Spikes in retinal bipolar cells phase-lock to visual stimuli with millisecond precision

Individual Differences Elucidate the Perceptual Benefits Associated with Robust Temporal Fine-Structure Processing

The auditory system is unique among sensory systems in its ability to phase lock to and precisely follow very fast cycle-by-cycle fluctuations in the phase of sound-driven cochlear vibrations. Yet, the perceptual role of this temporal fine structure (TFS) code is debated. This fundamental gap is attributable to our inability to experimentally manipulate TFS cues without altering other perceptually relevant cues. Here, we circumnavigated this limitation by leveraging individual differences across 200 participants to systematically compare variations in TFS sensitivity to performance in a range of speech perception tasks. TFS sensitivity was assessed through detection of interaural time/phase differences, while speech perception was evaluated by word identification under noise interference. Results suggest that greater TFS sensitivity is not associated with greater masking release from fundamental-frequency or spatial cues, but appears to contribute to resilience against the effects of reverberation. We also found that greater TFS sensitivity is associated with faster response times, indicating reduced listening effort. These findings highlight the perceptual significance of TFS coding for everyday hearing.

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically, the action potential is initiated once voltage-activated Na⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential's all-or-none nature. Here we demonstrate that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-activated Na⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-activated Ca²⁺ channel, enabling spikes of different amplitude. T-type Ca²⁺ channels generate small amplitude spikes, whereas larger amplitude spikes require high voltage-activated Cd²⁺-sensitive Ca²⁺ channels. We demonstrate that these different amplitude spikes can signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes whereas cholinergic inputs evoke larger spikes that do not rely on T-type channels. Different synaptic inputs to CSFcNs can therefore be signaled by the spike amplitude.

The Impact of Multivesicular Release on the Transmission of Sensory Information by Ribbon Synapses

The statistics of vesicle release determine how synapses transfer information, but the classical Poisson model of independent release does not always hold at the first stages of vision and hearing. There, ribbon synapses also encode sensory signals as events comprising two or more vesicles released simultaneously. The implications of such coordinated multivesicular release (MVR) for spike generation are not known. Here we investigate how MVR alters the transmission of sensory information compared with Poisson synapses using a pure rate-code. We used leaky integrate-and-fire models incorporating the statistics of release measured experimentally from glutamatergic synapses of retinal bipolar cells in zebrafish (both sexes) and compared these with models assuming Poisson inputs constrained to operate at the same average rates. We find that MVR can increase the number of spikes generated per vesicle while reducing interspike intervals and latency to first spike. The combined effect was to increase the efficiency of information transfer (bits per vesicle) over a range of conditions mimicking target neurons of different size. MVR was most advantageous in neurons with short time constants and reliable synaptic inputs, when less convergence was required to trigger spikes. In the special case of a single input driving a neuron, as occurs in the auditory system of mammals, MVR increased information transfer whenever spike generation required more than one vesicle. This study demonstrates how presynaptic integration of vesicles by MVR can increase the efficiency with which sensory information is transmitted compared with a rate-code described by Poisson statistics.SIGNIFICANCE STATEMENT Neurons communicate by the stochastic release of vesicles at the synapse and the statistics of this process will determine how information is represented by spikes. The classical model is that vesicles are released independently by a Poisson process, but this does not hold at ribbon-type synapses specialized to transmit the first electrical signals in vision and hearing, where two or more vesicles can fuse in a single event by a process termed coordinated multivesicular release. This study shows that multivesicular release can increase the number of spikes generated per vesicle and the efficiency of information transfer (bits per vesicle) over a range of conditions found in the retina and peripheral auditory system.

A theoretical model reveals specialized synaptic depressions and temporal frequency tuning in retinal parallel channels

In the Outer Plexiform Layer of a retina, a cone pedicle provides synaptic inputs for multiple cone bipolar cell (CBC) subtypes so that each subtype formats a parallelized processing channel to filter visual features from the environment. Due to the diversity of short-term depressions among cone-CBC contacts, these channels have different temporal frequency tunings. Here, we propose a theoretical model based on the hierarchy Linear-Nonlinear-Synapse framework to link the synaptic depression and the neural activities of the cone-CBC circuit. The model successfully captures various frequency tunings of subtype-specialized channels and infers synaptic depression recovery time constants inside circuits. Furthermore, the model can predict frequency-tuning behaviors based on synaptic activities. With the prediction of region-specialized UV cone parallel channels, we suggest the acute zone in the zebrafish retina supports detecting light-off events at high temporal frequencies.

More than the end: OFF response plasticity as a mnemonic signature of a sound’s behavioral salience

In studying how neural populations in sensory cortex code dynamically varying stimuli to guide behavior, the role of spiking after stimuli have ended has been underappreciated. This is despite growing evidence that such activity can be tuned, experience-and context-dependent and necessary for sensory decisions that play out on a slower timescale. Here we review recent studies, focusing on the auditory modality, demonstrating that this so-called OFF activity can have a more complex temporal structure than the purely phasic firing that has often been interpreted as just marking the end of stimuli. While diverse and still incompletely understood mechanisms are likely involved in generating phasic and tonic OFF firing, more studies point to the continuing post-stimulus activity serving a short-term, stimulus-specific mnemonic function that is enhanced when the stimuli are particularly salient. We summarize these results with a conceptual model highlighting how more neurons within the auditory cortical population fire for longer duration after a sound’s termination during an active behavior and can continue to do so even while passively listening to behaviorally salient stimuli. Overall, these studies increasingly suggest that tonic auditory cortical OFF activity holds an echoic memory of specific, salient sounds to guide behavioral decisions.

