Directionally selective calcium signals in dendrites of starburst amacrine cells

Symmetric interactions within a homogeneous starburst cell network can lead to robust asymmetries in dendrites of starburst amacrine cells

Starburst amacrine cells in the mammalian retina respond asymmetrically to movement along their dendrites; centrifugal movement elicits stronger responses in each dendrite than centripetal movement. It has been suggested that the asymmetrical response can be attributed to intrinsic properties of the processes themselves. But starburst cells are known to release and have receptors for both GABA and acetylcholine. We tested whether interactions within the starburst cell network can contribute to their directional response properties. In a computational model of interacting starburst amacrine cells, we simulated the response of individual dendrites to moving light stimuli. By setting the model parameters for "synaptic connection strength" (cs) to positive or negative values, overlapping starburst dendrites could either excite or inhibit each other. For some values of cs, we observed a very robust inward/outward asymmetry of the starburst dendrites consistent with the reported physiological findings. This is the case, for example, if a starburst cell receives inhibition from other starburst cells located in its surround. For other values of cs, individual dendrites can respond best either to inward movement or respond symmetrically. A properly wired network of starburst cells can therefore account for the experimentally observed asymmetry of their response to movement, independent of any internal biophysical or biochemical properties of starburst cell dendrites.

Stimulus-dependent correlated firing in directionally selective retinal ganglion cells

Synchronous spiking has been postulated to be a meta-signal in visual cortex and other CNS loci that tags neuronal spike responses to a single entity. In retina, however, synchronized spikes have been postulated to arise via mechanisms that would largely preclude their carrying such a code. One such mechanism is gap junction coupling, in which synchronous spikes would be a by-product of lateral signal sharing. Synchronous spikes have also been postulated to arise from common-source inputs to retinal ganglion cells having overlapping receptive fields, and thus code for stimulus location in the overlap area. On-Off directionally selective ganglion cells of the rabbit retina exhibit a highly precise tiling pattern in which gap junction coupling occurs between some neighboring, same-preferred-direction cells. Depending on how correlated spikes arise, and for what purpose, one could postulate that synchronized spikes in this system (1) always arise in some subset of same-direction cells because of gap junctions, but never in non-same-preferred-directional cells; (2) never arise in same-directional cells because their receptive fields do not overlap, but arise only in different-directional cells whose receptive fields overlap, as a code for location in the overlap region; or (3) arise in a stimulus-dependent manner for both same- and different-preferred-direction cells for a function similar to that postulated for neurons in visual cortex. Simultaneous, extracellular recordings were obtained from neighboring On-Off directionally selective (DS) ganglion cells having the same and different preferred directions in an isolated rabbit retinal preparation. Stimulation by large flashing spots elicited responses from DS ganglion-cell pairs that typically showed little synchronous firing. Movement of extended bars, however, often produced synchronous spikes in cells having similar or orthogonal preferred directions. Surprisingly, correlated firing could occur for the opposite contrast polarity edges of moving stimuli when the leading edge of a sweeping bar excited the receptive field of one cell as its trailing edge stimulated another. Pharmacological manipulations showed that the spike synchronization is enhanced by excitatory cholinergic amacrine-cell inputs, and reduced by inhibitory GABAergic inputs, in a motion-specific manner. One possible interpretation is that this synchronous firing could be a signal to higher centers that the outputs of the two DS ganglion cells should be "bound" together as responding to a contour of a common object.

Novel 2,1,3-Benzothiadiazole-Based Red-Fluorescent Dyes with Enhanced Two-Photon Absorption Cross-Sections

This paper reports the two-photon absorbing and orange-red fluorescence emitting properties of a series of new 2,1,3-benzothiadiazole (BTD)-based D-pi-A-pi-D-type and star-burst-type fluorescent dyes. In the D-pi-A-pi-D-type dyes 1-6, a central BTD core was connected with two terminal N,N-disubstituted amino groups via various pi-conjugated spacers. The star-burst-type dyes 8 and 10 have a three-branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine-containing BTD branches. All the BTD-based dyes displayed intense orange-red color fluorescence in a region of 550-689 nm, which was obtained by single-photon excitation with good fluorescent quantum yield up to 0.98 as well as by two-photon excitation. Large two-photon absorption (TPA) cross-sections (110-800 GM) of these BTD dyes were evaluated by open aperture Z-scan technique with a femtosecond Ti/sapphire laser. The TPA cross-sections of D-pi-A-pi-D-type dyes 2-6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional pi-conjugated spacer are about 1.5-2.5 times larger than that of 1c with only a benzene spacer. The TPA cross-sections significantly increased in three-branched star-burst-type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3-5 times larger than those of the corresponding one-dimensional sub-units 9 (170 GM) and 11 (230 GM), respectively. The ratios of sigma/e(pi) between three-branched and one-dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11), which are larger than those predicted simply on the basis of the chromophore number density (1:1), according to a cooperative enhancement of the two-photon absorbing nature in the three-branched system.

Application of multiline two-photon microscopy to functional in vivo imaging

High spatial resolution and low risks of photodamage make two-photon laser-scanning microscopy (TPLSM) the method of choice for biological imaging. However, the study of functional dynamics such as neuronal calcium regulation often also requires a high temporal resolution. Hitherto, acquisition speed is usually increased by line scanning, which restricts spatial resolution to structures along a single axis. To overcome this gap between high spatial and high temporal resolution we performed TPLSM with a beam multiplexer to generate multiple laser foci inside the sample. By detecting the fluorescence emitted from these laser foci with an electron-multiplying camera, it was possible to perform multiple simultaneous linescans. In addition to multiline scanning, the array of up to 64 laser beams could also be used in x-y scan mode to collect entire images at high frame rates. To evaluate the applicability of multiline TPLSM to functional in vivo imaging, calcium signals were monitored in visual motion-sensitive neurons in the brain of flies. The capacity of our method to simultaneously acquire signals at different cellular locations is exemplified by measurements at branched neurites and 'spine'-like structures. Calcium dynamics depended on branch size, but 'spines' did not systematically differ from their 'parent neurites'. The spatial resolution of our setup was critically evaluated by comparing it to confocal microscopy and the negative effect of scattering of emission light during image detection was assessed directly by running the setup in both imaging and point-scanning mode.

