Rod vision: pathways and processing in the mammalian retina

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Birth of cone bipolar cells, but not rod bipolar cells, is associated with existing RGCs

Retinal ganglion cells (RGCs) play important roles in retinogenesis. They are required for normal retinal histogenesis and retinal cell number balance. Developmental RGC loss is typically characterized by initial retinal neuronal number imbalance and subsequent loss of retinal neurons. However, it is not clear whether loss of a specific non-RGC cell type in the RGC-depleted retina is due to reduced cell production or subsequent degeneration. Taking advantage of three knockout mice with varying degrees of RGC depletion, we re-examined bipolar cell production in these retinas from various aspects. Results show that generation of the cone bipolar cells is correlated with the existing number of RGCs. However, generation of the rod bipolar cells is unaffected by RGC shortage. Results report the first observation that RGCs selectively influence the genesis of subsequent retinal cell types.

Reproducing retinal rod bipolar cell light response by mathematical model including neurotransmitter receptors

Detailed mathematical model of retinal cells is useful for the quantitative understanding of the subcellular processes of the visual system. Retinal bipolar cells receive information from photoreceptor cells, horizontal cells and amacrine cells, thus it can be considered as information integration system. Despite its importance, bipolar cell model including inputs and outputs has not been proposed. In this paper, we propose a rod bipolar cell model which can reproduce voltage response of light. The model includes TRPM1 channel which receives signal from photoreceptor cells, GABA channel which receives signal from surrounding amacrine cells, and cell body model which is based on the model proposed by Ishihara et al. The model was evaluated with several light signals, where experimentally obtained photoreceptor cell responses were used as TRPM1 channel input. Resulting bipolar cell membrane potential showed good agreement with the reported experimental results.

Processing of S-cone signals in the inner plexiform layer of the mammalian retina

Color information is encoded by two parallel pathways in the mammalian retina. One pathway compares signals from long- and middle-wavelength sensitive cones and generates red-green opponency. The other compares signals from short- and middle-/long-wavelength sensitive cones and generates blue-green (yellow) opponency. Whereas both pathways operate in trichromatic primates (including humans), the fundamental, phylogenetically ancient color mechanism shared among most mammals is blue-green opponency. In this review, we summarize the current understanding of how signals from short-wavelength sensitive cones are processed in the primate and nonprimate mammalian retina, with a focus on the inner plexiform layer where bipolar, amacrine, and ganglion cell processes interact to facilitate the generation of blue-green opponency.

Presynaptic Localization and Possible Function of Calcium-Activated Chloride Channel Anoctamin 1 in the Mammalian Retina

Calcium (Ca(2+))-activated chloride (Cl(-)) channels (CaCCs) play a role in the modulation of action potentials and synaptic responses in the somatodendritic regions of central neurons. In the vertebrate retina, large Ca(2+)-activated Cl(-) currents (ICl(Ca)) regulate synaptic transmission at photoreceptor terminals; however, the molecular identity of CaCCs that mediate ICl(Ca) remains unclear. The transmembrane protein, TMEM16A, also called anoctamin 1 (ANO1), has been recently validated as a CaCC and is widely expressed in various secretory epithelia and nervous tissues. Despite the fact that tmem16a was first cloned in the retina, there is little information on its cellular localization and function in the mammalian retina. In this study, we found that ANO1 was abundantly expressed as puncta in 2 synaptic layers. More specifically, ANO1 immunoreactivity was observed in the presynaptic terminals of various retinal neurons, including photoreceptors. ICl(Ca) was first detected in dissociated rod bipolar cells expressing ANO1. ICl(Ca) was abolished by treatment with the Ca(2+) channel blocker Co(2+), the L-type Ca(2+) channel blocker nifedipine, and the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and niflumic acid (NFA). More specifically, a recently discovered ANO1-selective inhibitor, T16Ainh-A01, and a neutralizing antibody against ANO1 inhibited ICl(Ca) in rod bipolar cells. Under a current-clamping mode, the suppression of ICl(Ca) by using NPPB and T16Ainh-A01 caused a prolonged Ca(2+) spike-like depolarization evoked by current injection in dissociated rod bipolar cells. These results suggest that ANO1 confers ICl(Ca) in retinal neurons and acts as an intrinsic regulator of the presynaptic membrane potential during synaptic transmission.

Mouse ganglion-cell photoreceptors are driven by the most sensitive rod pathway and by both types of cones

Intrinsically photosensitive retinal ganglion cells (iprgcs) are depolarized by light by two mechanisms: directly, through activation of their photopigment melanopsin; and indirectly through synaptic circuits driven by rods and cones. To learn more about the rod and cone circuits driving ipRGCs, we made multielectrode array (MEA) and patch-clamp recordings in wildtype and genetically modified mice. Rod-driven ON inputs to ipRGCs proved to be as sensitive as any reaching the conventional ganglion cells. These signals presumably pass in part through the primary rod pathway, involving rod bipolar cells and AII amacrine cells coupled to ON cone bipolar cells through gap junctions. Consistent with this interpretation, the sensitive rod ON input to ipRGCs was eliminated by pharmacological or genetic disruption of gap junctions, as previously reported for conventional ganglion cells. A presumptive cone input was also detectable as a brisk, synaptically mediated ON response that persisted after disruption of rod ON pathways. This was roughly three log units less sensitive than the rod input. Spectral analysis revealed that both types of cones, the M- and S-cones, contribute to this response and that both cone types drive ON responses. This contrasts with the blue-OFF, yellow-ON chromatic opponency reported in primate ipRGCs. The cone-mediated response was surprisingly persistent during steady illumination, echoing the tonic nature of both the rod input to ipRGCs and their intrinsic, melanopsin-based phototransduction. These synaptic inputs greatly expand the dynamic range and spectral bandpass of the non-image-forming visual functions for which ipRGCs provide the principal retinal input.

