Parallel cone bipolar pathways to a ganglion cell use different rates and amplitudes of quantal excitation

Efficient population coding depends on stimulus convergence and source of noise

Sensory organs transmit information to downstream brain circuits using a neural code comprised of spikes from multiple neurons. According to the prominent efficient coding framework, the properties of sensory populations have evolved to encode maximum information about stimuli given biophysical constraints. How information coding depends on the way sensory signals from multiple channels converge downstream is still unknown, especially in the presence of noise which corrupts the signal at different points along the pathway. Here, we calculated the optimal information transfer of a population of nonlinear neurons under two scenarios. First, a lumped-coding channel where the information from different inputs converges to a single channel, thus reducing the number of neurons. Second, an independent-coding channel when different inputs contribute independent information without convergence. In each case, we investigated information loss when the sensory signal was corrupted by two sources of noise. We determined critical noise levels at which the optimal number of distinct thresholds of individual neurons in the population changes. Comparing our system to classical physical systems, these changes correspond to first- or second-order phase transitions for the lumped- or the independent-coding channel, respectively. We relate our theoretical predictions to coding in a population of auditory nerve fibers recorded experimentally, and find signatures of efficient coding. Our results yield important insights into the diverse coding strategies used by neural populations to optimally integrate sensory stimuli in the presence of distinct sources of noise.

The neuronal organization of the retina

The mammalian retina consists of neurons of >60 distinct types, each playing a specific role in processing visual images. They are arranged in three main stages. The first decomposes the outputs of the rod and cone photoreceptors into ∼12 parallel information streams. The second connects these streams to specific types of retinal ganglion cells. The third combines bipolar and amacrine cell activity to create the diverse encodings of the visual world--roughly 20 of them--that the retina transmits to the brain. New transformations of the visual input continue to be found: at least half of the encodings sent to the brain (ganglion cell response selectivities) remain to be discovered. This diversity of the retina's outputs has yet to be incorporated into our understanding of higher visual function.

Amacrine and bipolar inputs to midget and parasol ganglion cells in marmoset retina

Retinal ganglion cells receive excitatory synapses from bipolar cells and inhibitory synapses from amacrine cells. Previous studies in primate suggest that the strength of inhibitory amacrine input is greater to cells in peripheral retina than to foveal (central) cells. A comprehensive study of a large number of ganglion cells at different eccentricities, however, is still lacking. Here, we compared the amacrine and bipolar input to midget and parasol ganglion cells in central and peripheral retina of marmosets (Callithrix jacchus). Ganglion cells were labeled by retrograde filling from the lateral geniculate nucleus or by intracellular injection. Presumed amacrine input was identified with antibodies against gephyrin; presumed bipolar input was identified with antibodies against the GluR4 subunit of the AMPA receptor. In vertical sections, about 40% of gephyrin immunoreactive (IR) puncta were colocalized with GABAA receptor subunits, whereas immunoreactivity for gephyrin and GluR4 was found at distinct sets of puncta. The density of gephyrin IR puncta associated with ganglion cell dendrites was comparable for midget and parasol cells at all eccentricities studied (up to 2 mm or about 16 degrees of visual angle for midget cells and up to 10 mm or >80 degrees of visual angle for parasol cells). In central retina, the densities of gephyrin IR and GluR4 IR puncta associated with the dendrites of midget and parasol cells are comparable, but the average density of GluR4 IR puncta decreased slightly in peripheral parasol cells. These anatomical results indicate that the ratio of amacrine to bipolar input does not account for the distinct functional properties of parasol and midget cells or for functional differences between cells of the same type in central and peripheral retina.

Encoding of luminance and contrast by linear and nonlinear synapses in the retina

Understanding how neural circuits transmit information is technically challenging because the neural code is contained in the activity of large numbers of neurons and synapses. Here, we use genetically encoded reporters to image synaptic transmission across a population of sensory neurons-bipolar cells in the retina of live zebrafish. We demonstrate that the luminance sensitivities of these synapses varies over 10(4) with a log-normal distribution. About half the synapses made by ON and OFF cells alter their polarity of transmission as a function of luminance to generate a triphasic tuning curve with distinct maxima and minima. These nonlinear synapses signal temporal contrast with greater sensitivity than linear ones. Triphasic tuning curves increase the dynamic range over which bipolar cells signal light and improve the efficiency with which luminance information is transmitted. The most efficient synapses signaled luminance using just 1 synaptic vesicle per second per distinguishable gray level.

