Light-evoked synaptic activity of retinal ganglion and amacrine cells is regulated in developing mouse retina

Light drives the developmental progression of outer retinal function

The complex nature of rod and cone photoreceptors and the light-evoked responsivity of bipolar cells in the mature rodent retina have been well characterized. However, little is known about the emergent light-evoked response properties of the mouse retina and the role light plays in shaping these emergent responses. We have previously demonstrated that the outer retina is responsive to green light as early as postnatal day 8 (P8). Here, we characterize the progression of both photoreceptors (rods and cones) and bipolar cell responses during development and into adulthood using ex vivo electroretinogram recordings. Our data show that the majority of photoreceptor response at P8 originates from cones and that these outputs drive second-order bipolar cell responses as early as P9. We find that the magnitude of the photoresponse increases concurrently with each passing day of postnatal development and that many functional properties of these responses, as well as the relative rod/cone contributions to the total light-evoked response, are age dependent. We compare these responses at eye opening and maturity to age-matched animals raised in darkness and found that the absence of light diminishes emergent and mature cone-to-bipolar cell signaling. Furthermore, we found cone-evoked responses to be significantly slower in dark-reared retinas. Together, this work characterizes the developmental photoresponsivity of the mouse retina while highlighting the importance of properly timed sensory input for the maturation of the first visual system synapse.

Graphene-based microfluidic perforated microelectrode arrays for retinal electrophysiological studies

Perforated microelectrode arrays (pMEAs) have become essential tools for ex vivo retinal electrophysiological studies. pMEAs increase the nutrient supply to the explant and alleviate the accentuated curvature of the retina, allowing for long-term culture and intimate contacts between the retina and electrodes for electrophysiological measurements. However, commercial pMEAs are not compatible with in situ high-resolution optical imaging and lack the capability of controlling the local microenvironment, which are highly desirable features for relating function to anatomy and probing physiological and pathological mechanisms in retina. Here we report on microfluidic pMEAs (μpMEAs) that combine transparent graphene electrodes and the capability of locally delivering chemical stimulation. We demonstrate the potential of μpMEAs by measuring the electrical response of ganglion cells to locally delivered high K+ stimulation under controlled microenvironments. Importantly, the capability for high-resolution confocal imaging of the retina tissue on top of the graphene electrodes allows for further analyses of the electrical signal source. The new capabilities provided by μpMEAs could allow for retinal electrophysiology assays to address key questions in retinal circuitry studies.

Visual Stimulation Induces Distinct Forms of Sensitization of On-Off Direction-Selective Ganglion Cell Responses in the Dorsal and Ventral Retina

Experience-dependent modulation of neuronal responses is a key attribute in sensory processing. In the mammalian retina, the On-Off direction-selective ganglion cell (DSGC) is well known for its robust direction selectivity. However, how the On-Off DSGC light responsiveness dynamically adjusts to the changing visual environment is underexplored. Here, we report that On-Off DSGCs tuned to posterior motion direction [i.e. posterior DSGCs (pDSGCs)] in mice of both sexes can be transiently sensitized by prior stimuli. Notably, distinct sensitization patterns are found in dorsal and ventral pDSGCs. Although responses of both dorsal and ventral pDSGCs to dark stimuli (Off responses) are sensitized, only dorsal cells show the sensitization of responses to bright stimuli (On responses). Visual stimulation to the dorsal retina potentiates a sustained excitatory input from Off bipolar cells, leading to tonic depolarization of pDSGCs. Such tonic depolarization propagates from the Off to the On dendritic arbor of the pDSGC to sensitize its On response. We also identified a previously overlooked feature of DSGC dendritic architecture that can support dendritic integration between On and Off dendritic layers bypassing the soma. By contrast, ventral pDSGCs lack a sensitized tonic depolarization and thus do not exhibit sensitization of their On responses. Our results highlight a topographic difference in Off bipolar cell inputs underlying divergent sensitization patterns of dorsal and ventral pDSGCs. Moreover, substantial crossovers between dendritic layers of On-Off DSGCs suggest an interactive dendritic algorithm for processing On and Off signals before they reach the soma.SIGNIFICANCE STATEMENT Visual neuronal responses are dynamically influenced by the prior visual experience. This form of plasticity reflects the efficient coding of the naturalistic environment by the visual system. We found that a class of retinal output neurons, On-Off direction-selective ganglion cells, transiently increase their responsiveness after visual stimulation. Cells located in dorsal and ventral retinas exhibit distinct sensitization patterns because of different adaptive properties of Off bipolar cell signaling. A previously overlooked dendritic morphologic feature of the On-Off direction-selective ganglion cell is implicated in the cross talk between On and Off pathways during sensitization. Together, these findings uncover a topographic difference in the adaptive encoding of upper and lower visual fields and the underlying neural mechanism in the dorsal and ventral retinas.

