Single-photon absorptions evoke synaptic depression in the retina to extend the operational range of rod vision

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions. The specialized downstream circuit for rod signalling, called the primary rod pathway, is well characterized in mammals, but circuitry for rod signalling in non-mammals is largely unknown. Here we demonstrate that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA sequencing, we identified two bipolar cell types in zebrafish that are related to mammalian rod bipolar cell (RBCs), the only bipolar type that directly carries rod signals from the outer to the inner retina in the primary rod pathway. By combining electrophysiology, histology and ultrastructural reconstruction of the zebrafish RBCs, we found that, similar to mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells postsynaptic to one RBC type is strikingly similar to that of mammalian RBCs and their amacrine partners, suggesting that the cell types and circuit design of the primary rod pathway emerged before the divergence of teleost fish and mammals. The second RBC type, which forms separate pathways, was either lost in mammals or emerged in fish.

A presynaptic source drives differing levels of surround suppression in two mouse retinal ganglion cell types

In early sensory systems, cell-type diversity generally increases from the periphery into the brain, resulting in a greater heterogeneity of responses to the same stimuli. Surround suppression is a canonical visual computation that begins within the retina and is found at varying levels across retinal ganglion cell types. Our results show that heterogeneity in the level of surround suppression occurs subcellularly at bipolar cell synapses. Using single-cell electrophysiology and serial block-face scanning electron microscopy, we show that two retinal ganglion cell types exhibit very different levels of surround suppression even though they receive input from the same bipolar cell types. This divergence of the bipolar cell signal occurs through synapse-specific regulation by amacrine cells at the scale of tens of microns. These findings indicate that each synapse of a single bipolar cell can carry a unique visual signal, expanding the number of possible functional channels at the earliest stages of visual processing.

Sensory deprivation arrests cellular and synaptic development of the night-vision circuitry in the retina

Experience regulates synapse formation and function across sensory circuits. How inhibitory synapses in the mammalian retina are sculpted by visual cues remains unclear. By use of a sensory deprivation paradigm, we find that visual cues regulate maturation of two GABA synapse types (GABAA and GABAC receptor synapses), localized across the axon terminals of rod bipolar cells (RBCs)-second-order retinal neurons integral to the night-vision circuit. Lack of visual cues causes GABAA synapses at RBC terminals to retain an immature receptor configuration with slower response profiles and prevents receptor recruitment at GABAC synapses. Additionally, the organizing protein for both these GABA synapses, LRRTM4, is not clustered at dark-reared RBC synapses. Ultrastructurally, the total number of ribbon-output/inhibitory-input synapses across RBC terminals remains unaltered by sensory deprivation, although ribbon synapse output sites are misarranged when the circuit develops without visual cues. Intrinsic electrophysiological properties of RBCs and expression of chloride transporters across RBC terminals are additionally altered by sensory deprivation. Introduction to normal 12-h light-dark housing conditions facilitates maturation of dark-reared RBC GABA synapses and restoration of intrinsic RBC properties, unveiling a new element of light-dependent retinal cellular and synaptic plasticity.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage. Thus, it has been long assumed that the primary rod pathway evolved in mammals. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs, suggesting that the cell types and circuit design of the primary rod pathway have emerged before the divergence of teleost fish and amniotes. The second RBC type, which forms separate pathways, is either lost in mammals or emerged in fish.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions1. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage2-6. Thus, it has been long assumed that the primary rod pathway evolved in mammals3,5-7. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs8, both zebrafish RBC types connect with all rods and red-cones in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs. This suggests that the cell types and circuit design of the primary rod pathway may have emerged before the divergence of teleost fish and amniotes (mammals, bird, reptiles). The second RBC type in zebrafish, which forms separate pathways from the first RBC type, is either lost in mammals or emerged in fish to serve yet unknown roles.

Light Adaptation of Retinal Rod Bipolar Cells

The sensitivity of retinal cells is altered in background light to optimize the detection of contrast. For scotopic (rod) vision, substantial adaptation occurs in the first two cells, the rods and rod bipolar cells (RBCs), through sensitivity adjustments in rods and postsynaptic modulation of the transduction cascade in RBCs. To study the mechanisms mediating these components of adaptation, we made whole-cell, voltage-clamp recordings from retinal slices of mice from both sexes. Adaptation was assessed by fitting the Hill equation to response-intensity relationships with the parameters of half-maximal response (I1/2 ), Hill coefficient (n), and maximum response amplitude (Rmax ). We show that rod sensitivity decreases in backgrounds according to the Weber-Fechner relation with an I1/2 of ∼50 R* s-1 The sensitivity of RBCs follows a near-identical function, indicating that changes in RBC sensitivity in backgrounds bright enough to adapt the rods are mostly derived from the rods themselves. Backgrounds too dim to adapt the rods can however alter n, relieving a synaptic nonlinearity likely through entry of Ca2+ into the RBCs. There is also a surprising decrease of Rmax , indicating that a step in RBC synaptic transduction is desensitized or that the transduction channels became reluctant to open. This effect is greatly reduced after dialysis of BAPTA at a membrane potential of +50 mV to impede Ca2+ entry. Thus the effects of background illumination in RBCs are in part the result of processes intrinsic to the photoreceptors and in part derive from additional Ca2+-dependent processes at the first synapse of vision.SIGNIFICANCE STATEMENT Light adaptation adjusts the sensitivity of vision as ambient illumination changes. Adaptation for scotopic (rod) vision is known to occur partly in the rods and partly in the rest of the retina from presynaptic and postsynaptic mechanisms. We recorded light responses of rods and rod bipolar cells to identify different components of adaptation and study their mechanisms. We show that bipolar-cell sensitivity largely follows adaptation of the rods but that light too dim to adapt the rods produces a linearization of the bipolar-cell response and a surprising decrease in maximum response amplitude, both mediated by a change in intracellular Ca2+ These findings provide a new understanding of how the retina responds to changing illumination.