Transience of the Retinal Output Is Determined by a Great Variety of Circuit Elements

Retinal ganglion cells (RGCs) encrypt stimulus features of the visual scene in action potentials and convey them toward higher visual centers in the brain. Although there are many visual features to encode, our recent understanding is that the ~46 different functional subtypes of RGCs in the retina share this task. In this scheme, each RGC subtype establishes a separate, parallel signaling route for a specific visual feature (e.g., contrast, the direction of motion, luminosity), through which information is conveyed. The efficiency of encoding depends on several factors, including signal strength, adaptational levels, and the actual efficacy of the underlying retinal microcircuits. Upon collecting inputs across their respective receptive field, RGCs perform further analysis (e.g., summation, subtraction, weighting) before they generate the final output spike train, which itself is characterized by multiple different features, such as the number of spikes, the inter-spike intervals, response delay, and the rundown time (transience) of the response. These specific kinetic features are essential for target postsynaptic neurons in the brain in order to effectively decode and interpret signals, thereby forming visual perception. We review recent knowledge regarding circuit elements of the mammalian retina that participate in shaping RGC response transience for optimal visual signaling.

Direction selectivity in retinal bipolar cell axon terminals

The ability to encode the direction of image motion is fundamental to our sense of vision. Direction selectivity along the four cardinal directions is thought to originate in direction-selective ganglion cells (DSGCs) because of directionally tuned GABAergic suppression by starburst cells. Here, by utilizing two-photon glutamate imaging to measure synaptic release, we reveal that direction selectivity along all four directions arises earlier than expected at bipolar cell outputs. Individual bipolar cells contained four distinct populations of axon terminal boutons with different preferred directions. We further show that this bouton-specific tuning relies on cholinergic excitation from starburst cells and GABAergic inhibition from wide-field amacrine cells. DSGCs received both tuned directionally aligned inputs and untuned inputs from among heterogeneously tuned glutamatergic bouton populations. Thus, directional tuning in the excitatory visual pathway is incrementally refined at the bipolar cell axon terminals and their recipient DSGC dendrites by two different neurotransmitters co-released from starburst cells.

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Cardinal direction selectivity emerges at types 7 and 2 bipolar cell axon terminals

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Starburst amacrine cells are necessary for direction selectivity in bipolar cells

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Cholinergic excitation and GABAergic inhibition are integrated at axon terminals

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Direction-selective ganglion cells receive directionally aligned glutamate inputs

A subset of cone bipolar cells expresses the Na+ channel SCN2A in the human retina

Some bipolar cells in the human retina are known to express voltage-gated Na⁺ channels. However, it is unclear which types of channels are expressed, and whether Na⁺ channel expression is limited to specific types of bipolar cells. In the present study, we examined the types of voltage-gated Na⁺ channels expressed in human bipolar cells and the morphology of bipolar cells with voltage-gated Na⁺ currents. To investigate the expression of voltage-gated Na⁺ channels in human bipolar cells, we examined whether Na⁺ channel transcripts could be detected in single bipolar cells using the reverse transcription polymerase chain reaction (RT-PCR) technique. The voltage-gated Na⁺ current was recorded from isolated bipolar cells using the patch-clamp recording technique. Types of bipolar cells that have the Na⁺ currents were investigated by analyzing their morphology after staining with Lucifer yellow. Using RT-PCR, the SCN2A Na⁺ channel was detected in 5 of 6 isolated bipolar cells. This suggests that a subset of human bipolar cells expresses the SCN2A Na⁺ channel. Under voltage-clamp conditions, depolarizing voltage steps induced a fast transient inward current in cone bipolar cells with axon terminal boutons that stratified at the ON layer, which includes the stratum 3, 4, and 5 of the inner plexiform layer (IPL, n = 2/11 cells). The fast transient inward current of isolated bipolar cells was blocked by 1 μM of tetrodotoxin (TTX), a voltage-gated Na⁺ channel blocker. No fast transient inward current was recorded with axon terminals that stratify at the OFF layer, which includes stratum 1 and 2 of the IPL (n = 4). Thus, a subset of ON cone bipolar cells at least expresses the putative voltage-gated Na⁺ channel SCN2A in the human retina. The Na⁺ channels in the bipolar cells may serve to amplify the release of neurotransmitter, glutamate, when membrane potential is rapidly depolarized and thereby selectively accelerating light responses.

Understanding the retinal basis of vision across species

The vertebrate retina first evolved some 500 million years ago in ancestral marine chordates. Since then, the eyes of different species have been tuned to best support their unique visuoecological lifestyles. Visual specializations in eye designs, large-scale inhomogeneities across the retinal surface and local circuit motifs mean that all species' retinas are unique. Computational theories, such as the efficient coding hypothesis, have come a long way towards an explanation of the basic features of retinal organization and function; however, they cannot explain the full extent of retinal diversity within and across species. To build a truly general understanding of vertebrate vision and the retina's computational purpose, it is therefore important to more quantitatively relate different species' retinal functions to their specific natural environments and behavioural requirements. Ultimately, the goal of such efforts should be to build up to a more general theory of vision.

Spatiotemporally Asymmetric Excitation Supports Mammalian Retinal Motion Sensitivity

The detection of visual motion is a fundamental function of the visual system. How motion speed and direction are computed together at the cellular level, however, remains largely unknown. Here, we suggest a circuit mechanism by which excitatory inputs to direction-selective ganglion cells in the mouse retina become sensitive to the motion speed and direction of image motion. Electrophysiological, imaging, and connectomic analyses provide evidence that the dendrites of ON direction-selective cells receive spatially offset and asymmetrically filtered glutamatergic inputs along motion-preference axis from asymmetrically wired bipolar and amacrine cell types with distinct release dynamics. A computational model shows that, with this spatiotemporal structure, the input amplitude becomes sensitive to speed and direction by a preferred direction enhancement mechanism. Our results highlight the role of an excitatory mechanism in retinal motion computation by which feature selectivity emerges from non-selective inputs.