Observations of synaptic structures: origins of the neuron doctrine and its current status

The neuron doctrine represents nerve cells as polarized structures that contact each other at specialized (synaptic) junctions and form the developmental, functional, structural and trophic units of nervous systems. The doctrine provided a powerful analytical tool in the past, but is now seldom used in educating neuroscientists. Early observations of, and speculations about, sites of neuronal communication, which were made in the early 1860s, almost 30 years before the neuron doctrine was developed, are presented in relation to later accounts, particularly those made in support of, or opposition to, the neuron doctrine. These markedly differing accounts are considered in relation to limitations imposed by preparative and microscopical methods, and are discussed briefly as representing a post-Darwinian, reductionist view, on the one hand, opposed to a holistic view of mankind as a special part of creation, on the other. The widely misunderstood relationship of the neuron doctrine to the cell theory is discussed, as is the degree to which the neuron doctrine is still strictly applicable to an analysis of nervous systems. Current research represents a 'post-neuronist' era. The neuron doctrine provided a strong analytical approach in the past, but can no longer be seen as central to contemporary advances in neuroscience.

Encoding light intensity by the cone photoreceptor synapse

How cone synapses encode light intensity determines the precision of information transmission at the first synapse on the visual pathway. Although it is known that cone photoreceptors hyperpolarize to light over 4-5 log units of intensity, the relationship between light intensity and transmitter release at the cone synapse has not been determined. Here, we use two-photon microscopy to visualize release of the synaptic vesicle dye FM1-43 from cone terminals in the intact lizard retina, in response to different stimulus light intensities. We then employ electron microscopy to translate these measurements into vesicle release rates. We find that from darkness to bright light, release decreases from 49 to approximately 2 vesicles per 200 ms; therefore, cones compress their 10,000-fold operating range for phototransduction into a 25-fold range for synaptic vesicle release. Tonic release encodes ten distinguishable intensity levels, skewed to most finely represent bright light, assuming release obeys Poisson statistics.

The interdependence and independence of amacrine cell dendrites: patch-clamp recordings and simulation studies on cultured GABAergic amacrine cells

Previously we reported that cultured rat GABAergic amacrine cells can evoke subthreshold graded depolarization and action potentials. Both types of electrical signals are thought to contribute to neurotransmitter release from their dendrites, because Ca(2+) channels in amacrine cells can be activated at a subthreshold level (around -50 mV). The aim of the present study is to describe the spatiotemporal pattern of the spread of these electrical signals in an amacrine cell, using a computer simulation study. The simulation is based on physiological data, obtained by dual whole-cell patch-clamp recordings on the soma and the dendrites of cultured rat GABAergic amacrine cells. We determined passive and active properties of amacrine cells from the physiological recordings. Then, using the NEURON simulator, we conducted computer simulations on a reconstructed model of amacrine cells. We show that graded potentials and action potentials spread through amacrine cells with distinct patterns, and discuss the electrical interrelationship among the dendrites of an amacrine cell. Subthreshold graded potentials applied to a distal dendrite were sufficiently localized, so that each dendrite could behave independently (dendritic independence). However, at a suprathreshold level, once action potentials were triggered, they propagated into every dendrite, exciting the entire cell (dendritic interdependence). We also showed that GABAergic inhibitory inputs on the dendrites suppress the dendritic interdependence of amacrine cells. These results suggest that an inhibitory amacrine cell can mediate both local and wide-field lateral inhibition, regulated by the spatiotemporal pattern of excitatory and inhibitory synaptic inputs on its dendrites.

Compartmental localization of gamma-aminobutyric acid type B receptors in the cholinergic circuitry of the rabbit retina

Although many effects of gamma-aminobutyric acid (GABA) on retinal function have been attributed to GABA(A) and GABA(C) receptors, specific retinal functions have also been shown to be mediated by GABA(B) receptors, including facilitation of light-evoked acetylcholine release from the rabbit retina (Neal and Cunningham [1995] J. Physiol. 482:363-372). To explain the results of a rich set of experiments, Neal and Cunningham proposed a model for this facilitation. In this model, GABA(B) receptor-mediated inhibition of glycinergic cells would reduce their own inhibition of cholinergic cells. In turn, muscarinic input from the latter to the glycinergic cells would complete a negative-feedback circuitry. In this study, we have used immunohistochemical techniques to test elements of this model. We report that glycinergic amacrine cells are GABA(B) receptor negative. In contrast, our data reveal the localization of GABA(B) receptors on cholinergic/GABAergic starburst amacrine cells. High-resolution localization of GABA(B) receptors on starburst amacrine cells shows that they are discretely localized to a limited population of its varicosities, the majority of likely synaptic-release sites being devoid of detectable levels of GABA(B) receptors. Finally, we identify a glycinergic cell that is a potential muscarinic receptor-bearing target of GABA(B)-modulated acetylcholine release. This target is the DAPI-3 cell. We propose, based on these data, a modification of the Neal and Cunningham model in which GABA(B) receptors are on starburst, not glycinergic amacrine cells.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

Direction-selective dendritic action potentials in rabbit retina

Dendritic spikes that propagate toward the soma are well documented, but their physiological role remains uncertain. Our in vitro patch-clamp recordings and two-photon calcium imaging show that direction-selective retinal ganglion cells (DSGCs) utilize orthograde dendritic spikes during physiological activity. DSGCs signal the direction of image motion. Excitatory subthreshold postsynaptic potentials are observed in DSGCs for motion in all directions and provide a weakly tuned directional signal. However, spikes are generated over only a narrow range of motion angles, indicating that spike generation greatly enhances directional tuning. Our results indicate that spikes are initiated at multiple sites within the dendritic arbors of DSGCs and that each dendritic spike initiates a somatic spike. We propose that dendritic spike failure, produced by local inhibitory inputs, might be a critical factor that enhances directional tuning of somatic spikes.

Dendritic computation

One of the central questions in neuroscience is how particular tasks, or computations, are implemented by neural networks to generate behavior. The prevailing view has been that information processing in neural networks results primarily from the properties of synapses and the connectivity of neurons within the network, with the intrinsic excitability of single neurons playing a lesser role. As a consequence, the contribution of single neurons to computation in the brain has long been underestimated. Here we review recent work showing that neuronal dendrites exhibit a range of linear and nonlinear mechanisms that allow them to implement elementary computations. We discuss why these dendritic properties may be essential for the computations performed by the neuron and the network and provide theoretical and experimental examples to support this view.