Fast visual adaptation to dim light in a cavity-nesting bird

Many birds move fast into dark nest cavities forcing the visual system to adapt to low light intensities. Their visual system takes between 15 and 60 min for complete dark adaptation, but little is known about the visual performance of birds during the first seconds in low light intensities. In a forced two-choice behavioural experiment we studied how well budgerigars can discriminate stimuli of different luminance directly after entering a darker environment. The birds made their choices within about 1 s and did not wait to adapt their visual system to the low light intensities. When moving from a bright facility into an environment with 0.5 log unit lower illuminance, the budgerigars detected targets with a luminance of 0.825 cd m^-2 on a black background. When moving into an environment with 1.7 or 3.5 log units lower illuminance, they detected targets with luminances between 0.106 and 0.136 cd m^-2. In tests with two simultaneously displayed targets, the birds discriminated similar luminance differences between the targets (Weber fraction of 0.41-0.54) in all light levels. Our results support the notion that partial adaptation of bird eyes to the lower illumination occurring within 1 s allows them to safely detect and feed their chicks.

Homeostatic plasticity in the retina

Vision begins in the retina, whose intricate neural circuits extract salient features of the environment from the light entering our eyes. Neurodegenerative diseases of the retina (e.g., inherited retinal degenerations, age-related macular degeneration, and glaucoma) impair vision and cause blindness in a growing number of people worldwide. Increasing evidence indicates that homeostatic plasticity (i.e., the drive of a neural system to stabilize its function) can, in principle, preserve retinal function in the face of major perturbations, including neurodegeneration. Here, we review the circumstances and events that trigger homeostatic plasticity in the retina during development, sensory experience, and disease. We discuss the diverse mechanisms that cooperate to compensate and the set points and outcomes that homeostatic retinal plasticity stabilizes. Finally, we summarize the opportunities and challenges for unlocking the therapeutic potential of homeostatic plasticity. Homeostatic plasticity is fundamental to understanding retinal development and function and could be an important tool in the fight to preserve and restore vision.

Artificial Vision Adaption Mimicked by an Optoelectrical In2O3 Transistor Array

Simulation of biological visual perception has gained considerable attention. In this paper, an optoelectrical In₂O₃ transistor array with a negative photoconductivity behavior is designed using a side-gate structure and a screen-printed ion-gel as the gate insulator. This paper is the first to observe a negative photoconductivity in electrolyte-gated oxide devices. Furthermore, an artificial visual perception system capable of self-adapting to environmental lightness is mimicked using the proposed device array. The transistor device array shows a self-adaptive behavior of light under different levels of light intensity, successfully demonstrating the visual adaption with an adjustable threshold range to the external environment. This study provides a new way to create an environmentally adaptive artificial visual perception system and has far-reaching significance for the future of neuromorphic electronics.

Connectomic analysis reveals an interneuron with an integral role in the retinal circuit for night vision

Night vision in mammals depends fundamentally on rod photoreceptors and the well-studied rod bipolar (RB) cell pathway. The central neuron in this pathway, the AII amacrine cell (AC), exhibits a spatially tuned receptive field, composed of an excitatory center and an inhibitory surround, that propagates to ganglion cells, the retina's projection neurons. The circuitry underlying the surround of the AII, however, remains unresolved. Here, we combined structural, functional and optogenetic analyses of the mouse retina to discover that surround inhibition of the AII depends primarily on a single interneuron type, the NOS-1 AC: a multistratified, axon-bearing GABAergic cell, with dendrites in both ON and OFF synaptic layers, but with a pure ON (depolarizing) response to light. Our study demonstrates generally that novel neural circuits can be identified from targeted connectomic analyses and specifically that the NOS-1 AC mediates long-range inhibition during night vision and is a major element of the RB pathway.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Mice Reach Higher Visual Sensitivity at Night by Using a More Efficient Behavioral Strategy

Circadian clocks predictively adjust the physiology of organisms to the day/night cycle. The retina has its own clock, and many diurnal changes in its physiology have been reported. However, their implications for retinal functions and visually guided behavior are largely unresolved. Here, we study the impact of diurnal rhythm on the sensitivity limit of mouse vision. A simple photon detection task allowed us to link well-defined retinal output signals directly to visually guided behavior. We show that visually guided behavior at its sensitivity limit is strongly under diurnal control, reaching the highest sensitivity and stability at night. The diurnal differences in visual sensitivity did not arise in the retina, as assessed by spike recordings from the most sensitive retinal ganglion cell types: ON sustained, OFF sustained, and OFF transient alpha ganglion cells. Instead, we found that mice, as nocturnal animals, use a more efficient search strategy for visual cues at night. Intriguingly, they can switch to the more efficient night strategy even at their subjective day after first having performed the task at night. Our results exemplify that the shape of visual psychometric functions depends robustly on the diurnal state of the animal, its search strategy, and even its diurnal history of performing the task. The results highlight the impact of the day/night cycle on high-level sensory processing, demonstrating a direct diurnal impact on the behavioral strategy of the animal.

Rod Photoreceptors Signal Fast Changes in Daylight Levels Using a Cx36-Independent Retinal Pathway in Mouse

