Stephen J. Eglen

Development of retinal ganglion cell structure and function.

In this review, we summarize the main stages of structural and functional development of retinal ganglion cells (RGCs). We first consider the various mechanisms that are involved in restructuring of dendritic trees. To date, many mechanisms have been implicated including target-dependent factors, interactions from neighboring RGCs, and afferent signaling. We also review recent evidence showing how rapidly such dendritic remodeling might occur, along with the intracellular signaling pathways underlying these rearrangements. Concurrent with such structural changes, the functional responses of RGCs also alter during maturation, from sub-threshold firing to reliable spiking patterns. Here we consider the development of intrinsic membrane properties and how they might contribute to the spontaneous firing patterns observed before the onset of vision. We then review the mechanisms by which this spontaneous activity becomes correlated across neighboring RGCs to form waves of activity. Finally, the relative importance of spontaneous versus light-evoked activity is discussed in relation to the emergence of mature receptive field properties.

Developmental Modulation of Retinal Wave Dynamics: Shedding Light on the GABA Saga

Embryonic spontaneous activity, in the form of propagating waves, is crucial for refining visual connections. To study what aspects of this correlated activity are instructive, we must first understand how their dynamics change with development and what factors trigger their disappearance after birth. Here we report that in the turtle retina, GABA, rather than glutamate and acetylcholine, influences developmental changes in wave dynamics. Using calcium imaging of the ganglion cell layer, we report how waves switch from fast and broad, when they emerge, to slow and narrow a few days before hatching, coinciding with the emergence of excitatory GABAA receptor-mediated activity. Around hatching, waves gradually become stationary patches, whereas GABAA shifts from excitatory to inhibitory, coinciding with the upregulation of the cotransporter KCC2, suggesting that changes in intracellular chloride underlie the shift. Dark-rearing from hatching causes correlated spontaneous activity to persist, whereas GABAA responses remain excitatory, and KCC2 expression is weaker. We conclude that GABA plays an important regulatory role during the maturation of retinal neural activity. Using a simple and elegant mechanism, namely the switch from excitatory to inhibitory, GABAA receptor-mediated activity is necessary and sufficient to cause retinal waves to stop propagating, ultimately leading to the disappearance of correlated spontaneous activity. Moreover, our results suggest that visual experience modulates the GABAergic switch.

Determinants of the exclusion zone in dopaminergic amacrine cell mosaics

A fundamental organizing feature of the retina is the presence of regularly spaced distributions of neurons, yet we have little knowledge of how this patterning emerges during development. Among these retinal mosaics, the spatial organization of the dopaminergic amacrine cells is unique: using nearest‐neighbor and Vornoi domain analysis, we found that the dopaminergic amacrine cells were neither randomly distributed, nor did they achieve the regularity documented for other retinal cell types. Autocorrelation analysis revealed the presence of an exclusion zone surrounding individual dopaminergic amacrine cells and modeling studies confirmed this organization, as the mosaic could be simulated by a minimal distance spacing rule defined by a broad set of parameters. Experimental studies determined the relative contributions of tangential dispersion, fate determination, and cell death in the establishment of this exclusion zone. Clonal boundary analysis and simulations of proximity‐driven movement discount tangential dispersion, while data from bcl‐2 overexpressing mice rule out feedback‐inhibitory fate‐deterministic accounts. Cell death, by contrast, appears to eliminate dopaminergic amacrine cells that are within close proximity, thereby establishing the exclusion zone surrounding individual cells and in turn creating their mosaic regularity. J. Comp. Neurol. 461:123–136, 2003.

Following the ontogeny of retinal waves: pan‐retinal recordings of population dynamics in the neonatal mouse

Novel pan‐retinal recordings of mouse retinal waves were obtained at near cellular resolution using a large‐scale, high‐density array of 4096 electrodes to investigate changes in wave spatiotemporal properties from postnatal day 2 to eye opening. Early cholinergic waves are large, slow and random, with low cellular recruitment. A developmental shift in GABAA signalling from depolarizing to hyperpolarizing influences the dynamics of cholinergic waves. Glutamatergic waves that occur just before eye opening are focused, faster, denser, non‐random and repetitive. These results provide a new, deeper understanding of developmental changes in retinal spontaneous activity patterns, which will help researchers in the investigation of the role of early retinal activity during wiring of the visual system.