The Zebrafish Visual System: From Circuits to Behavior

Visual stimuli can evoke complex behavioral responses, but the underlying streams of neural activity in mammalian brains are difficult to follow because of their size. Here, I review the visual system of zebrafish larvae, highlighting where recent experimental evidence has localized the functional steps of visuomotor transformations to specific brain areas. The retina of a larva encodes behaviorally relevant visual information in neural activity distributed across feature-selective ganglion cells such that signals representing distinct stimulus properties arrive in different areas or layers of the brain. Motor centers in the hindbrain encode motor variables that are precisely tuned to behavioral needs within a given stimulus setting. Owing to rapid technological progress, larval zebrafish provide unique opportunities for obtaining a comprehensive understanding of the intermediate processing steps occurring between visual and motor centers, revealing how visuomotor transformations are implemented in a vertebrate brain.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Neural mechanisms of contextual modulation in the retinal direction selective circuit

Contextual modulation of neuronal responses by surrounding environments is a fundamental attribute of sensory processing. In the mammalian retina, responses of On–Off direction selective ganglion cells (DSGCs) are modulated by motion contexts. However, the underlying mechanisms are unknown. Here, we show that posterior-preferring DSGCs (pDSGCs) are sensitive to discontinuities of moving contours owing to contextually modulated cholinergic excitation from starburst amacrine cells (SACs). Using a combination of synapse-specific genetic manipulations, patch clamp electrophysiology and connectomic analysis, we identified distinct circuit motifs upstream of On and Off SACs that are required for the contextual modulation of pDSGC activity for bright and dark contrasts. Furthermore, our results reveal a class of wide-field amacrine cells (WACs) with straight, unbranching dendrites that function as “continuity detectors” of moving contours. Therefore, divergent circuit motifs in the On and Off pathways extend the information encoding of On-Off DSGCs beyond their direction selectivity during complex stimuli.

The mechanisms of contextual modulation in direction selective ganglion cells in the retina remain unclear. Here, the authors find that that On-Off direction-selective ganglion cells are differentially sensitive to discontinuities of dark and bright moving edges in the visual environment and, using synapse-specific genetic manipulations with functional measurements, reveal the microcircuits underlying this contextual sensitivity.

Spikeling: A low-cost hardware implementation of a spiking neuron for neuroscience teaching and outreach

Understanding how neurons encode and compute information is fundamental to our study of the brain, but opportunities for hands-on experience with neurophysiological techniques on live neurons are scarce in science education. Here, we present Spikeling, an open source in silico implementation of a spiking neuron that costs £25 and mimics a wide range of neuronal behaviours for classroom education and public neuroscience outreach. Spikeling is based on an Arduino microcontroller running the computationally efficient Izhikevich model of a spiking neuron. The microcontroller is connected to input ports that simulate synaptic excitation or inhibition, to dials controlling current injection and noise levels, to a photodiode that makes Spikeling light sensitive, and to a light-emitting diode (LED) and speaker that allows spikes to be seen and heard. Output ports provide access to variables such as membrane potential for recording in experiments or digital signals that can be used to excite other connected Spikelings. These features allow for the intuitive exploration of the function of neurons and networks mimicking electrophysiological experiments. We also report our experience of using Spikeling as a teaching tool for undergraduate and graduate neuroscience education in Nigeria and the United Kingdom.

Direct measurement of bipolar cell responses to electrical stimulation in wholemount mouse retina

OBJECTIVE

This in vitro investigation examines the response of retinal bipolar cells to extracellular electrical stimulation.

APPROACH

In vitro investigations characterizing the response of retinal neurons to electrical stimulation have primarily focused on retinal ganglion cells because they are the output neurons of the retina and their superficial position in the retina makes them readily accessible to in vitro recording techniques. Thus, the majority of information regarding the response of inner retinal neurons has been inferred from ganglion cell activity. Here we use patch clamp electrophysiology to directly record electrically-evoked activity in bipolar cells within the inner retina of normal Tg(Gng13-EGFP)GI206Gsat and degenerate rd10 Tg(Gng13-EGFP)GI206Gsat mice using a wholemount preparation.

MAIN RESULTS

Bipolar cells respond to electrical stimulation with time-locked depolarizing voltage transients. The latency of the response declines with increases in stimulation amplitude. A desensitizing response is observed during repeated stimulation with 25 ms biphasic current pulses delivered at pulse rates greater than 6 pps. A burst of long-latency (200-1000 ms) inhibitory postsynaptic potentials are evoked by the stimulus and the burst exhibits evidence of a lower and upper stimulation threshold.

SIGNIFICANCE

These results provide insights into the various types of bipolar cell activity elicited by electrical stimulation and may be useful for future retinal prosthesis stimulation protocols. This investigation uses patch clamp electrophysiology to provide direct analysis of ON-type bipolar cell responses to electrical stimulation in a wholemount retina preparation. It explores the effects of variable stimulus amplitudes, pulse widths, and frequencies in both normal and degenerate retina. The analysis adds to a body of work largely based upon indirect measurements of bipolar cell activity, and the methodology demonstrates an alternative retina preparation technique in which to acquire single-cell activity.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

Diverse inhibitory and excitatory mechanisms shape temporal tuning in transient OFF α ganglion cells in the rabbit retina

KEY POINTS

Neurons combine excitatory and inhibitory signals to perform computations. In the retina, interactions between excitation and inhibition enable neurons to detect specific visual features. We describe how several excitatory and inhibitory mechanisms work together to allow transient OFF α ganglion cells in the rabbit retina to respond selectively to high temporal frequencies and thus detect faster image motion. The weightings of these different mechanisms change with the contrast and spatiotemporal properties of the visual input, and thereby support temporal tuning in α cells over a range of visual conditions. The results help us understand how ganglion cells selectively integrate excitatory and inhibitory signals to extract specific information from the visual input.

ABSTRACT

The 20 to 30 types of ganglion cell in the mammalian retina represent parallel signalling pathways that convey different information to the brain. α ganglion cells are selective for high temporal frequencies in visual inputs, which makes them particularly sensitive to rapid motion. Although α ganglion cells have been studied in several species, the synaptic basis for their selective temporal tuning remains unclear. Here, we analyse excitatory synaptic inputs to transient OFF α ganglion cells (t-OFF α GCs) in the rabbit retina. We show that convergence of excitatory and inhibitory synaptic inputs within the bipolar cell terminals presynaptic to the t-OFF α GCs shifts the temporal tuning to higher temporal frequencies. GABAergic inhibition suppresses the excitatory input at low frequencies, but potentiates it at high frequencies. Crossover glycinergic inhibition and sodium channel activity in the presynaptic bipolar cells also potentiate high frequency excitatory inputs. We found differences in the spatial and temporal properties, and contrast sensitivities of these mechanisms. These differences in stimulus selectivity allow these mechanisms to generate bandpass temporal tuning of t-OFF α GCs over a range of visual conditions.