Imaging light-modulated release of synaptic vesicles in the intact retina: retinal physiology at the dawn of the post-electrode era

Here, we illustrate an optical method for directly measuring the light-regulated synaptic output of neurons in the retina. The method allows simultaneous recording from many retinal neurons in intact flat-mount preparations of the vertebrate retina. These recordings depend on the use of FM1-43, an activity-dependent fluorescent dye that selectively labels synaptic vesicles. Release of the dye, which occurs upon vesicle exocytosis, is detected with 2-photon microscopy. This utilizes an infrared laser to trigger fluorescence excitation of the dye, while minimally perturbing retinal activity by activating phototransduction in rods and cones. Using this approach, one can measure activity of single neurons in the intact retinal network and populations of neurons in different layers of the retina, providing a new way to examine the function of retinal synapses and how visual information is processed.

Connexin45 mediates gap junctional coupling of bistratified ganglion cells in the mouse retina

Direction selectivity, a key feature of visual perception, originates in the retina and is transmitted by bistratified ganglion cells that, in the rabbit retina, exhibit a particular coupling pattern. We intracellularly labeled ganglion cells in different transgenic mouse lines, allowing a morphological classification of bistratified ganglion cells, an analysis of their coupling pattern, and the molecular identification of the connexins responsible for the coupling. Based on dendritic characteristics including co-fasciculation with the dendrites of cholinergic starburst amacrine cells, we were able to distinguish three types of bistratified ganglion cells. Two of these co-fasciculate with starburst amacrine cells and exhibit a specific homologous coupling pattern. Connexin45 (Cx45) appears to be the major component of the gap junctional channels because tracer coupling is absent in Cx45-deficient animals whereas it persists in Cx36-deficient animals. It is speculated that the transjunctional voltage dependence of Cx45 channels could support the transmission of direction selectivity.

BIOPHYSICAL MECHANISMS AND ESSENTIAL TOPOGRAPHY OF DIRECTIONALLY SELECTIVE SUBUNITS IN RABBIT'S RETINA

We commemorate the 40th anniversary of the classical study undertaken by Barlow-Levick with a new challenge: to show how direction selectivity in the dendritic plexus of starburst amacrine cells is being computed. In the rabbit retina, although the cellular locus of direction selectivity is known to occur predominantly in the dendrites of starburst amacrine cells, the biophysical mechanism by which this takes place and its essential topography are yet to be specified with precision. A cotransmission model, involving a conjoint release of excitation/inhibition (i.e., a bisynaptic relay of endogenous ACh and GABA) from the distal varicosities of individual starburst amacrines, will be non-diphasic when the vesicular release of Ach and the non-vesicular, carrier-mediated release of GABA by transporters in the anterograde direction are preferentially suppressed by a negative feedback mechanism involving autoreceptors. Such biophysical mechanisms, including the asymmetric distribution of chloride cotransporters, explain somatofugal motion bias in starburst amacrine cells leading to autonomous functioning "subunits" that underlie the formation of directional selectivity. However, the functional independence of starburst amacrine cell "subunits" is partly a question of their network organization. The topography of directionally selective "subunits" resides in the plexus of crisscrossing dendrites of juxtaposed starburst amacrines, consisting of (i) serial synapses of three or more starburst amacrines and a ON-OFF directionally selective ganglion cell; (ii) a synaptic couplet between two starburst amacrines; and (iii) a conventional synapse between a starburst amacrine and a ON-OFF directionally selective ganglion cell. Cholinergic and GABAergic monosynaptic interactions between starburst amacrine cells, including glutamatergic interactions with cone bipolar cells, are involved in the primary circuit underlying directional selectivity. Furthermore, the secondary circuit underlying directional selectivity, consists of starburst amacrine cells and cone bipolar cells arranged in a "push-pull" configuration, interacting synaptically onto ON-OFF directionally selective ganglion cells.

Long-term plasticity mediated by mGluR1 at a retinal reciprocal synapse

The flow of information across the retina is controlled by reciprocal synapses between bipolar cell terminals and amacrine cells. However, the synaptic delays and properties of plasticity at these synapses are not known. Here we report that glutamate release from goldfish Mb-type bipolar cell terminals can trigger fast (delay of 2-3 ms) and transient GABA(A) IPSCs and a much slower and more sustained GABA(C) feedback. Synaptically released glutamate activated mGluR1 receptors on amacrine cells and, depending on the strength of presynaptic activity, potentiated subsequent feedback. This poststimulus enhancement of GABAergic feedback lasted for up to 10 min. This form of mGluR1-mediated long-term synaptic plasticity may provide retinal reciprocal synapses with adaptive capabilities.

Synaptic organization of complex ganglion cells in rabbit retina: type and arrangement of inputs to directionally selective and local-edge-detector cells

The type and topographic distribution of synaptic inputs to a directionally selective (DS) rabbit retinal ganglion cell (GC) were examined and were compared with those received by two other complex GC types. The percentage of cone bipolar cell (BC) input, presumably an index of sustained responses and simple receptive field properties, is much higher than expected for complex GCs in reference to previous reports in other species: approximately 20% for the type 1 bistratified ON-OFF DS GC and for a multistratified GC, and approximately 40% for the small-tufted local-edge-detector GC. Consistent with a previous study (Famiglietti [1991] J. Comp. Neurol. 309:40-70), no ultrastructural evidence is found for inhibitory synapses from starburst amacrine cells to the ON-OFF DS GC. The density of inputs to the ON-OFF DS GC is high and rather evenly distributed over the dendritic tree. Clustering of inputs brings excitatory and inhibitory inputs into proximity, but the strict on-path condition of more proximal inhibitory inputs, favoring shunting inhibition, is not satisfied. Prominent BC input and its regional variation suggest that BCs play key roles in DS neural circuitry, both pre- and postsynaptic to the ON-OFF DS GC, according to a bilayer model (Famiglietti [1993] Invest. Ophthalmol. Vis. Sci. 34:S985). Asymmetry of inhibitory amacrine cell input may signify a region on the preferred side of the receptive field, the inhibition-free zone (Barlow and Levick [1965] J. Physiol. (Lond.) 178:477-504), supporting a role for postsynaptic integration in the DS mechanism. Prominent BC input to the local-edge-detector, often without accompanying amacrine cell input, indicates presynaptic integration in forming its trigger feature.

Directional selectivity is formed at multiple levels by laterally offset inhibition in the rabbit retina

The excitatory and inhibitory inputs to directionally selective (DS) ganglion cells are themselves directionally selective. Directionality is achieved because excitation is reduced during null-direction movement along a GABAergic pathway. Inhibition is reduced during preferred-direction movement along a pathway that includes cholinergic synapses. Both excitation and inhibition are made directional by laterally offset inhibitory signals similar to the spatial offset of the direct inhibitory input to the DS cell dendrites. Thus, spatially offset lateral inhibition generates directionality at three different levels in the DS circuitry. We also found that for stimuli falling within the dendritic field, cholinergic input is delivered to the OFF but not the ON dendrites. Cholinergic pathways from outside the dendritic field reach both ON and OFF dendrites, but both of these pathways are normally inactivated by GABAergic synapses.