Developmental regulation and activity-dependent maintenance of GABAergic presynaptic inhibition onto rod bipolar cell axonal terminals

Presynaptic inhibition onto axons regulates neuronal output, but how such inhibitory synapses develop and are maintained in vivo remains unclear. Axon terminals of glutamatergic retinal rod bipolar cells (RBCs) receive GABAA and GABAC receptor-mediated synaptic inhibition. We found that perturbing GABAergic or glutamatergic neurotransmission does not prevent GABAergic synaptogenesis onto RBC axons. But, GABA release is necessary for maintaining axonal GABA receptors. This activity-dependent process is receptor subtype specific: GABAC receptors are maintained, whereas GABAA receptors containing α1, but not α3, subunits decrease over time in mice with deficient GABA synthesis. GABAA receptor distribution on RBC axons is unaffected in GABAC receptor knockout mice. Thus, GABAA and GABAC receptor maintenance are regulated separately. Although immature RBCs elevate their glutamate release when GABA synthesis is impaired, homeostatic mechanisms ensure that the RBC output operates within its normal range after eye opening, perhaps to regain proper visual processing within the scotopic pathway.

Controlling gain one photon at a time

Adaptation is a salient property of sensory processing. All adaptational or gain control mechanisms face the challenge of obtaining a reliable estimate of the property of the input to be adapted to and obtaining this estimate sufficiently rapidly to be useful. Here, we explore how the primate retina balances the need to change gain rapidly and reliably when photons arrive rarely at individual rod photoreceptors. We find that the weakest backgrounds that decrease the gain of the retinal output signals are similar to those that increase human behavioral threshold, and identify a novel site of gain control in the retinal circuitry. Thus, surprisingly, the gain of retinal signals begins to decrease essentially as soon as background lights are detectable; under these conditions, gain control does not rely on a highly averaged estimate of the photon count, but instead signals from individual photon absorptions trigger changes in gain.

DOI:http://dx.doi.org/10.7554/eLife.00467.001

To process the sights and sounds around us, our senses must be attuned to a huge range of signals: from barely audible whispers to deafening rock concerts, and from dim glimmers of light to bright spotlights. Sensory neurons face the challenge of encoding this huge range of inputs within their much more restricted response range. Thus, neurons in our eyes and ears must continually adjust their gain or sensitivity to match changes in the light and sound inputs. These gain control processes must operate rapidly to keep up with the ever-changing input signals, but must also operate accurately so as not to distort the inputs.

The trade-off between rapid and accurate gain control can be illustrated by considering how the retina processes information at low light levels. There are two main types of light-sensitive cells in the retina: rods and cones. Vision at night relies on the ability of the rods to detect single photons—the smallest unit of light. In starlight, an individual rod will register photons only rarely, and most of the time, the majority of the rods will not register any photons. Neurons in the retinal circuits that read out the rod signals receive input from hundreds or thousands of rods, and those rod inputs are highly amplified to allow detection of the responses produced when a tiny fraction of the rods absorbs a photon. But this amplification is dangerous, as it could easily saturate retinal signals when light levels increase. Gain control mechanisms are needed to avoid such saturation.

Schwartz and Rieke now add to our understanding of this process by examining how the retinas of non-human primates behave in low light. They reveal that levels of background light that can only just be detected behaviorally trigger retinal gain controls; these gain controls operate when less than 1% of rods absorb a photon. Under these conditions, the physics of light itself will cause considerable variability in the stream of photons arriving at the retina, leading to high variability in the gain of retinal responses. Nonetheless, changes in gain occurred rapidly following changes in background, indicating that the underlying mechanisms spend little time averaging incident photons. Taken together, these findings will require revisiting our ideas about how adaptational mechanisms balance the competing demands of speed and reliability to help us see the world around us.

DOI:http://dx.doi.org/10.7554/eLife.00467.002

Inhibitory inputs tune the light response properties of dopaminergic amacrine cells in mouse retina

Dopamine (DA) is a neuromodulator that in the retina adjusts the circuitry for visual processing in dim and bright light conditions. It is synthesized and released from retinal interneurons called dopaminergic amacrine cells (DACs), whose basic physiology is not yet been fully characterized. To investigate their cellular and input properties as well as light responses, DACs were targeted for whole cell recording in isolated retina using two-photon fluorescence microscopy in a mouse line where the dopamine receptor 2 promoter drives green fluorescent protein (GFP) expression. Differences in membrane properties gave rise to cell-to-cell variation in the pattern of resting spontaneous spike activity ranging from silent to rhythmic to periodic burst discharge. All recorded DACs were light sensitive and generated responses that varied with intensity. The threshold response to light onset was a hyperpolarizing potential change initiated by rod photoreceptors that was blocked by strychnine, indicating a glycinergic amacrine input onto DACs at light onset. With increasing light intensity, the ON response acquired an excitatory component that grew to dominate the response to the strongest stimuli. Responses to bright light (photopic) stimuli also included an inhibitory OFF response mediated by GABAergic amacrine cells driven by the cone OFF pathway. DACs expressed GABA (GABA(A)α1 and GABA(A)α3) and glycine (α2) receptor clusters on soma, axon, and dendrites consistent with the light response being shaped by dual inhibitory inputs that may serve to tune spike discharge for optimal DA release.

Ambient illumination toggles a neuronal circuit switch in the retina and visual perception at cone threshold

VIDEO ABSTRACT

Gradual changes in the sensory environment can lead to abrupt changes in brain computations and perception. However, mechanistic understanding of the mediating microcircuits is missing. By sliding through light levels from starlight to daylight, we identify retinal ganglion cell types in the mouse that abruptly and reversibly switch the weighting of center and surround interactions in their receptive field around cone threshold. Two-photon-targeted recordings and genetic and viral tracing experiments revealed that the circuit element responsible for the switch is a large inhibitory neuron that provides direct inhibition to ganglion cells. Our experiments suggest that weak excitatory input via electrical synapses together with the spiking threshold in inhibitory cells act as a switch. We also reveal a switch-like component in the spatial integration properties of human vision at cone threshold. This work demonstrates that circuits in the retina can quickly and reversibly switch between two distinct states, implementing distinct perceptual regimes at different light levels.

Inhibition to retinal rod bipolar cells is regulated by light levels

The retina responds to a wide range of light stimuli by adaptation of retinal signaling to background light intensity and the use of two different photoreceptors: rods that sense dim light and cones that sense bright light. Rods signal to rod bipolar cells that receive significant inhibition from amacrine cells in the dark, especially from a rod bipolar cell-activated GABAergic amacrine cell. This inhibition modulates the output of rod bipolar cells onto downstream neurons. However, it was not clear how the inhibition of rod bipolar cells changes when rod signaling is limited by an adapting background light and cone signaling becomes dominant. We found that both light-evoked and spontaneous rod bipolar cell inhibition significantly decrease with light adaptation. This suggests a global decrease in the activity of amacrine cells that provide input to rod bipolar cells with light adaptation. However, inhibition to rod bipolar cells is also limited by GABAergic connections between amacrine cells, which decrease GABAergic input to rod bipolar cells. When we removed this serial inhibition, the light-evoked inhibition to rod bipolar cells remained after light adaptation. These results suggest that decreased inhibition to rod bipolar cells after light adaptation is due to decreased rod pathway activity as well as an active increase in inhibition between amacrine cells. Together these serve to limit rod bipolar cell inhibition after light adaptation, when the rod pathway is inactive and modulation of the signal is not required. This suggests an efficiency mechanism in the retina to limit unnecessary signaling.