Temporal contrast detected by rod photoreceptors is channeled into multiple retinal rod pathways that ultimately connect to cone photoreceptor pathways via Cx36 gap junctions or via chemical synapses. However, we do not yet understand how the different rod pathways contribute to the perception of temporal contrast (changes in luminance with time) at mesopic light levels, where both rods and cones actively respond to light. Here, we use a forced-choice, operant behavior assay to investigate rod-driven, temporal contrast sensitivity (TCS) in mice of either sex. Transgenic mice with desensitized cones (GNAT2 ^cpfl3 line) were used to identify rod contributions to TCS in mesopic lights. We found that at low mesopic lights (400 photons/s/μm² at the retina), control and GNAT2 ^cpfl3 mice had similar TCS. Surprisingly, at upper mesopic lights (8000 photons/s/μm²), GNAT2 ^cpfl3 mice exhibited a relative reduction in TCS to low (<12 Hz) while maintaining normal TCS to high (12-36 Hz) temporal frequencies. The rod-driven responses to high temporal frequencies developed gradually over time (>30 min). Furthermore, the TCS of GNAT2 ^cpfl3 and GNAT2 ^cpfl3 ::Cx36^-/- mice matched closely, indicating that transmission of high-frequency signals (1) does not require the rod-cone Cx36 gap junctions as has been proposed in the past; and (2) a Cx36-independent rod pathway(s) (e.g., direct rod to OFF cone bipolar cell synapses and/or glycinergic synapses from AII amacrine cells to OFF ganglion cells) is sufficient for fast, mesopic rod-driven vision. These findings extend our understanding of the link between visual circuits and perception in mouse.SIGNIFICANCE STATEMENT The contributions of specific retinal pathways to visual perception are not well understood. We found that the temporal processing properties of rod-driven vision in mice change significantly with light level. In dim lights, rods relay relatively slow temporal variations. However, in daylight conditions, rod pathways exhibit high sensitivity to fast but not to slow temporal variations, whereas cone-driven responses supplement the loss in rod-driven sensitivity to slow temporal variations. Our findings highlight the dynamic interplay of rod- and cone-driven vision as light levels rise from night to daytime levels. Furthermore, the fast, rod-driven signals do not require the rod-to-cone Cx36 gap junctions as proposed in the past, but rather, can be relayed by alternative Cx36-independent rod pathways.

Paradoxical Rules of Spike Train Decoding Revealed at the Sensitivity Limit of Vision

All sensory information is encoded in neural spike trains. It is unknown how the brain utilizes this neural code to drive behavior. Here, we unravel the decoding rules of the brain at the most elementary level by linking behavioral decisions to retinal output signals in a single-photon detection task. A transgenic mouse line allowed us to separate the two primary retinal outputs, ON and OFF pathways, carrying information about photon absorptions as increases and decreases in spiking, respectively. We measured the sensitivity limit of rods and the most sensitive ON and OFF ganglion cells and correlated these results with visually guided behavior using markerless head and eye tracking. We show that behavior relies only on the ON pathway even when the OFF pathway would allow higher sensitivity. Paradoxically, behavior does not rely on the spike code with maximal information but instead relies on a decoding strategy based on increases in spiking.

Range, routing and kinetics of rod signaling in primate retina

Stimulus- or context-dependent routing of neural signals through parallel pathways can permit flexible processing of diverse inputs. For example, work in mouse shows that rod photoreceptor signals are routed through several retinal pathways, each specialized for different light levels. This light-level-dependent routing of rod signals has been invoked to explain several human perceptual results, but it has not been tested in primate retina. Here, we show, surprisingly, that rod signals traverse the primate retina almost exclusively through a single pathway – the dedicated rod bipolar pathway. Identical experiments in mouse and primate reveal substantial differences in how rod signals traverse the retina. These results require reevaluating human perceptual results in terms of flexible computation within this single pathway. This includes a prominent speeding of rod signals with light level – which we show is inherited directly from the rod photoreceptors themselves rather than from different pathways with distinct kinetics.

A group I metabotropic glutamate receptor controls synaptic gain between rods and rod bipolar cells in the mouse retina

The canonical mGluR6-Trpm1 pathway that generates the sign-inverting signal between photoreceptors and ON bipolar cells has been well described. However, one type of ON bipolar cell, the rod bipolar cell (RBC), additionally is thought to express the group I mGluRs whose function is unknown. We examined the role of group I mGluRs in mouse RBCs and here provide evidence that it controls synaptic gain between rods and RBCs. In dark-adapted conditions, the mGluR1 antagonists LY367385 and (RS)-1-Aminoindan-1,5-dicarboxylic acid, but not the mGluR5 antagonist 2-Methyl-6-(phenylethynyl)pyridine hydrochloride reduced the light-evoked responses in RBCs indicating that mGluR1, but not mGluR5, serves to potentiate RBC responses. Perturbing the downstream phospholipase C (PLC)-protein kinase C (PKC) pathway by inhibiting PLC, tightly buffering intracellular Ca2+ , or preventing its release from intracellular stores reduced the synaptic potentiation by mGluR1. The effect of mGluR1 activation was dependent upon adaptation state, strongly increasing the synaptic gain in dark-, but not in light-adapted retinas, or in the presence of a moderate background light, consistent with the idea that mGluR1 activation requires light-dependent glutamate release from rods. Moreover, immunostaining revealed that protein kinase Cα (PKCα) is more strongly expressed in RBC dendrites in dark-adapted conditions, revealing an additional mechanism behind the loss of mGluR1 potentiation. In light-adapted conditions, exogenous activation of mGluR1 with the agonist 3,5-Dihydroxyphenylglycine increased the mGluR6 currents in some RBCs and decreased it in others, suggesting an additional action of mGluR1 that is unmasked in the light-adapted state. Elevating intracellular free Ca2+ , consistently resulted in a decrease in synaptic gain. Our results provide evidence that mGluR1 controls the synaptic gain in RBCs.

Parallel Processing of Rod and Cone Signals: Retinal Function and Human Perception

We know a good deal about the operation of the retina when either rod or cone photoreceptors provide the dominant input (i.e., under very dim or very bright conditions). However, we know much less about how the retina operates when rods and cones are coactive (i.e., under intermediate lighting conditions, such as dusk). Such mesopic conditions span 20-30% of the light levels over which vision operates and encompass many situations in which vision is essential (e.g., driving at night). These lighting conditions are challenging because rod and cone signals differ substantially: Rod responses are nearing saturation, while cone responses are weak and noisy. A rich history of perceptual studies guides our investigation of how the retina operates under mesopic conditions and in doing so provides a powerful opportunity to link general issues about parallel processing in neural circuits with computation and perception. We review some of the successes and challenges in understanding the retinal basis of perceptual rod-cone interactions.

