Normal and regenerating optic fibers in goldfish tectum: HRP-EM evidence for rapid synaptogenesis and optic fiber-fiber affinity

A simple model can unify a broad range of phenomena in retinotectal map development

A paradigm model system for studying the development of patterned connections in the nervous system is the topographic map formed by retinal axons in the optic tectum/superior colliculus. Starting in the 1970s, a series of computational models have been proposed to explain map development in both normal conditions, and perturbed conditions where the retina and/or tectum/superior colliculus are altered. This stands in contrast to more recent models that have often been simpler than older ones, and tend to address more limited data sets, but include more recent genetic manipulations. The original exploration of many of the early models was one-dimensional and limited by the computational resources of the time. This leaves open the ability of these early models to explain both map development in two dimensions, and the genetic manipulation data that have only appeared more recently. In this article, we show that a two-dimensional and updated version of the XBAM model (eXtended Branch Arrow Model), first proposed in 1982, reproduces a range of surgical map manipulations not yet demonstrated by more modern models. A systematic exploration of the parameter space of this model in two dimensions also reveals richer behavior than that apparent from the original one-dimensional versions. Furthermore, we show that including a specific type of axon-axon interaction can account for the map collapse recently observed when particular receptor levels are genetically manipulated in a subset of retinal ganglion cells. Together these results demonstrate that balancing multiple influences on map development seems to be necessary to explain many biological phenomena in retinotectal map formation, and suggest important constraints on the underlying biological variables.

Restoration of visual function following optic nerve regeneration in bluegill (Lepomis macrochirus) × pumpkinseed (Lepomis gibbosus) hybrid sunfish

Simple (dorsal light reflex) and complex (predator-prey interactions) visually mediated behaviors were used concurrently with morphological examination to assess restoration of visual function following optic nerve crush in bluegill (Lepomis macrochirus) × pumpkinseed (Lepomis gibbosus) hybrid sunfish. Regenerating optic nerve axons projected into thestratum opticum-stratum fibrosum et griseum superficialeby week 2, thestratum griseum centraleby week 4, andstratum album centraleby week 6. Initial projections into the laminae were diffuse and less stratified compared to controls. By week 12, the projection pattern of regenerating nerve fibers closely resembled the innervation of normal tecta. Visual improvements were correlated with increasing projections into the tectum. The dorsal light reflex improved from a 45° vertical deviation following nerve crush to 4.5° by week 16. Initial predator-prey interactions were exclusively mediated by the control eye. As regeneration progressed, there was a gradual expansion of the visual field. The reaction distance and attack angles within the visual field of the experimental eye were initially less than controls, however, these differences disappeared by week 10. Improvements in visual function were closely correlated with an increase of regenerating ganglion cell axons into the optic tectum indicating sufficient synaptogenesis to mediate both simple and complex visual behavior.

The development of retinotectal maps: a review of models based on molecular gradients

Information about the world is often represented in the brain in the form of topographic maps. A paradigm example is the topographic representation of the visual world in the optic tectum/superior colliculus. This map initially forms during neural development using activity-independent molecular cues, most notably some type of chemospecific matching between molecular gradients in the retina and corresponding gradients in the tectum/superior colliculus. Exactly how this process might work has been studied both experimentally and theoretically for several decades. This review discusses the experimental data briefly, and then in more detail the theoretical models proposed. The principal conclusions are that (1) theoretical models have helped clarify several important ideas in the field, (2) earlier models were often more sophisticated than more recent models, and (3) substantial revisions to current modelling approaches are probably required to account for more than isolated subsets of the experimental data.

Spontaneous retinal activity is tonic and does not drive tectal activity during activity-dependent refinement in regeneration

