Retinal Ganglion Cell Projections Research Articles

Leukocyte rolling and adhesion via ICAM-1 signals to endothelial permeability. Focus on “Leukocyte rolling and adhesion both contribute to regulation of microvascular permeability to albumin via ligation of ICAM-1”

THE VASCULAR ENDOTHELIUM plays a vital role in the inflammatory response by induction and surface expression of adhesion molecules and chemokines. At sites of acute inflammation, blood flow is increased and postcapillary venules exhibit increased permeability (leakiness) and support the influx of blood leukocytes, primarily neutrophils. The relationship between neutrophil influx and increased vessel permeability has been a topic of investigation for decades. Many investigations have reported a direct neutrophil-dependent increase in vessel permeability during inflammation using in vivo and in vitro models (reviewed in Ref. 4). One aspect that has received increased attention is whether endothelial cell adhesion molecules contribute to the regulation of vessel permeability, in addition to their well-characterized role in neutrophil rolling and adhesion. Intercellular adhesion molecule-1 (ICAM-1, also known as CD54) is an immunoglobulin superfamily member that is abundantly expressed on the endothelial cell surface and is enriched at endothelial cell borders in vivo and in vitro (2, 12, 14, 17). ICAM-1 surface expression is upregulated in endothelial cells by proinflammatory cytokines like TNF-, IL-1, IFN-, or bacterial endotoxins, and ICAM-1 serves as a receptor for leukocyte 2-integrins (LFA-1 and Mac-1) (reviewed in Ref. 15). In an experiment of nature, patients with leukocyte adhesion deficiency-1 (LAD-1) have a severe primary immune deficiency in which blood neutrophils fail to localize to sites of inflammation or injury. This defect occurs because patients’ neutrophils either lack 2-integrins or contain mutations in these molecules. Studies using neutrophils from LAD-1 patients or 2-integrin-deficient mice clearly demonstrate that these cells have normal selectin-dependent rolling on activated endothelium but fail to stably arrest and transmigrate. Additional studies have demonstrated that endothelial-expressed ICAM-1 contributes to both kinetics of leukocyte rolling and arrest in vivo as determined by intravital microscopy of the cremaster muscle microcirculation in ICAM-1 / mice (16). In this issue of American Journal of Physiology-Cell Physiology, Sumagin and colleagues (18) use elegant in vivo studies to demonstrate that microvascular permeability in the cremaster muscle model is regulated by leukocyte engagement of ICAM-1. The authors monitored permeability (Ps) in cremaster microcirculation by efflux of luminal fluorescent-tagged albumin by fluorescence confocal intravital microscopy. Prior studies by Sarelius and colleagues (19) in the same model demonstrated that ICAM-1 engagement induced a localized increase in vessel permeability in unstimulated arterioles and in TNF-stimulated venules. In agreement with this finding ,4ho f TNF- stimulation of arterioles and venules in ICAM-1 knockout (KO) animals did not result in increased vessel permeability as compared with wild-type (WT) mice under identical conditions. Similar results were seen with 2-integrin KO mice. In the current study, the authors provide further evidence linking ICAM-1 expression to regulation of endothelial permeability. Comparisons between unstimulated venules and TNF--treated venules reveal that neutrophil rolling regulates venule permeability in unstimulated venules while leukocyte adhesion regulates venule permeability in TNF--stimulated venules. The 2-integrin-blocking antibodies, which inhibit neutrophil arrest on endothelium, reduced the vessel permeability, and this was further reduced by treatment with a combination of 2-integrin and P-selectin blocking antibodies that inhibits both rolling and arrest. Interestingly, the reduction in venule permeability observed by blockade of P-selectin and 2-integrin function were suggested to be due to insufficient ICAM-1 engagement due to the absence of rolling leukocytes. To examine the role that neutrophils play, circulating neutrophils were depleted by antineutrophil GR-1 antibody treatment. Notably, depletion of neutrophils caused a marked decrease in

Genetic Dissection of EphA Receptor Signaling Dynamics during Retinotopic Mapping

