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Rachel O L Wong

Publications and source records attributed to Rachel O L Wong.

At least 19 recordsLinked to original sources
PubMed2006

In vivo imaging reveals dendritic targeting of laminated afferents by zebrafish retinal ganglion cells.

Targeting of axons and dendrites to particular synaptic laminae is an important mechanism by which precise patterns of neuronal connectivity are established. Although axons target specific laminae during development, dendritic lamination has been thought to occur largely by pruning of inappropriately placed arbors. We discovered by in vivo time-lapse imaging that retinal ganglion cell (RGC) dendrites in zebrafish show growth patterns implicating dendritic targeting as a mechanism for contacting appropriate synaptic partners. Populations of RGCs labeled in transgenic animals establish distinct dendritic strata sequentially, predominantly from the inner to outer retina. Imaging individual cells over successive days confirmed that multistratified RGCs generate strata sequentially, each arbor elaborating within a specific lamina. Simultaneous imaging of RGCs and subpopulations of presynaptic amacrine interneurons revealed that RGC dendrites appear to target amacrine plexuses that had already laminated. Dendritic targeting of prepatterned afferents may thus be a novel mechanism for establishing proper synaptic connectivity.

Afferent Pathways↗
PubMed2006

Failure to maintain eye-specific segregation in nob, a mutant with abnormally patterned retinal activity.

Axon terminals from the two eyes initially overlap in the dorsal-lateral geniculate nucleus (dLGN) but subsequently refine to occupy nonoverlapping territories. Retinal activity is required to establish and maintain this segregation. We show that despite the presence of retinal activity, segregated projections desegregate when the structure of activity is altered. Early in development, spontaneous retinal activity in the no b-wave (nob) mouse is indistinguishable from that of wild-type mice, and eye-specific segregation proceeds normally. But, around eye-opening, spontaneous and visually evoked activity in nob retinas become abnormal, coincident with a failure to preserve precise eye-specific territories. Dark-rearing studies suggest that altered visual experience is not responsible. Transgenic rescue of the mutated protein (nyctalopin) within nob retinal interneurons, without rescuing expression in either retinal projection neurons or their postsynaptic targets in the dLGN, restores spontaneous retinal activity patterns and prevents desegregation. Thus, normally structured spontaneous retinal activity stabilizes newly refined retinogeniculate circuitry.

Animals↗
PubMed2005

Axons and dendrites originate from neuroepithelial-like processes of retinal bipolar cells.

The cellular mechanisms underlying axogenesis and dendritogenesis are not completely understood. The axons and dendrites of retinal bipolar cells, which contact their synaptic partners within specific laminae in the inner and outer retina, provide a good system for exploring these issues. Using transgenic mice expressing enhanced green fluorescent protein (GFP) in a subset of bipolar cells, we determined that axonal and dendritic arbors of these interneurons develop directly from apical and basal processes attached to the outer and inner limiting membranes, respectively. Selective stabilization of processes contributed to stratification of axonal and dendritic arbors within the appropriate synaptic layer. This unusual mode of axogenesis and dendritogenesis from neuroepithelial-like processes may act to preserve neighbor-neighbor relationships in synaptic wiring between the outer and inner retina.

Animals↗
PubMed2005

A comparison of experience-dependent plasticity in the visual and somatosensory systems.

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

Action Potentials↗
PubMed2005

Disruption and recovery of patterned retinal activity in the absence of acetylcholine.

Many developing neural circuits generate synchronized bursting activity among neighboring neurons, a pattern thought to be important for sculpting precise neural connectivity. Network output remains relatively constant as the cellular and synaptic components of these immature circuits change during development, suggesting the presence of homeostatic mechanisms. In the retina, spontaneous waves of activity are present even before chemical synapse formation, needing gap junctions to propagate. However, as synaptogenesis proceeds, retinal waves become dependent on cholinergic neurotransmission, no longer requiring gap junctions. Later still in development, waves are driven by glutamatergic rather than cholinergic synapses. Here, we asked how retinal activity evolves in the absence of cholinergic transmission by using a conditional mutant in which the gene encoding choline acetyltransferase (ChAT), the sole synthetic enzyme for acetylcholine (ACh), was deleted from large retinal regions. ChAT-negative regions lacked retinal waves for the first few days after birth, but by postnatal day 5 (P5), ACh-independent waves propagated across these regions. Pharmacological analysis of the waves in ChAT knock-out regions revealed a requirement for gap junctions but not glutamate, suggesting that patterned activity may have emerged via restoration of previous gap-junctional networks. Similarly, in P5 wild-type retinas, spontaneous activity recovered after a few hours in nicotinic receptor antagonists, often as local patches of coactive cells but not waves. The rapid recovery of rhythmic spontaneous activity in the presence of cholinergic antagonists and the eventual emergence of waves in ChAT knock-out regions suggest that homeostatic mechanisms regulate retinal output during development.