During development, waves of activity periodically spread across retina to produce correlated activity that is thought to drive activity-dependent ordering in optic fibers. We asked whether similar waves of activity are produced in the retina of adult goldfish during activity-dependent refinement by regenerating optic fibers. Dual-electrode recordings of spontaneous activity were made at different distances across retina but revealed no evidence of retinal waves in normal retina or during regeneration. Retinal activity was tonic and lacked the episodic bursting associated with waves. Cross-correlation analysis showed that the correlated activity that was normally restricted to near neighbors (typically seen across 100-200 microm and absent at >500 microm) was not altered during regeneration. The only change associated with regeneration was a twofold reduction in ganglion cell firing rates. Because spontaneous retinal activity is known to be sufficient to generate refinement during regeneration in goldfish, we examined its effect on tectal activity. In normal fish, acutely eliminating retinal activity with TTX rapidly reduced tectal unit activity by >90%. Surprisingly, during refinement at 4-6 weeks, eliminating retinal activity had no detectable effect on tectal activity. Similar results were obtained in recordings from torus longitudinalis. After refinement at 3 months, tectal activity was again highly dependent on ongoing retinal activity. We conclude that spontaneous retinal activity drives tectal cells in normal fish and after regeneration but not during activity-dependent refinement. The implications of these results for the role of presynaptic activity in refinement are considered.

Regenerating optic fibers correct large-scale errors by random growth: evidence from in vivo imaging

Regenerating optic fibers in goldfish make large-scale errors when they invade tectum and subsequently correct these to generate a projection with moderate retinotopic order by 1 month. The behavior of fibers underlying these extensive rearrangements is not well understood. To clarify this, we have imaged optic fibers in living adult goldfish at 2-4 weeks of regeneration. A small number of neighboring retinal ganglion cells were labeled with microinjections of DiI and imaged in the dorsal tectum with a cooled CCD camera on a fluorescence microscope for 5 to 8 hours. Nearly all fibers were simple unbranched processes and had endings that were highly dynamic showing both growth and retraction. Fibers from dorsal retina that normally innervate ventral tectum were frequently observed in dorsal tectum. These ectopic fibers oscillated more frequently between growth and retraction and retracted more often than ventral optic fibers. Like retinotopic fibers, ectopic fibers exhibited net growth but they showed no apparent directional preference toward their retinotopic position. In contrast, large errors along the anterior-posterior axis corresponding to nasal-temporal retina were rare and there was no differential behavior that distinguished these fibers.

Activity-dependent refinement in the goldfish retinotectal system is mediated by the dynamic regulation of processes withdrawal: an in vivo imaging study

Imaging of regenerating optic fibers in living adult goldfish was used to visualize arbor restructuring during activity-dependent refinement. A small number of neighboring retinal ganglion cells were labeled with DiI and observed in the tectum of the living animal for 5-7 hours during the period of activity-dependent refinement. In contrast to earlier stages of regeneration, many optic arbors were surprisingly stable, showing little or no change. The observed changes were mainly retractions, and these were affected by retinotopic position and activity. Axon branches in retinotopic positions changed by much smaller amounts than ectopic axons, but in fish with retinal tetrodotoxin impulse blockade, no systematic difference was observed as a function of tectal position. Otherwise, impulse blockade had no notable effects.

The levels of retinal mRNA for gefiltin, a neuronal intermediate filament protein, are regulated by the tectum during optic fiber regeneration in the goldfish

Reorganization of the intermediate filament (IF) network during axonal regeneration is accompanied by changes in the expression of various IF proteins. An increase in expression of the neuronal IF subunit gefiltin in goldfish retinal ganglion cells (RGCs) has been linked to the unique ability of the goldfish optic nerve to regenerate following injury. Evidence suggests that the optic tectum, the target of optic fibers, may regulate the expression of gefiltin during regeneration. To address this issue we examined gefiltin mRNA levels during optic fiber regeneration in the presence or absence of the tectum. We found that gefiltin mRNA levels in the RGCs of animals that received an optic nerve crush (ONC group) began increasing by 10 days, peaked from 20 to 38 days at 5.5-fold over normal, and declined to near normal values by 115 days. In animals that had the entire tectum removed as well as an optic nerve crush (ETR group), gefiltin mRNA levels increased by 10 days, peaked at 20 days at 5.5 to 6.5-fold over normal, and although they dropped slightly thereafter, they remained elevated at 5-fold over normal for at least 115 days. When axons regenerated to the ipsilateral tectal lobe as a result of a left tectal lobe removal and left eye removal surgery (LTR/LER group), the expression pattern of gefiltin mRNA paralleled that of the ONC group. We also found that the abundance of gefiltin subunits in the retina was elevated at 30 days of regeneration in ONC and ETR animals, and that levels in the nerve were reconstituted to 80% of normal by 30 days. These results demonstrate that increases in gefiltin mRNA and protein levels during optic nerve regeneration are independent of the tectum, whereas the downregulation of gefiltin mRNA levels in the late stages of regeneration is entirely dependent upon the tectum.