Impact of light-adaptive mechanisms on mammalian retinal visual encoding at high light levels

A persistent change in illumination causes light-adaptive changes in retinal neurons. Light adaptation improves visual encoding by preventing saturation and by adjusting spatiotemporal integration to increase the signal-to-noise ratio (SNR) and utilize signaling bandwidth efficiently. In dim light, the visual input contains a greater relative amount of quantal noise, and vertebrate receptive fields are extended in space and time to increase SNR. Whereas in bright light, SNR of the visual input is high, the rate of synaptic vesicle release from the photoreceptors is low so that quantal noise in synaptic output may limit SNR postsynaptically. Whether and how reduced synaptic SNR impacts spatiotemporal integration in postsynaptic neurons remains unclear. To address this, we measured spatiotemporal integration in retinal horizontal cells and ganglion cells in the guinea pig retina across a broad illumination range, from low to high photopic levels. In both cell types, the extent of spatial and temporal integration changed according to an inverted U-shaped function consistent with adaptation to low SNR at both low and high light levels. We show how a simple mechanistic model with interacting, opponent filters can generate the observed changes in ganglion cell spatiotemporal receptive fields across light-adaptive states and postulate that retinal neurons postsynaptic to the cones in bright light adopt low-pass spatiotemporal response characteristics to improve visual encoding under conditions of low synaptic SNR.

Flexible Neural Hardware Supports Dynamic Computations in Retina

The ability of the retina to adapt to changes in mean light intensity and contrast is well known. Classically, however, adaptation is thought to affect gain but not to change the visual modality encoded by a given type of retinal neuron. Recent findings reveal unexpected dynamic properties in mouse retinal neurons that challenge this view. Specifically, certain cell types change the visual modality they encode with variations in ambient illumination or following repetitive visual stimulation. These discoveries demonstrate that computations performed by retinal circuits with defined architecture can change with visual input. Moreover, they pose a major challenge for central circuits that must decode properties of the dynamic visual signal from retinal outputs.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

Glutamatergic Retinal Waves

Spontaneous activity patterns propagate through many parts of the developing nervous system and shape the wiring of emerging circuits. Prior to vision, waves of activity originating in the retina propagate through the lateral geniculate nucleus (LGN) of the thalamus to primary visual cortex (V1). Retinal waves have been shown to instruct the wiring of ganglion cell axons in LGN and of thalamocortical axons in V1 via correlation-based plasticity rules. Across species, retinal waves mature in three stereotypic stages (I-III), in which distinct circuit mechanisms give rise to unique activity patterns that serve specific functions in visual system refinement. Here, I review insights into the patterns, mechanisms, and functions of stage III retinal waves, which rely on glutamatergic signaling. As glutamatergic waves spread across the retina, neighboring ganglion cells with opposite light responses (ON vs. OFF) are activated sequentially. Recent studies identified lateral excitatory networks in the inner retina that generate and propagate glutamatergic waves, and vertical inhibitory networks that desynchronize the activity of ON and OFF cells in the wavefront. Stage III wave activity patterns may help segregate axons of ON and OFF ganglion cells in the LGN, and could contribute to the emergence of orientation selectivity in V1.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

Functional Circuitry of the Retina

The mammalian retina is an important model system for studying neural circuitry: Its role in sensation is clear, its cell types are relatively well defined, and its responses to natural stimuli-light patterns-can be studied in vitro. To solve the retina, we need to understand how the circuits presynaptic to its output neurons, ganglion cells, divide the visual scene into parallel representations to be assembled and interpreted by the brain. This requires identifying the component interneurons and understanding how their intrinsic properties and synapses generate circuit behaviors. Because the cellular composition and fundamental properties of the retina are shared across species, basic mechanisms studied in the genetically modifiable mouse retina apply to primate vision. We propose that the apparent complexity of retinal computation derives from a straightforward mechanism-a dynamic balance of synaptic excitation and inhibition regulated by use-dependent synaptic depression-applied differentially to the parallel pathways that feed ganglion cells.

Complex inhibitory microcircuitry regulates retinal signaling near visual threshold

Neuronal microcircuits, small, localized signaling motifs involving two or more neurons, underlie signal processing and computation in the brain. Compartmentalized signaling within a neuron may enable it to participate in multiple, independent microcircuits. Each A17 amacrine cell in the mammalian retina contains within its dendrites hundreds of synaptic feedback microcircuits that operate independently to modulate feedforward signaling in the inner retina. Each of these microcircuits comprises a small (<1 μm) synaptic varicosity that typically receives one excitatory synapse from a presynaptic rod bipolar cell (RBC) and returns two reciprocal inhibitory synapses back onto the same RBC terminal. Feedback inhibition from the A17 sculpts the feedforward signal from the RBC to the AII, a critical component of the circuitry mediating night vision. Here, we show that the two inhibitory synapses from the A17 to the RBC express kinetically distinct populations of GABA receptors: rapidly activating GABA(A)Rs are enriched at one synapse while more slowly activating GABA(C)Rs are enriched at the other. Anatomical and electrophysiological data suggest that macromolecular complexes of voltage-gated (Cav) channels and Ca(2+)-activated K(+) channels help to regulate GABA release from A17 varicosities and limit GABA(C)R activation under certain conditions. Finally, we find that selective elimination of A17-mediated feedback inhibition reduces the signal to noise ratio of responses to dim flashes recorded in the feedforward pathway (i.e., the AII amacrine cell). We conclude that A17-mediated feedback inhibition improves the signal to noise ratio of RBC-AII transmission near visual threshold, thereby improving visual sensitivity at night.

Simultaneous encoding of odors by channels with diverse sensitivity to inhibition

Odorant receptors in the periphery map precisely onto olfactory glomeruli ("coding channels") in the brain. However, the odor tuning of a glomerulus is not strongly correlated with its spatial position. This raises the question of whether lateral inhibition between glomeruli is specific or nonspecific. Here we show that, in the Drosophila brain, focal activation of even a single glomerulus recruits GABAergic interneurons in all glomeruli. Moreover, the relative level of interneuron activity in different glomeruli is largely odor invariant. Although interneurons are recruited nonspecifically, glomeruli differ dramatically in their sensitivity to interneuron activity, and this is explained by their varying sensitivity to GABA. Interestingly, a stimulus is typically encoded in parallel by channels having high and low sensitivity to inhibition. Because lateral inhibition confers both costs and benefits, the brain might rely preferentially on "high" and "low" channels in different behavioral contexts.