Method to remove photoreceptors from whole mount retina in vitro

Patch clamp recordings of neurons in the inner nuclear layer of the retina are difficult to conduct in a whole mount retina preparation because surrounding neurons block the path of the patch pipette. Vertical slice preparations or dissociated retinal cells provide access to bipolar cells at the cost of severing the lateral connection between neurons. We have developed a technique to remove photoreceptors from the rodent retina that exposes inner nuclear layer neurons, allowing access for patch clamp recording. Repeated application to and removal of filter paper from the photoreceptor side of an isolated retina effectively and efficiently removes photoreceptor cells and, in degenerate retina, hypertrophied Müller cell end feet. Live-dead assays applied to neurons remaining after photoreceptor removal demonstrated mostly viable cells. Patch clamp recordings from bipolar cells reveal responses similar to those recorded in traditional slice and dissociated cell preparations. An advantage of the photoreceptor peel technique is that it exposes inner retinal neurons in a whole mount retina preparation for investigation of signal processing. A disadvantage is that photoreceptor removal alters input to remaining retinal neurons. The technique may be useful for investigations of extracellular electrical stimulation, photoreceptor DNA analysis, and nonpharmacological removal of light input.NEW & NOTEWORTHY This study reports a method for removing photoreceptors from rodent whole mount retina while preserving the architecture of the inner retina. The method enables easier access to the inner retina for studies of neural processing, such as by patch clamp recording.

Retinal Circuitry Balances Contrast Tuning of Excitation and Inhibition to Enable Reliable Computation of Direction Selectivity

Feedforward (FF) inhibition is a common motif in many neural networks. Typically, excitatory inputs drive both principal neurons and interneurons; the interneurons then inhibit the principal neurons, thereby regulating the strength and timing of the FF signal. The interneurons introduce a likely nonlinear processing step that could distort the excitation/inhibition (E/I) ratio in the principal neuron, potentially degrading the reliability of computation in the circuit. In the retina, FF inhibition is an essential feature of the circuitry underlying direction selectivity (DS): glutamatergic bipolar cells (BCs) provide excitatory input to direction-selective ganglion cells (DSGCs) and GABAergic starburst amacrine cells (SACs), and the SACs then provide FF inhibition onto DSGCs. Robust DS computation requires a consistent synaptic E/I ratio in the DSGC in various visual conditions. Here, we show in mouse retina that the E/I ratio is maintained in DSGCs over a wide stimulus contrast range due to compensatory mechanisms in the diverse population of presynaptic BCs. BC inputs to SACs exhibit higher contrast sensitivity, so that the subsequent nonlinear transformation in SACs reduces the contrast sensitivity of FF inhibition to match the sensitivity of direct excitatory inputs onto DSGCs. Measurements of light-evoked responses from individual BC synaptic terminals suggest that the distinct sensitivity of BC inputs reflects different contrast sensitivity between BC subtypes. Numerical simulations suggest that this network arrangement is crucial for reliable DS computation.

SIGNIFICANCE STATEMENT

Properly balanced excitation and inhibition are essential for many neuronal computations across brain regions. Feedforward inhibition circuitry, in which a common excitatory source drives both the principal cell and an interneuron, is a typical mechanism by which neural networks maintain this balance. Feedforward circuits may become imbalanced at low stimulation levels, however, if the excitatory drive is too weak to overcome the activation threshold in the interneuron. Here we reveal how excitation and inhibition remain balanced in direction selective ganglion cells in the mouse retina over a wide visual stimulus range.

Cone signals in monostratified and bistratified amacrine cells of adult zebrafish retina

Strata within the inner plexiform layer (IPL) of vertebrate retinas are suspected to be distinct signaling regions. Functions performed within adult zebrafish IPL strata were examined through microelectrode recording and staining of stratified amacrine types. The stimulus protocol and analysis discriminated the pattern of input from red, green, blue, and UV cones as well as the light-response waveforms in this tetrachromatic species. A total of 36 cells were analyzed. Transient depolarizing waveforms at ON and OFF originated with bistratified amacrine types, whose dendritic planes branched either in IPL sublaminas a & b, or only within sublamina a. Monophasic-sustained depolarizing waveforms originated with types monostratified in IPL s4 (sublamina b). OFF responses hyperpolarized at onset, depolarized at offset, and in some cases depolarized during mid-stimulus. These signals originated with types monostratified in s1 or s2 (sublamina a). Bistratified amacrines received depolarizing signals only from red cones, at both ON and OFF, while s4 stratified ON cells combined red and green cone signals. The s1/s2 stratified OFF cells utilized hyperpolarizing signals from red, red and green, or red and blue cones at ON, but only depolarizing red cone signals at OFF. ON and OFF depolarizing transients from red cones appear widely distributed within IPL strata. "C-type" physiologies, depolarized by some wavelengths, hyperpolarized by others, in biphasic or triphasic spectral patterns, originated with amacrine cells monostratified in s5. Collectively, cells in this stratum processed signals from all cone types. J. Comp. Neurol. 525:1532-1557, 2017. © 2016 Wiley Periodicals, Inc.

Probing electrical tuning of hair cells with a Zap current method in the intact amphibian papilla of bullfrogs

Most, if not all, modern vertebrate species have evolved exquisite inner ears to discriminate acoustic signals of different frequencies, through a process called frequency tuning. For non-mammalian species, at least part of frequency tuning has been attributed to intrinsic electrical properties of hair cells, i.e. electrical tuning. Since it was first discovered, the traditional method to assess electrical tuning has been to inject step current into hair cells and examine dampened membrane voltage oscillation. However, this method is not applicable for hair cells that do not oscillate. In this study, we developed a Zap current method that can be unbiasedly applied to all hair cells regardless of their oscillating behavior. Similar to a chirp sound in acoustic stimulation, a Zap current is a sinusoidal current with the frequency increased linearly with time. We first validated this new method with the traditional step current method on hair cells with dampened membrane voltage oscillation, and then applied it to all hair cells in the intact amphibian papilla of bullfrogs. We found that while hair cells with dampened membrane voltage oscillation are sharply tuned, non-oscillating hair cells are broadly tuned. In addition, we found a third type of hair cells, which oscillate continuously and are extremely sharply tuned, with multiple peaks that are reminiscent of harmonics in the mammalian cochlea. In conclusion, the new Zap current method provides an unbiased way to assess electrical tuning, and it reveals an underappreciated heterogeneity of electrical tuning in the bullfrog amphibian papilla.