Influence of cell fate mechanisms upon retinal mosaic formation: a modelling study

Many types of retinal neurone are arranged in a spatially regular manner so that the visual scene is uniformly sampled. Several mechanisms are thought to be involved in the development of regular cellular positioning. One early-acting mechanism is the lateral inhibition of neighbouring cells from acquiring the same fate, mediated by Delta-Notch signalling. We have used computer modelling to test whether lateral inhibition might transform an initial population of undifferentiated cells into more regular populations of two types of differentiated cells. Initial undifferentiated cells were positioned randomly, subject only to a minimal distance constraint. Each undifferentiated cell then acquired either primary or secondary fate using one of several lateral inhibition mechanisms. Mosaic regularity was assessed using the regularity index and the packing factor. We found that for irregular undifferentiated mosaics, the arrangement of resulting primary (but not secondary) fate cells was more regular than in the initial undifferentiated population. However, for regular undifferentiated mosaics, no further increases in the regularity of the primary fate mosaics were observed. We have used this model to test the specific hypothesis that on- and off-centre retinal ganglion cells emerge from an initial, irregular undifferentiated population of ganglion cells. Lateral inhibition can subdivide an initially irregular population into two types of cell that are mildly regular. However, lateral inhibition alone is insufficient to produce mosaics of the same regularity as observed experimentally. Likewise, and in contrast to earlier reports, cell death alone is insufficient to match the regularity of experimental mosaics. We conclude that lateral inhibition can transform irregular distributions into regular mosaics, upon which subsequent processes (such as lateral cell movement or cell death) can further refine mosaic regularity.

Lateral cell movement driven by dendritic interactions is sufficient to form retinal mosaics

The formation of retinal mosaics is thought to involve lateral movement of retinal cells from their clonal column of origin. The forces underlying this lateral cell movement are currently unknown. We have used a model of neurite outgrowth combined with cell movement to investigate the hypothesis that lateral cell movement is guided by dendritic interactions. We have assumed that cells repel each other in proportion to the degree of dendritic overlap between neighbouring cells. Our results first show that small cell movements are sufficient to transform random cell distributions into regular mosaics, and that all cells within the population move. When dendritic fields are allowed to grow, the model produces regular mosaics across all cell densities tested. We also find that the model can produce constant coverage of visual space over varying cell densities. However, if dendritic field sizes are fixed, mosaic regularity is proportional to the cell density and dendritic field size. Our model suggests that dendritic mechanisms may therefore provide sufficient information for rearrangement of cells into regular mosaics. We conclude by mentioning possible future experiments that might suggest whether dendritic interactions are adaptive or fixed during mosaic formation.

Detecting Pairwise Correlations in Spike Trains: An Objective Comparison of Methods and Application to the Study of Retinal Waves

Correlations in neuronal spike times are thought to be key to processing in many neural systems. Many measures have been proposed to summarize these correlations and of these the correlation index is widely used and is the standard in studies of spontaneous retinal activity. We show that this measure has two undesirable properties: it is unbounded above and confounded by firing rate. We list properties needed for a measure to fairly quantify and compare correlations and we propose a novel measure of correlation—the spike time tiling coefficient. This coefficient, the correlation index, and 33 other measures of correlation of spike times are blindly tested for the required properties on synthetic and experimental data. Based on this, we propose a measure (the spike time tiling coefficient) to replace the correlation index. To demonstrate the benefits of this measure, we reanalyze data from seven key studies, which previously used the correlation index to investigate the nature of spontaneous activity. We reanalyze data from β2(KO) and β2(TG) mutants, mutants lacking connexin isoforms, and also the age-dependent changes in wild-type and β2(KO) correlations. Reanalysis of the data using the proposed measure can significantly change the conclusions. It leads to better quantification of correlations and therefore better inference from the data. We hope that the proposed measure will have wide applications, and will help clarify the role of activity in retinotopic map formation.

