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

Specific retinal neurons regulate context-dependent defensive responses to visual threat

While encountering a visual threat, an animal assesses multiple factors to choose an appropriate defensive strategy. For example, when a rodent detects a looming aerial predator, its behavioral response can be influenced by a specific environmental context, such as the availability of a shelter. Indeed, rodents typically escape from a looming stimulus when a shelter is present; otherwise, they typically freeze. Here we report that context-dependent behavioral responses can be initiated at the earliest stage of the visual system by distinct types of retinal ganglion cells (RGCs), the retina's output neurons. Using genetically defined cell ablation in mature mice, we discovered that some RGC types were necessary for either escaping (alpha RGCs) or freezing (intrinsically photosensitive RGCs) in response to a looming stimulus but not for both behaviors; whereas other RGC types were not required for either behavior (direction-selective RGCs preferring vertical motion). Altogether, our results suggest that specific RGC types regulate distinct behavioral responses elicited by the same threatening stimulus depending on contextual signals in the environment. These findings emphasize the unique contribution of early visual pathways to evolutionally conserved behavioral reactions.

Retinal Connectomics: A Review

The retina is an ideal model for understanding the fundamental rules for how neural networks are constructed. The compact neural networks of the retina perform all of the initial processing of visual information before transmission to higher visual centers in the brain. The field of retinal connectomics uses high-resolution electron microscopy datasets to map the intricate organization of these networks and further our understanding of how these computations are performed by revealing the fundamental topologies and allowable networks behind retinal computations. In this article, we review some of the notable advances that retinal connectomics has provided in our understanding of the specific cells and the organization of their connectivities within the retina, as well as how these are shaped in development and break down in disease. Using these anatomical maps to inform modeling has been, and will continue to be, instrumental in understanding how the retina processes visual signals.

The Structural and Functional Integrity of Rod Photoreceptor Ribbon Synapses Depends on Redundant Actions of Dynamins 1 and 3

Vertebrate vision begins with light absorption by rod and cone photoreceptors, which transmit signals from their synaptic terminals to second-order neurons: bipolar and horizontal cells. In mouse rods, there is a single presynaptic ribbon-type active zone at which the release of glutamate occurs tonically in the dark. This tonic glutamatergic signaling requires continuous exo- and endocytosis of synaptic vesicles. At conventional synapses, endocytosis commonly requires dynamins: GTPases encoded by three genes (Dnm1-3), which perform membrane scission. Disrupting endocytosis by dynamin deletions impairs transmission at conventional synapses, but the impact of disrupting endocytosis and the role(s) of specific dynamin isoforms at rod ribbon synapses are understood incompletely. Here, we used cell-specific knock-outs (KOs) of the neuron-specific Dnm1 and Dnm3 to investigate the functional roles of dynamin isoforms in rod photoreceptors in mice of either sex. Analysis of synaptic protein expression, synapse ultrastructure, and retinal function via electroretinograms (ERGs) showed that dynamins 1 and 3 act redundantly and are essential for supporting the structural and functional integrity of rod ribbon synapses. Single Dnm3 KO showed no phenotype, and single Dnm1 KO only modestly reduced synaptic vesicle density without affecting vesicle size and overall synapse integrity, whereas double Dnm1/Dnm3 KO impaired vesicle endocytosis profoundly, causing enlarged vesicles, reduced vesicle density, reduced ERG responses, synaptic terminal degeneration, and disassembly and degeneration of postsynaptic processes. Concurrently, cone function remained intact. These results show the fundamental redundancy of dynamins 1 and 3 in regulating the structure and function of rod ribbon synapses.

Molecular identification of wide-field amacrine cells in mouse retina that encode stimulus orientation

Visual information processing is sculpted by a diverse group of inhibitory interneurons in the retina called amacrine cells. Yet, for most of the >60 amacrine cell types, molecular identities and specialized functional attributes remain elusive. Here, we developed an intersectional genetic strategy to target a group of wide-field amacrine cells (WACs) in mouse retina that co-express the transcription factor Bhlhe22 and the Kappa Opioid Receptor (KOR; B/K WACs). B/K WACs feature straight, unbranched dendrites spanning over 0.5 mm (∼15° visual angle) and produce non-spiking responses to either light increments or decrements. Two-photon dendritic population imaging reveals Ca 2+ signals tuned to the physical orientations of B/K WAC dendrites, signifying a robust structure-function alignment. B/K WACs establish divergent connections with multiple retinal neurons, including unexpected connections with non-orientation-tuned ganglion cells and bipolar cells. Our work sets the stage for future comprehensive investigations of the most enigmatic group of retinal neurons: WACs.