The rod pathway of the microbat retina has bistratified rod bipolar cells and tristratified AII amacrine cells

We studied the retinal rod pathway of Carollia perspicillata and Glossophaga soricina, frugivorous microbats of the phyllostomid family. Protein kinase Cα (PKCα) immunolabeling revealed abundant rod bipolar cells (RBCs) with axon terminals in the innermost sublamina of the inner plexiform layer (IPL), which is typical for mammals. Extraordinarily, the RBC axons showed additional synaptic contacts in a second sublamina further out in the IPL. Dye injections of PKCα-prelabeled RBCs of C. perspicillata confirmed the bistratified axon morphology. The functional partition of the IPL into ON and OFF sublayers was shown by using antibodies against vesicular glutamate transporter 1 [labeling all ON and OFF bipolar cell (BC) axon terminals] and G-protein γ13 (labeling all ON BCs). The ON sublayer occupied 75% of the IPL thickness, including both strata of the RBC axons. RBC output onto putative AII amacrine cells (ACs), the crucial interneurons of the rod pathway, was identified by calretinin, PKCα, and CtBP2 triple immunolabeling. Dye injections of calretinin-prelabeled ACs revealed tristratification of the AII ACs corresponding to the bistratified RBCs. Triple immunolabeling for PKCα, nitric oxide synthetase (NOS), and either GABA(C) or CtBP2 indicated GABAergic feedback onto RBCs via NOS-immunoreactive ACs. AII output analysis showed glycineric synapses with glycine receptor α1 expression between AII cells and OFF cone BCs and connexin 36-labeled gap junctions between AII cells and ON cone BCs. We conclude that microbats have a well developed rod pathway with great similarities to that of other mammals, but with an unusual IPL stratification pattern of RBCs and AIIs.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Electrical synapses between AII amacrine cells in the retina: Function and modulation

Adaptation enables the visual system to operate across a large range of background light intensities. There is evidence that one component of this adaptation is mediated by modulation of gap junctions functioning as electrical synapses, thereby tuning and functionally optimizing specific retinal microcircuits and pathways. The AII amacrine cell is an interneuron found in most mammalian retinas and plays a crucial role for processing visual signals in starlight, twilight and daylight. AII amacrine cells are connected to each other by gap junctions, potentially serving as a substrate for signal averaging and noise reduction, and there is evidence that the strength of electrical coupling is modulated by the level of background light. Whereas there is extensive knowledge concerning the retinal microcircuits that involve the AII amacrine cell, it is less clear which signaling pathways and intracellular transduction mechanisms are involved in modulating the junctional conductance between electrically coupled AII amacrine cells. Here we review the current state of knowledge, with a focus on the recent evidence that suggests that the modulatory control involves activity-dependent changes in the phosphorylation of the gap junction channels between AII amacrine cells, potentially linked to their intracellular Ca(2+) dynamics. This article is part of a Special Issue entitled Electrical Synapses.

Intrinsic properties and functional circuitry of the AII amacrine cell

Amacrine cells represent the most diverse class of retinal neuron, comprising dozens of distinct cell types. Each type exhibits a unique morphology and generates specific visual computations through its synapses with a subset of excitatory interneurons (bipolar cells), other amacrine cells, and output neurons (ganglion cells). Here, we review the intrinsic and network properties that underlie the function of the most common amacrine cell in the mammalian retina, the AII amacrine cell. The AII connects rod and cone photoreceptor pathways, forming an essential link in the circuit for rod-mediated (scotopic) vision. As such, the AII has become known as the rod-amacrine cell. We, however, now understand that AII function extends to cone-mediated (photopic) vision, and AII function in scotopic and photopic conditions utilizes the same underlying circuit: AIIs are electrically coupled to each other and to the terminals of some types of ON cone bipolar cells. The direction of signal flow, however, varies with illumination. Under photopic conditions, the AII network constitutes a crossover inhibition pathway that allows ON signals to inhibit OFF ganglion cells and contributes to motion sensitivity in certain ganglion cell types. We discuss how the AII's combination of intrinsic and network properties accounts for its unique role in visual processing.

Multiple episodes of convergence in genes of the dim light vision pathway in bats

The molecular basis of the evolution of phenotypic characters is very complex and is poorly understood with few examples documenting the roles of multiple genes. Considering that a single gene cannot fully explain the convergence of phenotypic characters, we choose to study the convergent evolution of rod vision in two divergent bats from a network perspective. The Old World fruit bats (Pteropodidae) are non-echolocating and have binocular vision, whereas the sheath-tailed bats (Emballonuridae) are echolocating and have monocular vision; however, they both have relatively large eyes and rely more on rod vision to find food and navigate in the night. We found that the genes CRX, which plays an essential role in the differentiation of photoreceptor cells, SAG, which is involved in the desensitization of the photoactivated transduction cascade, and the photoreceptor gene RH, which is directly responsible for the perception of dim light, have undergone parallel sequence evolution in two divergent lineages of bats with larger eyes (Pteropodidae and Emballonuroidea). The multiple convergent events in the network of genes essential for rod vision is a rare phenomenon that illustrates the importance of investigating pathways and networks in the evolution of the molecular basis of phenotypic convergence.