Retinal ganglion cells (RGCs) project axons from their cell bodies in the eye to targets in the superior colliculus of the midbrain. The wiring of these axons to their synaptic targets creates an ordered representation, or "map," of retinal space within the brain. Many lines of experiments have demonstrated that the development of this map requires complementary gradients of EphA receptor tyrosine kinases and their ephrin-A ligands, yet basic features of EphA signaling during mapping remain to be resolved. These include the individual roles played by the multiple EphA receptors that make up the retinal EphA gradient. We have developed a set of ratiometric "relative signaling" (RS) rules that quantitatively predict how the composite low-nasal-to-high-temporal EphA gradient is translated into topographic order among RGCs. A key feature of these rules is that the component receptors of the gradient--in the mouse, EphA4, EphA5, and EphA6--must be functionally equivalent and interchangeable. To test this aspect of the model, we generated compound mutant mice in which the periodicity, slope, and receptor composition of the gradient are systematically altered with respect to the levels of EphA4, EphA5, and a closely related receptor, EphA3, that we ectopically express. Analysis of the retinotopic maps of these new mouse mutants establishes the general utility of the RS rules for predicting retinocollicular topography, and demonstrates that individual EphA gene products are approximately equivalent with respect to axon guidance and target selection.

VEGF Signaling through Neuropilin 1 Guides Commissural Axon Crossing at the Optic Chiasm

SummaryDuring development, the axons of retinal ganglion cell (RGC) neurons must decide whether to cross or avoid the midline at the optic chiasm to project to targets on both sides of the brain. By combining genetic analyses with in vitro assays, we show that neuropilin 1 (NRP1) promotes contralateral RGC projection in mammals. Unexpectedly, the NRP1 ligand involved is not an axon guidance cue of the class 3 semaphorin family, but VEGF164, the neuropilin-binding isoform of the classical vascular growth factor VEGF-A. VEGF164 is expressed at the chiasm midline and is required for normal contralateral growth in vivo. In outgrowth and growth cone turning assays, VEGF164 acts directly on NRP1-expressing contralateral RGCs to provide growth-promoting and chemoattractive signals. These findings have identified a permissive midline signal for axons at the chiasm midline and provide in vivo evidence that VEGF-A is an essential axon guidance cue.

An Instructive Role for Patterned Spontaneous Retinal Activity in Mouse Visual Map Development

Complex neural circuits in the mammalian brain develop through a combination of genetic instruction and activity-dependent refinement. The relative role of these factors and the form of neuronal activity responsible for circuit development is a matter of significant debate. In the mammalian visual system, retinal ganglion cell projections to the brain are mapped with respect to retinotopic location and eye of origin. We manipulated the pattern of spontaneous retinal waves present during development without changing overall activity levels through the transgenic expression of β2-nicotinic acetylcholine receptors in retinal ganglion cells of mice. We used this manipulation to demonstrate that spontaneous retinal activity is not just permissive, but instructive in the emergence of eye-specific segregation and retinotopic refinement in the mouse visual system. This suggests that specific patterns of spontaneous activity throughout the developing brain are essential in the emergence of specific and distinct patterns of neuronal connectivity.

Extracellular sodium modulates the excitability of cultured hippocampal pyramidal cells