Imaging techniques in retinal research

In recent years, retinal research has benefited from major advances in optical imaging approaches. Investigations of the structural and functional organization of the vertebrate retina using live preparations have been facilitated by improvements in cell labeling methods, and by microscopy techniques that permit high-resolution of cells in vitro and in vivo. In particular, the generation of transgenic animals with fluorescently labeled retinal cells has permitted real-time visualization of cell generation, migration, differentiation and growth in the developing retina. Neuronal activity can also be examined by optical imaging using activity reporters directed to specific retinal cell types. Optical techniques such as multiphoton microscopy and total internal reflection fluorescence microscopy (TIRFM) have helped unravel the physiological properties and function of retinal cells. Here, we focus on the latest cell labeling methods that have proven highly useful in many aspects of retinal research. We also highlight several examples of how newly developed imaging technology itself has facilitated investigations that have advanced our understanding of retinal circuits and their development.

Dynamic multiphoton imaging: a live view from cells to systems

Leaps in scientific technology often occur at the interface of seemingly disparate disciplines. This holds true with the recent application of multiphoton microscopy to the biological sciences, leading to a new generation of imaging-based studies extending from the tracking of individual molecules within living cells to the observation of whole organisms.

Characterization of inhibitory postsynaptic currents in rod bipolar cells of the mouse retina

The synaptic terminals of mammalian rod bipolar cells are the targets of multiple presynaptic inhibitory inputs arriving from glycinergic and GABAergic amacrine cells. To investigate the contribution of these different inhibitory receptor types, we have applied the patch-clamp technique in acutely isolated slices of the adult mouse retina. By using the whole-cell configuration, we measured and analyzed the spontaneous postsynaptic currents (PSCs) in rod bipolar cells. The spontaneous synaptic activity of rod bipolar cells was very low. However, when amacrine cells were depolarized by AMPA or kainate, the PSC frequency in rod bipolar cells increased significantly. These PSCs comprised several types that could be distinguished by pharmacological and kinetic criteria. Strychnine-sensitive, glycinergic PSCs were characterized by a mean peak amplitude of -43.5 pA and a weighted decay time constant (tauw) of 10.9 ms. PSCs that persisted in the presence of strychnine, but were completely inhibited by bicuculline, were mediated by GABAARs. They had a mean peak amplitude of -20.0 pA and a significantly faster tauw of 5.8 ms. Few PSCs remained in the presence of strychnine and bicuculline, suggesting that they were mediated by GABACRs. These PSCs were characterized by much smaller amplitudes (-6.2 pA) and a significantly slower decay kinetics (tauw=51.0 ms). We conclude that rod bipolar cells express at least three types of functionally different inhibitory receptors, namely GABAARs, GABACRs, and GlyRs that may ultimately regulate the Ca2+ influx into rod bipolar cell terminals, thereby modulating their glutamate release.

A developmental switch in the excitability and function of the starburst network in the mammalian retina

Dual patch-clamp recording and Ca2+ uncaging revealed Ca2+-dependent corelease of ACh and GABA from, and the presence of reciprocal nicotinic and GABAergic synapses between, starburst cells in the perinatal rabbit retina. With maturation, the nicotinic synapses between starburst cells dramatically diminished, whereas the GABAergic synapses remained and changed from excitatory to inhibitory, indicating a coordinated conversion of the starburst network excitability from an early hyperexcitatory to a mature nonepileptic state. We show that this transition allows the starburst cells to use their neurotransmitters for two completely different functions. During early development, the starburst network mediates recurrent excitation and spontaneous retinal waves, which are important for visual system development. After vision begins, starburst cells release GABA in a prolonged and Ca2+-dependent manner and inhibit each other laterally via direct GABAergic synapses, which may be important for visual integration, such as the detection of motion direction.

Multiplication and stimulus invariance in a looming-sensitive neuron

Multiplicative operations and invariance of neuronal responses are thought to play important roles in the processing of neural information in many sensory systems. Yet the biophysical mechanisms that underlie both multiplication and invariance of neuronal responses in vivo, either at the single cell or at the network level, remain to a large extent unknown. Recent work on an identified neuron in the locust visual system (the LGMD neuron) that responds well to objects looming on a collision course towards the animal suggests that this cell represents a good model to investigate the biophysical basis of multiplication and invariance at the single neuron level. Experimental and theoretical results are consistent with multiplication being implemented by subtraction of two logarithmic terms followed by exponentiation via active membrane conductances, according to a x 1/b = exp(log(a) - log(b)). Invariance appears to be in part due to non-linear integration of synaptic inputs within the dendritic tree of this neuron.

Multiple spatiotemporal patterns of dendritic Ca2+ signals in goldfish retinal amacrine cells

Although it has been reported that dendritic neurotransmitter releases from amacrine cells are regulated by the intracellular Ca(2+) concentration ([Ca(2+)](i)), their spatiotemporal patterns are not well explained. Fast Ca(2+) imagings of amacrine cells in the horizontal slice preparation of goldfish retinas under whole-cell patch-clamp recordings were undertaken to better investigate the spatiotemporal patterns of dendritic [Ca(2+)](i). We found that amacrine cell dendrites showed inhomogeneous [Ca(2+)](i) increases in both Na(+) spiking cells and cells without Na(+) spikes. The spatiotemporal properties of inhomogeneous [Ca(2+)](i) increases were classified into three patterns: local, regional and global. Local [Ca(2+)](i) increases were observed in very discrete regions and appeared as discontinuous patches, presumably evoked by local excitatory postsynaptic potentials. Regional [Ca(2+)](i) increases were observed in either a single or a small number of dendrites, presumably reflecting the result of dendritic action potentials. Global [Ca(2+)](i) increases were observed in the entire dendrites of a cell and were mediated by Na(+) action potentials or multiple Na(+) action potentials riding on slow depolarization. Ca(2+)-mediated potentials also evoked global [Ca(2+)](i) increase in cells without Na(+) spikes. These spatiotemporal dynamics of dendritic Ca(2+) signals may reflect multiple modes of synaptic integration on the dendrites of amacrine cells.