Immunohistochemical identification and synaptic inputs to the diffuse bipolar cell type DB1 in macaque retina

Detailed analysis of the synaptic inputs to the primate DB1 bipolar cell has been precluded by the absence of a suitable immunohistochemical marker. Here we demonstrate that antibodies for the EF-hand calcium-binding protein, secretagogin, strongly label the DB1 bipolar cell as well as a mixed population of GABAergic amacrine cells in the macaque retina. Using secretagogin as a marker, we show that the DB1 bipolar makes synaptic contact with both L/M as well as S-cone photoreceptors and only minimal contact with rod photoreceptors. Electron microscopy showed that the DB1 bipolar makes flat contacts at both triad-associated and nontriad-associated positions on the cone pedicle. Double labeling with various glutamate receptor subunit antibodies failed to conclusively determine the subunit composition of the glutamate receptors on DB1 bipolar cells. In the IPL, DB1 bipolar cell axon terminals expressed the glycine receptor, GlyRα1, at sites of contact with AII amacrine cells, suggesting that these cells receive input from the rod pathway.

CNGA3 deficiency affects cone synaptic terminal structure and function and leads to secondary rod dysfunction and degeneration

PURPOSE

To investigate rod function and survival after cone dysfunction and degeneration in a mouse model of cone cyclic nucleotide-gated (CNG) channel deficiency.

METHODS

Rod function and survival in mice with cone CNG channel subunit CNGA3 deficiency (CNGA3-/- mice) were evaluated by electroretinographic (ERG), morphometric, and Western blot analyses. The arrangement, integrity, and ultrastructure of photoreceptor terminals were investigated by immunohistochemistry and electron microscopy.

RESULTS

The authors found loss of cone function and cone death accompanied by impairment of rods and rod-driven signaling in CNGA3-/- mice. Scotopic ERG b-wave amplitudes were reduced by 15% at 1 month, 30% at 6 months, and 40% at 9 months and older, while scotopic a-wave amplitudes were decreased by 20% at 9 months, compared with ERGs of age-matched wild-type mice. Outer nuclear layer thickness in CNGA3-/- retina was reduced by 15% at 12 months compared with age-matched wild-type controls. This was accompanied by a 30%-40% reduction in expression of rod-specific proteins, including rhodopsin, rod transducin α-subunit, and glutamic acid-rich protein (GARP). Cone terminals in the CNGA3-/- retina showed a progressive loss of neurochemical and ultrastructural integrity. Abnormalities were observed as early as 1 month. Disorganized rod terminal ultrastructure was noted by 12 months.

CONCLUSIONS

These findings demonstrate secondary rod impairment and degeneration after cone degeneration in mice with cone CNG channel deficiency. Loss of cone phototransduction accompanies the compromised integrity of cone terminals. With time, rod synaptic structure, function, and viability also become compromised.

Dark-adapted response threshold of OFF ganglion cells is not set by OFF bipolar cells in the mouse retina

The nervous system frequently integrates parallel streams of information to encode a broad range of stimulus strengths. In mammalian retina it is generally believed that signals generated by rod and cone photoreceptors converge onto cone bipolar cells prior to reaching the retinal output, the ganglion cells. Near absolute visual threshold a specialized mammalian retinal circuit, the rod bipolar pathway, pools signals from many rods and converges on depolarizing (AII) amacrine cells. However, whether subsequent signal flow to OFF ganglion cells requires OFF cone bipolar cells near visual threshold remains unclear. Glycinergic synapses between AII amacrine cells and OFF cone bipolar cells are believed to relay subsequently rod-driven signals to OFF ganglion cells. However, AII amacrine cells also make glycinergic synapses directly with OFF ganglion cells. To determine the route for signal flow near visual threshold, we measured the effect of the glycine receptor antagonist strychnine on response threshold in fully dark-adapted retinal cells. As shown previously, we found that response threshold for OFF ganglion cells was elevated by strychnine. Surprisingly, strychnine did not elevate response threshold in any subclass of OFF cone bipolar cell. Instead, in every OFF cone bipolar subclass strychnine suppressed tonic glycinergic inhibition without altering response threshold. Consistent with this lack of influence of strychnine, we found that the dominant input to OFF cone bipolar cells in darkness was excitatory and the response threshold of the excitatory input varied by subclass. Thus, in the dark-adapted mouse retina, the high absolute sensitivity of OFF ganglion cells cannot be explained by signal transmission through OFF cone bipolar cells.

Lateral suppression of mesopic rod and cone flicker detection

This study investigated the mechanisms of flicker detection suppression by measuring mesopic rod and cone critical flicker frequencies (CFFs) at different center and surround illuminance levels. Stimuli were generated with a four-primary photostimulator that provided independent control of rod and cone excitations. The results showed that dim surrounds ≤0.2 Td suppressed cone-mediated CFFs at ≥20 Td but not rod-mediated CFFs. These results can be understood in terms of peak amplitudes of photoreceptor impulse response functions under different stimulation conditions.

Light-evoked lateral GABAergic inhibition at single bipolar cell synaptic terminals is driven by distinct retinal microcircuits

Inhibitory amacrine cells (ACs) filter visual signals crossing the retina by modulating the excitatory, glutamatergic output of bipolar cells (BCs) on multiple temporal and spatial scales. Reciprocal feedback from ACs provides focal inhibition that is temporally locked to the activity of presynaptic BC activity, whereas lateral feedback originates from ACs excited by distant BCs. These distinct feedback mechanisms permit temporal and spatial computation at BC terminals. Here, we used a unique preparation to study light-evoked IPSCs recorded from axotomized terminals of ON-type mixed rod/cone BCs (Mb) in goldfish retinal slices. In this preparation, light-evoked IPSCs could only reach axotomized BC terminals via the lateral feedback pathway, allowing us to study lateral feedback in the absence of overlapping reciprocal feedback components. We found that light evokes ON and OFF lateral IPSCs (L-IPSCs) in Mb terminals having different temporal patterns and conveyed via distinct retinal pathways. The relative contribution of rods versus cones to ON and OFF L-IPSCs was light intensity dependent. ACs presynaptic to Mb BC terminals received inputs via AMPA/KA- and NMDA-type receptors in both the ON and OFF pathways, and used TTX-sensitive sodium channels to boost signal transfer along their processes. ON and OFF L-IPSCs, like reciprocal feedback IPSCs, were mediated by both GABA(A) and GABA(C) receptors. However, our results suggest that lateral and reciprocal feedback do not cross-depress each other, and are therefore mediated by distinct populations of ACs. These findings demonstrate that retinal inhibitory circuits are highly specialized to modulate BC output at different light intensities.