Recent studies demonstrated a photophobia mechanism with modulation of nociceptive, cortico-thalamic neurons by retinal ganglion cell projections; however, little is known about how their neuronal homeostasis is disrupted. Since we have found that lumbar cerebrospinal fluid (CSF) sodium increases during migraine and that cranial sodium increases in a rat migraine model, the purpose of this study was to examine the effects of extracellular sodium ([Na +] o) on the intrinsic excitability of hippocampal pyramidal neurons. We monitored excitability by whole cell patch using a multiplex micropipette with a common outlet to change artificial CSF (ACSF) [Na +] o at cultured neurons accurately (SD < 7 mM) and rapidly (< 5 s) as determined by a sodium-selective micro-electrode of the same size and at the same location as a neuronal soma. Changing [Na +] o in ACSF from 100 to 160 mM, choline-balanced at 310–320 mOsm, increased the action potential (AP) amplitude, decreased AP width, and augmented firing rate by 28%. These effects were reversed on returning the ACSF [Na +] o to 100 mM. Testing up to 180 mM [Na +] o required ACSF with higher osmolarity (345–355 mOsm), at which the firing rate increased by 36% between 100 and 180 mM [Na +] o, with higher amplitude and narrower APs. In voltage clamp mode, the sodium and potassium currents increased significantly at higher [Na +] o. These results demonstrate that fluctuations in [Na +] o modulate neuronal excitability by a sodium current mechanism and that excessively altered neuronal excitability may contribute to hypersensitivity symptoms.

Foxn4 is required for retinal ganglion cell distal axon patterning

The regulation of retinal ganglion cell (RGC) axon growth and patterning in vivo is thought to be largely dependent on interactions with visual pathway and target cells. Here we address the hypothesis that amacrine cells, RGCs' presynaptic partners, regulate RGC axon growth or targeting. We asked whether amacrine cells play a role in RGC axon growth in vivo using Foxn4 −/− mice, which have fewer amacrine cells, but a normal complement of RGCs. We found that Foxn4 −/− mice have a similar reduction in most subtypes of amacrine cells examined. Remarkably, spontaneous retinal waves were not affected by the reduction of amacrine cells in the Foxn4 −/− mice. There was, however, a developmental delay in the distribution of RGC projections to the superior colliculus. Furthermore, RGC axons failed to penetrate into the retinorecipient layers in the Foxn4 −/− mice. Foxn4 is not expressed by RGCs and was not detectable in the superior colliculus itself. These findings suggest that amacrine cells are critical for proper RGC axon growth in vivo, and support the hypothesis that the amacrine cell–RGC interaction may contribute to the regulation of distal projections and axon patterning.

Phr1 is required for proper retinocollicular targeting of nasal–dorsal retinal ganglion cells

Precise targeting of retinal projections is required for the normal development of topographic maps in the mammalian primary visual system. During development, retinal axons project to and occupy topographically appropriate positions in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC). Phr1 retinal mutant mice, which display mislocalization of the ipsilateral retinogeniculate projection independent of activity and ephrin-A signaling, were found to have a more global disruption of topographic specificity of retinofugal inputs. The retinocollicular projection lacks local refinement of terminal zones and multiple ectopic termination zones originate from the dorsal-nasal (DN) retinal quadrant. Similarly, in the dLGN, the inputs originating from the contralateral DN retina are poorly refined in the Phr1 mutant. These results show that Phr1 is an essential regulator of retinal ganglion cell projection during both dLGN and SC topographic map development.

Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons

BackgroundBrimonidine is a common drug for lowering ocular pressure and may directly protect retinal ganglion cells in glaucoma. The disease involves early loss of retinal ganglion cell transport to brain targets followed by axonal and somatic degeneration. We examined whether brimonidine preserves ganglion cell axonal transport and abates degeneration in rats with elevated ocular pressure induced by laser cauterization of the episcleral veins.ResultsOcular pressure was elevated unilaterally by 90% for a period of 8 weeks post- cauterization. During this time, brimonidine (1mg/kg/day) or vehicle (phosphate-buffered saline) was delivered systemically and continuously via subcutaneous pump. Animals received bilateral intravitreal injections of fluorescent cholera toxin subunit β (CTB) two days before sacrifice to assess anterograde transport. In retinas from the vehicle group, elevated pressure induced a 44% decrease in the fraction of ganglion cells with intact uptake of CTB and a 14-42% reduction in the number of immuno-labelled ganglion cell bodies, with the worst loss occurring nasally. Elevated pressure also caused a 33% loss of ganglion cell axons in vehicle optic nerves and a 70% decrease in CTB transport to the superior colliculus. Each of these components of ganglion cell degeneration was either prevented or significantly reduced in the brimonidine treatment group.ConclusionsContinuous and systemic treatment with brimonidine by subcutaneous injection significantly improved retinal ganglion cell survival with exposure to elevated ocular pressure. This effect was most striking in the nasal region of the retina. Brimonidine treatment also preserved ganglion cell axon morphology, sampling density and total number in the optic nerve with elevated pressure. Consistent with improved outcome in the optic projection, brimonidine also significantly reduced the deficits in axonal transport to the superior colliculus associated with elevated ocular pressure. As transport deficits to and from retinal ganglion cell projection targets in the brain are relevant to the progression of glaucoma, the ability of brimonidine to preserve optic nerve axons and active transport suggests its neuroprotective effects are relevant not only at the cell body, but throughout the entire optic projection.

