The role of starburst amacrine cells in visual signal processing

Does heterogeneity of intracellular Ca[Formula: see text] dynamics underlie speed tuning of direction-selective responses in starburst amacrine cells?

The starburst amacrine cell (SAC) plays a fundamental role in retinal motion perception. In the vertebrate retina, SAC dendrites have been shown to be directionally selective in terms of their Ca[Formula: see text] responses for stimuli that move centrifugally from the soma. The mechanism by which SACs show Ca[Formula: see text] bias for centrifugal motion is yet to be determined with precision. Recent morphological studies support a presynaptic delay in glutamate receptor activation induced Ca[Formula: see text] release from bipolar cells preferentially contacting SACs. However, bipolar cells are known to be electrotonically coupled so time delays between the bipolar cells that provide input to SACs seem unlikely. Using fluorescent microscopy and imunnostaining, we found that the endoplasmic reticulum (ER) is omnipresent in the soma extending to the distal processes of SACs. Consequently, a working hypothesis on heterogeneity of intracellular Ca[Formula: see text] dynamics from ER is proposed as a possible explanation for the cause of speed tuning of direction-selective Ca[Formula: see text] responses in dendrites of SACs.

Protocadherin-dependent dendritic self-avoidance regulates neural connectivity and circuit function

Dendritic and axonal arbors of many neuronal types exhibit self-avoidance, in which branches repel each other. In some cases, these neurites interact with those of neighboring neurons, a phenomenon called self/non-self discrimination. The functional roles of these processes remain unknown. In this study, we used retinal starburst amacrine cells (SACs), critical components of a direction-selective circuit, to address this issue. In SACs, both processes are mediated by the gamma-protocadherins (Pcdhgs), a family of 22 recognition molecules. We manipulated Pcdhg expression in SACs and recorded from them and their targets, direction-selective ganglion cells (DSGCs). SACs form autapses when self-avoidance is disrupted and fail to form connections with other SACs when self/non-self discrimination is perturbed. Pcdhgs are also required to prune connections between closely spaced SACs. These alterations degrade the direction selectivity of DSGCs. Thus, self-avoidance, self/non-self discrimination, and synapse elimination are essential for proper function of a circuit that computes directional motion.

DOI:http://dx.doi.org/10.7554/eLife.08964.001

Nerve cells (or neurons) connect to one another to form circuits that control the animal's behavior. Typically, each neuron receives signals from other cells via branch-like structures called dendrites. Each specific type of neuron has a characteristic pattern of branched dendrites, which is different from the pattern of other types of neuron. Therefore, it is reasonable to imagine that the shape of these branches can influence how the neuron works; however, this idea has rarely been tested experimentally.

Different processes are known to act together to control the pattern of the branched dendrites. For example, dendrites in some neurons avoid other dendrites from the same neuron. This phenomenon is referred to as ‘self-avoidance’. In some of these cases, the same dendrites freely interact with the dendrites of neighboring neurons of the same type; this is called ‘self/non-self discrimination’. It is not clear, however, how these two processes influence the activity of neural circuits.

Both self-avoidance and self/non-self discrimination rely on the expression of genes that encode so-called recognition molecules. Kostadinov and Sanes have now altered the expression of these genes in mice to see the effect that disrupting these two phenomena has on a set of neurons called ‘starburst amacrine cells’ that are found at the back the eye. The dendrites of starburst amacrine cells generate signals when objects move across the animal's field of vision. These dendrites then signal to other starburst amacrine cells and to so-called ‘direction-selective ganglion cells’, which in turn send this information to the brain for further processing. The experiments revealed that these disruptions affected the connections between the dendrites. Starburst amacrine cells that lacked self-avoidance mistakenly formed connections with themselves—as if they mistook their own dendrites for those of other starburst cells. In contrast, neurons that lacked self/non-self discrimination made the opposite mistake, and rarely formed connections with each other—as if they mistook the dendrites of other starbursts for their own. Disruptions to either phenomenon interfered with the activity of the direction-selective ganglion cells.

Following on from the work of Kostadinov and Sanes, the next challenges include uncovering how the recognition molecules help with self-avoidance and self/non-self discrimination. It will also be important to examine whether the conclusions based on one type of neurons can be generalized to others that also exhibit these two phenomena.

DOI:http://dx.doi.org/10.7554/eLife.08964.002

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.