Action potential generation at an axon initial segment-like process in the axonless retinal AII amacrine cell

In axon-bearing neurons, action potentials conventionally initiate at the axon initial segment (AIS) and are important for neuron excitability and cell-to-cell communication. However in axonless neurons, spike origin has remained unclear. Here we report in the axonless, spiking AII amacrine cell of the mouse retina a dendritic process sharing organizational and functional similarities with the AIS. This process was revealed through viral-mediated expression of channelrhodopsin-2-GFP with the AIS-targeting motif of sodium channels (Na(v)II-III). The AII processes showed clustering of voltage-gated Na+ channel 1.1 (Na(v)1.1) as well as AIS markers ankyrin-G and neurofascin. Furthermore, Na(v)II-III targeting disrupted Na(v)1.1 clustering in the AII process, which drastically decreased Na+ current and abolished the ability of the AII amacrine cell to generate spiking. Our findings indicate that, despite lacking an axon, spiking in the axonless neuron can originate at a specialized AIS-like process.

ApoER2 function in the establishment and maintenance of retinal synaptic connectivity

The cellular and molecular mechanisms responsible for the development of inner retinal circuitry are poorly understood. Reelin and apolipoprotein E (apoE), ligands of apoE receptor 2 (ApoER2), are involved in retinal development and degeneration, respectively. Here we describe the function of ApoER2 in the developing and adult retina. ApoER2 expression was highest during postnatal inner retinal synaptic development and was considerably lower in the mature retina. Both during development and in the adult, ApoER2 was expressed by A-II amacrine cells. ApoER2 knock-out (KO) mice had rod bipolar morphogenic defects, altered A-II amacrine dendritic development, and impaired rod-driven retinal responses. The presence of an intact ApoER2 NPxY motif, necessary for binding Disabled-1 and transducing the Reelin signal, was also necessary for development of the rod bipolar pathway, while the alternatively spliced exon 19 was not. Mice deficient in another Reelin receptor, very low-density lipoprotein receptor (VLDLR), had normal rod bipolar morphology but altered A-II amacrine dendritic development. VLDLR KO mice also had reductions in oscillatory potentials and delayed synaptic response intervals. Interestingly, age-related reductions in rod and cone function were observed in both ApoER2 and VLDLR KOs. These results support a pivotal role for ApoER2 in the establishment and maintenance of normal retinal synaptic connectivity.

Electrical synaptic input to ganglion cells underlies differences in the output and absolute sensitivity of parallel retinal circuits

Parallel circuits throughout the CNS exhibit distinct sensitivities and responses to sensory stimuli. Ambiguities in the source and properties of signals elicited by physiological stimuli, however, frequently obscure the mechanisms underlying these distinctions. We found that differences in the degree to which activity in two classes of Off retinal ganglion cell (RGC) encode information about light stimuli near detection threshold were not due to obvious differences in the cells' intrinsic properties or the chemical synaptic input the cells received; indeed, differences in the cells' light responses were largely insensitive to block of fast ionotropic glutamate receptors. Instead, the distinct responses of the two types of RGCs likely reflect differences in light-evoked electrical synaptic input. These results highlight a surprising strategy by which the retina differentially processes and routes visual information and provide new insight into the circuits that underlie responses to stimuli near detection threshold.

An extended 15 Hz ERG protocol (2): data of normal subjects and patients with achromatopsia, CSNB1, and CSNB2

The amplitude versus flash strength curve of 15 Hz electroretinograms (ERGs) shows two minima. The minima are caused by interactions between the primary and the secondary rod pathways (first minimum), and the secondary rod pathway and the cone-driven pathway (second minimum). Furthermore, cone pathway contributions cause higher-order harmonics to occur in the responses. We measured 15 Hz ERGs in 20 healthy subjects to determine normal ranges and in patients to verify our hypotheses on the contributions of the different pathways and to investigate the clinical application. We analyzed the amplitudes and phases of the 15, 30, and 45 Hz components in the ERGs. The overall shape of the 15 Hz amplitude curves was similar in all normal subjects and showed two minima. The 30 and 45 Hz amplitude curves increased for stimuli of high flash strengths indicating cone pathway contributions. The 15 Hz amplitude curve of the responses of an achromat was similar to that of the normal subjects for low flash strengths and showed a minimum, indicating normal primary and secondary rod pathway function. There was no second minimum, and there were no higher-order harmonics, consistent with absent cone pathway function. The 15 Hz ERGs in CSNB1 and CSNB2 patients were similar and of low amplitude for flash strengths just above where the first minimum normally occurs. We could determine that in the CSNB1 patients, the responses originate from the cone pathway, while in the CSNB2 patients, the responses originate from the secondary rod pathway.

An extended 15 Hz ERG protocol (1): the contributions of primary and secondary rod pathways and the cone pathway

The minimum in the amplitude versus flash strength curve of dark-adapted 15 Hz electroretinograms (ERGs) has been attributed to interactions between the primary and secondary rod pathways. The 15 Hz ERGs can be used to examine the two rod pathways in patients. However, previous studies suggested that the cone-driven pathway also contributes to the 15 Hz ERGs for flash strengths just above that of the minimum. We investigated cone pathway contributions to improve upon the interpretation of (abnormal) 15 Hz ERGs measured in patients. We recorded 15 Hz ERGs in five healthy volunteers, using a range of flash strengths that we extended to high values. The stimuli were varied in both colour (blue, green, amber, and red) and flash duration (short flash and square wave) in order to stimulate rods and cones in various ways. The differences in the responses to the four colours could be fully explained by the spectral sensitivity of rods for flash strengths up to approximately 12.5 log quanta·deg(-2). At higher flash strengths, higher-order harmonics appeared in the responses which could be attributed to cones being more sensitive than rods to higher frequencies. Furthermore, the amplitude curves of the blue and green responses showed a second minimum suggesting rod to cone interactions. We present a descriptive model of the contributions of the rod and cone pathways. In clinical application, we would advise using the short flash flicker instead of the square wave flicker, as the responses are of larger amplitude, and cone pathway contributions can be recognized from large higher-order harmonics.