GPGPU implementation of a synaptically optimized, anatomically accurate spiking network simulator

Simulation of biological spiking networks is becoming more relevant in understanding neuronal processes. An increasing proportion of these simulations focuses on large scale modeling efforts. Unfortunately the size of large networks is often limited by both computational power and memory. Computational power constrains both the maximum number of differential equations and the maximum number of spikes that can be processed per unit time. Memory size limits the maximum number of neurons and synapses that can be simulated. To solve for the computational bottleneck, a neuronal simulator is implemented on a CUDA-based General Purpose Graphic Processing Unit (GPGPU). CUDA provides a C-like environment to harness the computational power of specialized video cards from NVIDIA (these cards provide a computational peak power on single precision floats of 1TFLOPS, at least an order of magnitude higher than the fastest CPU). To solve for the memory bottleneck, a just-in-time synapse storing algorithm is implemented requiring only 4 bytes per synapse. Only the synaptic weight is stored, while both post-synaptic contact and delay are recomputed at run-time. This allows a resource shift from memory to computation which fits with the peculiar GPGPU architecture, where an abundance of compute nodes access the memory via a bandwidth-limited bus. Neurons are represented by a single compartment whose activity is modeled by the Izhikevich formalism. Excitatory synapses are plastic and follow both a spike-timing-dependent plasticity rule and a short term potentiation/depression rule. We are able to simulate networks with up to a million neurons and up to 100 million synapses on a single GPGPU card. Networks of this size cannot be simulated on desktop computers. For smaller networks the speedup obtained is at least of an order of magnitude compared to traditional CPU platform. As an example of possible use of the present work we present preliminary results on the simulation of early stage of the visual pathway. In the retina, Retinal Ganglion Cells (RGC) project to the Lateral Geniculate Nucleus (LGN). LGN projects to the cortical area V1. V1 projects back to LGN. The network is represented by 100,000 neurons, 10 million synapses, and 32 different morphological classes with >350 topological projections. Input to the network is provided by current injection in the RGC layer. The RGC layer models midget and parasol ganglion cells (representing 80% of the RGC in primates). Each ganglion type is then subdivided into on- and off-center cells for a total of 4 different types of RGC. Training is performed via natural stimuli while testing is done with vertical and horizontal bars. The network average spiking frequency is within biological limits. Testing performed with both vertical and horizontal bars shows each pattern propagating along the network’s anatomical projections. At each stage the pattern is progressively elaborated and modified. In conclusion we present a novel simulator that is fast, synaptically optimized and anatomically accurate. At an additional cost to an available desktop PC of few hundred dollars we think the GPGPU is an ideal platform to simulate large spiking networks.