Cross-synaptic synchrony and transmission of signal and noise across the mouse retina

Cross-synaptic synchrony—correlations in transmitter release across output synapses of a single neuron—is a key determinant of how signal and noise traverse neural circuits. The anatomical connectivity between rod bipolar and A17 amacrine cells in the mammalian retina, specifically that neighboring A17s often receive input from many of the same rod bipolar cells, provides a rare technical opportunity to measure cross-synaptic synchrony under physiological conditions. This approach reveals that synchronization of rod bipolar cell synapses is near perfect in the dark and decreases with increasing light level. Strong synaptic synchronization in the dark minimizes intrinsic synaptic noise and allows rod bipolar cells to faithfully transmit upstream signal and noise to downstream neurons. Desynchronization in steady light lowers the sensitivity of the rod bipolar output to upstream voltage fluctuations. This work reveals how cross-synaptic synchrony shapes retinal responses to physiological light inputs and, more generally, signaling in complex neural networks.

DOI:http://dx.doi.org/10.7554/eLife.03892.001

The human eye is capable of detecting a single photon of starlight. This level of sensitivity is made possible by the high sensitivity of photoreceptors called rods. There are around 120 million rods in the retina, and they support vision in levels of light that are too low to activate the photoreceptors called cones that allow us to see in color. This is why we cannot see colors in the dark.

Signals are relayed through the retina via a circuit made up of multiple types of neurons. The activation of rods leads to activation of cells known as ‘rod bipolar cells’ which, in turn, activate amacrine cells and ganglion cells, with the latter sending signals via the optic nerve to the brain. All of these neurons communicate with one another at junctions called synapses. Activation of a rod bipolar cell, for example, triggers the release of molecules called neurotransmitters: these molecules bind to and activate receptors on the amacrine cells, enabling the signal to be transmitted.

For the brain to detect that a single photon has struck a rod, the eye must transmit information along this chain of neurons in a way that is highly reliable while adding very little noise to the signal. Grimes et al. have now revealed a key step in how this is achieved.

Electrical recordings from the mouse retina revealed that, in the dark, small fluctuations in the activity of rod bipolar cells lead to the near-deterministic release of neurotransmitters. This reduces the impact of random fluctuations in neurotransmitter release produced at individual synapses and ensures that the signals from rod bipolar cells (and thus from rods) are transmitted faithfully through the circuit with minimal added noise. As light levels increase, this tight synchrony of transmitter release breaks down, reducing the sensitivity to individual photons.

Given that many other brain regions share the features that enable retinal cells to coordinate the release of neurotransmitters, this mechanism might be used throughout the brain to increase the signal-to-noise ratio for the transmission of information through neural circuits.

DOI:http://dx.doi.org/10.7554/eLife.03892.002

The synaptic and circuit mechanisms underlying a change in spatial encoding in the retina

Components of neural circuits are often repurposed so that the same biological hardware can be used for distinct computations. This flexibility in circuit operation is required to account for the changes in sensory computations that accompany changes in input signals. Yet we know little about how such changes in circuit operation are implemented. Here we show that a single retinal ganglion cell performs a different computation in dim light--averaging contrast within its receptive field--than in brighter light, when the cell becomes sensitive to fine spatial detail. This computational change depends on interactions between two parallel circuits that control the ganglion cell's excitatory synaptic inputs. Specifically, steady-state interactions through dendro-axonal gap junctions control rectification of the synapses providing excitatory input to the ganglion cell. These findings provide a clear example of how a simple synaptic mechanism can repurpose a neural circuit to perform diverse computations.

Functional allocation of synaptic contacts in microcircuits from rods via rod bipolar to AII amacrine cells in the mouse retina

Retinal microcircuits for night vision at the absolute threshold are required to relay a single-photon rod signal reliably to ganglion cells via rod bipolar (RB) cells and AII amacrine cells. To assess the noise reduction of intercellular signal transmission in this rod-specific pathway, we quantified its synaptic connectivity by 3D reconstruction of a series of electron micrographs. In most cases (94%), each rod made ribbon synaptic contacts onto two adjacent RB cells. Conversely, each RB cell was contacted by 25 rods. Each RB axon terminal contacted four or five AII amacrine cells via 53 ribbon synapses. Thus, the signal from one rod may be represented as 106 replicates at two RB axons. Moreover, the two adjacent RB cells contacted two to four AII amacrine cells in common, where the signals relayed by two RB cells were reunited. In more detail, over 50% of each RB output was directed predominantly to a single, preferred AII amacrine cell, although each RB cell also separately contacted another one to three AII amacrine cells. Most of the replicate signals at two RB axons were collected on a few AII amacrine cells via reunions, dominant connections, and electrical coupling by AII-AII gap junctions. Thus the original signal may be reliably represented by signal amplification with focal accumulation without gathering unnecessary noise from a wide surrounding area. This allocation of RB-AII synaptic contacts may serve as the structural basis for the physiological properties of the AII single-photon response that include high amplification, local adaptation, and regenerative acceleration.