Synaptic inputs to two types of koniocellular pathway ganglion cells in marmoset retina

The retinal connectivity of the diverse group of cells contributing to koniocellular visual pathways (widefield ganglion cells) is largely unexplored. Here we examined the synaptic inputs onto two koniocellular-projecting ganglion cell types named large sparse and broad thorny cells. Ganglion cells were labeled by retrograde tracer injections targeted to koniocellular layer K3 in the lateral geniculate nucleus in marmosets (Callithrix jacchus) and subsequently photofilled. Retinal preparations were processed with antibodies against the C-terminal binding protein 2, the AMPA receptor subunit GluR4, and against CD15 to identify bipolar (excitatory) and/or antibodies against gephyrin to identify amacrine (inhibitory) input. Large sparse cells are narrowly stratified close to the ganglion cell layer. Broad thorny ganglion cells are broadly stratified in the center of the inner plexiform layer. Bipolar input to large sparse cells derives from DB6 and maybe other ON bipolar types, whereas that to broad thorny cells derives from ON and OFF bipolar cell types. The total number of putative synapses on broad thorny cells is higher than the number on large sparse cells but the density of inputs (between 2 and 5 synapses per 100 μm(2) dendritic area) is similar for the two cell types, indicating that the larger number of synapses on broad thorny cells is attributable to the larger membrane surface area of this cell type. Synaptic input density is comparable to previous values for midget-parvocellular and parasol-magnocellular pathway cells. This suggests functional differences between koniocellular, parvocellular, and magnocellular pathways do not arise from variation in synaptic input densities.

Development of cell type-specific connectivity patterns of converging excitatory axons in the retina

To integrate information from different presynaptic cell types, dendrites receive distinct patterns of synapses from converging axons. How different afferents in vivo establish specific connectivity patterns with the same dendrite is poorly understood. Here, we examine the synaptic development of three glutamatergic bipolar cell types converging onto a common postsynaptic retinal ganglion cell. We find that after axons and dendrites target appropriate synaptic layers, patterns of connections among these neurons diverge through selective changes in the conversion of axo-dendritic appositions to synapses. This process is differentially regulated by neurotransmission, which is required for the shift from single to multisynaptic appositions of one bipolar cell type but not for maintenance and elimination, respectively, of connections from the other two types. Thus, synaptic specificity among converging excitatory inputs in the retina emerges via differential synaptic maturation of axo-dendritic appositions and is shaped by neurotransmission in a cell type-dependent manner.

Ret-PCP2 colocalizes with protein kinase C in a subset of primate ON cone bipolar cells

Purkinje cell protein 2 (PCP2), a member of the family of guanine dissociation inhibitors and a strong interactor with the G-protein subunit G alpha(o), localizes to retinal ON bipolar cells. The retina-specific splice variant of PCP2, Ret-PCP2, accelerates the light response of rod bipolar cells by modulating the mGluR6 transduction cascade. All ON cone bipolar cells express mGluR6 and G alpha(o), but only a subset expresses Ret-PCP2. Here we test the hypothesis that Ret-PCP2 contributes to shaping the various temporal bandwidths of ON cone bipolar cells in monkey retina. We found that the retinal splice variants in monkey and mouse are similar and longer than the cerebellar variants. Ret-PCP2 is strongly expressed by diffuse cone bipolar type 4 cells (DB4; marked with anti-PKCalpha) and weakly expressed by midget bipolar dendrites (labeled by antibodies against G alpha(o), G gamma 13, or mGluR6). Ret-PCP2 is absent from diffuse cone bipolar type 6 (DB6; marked with anti-CD15) and blue cone bipolar cells (marked with anti-CCK precursor). Thus, cone bipolar cells that terminate in stratum 3 of the inner plexiform layer (DB4) express more Ret-PCP2 than those that terminate in strata 3 + 4 (midget bipolar cells), and these in turn express more than those that terminate in stratum 5 (DB6 and blue cone bipolar cells). This expression pattern approximates the arborization of ganglion cells (GC) with different temporal bandwidths: parasol GCs stratifying near stratum 3 are faster than midget GCs stratifying in strata 3 + 4, and these are probably faster than the sluggish GCs that arborize in stratum 5.