Visual Deprivation Retards the Maturation of Dendritic Fields and Receptive Fields of Mouse Retinal Ganglion Cells

It was well documented that both the size of the dendritic field and receptive field of retinal ganglion cells (RGCs) are developmentally regulated in the mammalian retina, and visual stimulation is required for the maturation of the dendritic and receptive fields of mouse RGCs. However, it is not clear whether the developmental changes of the RGC receptive field correlate with the dendritic field and whether visual stimulation regulates the maturation of the dendritic field and receptive field of RGCs in a correlated manner. The present work demonstrated that both the dendritic and receptive fields of RGCs continuously develop after eye opening. However, the correlation between the developmental changes in the receptive field size and the dendritic field varies among different RGC types. These results suggest a continuous change of synaptic converging of RGC synaptic inputs in an RGC type-dependent manner. Besides, light deprivation impairs both the development of dendritic and receptive fields.

Decellularised extracellular matrix-derived peptides from neural retina and retinal pigment epithelium enhance the expression of synaptic markers and light responsiveness of human pluripotent stem cell derived retinal organoids

Tissue specific extracellular matrices (ECM) provide structural support and enable access to molecular signals and metabolites, which are essential for directing stem cell renewal and differentiation. To mimic this phenomenon in vitro, tissue decellularisation approaches have been developed, resulting in the generation of natural ECM scaffolds that have comparable physical and biochemical properties of the natural tissues and are currently gaining traction in tissue engineering and regenerative therapies due to the ease of standardised production, and constant availability. In this manuscript we report the successful generation of decellularised ECM-derived peptides from neural retina (decel NR) and retinal pigment epithelium (decel RPE), and their impact on differentiation of human pluripotent stem cells (hPSCs) to retinal organoids. We show that culture media supplementation with decel RPE and RPE-conditioned media (CM RPE) significantly increases the generation of rod photoreceptors, whilst addition of decel NR and decel RPE significantly enhances ribbon synapse marker expression and the light responsiveness of retinal organoids. Photoreceptor maturation, formation of correct synapses between retinal cells and recording of robust light responses from hPSC-derived retinal organoids remain unresolved challenges for the field of regenerative medicine. Enhanced rod photoreceptor differentiation, synaptogenesis and light response in response to addition of decellularised matrices from RPE and neural retina as shown herein provide a novel and substantial advance in generation of retinal organoids for drug screening, tissue engineering and regenerative medicine.

Pan-retinal characterisation of Light Responses from Ganglion Cells in the Developing Mouse Retina

We have investigated the ontogeny of light-driven responses in mouse retinal ganglion cells (RGCs). Using a large-scale, high-density multielectrode array, we recorded from hundreds to thousands of RGCs simultaneously at pan-retinal level, including dorsal and ventral locations. Responses to different contrasts not only revealed a complex developmental profile for ON, OFF and ON-OFF responses, but also unveiled differences between dorsal and ventral RGC responses. At eye-opening, dorsal RGCs of all types were more responsive to light, perhaps indicating an environmental priority to nest viewing for pre-weaning pups. The developmental profile of ON and OFF responses exhibited antagonistic behaviour, with the strongest ON responses shortly after eye-opening, followed by an increase in the strength of OFF responses later on. Further, we found that with maturation receptive field (RF) center sizes decrease, spike-triggered averaged responses to white noise become stronger, and centers become more circular while maintaining differences between RGC types. We conclude that the maturation of retinal functionality is not spatially homogeneous, likely reflecting ecological requirements that favour earlier maturation of the dorsal retina.

