higher than to corneal irradiance. When plotted against corneal irradiance, pharmacological pupil constriction reduces the light sensitivity of the circadian system, but pupil dilation has no effect. As expected, when plotted against retinal irradiance all dose-response curves superimposed, confirming that the circadian system responds to photon flux on the retina. Pupil dilation does not increase the circadian response to increasing irradiance, since the response of the circadian system attains saturation at irradiance levels lower than those required to induce pupil constriction. The main finding shows that due to the different response sensitivities, the effect of pupil constriction on the light sensitivity of the circadian system in the hamster under natural conditions is virtually negligible. We further suggest the existence of distinct modulating mechanisms for the differential retinal irradiance sensitivity of the pupil system and the circadian system, which enables the different responses to be tuned to their specific tasks while using similar photoreceptive input.

Characterization and synaptic connectivity of melanopsin-containing ganglion cells in the primate retina

Melanopsin is a photopigment expressed in retinal ganglion cells, which are intrinsically photosensitive and are also involved in retinal circuits arising from rod and cone photoreceptors. This circuitry, however, is poorly understood. Here, we studied the morphology, distribution and synaptic input to melanopsin-containing ganglion cells in a New World monkey, the common marmoset (Callithrix jacchus). The dendrites of melanopsin-containing cells in marmoset stratify either close to the inner nuclear layer (outer stratifying), or close to the ganglion cell layer (inner stratifying). The dendritic fields of outer-stratifying cells tile the retina, with little overlap. However, the dendritic fields of outer-stratifying cells largely overlap with the dendritic fields of inner-stratifying cells. Thus, inner-stratifying and outer-stratifying cells may form functionally independent populations. The synaptic input to melanopsin-containing cells was determined using synaptic markers (antibodies to C-terminal binding protein 2, CtBP2, for presumed bipolar synapses, and antibodies to gephyrin for presumed amacrine synapses). Both outer-stratifying and inner-stratifying cells show colocalized immunoreactive puncta across their entire dendritic tree for both markers. The density of CtBP2 puncta on inner dendrites was about 50% higher than that on outer dendrites. The density of gephyrin puncta was comparable for outer and inner dendrites but higher than the density of CtBP2 puncta. The inner-stratifying cells may receive their input from a type of diffuse bipolar cell (DB6). Our results are consistent with the idea that both outer and inner melanopsin cells receive bipolar and amacrine input across their dendritic tree.

Melanopsin-dependent nonvisual responses: evidence for photopigment bistability in vivo

In mammals, nonvisual responses to light have been shown to involve intrinsically photosensitive retinal ganglion cells (ipRGC) that express melanopsin and that are modulated by input from both rods and cones. Recent in vitro evidence suggests that melanopsin possesses dual photosensory and photoisomerase functions, previously thought to be a unique feature of invertebrate rhabdomeric photopigments. In cultured cells that normally do not respond to light, heterologous expression of mammalian melanopsin confers light sensitivity that can be restored by prior stimulation with appropriate wavelengths. Using three different physiological and behavioral assays, we show that this in vitro property translates to in vivo, melanopsin-dependent nonvisual responses. We find that prestimulation with long-wavelength light not only restores but enhances single-unit responses of SCN neurons to 480-nm light, whereas the long-wavelength stimulus alone fails to elicit any response. Recordings in Opn4-/- mice confirm that melanopsin provides the main photosensory input to the SCN, and furthermore, demonstrate that melanopsin is required for response enhancement, because this capacity is abolished in the knockout mouse. The efficiency of the light-enhancement effect depends on wavelength, irradiance, and duration. Prior long-wavelength light exposure also enhances short-wavelength-induced phase shifts of locomotor activity and pupillary constriction, consistent with the expression of a photoisomerase-like function in nonvisual responses to light.