Acetylcholine↗
PubMed2005

Regulation of dendritic growth and plasticity by local and global calcium dynamics.

The dendritic arbors of neurons are organized into unique spatial patterns that are well suited for their specific functions. Although the intracellular signaling pathways that determine dendritic arbor size and branching patterns during development are not completely understood, it is evident that neurotransmission-mediated elevation in intracellular calcium levels ([Ca2+]i) plays a major role. Recent advances in calcium imaging and genetic approaches have provided new insight into how calcium acts to regulate dendritic growth and remodeling. Global increases in [Ca2+]i that occur upon neuronal depolarization control dendritic arbor growth by regulating transcription. However, the detailed branching patterns of dendritic arbors are regulated by local calcium signaling. Calcium-induced calcium release (CICR) from intracellular stores acts to locally stabilize dendritic branches, presumably triggered by neurotransmission upon contact with newly established inputs. The effects of global and local increases in [Ca2+]i on dendritic structure are cell type- and age-dependent. Unraveling the signaling pathways evoked by local and global rises in [Ca2+]i that shape the developing dendritic arbor at each developmental stage remains challenging but highly important.

Calcium↗
PubMed2005

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.

Animals↗
PubMed2005

Laminar circuit formation in the vertebrate retina.

Neuronal function depends on the accurate wiring between pre- and postsynaptic cells. Determining the mechanisms underlying precision in neuronal connectivity is challenging because of the complexity of the nervous system. In diverse parts of the nervous system, regions of synaptic contact are organized into distinct parallel layers, or laminae, that are correlated with distinct functions. Such an arrangement enables the development of synapse specificity to be more readily investigated. Here, we present an overview of the developmental mechanisms that are thought to underlie the formation of synaptic layers in the vertebrate retina, a highly laminated CNS structure. We will contrast the roles of activity-dependent and activity-independent mechanisms in establishing functionally discrete sublaminae in the inner retina, where circuits involving many subtypes of retinal neurons are assembled precisely. In addition, we will discuss new optical imaging approaches for elucidating how retinal synaptic lamination occurs in vivo.

Animals↗
PubMed2005

Cone survival despite rod degeneration in XOPS-mCFP transgenic zebrafish.

PURPOSE: In animal models of retinitis pigmentosa, rod photoreceptor degeneration eventually leads to loss of cone photoreceptors. The purpose of this study was to characterize a transgenic model of rod degeneration in zebrafish. METHODS: Zebrafish transgenic for XOPS-mCFP, a membrane-targeted form of cyan fluorescent protein driven by the Xenopus rhodopsin promoter, were generated by plasmid injection. Immunohistochemistry was used to detect cell type, proliferation, and TUNEL markers in larval and adult retinas. Rod- and cone-specific transcripts were detected by RT-PCR. Visual responses in transgenic adults were measured by electroretinogram. RESULTS: The XOPS promoter directed specific expression of mCFP in rods by 55 hours post fertilization (hpf). Rods in XOPS-mCFP heterozygotes began dying at 3.5 days post fertilization (dpf) and were almost completely absent by 5 dpf. A few rods were observed at the retinal margin, and numerous immature rods were observed in the outer nuclear layer (ONL) of transgenic adults. Apoptosis was increased in the ONL of larval and adult transgenic animals, and an elevation of rod precursor proliferation in adults was observed. ERG analysis confirmed that rod responses were absent in this line. Cone morphology and electrophysiology appeared normal in transgenic animals up to 7 months of age. CONCLUSIONS: The XOPS-mCFP transgene causes selective degeneration of rods without secondary loss of cones in animals up to 7 months of age. This raises important questions about the significance of rod-cone interactions in zebrafish and their potential as a model of human inherited retinal degenerations.

Animals↗
PubMed2005

Targeting of amacrine cell neurites to appropriate synaptic laminae in the developing zebrafish retina.