Increased proliferation of late-born retinal progenitor cells by gestational lead exposure delays rod and bipolar cell differentiation

PURPOSE

Studies of neuronal development in the retina often examine the stages of proliferation, differentiation, and synaptic development, albeit independently. Our goal was to determine if a known neurotoxicant insult to a population of retinal progenitor cells (RPCs) would affect their eventual differentiation and synaptic development. To that end, we used our previously published human equivalent murine model of low-level gestational lead exposure (GLE). Children and animals with GLE exhibit increased scotopic electroretinogram a- and b-waves. Adult mice with GLE exhibit an increased number of late-born RPCs, a prolonged period of RPC proliferation, and an increased number of late-born rod photoreceptors and rod and cone bipolar cells (BCs), with no change in the number of late-born Müller glial cells or early-born neurons. The specific aims of this study were to determine whether increased and prolonged RPC proliferation alters the spatiotemporal differentiation and synaptic development of rods and BCs in early postnatal GLE retinas compared to control retinas.

METHODS

C57BL/6N mouse pups were exposed to lead acetate via drinking water throughout gestation and until postnatal day 10, which is equivalent to the human gestation period for retinal neurogenesis. RT-qPCR, immunohistochemical analysis, and western blots of well-characterized, cell-specific genes and proteins were performed at embryonic and early postnatal ages to assess rod and cone photoreceptor differentiation, rod and BC differentiation and synaptic development, and Müller glial cell differentiation.

RESULTS

Real-time quantitative PCR (RT-qPCR) with the rod-specific transcription factors Nrl, Nr2e3, and Crx and the rod-specific functional gene Rho, along with central retinal confocal studies with anti-recoverin and anti-rhodopsin antibodies, revealed a two-day delay in the differentiation of rod photoreceptors in GLE retinas. Rhodopsin immunoblots supported this conclusion. No changes in glutamine synthetase gene or protein expression, a marker for late-born Müller glial cells, were observed in the developing retinas. In the retinas from the GLE mice, anti-PKCα, -Chx10 (Vsx2) and -secretagogin antibodies revealed a two- to three-day delay in the differentiation of rod and cone BCs, whereas the expression of the proneural and BC genes Otx2 and Chx10, respectively, increased. In addition, confocal studies of proteins associated with functional synapses (e.g., vesicular glutamate transporter 1 [VGluT1], plasma membrane calcium ATPase [PMCA], transient receptor potential channel M1 [TRPM1], and synaptic vesicle glycoprotein 2B [SV2B]) revealed a two-day delay in the formation of the outer and inner plexiform layers of the GLE retinas. Moreover, several markers revealed that the initiation of the differentiation and intensity of the labeling of early-born cells in the retinal ganglion cell and inner plexiform layers were not different in the control retinas.

CONCLUSIONS

Our combined gene, confocal, and immunoblot findings revealed that the onset of rod and BC differentiation and their subsequent synaptic development is delayed by two to three days in GLE retinas. These results suggest that perturbations during the early proliferative stages of late-born RPCs fated to be rods and BCs ultimately alter the coordinated time-dependent progression of rod and BC differentiation and synaptic development. These GLE effects were selective for late-born neurons. Although the molecular mechanisms are unknown, alterations in soluble neurotrophic factors and/or their receptors are likely to play a role. Since neurodevelopmental delays and altered synaptic connectivity are associated with neuropsychiatric and behavioral disorders as well as cognitive deficits, future work is needed to determine if similar effects occur in the brains of GLE mice and whether children with GLE experience similar delays in retinal and brain neuronal differentiation and synaptic development.