Spatial organization of AMPAR subtypes in ON RGCs

Retinal ganglion cells (RGCs) receive glutamatergic input from bipolar cells through NMDA- and AMPA-type glutamate receptors. Both GluA2-containing, Ca(2+)-impermeable AMPA receptors (CI-AMPARs) and GluA2-lacking, Ca(2+)-permeable AMPA receptors (CP-AMPARs) contribute to light-evoked responses in ON RGCs; however, specific roles for each subtype are not well understood. Here, we present evidence that light intensity determines the subtype of AMPAR that is activated during the synaptic response in ON RGCs. Using current voltage analysis of the EPSC we show that light intensities near RGC threshold, intensities that travel through the well described primary rod pathway, evoke synaptic currents that are preferentially mediated by CP-AMPARs. Synaptic responses evoked by spontaneous release of transmitter from bipolar cell terminals also preferentially activate CP-AMPARs. Conversely, higher light intensities, most likely carried by secondary rod pathways, activate CI-AMPARs. The same pattern of CP-AMPAR and CI-AMPAR activation was observed in mice containing only functional rods, suggesting that the recruitment of CI-AMPARs at higher light intensity does not require cone stimulation. When glutamate spillover was induced by blocking transporters with TBOA, both the near threshold and spontaneous EPSCs contained a significant CI-AMPAR component. We propose that CI-AMPARs are activated by "spillover" of synaptic glutamate only during bright illumination, or when glutamate uptake is blocked. Glutamate may spill over to more distant sites at the same synapse, or perhaps as far as neighboring synapses. Together, our data suggest that the spatial organization of AMPARs at ON RGCs synapses allows for selective, intensity-dependent activation of AMPARs with distinct subunit composition.

Adaptation to background light enables contrast coding at rod bipolar cell synapses

Rod photoreceptors contribute to vision over an ∼ 6-log-unit range of light intensities. The wide dynamic range of rod vision is thought to depend upon light intensity-dependent switching between two parallel pathways linking rods to ganglion cells: a rod → rod bipolar (RB) cell pathway that operates at dim backgrounds and a rod → cone → cone bipolar cell pathway that operates at brighter backgrounds. We evaluated this conventional model of rod vision by recording rod-mediated light responses from ganglion and AII amacrine cells and by recording RB-mediated synaptic currents from AII amacrine cells in mouse retina. Contrary to the conventional model, we found that the RB pathway functioned at backgrounds sufficient to activate the rod → cone pathway. As background light intensity increased, the RB's role changed from encoding the absorption of single photons to encoding contrast modulations around mean luminance. This transition is explained by the intrinsic dynamics of transmission from RB synapses.

Identifying cell class specific losses from serially generated electroretinogram components

Purpose. Processing of information through the cellular layers of the retina occurs in a serial manner. In the electroretinogram (ERG), this complicates interpretation of inner retinal changes as dysfunction may arise from “upstream” neurons or may indicate a direct loss to that neural generator. We propose an approach that addresses this issue by defining ERG gain relationships. Methods. Regression analyses between two serial ERG parameters in a control cohort of rats are used to define gain relationships. These gains are then applied to two models of retinal disease. Results. The PIII_amp to PII_amp gain is unity whereas the PII_amp to pSTR_amp and PII_amp to nSTR_amp gains are greater than unity, indicating “amplification” (P < 0.05). Timing relationships show amplification between PIII_it to PII_it and compression for PII_it to pSTR_it and PII_it to nSTR_it, (P < 0.05). Application of these gains to ω-3-deficiency indicates that all timing changes are downstream of photoreceptor changes, but a direct pSTR amplitude loss occurs (P < 0.05). Application to diabetes indicates widespread inner retinal dysfunction which cannot be attributed to outer retinal changes (P < 0.05). Conclusions. This simple approach aids in the interpretation of inner retinal ERG changes by taking into account gain characteristics found between successive ERG components of normal animals.

Light adaptation alters the source of inhibition to the mouse retinal OFF pathway

Sensory systems must avoid saturation to encode a wide range of stimulus intensities. One way the retina accomplishes this is by using both dim-light-sensing rod and bright-light-sensing cone photoreceptor circuits. OFF cone bipolar cells are a key point in this process, as they receive both excitatory input from cones and inhibitory input from AII amacrine cells via the rod pathway. However, in addition to AII amacrine cell input, other inhibitory inputs from cone pathways also modulate OFF cone bipolar cell light signals. It is unknown how these inhibitory inputs to OFF cone bipolar cells change when switching between rod and cone pathways or whether all OFF cone bipolar cells receive rod pathway input. We found that one group of OFF cone bipolar cells (types 1, 2, and 4) receive rod-mediated inhibitory inputs that likely come from the rod-AII amacrine cell pathway, while another group of OFF cone bipolar cells (type 3) do not. In both cases, dark-adapted rod-dominant light responses showed a significant contribution of glycinergic inhibition, which decreased with light adaptation and was, surprisingly, compensated by an increase in GABAergic inhibition. As GABAergic input has distinct timing and spatial spread from glycinergic input, a shift from glycinergic to GABAergic inhibition could significantly alter OFF cone bipolar cell signaling to downstream OFF ganglion cells. Larger GABAergic input could reflect an adjustment of OFF bipolar cell spatial inhibition, which may be one mechanism that contributes to retinal spatial sensitivity in the light.

Transformation of visual signals by inhibitory interneurons in retinal circuits

One of the largest mysteries of the brain lies in understanding how higher-level computations are implemented by lower-level operations in neurons and synapses. In particular, in many brain regions inhibitory interneurons represent a diverse class of cells, the individual functional roles of which are unknown. We discuss here how the operations of inhibitory interneurons influence the behavior of a circuit, focusing on recent results in the vertebrate retina. A key role in this understanding is played by a common representation of the visual stimulus that can be applied at different stages. By considering how this stimulus representation changes at each location in the circuit, we can understand how neuron-level operations such as thresholds and inhibition yield circuit-level computations such as how stimulus selectivity and gain are controlled by local and peripheral visual stimuli.

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

Alert response to motion onset in the retina

Previous studies have shown that motion onset is very effective at capturing attention and is more salient than smooth motion. Here, we find that this salience ranking is present already in the firing rate of retinal ganglion cells. By stimulating the retina with a bar that appears, stays still, and then starts moving, we demonstrate that a subset of salamander retinal ganglion cells, fast OFF cells, responds significantly more strongly to motion onset than to smooth motion. We refer to this phenomenon as an alert response to motion onset. We develop a computational model that predicts the time-varying firing rate of ganglion cells responding to the appearance, onset, and smooth motion of a bar. This model, termed the adaptive cascade model, consists of a ganglion cell that receives input from a layer of bipolar cells, represented by individual rectified subunits. Additionally, both the bipolar and ganglion cells have separate contrast gain control mechanisms. This model captured the responses to our different motion stimuli over a wide range of contrasts, speeds, and locations. The alert response to motion onset, together with its computational model, introduces a new mechanism of sophisticated motion processing that occurs early in the visual system.