The retina-attached SCN slice preparation: an in vitro mammalian circadian visual system

The suprachiasmatic nucleus (SCN), the mammalian circadian pacemaker, receives information about ambient light levels through the retinohypothalamic tract. This information resets the molecular clock of SCN neurons, thereby entraining overt animal behavior and physiology to the solar cycle. Progress toward functional characterization of retinal influences on the SCN has been hampered by limitations of established experimental paradigms. To overcome this hurdle, the authors have developed a novel in vitro preparation of the rat retinohypothalamic circuit that maintains functional connectivity between the retinas and the SCN. This method permits whole-cell patch-clamp recordings from visually identified, light-responsive SCN neurons. Using this preparation, the authors have found that in the SCN, light-evoked responses are partly driven by the melanopsin photosensory system of the intrinsically photosensitive retinal ganglion cells and that SCN neurons exhibit light adaptation. The authors have also been able to generate this preparation from mice, demonstrating the feasibility of applying this method to transgenic mice.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+]i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i, suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

Responses of suprachiasmatic nucleus neurons to light and dark adaptation: relative contributions of melanopsin and rod-cone inputs

The circadian oscillator in the suprachiasmatic nucleus (SCN) is entrained to the environmental light/dark cycle through photic information conveyed from the retina. The vast majority of projections to the SCN arise from melanopsin-expressing ganglion cells that are intrinsically light sensitive and that receive inputs from both rods and cones. To investigate the relative contributions of the different photoreceptive systems in shaping the photic signal influencing the circadian clock, we analyzed neuronal responses of single SCN neurons using extracellular electrophysiological recordings under different conditions of light adaptation. In the majority of neurons (78%), the spike rate is increased by light stimulation whereas the remainder are light-inhibited. The neuronal response to light is composed of several components distinguished by their temporal dynamics and degree of alteration after previous light exposure. SCN neurons display a sustained response to light followed by persistence of the response after light offset. These responses are sluggish and relatively unaffected by previous light exposures. Neurons also respond with a brisk, excitatory ON response and often an OFF response that is either excitatory or inhibitory. ON-OFF responses are transient and strongly reduced by previous bright white light exposure. Furthermore, two types of neuronal response patterns can be distinguished by the presence or absence of a slow-transient component that follows the transient ON response. The transient ON-OFF components express light adaptation properties characteristic of retinal channels involving cones, whereas the sustained and persistent components are consistent with in vitro response properties reported for melanopsin-expressing ganglion cells.

Vesicular glutamate transporter 1 is required for photoreceptor synaptic signaling but not for intrinsic visual functions

Glutamatergic neurotransmission requires vesicular glutamate transporters (VGLUTs) to sequester glutamate into synaptic vesicles. Generally, VGLUT1 and VGLUT2 isoforms show complementary expression in the CNS and retina. However, little is known about whether isoform-specific expression serves distinct pathways and physiological functions. Here, by examining visual functions in VGLUT1-null mice, we demonstrate that visual signaling from photoreceptors to retinal output neurons requires VGLUT1. However, photoentrainment and pupillary light responses are preserved. We provide evidence that melanopsin-containing, intrinsically photosensitive retinal ganglion cells (RGCs), signaling via VGLUT2 pathways, support these non-image-forming functions. We conclude that VGLUT1 is essential for transmitting visual signals from photoreceptors to second- and third-order neurons, but VGLUT1 is not necessary for intrinsic visual functions. Furthermore, melanopsin and VGLUT2 expression in a subset of RGCs immediately after birth strongly supports the idea that intrinsic vision can function well before rod- and cone-mediated signaling has matured.

Synaptic influences on rat ganglion-cell photoreceptors

The intrinsically photosensitive retinal ganglion cells (ipRGCs) provide a conduit through which rods and cones can access brain circuits mediating circadian entrainment, pupillary constriction and other non-image-forming visual functions. We characterized synaptic inputs to ipRGCs in rats using whole-cell and multielectrode array recording techniques. In constant darkness all ipRGCs received spontaneous excitatory and inhibitory synaptic inputs. Light stimulation evoked in all ipRGCs both synaptically driven ('extrinsic') and autonomous melanopsin-based ('intrinsic') responses. The extrinsic light responses were depolarizing, about 5 log units more sensitive than the intrinsic light response, and transient near threshold but sustained to brighter light. Pharmacological data showed that ON bipolar cells and amacrine cells make the most prominent direct contributions to these extrinsic light responses, whereas OFF bipolar cells make a very weak contribution. The spatial extent of the synaptically driven light responses was comparable to that of the intrinsic photoresponse, suggesting that synaptic contacts are made onto the entire dendritic field of the ipRGCs. These synaptic influences increase the sensitivity of ipRGCs to light, and also extend their temporal bandpass to higher frequencies. These extrinsic ipRGC light responses can explain some of the previously reported properties of circadian photoentrainment and other non-image-forming visual behaviours.