Reliability and frequency response of excitatory signals transmitted to different types of retinal ganglion cell

The same visual stimulus evokes a different pattern of neural signals each time the stimulus is presented. Because this unreliability reduces visual performance, it is important to understand how it arises from neural circuitry. We asked whether different types of ganglion cell receive excitatory signals with different reliability and frequency content and, if so, how retinal circuitry contributes to these differences. If transmitter release is governed by Poisson statistics, the SNR of the postsynaptic currents (ratio of signal power to noise power) should grow linearly with quantal rate (qr), a prediction that we confirmed experimentally. Yet ganglion cells of the same type receive quanta at different rates. Thus to obtain a measure of reliability independent of quantal rate, we calculated the ratio SNR/qr, and found this measure to be type-specific. We also found type-specific differences in the frequency content of postsynaptic currents, although types whose dendrites branched at nearby levels of the inner plexiform layer (IPL) had similar frequency content. As a result, there was an orderly distribution of frequency response through the depth of the IPL, with alternating layers of broadband and high-pass signals. Different types of bipolar cell end at different depths of the IPL and provide excitatory synapses to ganglion cell dendrites there. Thus these findings indicate that a bipolar cell synapse conveys signals whose temporal message and reliability (SNR/qr) are determined by neuronal type. The final SNR of postsynaptic currents is set by the dendritic membrane area of a ganglion cell, which sets the numbers of bipolar cell synapses and thus the rate at which it receives quanta [SNR = qr x (SNR/qr)].

Different types of ganglion cell share a synaptic pattern

Retinal ganglion cells comprise about 10 morphological types that also differ functionally. To determine whether functional differences might arise partially from differences in excitatory input, we quantified the distributions of ribbon contacts to four mammalian ganglion cell types [brisk-transient (BT), brisk-sustained (BS), local edge (LE), directionally selective (DS)], comparing small vs. large and "sluggish" vs. "brisk." Cells in guinea pig retina were filled with fluorescent dye, immunostained for synaptic ribbons, and reconstructed with their ribbon contacts by confocal microscopy. False-positive contacts were corrected by performing the same analysis on processes that lack synapses: glial stalks and rod bipolar axons. All types shared a domed distribution of membrane that was well fit by a Gaussian function (R(2) = 0.96 +/- 0.01); they also shared a constant density of contacts on the dendritic membrane, both across each arbor and across cell types (19 +/- 1 contacts/100 microm(2) membrane). However, the distributions of membrane across the retina differed markedly in width (BT > DS approximately BS > LE) and peak density (BS > DS > LE > BT). Correspondingly, types differed in peak density of contacts (BS > DS approximately LE > BT) and total number (BS approximately BT > DS > LE). These differences between cell types in spatial extent and local concentration of membrane and synapses help to explain certain functional differences.

How robust is a neural circuit?

Design in engineering begins with the problem of robustness-by what factor should intrinsic capacity exceed normal demand? Here we consider robustness for a neural circuit that crosses the retina from cones to ganglion cells. The circuit's task is to represent the visual scene at many successive stages, each time by modulating a stream of stochastic events: photoisomerizations, then transmitter quanta, then spikes. At early stages, the event rates are high to achieve some critical signal-to-noise ratio and temporal bandwidth, which together set the information rate. Then neural circuits concentrate the information and repackage it, so that nearly the same total information can be represented by modulating far lower event rates. This is important for spiking because of its high metabolic cost. Considering various measurements at the outer and inner retina, we conclude that the "safety factors" are about 2-10, similar to other tissues.

Inferring network activity from synaptic noise

During intense network activity in vivo, cortical neurons are in a high-conductance state, in which the membrane potential (V(m)) is subject to a tremendous fluctuating activity. Clearly, this "synaptic noise" contains information about the activity of the network, but there are presently no methods available to extract this information. We focus here on this problem from a computational neuroscience perspective, with the aim of drawing methods to analyze experimental data. We start from models of cortical neurons, in which high-conductance states stem from the random release of thousands of excitatory and inhibitory synapses. This highly complex system can be simplified by using global synaptic conductances described by effective stochastic processes. The advantage of this approach is that one can derive analytically a number of properties from the statistics of resulting V(m) fluctuations. For example, the global excitatory and inhibitory conductances can be extracted from synaptic noise, and can be related to the mean activity of presynaptic neurons. We show here that extracting the variances of excitatory and inhibitory synaptic conductances can provide estimates of the mean temporal correlation-or level of synchrony-among thousands of neurons in the network. Thus, "probing the network" through intracellular V(m) activity is possible and constitutes a promising approach, but it will require a continuous effort combining theory, computational models and intracellular physiology.