Layers of inhibitory networks shape receptive field properties of AII amacrine cells

In the retina, rod and cone pathways mediate visual signals over a billion-fold range in luminance. AII ("A-two") amacrine cells (ACs) receive signals from both pathways via different bipolar cells, enabling AIIs to operate at night and during the day. Previous work has examined luminance-dependent changes in AII gap junction connectivity, but less is known about how surrounding circuitry shapes AII receptive fields across light levels. Here, we report that moderate contrast stimuli elicit surround inhibition in AIIs under all but the dimmest visual conditions, due to actions of horizontal cells and at least two ACs that inhibit presynaptic bipolar cells. Under photopic (daylight) conditions, surround inhibition transforms AII response kinetics, which are inherited by downstream ganglion cells. Ablating neuronal nitric oxide synthase type-1 (nNOS-1) ACs removes AII surround inhibition under mesopic (dusk/dawn), but not photopic, conditions. Our findings demonstrate how multiple layers of neural circuitry interact to encode signals across a wide physiological range.

New insights into retinal circuits through EM connectomics: what we have learnt and what remains to be learned

The retinal neural circuit is intricately wired for efficient processing of visual signals. This is well-supported by the specialized connections between retinal neurons at both the functional and ultrastructural levels. Through 3D electron microscopic (EM) reconstructions of retinal neurons and circuits we have learnt much about the specificities of connections within the retinal layers including new insights into how retinal neurons establish connections and perform sophisticated visual computations. This mini-review will summarize the retinal circuitry and provide details about the novel insights EM connectomics has brought into our understanding of the retinal circuitry. We will also discuss unresolved questions about the retinal circuitry that can be addressed by EM connectomics in the future.

Functional properties of GABAA receptors of AII amacrine cells of the rat retina

Amacrine cells are a highly diverse group of inhibitory retinal interneurons that sculpt the responses of bipolar cells, ganglion cells, and other amacrine cells. They integrate excitatory inputs from bipolar cells and inhibitory inputs from other amacrine cells, but for most amacrine cells, little is known about the specificity and functional properties of their inhibitory inputs. Here, we have investigated GABAA receptors of the AII amacrine, a critical neuron in the rod pathway microcircuit, using patch-clamp recording in rat retinal slices. Puffer application of GABA evoked robust responses, but, surprisingly, spontaneous GABAA receptor-mediated postsynaptic currents were not observed, neither under control conditions nor following application of high-K+ solution to facilitate release. To investigate the biophysical and pharmacological properties of GABAA receptors in AIIs, we therefore used nucleated patches and a fast application system. Both brief and long pulses of GABA (3 mM) evoked GABAA receptor-mediated currents with slow, multi-exponential decay kinetics. The average weighted time constant (τw) of deactivation was ~163 ms. Desensitization was even slower, with τw ~330 ms. Non-stationary noise analysis of patch responses and directly observed channel gating yielded a single-channel conductance of ~23 pS. Pharmacological investigation suggested the presence of α2 and/or α3 subunits, as well as the γ2 subunit. Such subunit combinations are typical of GABAA receptors with slow kinetics. If synaptic GABAA receptors of AII amacrines have similar functional properties, the slow deactivation and desensitization kinetics will facilitate temporal summation of GABAergic inputs, allowing effective summation and synaptic integration to occur even for relatively low frequencies of inhibitory inputs.

Using optogenetics to dissect rod inputs to OFF ganglion cells in the mouse retina

Introduction

Light responses of rod photoreceptor cells traverse the retina through three pathways. The primary pathway involves synapses from rods to ON-type rod bipolar cells with OFF signals reaching retinal ganglion cells (RGCs) via sign-inverting glycinergic synapses. Secondly, rod signals can enter cones through gap junctions. Finally, rods can synapse directly onto cone OFF bipolar cells.

Methods

To analyze these pathways, we obtained whole cell recordings from OFF-type α RGCs in mouse retinas while expressing channelrhodopsin-2 in rods and/or cones.

Results

Optogenetic stimulation of rods or cones evoked large fast currents in OFF RGCs. Blocking the primary rod pathway with L-AP4 and/or strychnine reduced rod-driven optogenetic currents in OFF RGCs by ~1/3. Blocking kainate receptors of OFF cone bipolar cells suppressed both rod- and cone-driven optogenetic currents in OFF RGCs. Inhibiting gap junctions between rods and cones with mecloflenamic acid or quinpirole reduced rod-driven responses in OFF RGCs. Eliminating the exocytotic Ca²⁺ sensor, synaptotagmin 1 (Syt1), from cones abolished cone-driven optogenetic responses in RGCs. Rod-driven currents were not significantly reduced after isolating the secondary pathway by eliminating Syt1 and synaptotagmin 7 (Syt7) to block synaptic release from rods. Eliminating Syt1 from both rods and cones abolished responses to optogenetic stimulation. In Cx36 KO retinas lacking rod-cone gap junctions, optogenetic activation of rods evoked small and slow responses in most OFF RGCs suggesting rod signals reached them through an indirect pathway. Two OFF cells showed faster responses consistent with more direct input from cone OFF bipolar cells.