Odorant-induced membrane potential depolarization of AIY interneuron in Caenorhabditis elegans

Although some interneurons in C. elegans have been shown to have unusual region-specific Ca(2+) dynamics, the region-specific Ca(2+) and membrane potential response properties of these neurons are largely unknown due to technical limitations. In this report, we focused on one of these neurons, AIY interneuron, where Ca(2+) dynamics have been detected only in neurites, and not the soma, during odor and temperature stimulation to determine whether membrane potential and Ca(2+) are region-specific dynamics and distinct from one another. To visualize voltage change both in the soma and neurites of AIY, we used voltage-sensitive fluorescent protein (VSFP) 2.42. First, we confirmed that the sensor protein worked correctly in C. elegans by depolarizing AIY interneuron with high concentrations of KCl. Next, we observed membrane potential depolarization during odor (isoamyl alcohol) stimulation in both neurites and the soma. Additionally, depolarization of membrane potential with direct application of high KCl induced a Ca(2+) increase in the soma. From these results, we conclude that membrane potential behavior and Ca(2+) dynamics in AIY differ in its subcellular regions and that VSFP2.42 can be a useful tool for studying information processing in single neurons.

Nonlinear interactions between excitatory and inhibitory retinal synapses control visual output

The visual system is highly sensitive to dynamic features in the visual scene. However, it is not known how or where this enhanced sensitivity first occurs. We investigated this phenomenon by studying interactions between excitatory and inhibitory synapses in the second synaptic layer of the mouse retina. We found that these interactions showed activity-dependent changes that enhanced signaling of dynamic stimuli. Excitatory signaling from cone bipolar cells to ganglion cells exhibited strong synaptic depression, attributable to reduced glutamate release from bipolar cells. This depression was relieved by amacrine cell inhibitory feedback that activated presynaptic GABA(C) receptors. We found that the balance between excitation and feedback inhibition depended on stimulus frequency; at short interstimulus intervals, excitation was enhanced, attributable to reduced inhibitory feedback. This dynamic interplay may enrich visual processing by enhancing retinal responses to closely spaced temporal events, representing rapid changes in the visual environment.

Ribbon synapses compute temporal contrast and encode luminance in retinal rod bipolar cells

Contrast is computed throughout the nervous system to encode changing inputs efficiently. The retina encodes luminance and contrast over a wide range of visual conditions and must adapt its responses to maintain sensitivity and to avoid saturation. We examined the means by which one type of adaptation allows individual synapses to compute contrast and encode luminance in biphasic responses to step changes in light levels. Light-evoked depletion of the readily releasable vesicle pool (RRP) at rod bipolar cell ribbon synapses in rat retina limited the dynamic range available to encode transient, but not sustained, responses, thereby allowing the transient and sustained components of release to compute temporal contrast and encode mean light levels, respectively. A release/replenishment model revealed that a single, homogeneous pool of synaptic vesicles is sufficient to generate this behavior and that a partial depletion of the RRP is the dominant mechanism for shaping the biphasic contrast/luminance response.

A synaptic mechanism for retinal adaptation to luminance and contrast

The gain of signaling in primary sensory circuits is matched to the stimulus intensity by the process of adaptation. Retinal neural circuits adapt to visual scene statistics, including the mean (background adaptation) and the temporal variance (contrast adaptation) of the light stimulus. The intrinsic properties of retinal bipolar cells and synapses contribute to background and contrast adaptation, but it is unclear whether both forms of adaptation depend on the same cellular mechanisms. Studies of bipolar cell synapses identified synaptic mechanisms of gain control, but the relevance of these mechanisms to visual processing is uncertain because of the historical focus on fast, phasic transmission rather than the tonic transmission evoked by ambient light. Here, we studied use-dependent regulation of bipolar cell synaptic transmission evoked by small, ongoing modulations of membrane potential (V(M)) in the physiological range. We made paired whole-cell recordings from rod bipolar (RB) and AII amacrine cells in a mouse retinal slice preparation. Quasi-white noise voltage commands modulated RB V(M) and evoked EPSCs in the AII. We mimicked changes in background luminance or contrast, respectively, by depolarizing the V(M) or increasing its variance. A linear systems analysis of synaptic transmission showed that increasing either the mean or the variance of the presynaptic V(M) reduced gain. Further electrophysiological and computational analyses demonstrated that adaptation to mean potential resulted from both Ca channel inactivation and vesicle depletion, whereas adaptation to variance resulted from vesicle depletion alone. Thus, background and contrast adaptation apparently depend in part on a common synaptic mechanism.

An intrinsic neural oscillator in the degenerating mouse retina

The loss of photoreceptors during retinal degeneration (RD) is known to lead to an increase in basal activity in remnant neural networks. To identify the source of activity, we combined two-photon imaging with patch-clamp techniques to examine the physiological properties of morphologically identified retinal neurons in a mouse model of RD (rd1). Analysis of activity in rd1 ganglion cells revealed sustained oscillatory (∼10 Hz) synaptic activity in ∼30% of all classes of cells. Oscillatory activity persisted after putative inputs from residual photoreceptor, rod bipolar cell, and inhibitory amacrine cell synapses were pharmacologically blocked, suggesting that presynaptic cone bipolar cells were intrinsically active. Examination of presynaptic rd1 ON and OFF bipolar cells indicated that they rested at relatively negative potentials (less than -50 mV). However, in approximately half the cone bipolar cells, low-amplitude membrane oscillation (∼5 mV, ∼10 Hz) were apparent. Such oscillations were also observed in AII amacrine cells. Oscillations in ON cone bipolar and AII amacrine cells exhibited a weak apparent voltage dependence and were resistant to blockade of synaptic receptors, suggesting that, as in wild-type retina, they form an electrically coupled network. In addition, oscillations were insensitive to blockers of voltage-gated Ca(2+) channels (0.5 mm Cd(2+) and 0.5 mm Ni(2+)), ruling out known mechanisms that underlie oscillatory behavior in bipolar cells. Together, these results indicate that an electrically coupled network of ON cone bipolar/AII amacrine cells constitutes an intrinsic oscillator in the rd1 retina that is likely to drive synaptic activity in downstream circuits.