Compensatory plasticity in the olfactory epithelium: age, timing, and reversibility

Like other biological systems, olfaction responds "homeostatically" to enduring change in the stimulus environment. This adaptive mechanism, referred to as compensatory plasticity, has been studied almost exclusively in developing animals. Thus it is unknown if this phenomenon is limited to ontogenesis and irreversible, characteristics common to some other forms of plasticity. Here we explore the effects of odor deprivation on the adult mouse olfactory epithelium (OE) using nasal plugs to eliminate nasal airflow unilaterally. Plugs were in place for 2-6 wk after which electroolfactograms (EOGs) were recorded from the occluded and open sides of the nasal cavity. Mean EOG amplitudes were significantly greater on the occluded than on the open side. The duration of plugging did not affect the results, suggesting that maximal compensation occurs within 2 wk or less. The magnitude of the EOG difference between the open and occluded side in plugged mice was comparable to adults that had undergone surgical naris occlusion as neonates. When plugs were removed after 4 wk followed by 2 wk of recovery, mean EOG amplitudes were not significantly different between the always-open and previously plugged sides of the nasal cavity suggesting that this form of plasticity is reversible. Taken together, these results suggest that compensatory plasticity is a constitutive mechanism of olfactory receptor neurons that allows these cells to recalibrate their stimulus-response relationship to fit the statistics of their current odor environment.

Developmental changes in NMDA receptor subunit composition at ON and OFF bipolar cell synapses onto direction-selective retinal ganglion cells

In the developing mouse retina, spontaneous and light-driven activity shapes bipolar→ganglion cell glutamatergic synapse formation, beginning around the time of eye-opening (P12-P14) and extending through the first postnatal month. During this time, glutamate release can spill outside the synaptic cleft and possibly stimulate extrasynaptic NMDA-type glutamate receptors (NMDARs) on ganglion cells. Furthermore, the role of NMDARs during development may differ between ON and OFF bipolar synapses as in mature retina, where ON synapses reportedly include extrasynaptic NMDARs with GluN2B subunits. To better understand the function of glutamatergic synapses during development, we made whole-cell recordings of NMDAR-mediated responses, in vitro, from two types of genetically identified direction-selective ganglion cells (dsGCs): TRHR (thyrotropin-releasing hormone receptor) and Drd4 (dopamine receptor 4). Both dsGC types responded to puffed NMDA between P7 and P28; and both types exhibited robust light-evoked NMDAR-mediated responses at P14 and P28 that were quantified by conductance analysis during nicotinic and GABA(A) receptor blockade. For a given cell type and at a given age, ON and OFF bipolar cell inputs evoked similar NMDAR-mediated responses, suggesting that ON-versus-OFF differences in mature retina do not apply to the cell types or ages studied here. At P14, puff- and light-evoked NMDAR-mediated responses in both dsGCs were partially blocked by the GluN2B antagonist ifenprodil, whereas at P28 only TRHR cells remained ifenprodil-sensitive. NMDARs contribute at both ON and OFF bipolar cell synapses during a period of robust activity-dependent synaptic development, with declining GluN2B involvement over time in specific ganglion cell types.

Sensory experience shapes the development of the visual system's first synapse

Specific connectivity patterns among neurons create the basic architecture underlying parallel processing in our nervous system. Here we focus on the visual system's first synapse to examine the structural and functional consequences of sensory deprivation on the establishment of parallel circuits. Dark rearing reduces synaptic strength between cones and cone bipolar cells, a previously unappreciated effect of sensory deprivation. In contrast, rod bipolar cells, which utilize the same glutamate receptor to contact rods, are unaffected by dark rearing. Underlying the physiological changes, we find the localization of metabotropic glutamate receptors within cone bipolar, but not rod bipolar, cell dendrites is a light-dependent process. Furthermore, although cone bipolar cells share common cone partners, each bipolar cell type that we examined depends differentially on sensory input to achieve mature connectivity. Thus, visual experience differentially affects maturation of rod versus cone pathways and of cell types within the cone pathway.

Dark rearing alters the normal development of spatiotemporal response properties but not of contrast detection threshold in mouse retinal ganglion cells

The mouse visual system is immature when the eyes open two weeks after birth. As in other mammals, some of the maturation that occurs in the subsequent weeks is known to depend on visual experience. Development of the retina, which as the first stage of vision provides the visual information to the brain, also depends on light-driven activity for proper development but has been less well studied than visual cortical development. The critical properties for retinal encoding of images include detection of contrast and responsiveness to the broad range of temporal stimulus frequencies present in natural stimuli. Here we show that contrast detection threshold and temporal frequency response characteristics of ON and OFF retinal ganglion cells (RGCs), which are poor at eye opening, subsequently undergo maturation, improving RGC performance. Further, we find that depriving mice of visual experience from before birth by rearing them in the dark causes ON and OFF RGCs to have smaller receptive field centers but does not affect their contrast detection threshold development. The modest developmental increase in temporal frequency responsiveness of RGCs in mice reared on a normal light cycle was inhibited by dark rearing only in ON but not OFF RGCs. Thus, these RGC response characteristics are in many ways unaffected by the experience-dependent changes to synaptic and spontaneous activity known to occur in the mouse retina in the two weeks after eye opening, but specific differences are apparent in the ON vs. OFF RGC populations.