Quantal Encoding of Information in a Retinal Ganglion Cell

A retinal ganglion cell receives information about a white-noise stimulus as a flickering pattern of glutamate quanta. The ganglion cell reencodes this information as brief bursts of one to six spikes separated by quiescent periods. When the stimulus is repeated, the number of spikes in a burst is highly reproducible (variance < mean) and spike timing is precise to within 10 ms, leading to an estimate that each spike encodes about 2 bits. To understand how the ganglion cell reencodes information, we studied the quantal patterns by repeating a white-noise stimulus and recording excitatory currents from a voltage-clamped, brisk-sustained ganglion cell. Quanta occurred in synchronous bursts of 3 to 65; the resulting postsynaptic currents summed to form excitatory postsynaptic currents (EPSCs). The number of quanta in an EPSC was only moderately reproducible (variance = mean), quantal timing was precise to within 14 ms, and each quantum encoded 0.1–0.4 bit. In conclusion, compared to a spike, a quantum has similar temporal precision, but is less reproducible and encodes less information. Summing multiple quanta into discrete EPSCs improves the reproducibility of the overall quantal pattern and contributes to the reproducibility of the spike train.

Sluggish and brisk ganglion cells detect contrast with similar sensitivity

Roughly half of all ganglion cells in mammalian retina belong to the broad class, termed "sluggish." Many of these cells have small receptive fields and project via lateral geniculate nuclei to visual cortex. However, their possible contributions to perception have been largely ignored because sluggish cells seem to respond weakly compared with the more easily studied "brisk" cells. By selecting small somas under infrared DIC optics and recording with a loose seal, we could routinely isolate sluggish cells. When a spot was matched spatially and temporally to the receptive field center, most sluggish cells could detect the same low contrasts as brisk cells. Detection thresholds for the two groups determined by an "ideal observer" were similar: threshold contrast for sluggish cells was 4.7 +/- 0.5% (mean +/- SE), and for brisk cells was 3.4 +/- 0.3% (Mann-Whitney test: P > 0.05). Signal-to-noise ratios for the two classes were also similar at low contrast. However, sluggish cells saturated at somewhat lower contrasts (contrast for half-maximum response was 14 +/- 1 vs. 19 +/- 2% for brisk cells) and were less sensitive to higher temporal frequencies (when the stimulus frequency was increased from 2 to 4 Hz, the response rate fell by 1.6-fold). Thus the sluggish cells covered a narrower dynamic range and a narrower temporal bandwidth, consistent with their reported lower information rates. Because information per spike is greater at lower firing rates, sluggish cells may represent "cheaper" channels that convey less urgent visual information at a lower energy cost.

Structure and function of ribbon synapses

Sensory neurons with short conduction distances can use nonregenerative, graded potentials to modulate transmitter release continuously. This mechanism can transmit information at much higher rates than spiking. Graded signaling requires a synapse to sustain high rates of exocytosis for relatively long periods, and this capacity is the special virtue of ribbon synapses. Vesicles tethered to the ribbon provide a pool for sustained release that is typically fivefold greater than the docked pool available for fast release. The current article, which is part of the TINS Synaptic Connectivity series, reviews recent evidence for this fundamental computational strategy and its underlying cell biology.