Sox2 regulates cholinergic amacrine cell positioning and dendritic stratification in the retina

The retina contains two populations of cholinergic amacrine cells, one positioned in the ganglion cell layer (GCL) and the other in the inner nuclear layer (INL), that together comprise ∼1/2 of a percent of all retinal neurons. The present study examined the genetic control of cholinergic amacrine cell number and distribution between these two layers. The total number of cholinergic amacrine cells was quantified in the C57BL/6J and A/J inbred mouse strains, and in 25 recombinant inbred strains derived from them, and variations in their number and ratio (GCL/INL) across these strains were mapped to genomic loci. The total cholinergic amacrine cell number was found to vary across the strains, from 27,000 to 40,000 cells, despite little variation within individual strains. The number of cells was always lower within the GCL relative to the INL, and the sizes of the two populations were strongly correlated, yet there was variation in their ratio between the strains. Approximately 1/3 of that variation in cell ratio was mapped to a locus on chromosome 3, where Sex determining region Y box 2 (Sox2) was identified as a candidate gene due to the presence of a 6-nucleotide insertion in the protein-coding sequence in C57BL/6J and because of robust and selective expression in cholinergic amacrine cells. Conditionally deleting Sox2 from the population of nascent cholinergic amacrine cells perturbed the normal ratio of cells situated in the GCL versus the INL and induced a bistratifying morphology, with dendrites distributed to both ON and OFF strata within the inner plexiform layer.

Visual stimulation switches the polarity of excitatory input to starburst amacrine cells

Direction-selective ganglion cells (DSGCs) are tuned to motion in one direction. Starburst amacrine cells (SACs) are thought to mediate this direction selectivity through precise anatomical wiring to DSGCs. Nevertheless, we previously found that visual adaptation can reverse DSGCs's directional tuning, overcoming the circuit anatomy. Here we explore the role of SACs in the generation and adaptation of direction selectivity. First, using pharmacogenetics and two-photon calcium imaging, we validate that SACs are necessary for direction selectivity. Next, we demonstrate that exposure to an adaptive stimulus dramatically alters SACs' synaptic inputs. Specifically, after visual adaptation, On-SACs lose their excitatory input during light onset but gain an excitatory input during light offset. Our data suggest that visual stimulation alters the interactions between rod- and cone-mediated inputs that converge on the terminals of On-cone BCs. These results demonstrate how the sensory environment can modify computations performed by anatomically defined neuronal circuits.

Development of Retinal Amacrine Cells and Their Dendritic Stratification

Themammalian retina containsmultiple neurons, each of which contributes differentially to visual processing. Of these retinal neurons, amacrine cells have recently come to prime light since they facilitate majority of visual processing that takes place in the retina. Amacrine cells are also the most diverse group of neurons in the retina, classified majorly based on the neurotransmitter type they express and morphology of their dendritic arbors. Currently, little is known about the molecular basis contributing to this diversity during development. Amacrine cells also contribute to most of the synapses in the inner plexiform layer and mediate visual information input from bipolar cells onto retinal ganglion cells. In this review, we will describe the current understanding of amacrine cell and cell subtype development. Furthermore, we will address the molecular basis of retinal lamination at the inner plexiform layer. Overall, our review will provide a developmental perspective of amacrine cell subtype classification and their dendritic stratification.

Molecular changes and vision loss in a mouse model of closed-globe blast trauma

PURPOSE

To characterize retinal changes and assess vision after an eye-directed air blast.

METHODS

Adult C57Bl/6 mice were exposed to a blast directed at one eye. Optical coherence tomography and histology were performed to assess retina and optic nerve integrity. Cell death, oxidative stress, and glial reactivity were examined by immunohistochemistry. Visual changes were measured by ERG recordings and the optokinetic reflex.

RESULTS

In the outer retina, eye blast caused retinal pigment epithelium vacuoles and rare retinal detachments followed by regional cell death. Labeling for nitrotyrosine and markers of pyroptosis (caspase-1) and necroptosis (receptor-interacting protein kinases-1, -3) increased, primarily in the inner retina, after blast. Caspase-1 labeling was restricted primarily to the starburst amacrine cells. A few degenerating axons were detected at 28 days post blast. Despite a lack of substantial cell death or decreased ERG, there was a deficit in visual acuity after blast.