Nev ( cyfip2) is required for retinal lamination and axon guidance in the zebrafish retinotectal system

In the zebrafish retinotectal system, retinal ganglion cells (RGCs) project topographically along anterior–posterior (A-P) and dorsal–ventral (D-V) axes to innervate their primary target, the optic tectum. In the nevermind ( nev) mutant, D-V positional information is not maintained by dorsonasal retinal axons as they project through the optic tract to the tectum. Here we present a detailed phenotypic analysis of the retinotectal projection in nev and show that dorsonasal axons do eventually find their correct location on the tectum, albeit after taking a circuitous path. Interestingly, nev seems to be specifically required for retinal axons but not for several non-retinal axon tracts. In addition, we find that nev is required both cell autonomously and cell nonautonomously for proper lamination of the retina. We show that nev encodes Cyfip2 ( Cytoplasmic FMRP interacting protein 2) and is thus the first known mutation in a vertebrate Cyfip family member. Finally, we show that CYFIP2 acts cell autonomously in the D-V sorting of dorsonasal RGC axons in the optic tract. CYFIP2 is a highly conserved protein that lacks known domains or structural motifs but has been shown to interact with Rac and the fragile-X mental retardation protein, suggesting intriguing links to cytoskeletal dynamics and RNA regulation.

The influence of intrinsically-photosensitive retinal ganglion cells on the spectral sensitivity and response dynamics of the human pupillary light reflex

Historically, it was assumed that the light-evoked neural signals driving the human pupillary light reflex (PLR) originated exclusively from rod and cone photoreceptors. However, a novel melanopsin-containing photoreceptive cell class has recently been discovered in the mammalian retina. These intrinsically-photosensitive retinal ganglion cells (ipRGCs) project to the pretectum, the retinorecipient area of the brain responsible for the PLR. This study was therefore designed to examine the relative contribution of rod, cone and the melanopsin photoresponses of ipRGCs to the human PLR. We establish that the melanopsin photoresponse of ipRGCs contributes significantly to the maintenance of half maximal pupilloconstriction in response to light stimuli of 30 s or longer, even at low photopic irradiances. Furthermore, we show that the melanopsin photoresponse contributes significantly to three-quarter maximal pupilloconstriction in response to light stimuli as short as 2 s. We also demonstrate that cone photoresponses driving pupilloconstriction adapt considerably and contribute little after 30 s, but rod photoresponses adapt less and contribute significantly to the maintenance of pupilloconstriction in response to steady-state light stimuli at irradiance levels which are below the threshold of the melanopsin photoresponse.

Circadian Modulation of Melanopsin-Driven Light Response in Rat Ganglion-Cell Photoreceptors

Intrinsically photosensitive retinal ganglion cells (ipRGCs) project to the suprachiasmatic nucleus (SCN) and are essential for normal photic entrainment of global circadian rhythms in physiology and behavior. The effect of light on the central clock is dependent on circadian phase, and the retina itself contains intrinsic circadian oscillators that can alter its sensitivity to light. This raises the possibility that the ipRGCs, and hence the photoentraining signals in the retinohypothalamic tract, are subject to circadian modulation. Although the ipRGC photopigment melanopsin reportedly exhibits circadian variations in expression, there has been no direct test of the hypothesis that ipRGC sensitivity is under circadian control. Here, the authors provide such a test by measuring the sensitivity of intrinsic photoresponses of rat ipRGCs at 4 circadian times (CTs) using multielectrode array recording. There was little if any circadian modulation in the threshold of intrinsic ipRGC photoresponses. However, very bright light evoked significantly more spiking early in the subjective night (CT12-13) than at other circadian phases. Thus, the gain of the melanopsin-driven response is slightly increased in the early night, at roughly the circadian phase when melanopsin synthesis is thought to be elevated. However, this gain change is probably too modest to contribute much to shape the phase response curve (PRC) for behavioral photoentrainment.

Specificity and Sufficiency of EphB1 in Driving the Ipsilateral Retinal Projection