NeuroD factors regulate cell fate and neurite stratification in the developing retina

Members of the basic helix-loop-helix (bHLH) family of transcription factors have been shown to control critical aspects of development in many tissues. To identify bHLH genes that might regulate specific aspects of retinal cell development, we investigated the expression of bHLH genes in single, developing mouse retinal cells, with particular emphasis on the NeuroD family. Two of these factors, NeuroD2 and NeuroD6/NEX, had not been previously reported as expressed in the retina. A series of loss- and gain-of-function experiments was performed, which suggested that NeuroD genes have both similarities and differences in their activities. Notably, misexpression of NeuroD genes can direct amacrine cell processes to two to three specific sublaminae in the inner plexiform layer. This effect is specific to cell type and NeuroD gene, as the AII amacrine cell type is refractory to the effects of NeuroD1 and NeuroD6, but uniquely sensitive to the effect of NeuroD2 on neurite targeting. Additionally, NeuroD2 is endogenously expressed in AII amacrine cells, among others, and loss of NeuroD2 function results in a partial loss of AII amacrine cells. The effects of misexpressing NeuroD genes on retinal cell fate determination also suggested shared and divergent functions. Remarkably, NeuroD2 misexpression induced ganglion cell production even after the normal developmental window of ganglion cell genesis. Together, these data suggest that members of the NeuroD family are important for neuronal cell type identity and may be involved in several cell type-specific aspects of retinal development, including fate determination, differentiation, morphological development, and circuit formation.

Visual threshold is set by linear and nonlinear mechanisms in the retina that mitigate noise: how neural circuits in the retina improve the signal-to-noise ratio of the single-photon response

In sensory biology, a major outstanding question is how sensory receptor cells minimize noise while maximizing signal to set the detection threshold. This optimization could be problematic because the origin of both the signals and the limiting noise in most sensory systems is believed to lie in stimulus transduction. Signal processing in receptor cells can improve the signal-to-noise ratio. However, neural circuits can further optimize the detection threshold by pooling signals from sensory receptor cells and processing them using a combination of linear and nonlinear filtering mechanisms. In the visual system, noise limiting light detection has been assumed to arise from stimulus transduction in rod photoreceptors. In this context, the evolutionary optimization of the signal-to-noise ratio in the retina has proven critical in allowing visual sensitivity to approach the limits set by the quantal nature of light. Here, we discuss how noise in the mammalian retina is mitigated to allow for highly sensitive night vision.

Visible light induced ocular delayed bioluminescence as a possible origin of negative afterimage

The delayed luminescence of biological tissues is an ultraweak reemission of absorbed photons after exposure to external monochromatic or white light illumination. Recently, Wang, Bókkon, Dai and Antal (2011) [10] presented the first experimental proof of the existence of spontaneous ultraweak biophoton emission and visible light induced delayed ultraweak photon emission from in vitro freshly isolated rat's whole eye, lens, vitreous humor and retina. Here, we suggest that the photobiophysical source of negative afterimage can also occur within the eye by delayed bioluminescent photons. In other words, when we stare at a colored (or white) image for few seconds, external photons can induce excited electronic states within different parts of the eye that is followed by a delayed reemission of absorbed photons for several seconds. Finally, these reemitted photons can be absorbed by non-bleached photoreceptors that produce a negative afterimage. Although this suggests the photobiophysical source of negative afterimages is related retinal mechanisms, cortical neurons have also essential contribution in the interpretation and modulation of negative afterimages.

Visual responses in the lateral geniculate evoked by Cx36-independent rod pathways

Emerging evidence indicates rods can communicate with retinal ganglion cells (RGCs) via pathways that do not involve gap-junctions. Here we investigated the significance of such pathways for central visual responses, using mice lacking a key gap junction protein (Cx36(-/-)) and carrying a mutation that disrupts cone phototransduction (Gnat2(cpfl3)). Electrophysiological recordings spanning the lateral geniculate revealed rod-mediated ON and OFF visual responses in virtually every cell from all major anatomical sub-compartments of this nucleus. Hence, we demonstrate that one or more classes of RGC receive input from Cx36-independent rod pathways and drive extensive ON and OFF responses across the visual thalamus.

Changes to tear cytokines of type 2 diabetic patients with or without retinopathy

PURPOSE

To investigate changes in cytokine levels in tears of type 2 diabetics with or without retinopathy.

METHODS

Tears were collected from 15 type 2 diabetics without retinopathy (DNR), 15 patients with retinopathy (DR), and 15 age and gender matched non-diabetic controls. Tear concentrations of 27 cytokines were measured by multiplex bead immunoassay. Cytokine differences between groups, ratios of type-1 T helper (Th1)/type-2 T helper (Th2) cytokines and anti-angiogenic/pro-angiogenic cytokines were analyzed statistically.

RESULTS

The most abundant cytokine detected in tears was interferon-induced protein-10 (IP-10). In comparison with controls, IP-10 and monocyte chemoattracant protein-1 (MCP-1) levels were significantly elevated in DR (p=0.016 and 0.036, respectively) and DNR groups (p=0.021 and 0.026, respectively). Interleukin-1 (IL-1) receptor antagonist (IL-1ra) levels were significantly increased in DNR (p=0.016). Th1/Th2 cytokines interferon-gamma (IFN-γ)/IL-5 and IL-2/IL-5 ratios were significantly increased in DR compared to controls (p=0.037 and 0.031, respectively). Anti-angiogenic/angiogenic cytokines IFN-γ/MCP-1 and IL-4/MCP-1 ratios in DR and DNR were significantly decreased compared to controls (p<0.05). IL-4/IL-8 and IL-12p70/IL-8 ratios were also significantly decreased in DR compared to controls (p=0.02 and 0.045, respectively). No significant correlation was demonstrated between tear cytokine concentrations and glycosylated hemoglobin (HbA1c) or fasting plasma glucose (FPG).

CONCLUSIONS

Diabetic tears exhibited elevated levels of IP-10 and MCP-1. The Th1/Th2 cytokine balance may shift to a predominantly Th1 state in DR patients. Pro-angiogenic cytokines are more highly represented than anti-angiogenic cytokines in the tears of diabetic patients.