CONCLUSIONS

Oxidative stress, neuroinflammation, and cell death became increasingly prevalent, over time post blast suggestive of an ongoing neurodegenerative response. Outer retinal changes either resolved or remained focal. In contrast, inner retinal changes were more robust and spread from focal regions to the entire retina over time post blast. Our model of eye blast trauma causes molecular changes and a decrease in visual acuity within the first month post blast despite a lack of overt eye injury. This subtle response matches the delayed presentation of visual deficits in some blast-exposed Veterans.

Neuronal expression of muscle LIM protein in postnatal retinae of rodents

Muscle LIM protein (MLP) is a member of the cysteine rich protein family and has so far been regarded as a muscle-specific protein that is mainly involved in myogenesis and the organization of cytoskeletal structure in myocytes, respectively. The current study demonstrates for the first time that MLP expression is not restricted to muscle tissue, but is also found in the rat naive central nervous system. Using quantitative PCR, Western blot and immunohistochemical analyses we detected MLP in the postnatal rat retina, specifically in the somas and dendritic arbors of cholinergic amacrine cells (AC) of the inner nuclear layer and the ganglion cell layer (displaced AC). Induction of MLP expression started at embryonic day 20 and peaked between postnatal days 7 and 14. It subsequently decreased again to non-detectable protein levels after postnatal day 28. MLP was identified in the cytoplasm and dendrites but not in the nucleus of AC. Thus, retinal MLP expression correlates with the morphologic and functional development of cholinergic AC, suggesting a potential role of this protein in postnatal maturation and making MLP a suitable marker for these neurons.

Attractor dynamics in local neuronal networks

Patterns of synaptic connectivity in various regions of the brain are characterized by the presence of synaptic motifs, defined as unidirectional and bidirectional synaptic contacts that follow a particular configuration and link together small groups of neurons. Recent computational work proposes that a relay network (two populations communicating via a third, relay population of neurons) can generate precise patterns of neural synchronization. Here, we employ two distinct models of neuronal dynamics and show that simulated neural circuits designed in this way are caught in a global attractor of activity that prevents neurons from modulating their response on the basis of incoming stimuli. To circumvent the emergence of a fixed global attractor, we propose a mechanism of selective gain inhibition that promotes flexible responses to external stimuli. We suggest that local neuronal circuits may employ this mechanism to generate precise patterns of neural synchronization whose transient nature delimits the occurrence of a brief stimulus.

An allosteric regulator of R7-RGS proteins influences light-evoked activity and glutamatergic waves in the inner retina

In the outer retina, G protein-coupled receptor (GPCR) signaling mediates phototransduction and synaptic transmission between photoreceptors and ON bipolar cells. In contrast, the functions of modulatory GPCR signaling networks in the inner retina are less well understood. We addressed this question by determining the consequences of augmenting modulatory Gi/o signaling driven by endogenous transmitters. This was done by analyzing the effects of genetically ablating the R7 RGS-binding protein (R7BP), a membrane-targeting protein and positive allosteric modulator of R7-RGS (regulator of the G protein signaling 7) family that deactivates Gi/oα subunits. We found that R7BP is expressed highly in starburst amacrine cells and retinal ganglion cells (RGCs). As indicated by electroretinography and multielectrode array recordings of adult retina, ablation of R7BP preserved outer retina function, but altered the firing rate and latency of ON RGCs driven by rods and cones but not rods alone. In developing retina, R7BP ablation increased the burst duration of glutamatergic waves whereas cholinergic waves were unaffected. This effect on glutamatergic waves did not result in impaired segregation of RGC projections to eye-specific domains of the dorsal lateral geniculate nucleus. R7BP knockout mice exhibited normal spatial contrast sensitivity and visual acuity as assessed by optomotor reflexes. Taken together these findings indicate that R7BP-dependent regulation of R7-RGS proteins shapes specific aspects of light-evoked and spontaneous activity of RGCs in mature and developing retina.