Postsynaptic current bursts instruct action potential firing at a graded synapse

Nematode neurons generally produce graded potentials instead of action potentials (APs). It is unclear how the graded potentials control postsynaptic cells under physiological conditions. Here we show that postsynaptic currents (PSCs) frequently occur in bursts at the neuromuscular junction of C. elegans. Cholinergic bursts concur with facilitated AP firing, elevated cytosolic [Ca²⁺], and contraction of the muscle whereas GABA ergic bursts suppress AP firing. The bursts, distinct from artificially evoked responses, are characterized by a persistent current (the primary component of burst-associated charge transfer)and increased frequency and mean amplitude of PSC events. The persistent current of cholinergic PSC bursts is mostly mediated by levamisole-sensitive acetylcholine receptors, which correlates well with locomotory phenotypes of receptor mutants. Eliminating command interneurons abolishes the bursts whereas mutating SLO-1 K⁺ channel, a potent presynaptic inhibitor of exocytosis, greatly increases the mean burst duration. These observations suggest that motoneurons control muscle by producing PSC bursts.

Dopamine D1 receptors regulate the light dependent development of retinal synaptic responses

Retinal synaptic connections and function are developmentally regulated. Retinal synaptic activity plays critical roles in the development of retinal synaptic circuitry. Dopamine receptors have been thought to play important roles in the activity-dependent synaptic plasticity in central nervous system. The primary goal of this study is to determine whether dopamine D1 receptor regulates the activity-dependent development of retinal light responsiveness. Accordingly, we recorded electroretinogram from wild type mice and mice with genetic deletion of D1 dopamine receptor (D1-/- mice) raised under cyclic light conditions and constant darkness. Our results demonstrated that D1-/- mice have reduced amplitudes of all three major components of electroretinogram in adulthood. When the relative strength of the responses is considered, the D1-/- mice have selective reduction of the amplitudes of a-wave and oscillatory potentials evoked by low-intermediate intensities of lights. During postnatal development, D1-/- mice have increased amplitude of b-wave at the time of eye-opening but reduced developmental increase of the amplitude of b-wave after eye opening. Light deprivation from birth significantly reduced the amplitudes of b-wave and oscillatory potentials, increased the outer retinal light response gain and altered the light response kinetics of both a- and b-waves of wild type mice. In D1-/- mice, the effect of dark rearing on the amplitude of oscillatory potentials was diminished and dark rearing induced effects on the response gain of outer retina and the kinetics of a-wave were reversed. These results demonstrated roles of dopamine D1 receptor in the activity-dependent functional development of mouse retina.

Refinement but not maintenance of visual receptive fields is independent of visual experience

Visual deprivation is reported to prevent or delay the development of mature receptive field (RF) properties in primary visual cortex (V1) in several species. In contrast, visual deprivation neither prevents nor delays refinement of RF size in the superior colliculus (SC) of Syrian hamsters, although vision is required for RF maintenance in the SC. Here, we report that, contrary to expectation, visual cortical RF refinement occurs normally in dark-reared animals. As in the SC, a brief period of visual experience is required to maintain V1 RF refinement in adulthood. Whereas in the SC, 3 days of visual experience within a sensitive period (P37-40) was sufficient to protect RFs from deprivation-induced enlargement in adulthood, 7 days (P33-40) were required for RF size maintenance in V1. Thus, spontaneous activity is sufficient for RF refinement at these 2 levels of the visual pathway, and visual input is necessary only to prevent deprivation-induced RF enlargement in adulthood. These studies show that sensory experience during a late juvenile sensitive period protects the visual pathway against sensory deprivation in adulthood, and suggest that more importance may have been placed on the role of early visual experience in visual RF development than is warranted.