Patch clamp recordings of retinal bipolar cells in response to extracellular electrical stimulation in wholemount mouse retina

Retinitis pigmentosa is a family of inherited retinal diseases identified by the degeneration of photoreceptors, which leads to blindness. In efforts to restore vision lost to retinitis pigmentosa, retinal prostheses have been developed to generate visual percepts by electrically stimulating the surviving retinal bipolar and ganglion cells. The response of retinal ganglion cells to electrical stimulation has been characterized through direct measurement. However, the response of bipolar cells has only been inferred by measuring retinal ganglion cell activity. This investigation reports on a novel tissue preparation technique facilitating bipolar cell patch clamp recordings in wholemount retina. We find that bipolar cells respond to extracellular electrical stimuli with time-locked voltage spike depolarizations, which are likely mediated by voltage-gated calcium channels.

Crossover Inhibition Generates Sustained Visual Responses in the Inner Retina

In daylight, the input to the retinal circuit is provided primarily by cone photoreceptors acting as band-pass filters, but the retinal output also contains neuronal populations transmitting sustained signals. Using in vivo imaging of genetically encoded calcium reporters, we investigated the circuits that generate these sustained channels within the inner retina of zebrafish. In OFF bipolar cells, sustained transmission was found to depend on crossover inhibition from the ON pathway through GABAergic amacrine cells. In ON bipolar cells, the amplitude of low-frequency signals was regulated by glycinergic amacrine cells, while GABAergic inhibition regulated the gain of band-pass signals. We also provide the first functional description of a subset of sustained ON bipolar cells in which synaptic activity was suppressed by fluctuations at frequencies above ∼0.2 Hz. These results map out the basic circuitry by which the inner retina generates sustained visual signals and describes a new function of crossover inhibition.

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.

Inhibitory input to the direction-selective ganglion cell is saturated at low contrast

Direction-selective ganglion cells (DSGCs) respond selectively to motion toward a "preferred" direction, but much less to motion toward the opposite "null" direction. Directional signals in the DSGC depend on GABAergic inhibition and are observed over a wide range of speeds, which precludes motion detection based on a fixed temporal correlation. A voltage-clamp analysis, using narrow bar stimuli similar in width to the receptive field center, demonstrated that inhibition to DSGCs saturates rapidly above a threshold contrast. However, for wide bar stimuli that activate both the center and surround, inhibition depends more linearly on contrast. Excitation for both wide and narrow bars was also more linear. We propose that positive feedback, likely within the starburst amacrine cell or its network, produces steep saturation of inhibition at relatively low contrast. This mechanism renders GABA release essentially contrast and speed invariant, which enhances directional signals for small objects and thereby increases the signal-to-noise ratio for direction-selective signals in the spike train over a wide range of stimulus conditions. The steep saturation of inhibition confers to a neuron immunity to noise in its spike train, because when inhibition is strong no spikes are initiated.

Chemical stimulation of rat retinal neurons: feasibility of an epiretinal neurotransmitter-based prosthesis

OBJECTIVE

No cure currently exists for photoreceptor degenerative diseases, which cause partial or total blindness in millions of people worldwide. Electrical retinal prostheses have been developed by several groups with the goal of restoring vision lost to these diseases, but electrical stimulation has limitations. It excites both somas and axons, activating retinal pathways nonphysiologically, and limits spatial resolution because of current spread. Chemical stimulation of retinal ganglion cells (RGCs) using the neurotransmitter glutamate has been suggested as an alternative to electrical stimulation with some significant advantages. However, sufficient scientific data to support developing a chemical-based retinal prosthesis is lacking. The goal of this study was to investigate the feasibility of a neurotransmitter-based retinal prosthesis and determine therapeutic stimulation parameters.

APPROACH

We injected controlled amounts of glutamate into rat retinas from the epiretinal side ex vivo via micropipettes using a pressure injection system and recorded RGC responses with a multielectrode array. Responsive units were identified using a spike rate threshold of 3 Hz.

MAIN RESULTS

We recorded both somal and axonal units and demonstrated successful glutamatergic stimulation across different RGC subtypes. Analyses show that exogenous glutamate acts on RGC synapses similar to endogenous glutamate and, unlike electrical prostheses, stimulates only RGC somata. The spatial spread of glutamate stimulation was ≈ 290 μm from the injection site, comparable to current electrical prostheses. Further, the glutamate injections produced spatially differential responses in OFF, ON, and ON-OFF RGC subtypes, suggesting that differential stimulation of the OFF and ON systems may be possible. A temporal resolution of 3.2 Hz was obtained, which is a rate suitable for spatial vision.

SIGNIFICANCE

We provide strong support for the feasibility of an epiretinal neurotransmitter-based retinal prosthesis. Our findings suggest that chemical stimulation of RGCs is a viable alternative to electrical stimulation and could offer distinct advantages such as the selective stimulation of RGC somata.

Seeing the whole picture: A comprehensive imaging approach to functional mapping of circuits in behaving zebrafish

In recent years, the zebrafish has emerged as an appealing model system to tackle questions relating to the neural circuit basis of behavior. This can be attributed not just to the growing use of genetically tractable model organisms, but also in large part to the rapid advances in optical techniques for neuroscience, which are ideally suited for application to the small, transparent brain of the larval fish. Many characteristic features of vertebrate brains, from gross anatomy down to particular circuit motifs and cell-types, as well as conserved behaviors, can be found in zebrafish even just a few days post fertilization, and, at this early stage, the physical size of the brain makes it possible to analyze neural activity in a comprehensive fashion. In a recent study, we used a systematic and unbiased imaging method to record the pattern of activity dynamics throughout the whole brain of larval zebrafish during a simple visual behavior, the optokinetic response (OKR). This approach revealed the broadly distributed network of neurons that were active during the behavior and provided insights into the fine-scale functional architecture in the brain, inter-individual variability, and the spatial distribution of behaviorally relevant signals. Combined with mapping anatomical and functional connectivity, targeted electrophysiological recordings, and genetic labeling of specific populations, this comprehensive approach in zebrafish provides an unparalleled opportunity to study complete circuits in a behaving vertebrate animal.

Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells

Most retinal bipolar cells (BCs) transmit visual input from photoreceptors to ganglion cells using graded potentials, but some also generate calcium or sodium spikes. Sodium spikes are thought to increase temporal precision of light-evoked BC signaling; however, the role of calcium spikes in BCs is not fully understood. Here we studied how calcium spikes and graded responses mediate neurotransmitter release from Mb-type BCs, known to produce both. In dark-adapted goldfish retinal slices, light induced spikes in 40% of the axon terminals of intact Mbs; in the rest, light generated graded responses. These light-evoked membrane potentials were used to depolarize axotomized Mb terminals where depolarization-evoked calcium current (ICa) and consequent exocytosis-associated membrane capacitance increases (ΔCm) could be precisely measured. When evoked by identical dim light intensities, spiking responses transferred more calcium (Q(Ca)) and triggered larger exocytosis with higher efficiency (ΔCm/Q(Ca)) than graded potentials. Q(Ca) was translated into exocytosis linearly when transferred with spikes and supralinearly when transferred with graded responses. At the Mb output (ΔCm), spiking responses coded light intensity with numbers and amplitude whereas graded responses coded with amplitude, duration, and steepness. Importantly, spiking responses saturated exocytosis within scotopic range but graded potentials did not. We propose that calcium spikes in Mbs increase signal input-output ratio by boosting Mb glutamate release at threshold intensities. Therefore, spiking Mb responses are suitable to transfer low-light-intensity signals to ganglion cells with higher gain, whereas graded potentials signal for light over a wider range of intensities at the Mb output.

A synaptic mechanism for temporal filtering of visual signals

The visual system transmits information about fast and slow changes in light intensity through separate neural pathways. We used in vivo imaging to investigate how bipolar cells transmit these signals to the inner retina. We found that the volume of the synaptic terminal is an intrinsic property that contributes to different temporal filters. Individual cells transmit through multiple terminals varying in size, but smaller terminals generate faster and larger calcium transients to trigger vesicle release with higher initial gain, followed by more profound adaptation. Smaller terminals transmitted higher stimulus frequencies more effectively. Modeling global calcium dynamics triggering vesicle release indicated that variations in the volume of presynaptic compartments contribute directly to all these differences in response dynamics. These results indicate how one neuron can transmit different temporal components in the visual signal through synaptic terminals of varying geometries with different adaptational properties.

Building blocks of temporal filters in retinal synapses

We discuss recent findings that uncover how the physical size of synaptic terminals contributes to the temporal filtering of retinal synapses.

Sensory systems must be able to extract features of a stimulus to detect and represent properties of the world. Because sensory signals are constantly changing, a critical aspect of this transformation relates to the timing of signals and the ability to filter those signals to select dynamic properties, such as visual motion. At first assessment, one might think that the primary biophysical properties that construct a temporal filter would be dynamic mechanisms such as molecular concentration or membrane electrical properties. However, in the current issue of PLOS Biology, Baden et al. identify a mechanism of temporal filtering in the zebrafish and goldfish retina that is not dynamic but is in fact a structural building block—the physical size of a synapse itself. The authors observe that small, bipolar cell synaptic terminals are fast and highly adaptive, whereas large ones are slower and adapt less. Using a computational model, they conclude that the volume of the synaptic terminal influences the calcium concentration and the number of available vesicles. These results indicate that the size of the presynaptic terminal is an independent control for the dynamics of a synapse and may reveal aspects of synaptic function that can be inferred from anatomical structure.

Rapid mapping of visual receptive fields by filtered back projection: application to multi-neuronal electrophysiology and imaging

Neurons in the visual system vary widely in the spatiotemporal properties of their receptive fields (RFs), and understanding these variations is key to elucidating how visual information is processed. We present a new approach for mapping RFs based on the filtered back projection (FBP), an algorithm used for tomographic reconstructions. To estimate RFs, a series of bars were flashed across the retina at pseudo-random positions and at a minimum of five orientations. We apply this method to retinal neurons and show that it can accurately recover the spatial RF and impulse response of ganglion cells recorded on a multi-electrode array. We also demonstrate its utility for in vivo imaging by mapping the RFs of an array of bipolar cell synapses expressing a genetically encoded Ca²⁺ indicator. We find that FBP offers several advantages over the commonly used spike-triggered average (STA): (i) ON and OFF components of a RF can be separated; (ii) the impulse response can be reconstructed at sample rates of 125 Hz, rather than the refresh rate of a monitor; (iii) FBP reveals the response properties of neurons that are not evident using STA, including those that display orientation selectivity, or fire at low mean spike rates; and (iv) the FBP method is fast, allowing the RFs of all the bipolar cell synaptic terminals in a field of view to be reconstructed in under 4 min. Use of the FBP will benefit investigations of the visual system that employ electrophysiology or optical reporters to measure activity across populations of neurons.

Roles of ON cone bipolar cell subtypes in temporal coding in the mouse retina

In the visual system, diverse image processing starts with bipolar cells, which are the second-order neurons of the retina. Thirteen subtypes of bipolar cells have been identified, which are thought to encode different features of image signaling and to initiate distinct signal-processing streams. Although morphologically identified, the functional roles of each bipolar cell subtype in visual signal encoding are not fully understood. Here, we investigated how ON cone bipolar cells of the mouse retina encode diverse temporal image signaling. We recorded bipolar cell voltage changes in response to two different input functions: sinusoidal light and step light stimuli. Temporal tuning in ON cone bipolar cells was diverse and occurred in a subtype-dependent manner. Subtypes 5s and 8 exhibited low-pass filtering property in response to a sinusoidal light stimulus, and responded with sustained fashion to step-light stimulation. Conversely, subtypes 5f, 6, 7, and XBC exhibited bandpass filtering property in response to sinusoidal light stimuli, and responded transiently to step-light stimuli. In particular, subtypes 7 and XBC were high-temporal tuning cells. We recorded responses in different ways to further examine the underlying mechanisms of temporal tuning. Current injection evoked low-pass filtering, whereas light responses in voltage-clamp mode produced bandpass filtering in all ON bipolar cells. These findings suggest that cone photoreceptor inputs shape bandpass filtering in bipolar cells, whereas intrinsic properties of bipolar cells shape low-pass filtering. Together, our results demonstrate that ON bipolar cells encode diverse temporal image signaling in a subtype-dependent manner to initiate temporal visual information-processing pathways.