Quantitative analysis of neuronal morphologies in the mouse retina visualized by using a genetically directed reporter

An alkaline phosphatase (AP) reporter has been used to visualize detailed morphologies for all major classes of retinal neurons in the adult mouse. The analysis was performed on retinas in which AP expression was activated by Cre-mediated DNA recombination in a small fraction of cells. Recombination was controlled pharmacologically and, to a first approximation, appears to have occurred randomly. The morphologies of 794 inner retinal neurons have been analyzed by measuring arbor area, stratification level, and neurite branching patterns. When analyzed in this multidimensional parametric space, the cells can be clustered into subgroups by visual inspection and by using the Ward's and K-means algorithms. One application of this cell morphology data set and cluster analysis is as a standard for comparison with the retinas of genetically altered mice. This work illustrates the utility and feasibility of genetically directed marking methods for large-scale surveys of neuronal morphology.

Contribution of a glial glutamate transporter to GABA synthesis in the retina

Neuronal glutamate transporters have been shown to play a role in GABA synthesis by enhancing glutamate uptake. In the present study, we have examined whether a glial glutamate transporter, GLAST, has a role in GABA synthesis in the mammalian retina. We found that the retinal GABA level was about two-fold higher in the GLAST-/- mouse retina compared to that in the wild type. Endogenous glutamate level was also increased about 2-fold in the mutant. Therefore, loss of GLAST results in a higher retinal GABA level, probably due to increased availability of its precursor, glutamate. An increase in GABAergic activity can be expected to affect trigger features such as directional selective response of neurons in the GLAST-/- mouse retina.

Analytical solutions of the Frankenhaeuser-Huxley equations I: minimal model for backpropagation of action potentials in sparsely excitable dendrites

Hodgkin and Huxley's ionic theory of the nerve impulse embodies principles, applicable also to the impulses in vertebrate nerve fibers, as demonstrated by Bernhard Frankenhaeuser and Andrew Huxley 40 years ago. Frankenhaeuser and Huxley reformulated the classical Hodgkin-Huxley equations, in terms of electrodiffusion theory, and computed action potentials specifically for saltatory conduction in myelinated axons. In this paper, we obtain analytical solutions to the most difficult nonlinear partial differential equations in classical neurophysiology. We solve analytically the Frankenhaeuser-Huxley equations pertaining to a model of sparsely excitable, nonlinear dendrites with clusters of transiently activating, TTX-sensitive Na(+) channels, discretely distributed as point sources of inward current along a continuous (non-segmented) leaky cable structure. Each cluster or hot-spot, corresponding to a mesoscopic level description of Na(+) ion channels, includes known cumulative inactivation kinetics observed at the microscopic level. In such a third-order system, the 'recovery' variable is an electrogenic sodium-pump imbedded in the passive membrane, and the system is stabilized by the presence of a large leak conductance mediated by a composite number of ligand-gated channels permeable to monovalent cations Na(+) and K(+). In order to reproduce antidromic propagation and attenuation of action potentials, a nonlinear integral equation must be solved (in the presence of suprathreshold input, and a constant-field equation of electrodiffusion at each hot-spot with membrane gates controlling the flow of current). A perturbative expansion of the non-dimensional membrane potential (Phi) is used to obtain time-dependent analytical solutions, involving a voltage-dependent Na(+) activation (micro) and a state-dependent inactivation (eta) gating variables. It is shown that action potentials attenuate in amplitude in accordance with experimental findings, and that the spatial density distribution of transient Na(+) channels along a long dendrite contributes significantly to their discharge patterns. A major significance of the analytical modeling, in contrast to the computational modeling of backpropagating action potentials, is the provision of a continuous description of the voltage as a function of position, allowing for greater feasibility in developing large-scale biophysical neural networks, without the need for ad hoc computational modeling.

Identification of ON-OFF direction-selective ganglion cells in the mouse retina

We identified the ON-OFF direction-selective ganglion cells (DSGCs) in the mouse retina and characterized their physiological, morphological and pharmacological properties. These cells showed transient responses to the onset and termination of a stationary flashing spot, and strong directional selectivity to a moving rectangle. Application of various pharmacological reagents demonstrated that the ON-OFF DSGCs in the mouse retina utilize a similar array of transmitters and receptors to compute motion direction to their counterparts in the rabbit retina. Voltage clamp recording showed that ON-OFF DSGCs in the mouse retina receive a larger inhibitory input when the stimulus is moving in the null direction and a larger excitatory input when the stimulus is moving in the preferred direction. Finally, intracellular infusion of neurobiotin revealed a bistratified dendritic field with recursive dendrites forming loop-like structures, previously classified as RG(D2) by morphology. Overall, the ON-OFF DSGCs in the mouse retina exhibit almost identical properties to their counterparts in the rabbit retina, indicating that the mechanisms for computing motion direction are conserved from mouse to rabbit, and probably also to higher mammals. This first detailed characterization of ON-OFF DSGCs in the mouse retina provides fundamental information for further study of maturation and regulation of the neuronal circuitry underlying computation of direction.

Fly motion vision is based on Reichardt detectors regardless of the signal-to-noise ratio

The computational structure of an optimal motion detector was proposed to depend on the signal-to-noise ratio (SNR) of the stimulus: At low SNR, the optimal motion detector should be a correlation or "Reichardt" type, whereas at high SNR, the detector would employ a gradient scheme [Potters, M. & Bialek, W. (1994) J. Physiol. (Paris) 4, 1755-1775]. Although a large body of experiments supports the Reichardt detector as the processing scheme leading to direction selectivity in fly motion vision, in most of these studies the SNR was rather low. We therefore reinvestigated the question over a much larger SNR range. Using 2-photon microscopy, we found that local dendritic [Ca(2+)] modulations, which are characteristic of Reichardt detectors, occur in response to drifting gratings over a wide range of luminance levels and contrasts. We also explored, as another fingerprint of Reichardt detectors, the dependence of the velocity optimum on the pattern wavelength. Again, we found Reichardt-typical behavior throughout the whole luminance and contrast range tested. Our results, therefore, provide strong evidence that only a single elementary processing scheme is used in fly motion vision.