Distribution and development of P2Y1-purinoceptors in the mouse retina

There is increasing evidence that ATP acts on purinergic receptors and mediates synaptic transmission in the retina. In a previous study, we raised the possibility that P2X-purinoceptors, presumably P2X(2)-purinoceptors in OFF-cholinergic amacrine cells, play a key role in the formation of OFF pathway-specific modulation. In this study, we examined whether the P2Y(1)-purinoceptors can function in cholinergic amacrine cells in the mouse retina since cholinergic amacrine cells in the rat retina express P2Y(1)-purinoceptors. P2Y(1)-purinoceptors were shown to be expressed in dendrites of both ON- and OFF-cholinergic amacrine cells in adults. At postnatal day 7, there was immunoreactivity for P2Y(1)-purinoceptors in the soma of cholinergic amacrine cells. At postnatal day 14, weak immunoreactivity for P2Y(1)-purinoceptors was detected in the dendrites but not in the soma of cholinergic amacrine cells. At postnatal day 21, strong immunoreactivity for P2Y(1)-purinoceptors was detected in dendrites of cholinergic amacrine cells. The expression pattern of P2Y(1)-purinoceptors was not affected by visual experience. We concluded that P2Y(1)-purinoceptors are not involved in the OFF-pathway-specific signal transmission in cholinergic amacrine cells of the mouse retina.

Spatial relationships between GABAergic and glutamatergic synapses on the dendrites of distinct types of mouse retinal ganglion cells across development

Neuronal output requires a concerted balance between excitatory and inhibitory (I/E) input. Like other circuits, inhibitory synaptogenesis in the retina precedes excitatory synaptogenesis. How then do neurons attain their mature balance of I/E ratios despite temporal offset in synaptogenesis? To directly compare the development of glutamatergic and GABAergic synapses onto the same cell, we biolistically transfected retinal ganglion cells (RGCs) with PSD95CFP, a marker of glutamatergic postsynaptic sites, in transgenic Thy1­YFPγ2 mice in which GABA_A receptors are fluorescently tagged. We mapped YFPγ2 and PSD95CFP puncta distributions on three RGC types at postnatal day P12, shortly before eye opening, and at P21 when robust light responses in RGCs are present. The mature I_GABA/E ratios varied among ON-Sustained (S) A-type, OFF-S A-type, and bistratified direction selective (DS) RGCs. These ratios were attained at different rates, before eye-opening for ON-S and OFF-S A-type, and after eye-opening for DS RGCs. At both ages examined, the I_GABA/E ratio was uniform across the arbors of the three RGC types. Furthermore, measurements of the distances between neighboring PSD95CFP and YFPγ2 puncta on RGC dendrites indicate that their local relationship is established early in development, and cannot be predicted by random organization. These close spatial associations between glutamatergic and GABAergic postsynaptic sites appear to represent local synaptic arrangements revealed by correlative light and EM reconstructions of a single RGC's dendrites. Thus, although RGC types have different I_GABA/E ratios and establish these ratios at separate rates, the local relationship between excitatory and inhibitory inputs appear similarly constrained across the RGC types studied.

Vision drives correlated activity without patterned spontaneous activity in developing Xenopus retina

Developing amphibians need vision to avoid predators and locate food before visual system circuits fully mature. Xenopus tadpoles can respond to visual stimuli as soon as retinal ganglion cells (RGCs) innervate the brain, however, in mammals, chicks and turtles, RGCs reach their central targets many days, or even weeks, before their retinas are capable of vision. In the absence of vision, activity-dependent refinement in these amniote species is mediated by waves of spontaneous activity that periodically spread across the retina, correlating the firing of action potentials in neighboring RGCs. Theory suggests that retinorecipient neurons in the brain use patterned RGC activity to sharpen the retinotopy first established by genetic cues. We find that in both wild type and albino Xenopus tadpoles, RGCs are spontaneously active at all stages of tadpole development studied, but their population activity never coalesces into waves. Even at the earliest stages recorded, visual stimulation dominates over spontaneous activity and can generate patterns of RGC activity similar to the locally correlated spontaneous activity observed in amniotes. In addition, we show that blocking AMPA and NMDA type glutamate receptors significantly decreases spontaneous activity in young Xenopus retina, but that blocking GABA(A) receptor blockers does not. Our findings indicate that vision drives correlated activity required for topographic map formation. They further suggest that developing retinal circuits in the two major subdivisions of tetrapods, amphibians and amniotes, evolved different strategies to supply appropriately patterned RGC activity to drive visual circuit refinement.