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.

Shape recognition elicited by microsecond flashes is not based on photon quantity

It is generally thought that the perceptual impact of a brief flash of light is determined by the quantity of photons the flash delivers. This means that only the total quantity of photons is important below a critical duration of about 30-100 ms. Recent findings have challenged this concept and the present work provides additional evidence that it is not correct. The first experiment reported here delivered a given quantity of photons in under 200 μs, either as a single threshold-intensity flash or as multiple flashes at the same intensity. The single flash was ineffective at eliciting recognition, but multiple flashes became progressively more effective as the number of flashes was increased. A second experiment varied the number of 10 μs flashes. The effectiveness of multiple flashes was far higher than would be expected on the basis of the total quantity of photons being delivered. The results of both experiments suggest that the brief transitions of intensity provided by the flashes are far more important than the quantity of photons. A final experiment examined the combined impact from two threshold-intensity flashes as the interstimulus interval was increased. The pair members were able to combine their influence for at least 100 ms. These results call for more attention to how very brief light flashes generate signals that convey image content.

Nitric oxide mediates activity-dependent plasticity of retinal bipolar cell output via S-nitrosylation

Coding a wide range of light intensities in natural scenes poses a challenge for the retina: adaptation to bright light should not compromise sensitivity to dim light. Here we report a novel form of activity-dependent synaptic plasticity, specifically, a "weighted potentiation" that selectively increases output of Mb-type bipolar cells in the goldfish retina in response to weak inputs but leaves the input-output ratio for strong stimuli unaffected. In retinal slice preparation, strong depolarization of bipolar terminals significantly lowered the threshold for calcium spike initiation, which originated from a shift in activation of voltage-gated calcium currents (ICa) to more negative potentials. The process depended upon glutamate-evoked retrograde nitric oxide (NO) signaling as it was eliminated by pretreatment with an NO synthase blocker, TRIM. The NO-dependent ICa modulation was cGMP independent but could be blocked by N-ethylmaleimide (NEM), indicating that NO acted via an S-nitrosylation mechanism. Importantly, the NO action resulted in a weighted potentiation of Mb output in response to small (≤-30 mV) depolarizations. Coincidentally, light flashes with intensity ≥ 2.4 × 10(8) photons/cm(2)/s lowered the latency of scotopic (≤ 2.4 × 10(8) photons/cm(2)/s) light-evoked calcium spikes in Mb axon terminals in an NEM-sensitive manner, but light responses above cone threshold (≥ 3.5 × 10(9) photons/cm(2)/s) were unaltered. Under bright scotopic/mesopic conditions, this novel form of Mb output potentiation selectively amplifies dim retinal inputs at Mb → ganglion cell synapses. We propose that this process might counteract decreases in retinal sensitivity during light adaptation by preventing the loss of visual information carried by dim scotopic signals.

NaV1.1 channels in axon initial segments of bipolar cells augment input to magnocellular visual pathways in the primate retina

In the primate visual system, the ganglion cells of the magnocellular pathway underlie motion and flicker detection and are relatively transient, while the more sustained ganglion cells of the parvocellular pathway have comparatively lower temporal resolution, but encode higher spatial frequencies. Although it is presumed that functional differences in bipolar cells contribute to the tuning of the two pathways, the properties of the relevant bipolar cells have not yet been examined in detail. Here, by making patch-clamp recordings in acute slices of macaque retina, we show that the bipolar cells within the magnocellular pathway, but not the parvocellular pathway, exhibit voltage-gated sodium (NaV), T-type calcium (CaV), and hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents, and can generate action potentials. Using immunohistochemistry in macaque and human retinae, we show that NaV1.1 is concentrated in an axon initial segment (AIS)-like region of magnocellular pathway bipolar cells, a specialization not seen in transient bipolar cells of other vertebrates. In contrast, CaV3.1 channels were localized to the somatodendritic compartment and proximal axon, but were excluded from the AIS, while HCN1 channels were concentrated in the axon terminal boutons. Simulations using a compartmental model reproduced physiological results and indicate that magnocellular pathway bipolar cells initiate spikes in the AIS. Finally, we demonstrate that NaV channels in bipolar cells augment excitatory input to parasol ganglion cells of the magnocellular pathway. Overall, the results demonstrate that selective expression of voltage-gated channels contributes to the establishment of parallel processing in the major visual pathways of the primate retina.

Light adaptation increases response latency of alpha ganglion cells via a threshold-like nonlinearity

Adaptation is an important process of sensory systems to adjust sensitivity to ensure the appropriate information encoding. Sensitivity and kinetics of retinal ganglion cell (RGC) responses have been studied extensively using a brief flash superimposed on different but steady backgrounds. However, it is still unclear if light adaptation exerts any effect on more complex response properties, such as response nonlinearity. In this study, we found that the latency of spike responses to a repeated flashing spot stimulation increased by 30 ms in the mouse ON α RGCs (An ON-type RGC is excited when a spot is turned on in the center of its receptive field). A single dimming event preceding the test flash on a steady adapting background could also produce similar effect in increasing latency of light responses. A simple computational model with a linear transformation of the light stimulus and a threshold-like nonlinearity could account for the experimental data. Moreover, the strength of the measured nonlinearity and the response latency were affected by the duration of light adaptation. The possible biological processes underlying this nonlinearity were explored. Voltage clamp recording revealed the presence of the increase in latency and threshold-like nonlinearity in the excitatory input of RGCs. However, no comparable nonlinearity was observed in the light responses of the ON cone bipolar cells. We further excluded GABAergic and glycinergic inhibition, N-methyl-D-aspartate receptor rectification and voltage-gated Na(+) channels as potential sources of this nonlinearity by pharmacological experiments. Our results indicate the bipolar cell terminals as the potential site of nonlinearity. Computational modeling constrained by experimental data supports that conclusion and suggests the voltage-sensitive Ca(++) channels and Ca(++)-dependent vesicle release in the bipolar cell terminals as mechanistic basis.