Morphological and physiological properties of enhanced green fluorescent protein (EGFP)-expressing wide-field amacrine cells in the ChAT-EGFP mouse line

Mammalian retinas comprise a variety of interneurons, among which amacrine cells represent the largest group, with more than 30 different cell types each exhibiting a rather distinctive morphology and carrying out a unique function in retinal processing. However, many amacrine types have not been studied systematically because, in particular, amacrine cells with large dendritic fields, i.e. wide-field amacrine cells, have a low abundance and are therefore difficult to target. Here, we used a transgenic mouse line expressing the coding sequence of enhanced green fluorescent protein under the promoter for choline acetyltransferase (ChAT-EGFP mouse) and characterized a single wide-field amacrine cell population monostratifying in layer 2/3 of the inner plexiform layer (WA-S2/3 cell). Somata of WA-S2/3 cells are located either in the inner nuclear layer or are displaced to the ganglion cell layer and exhibit a low cell density. Using immunohistochemistry, we show that WA-S2/3 cells are presumably GABAergic but may also release acetylcholine as their somata are weakly positive for ChAT. Two-photon-guided patch-clamp recordings from intact retinas revealed WA-S2/3 cells to be ON-OFF cells with a homogenous receptive field even larger than the dendritic field. The large spatial extent of the receptive field is most likely due to the extensive homologous and heterologous coupling among WA-S2/3 cells and to other amacrine cells, respectively, as indicated by tracer injections. In summary, we have characterized a novel type of GABAergic ON-OFF wide-field amacrine cell which is ideally suited to providing long-range inhibition to ganglion cells due to its strong coupling.

Survey on amacrine cells coupling to retrograde-identified ganglion cells in the mouse retina

PURPOSE

Retinal amacrine cells (ACs) may make inhibitory chemical synapses and potentially excitatory gap junctions on ganglion cells (GCs). The total number and subtypes of ACs coupled to the entire GC population were investigated in wild-type and three lines of transgenic mice.

METHODS

GCs and GC-coupled ACs were identified by the previously established LY-NB (Lucifer yellow-Neurobiotin) retrograde double-labeling technique, in conjunction with specific antibodies and confocal microscopy.

RESULTS

GC-coupled ACs (NB-positive and LY-negative) comprised nearly 11% of displaced ACs and 4% of conventional ACs in wild-type mice, and were 9% and 4% of displaced ACs in Cx45(-/-) and Cx36/45(-/-) mice, respectively. Their somas were small in Cx36/45(-/-) mice, but variable in other strains. They were mostly γ-aminobutyric acid (GABA)-immunoreactive (IR) and located in the GC layer. They comprised only a small portion in the AC subpopulations, including GABA-IR, glycine-IR, calretinin-IR, 5-HT-accumulating, and ON-type choline acetyltransferase (ChAT) ACs in wild-type and ChAT transgenic mice (ChAT- tdTomato). In the distal 80% of the inner plexiform layer (IPL), dense GC dendrites coexisted with rich glycine-IR and GABA-IR. In the inner 20% of the IPL, sparse GC dendrites presented with a major GABA band and sparse glycine-IR.

CONCLUSIONS

Various subtypes of ACs may couple to GCs. ACs of the same immunoreactivity may either couple or not couple to GCs. Cx36 and Cx45 dominate GC-AC coupling except for small ACs. The overall potency of GC-AC coupling is moderate, especially in the proximal 20% of the IPL, where inhibitory chemical signals are dominated by GABA ACs.

Spatial relationships between GABAergic and glutamatergic synapses on the dendrites of distinct types of mouse retinal ganglion cells across development

Neuronal output requires a concerted balance between excitatory and inhibitory (I/E) input. Like other circuits, inhibitory synaptogenesis in the retina precedes excitatory synaptogenesis. How then do neurons attain their mature balance of I/E ratios despite temporal offset in synaptogenesis? To directly compare the development of glutamatergic and GABAergic synapses onto the same cell, we biolistically transfected retinal ganglion cells (RGCs) with PSD95CFP, a marker of glutamatergic postsynaptic sites, in transgenic Thy1­YFPγ2 mice in which GABA_A receptors are fluorescently tagged. We mapped YFPγ2 and PSD95CFP puncta distributions on three RGC types at postnatal day P12, shortly before eye opening, and at P21 when robust light responses in RGCs are present. The mature I_GABA/E ratios varied among ON-Sustained (S) A-type, OFF-S A-type, and bistratified direction selective (DS) RGCs. These ratios were attained at different rates, before eye-opening for ON-S and OFF-S A-type, and after eye-opening for DS RGCs. At both ages examined, the I_GABA/E ratio was uniform across the arbors of the three RGC types. Furthermore, measurements of the distances between neighboring PSD95CFP and YFPγ2 puncta on RGC dendrites indicate that their local relationship is established early in development, and cannot be predicted by random organization. These close spatial associations between glutamatergic and GABAergic postsynaptic sites appear to represent local synaptic arrangements revealed by correlative light and EM reconstructions of a single RGC's dendrites. Thus, although RGC types have different I_GABA/E ratios and establish these ratios at separate rates, the local relationship between excitatory and inhibitory inputs appear similarly constrained across the RGC types studied.