Developmental mechanisms that regulate retinal ganglion cell dendritic morphology

One of the fundamental features of retinal ganglion cells (RGCs) is that dendrites of individual RGCs are confined to one or a few narrow strata within the inner plexiform layer (IPL), and each RGC synapses only with a small group of presynaptic bipolar and amacrine cells with axons/dendrites ramified in the same strata to process distinct visual features. The underlying mechanisms which control the development of this laminar-restricted distribution pattern of RGC dendrites have been extensively studied, and it is still an open question whether the dendritic pattern of RGCs is determined by molecular cues or by activity-dependent refinement. Accumulating evidence suggests that both molecular cues and activity-dependent refinement might regulate RGC dendrites in a cell subtype-specific manner. However, identification of morphological subtypes of RGCs before they have achieved their mature dendritic pattern is a major challenge in the study of RGC dendritic development. This problem is now being circumvented through the use of molecular markers in genetically engineered mouse lines to identify RGC subsets early during development. Another unanswered fundamental question in the study of activity-dependent refinement of RGC dendrites is how changes in synaptic activity lead to the changes in dendritic morphology. Recent studies have started to shed light on the molecular basis of activity-dependent dendritic refinement of RGCs by showing that some molecular cascades control the cytoskeleton reorganization of RGCs.

Receptive field center size decreases and firing properties mature in ON and OFF retinal ganglion cells after eye opening in the mouse

Development of the mammalian visual system is not complete at birth but continues postnatally well after eye opening. Although numerous studies have revealed changes in the development of the thalamus and visual cortex during this time, less is known about the development of response properties of retinal ganglion cells (RGCs). Here, we mapped functional receptive fields of mouse RGCs using a Gaussian white noise checkerboard stimulus and a multielectrode array to record from retinas at eye opening, 3 days later, and 4 wk after birth, when visual responses are essentially mature. Over this time, the receptive field center size of ON and OFF RGC populations decreased. The average receptive field center size of ON RGCs was larger than that of OFF RGCs at eye opening, but they decreased to the same size in the adult. Firing properties were also immature at eye opening. RGCs had longer latencies, lower frequencies of firing, and lower sensitivity than in the adult. Hence, the dramatic maturation of the visual system during the first weeks of visual experience includes the retina.

Developmental time course distinguishes changes in spontaneous and light-evoked retinal ganglion cell activity in rd1 and rd10 mice

In a subset of hereditary retinal diseases, early photoreceptor degeneration causes rapidly progressive blindness in children. To better understand how retinal development may interact with degenerative processes, we compared spontaneous and light-evoked activity among retinal ganglion cells in rd1 and rd10 mice, strains with closely related retinal disease. In each, a mutation in the Pde6b gene causes photoreceptor dysfunction and death, but in rd10 mice degeneration starts after a peak in developmental plasticity of retinal circuitry and thereafter progresses more slowly. In vitro multielectrode action potential recordings revealed that spontaneous waves of correlated ganglion cell activity comparable to those in wild-type mice were present in rd1 and rd10 retinas before eye opening [postnatal day (P) 7 to P8]. In both strains, spontaneous firing rates increased by P14-P15 and were many times higher by 4-6 wk of age. Among rd1 ganglion cells, all responses to light had disappeared by ~P28, yet in rd10 retinas vigorous ON and OFF responses were maintained well beyond this age and were not completely lost until after P60. This difference in developmental time course separates mechanisms underlying the hyperactivity from those that alter light-driven responses in rd10 retinas. Moreover, several broad physiological groups of cells remained identifiable according to response polarity and time course as late as P60. This raises hope that visual function might be preserved or restored despite ganglion cell hyperactivity seen in inherited retinal degenerations, particularly if treatment or manipulation of early developmental plasticity were to be timed appropriately.