Dynamics of retinotectal synaptogenesis in normal and 3-eyed frogs: evidence for the postsynaptic regulation of synapse number

Quantitative stereological methods were used to determine if the number, density, and types of synaptic connections formed during development are regulated by presynaptic input or by postsynaptic target cells in the optic tectum of normal and 3-eyed Rana pipiens tadpoles and frogs. Our analysis indicates that the number and size of synapses is approximately the same in both tecta of 3-eyed tadpoles and frogs, even though one tectal lobe is receiving input from twice the normal complement of retinal ganglion cells. Moreover, the number and size of synapses in the tectal lobes of 3-eyed animals did not differ significantly from values determined for normal tadpoles and frogs of the same developmental stage. These data suggest strongly that developing tectal cells regulate the number of synaptic contacts they will form. Differences in several morphological features between singly and doubly innervated tecta, however, including synapse density, distribution and complexity, amount of extracellular space, and number of myelin figures, suggest that the presence of supernumerary input retards tectal maturation. We propose that the noncorrelated activity of retinal ganglion cell terminals in the doubly innervated tectum results in fewer stabilized synapses per unit volume of neuropil and in the delayed maturation of the tectal neuropil. Taken together, our data suggest a complex dynamic interaction between retina and tectum during development.

Regulation of retinal ganglion cell axon arbor size by target availability: mechanisms of compression and expansion of the retinotectal projection

The ability of pre- and postsynaptic populations to achieve the proper convergence ratios during development is especially critical in topographically mapped systems such as the retinotectal system. The ratio of retinal ganglion cells to their target cells in the optic tectum can be altered experimentally either by early partial tectal ablation, which results in an orderly compression of near-normal numbers of retinal projections into a smaller tectal area, or by early monocular enucleation, which results in the expansion of a reduced number of axons in a near-normal tectal volume. Our previous studies showed that changes in cell death and synaptic density consequent to these manipulations can account for only a minor component of this compensation for the population mismatch. In this study, we examine other mechanisms of population matching in the hamster retinotectal system. We used an in vitro horseradish peroxidase labeling method to trace individual retinal ganglion cell axons in superior colliculi partially ablated on the day of birth, as well as in colliculi contralateral to a monocular enucleation. We found that individual axon arbors within the partially lesioned tectum occupy a smaller area, with fewer branches and fewer terminal boutons, but preserve a normal bouton density. In contrast, ipsilaterally projecting axon arbors in monocularly enucleated animals occupy a greater area than in the normal condition, with a much larger arbor length and greater number of boutons and branches compared with normal ipsilaterally projecting cells. Alteration of axonal arborization of retinal ganglion cells is the main factor responsible for matching the retinal and tectal cell populations within the tectum. This process conserves normal electrophysiological function over a wide range of convergence ratios and may occur through strict selectivity of tectal cells for their normal number of inputs.

Changes in synaptic density after developmental compression or expansion of retinal input to the superior colliculus

The retinal projection to the superior colliculus can be made abnormally dense by inducing a "compressed" retinal projection into a subnormal tectal volume, or abnormally sparse by monocular enucleation early in development. Any or all of the features of cell number, axonal arbor, dendritic arbor, and synaptic density could potentially be adjusted to compensate for such variations in the convergence of one cell population on another. We have examined the consequences of neonatal partial tectal ablation or monocular enucleation for synaptic length, density, and relative numbers of synapse classes in the superficial gray layer of the hamster superior colliculus. Monocular enucleation resulted in a reduction of synaptic density in the superficial gray layer of the colliculus ipsilateral to the remaining eye. This decrease in density was entirely accounted for by a reduction of the number of synapses with round vesicles, large asymmetric terminal specializations, and pale mitochondria characteristic of retinocollicular terminals (RLP synapses). There was no compensatory increase in any other synaptic class. RLP synapses were larger in monocular enucleates. Partial tectal ablation had no effect on synaptic density, nor on the relative proportions of different synaptic types. Synapses of the RLP class were slightly smaller than normal. These results suggest that synaptic density is normally at a maximum that cannot be altered by increases in potential input. However, density may be reduced by decreasing the number of inputs. Terminal classes do not appear to compete with each other within the collicular volume, suggesting that postsynaptic cells controls both the classes and numbers of their potential inputs.

