Plasticity in a central nervous pathway in xenopus: anatomical changes in the isthmotectal projection after larval eye rotation

The Impact of Ecological Niche on Adaptive Flexibility of Sensory Circuitry

Evolution and development are interdependent, particularly with regard to the construction of the nervous system and its position as the machine that produces behavior. On the one hand, the processes directing development and plasticity of the brain provide avenues through which natural selection can sculpt neural cell fate and connectivity, and on the other hand, they are themselves subject to selection pressure. For example, mutations that produce heritable perturbations in neuronal birth and death rates, transcription factor expression, or availability of axon guidance factors within sensory pathways can markedly affect the development of form and thus the function of stimulus decoding circuitry. This evolvability of flexible circuits makes them more adaptable to environmental variation. Although there is general agreement on this point, whether the sensitivity of circuits to environmental influence and the mechanisms underlying development and plasticity of sensory pathways are similar across species from different ecological niches has received almost no attention. Neural circuits are generally more sensitive to environmental influences during an early critical period, but not all niches afford the same access to stimuli in early life. Furthermore, depending on predictability of the habitat and ecological niche, sensory coding circuits might be more susceptible to sensory experience in some species than in others. Despite decades of work on understanding the mechanisms underlying critical period plasticity, the importance of ecological niche in visual pathway development has received little attention. Here, I will explore the relationship between critical period plasticity and ecological niche in mammalian sensory pathways.

Rem2 in the bullfrog (Rana catesbeiana): Patterns of expression within the central nervous system and brain expression at different ontogenetic stages

Rem2 is a member of the RGK (Rem, Rad, and Gem/Kir) subfamily of the Ras superfamily of GTP binding proteins. In mammals, Rem2 has been found to be unique in not only its structure, but also its tissue specificity, as it is the first member to be found at high levels in neuronal tissue. Because Rem2 has previously been implicated in neuronal cell proliferation, and amphibians maintain relatively high neuronal proliferative activity as adults, we sought to isolate and acquire the full-length sequence of the rem2 gene from the brain of the bullfrog (Rana catesbeiana). Furthermore, we used real time PCR (rtPCR) to characterize its tissue specificity, regional brain expression, and brain expression levels at different stages of development. Deduced amino acid sequence analysis showed that the bullfrog Rem2 protein possesses the unique 5' extension characteristic of mammalian Rem2 and the RGK subfamily to which it belongs. Tissue specificity of the bullfrog rem2 gene showed that the bullfrog is similar to both mammals and fish in that the levels of rem2 gene expression were significantly greater in the brain than all other tissues assayed. In the brain itself, differential rem2 expression patterns were observed between six major brain areas assayed and the spinal cord, with expression significantly high in the cerebrum and low in the cerebellum. Finally, examination of whole brain rem2 expression levels in bullfrogs at different stages of development revealed greater expression after metamorphic climax.

Binocular maps in Xenopus tectum: Visual experience and the development of isthmotectal topography

Xenopus frogs have a prominent binocular field that develops as a consequence of the migration of the eyes during the remodeling of the head during and after metamorphosis. In the optic tectum, a topographic representation of the ipsilateral eye develops during this same period. It is relayed indirectly, via the nucleus isthmi. In the early stages of binocular development, the topographic matching of the ipsilateral input to the retinotectal input from the contralateral eye is largely governed by chemical cues, but the ultimate determinant of the ipsilateral map is binocular visual input. Visual input is such a dominant factor that abnormal visual input resulting from unilateral eye rotation can induce isthmotectal axons to alter their trajectories dramatically, even shifting their terminal zones from one pole of the tectum to the other. This plasticity normally is high only during a 3-4-month critical period of late tadpole-early juvenile life, but the critical period can be extended indefinitely by dark-rearing. N-methyl-D-aspartate (NMDA) receptors are involved in this process; plasticity can be blocked or promoted by chronic treatment with NMDA antagonists or agonists, respectively. Cholinergic nicotinic receptors on retinotectal axons are likely to play an essential role as well. Modifications in the polysialylation of neural cell adhesion molecule are correlated with the state of plasticity. The circuitry underlying binocular plasticity is not yet fully understood but has proved not to be a simple convergence of ipsilateral and contralateral inputs onto the same targets.

In vivo spike-timing-dependent plasticity in the optic tectum of Xenopus laevis

Spike-timing-dependent plasticity (STDP) is found in vivo in a variety of systems and species, but the first demonstrations of in vivo STDP were carried out in the optic tectum of Xenopus laevis embryos. Since then, the optic tectum has served as an excellent experimental model for studying STDP in sensory systems, allowing researchers to probe the developmental consequences of this form of synaptic plasticity during early development. In this review, we will describe what is known about the role of STDP in shaping feed-forward and recurrent circuits in the optic tectum with a focus on the functional implications for vision. We will discuss both the similarities and differences between the optic tectum and mammalian sensory systems that are relevant to STDP. Finally, we will highlight the unique properties of the embryonic tectum that make it an important system for researchers who are interested in how STDP contributes to activity-dependent development of sensory computations.