Synaptic contacts between an identified type of ON cone bipolar cell and ganglion cells in the mouse retina

We surveyed the potential contacts between an identified type of bipolar cell and retinal ganglion cells in the mouse. By crossing two existing mouse strains (line 357 and line GFP-M), we created a double transgenic strain in which GFP is expressed by all members of a single type of ON cone bipolar cell and a sparse, mixed population of retinal ganglion cells. The GFP-expressing bipolar cells appear to be those termed CB4a of Pignatelli & Strettoi [(2004) J. Comp. Neurol., 476, 254-266] and type 7 of Ghosh et al. [(2004) J. Comp. Neurol., 469, 70-82 and J. Comp. Neurol., 476, 202-203]. The labelled ganglion cells include examples of most or all types of ganglion cells present in the mouse. By studying the juxtaposition of their processes in three dimensions, we could learn which ganglion cell types are potential synaptic targets of the line 357 bipolar cell. Of 12 ganglion cell types observed, 10 types could be definitively ruled out as major synaptic targets of the line 357 bipolar cells. One type of monostratified ganglion cell and one bistratified cell tightly cofasciculate with axon terminals of the line 357 bipolar cells. Double labelling for kinesin II demonstrates colocalization of bipolar cell ribbons at the sites of contact between these two types of ganglion cell and the line 357 bipolar cells.

A computational model of the ribbon synapse

A model of the ribbon synapse was developed to replicate both pre- and postsynaptic functions of this glutamatergic juncture. The presynaptic portion of the model is rich in anatomical and physiological detail and includes multiple release sites for each ribbon based on anatomical studies of presynaptic terminals, presynaptic voltage at the terminal, the activation of voltage-gated calcium channels and a calcium-dependent release mechanism whose rate varies as a function of the calcium concentration that is monitored at two different sites which control both an ultrafast, docked pool of vesicles and a release ready pool of tethered vesicles. The postsynaptic portion of the program models diffusion of glutamate and the physiological properties of glutamatergic neurotransmission in target cells. We demonstrate the behavior of the model using the retinal bipolar cell to ganglion cell ribbon synapse. The model was constrained by the anatomy of salamander bipolar terminals based on the ultrastructure of these synapses and presynaptic contacts were placed onto realistic ganglion cell morphology activated by a range of ribbon synapses (46-138). These inputs could excite the cell in a manner consistent with physiological observations. This model is a comprehensive, first-generation attempt to assemble our present understanding of the ribbon synapse into a domain that permits testing our understanding of this important structure. We believe that with minor modifications of this model, it can be fine tuned for other ribbon synapses.

Bipolar cells of the mouse retina: a gene gun, morphological study

One of the key elements concerning our understanding of the organization of the mouse retina is the complete classification of the various types of bipolar cells. With the present study, we tried to contribute to this important issue. Unfortunately, most of the antibodies that stain specifically bipolar cells in the retina of other mammals hardly work for the retina of the mouse. We succeeded in overcoming this limitation by using a relatively novel technique based on the gene gun transfer of fluorescent dyes to cells. Hence, we were able to stain a considerable number of bipolar cells that could be characterized according to morphological and comparative criteria. We also performed a complete morphometric analysis of a subset of bipolar cells stained by anti-neurokinin-3 receptor antibodies. We found nine types of cone bipolar cells and one type of rod bipolar cell; these data are consistent with the findings of previous studies on the retinas of other mammals, such as rabbits, rats, and monkeys and with a recent study based on the mouse retina (Ghosh et al. [2004] J Comp Neurol 469:70-82). Our results also confirm the existence of a common structural similarity among mammalian retinas. It remains to be elucidated what is exactly the functional role of the various types of cone bipolar cells and what is the specific contribution they provide to the perception of a given visual stimulus. Most probably, each bipolar cell type constitutes a specialized channel for the computation of a selected component of the visual stimulus. More complex signal coding, involving the coordinated activity of various types of bipolar cells, could also be postulated, as it has been shown for ganglion cells (Meister [1996] Proc Natl Acad Sci U S A 93:609-614).

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'.