A unique role for Kv3 voltage-gated potassium channels in starburst amacrine cell signaling in mouse retina

Direction-selective retinal ganglion cells show an increased activity evoked by light stimuli moving in the preferred direction. This selectivity is governed by direction-selective inhibition from starburst amacrine cells occurring during stimulus movement in the opposite or null direction. To understand the intrinsic membrane properties of starburst cells responsible for direction-selective GABA release, we performed whole-cell recordings from starburst cells in mouse retina. Voltage-clamp recordings revealed prominent voltage-dependent K(+) currents. The currents were mostly blocked by 1 mm TEA, activated rapidly at voltages more positive than -20 mV, and deactivated quickly, properties reminiscent of the currents carried by the Kv3 subfamily of K+ channels. Immunoblots confirmed the presence of Kv3.1 and Kv3.2 proteins in retina and immunohistochemistry revealed their expression in starburst cell somata and dendrites. The Kv3-like current in starburst cells was absent in Kv3.1-Kv3.2 knock-out mice. Current-clamp recordings showed that the fast activation of the Kv3 channels provides a voltage-dependent shunt that limits depolarization of the soma to potentials more positive than -20 mV. This provides a mechanism likely to contribute to the electrical isolation of individual starburst cell dendrites, a property thought essential for direction selectivity. This function of Kv3 channels differs from that in other neurons where they facilitate high-frequency repetitive firing. Moreover, we found a gradient in the intensity of Kv3.1b immunolabeling favoring proximal regions of starburst cells. We hypothesize that this Kv3 channel gradient contributes to the preference for centrifugal signal flow in dendrites underlying direction-selective GABA release from starburst amacrine cells

Serial block-face scanning electron microscopy to reconstruct three-dimensional tissue nanostructure

Three-dimensional (3D) structural information on many length scales is of central importance in biological research. Excellent methods exist to obtain structures of molecules at atomic, organelles at electron microscopic, and tissue at light-microscopic resolution. A gap exists, however, when 3D tissue structure needs to be reconstructed over hundreds of micrometers with a resolution sufficient to follow the thinnest cellular processes and to identify small organelles such as synaptic vesicles. Such 3D data are, however, essential to understand cellular networks that, particularly in the nervous system, need to be completely reconstructed throughout a substantial spatial volume. Here we demonstrate that datasets meeting these requirements can be obtained by automated block-face imaging combined with serial sectioning inside the chamber of a scanning electron microscope. Backscattering contrast is used to visualize the heavy-metal staining of tissue prepared using techniques that are routine for transmission electron microscopy. Low-vacuum (20–60 Pa H₂O) conditions prevent charging of the uncoated block face. The resolution is sufficient to trace even the thinnest axons and to identify synapses. Stacks of several hundred sections, 50–70 nm thick, have been obtained at a lateral position jitter of typically under 10 nm. This opens the possibility of automatically obtaining the electron-microscope-level 3D datasets needed to completely reconstruct the connectivity of neuronal circuits.

A new method combines automated imaging with serial sectioning inside the chamber of a scanning electron microscope

Parallel processing in the mammalian retina

Our eyes send different 'images' of the outside world to the brain - an image of contours (line drawing), a colour image (watercolour painting) or an image of moving objects (movie). This is commonly referred to as parallel processing, and starts as early as the first synapse of the retina, the cone pedicle. Here, the molecular composition of the transmitter receptors of the postsynaptic neurons defines which images are transferred to the inner retina. Within the second synaptic layer - the inner plexiform layer - circuits that involve complex inhibitory and excitatory interactions represent filters that select 'what the eye tells the brain'.

Analysis of perfect mappings of the stimuli through neural temporal sequences

The analysis of an optimal neural system that maps stimuli into unique sequences of activations of fundamental atoms or functional clusters (FCs) is carried out. We say that it is perfect because the system maps with an injective function every stimulus in minimum time with the least number of FCs, such that every FC is activated only once. The neural system has the possibility to sustain several sequences in parallel. In this framework, we study the capacity achievable by the system, minimal completion time and complexity in terms of the number of parallel sequences. We show that the maximum capacity of the system is achieved without using parallel sequences at the expense of long completion times. However, when the capacity value is fixed, the largest possible number of parallel sequences is optimal because it requires short completion times. The complexity measure adds to important points: (i) the largest complexity of the system is achieved without parallel sequences, and (ii) the capacity estimation is a good estimation of the complexity of the system.

Expression of calcium transporters in the retina of the tiger salamander (Ambystoma tigrinum)

Changes in intracellular calcium concentration, [Ca2+]i, modulate the flow of visual signals across all stages of processing in the retina, yet the identities of Ca2+ transporters responsible for these changes are still largely unknown. In the current study, the distribution of plasma membrane and intracellular Ca2+ transporters in the retina of tiger salamander, a model system for physiological studies of retinal function, was determined. Plasma membrane calcium ATPases (PMCAs), responsible for high-affinity Ca2+ extrusion, were highly expressed in the salamander retina. PMCA isoforms 1, 2, and 4 were localized to photoreceptors, whereas the inner retina expressed all four isoforms. PMCA3 was expressed in a sparse population of amacrine and ganglion neurons, whereas PMCA2 was expressed in most amacrine and ganglion cells. Na+/Ca2+ exchangers, a high-capacity Ca2+ extrusion system, were expressed in the outer plexiform layer and in a subset of inner nuclear and ganglion layer cells. Intracellular Ca2+ store transporters were also represented prominently. SERCA2a, a splice variant of the sarcoplasmic-endoplasmic Ca2+ ATPase, was found mostly in photoreceptors, whereas SERCA2b was found in the majority of retinal neurons and in glial cells. The predominant endoplasmic reticulum (ER) Ca2+ channels in the salamander retina are represented by the isoform 2 of the IP3 receptor family and the isoform 2 of the ryanodine receptor family. These results indicate that Ca2+ transporters in the salamander retina are expressed in a cell type-specific manner.

Two-Photon Sensor for Metal Ions Derived from Azacrown Ether

A two-photon sensor for the metal ions derived from azacrown ether as the receptor is reported. The sensor emits strong two-photon fluorescence when excited by 800 nm laser photons. Moreover, the binding constants measured by the one- and two-photon fluorescence are similar. This result may be useful for the design of efficient two-photon fluorescence probes for biological substrates.