Two-photon imaging of nonlinear glutamate release dynamics at bipolar cell synapses in the mouse retina

Alpha/Y-type retinal ganglion cells encode visual information with a receptive field composed of nonlinear subunits. This nonlinear subunit structure enhances sensitivity to patterns composed of high spatial frequencies. The Y-cell's subunits are the presynaptic bipolar cells, but the mechanism for the nonlinearity remains incompletely understood. We investigated the synaptic basis of the subunit nonlinearity by combining whole-cell recording of mouse Y-type ganglion cells with two-photon fluorescence imaging of a glutamate sensor (iGluSnFR) expressed on their dendrites and throughout the inner plexiform layer. A control experiment designed to assess iGluSnFR's dynamic range showed that fluorescence responses from Y-cell dendrites increased proportionally with simultaneously recorded excitatory current. Spatial resolution was sufficient to readily resolve independent release at intermingled ON and OFF bipolar terminals. iGluSnFR responses at Y-cell dendrites showed strong surround inhibition, reflecting receptive field properties of presynaptic release sites. Responses to spatial patterns located the origin of the Y-cell nonlinearity to the bipolar cell output, after the stage of spatial integration. The underlying mechanism differed between OFF and ON pathways: OFF synapses showed transient release and strong rectification, whereas ON synapses showed relatively sustained release and weak rectification. At ON synapses, the combination of fast release onset with slower release offset explained the nonlinear response of the postsynaptic ganglion cell. Imaging throughout the inner plexiform layer, we found transient, rectified release at the central-most levels, with increasingly sustained release near the borders. By visualizing glutamate release in real time, iGluSnFR provides a powerful tool for characterizing glutamate synapses in intact neural circuits.

Olfactory stimulation selectively modulates the OFF pathway in the retina of zebrafish

Cross-modal regulation of visual performance by olfactory stimuli begins in the retina, where dopaminergic interneurons receive projections from the olfactory bulb. However, we do not understand how olfactory stimuli alter the processing of visual signals within the retina. We investigated this question by in vivo imaging activity in transgenic zebrafish expressing SyGCaMP2 in bipolar cell terminals and GCaMP3.5 in ganglion cells. The food-related amino acid methionine reduced the gain and increased sensitivity of responses to luminance and contrast transmitted through OFF bipolar cells but not ON. The effects of olfactory stimulus were blocked by inhibiting dopamine uptake and release. Activation of dopamine receptors increased the gain of synaptic transmission in vivo and potentiated synaptic calcium currents in isolated bipolar cells. These results indicate that olfactory stimuli alter the sensitivity of the retina through the dopaminergic regulation of presynaptic calcium channels that control the gain of synaptic transmission through OFF bipolar cells.

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Olfactory stimuli regulate transmission of signals through retinal bipolar cells

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Modulation of synaptic gain and sensitivity occur in OFF bipolar cells but not ON

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An inhibitor of dopamine uptake blocks odor-induced changes in synaptic gain

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Dopamine potentiates presynaptic calcium channels in isolated bipolar cells

Synaptic mechanisms of adaptation and sensitization in the retina

Sensory systems continually adjust the way stimuli are processed. What are the circuit mechanisms underlying this plasticity? We investigated how synapses in the retina of zebrafish adjust to changes in the temporal contrast of a visual stimulus by imaging activity in vivo. Following an increase in contrast, bipolar cell synapses with strong initial responses depressed, whereas synapses with weak initial responses facilitated. Depression and facilitation predominated in different strata of the inner retina, where bipolar cell output was anticorrelated with the activity of amacrine cell synapses providing inhibitory feedback. Pharmacological block of GABAergic feedback converted facilitating bipolar cell synapses into depressing ones. These results indicate that depression intrinsic to bipolar cell synapses causes adaptation of the ganglion cell response to contrast, whereas depression in amacrine cell synapses causes sensitization. Distinct microcircuits segregating to different layers of the retina can cause simultaneous increases or decreases in the gain of neural responses.

Two-photon imaging of neural population activity in zebrafish

Rapidly developing imaging technologies including two-photon microscopy and genetically encoded calcium indicators have opened up new possibilities for recording neural population activity in awake, behaving animals. In the small, transparent zebrafish, it is even becoming possible to image the entire brain of a behaving animal with single-cell resolution, creating brain-wide functional maps. In this chapter, we comprehensively review past functional imaging studies in zebrafish, and the insights that they provide into the functional organization of neural circuits. We further offer a basic primer on state-of-the-art methods for in vivo calcium imaging in the zebrafish, including building a low-cost two-photon microscope and highlight possible challenges and technical considerations.

Features and functions of nonlinear spatial integration by retinal ganglion cells

Ganglion cells in the vertebrate retina integrate visual information over their receptive fields. They do so by pooling presynaptic excitatory inputs from typically many bipolar cells, which themselves collect inputs from several photoreceptors. In addition, inhibitory interactions mediated by horizontal cells and amacrine cells modulate the structure of the receptive field. In many models, this spatial integration is assumed to occur in a linear fashion. Yet, it has long been known that spatial integration by retinal ganglion cells also incurs nonlinear phenomena. Moreover, several recent examples have shown that nonlinear spatial integration is tightly connected to specific visual functions performed by different types of retinal ganglion cells. This work discusses these advances in understanding the role of nonlinear spatial integration and reviews recent efforts to quantitatively study the nature and mechanisms underlying spatial nonlinearities. These new insights point towards a critical role of nonlinearities within ganglion cell receptive fields for capturing responses of the cells to natural and behaviorally relevant visual stimuli. In the long run, nonlinear phenomena of spatial integration may also prove important for implementing the actual neural code of retinal neurons when designing visual prostheses for the eye.