How Retinal Ganglion Cells Prevent Synaptic Noise From Reaching the Spike Output

Synaptic vesicles are released stochastically, and therefore stimuli that increase a neuron's synaptic input might increase noise at its spike output. Indeed this appears true for neurons in primary visual cortex, where spike output variability increases with stimulus contrast. But in retinal ganglion cells, although intracellular recordings (with spikes blocked) showed that stronger stimuli increase membrane fluctuations, extracellular recordings showed that noise at the spike output is constant. Here we show that these seemingly paradoxical findings occur in the same cell and explain why. We made intracellular recordings from ganglion cells, in vitro, and presented periodic stimuli of various contrasts. For each stimulus cycle, we measured the response at the stimulus frequency (F1) for both membrane potential and spikes as well as the spike rate. The membrane and spike F1 response increased with contrast, but noise (SD) in the F1 responses and the spike rate was constant. We also measured membrane fluctuations (with spikes blocked) during the response depolarization and found that they did increase with contrast. However, increases in fluctuation amplitude were small relative to the depolarization (<10% at high contrast). A model based on estimated synaptic convergence, release rates, and membrane properties accounted for the relative magnitudes of fluctuations and depolarization. Furthermore, a cell's peak spike response preceded the peak depolarization, and therefore fluctuation amplitude peaked as the spike response declined. We conclude that two extremely general properties of a neuron, synaptic convergence and spike generation, combine to minimize the effects of membrane fluctuations on spiking.

Primate Retinal Signaling Pathways: Suppressing ON-Pathway Activity in Monkey With Glutamate Analogues Mimics Human CSNB1-NYX Genetic Night Blindness

Retinal on-pathway dysfunction is implicated in human complete-type congenital stationary night blindness (CSNB1), a Mendelian genetic condition that results from mutations in the NYX gene encoding the protein nyctalopin. We probed cone pathway dysfunction in four human genotyped CSNB1 affected males by electroretinogram (ERG) recordings elicited with photopic sinusoidal and rapid-on/off-ramp flicker stimuli that are reputed to elicit on/off-pathway activity selectively. Results were analyzed in relation to ERG abnormalities created in anesthetized non-human primates by intravitreal application of glutamate analogues that selectively suppress retinal on- or off-pathway bipolar cell activity. 2-amino-4-phosphonobutyric acid (APB), which selectively blocks light responses of on-pathway depolarizing bipolar cells, fully recreated the essential ERG abnormalities found for human CSNB1 under the condition that the off-pathway remained active. Both CSNB1-NYX humans and APB-treated monkey retina lacked the normal amplitude dip and the phase deflection that occurs in the fundamental component near 12 Hz for sinusoidal flicker stimuli. The off-pathway suppressing agent, cis-2,3-piperidine-dicarboxylic acid (PDA), gave results in monkey quite discordant to CSNB1 human for sinusoidal stimulation. The results implicated a specific on-pathway signaling deficiency in CSNB1-NYX males with no evidence of off-pathway involvement. Likewise, rapid-on/off ramping stimuli also indicated that the functional deficit was localized to the on pathway. Analysis of non-human primate retinal responses after drug application demonstrated a complexity to on/off-pathway contributions to ramping on/off ERG responses not previously anticipated. These results support the hypothesis that nyctalopin acts principally or exclusively within the on pathway at the level of depolarizing bipolar cells, and thus human CSNB1-NYX subjects provide an opportunity to probe the primate visual system for consequences of on-pathway deficits.

Temporal modulation sensitivity of tree shrew retinal ganglion cells

Tree shrews (Tupaia belangeri) are small diurnal mammals capable of quick and agile navigation. Electroretinographic and behavioral studies have indicated that tree shrews possess very good temporal vision, but the neuronal mechanisms underlying that temporal vision are not well understood. We used single-unit extracellular recording techniques to characterize the temporal response properties of individual retinal ganglion cell axons recorded from the optic tract. A prominent characteristic of most cells was their sustained or transient nature in responding to the flashing spot. Temporal modulation sensitivity functions were obtained using a Gaussian spot that was temporally modulated at different frequencies (2–60 Hz). Sustained cells respond linearly to contrast. They showed an average peak frequency of 6.9 Hz, a high-frequency cutoff at 31.3 Hz, and low-pass filtering. Transient cells showed nonlinear response to contrast. They had a peak frequency of 19.3 Hz, a high-frequency cutoff at about 47.6 Hz, band-pass filtering, and higher overall sensitivity than sustained cells. The responses of transient cells also showed a phase advance of about 88 deg whereas the phase advance for sustained cells was about 43 deg. Comparison with behavioral temporal modulation sensitivity results suggested that transient retinal ganglion cells may underlie detection for a wide range of temporal frequencies, with sustained ganglion cells possibly mediating detection below 4 Hz. These data suggest that two well-separated temporal channels exist at the retinal ganglion cell level in the tree shrew retina, with the transient channel playing a major role in temporal vision.