Dopaminergic Amacrine Cells in the Inner Nuclear Layer and Ganglion Cell Layer Comprise a Single Functional Retinal Mosaic

Many types of retinal neuron are distributed in an orderly manner across the surface of the retina. Indeed, the existence of such regularity amongst a population of neurons, termed a retinal mosaic, may be a defining feature of functionally independent types of retinal neuron. We have examined the spatial distribution of dopaminergic amacrine cells in the ferret retina both in the inner nuclear layer (INL) and in the ganglion cell layer (GCL) to determine whether the cells in each layer form an independent retinal mosaic as evidence of whether they should be considered as two separate types. Ferret retinas contain approximately 1,900 dopaminergic amacrine cells, of which 27% are located in the GCL, and the rest in the INL. Based on analysis of their Voronoi domains as well as autocorrelation analysis and tests for complete spatial randomness, we found that the distribution of INL cells was statistically regular, while that of the GCL cells was not. However, by using cross‐correlation analysis, these two groups of cells were found to be spatially dependent: an exclusion zone was detected in the cross‐correlogram of roughly the same size as that found in the autocorrelograms of both INL and GCL cells. Such a pattern would be expected if dopaminergic amacrine cells in the INL and GCL were members of a single regular population differing only in their somatic depth. By using computer simulations, we tested this hypothesis directly, confirming that a random assignment of 27% from the total population produces cross‐correlograms that are indistinguishable from those of the biological mosaics. We conclude, therefore, that the cells in the two layers form a single functional population; those in the GCL appear to be misplaced. Somatic positioning with respect to depth within the retina is not, by itself, a reliable guide for functional classification. J. Comp. Neurol. 466:343–355, 2003.

A Multi-Component Model of the Developing Retinocollicular Pathway Incorporating Axonal and Synaptic Growth

During development, neurons extend axons to different brain areas and produce stereotypical patterns of connections. The mechanisms underlying this process have been intensively studied in the visual system, where retinal neurons form retinotopic maps in the thalamus and superior colliculus. The mechanisms active in map formation include molecular guidance cues, trophic factor release, spontaneous neural activity, spike-timing dependent plasticity (STDP), synapse creation and retraction, and axon growth, branching and retraction. To investigate how these mechanisms interact, a multi-component model of the developing retinocollicular pathway was produced based on phenomenological approximations of each of these mechanisms. Core assumptions of the model were that the probabilities of axonal branching and synaptic growth are highest where the combined influences of chemoaffinity and trophic factor cues are highest, and that activity-dependent release of trophic factors acts to stabilize synapses. Based on these behaviors, model axons produced morphologically realistic growth patterns and projected to retinotopically correct locations in the colliculus. Findings of the model include that STDP, gradient detection by axonal growth cones and lateral connectivity among collicular neurons were not necessary for refinement, and that the instructive cues for axonal growth appear to be mediated first by molecular guidance and then by neural activity. Although complex, the model appears to be insensitive to variations in how the component developmental mechanisms are implemented. Activity, molecular guidance and the growth and retraction of axons and synapses are common features of neural development, and the findings of this study may have relevance beyond organization in the retinocollicular pathway.

Neural circuits for peristaltic wave propagation in crawling Drosophila larvae: analysis and modeling

Drosophila larvae crawl by peristaltic waves of muscle contractions, which propagate along the animal body and involve the simultaneous contraction of the left and right side of each segment. Coordinated propagation of contraction does not require sensory input, suggesting that movement is generated by a central pattern generator (CPG). We characterized crawling behavior of newly hatched Drosophila larvae by quantifying timing and duration of segmental boundary contractions. We developed a CPG network model that recapitulates these patterns based on segmentally repeated units of excitatory and inhibitory (EI) neuronal populations coupled with immediate neighboring segments. A single network with symmetric coupling between neighboring segments succeeded in generating both forward and backward propagation of activity. The CPG network was robust to changes in amplitude and variability of connectivity strength. Introducing sensory feedback via “stretch-sensitive” neurons improved wave propagation properties such as speed of propagation and segmental contraction duration as observed experimentally. Sensory feedback also restored propagating activity patterns when an inappropriately tuned CPG network failed to generate waves. Finally, in a two-sided CPG model we demonstrated that two types of connectivity could synchronize the activity of two independent networks: connections from excitatory neurons on one side to excitatory contralateral neurons (E to E), and connections from inhibitory neurons on one side to excitatory contralateral neurons (I to E). To our knowledge, such I to E connectivity has not yet been found in any experimental system; however, it provides the most robust mechanism to synchronize activity between contralateral CPGs in our model. Our model provides a general framework for studying the conditions under which a single locally coupled network generates bilaterally synchronized and longitudinally propagating waves in either direction.