At the optic chiasm, retinal ganglion cell (RGC) axons make the decision to either avoid or traverse the midline, a maneuver that establishes the binocular pathways. In mice, the ipsilateral retinal projection arises from RGCs in the peripheral ventrotemporal (VT) crescent of the retina. These RGCs express the guidance receptor EphB1, which interacts with ephrin-B2 on radial glia cells at the optic chiasm to repulse VT axons away from the midline and into the ipsilateral optic tract. However, because VT RGCs express more than one EphB receptor, the sufficiency and specificity of the EphB1 receptor in directing the ipsilateral projection is unclear. In this study, we use in utero retinal electroporation to demonstrate that ectopic EphB1 expression can redirect RGCs with a normally crossed projection to an ipsilateral trajectory. Moreover, EphB1 is specifically required for rerouting RGC projections ipsilaterally, because introduction of the highly similar EphB2 receptor is much less efficient in redirecting RGC fibers, even when expressed at higher surface levels. Introduction of EphB1-EphB2 chimeric receptors into RGCs reveals that both extracellular and juxtamembrane domains of EphB1 are required to efficiently convert RGC projections ipsilaterally. Together, these data describe for the first time functional differences between two highly similar Eph receptors at a decision point in vivo, with EphB1 displaying unique properties that efficiently drives the uncrossed retinal projection.

Retinal Ganglion Cells Survive and Maintain Normal Dendritic Morphology in a Mouse Model of Inherited Photoreceptor Degeneration

Retinitis pigmentosa (RP), a family of inherited disorders characterized by progressive photoreceptor death, is a leading cause of blindness with no available cure. Despite the genetic heterogeneity underlying the disease, recent data on animal models show that the degeneration of photoreceptors triggers stereotyped remodeling among their postsynaptic partners. In particular, bipolar and horizontal cells might undergo dendritic atrophy and secondary death. The aim of this study was to investigate whether or not concomitant changes also occur in retinal ganglion cells (RGCs), the only retinal projection neurons to the brain and the proposed substrate for various therapeutic approaches for RP. We assessed the retention of morphology, overall architecture, and survival of RGCs in a mouse model of RP at various stages of the disease. To study the morphology of single RGCs, we generated a new mouse line by crossing Thy1-GFP-M mice (Feng et al., 2000), which express GFP (green fluorescent protein) in a small number of heterogeneous RGCs types, and rd10 mutants, a model of autosomal recessive RP, which exhibit a typical rod-cone degeneration (Chang et al., 2002). We show remarkable preservation of RGC structure, survival, and projections to higher visual centers in the time span from 3 to 9 months of life, well beyond the death of photoreceptors. Thus, unlike second-order neurons, RGCs appear as a considerably stable population of cells, potentially constituting a favorable substrate for restoring vision in RP individuals by means of electronic prostheses or direct expression of photosensitive proteins.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior–posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Functional Distribution in Area V1 Revealed by Spatially Uniform Stimuli

We measured responses to pure luminance changes in area V1 using functional magnetic resonance imaging. We fully dilated the subjects' pupils with a mydriatic agent; placed semitransparent, hemispheric covers over the subjects' eyes; and varied luminance levels of stimuli gradually or instantaneously. We found that the profile of cortical responses varied at about 10 degrees of eccentricity on the representation in area V1 despite the use of uniform achromatic stimuli. This variation was possibly caused by differences between response profiles in the central and peripheral zones of the retina. Our results might reflect the functional distribution of retinal ganglion cell projections.

Impact de la pathologie glaucomateuse sur les voies visuelles centrales et les structures circadiennes

Glaucoma is a chronic optic neuropathy leading to a degeneration of retinal ganglion cells. There is accumulating evidence that glaucomatous damage extends from retinal ganglion cells to vision centers in the brain. Degenerative changes are observed in magnocellular, parvocellular, and koniocellular pathways in the lateral geniculate nucleus, and these changes are related to intraocular pressure and the severity of optic nerve damage. In addition, recent studies show that there are also changes in the visual cortex in relation to varying degrees of retinal ganglion cell loss. In a rat model of glaucoma, we have recently demonstrated a reduction of retinal projections of retinal ganglion cells, not only on the visual system but also on the suprachiasmatic nucleus. Human studies suggest that the ganglion cell degeneration caused by glaucoma could lead to a lesion of the retinohypothalamic tract, which permits the synchronization of circadian rhythms.