Discussion

These data show that the secondary rod pathway supports robust inputs into OFF α RGCs and suggests the tertiary pathway recruits both direct and indirect inputs.

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogenous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signaling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA_A receptors of A17 amacrines, using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10-90% rise time ~740 μs) and double-exponential decays (τ_fast ~4.5 ms [43% of amplitude]; τ_slow ~22 ms). Ultrafast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ₁ ~5.3 ms [55% of amplitude]; τ₂ ~48 ms [32% amplitude]; τ₃ ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1βγ2 subunit composition and the somatic receptors have an α2βγ2 and/or α3βγ2 composition. These results demonstrate the presence of synaptic GABA_A receptors on A17s, which may play an important role in signal integration in these cells.

Dendro-somatic synaptic inputs to ganglion cells contradict receptive field and connectivity conventions in the mammalian retina

The morphology of retinal neurons strongly influences their physiological function. Ganglion cell (GC) dendrites ramify in distinct strata of the inner plexiform layer (IPL) so that GCs responding to light increments (ON) or decrements (OFF) receive appropriate excitatory inputs. This vertical stratification prescribes response polarity and ensures consistent connectivity between cell types, whereas the lateral extent of GC dendritic arbors typically dictates receptive field (RF) size. Here, we identify circuitry in mouse retina that contradicts these conventions. AII amacrine cells are interneurons understood to mediate "crossover" inhibition by relaying excitatory input from the ON layer to inhibitory outputs in the OFF layer. Ultrastructural and physiological analyses show, however, that some AIIs deliver powerful inhibition to OFF GC somas and proximal dendrites in the ON layer, rendering the inhibitory RFs of these GCs smaller than their dendritic arbors. This OFF pathway, avoiding entirely the OFF region of the IPL, challenges several tenets of retinal circuitry. These results also indicate that subcellular synaptic organization can vary within a single population of neurons according to their proximity to potential postsynaptic targets.

Dopamine D1 and D4 receptors contribute to light adaptation in ON-sustained retinal ganglion cells

The adaptation of ganglion cells to increasing light levels is a crucial property of the retina. The retina must respond to light intensities that vary by 10-12 orders of magnitude, but the dynamic range of ganglion cell responses covers only ∼3 orders of magnitude. Dopamine is a crucial neuromodulator for light adaptation and activates receptors in the D1 and D2 families. Dopamine type D1 receptors (D1Rs) are expressed on horizontal cells and some bipolar, amacrine, and ganglion cells. In the D2 family, D2Rs are expressed on dopaminergic amacrine cells and D4Rs are primarily expressed on photoreceptors. However, the roles of activating these receptors to modulate the synaptic properties of the inputs to ganglion cells are not yet clear. Here, we used single-cell retinal patch-clamp recordings from the mouse retina to determine how activating D1Rs and D4Rs changed the light-evoked and spontaneous excitatory inputs to ON-sustained (ON-s) ganglion cells. We found that both D1R and D4R activation decrease the light-evoked excitatory inputs to ON-s ganglion cells, but that only the sum of the peak response decrease due to activating the two receptors was similar to the effect of light adaptation to a rod-saturating background. The largest effects on spontaneous excitatory activity of both D1R and D4R agonists was on the frequency of events, suggesting that both D1Rs and D4Rs are acting upstream of the ganglion cells.NEW & NOTEWORTHY Dopamine by bright light conditions allows retinal neurons to reduce sensitivity to adapt to bright light conditions. It is not clear how and why dopamine receptors modulate retinal ganglion cell signaling. We found that both D1 and D4 dopamine receptors in photoreceptors and inner retinal neurons contribute significantly to the reduction in sensitivity of ganglion cells with light adaptation. However, light adaptation also requires dopamine-independent mechanisms that could reflect inherent sensitivity changes in photoreceptors.