Comparative effects of posterior eye cup tissues from myopic and hyperopic chick eyes on cultured scleral fibroblasts

The role of individual ocular tissues in mediating changes to the sclera during myopia development is unclear. The aim of this study was to examine the effects of retina, RPE and choroidal tissues from myopic and hyperopic chick eyes on the DNA and glycosaminoglycan (GAG) content in cultures of chick scleral fibroblasts. Primary cultures of fibroblastic cells expressing vimentin and α-smooth muscle actin were established in serum-supplemented growth medium from 8-day-old normal chick sclera. The fibroblasts were subsequently co-cultured with posterior eye cup tissue (full thickness containing retina, RPE and choroid) obtained from untreated eyes and eyes wearing translucent diffusers (form-deprivation myopia, FDM) or -15D lenses (lens-induced myopia, LIM) for 3 days (post-hatch day 5-8) (n = 6 per treatment group). The effect of tissues (full thickness and individual retina, RPE, and choroid layers) from -15D (LIM) versus +15D (lens-induced hyperopia, LIH) treated eyes was also determined. Refraction changes in the direction predicted by the visual treatments were confirmed by retinoscopy prior to tissue collection. Glycosaminoglycan (GAG) and DNA content of the scleral fibroblast cultures were measured using GAG and PicoGreen assays. There was no significant difference in the effect of full thickness tissue from either FDM or LIM treated eyes on DNA and GAG content of scleral fibroblasts (DNA 8.9 ± 2.6 μg and 8.4 ± 1.1 μg, p = 0.12; GAG 11.2 ± 0.6 μg and 10.1 ± 1.0 μg, p = 0.34). Retina from LIM eyes did not alter fibroblast DNA or GAG content compared to retina from LIH eyes (DNA 27.2 ± 1.7 μg versus 23.2 ± 1.5 μg, p = 0.21; GAG 28.1 ± 1.7 μg versus. 28.7 ± 1.2 μg, p = 0.46). Similarly, the choroid from LIH and LIM eyes did not produce a differential effect on DNA content (DNA LIM 46.9 ± 6.4 versus LIH 51.5 ± 4.7 μg, p = 0.31). In contrast, scleral fibroblast DNA was greater in co-culture with RPE from LIM eyes than the empty basket and DNA content less for co-culture with RPE from LIH eyes (LIM: 72.4 ± 6.3 μg versus empty basket: 46.03 ± 1.0 μg; p = 0.0005 and LIH: 27.9 ± 2.3 μg versus empty basket: 46.03 ± 1.0 μg; p = 0.0004). GAG content was lower with RPE from LIM eyes (LIM: 27.7 ± 0.9 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.021) and was higher with RPE from LIH eyes (LIH: 33.7 ± 1.9 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.010). Choroid from LIM eyes induce a relative increase in scleral GAG content e.g. (LIM: 32.5 ± 0.7 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.0004) while, choroid from LIH eyes induced a relative decrease in scleral GAG content (LIH: 18.9 ± 1.2 μg versus empty basket: 29.5 ± 0.8 μg, p = 0.0034). GAG content of cells in co-culture with choroid from LIM versus LIH treated eyes was significantly different (32.5 ± 0.7 μg versus 18.9 ± 1.2 μg respectively, p = 0.0002). In conclusion, these experiments provide an evidence for a directional growth signal that is present (and remains) in the ex-vivo RPE/choroid, but that does not remain in the ex-vivo retina. The identity of this factor(s) that can modify scleral cell DNA and GAG content requires further research.

The tasks of amacrine cells

Their unique patterns of size, numbers, and stratification indicate that amacrine cells have diverse functions. These are mostly unknown, as studies using imaging and electrophysiological methods have only recently begun. However, some of the events that occur within the amacrine cell population—and some important unresolved puzzles—can be stated purely from structural reasoning.