Synaptic inputs to retinal ganglion cells that set the circadian clock

Melanopsin-containing retinal ganglion cells (RGCs) project to the suprachiasmatic nuclei (SCN) and mediate photoentrainment of the circadian system. Melanopsin is a novel retinal-based photopigment that renders these cells intrinsically photosensitive (ip). Although genetic ablation of melanopsin abolishes the intrinsic light response, it has a surprisingly minor effect on circadian photoentrainment. This and other non-visual responses to light are lost only when the melanopsin deficiency is coupled with mutations that disable classical rod and cone photoreceptors, suggesting that melanopsin-containing RGCs also receive rod- and cone-driven synaptic inputs. Using whole-cell patch-clamp recording, we demonstrate that light triggers synaptic currents in ipRGCs via activation of ionotropic glutamate and gamma-aminobutyric acid (GABA) receptors. Miniature postsynaptic currents (mPSCs) were clearly observed in ipRGCs, although they were less robust and were seen less frequently than those seen in non-ip cells. Pharmacological treatments revealed that the majority of ipRGCs receive excitatory glutamatergic inputs that were blocked by DNQX and/or kynurenic acid, as well as inhibitory GABAergic inputs that were blocked by bicuculline. Other ipRGCs received either glutamatergic or GABAergic inputs nearly exclusively. Although strychnine (Strych)-sensitive mPSCs were evident on many non-ipRGCs, indicating the presence of glycinergic inputs, we saw no evidence of Strych-sensitive events in ipRGCs. Based on these results, it is clear that SCN-projecting RGCs can respond to light both via an intrinsic melanopsin-based signaling cascade and via a synaptic pathway driven by classical rod and/or cone photoreceptors. It remains to be determined how the ipRGCs integrate these temporally distinct inputs to generate the signals that mediate circadian photoentrainment and other non-visual responses to light.

Neuronal Pentraxins Mediate Synaptic Refinement in the Developing Visual System

Neuronal pentraxins (NPs) define a family of proteins that are homologous to C-reactive and acute-phase proteins in the immune system and have been hypothesized to be involved in activity-dependent synaptic plasticity. To investigate the role of NPs in vivo, we generated mice that lack one, two, or all three NPs. NP1/2 knock-out mice exhibited defects in the segregation of eye-specific retinal ganglion cell (RGC) projections to the dorsal lateral geniculate nucleus, a process that involves activity-dependent synapse formation and elimination. Retinas from mice lacking NP1 and NP2 had cholinergically driven waves of activity that occurred at a frequency similar to that of wild-type mice, but several other parameters of retinal activity were altered. RGCs cultured from these mice exhibited a significant delay in functional maturation of glutamatergic synapses. Other developmental processes, such as pathfinding of RGCs at the optic chiasm and hippocampal long-term potentiation and long-term depression, appeared normal in NP-deficient mice. These data indicate that NPs are necessary for early synaptic refinements in the mammalian retina and dorsal lateral geniculate nucleus. We speculate that NPs exert their effects through mechanisms that parallel the known role of short pentraxins outside the CNS.

Temporal regulation of EphA4 in astroglia during murine retinal and optic nerve development

Eph receptors and their ephrin ligands play important roles in many aspects of visual system development. In this study, we characterized the spatial and temporal expression pattern of EphA4 in astrocyte precursor cell (APC) and astrocyte populations in the murine retina and optic nerve. EphA4 is expressed by immotile optic disc astrocyte precursor cells (ODAPS), but EphA4 is downregulated as these cells migrate into the retina. Surprisingly, mature astrocytes in the adult retina re-express EphA4. Within the optic nerve, EphA4 is expressed in specialized astrocytes that form a meshwork at the optic nerve head (ONH). Our in vitro and in vivo data indicate that EphA4 is dispensable for retinal ganglion cell (RGC) axon growth and projections through the chiasm. While optic stalk structure, APC proliferation and migration, retinal vascularization, and oligodendrocyte migration appear normal in EphA4 mutants, the expression of EphA4 in APCs and in the astrocyte meshwork at the ONH has implications for optic nerve pathologies.