Modeling the role of mid-wavelength cones in circadian responses to light

Nonvisual responses to light, such as photic entrainment of the circadian clock, involve intrinsically light-sensitive melanopsin-expressing ganglion cells as well as rod and cone photoreceptors. However, previous studies have been unable to demonstrate a specific contribution of cones in the photic control of circadian responses to light. Using a mouse model that specifically lacks mid-wavelength (MW) cones we show that these photoreceptors play a significant role in light entrainment and in phase shifting of the circadian oscillator. The contribution of MW cones is mainly observed for light exposures of short duration and toward the longer wavelength region of the spectrum, consistent with the known properties of this opsin. Modeling the contributions of the various photoreceptors stresses the importance of considering the particular spectral, temporal, and irradiance response domains of the photopigments when assessing their role and contribution in circadian responses to light.

Phototransduction in ganglion-cell photoreceptors

A third class of photoreceptors has recently been identified in the mammalian retina. They are a rare cell type within the class of ganglion cells, which are the output cells of the retina. These intrinsically photosensitive retinal ganglion cells support a variety of physiological responses to daylight, including synchronization of circadian rhythms, modulation of melatonin release, and regulation of pupil size. The goal of this review is to summarize what is currently known concerning the cellular and biochemical basis of phototransduction in these cells. I summarize the overwhelming evidence that melanopsin serves as the photopigment in these cells and review the emerging evidence that the downstream signaling cascade, including the light-gated channel, might resemble those found in rhabdomeric invertebrate photoreceptors.

Melanopsin changes in neonatal albino rat independent of rods and cones

Intrinsically photosensitive retinal ganglion cells employ the photopigment melanopsin and provide light information to brain areas responsible for the regulation of circadian rhythms. The expression of melanopsin is regulated by environmental illumination, but it remains to be clarified whether the rods and cones are involved. Here, we examined the influence of 5 days of constant light and dark conditions on melanopsin mRNA and protein expression in newborn albino rats, in which functional rods and cones have not yet been developed. We found that the melanopsin mRNA level was unaffected, whereas the melanopsin protein level was more than two-fold higher in the darkness-adapted group than in pups raised in constant light. In pups raised during 12 : 12 h light/dark cycles, the melanopsin protein level was significantly higher during the day than at night. Our findings indicate that melanopsin protein changes are independent of input from the rods and cones.

Inner retinal photoreception independent of the visual retinoid cycle

Mice lacking the visual cycle enzymes RPE65 or lecithin-retinol acyl transferase (Lrat) have pupillary light responses (PLR) that are less sensitive than those of mice with outer retinal degeneration (rd/rd or rdta). Inner retinal photoresponses are mediated by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting that the melanopsin-dependent photocycle utilizes RPE65 and Lrat. To test this hypothesis, we generated rpe65(-/-); rdta and lrat(-/-); rd/rd mutant mice. Unexpectedly, both rpe65(-/-); rdta and lrat(-/-); rd/rd mice demonstrate paradoxically increased PLR photosensitivity compared with mice mutant in visual cycle enzymes alone. Acute pharmacologic inhibition of the visual cycle of melanopsin-deficient mice with all-trans-retinylamine results in a near-total loss of PLR sensitivity, whereas treatment of rd/rd mice has no effect, demonstrating that the inner retina does not require the visual cycle. Treatment of rpe65(-/-); rdta with 9-cis-retinal partially restores PLR sensitivity. Photic sensitivity in P8 rpe65(-/-) and lrat(-/-) ipRGCs is intact as measured by ex vivo multielectrode array recording. These results demonstrate that the melanopsin-dependent ipRGC photocycle is independent of the visual retinoid cycle.

Ectopic expression of a microbial-type rhodopsin restores visual responses in mice with photoreceptor degeneration

The death of photoreceptor cells caused by retinal degenerative diseases often results in a complete loss of retinal responses to light. We explore the feasibility of converting inner retinal neurons to photosensitive cells as a possible strategy for imparting light sensitivity to retinas lacking rods and cones. Using delivery by an adeno-associated viral vector, here, we show that long-term expression of a microbial-type rhodopsin, channelrhodopsin-2 (ChR2), can be achieved in rodent inner retinal neurons in vivo. Furthermore, we demonstrate that expression of ChR2 in surviving inner retinal neurons of a mouse with photoreceptor degeneration can restore the ability of the retina to encode light signals and transmit the light signals to the visual cortex. Thus, expression of microbial-type channelrhodopsins, such as ChR2, in surviving inner retinal neurons is a potential strategy for the restoration of vision after rod and cone degeneration.