Nonlinearity and noise at the rod-rod bipolar cell synapse

In the retina, rod bipolar (RBP) cells synapse with many rods, and suppression of rod outer segment and synaptic noise is necessary for their detection of rod single-photon responses (SPRs). Depending on the rods' signal-to-noise ratio (SNR), the suppression mechanism will likely eliminate some SPRs as well, resulting in decreased quantum efficiency. We examined this synapse in rabbit, where 100 rods converge onto each RBP. Suction electrode recordings showed that rabbit rod SPRs were difficult to distinguish from noise (independent SNR estimates were 2.3 and 2.8). Nonlinear transmission from rods to RBPs improved response detection (SNR = 8.7), but a large portion of the rod SPRs was discarded. For the dimmest flashes, the loss approached 90%. Despite the high rejection ratio, noise of two distinct types was apparent in the RBP traces: low-amplitude rumblings and discrete events that resembled the SPR. The SPR-like event frequency suggests that they result from thermal isomerizations of rhodopsin, which occurred at the rate 0.033/s/rod. The presence of low-amplitude noise is explained by a sigmoidal input-output relationship at the rod-RBP synapse and the input of noisy rods. The rabbit rod SNR and RBP quantum efficiency are the lowest yet reported, suggesting that the quantum efficiency of the rod-RBP synapse may depend on the SNR in rods. These results point to the possibility that fewer photoisomerizations are discarded for species such as primate, which has a higher rod SNR.

Nanodomain control of exocytosis is responsible for the signaling capability of a retinal ribbon synapse

Primary sensory circuits encode both weak and intense stimuli reliably, requiring that their synapses signal over a wide dynamic range. In the retinal circuitry subserving night vision, processes intrinsic to the rod bipolar (RB) cell presynaptic active zone (AZ) permit the RB synapse to encode signals generated by the absorption of single photons as well as by more intense stimuli. In a study using an in vitro slice preparation of the mouse retina, we provide evidence that the location of Ca channels with low open probability within nanometers of the release sites is a critical determinant of the physiological behavior of the RB synapse. This gives rise to apparent one-to-one coupling between Ca channel opening and vesicle release, allowing presynaptic potential to be encoded linearly over a wide dynamic range. Further, it permits a transition from univesicular to multivesicular release (MVR) when two Ca channels/AZ open at potentials above the threshold for exocytosis. MVR permits small presynaptic voltage changes to elicit postsynaptic responses larger than quantal synaptic noise.

Retinal processing: global players like it local

A recent study of a specific type of retinal amacrine cell shows how a single interneuron can implement a large number of parallel feedback circuits, illustrating how highly complex circuits can be generated by a small number of neurons.

Symmetry breakdown in the ON and OFF pathways of the retina at night: functional implications

Several recent studies have shown that the ON and OFF channels of the visual system are not simple mirror images of each other, that their response characteristics are asymmetric (Chichilnisky and Kalmar, 2002; Sagdullaev and McCall, 2005). How the asymmetries bear on visual processing is not well understood. Here, we show that ON and OFF ganglion cells show a strong asymmetry in their temporal adaptation to photopic (day) and scotopic (night) conditions and that the asymmetry confers a functional advantage. Under photopic conditions, the ON and OFF ganglion cells show similar temporal characteristics. Under scotopic conditions, the two cell classes diverge-ON cells shift their tuning to low temporal frequencies, whereas OFF cells continue to respond to high. This difference in processing corresponds to an asymmetry in the natural world, one produced by the Poisson nature of photon capture and persists over a broad range of light levels. This work characterizes a previously unknown divergence in the ON and OFF pathways and its utility to visual processing. Furthermore, the results have implications for downstream circuitry and thus offer new constraints for models of downstream processing, since ganglion cells serve as building blocks for circuits in higher brain areas. For example, if simple cells in visual cortex rely on complementary interactions between the two pathways, such as push-pull interactions (Alonso et al., 2001; Hirsch, 2003), their receptive fields may be radically different under scotopic conditions, when the ON and OFF pathways are out of sync.

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

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

Identification of a circadian clock-controlled neural pathway in the rabbit retina

BACKGROUND

Although the circadian clock in the mammalian retina regulates many physiological processes in the retina, it is not known whether and how the clock controls the neuronal pathways involved in visual processing.

METHODOLOGY/PRINCIPAL FINDINGS

By recording the light responses of rabbit axonless (A-type) horizontal cells under dark-adapted conditions in both the day and night, we found that rod input to these cells was substantially increased at night under control conditions and following selective blockade of dopamine D(2), but not D(1), receptors during the day, so that the horizontal cells responded to very dim light at night but not in the day. Using neurobiotin tracer labeling, we also found that the extent of tracer coupling between rabbit rods and cones was more extensive during the night, compared to the day, and more extensive in the day following D(2) receptor blockade. Because A-type horizontal cells make synaptic contact exclusively with cones, these observations indicate that the circadian clock in the mammalian retina substantially increases rod input to A-type horizontal cells at night by enhancing rod-cone coupling. Moreover, the clock-induced increase in D(2) receptor activation during the day decreases rod-cone coupling so that rod input to A-type horizontal cells is minimal.

CONCLUSIONS/SIGNIFICANCE

Considered together, these results identify the rod-cone gap junction as a key site in mammals through which the retinal clock, using dopamine activation of D(2) receptors, controls signal flow in the day and night from rods into the cone system.

Expression of sigma receptor 1 mRNA and protein in rat retina

Sigma receptor (sigmaR), known as a unique nonopiate, nonphencyclidine brain receptor, can bind diverse classes of psychotropic drugs, neurosteroids and other synthetic compounds, such as (+)pentazocine, etc. Two types of sigmaRs have been identified: sigmaR1 and sigmaR2. In this work, we examined the expression of sigmaR1 in rat retina by reverse transcription-polymerase chain reactive (RT-PCR) analysis and immunofluorescence double labeling. RT-PCR analysis showed that sigmaR1 mRNA was present in rat retina. Furthermore, labeling for sigmaR1 was diffusely distributed in the outer and inner plexiform layers. The sigmaR1-immunoreactivity (IR) was also observed in many cells in the inner nuclear layer and the ganglion cell layer. In the outer retina sigmaR1 was expressed in all horizontal cells labeled by calbindin. In contrast, no sigmaR1-IR was detected in several subtypes of bipolar cells, including rod-dominant ON-type bipolar cells, types 2, 3, 5 and 8 bipolar cells, labeled by protein kinase C (PKC), recoverin and hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4) respectively. In the inner retina, most of GABAergic amacrine cells, including dopaminergic and cholinergic ones, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) respectively, expressed sigmaR1. Some glycinergic amacrine cells were also labeled by sigmaR1, but glycinergic AII amacrine cells were not labeled. In addition, sigmaR1-IR was seen in almost all somata of the ganglion cells retrogradely labeled by fluorogold. These results suggest that sigmaR1 may have neuromodulatory and neuroprotective roles in the retina.