Effects of noise on the spike timing precision of retinal ganglion cells

Information in a spike train is limited by variability in the spike timing. This variability is caused by noise from several sources including synapses and membrane channels; but how deleterious each noise source is and how they affect spike train coding is unknown. Combining physiology and a multicompartment model, we studied the effect of synaptic input noise and voltage-gated channel noise on spike train reliability for a mammalian ganglion cell. For tonic stimuli, the SD of the interspike intervals increased supralinearly with increasing interspike interval. When the cell was driven by current injection, voltage-gated channel noise and background synaptic noise caused fluctuations in the interspike interval of comparable amplitude. Spikes initiated on the dendrites could cause additional spike timing fluctuations. For transient stimuli, synaptic noise was dominant and spontaneous background activity strongly increased fluctuations in spike timing but decreased the latency of the first spike.

Synaptic ribbon. Conveyor belt or safety belt?

The synaptic ribbon in neurons that release transmitter via graded potentials has been considered as a conveyor belt that actively moves vesicles toward their release sites. But evidence has accumulated to the contrary, and it now seems plausible that the ribbon serves instead as a safety belt to tether vesicles stably in mutual contact and thus facilitate multivesicular release by compound exocytosis.

The influence of different retinal subcircuits on the nonlinearity of ganglion cell behavior

Y-type retinal ganglion cells show a pronounced, nonlinear, frequency-doubling behavior in response to modulated sinewave gratings. This is not observed in X-type cells. The source of this spatial nonlinear summation is still under debate. We have designed a realistic biophysical model of the cat retina to test the influence of different retinal cell classes and subcircuits on the linearity of ganglion cell responses. The intraretinal connectivity consists of the fundamental feedforward pathway via bipolar cells, lateral horizontal cell connectivity, and two amacrine circuits. The wiring diagram of X- and Y-cells is identical apart from two aspects: (1) Y-cells have a wider receptive field and (2) they receive input from a nested amacrine circuit consisting of narrow- and wide-field amacrine cells. The model was tested with contrast-reversed gratings. First and second harmonic response components were determined to estimate the degree of nonlinearity. By means of circuit dissection, we found that a high degree of the Y-cell nonlinear behavior arises from the spatial integration of temporal photoreceptor nonlinearities. Furthermore, we found a weaker and less uniform influence of the nested amacrine circuit. Different sources of nonlinearities interact in a multiplicative manner, and the influence of the amacrine circuit is approximately 25% weaker than that of the photoreceptor. The model predicts that significant nonlinearities occur already at the level of horizontal cell responses. Pharmacological inactivation of the amacrine circuit is expected to exert a milder effect in reducing ganglion cell nonlinearity.

Light response of retinal ON bipolar cells requires a specific splice variant of Galpha(o)

Glutamate released onto retinal ON bipolar neurons binds to a metabotropic receptor to activate a heterotrimeric G-protein (G(o)) that ultimately closes a nonspecific cation channel. Signaling requires the alpha subunit (Galpha(o)), but its effector is unknown. Because Galpha(o) is transcribed into two splice variants (alpha(o1) and alpha(o2)) that differ in the key GTPase domain, the next step in elucidating this pathway was to determine which splice variant carries the signal. Here we show by reverse transcription-PCR and Western blots that retina expresses both splice variants. Furthermore, in situ hybridization and immunostaining on mouse retina deficient in one splice variant or the other show that both alpha(o1) and alpha(o2) are expressed by ON bipolar cells but that alpha(o1) is much more abundant. Finally, electroretinography performed on mice deficient for one splice variant or the other shows that the positive b-wave (response of ON bipolar cells to rod and cone input) requires alpha(o1) but not alpha(o2). Thus, the light response of the ON bipolar cell is probably carried by its strongly expressed splice variant, Galpha(o1).

The receptive fields of cat retinal ganglion cells in physiological and pathological states: where we are after half a century of research

Studies on the receptive field properties of cat retinal ganglion cells over the past half-century are reviewed within the context of the role played by the receptive field in visual information processing. Emphasis is placed on the work conducted within the past 20 years, but a summary of key contributions from the 1950s to 1970s is provided. We have sought to review aspects of the ganglion cell receptive field that have not been featured prominently in previous review articles. Our review of the receptive field properties of X- and Y-cells focuses on quantitative studies and includes consideration of the function of the receptive field in visual signal processing. We discuss the non-classical as well as the classical receptive field. Attention is also given to the receptive field properties of the less well-studied cat ganglion cells-the W-cells-and the effect of pathology on cat ganglion cell properties. Although work from our laboratories is highlighted, we hope that we have given a reasonably balanced view of the current state of the field.