Computational and Molecular Properties of Starburst Amacrine Cell Synapses Differ With Postsynaptic Cell Type

A presynaptic neuron can increase its computational capacity by transmitting functionally distinct signals to each of its postsynaptic cell types. To determine whether such computational specialization occurs over fine spatial scales within a neurite arbor, we investigated computation at output synapses of the starburst amacrine cell (SAC), a critical component of the classical direction-selective (DS) circuit in the retina. The SAC is a non-spiking interneuron that co-releases GABA and acetylcholine and forms closely spaced (<5 μm) inhibitory synapses onto two postsynaptic cell types: DS ganglion cells (DSGCs) and neighboring SACs. During dynamic optogenetic stimulation of SACs in mouse retina, whole-cell recordings of inhibitory postsynaptic currents revealed that GABAergic synapses onto DSGCs exhibit stronger low-pass filtering than those onto neighboring SACs. Computational analyses suggest that this filtering difference can be explained primarily by presynaptic properties, rather than those of the postsynaptic cells per se. Consistent with functionally diverse SAC presynapses, blockade of N-type voltage-gated calcium channels abolished GABAergic currents in SACs but only moderately reduced GABAergic and cholinergic currents in DSGCs. These results jointly demonstrate how specialization of synaptic outputs could enhance parallel processing in a compact interneuron over fine spatial scales. Moreover, the distinct transmission kinetics of GABAergic SAC synapses are poised to support the functional diversity of inhibition within DS circuitry.

Photoreceptive Ganglion Cells Drive Circuits for Local Inhibition in the Mouse Retina

Intrinsically photosensitive retinal ganglion cells (ipRGCs) exhibit melanopsin-dependent light responses that persist in the absence of rod and cone photoreceptor-mediated input. In addition to signaling anterogradely to the brain, ipRGCs signal retrogradely to intraretinal circuitry via gap junction-mediated electrical synapses with amacrine cells (ACs). However, the targets and functions of these intraretinal signals remain largely unknown. Here, in mice of both sexes, we identify circuitry that enables M5 ipRGCs to locally inhibit retinal neurons via electrical synapses with a nonspiking GABAergic AC. During pharmacological blockade of rod- and cone-mediated input, whole-cell recordings of corticotropin-releasing hormone-expressing (CRH+) ACs reveal persistent visual responses that require both melanopsin expression and gap junctions. In the developing retina, ipRGC-mediated input to CRH+ ACs is weak or absent before eye opening, indicating a primary role for this input in the mature retina (i.e., in parallel with rod- and cone-mediated input). Among several ipRGC types, only M5 ipRGCs exhibit consistent anatomical and physiological coupling to CRH+ ACs. Optogenetic stimulation of local CRH+ ACs directly drives IPSCs in M4 and M5, but not M1-M3, ipRGCs. CRH+ ACs also inhibit M2 ipRGC-coupled spiking ACs, demonstrating direct interaction between discrete networks of ipRGC-coupled interneurons. Together, these results demonstrate a functional role for electrical synapses in translating ipRGC activity into feedforward and feedback inhibition of local retinal circuits.SIGNIFICANCE STATEMENT Melanopsin directly generates light responses in intrinsically photosensitive retinal ganglion cells (ipRGCs). Through gap junction-mediated electrical synapses with retinal interneurons, these uniquely photoreceptive RGCs may also influence the activity and output of neuronal circuits within the retina. Here, we identified and studied an electrical synaptic circuit that, in principle, could couple ipRGC activity to the chemical output of an identified retinal interneuron. Specifically, we found that M5 ipRGCs form electrical synapses with corticotropin-releasing hormone-expressing amacrine cells, which locally release GABA to inhibit specific RGC types. Thus, ipRGCs are poised to influence the output of diverse retinal circuits via electrical synapses with interneurons.

Calmodulin Bidirectionally Regulates Evoked and Spontaneous Neurotransmitter Release at Retinal Ribbon Synapses

For decades, a role for the Ca²⁺-binding protein calmodulin (CaM) in Ca²⁺-dependent presynaptic modulation of synaptic transmission has been recognized. Here, we investigated the influence of CaM on evoked and spontaneous neurotransmission at rod bipolar (RB) cell→AII amacrine cell synapses in the mouse retina. Our work was motivated by the observations that expression of CaM in RB axon terminals is extremely high and that [Ca²⁺] in RB terminals normally rises sufficiently to saturate endogenous buffers, making tonic CaM activation likely. Taking advantage of a model in which RBs can be stimulated by expressed channelrhodopsin-2 (ChR2) to avoid dialysis of the presynaptic terminal, we found that inhibition of CaM dramatically decreased evoked release by inhibition of presynaptic Ca channels while at the same time potentiating both Ca²⁺-dependent and Ca²⁺-independent spontaneous release. Remarkably, inhibition of myosin light chain kinase (MLCK), but not other CaM-dependent targets, mimicked the effects of CaM inhibition on evoked and spontaneous release. Importantly, initial antagonism of CaM occluded the effect of subsequent inhibition of MLCK on spontaneous release. We conclude that CaM, by acting through MLCK, bidirectionally regulates evoked and spontaneous release at retinal ribbon synapses.