Cellular mechanisms underlying the precision by which neurons target their synaptic partners have largely been determined based on the study of projection neurons. By contrast, little is known about how interneurons establish their local connections in vivo. Here, we investigated how developing amacrine interneurons selectively innervate the appropriate region of the synaptic neuropil in the inner retina, the inner plexiform layer (IPL). Increases (ON) and decreases (OFF) in light intensity are processed by circuits that are structurally confined to separate ON and OFF synaptic sublaminae within the IPL. Using transgenic zebrafish in which the majority of amacrine cells express fluorescent protein, we determined that the earliest amacrine-derived neuritic plexus formed between two cell populations whose somata, at maturity, resided on opposite sides of this plexus. When we followed the behavior of individual amacrine cells over time, we discovered that they exhibited distinct patterns of structural dynamics at different stages of development. During cellular migration, amacrine cells exhibited an exuberant outgrowth of neurites that was undirected. Upon reaching the forming IPL, neurites extending towards the ganglion cell layer were relatively more stable. Importantly, when an arbor first formed, it preferentially ramified in either the inner or outer IPL corresponding to the future ON and OFF sublaminae, and maintained this stratification pattern. The specificity by which ON and OFF amacrine interneurons innervate their respective sublaminae in the IPL contrasts with that observed for projection neurons in the retina and elsewhere in the central nervous system.

Amacrine Cells↗
PubMed2004

Transient requirement for ganglion cells during assembly of retinal synaptic layers.

The inner plexiform layer (IPL) of the vertebrate retina comprises functionally specialized sublaminae, representing connections between bipolar, amacrine and ganglion cells with distinct visual functions. Developmental mechanisms that target neurites to the correct synaptic sublaminae are largely unknown. Using transgenic zebrafish expressing GFP in subsets of amacrine cells, we imaged IPL formation and sublamination in vivo and asked whether the major postsynaptic cells in this circuit, the ganglion cells, organize the presynaptic inputs. We found that in the lak/ath5 mutant retina, where ganglion cells are never born, formation of the IPL is delayed, with initial neurite outgrowth ectopically located and grossly disorganized. Over time, the majority of early neurite projection errors are corrected, and major ON and OFF sublaminae do form. However, focal regions of disarray persist where sublaminae do not form properly. Bipolar axons, which arrive later, are targeted correctly, except at places where amacrine stratification is disrupted. The lak mutant phenotype reveals that ganglion cells have a transient role organizing the earliest amacrine projections to the IPL. However, it also suggests that amacrine cells interact with each other during IPL formation; these interactions alone appear sufficient to form the IPL. Furthermore, our results suggest that amacrines may guide IPL sublamination by providing stratification cues for other cell types.

Animals↗
PubMed2003

Mapping by waves. Patterned spontaneous activity regulates retinotopic map refinement.

A role for spontaneous spiking activity in shaping neuronal circuits has frequently been debated. Analyses of retinotopy in mutant mice with reduced correlated firing among neighboring retinal cells by Grubb et al. and McLaughlin et al. in this issue of Neuron indicate the importance of patterned spontaneous activity for retinotopic map refinement in subcortical visual targets.

Action Potentials↗
PubMed2003

Developmental loss of synchronous spontaneous activity in the mouse retina is independent of visual experience.

In the immature retina, correlated spontaneous activity in the form of propagating waves is thought to be necessary for the refinement of connections between the retina and its targets. The continued presence of this activity in the mature retina would interfere with the transmission of information about the visual scene. The mechanisms responsible for the disappearance of retinal waves are not well understood, but one hypothesis is that visual experience is important. To test this hypothesis, we monitored the developmental changes in spontaneous retinal activity of both normal mice and mice reared in the dark. Using multi-electrode array recordings, we found that retinal waves in normally reared mice are present at postnatal day (P) 9 and begin to break down shortly after eye opening, around P15. By P21, waves have disappeared, and synchronous firing is comparable with that observed in the adult (6 weeks). In mice raised in the dark, we found a similar time course for the disappearance of waves. However, at P15, dark-reared retinas occasionally showed abnormally long periods of relative inactivity, not seen in controls. Apart from this quiescence, we found no striking differences between the patterns of spontaneous retinal activity from normal and dark-reared mice. We therefore suggest that visual experience is not required for the loss of synchronous spontaneous activity.

Action Potentials↗
PubMed2002

Imaging calcium dynamics in the nervous system by means of ballistic delivery of indicators.