Direction selectivity in a model of the starburst amacrine cell

The starburst amacrine cell (SBAC), found in all mammalian retinas, is thought to provide the directional inhibitory input recorded in On–Off direction-selective ganglion cells (DSGCs). While voltage recordings from the somas of SBACs have not shown robust direction selectivity (DS), the dendritic tips of these cells display direction-selective calcium signals, even when γ-aminobutyric acid (GABAa,c) channels are blocked, implying that inhibition is not necessary to generate DS. This suggested that the distinctive morphology of the SBAC could generate a DS signal at the dendritic tips, where most of its synaptic output is located. To explore this possibility, we constructed a compartmental model incorporating realistic morphological structure, passive membrane properties, and excitatory inputs. We found robust DS at the dendritic tips but not at the soma. Two-spot apparent motion and annulus radial motion produced weak DS, but thin bars produced robust DS. For these stimuli, DS was caused by the interaction of a local synaptic input signal with a temporally delayed “global” signal, that is, an excitatory postsynaptic potential (EPSP) that spread from the activated inputs into the soma and throughout the dendritic tree. In the preferred direction the signals in the dendritic tips coincided, allowing summation, whereas in the null direction the local signal preceded the global signal, preventing summation. Sine-wave grating stimuli produced the greatest amount of DS, especially at high velocities and low spatial frequencies. The sine-wave DS responses could be accounted for by a simple mathematical model, which summed phase-shifted signals from soma and dendritic tip. By testing different artificial morphologies, we discovered DS was relatively independent of the morphological details, but depended on having a sufficient number of inputs at the distal tips and a limited electrotonic isolation. Adding voltage-gated calcium channels to the model showed that their threshold effect can amplify DS in the intracellular calcium signal.

Functional fluorescent Ca2+ indicator proteins in transgenic mice under TET control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.

Winfred Denk and colleagues succeed in generating transgenic mice that express one of two calcium indicators in their cells, creating a valuable tool to study neuronal activity

Computational subunits in thin dendrites of pyramidal cells

The thin basal and oblique dendrites of cortical pyramidal neurons receive most of the synaptic inputs from other cells, but their integrative properties remain uncertain. Previous studies have most often reported global linear or sublinear summation. An alternative view, supported by biophysical modeling studies, holds that thin dendrites provide a layer of independent computational 'subunits' that sigmoidally modulate their inputs prior to global summation. To distinguish these possibilities, we combined confocal imaging and dual-site focal synaptic stimulation of identified thin dendrites in rat neocortical pyramidal neurons. We found that nearby inputs on the same branch summed sigmoidally, whereas widely separated inputs or inputs to different branches summed linearly. This strong spatial compartmentalization effect is incompatible with a global summation rule and provides the first experimental support for a two-layer 'neural network' model of pyramidal neuron thin-branch integration. Our findings could have important implications for the computing and memory-related functions of cortical tissue.

Burst generation in rat pyramidal neurones by regenerative potentials elicited in a restricted part of the basilar dendritic tree

The common preconception about central nervous system neurones is that thousands of small postsynaptic potentials sum across the entire dendritic tree to generate substantial firing rates, previously observed in in vivo experiments. We present evidence that local inputs confined to a single basal dendrite can profoundly influence the neuronal output of layer V pyramidal neurones in the rat prefrontal cortical slices. In our experiments, brief glutamatergic stimulation delivered in a restricted part of the basilar dendritic tree invariably produced sustained plateau depolarizations of the cell body, accompanied by bursts of action potentials. Because of their small diameters, basolateral dendrites are not routinely accessible for glass electrode measurements, and very little is known about their electrical properties and their role in information processing. Voltage-sensitive dye recordings were used to follow membrane potential transients in distal segments of basal branches during sub- and suprathreshold glutamate and synaptic stimulations. Recordings were obtained simultaneously from multiple dendrites and multiple points along individual dendrites, thus showing in a direct way how regenerative potentials initiate at the postsynaptic site and propagate decrementally toward the cell body. The glutamate-evoked dendritic plateau depolarizations described here are likely to occur in conjunction with strong excitatory drive during so-called 'UP states', previously observed in in vivo recordings from mammalian cortices.

Activity-dependent expression of acyl-coenzyme a-binding protein in retinal muller glial cells evoked by optokinetic stimulation

Long-term horizontal optokinetic stimulation (HOKS) decreases the gain of the horizontal optokinetic reflex and evokes the second phase of optokinetic afternystagmus (OKAN-II). We investigated the possible molecular constituents of this adaptation. We used a differential display reverse transcriptase-PCR screen for mRNAs isolated from retinas of rabbits that received HOKS. In each rabbit, we compared mRNAs from the retina stimulated in the posterior-->anterior (preferred) direction with mRNAs from the retina stimulated in the anterior-->posterior (null) direction. Acyl-CoA-binding protein (ACBP) mRNA was one of four mRNAs selected by this screen, the proteins of which interact with GABA receptors. HOKS in the preferred direction increased ACBP mRNA transcription and ACBP protein expression. ACBP was localized to Muller glial cells by hybridization histochemistry and by immunohistochemistry. ACBP interacts with the alpha1-subunit of the GABA(A) receptor, as determined by a yeast two-hybrid technique. This interaction was confirmed by coimmunoprecipitation of ACBP and the alpha1-subunit of the GABA(A) receptor using an antibody to GABA(A)alpha1. The interaction was also confirmed by a "pull-down" assay in which histidine-tagged ACBP was used to pull down the GABA(A)alpha1. ACBP does not cross the blood-brain barrier. However, smaller truncated proteolytic fragments of ACBP do, increasing the excitability of central cortical neurons. Muller cells may secrete ACBP in the inner plexiform layer, thereby decreasing the sensitivity of GABA(A) receptors expressed on the surface of ganglion cell dendrites. Because retinal directional sensitivity is linked to GABAergic transmission, HOKS-induced expression of ACBP could provide a molecular basis for adaptation to HOKS and for the genesis of OKAN-II.

Directional sensitivity of dendritic calcium responses to wind stimuli in the cricket giant interneuron

We examined directional sensitivities in the dendritic activity of the identified giant interneurons (GIs) in the cricket, using in vivo Ca(2+) imaging during different directional air-current stimuli. Air current stimulus evoked action potential burst and quick Ca(2+) increase in GI. The stimulus direction of the maximal Ca(2+) responses corresponded to that of the maximal voltage response. However, the shapes of the directional tuning curves based on the Ca(2+) responses for each dendritic branch were different from the overall tuning curve based on spike counts for the cell. Moreover, different dendritic branches displayed distinct directional sensitivity profiles to the air-current stimuli. We propose that postsynaptic activities will influence the local Ca(2+) signals in the distal dendrites, and produce the difference in directional sensitivity of the dendritic Ca(2+) response.

Dendritic relationship between starburst amacrine cells and direction-selective ganglion cells in the rabbit retina

We investigated the dendritic relationship between starburst amacrine cells (SAs) and morphologically and physiologically characterized ON and ON-OFF direction-selective ganglion cells (DSGCs) in the rabbit retina. ON and ON-OFF DSGCs were found to exhibit tight dendritic cofasciculation with the SA plexus, visualized by immunolabelling of the vesicular acetylcholine transporter (VAChT). The degree of cofasciculation of both types of DSGC dendrites and SA plexus was found to be significant, unlike the relationship between non-DS cells and the SA plexus, which was close to chance distribution. No difference in the degree of cofasciculation in different regions of the DS dendritic field was observed. Individual SAs intracellularly injected both on the 'preferred' and 'null' side of the DSGCs showed the same degree of cofasciculation with the DSGCs. Therefore, the computation of motion direction is unlikely to result from apparent asymmetry in geometric proximity between SAs and DSGCs. Highly selective synaptic connections between SAs and DSGCs are necessary.