Signal processing in a simple visual system: the locust ocellar system and its synapses

The neurons with the widest axons that carry information into a locust brain belong to L-neurons, the large, second-order neurons of the ocelli. L-neurons play roles in flight control and boosting visual sensitivity. Their morphology is simple, and their axons convey graded potentials from the ocellus with little decrement to the brain, which makes them good subjects in which to study transmission of graded potentials. L-neurons are very sensitive to changes in light, due to an abnormally high gain in the sign inverting synapses they receive from photoreceptors. Adaptation ensures that L-neurons signal contrast in a light signal when average light intensity changes, and that their responses depend on the speed of change in light. Neurons L1-3 make excitatory output synapses with third-order neurons and with L4-5. These synapses transmit tonically, but are unable to convey hyperpolarising signals about large increases in light. Graded rebound spikes enhance depolarising responses. L1-3 also make reciprocal inhibitory synapses with each other and transmission at these decrements so rapidly that it normally requires a presynaptic spike. The resolution with which graded potentials can be transferred has been studied at the inhibitory synapses, and is limited by intrinsic variability in the mechanism that determines neurotransmitter release. Electron microscopy has shown that each excitatory connection made from an L-neuron to a postsynaptic partner consists of thousands of discrete synaptic contacts, in which individual dense-staining bars in the presynaptic neuron are associated with clouds of vesicles. Acetylcholine is likely to be a neurotransmitter released by L-neurons.

Microcircuits for night vision in mouse retina

Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod --> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod --> rod bipolar --> AII amacrine --> cone bipolar --> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).

Bipolar cells contribute to nonlinear spatial summation in the brisk-transient (Y) ganglion cell in mammalian retina

The receptive field of the Y-ganglion cell comprises two excitatory mechanisms: one integrates linearly over a narrow field, and the other integrates nonlinearly over a wide field. The linear mechanism has been attributed to input from bipolar cells, and the nonlinear mechanism has been attributed to input from a class of amacrine cells whose nonlinear "subunits" extend across the linear receptive field and beyond. However, the central component of the nonlinear mechanism could in theory be driven by bipolar input if that input were rectified. Recording intracellularly from the Y-cell in guinea pig retina, we blocked the peripheral component of the nonlinear mechanism with tetrodotoxin and found the remaining nonlinear receptive field to be precisely co-spatial with the central component of the linear receptive field. Both linear and nonlinear mechanisms were caused by an excitatory postsynaptic potential that reversed near 0 mV. The nonlinear mechanism depended neither on acetylcholine nor on feedback involving GABA or glycine. Thus the central components of the ganglion cell's linear and nonlinear mechanisms are apparently driven by synapses from the same rectifying bipolar cell.

The fundamental plan of the retina

The retina, like many other central nervous system structures, contains a huge diversity of neuronal types. Mammalian retinas contain approximately 55 distinct cell types, each with a different function. The census of cell types is nearing completion, as the development of quantitative methods makes it possible to be reasonably confident that few additional types exist. Although much remains to be learned, the fundamental structural principles are now becoming clear. They give a bottom-up view of the strategies used in the retina's processing of visual information and suggest new questions for physiological experiments and modeling.

Bipolar cells use kainate and AMPA receptors to filter visual information into separate channels

Unlike cone photoreceptors, whose light responses have a uniform time course, retinal ganglion cells are tuned to respond to different temporal components in a changing visual scene. The signals in a mammalian cone flow to three to five morphologically distinct "OFF" bipolar cells at a sign-conserving, glutamatergic synapse. By recording simultaneously from pairs of synaptically connected cones and OFF bipolar cells, I now show that each morphological type of OFF bipolar cell receives its signal through a different AMPA or kainate receptor. The characteristic rate at which each receptor recovers from desensitization divides the cone signal into temporal components. Temporal processing begins at the first synapse in the visual system.