The use of fluorescence-based calcium indicators has, over the years, unraveled important calcium-dependent mechanisms underlying neuronal function and development. However, difficulties associated with the loading of calcium indicators have limited their widespread use, particularly for the study of neuronal processing in the adult nervous system. Here, we show that in the central and peripheral nervous systems, populations of neurons and their processes, including dendritic spines and filopodia, can be labeled rapidly and efficiently by delivering calcium indicator-coated particles using a 'gene gun'. Importantly, neuronal labeling occurred both in vitro and in vivo, and across a wide range of ages and preparations. The labeled cells demonstrate spontaneous and evoked calcium transients, indicating that particle-mediated delivery is not deleterious to neuronal function. Furthermore, unlike loading with patch pipettes, cytoplasmic content is preserved following ballistic loading. This enables the study of calcium-dependent second messenger pathways without loss of signaling components. The ballistic delivery of calcium indicators thus opens up many new avenues for further exploration of the structure and function of the nervous system from single spines to neuronal networks.

Animals↗
PubMed2002

Transmitter-evoked local calcium release stabilizes developing dendrites.

In the central nervous system, dendritic arborizations of neurons undergo dynamic structural remodelling during development. Processes are elaborated, maintained or eliminated to attain the adult pattern of synaptic connections. Although neuronal activity influences this remodelling, it is not known how activity exerts its effects. Here we show that neurotransmission-evoked calcium (Ca(2+)) release from intracellular stores stabilizes dendrites during the period of synapse formation. Using a ballistic labelling method to load cells with Ca(2+) indicator dyes, we simultaneously monitored dendritic activity and structure in the intact retina. Two distinct patterns of spontaneous Ca(2+) increases occurred in developing retinal ganglion cells--global increases throughout the arborization, and local 'flashes' of activity restricted to small dendritic segments. Blockade of local, but not global, activity caused rapid retraction of dendrites. This retraction was prevented locally by focal uncaging of caged Ca(2+) that triggered Ca(2+) release from internal stores. Thus, local Ca(2+) release is a mechanism by which afferent activity can selectively and differentially regulate dendritic structure across the developing arborization.

Acetylcholine↗
PubMed2002

Ventricle-directed migration in the developing cerebral cortex.

It is believed that postmitotic neurons migrate away from their sites of origin in the germinal zones to populate distant targets. Contrary to this notion, we found, using time-lapse imaging of brain slices, populations of neurons positioned at various levels of the developing neocortex that migrate towards the cortical ventricular zone. After a pause in this proliferative zone, they migrate radially in the direction of the pial surface to take up positions in the cortical plate. Immunohistochemical analysis together with tracer labeling in brain slices showed that cells showing ventricle-directed migration in the developing cortex are GABAergic interneurons originating in the ganglionic eminence in the ventral telencephalon. We speculate that combinations of chemoattractant and chemorepellent molecules are involved in this ventricle-directed migration and that interneurons may seek the cortical ventricular zone to receive layer information.

Animals↗
PubMed2002

Segregation of ON and OFF retinogeniculate connectivity directed by patterned spontaneous activity.

In many parts of the developing nervous system, the early patterns of connectivity are refined by processes that require neuronal activity. These processes are thought to involve Hebbian mechanisms that lead to strengthening and maintenance of inputs that display correlated pre- and postsynaptic activity and elimination of inputs that fire asynchronously. Here we investigated the role of patterned spontaneous retinal activity and Hebbian synaptic mechanisms on segregation of ON and OFF retinal afferents in the dorsal lateral geniculate nucleus (dLGN) of the developing ferret visual system. We recorded extracellularly the spontaneous spike activity of neighboring pairs of ganglion cells and found that OFF cells have significantly higher mean firing rates than ON cells. Spiking is best correlated between cells of the same sign (ON, ON; OFF, OFF) compared with cells of opposite sign (ON, OFF). We then constructed a simple Hebbian model of retinogeniculate synaptic development based on a correlational framework. Using our recorded activity patterns, together with previous calcium-imaging data, we show that endogenous retinal activity, coupled with Hebbian mechanisms of synaptic development, can drive the segregation of ON and OFF retinal inputs to the dLGN. Segregation occurs robustly when heterosynaptic competition is present within time windows of 50-500 ms. In addition, our results suggest that the initial patterns of connectivity (biases in convergence of inputs) and the strength of inhibition in the network each play a crucial role in determining whether ON or OFF inputs dominate at maturity.

Algorithms↗