Cooperative LTP can map memory sequences on dendritic branches

Hebbian synaptic learning requires co-activation of presynaptic and postsynaptic neurons. However, under some conditions, information regarding the postsynaptic action potential, carried by backpropagating action potentials, can be strongly degraded before it reaches the distal exhibit Hebbian long-term potentiation (LTP)? Recent results show that LTP can indeed occur at synapses on distal dendrites of hippocamal CA1 neurons, even in the absence of a postsynaptic somatic spike. Instead. local dendritic spikes contribute to the depolarization required to induce LTP. Here, a dendritically constrained synaptic learning rule is proposed, which suggests that nearby synapses can encode temporally contiguous events.

Noninvasive two-photon imaging reveals retinyl ester storage structures in the eye

Visual sensation in vertebrates is triggered when light strikes retinal photoreceptor cells causing photoisomerization of the rhodopsin chromophore 11-cis-retinal to all-trans-retinal. The regeneration of preillumination conditions of the photoreceptor cells requires formation of 11-cis-retinal in the adjacent retinal pigment epithelium (RPE). Using the intrinsic fluorescence of all-trans-retinyl esters, noninvasive two-photon microscopy revealed previously uncharacterized structures (6.9 +/- 1.1 microm in length and 0.8 +/- 0.2 microm in diameter) distinct from other cellular organelles, termed the retinyl ester storage particles (RESTs), or retinosomes. These structures form autonomous all-trans-retinyl ester-rich intracellular compartments distinct from other organelles and colocalize with adipose differentiation-related protein. As demonstrated by in vivo experiments using wild-type mice, the RESTs participate in 11-cis-retinal formation. RESTs accumulate in Rpe65-/- mice incapable of carrying out the enzymatic isomerization, and correspondingly, are absent in the eyes of Lrat-/- mice deficient in retinyl ester synthesis. These results indicate that RESTs located close to the RPE plasma membrane are essential components in 11-cis-retinal production.

Two-photon imaging of calcium accumulation in rat cerebellar granule cells

Topical accumulation of calcium ions in neurites and cell bodies of rat cerebellar granule cells was studied by two-photon microscopy in neurons loaded with the Ca-sensitive fluorescent indicator Oregon Green 488 Bapta. High potassium caused a rapid surge of internal calcium ([Ca2+]i) in the cell body, followed by a plateau. In neurites, [Ca2+]i reached a peak and then decreased back to the control level. In contrast, in neurons stimulated by NMDA, [Ca2+]i reached a steady level and remained constant as long as the agonist was present in the bath, either in the cell bodies or in neurites. In the latter, the response to NMDA treatment was smaller and heterogeneous, and [Ca2+]i increased in certain segments of the neurite, but not in others.

Cation--chloride cotransporters mediate neural computation in the retina

The ability of directionally selective (DS) retinal ganglion cells to respond selectively to stimulus motion in one direction is a classic unresolved example of computation in a local neural circuit. Recent evidence indicates that DS responses occur first in the retina in the dendrites of starburst amacrine cells (interneurons presynaptic to the ganglion cells). We report that the directional responses of starburst-cell dendrites and DS ganglion cells are highly sensitive to the polarity of the transmembrane chloride gradient. Reducing the transmembrane chloride gradient by ion substitution or by blocking the K-Cl cotransporter resulted in the starburst cells responding equally to light moving in opposite directions. Conversely, increasing the chloride gradient by blocking the Na-K-Cl cotransporter eliminated responses to light moving in either direction. Moreover, in each case, blocking the chloride cotransporters or reducing the transmembrane chloride gradient eliminated the directional responses of DS ganglion cells in a manner opposite that of the starburst cells. These results indicate that chloride cotransporters play a key role in the generation of direction selectivity and that the directional responses of starburst cells and DS ganglion cells are exquisitely sensitive to the chloride equilibrium potential. The findings further suggest that the directional responses of DS ganglion cells are mediated in part by the directional release of gamma-aminobutyric acid from starburst dendrites and that the asymmetric distribution of the two cotransporters along starburst-cell dendrites mediates direction selectivity. A model of direction selectivity in the retina that incorporates these and other findings is discussed.

The control of dendrite development

Dendrite development is an important and unsolved problem in neuroscience. The nervous system is composed of a vast number of neurons with strikingly different morphology. Neurons are highly polarized cells with distinct subcellular compartments, including one or multiple dendritic processes arising from the cell body, and a single, extended axon. Communications between neurons involve synapses formed between axons of the presynaptic neurons and dendrites of the postsynaptic neurons. Extensive studies over the past decade have identified many molecules underlying axonal outgrowth and pathfinding. In contrast, the control of dendrite development is still much less well understood. However, recent progress has begun to shed light on the molecular mechanisms that orchestrate dendrite growth, arborization, and guidance.

Contextual tuning of direction-selective retinal ganglion cells

A direction-selective (DS) retinal ganglion cell responds well to a small object moving within its receptive field center, but less well when there is also a moving stimulus in the surrounding area; this has been described as tuning for local motion. We show here an additional selectivity, such that the surround has less effect if there is a discontinuity--that is, a difference in spatial phase, spatial frequency or velocity--between the center stimulus and that present in the surround.

Seeing More Clearly: Recent Advances in Understanding Retinal Circuitry

Among 10 breakthroughs that Science announced at the end of 2002 was the discovery of a photosensing (melanopsin-containing) retinal ganglion cell (RGC) and its role in entraining the circadian clock. This breakthrough exemplifies the ultimate goal of neuroscience: to understand the nervous system from molecules to behavior. Light-sensing RGCs constitute one of a dozen discrete RGC populations coding various aspects of visual scenes by virtue of their unique morphology, physiology, and coverage of the retina. Interestingly, the function of the melanopsin-containing RGCs in entraining the circadian clock need not involve much retinal processing, making it the simplest form of processing in the retina. This review focuses on recent advances in our understanding of retinal circuitry, visual processing, and retinal development demonstrated by innovative experimental techniques. It also discusses the advantages of using the retina as a model system to address some of the key questions in neuroscience.