Isthmotectal axons make ectopic synapses in monocular regions of the tectum in developing Xenopus laevis frogs

During the development of binocular maps in the tectum of Xenopus laevis, axons that relay input from the ipsilateral eye via the nucleus isthmi undergo a prolonged period of shifting connections. This shifting accompanies the dramatic change in eye position that takes place as the laterally placed eyes of the tadpole move dorsofrontally. There is a concomitant expansion of the proportion of tectum that receives contralateral retinotectal input corresponding to the binocular portion of the visual field. Electrophysiological recording demonstrates that ipsilateral units are present in those rostral tectal zones, and anatomical methods show that the isthmotectal axons arborize densely in the rostral region but also extend sparser branches into the caudal zone, which is occupied by contralateral inputs with receptive fields in the monocular zone of the visual field. A mechanism that aligns the ipsilateral and contralateral maps is activity-dependent stabilization of isthmotectal axons that exhibit firing patterns correlated with those of nearby retinotectal axons. In order for activity patterns to function in stabilizing correct connections and promoting the withdrawal of incorrect connections, synaptic communication of some sort is hypothesized to be essential. We have investigated whether isthmotectal axons make morphologically identifiable synapses during development and where such synapses are located. We find evidence for morphologically identifiable synapses in all regions of the tectum, along with many growth cones and structures that are probably immature synapses. As in the adult, the synapses contain round, clear vesicles, have asymmetric specializations, and terminate on structures that appear to be dendrites. In both adult and tadpole, the rarity of serial synapses involving isthmotectal terminals suggests that the interactions between retinotectal and isthmotectal inputs are mediated by postsynaptic dendrites.

Recovery of contrast sensitivity during optic nerve regeneration in fish

Psychophysical experiments on goldfish and sunfish studied the recovery time course of visual contrast detection during optic nerve regeneration. The results showed delayed recovery of detection of positive as compared to negative contrasts, and of high as compared to low spatial frequencies. The findings are related to previous electrophysiological and anatomical results in the fish retinotectal system.

Tectectomy in the cyclopean salamander

In an Ambystoma larva with both natural eyes removed and one eye grafted atop the head (Cyclops preparation), vision-dependent behavior usually recovers from the enucleation inherent in the operation, but the optically activated skin blanching reaction reappears in a very small number of instances. In the present studies, while the latter trend continued for the conventional Cyclops preparation, tectectomy concurrent with the ectopic eye transplantation resulted in a several-fold increase in the recovery of blanching competency. Some 60 percent of the tectectomized Cyclops animals exhibited the same Hogben-Slome pigmentation indices as larvae with one natural eye intact (controls). As measured planimetrically with an image analyzer, the pigment spots (melanosome containing portions of dermal melanocytes) contracted to the same extent in the blanch-competent Cyclops animals as in controls with a single natural eye.

Glutamate-immunoreactivity in the retina and optic tectum of goldfish

Glutamate was immunohistochemically localized in the goldfish retina and tectum at the light and electron microscopic (E.M.) levels using double affinity purified antisera against glutaraldehyde conjugated L-glutamate. In retina, glutamate-immunoreactivity (Glu+) was observed in cone inner segments, cone pedicles, bipolar cells, a small number of amacrine cells and the majority of cells in the ganglion cell layer. The latter were shown to be ganglion cells by simultaneous retrograde labeling. Centrally, Glu+ was observed in axons in the optic nerve and tract, and in stratum opticum and stratum fibrosum et griseum superficialis (SFGS) of the tectum. The Glu+ in the optic pathway disappeared four days after optic denervation and was restored by regeneration without affecting the Glu+ of intrinsic tectal neurons. In tectum, Glu+ was also observed in torus longitudinalis granule cells, toral terminals in stratum marginale, some pyramidal neurons in the SFGS, multipolar and fusiform neurons in stratum griseum centrale, large multipolar and pyriform projection neurons in stratum album centrale, and many periventricular neurons. Glu+ was also localized within unidentified puncta throughout the tectum and within radially oriented dendrites of periventricular neurons. At the E.M. level, a variety of Glu+ terminals were observed. Glu+ toral terminals formed axospinous synapses with dendritic spines of pyramidal neurons. Ultrastructurally identifiable Glu+ putative optic terminals formed synapses with either Glu+ or Glu- dendritic profiles, and with Glu- vesicle-containing profiles, presumed to be GABAergic. These findings are consistent with the hypothesis that a number of intrinsic and projection neurons in the goldfish retinotectal system, including most ganglion cells, may use glutamate as a neurotransmitter.