Illuminating synapses and circuitry in the retina

In the central nervous system, space is at a premium. This is especially true in the retina, where synapses, cells, and circuitry have evolved to maximize signal-processing capacity within a thin, optically transparent tissue. For example, at some retinal synapses, single presynaptic active zones contact multiple postsynaptic targets; some individual neurons perform completely different tasks depending on visual conditions, while others execute hundreds of circuit computations in parallel; and the retinal network adapts, at various levels, to the ever-changing visual world. Each of these features reflects efficient use of limited cellular resources to optimally encode visual information.

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.

Coordinated control of sensitivity by two splice variants of Gα(o) in retinal ON bipolar cells

The high sensitivity of scotopic vision depends on the efficient retinal processing of single photon responses generated by individual rod photoreceptors. At the first synapse in the mammalian retina, rod outputs are pooled by a rod “ON” bipolar cell, which uses a G-protein signaling cascade to enhance the fidelity of the single photon response under conditions where few rods absorb light. Here we show in mouse rod bipolar cells that both splice variants of the G_o α subunit, Gα_o1 and Gα_o2, mediate light responses under the control of mGluR6 receptors, and their coordinated action is critical for maximizing sensitivity. We found that the light response of rod bipolar cells was primarily mediated by Gα_o1, but the loss of Gα_o2 caused a reduction in the light sensitivity. This reduced sensitivity was not attributable to the reduction in the total number of G_o α subunits, or the altered balance of expression levels between the two splice variants. These results indicate that Gα_o1 and Gα_o2 both mediate a depolarizing light response in rod bipolar cells without occluding each other’s actions, suggesting they might act independently on a common effector. Thus, Gα_o2 plays a role in improving the sensitivity of rod bipolar cells through its action with Gα_o1. The coordinated action of two splice variants of a single Gα may represent a novel mechanism for the fine control of G-protein activity.

Phosducin regulates transmission at the photoreceptor-to-ON-bipolar cell synapse

The rate of synaptic transmission between photoreceptors and bipolar cells has been long known to depend on conditions of ambient illumination. However, the molecular mechanisms that mediate and regulate transmission at this ribbon synapse are poorly understood. We conducted electroretinographic recordings from dark- and light-adapted mice lacking the abundant photoreceptor-specific protein phosducin and found that the ON-bipolar cell responses in these animals have a reduced light sensitivity in the dark-adapted state. Additional desensitization of their responses, normally caused by steady background illumination, was also diminished compared with wild-type animals. This effect was observed in both rod- and cone-driven pathways, with the latter affected to a larger degree. The underlying mechanism is likely to be photoreceptor specific because phosducin is not expressed in other retina neurons and transgenic expression of phosducin in rods of phosducin knock-out mice rescued the rod-specific phenotype. The underlying mechanism functions downstream from the phototransduction cascade, as evident from the sensitivity of phototransduction in phosducin knock-out rods being affected to a much lesser degree than b-wave responses. These data indicate that a major regulatory component responsible for setting the sensitivity of signal transmission between photoreceptors and ON-bipolar cells is confined to photoreceptors and that phosducin participates in the underlying molecular mechanism.

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.

Retinal parallel processors: more than 100 independent microcircuits operate within a single interneuron

Most neurons are highly polarized cells with branched dendrites that receive and integrate synaptic inputs and extensive axons that deliver action potential output to distant targets. By contrast, amacrine cells, a diverse class of inhibitory interneurons in the inner retina, collect input and distribute output within the same neuritic network. The extent to which most amacrine cells integrate synaptic information and distribute their output is poorly understood. Here, we show that single A17 amacrine cells provide reciprocal feedback inhibition to presynaptic bipolar cells via hundreds of independent microcircuits operating in parallel. The A17 uses specialized morphological features, biophysical properties, and synaptic mechanisms to isolate feedback microcircuits and maximize its capacity to handle many independent processes. This example of a neuron employing distributed parallel processing rather than spatial integration provides insights into how unconventional neuronal morphology and physiology can maximize network function while minimizing wiring cost.

Transient release kinetics of rod bipolar cells revealed by capacitance measurement of exocytosis from axon terminals in rat retinal slices

Presynaptic transmitter release has mostly been studied through measurements of postsynaptic responses, but a few synapses offer direct access to the presynaptic terminal, thereby allowing capacitance measurements of exocytosis. For mammalian rod bipolar cells, synaptic transmission has been investigated in great detail by recording postsynaptic currents in AII amacrine cells. Presynaptic measurements of the dynamics of vesicular cycling have so far been limited to isolated rod bipolar cells in dissociated preparations. Here, we first used computer simulations of compartmental models of morphologically reconstructed rod bipolar cells to adapt the 'Sine + DC' technique for capacitance measurements of exocytosis at axon terminals of intact rod bipolar cells in retinal slices. In subsequent physiological recordings, voltage pulses that triggered presynaptic Ca(2+) influx evoked capacitance increases that were proportional to the pulse duration. With pulse durations 100 ms, the increase saturated at 10 fF, corresponding to the size of a readily releasable pool of vesicles. Pulse durations 400 ms evoked additional capacitance increases, probably reflecting recruitment from additional pools of vesicles. By using Ca(2+) tail current stimuli, we separated Ca(2+) influx from Ca(2+) channel activation kinetics, allowing us to estimate the intrinsic release kinetics of the readily releasable pool, yielding a time constant of 1.1 ms and a maximum release rate of 2-3 vesicles (release site)(1) ms(1). Following exocytosis, we observed endocytosis with time constants ranging from 0.7 to 17 s. Under physiological conditions, it is likely that release will be transient, with the kinetics limited by the activation kinetics of the voltage-gated Ca(2+) channels.