Voltage-gated Na channels in AII amacrine cells accelerate scotopic light responses mediated by the rod bipolar cell pathway

During night (i.e., scotopic) vision in mammals, rod photoreceptor output is conveyed to ganglion cells (GCs), the output cells of the retina, by a specialized neural circuit comprising rod bipolar (RB) and AII amacrine cells. Here, we examined how intrinsic postsynaptic conductances in AIIs contribute to transmission of rod-derived signals. Using paired recordings from synaptically coupled RBs and AIIs, we found that a voltage-gated Na conductance in AII amacrines accelerated EPSPs arising from RB synaptic input. EPSPs also could be amplified by the Na conductance when AIIs were hyperpolarized below resting membrane potential, thereby increasing the availability of Na channels. AII amacrines are coupled electrically, and coupled AII amacrines likely receive common input from individual RBs. Na channel-mediated effects on EPSPs, however, appeared to occur at the single-cell rather than the AII network level. By recording light-evoked synaptic currents from GCs, we determined that the Na channel-dependent acceleration, but not amplification, of RB output by AII amacrines is reflected in the dynamics of AII synaptic output to retinal ganglion cells: synaptic inputs to both ON and OFF GCs are slowed equivalently, although not attenuated in amplitude, when Na channels in AIIs are blocked. Thus, during scotopic vision, Na conductances in AIIs serve to accelerate RB output.

Optimal processing of photoreceptor signals is required to maximize behavioural sensitivity

The sensitivity of receptor cells places a fundamental limit upon the sensitivity of sensory systems. For example, the signal-to-noise ratio of sensory receptors has been suggested to limit absolute thresholds in the visual and auditory systems. However, the necessity of optimally processing sensory receptor signals for behaviour to approach this limit has received less attention. We investigated the behavioural consequences of increasing the signal-to-noise ratio of the rod photoreceptor single-photon response in a transgenic mouse, the GCAPs-/- knockout. The loss of fast Ca2+ feedback to cGMP synthesis in phototransduction for GCAPs-/- mice increases the magnitude of the rod single-photon response and dark noise, with the increase in size of the single-photon response outweighing the increase in noise. Surprisingly, despite the increased rod signal-to-noise ratio, behavioural performance for GCAPs-/- mice was diminished near absolute visual threshold. We demonstrate in electrophysiological recordings that the diminished performance compared to wild-type mice is explained by poorly tuned postsynaptic processing of the rod single-photon response at the rod bipolar cell. In particular, the level of postsynaptic saturation in GCAPs-/- rod bipolar cells is not sufficient to eliminate rod noise, and degrades the single-photon response signal-to-noise ratio. Thus, it is critical for retinal processing to be optimally tuned near absolute threshold; otherwise the visual system fails to utilize fully the signals present in the rods.

The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.

Eye smarter than scientists believed: neural computations in circuits of the retina

We rely on our visual system to cope with the vast barrage of incoming light patterns and to extract features from the scene that are relevant to our well-being. The necessary reduction of visual information already begins in the eye. In this review, we summarize recent progress in understanding the computations performed in the vertebrate retina and how they are implemented by the neural circuitry. A new picture emerges from these findings that helps resolve a vexing paradox between the retina's structure and function. Whereas the conventional wisdom treats the eye as a simple prefilter for visual images, it now appears that the retina solves a diverse set of specific tasks and provides the results explicitly to downstream brain areas.

Histamine reduces flash sensitivity of on ganglion cells in the primate retina

PURPOSE. In Old World primates, the retina receives input from histaminergic neurons in the posterior hypothalamus. They are a subset of the neurons that project throughout the central nervous system and fire maximally during the day. The contribution of these neurons to vision, was examined by applying histamine to a dark-adapted, superfused baboon eye cup preparation while making extracellular recordings from peripheral retinal ganglion cells. METHODS. The stimuli were 5-ms, 560-nm, weak, full-field flashes in the low scotopic range. Ganglion cells with sustained and transient ON responses and two cell types with OFF responses were distinguished; their responses were recorded with a 16-channel microelectrode array. RESULTS. Low micromolar doses of histamine decreased the rate of maintained firing and the light sensitivity of ON ganglion cells. Both sustained and transient ON cells responded similarly to histamine. There were no statistically significant effects of histamine in a more limited study of OFF ganglion cells. The response latencies of ON cells were approximately 5 ms slower, on average, when histamine was present. Histamine also reduced the signal-to-noise ratio of ON cells, particularly in those cells with a histamine-induced increase in maintained activity. CONCLUSIONS. A major action of histamine released from retinopetal axons under dark-adapted conditions, when rod signals dominate the response, is to reduce the sensitivity of ON ganglion cells to light flashes. These findings may relate to reports that humans are less sensitive to light stimuli in the scotopic range during the day, when histamine release in the retina is expected to be at its maximum.

Electrical Coupling and Passive Membrane Properties of AII Amacrine Cells

AII amacrine cells in the mammalian retina are connected via electrical synapses to on-cone bipolar cells and to other AII amacrine cells. To understand synaptic integration in these interneurons, we need information about the junctional conductance ( gj), the membrane resistance ( rm), the membrane capacitance ( Cm), and the cytoplasmic resistivity ( Ri). Due to the extensive electrical coupling, it is difficult to obtain estimates of rm, as well as the relative contribution of the junctional and nonjunctional conductances to the total input resistance of an AII amacrine cell. Here we used dual voltage-clamp recording of pairs of electrically coupled AII amacrine cells in an in vitro slice preparation from rat retina and applied meclofenamic acid (MFA) to block the electrical coupling and isolate single AII amacrines electrically. In the control condition, the input resistance ( Rin) was ∼620 MΩ and the apparent rm was ∼760 MΩ. After block of electrical coupling, determined by estimating gj in the dual recordings, Rin and rm were ∼4,400 MΩ, suggesting that the nongap junctional conductance of an AII amacrine cell is ∼16% of the total input conductance. Control experiments with nucleated patches from AII amacrine cells suggested that MFA had no effect on the nongap junctional membrane of these cells. From morphological reconstructions of AII amacrine cells filled with biocytin, we obtained a surface area of ∼900 μm2 which, with a standard value for Cm of 0.01 pF/μm2, corresponds to an average capacitance of ∼9 pF and a specific membrane resistance of ∼41 kΩ cm2. Together with information concerning synaptic connectivity, these data will be important for developing realistic compartmental models of the network of AII amacrine cells.