Laminar and sublaminar ultracytochemical localization of cytochrome oxidase in the optic tectum of normal goldfish

Distinct laminae and sublaminae in the goldfish optic tectum exhibit substantial differences in cytochrome oxidase (C.O.) reactivity. To determine whether these differences are associated with differential reactivity of different neuronal profiles, each tectal sublamina was examined at the ultrastructural level following C.O. treatment. The greatest abundance of darkly reactive mitochondria was found in the optically innervated layers within both pre- and postsynaptic profiles in correspondence with the most intense staining of these layers at the light microscopic level. Many reactive mitochondria were localized within terminals that were presumed to be optic on the basis of cytological criteria or were shown to be optic by filling optic fibers with HRP and processing so as to simultaneously demonstrate both mitochondrial C.O. reactivity and HRP labeling. These optic terminals tended to differ from each other in size and level of reactivity. The largest terminals were located within sublamina d of the stratum fibrosum et griseum superficials (SFGSd), and these were the most intensely reactive and contained the greatest number of darkly reactive mitochondria. Medium-sized terminals were found within sublaminae SFGSa, SFGSb, and a and c of the stratum album centrale (SACa,c). These were also darkly reactive but contained fewer mitochondria. Other medium-to-small optic terminals were found in stratum opticum a and b (60a,b), SFGSb, SFGSc, and stratum griseum centrale c (SGCc). These typically contained fewer mitochondria that also tended to be relatively less reactive, although darkly reactive mitochondria were also present. We suggest that the metalbolic differences within optic terminals of different size and sublaminar stratification arise from different ganglion cell classes and that the different optic layers of tectum are functionally substratified. As expected, darkly reactive mitochondria were most abundant in th intensely stained sublaminae, which included the optic lamina SFGS and nonoptic sublamina SGCa, and they were found not only within optic terminals but also within dendrites, presynaptic dendrites, and nonoptic terminals as well. Glial processes tended to contain less reactive mitochondria. The most prominent of the nonoptic terminals were the large-diameter P1 terminals, which contained pleomorphic vesicles and formed symmetric (presumed inhibitory) synapses. In stratum marginale most of the darkly reactive mitochondria were localized within dendrites. In the rest of the tectal layers most of the darkly reactive mitochondria were found in both presynaptic terminals and dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Optic synapse number but not density is constrained during regeneration onto surgically halved tectum in goldfish: HRP-EM evidence that optic fibers compete for fixed numbers of postsynaptic sites on the tectum

The number of optic synapses in the half tectum of goldfish was counted by using an improved HRP-labeling protocol and a columnar sampling method that spanned the entire optic innervation layer, S-SO-SFGS. It was previously found by using this procedure in intact tectum that the normal number of optic synapses was regenerated by 30 days and maintained thereafter even in the absence of impulse activity. This suggested that the number of synapses in this system was intrinsically fixed. In order to examine whether this limit was imposed by optic fibers or by target cells, optic synapses were counted in surgically halved tecta which received compressed optic projections consisting of regenerating optic fibers from the entire retina. We reasoned that if synapse number is a function of the number of afferents, then there should be twice the normal number of optic synapses per column; on the other hand, if their number is fixed by target, then their number per column should be normal. We found that the number of optic (labeled) synapses was normal in sample columns from fish at 70 days and 160 days after optic nerve crush. Thus, retinal ganglion cells, on average, formed half as many synapses on the half tectum compared to intact tectum, indicating the number of optic synapses was limited by the tectum. The number of nonoptic (unlabeled) synapses was also found to be normal. By contrast, the S-SO-SFGS was found to be 88-103% thicker compared to normal fish, apparently because of a 20-fold increase in the number of optic fibers. As a result, the density of synapses was about half normal in half tecta, and so, in contrast to synapse number, synaptic density is not constrained during regeneration. We infer from these data that optic fibers compete for limited numbers of postsynaptic sites during regeneration and suggest that this competition promotes neural map refinement and the various plasticities described for this projection.