Supervised Learning in Spiking Neural Networks with ReSuMe: Sequence Learning, Classification, and Spike Shifting

Learning from instructions or demonstrations is a fundamental property of our brain necessary to acquire new knowledge and develop novel skills or behavioral patterns. This type of learning is thought to be involved in most of our daily routines. Although the concept of instruction-based learning has been studied for several decades, the exact neural mechanisms implementing this process remain unrevealed. One of the central questions in this regard is, How do neurons learn to reproduce template signals (instructions) encoded in precisely timed sequences of spikes?

Here we present a model of supervised learning for biologically plausible neurons that addresses this question. In a set of experiments, we demonstrate that our approach enables us to train spiking neurons to reproduce arbitrary template spike patterns in response to given synaptic stimuli even in the presence of various sources of noise.

We show that the learning rule can also be used for decision-making tasks. Neurons can be trained to classify categories of input signals based on only a temporal configuration of spikes. The decision is communicated by emitting precisely timed spike trains associated with given input categories. Trained neurons can perform the classification task correctly even if stimuli and corresponding decision times are temporally separated and the relevant information is consequently highly overlapped by the ongoing neural activity.

Finally, we demonstrate that neurons can be trained to reproduce sequences of spikes with a controllable time shift with respect to target templates. A reproduced signal can follow or even precede the targets. This surprising result points out that spiking neurons can potentially be applied to forecast the behavior (firing times) of other reference neurons or networks.

Prey-capture in the African clawed toad (Xenopus laevis): comparison of turning to visual and lateral line stimuli

Separately delivered visual and lateral line stimuli elicit similar but not identical orientation and approach by intact, sighted Xenopus. Response frequencies for visual stimuli declined sharply for distant or caudal stimuli while those for lateral line stimuli changed little. Turn angles correlated highly with stimulus angles but were smaller on average, so regression slopes were less than one. Regression slopes were smaller for visual than for lateral line stimuli, but this apparent difference was due to different distributions of stimulus distance interacting with the toad's rotation center. Errors in final headings, most often under-rotations, did not differ by modality. Frequencies of lunges and arm capture movements were higher for visual stimuli both overall and especially for rostral proximal stimuli. The results demonstrate accurate orientation by sighted Xenopus to visual and lateral line stimuli; they are consistent with expectations based on in-register tectal maps. Orientation to lateral line stimuli is similar to previous results with blinded animals, revealing no heightened acuity in the latter. Modality differences indicate that the lateral line system is better for omnidirectional orientation and approach to distant stimuli whereas the visual system is more attuned to nearby rostral stimuli and more apt to mediate strikes.

Connections of contralaterally projecting isthmotectal axons and GABA-immunoreactive neurons in Xenopus tectum: An ultrastructural study

To investigate the circuitry that mediates binocular interactions in the tectum of Xenopus frogs, we have begun to identify the tectal cells that receive ipsilateral eye input relayed via the nucleus isthmi. Isthmotectal axons were labeled with horseradish peroxidase, and thin sections were labeled by postembedding immunogold reaction with antibodies to γ-aminobutyric acid (GABA). Ultrastructural examination reveals that many isthmotectal axons terminate on GABA-immunoreactive dendrites. Other isthmotectal axons contact postsynaptic structures that are unlabeled but have an appearance consistent with previously described GABA-poor zones of GABA-immunoreactive dendrites. We also examined the unlabeled inputs to the dendrites that were postsynaptic to filled isthmotectal axons. The most common nonisthmic inputs to those dendrites were GABA-immunoreactive processes with symmetric morphology. Surprisingly, we found only one input with the retinotectal characteristics of densely packed round, clear vesicles and minimal GABA immunoreactivity. These results indicate that isthmotectal axons synapse onto inhibitory interneurons, that retinotectal and isthmotectal axons do not synapse close to each other on the same dendrites, and that inhibitory connections are the closest neighbors to isthmotectal synapses.

Stage-specific activity patterns affect motoneuron axonal retraction and outgrowth during the metamorphosis of Manduca sexta

During the metamorphosis of holometabolous insects, most larval muscles and sensory neurons are replaced by new adult elements, whereas most motoneurons persist and are remodelled to serve new adult functions. In Manduca sexta, the formation of the anlagen of the adult dorsal longitudinal flight muscle (DLM) is characterized by retraction of axonal terminals and dendrites of persisting larval motoneurons, partial target muscle degeneration and myoblast accumulation during late larval life. Most of these structural changes have been attributed to hormonal control, not only because ecdysteroids govern metamorphosis, but also because motoneurons express ecdysteroid receptors and experimental manipulations of ecdysteroid titres perturb normal development. To test whether activity-dependent mechanisms also came into play, chronic extracellular recordings were conducted in vivo from the five future DLM motoneurons throughout the last 3 days of larval life. Motoneuron activity is regulated developmentally. The types of motoneurons recruited, the number of motor spikes and the duration of bursts change in a stereotypical fashion during different stages, indicating an internal control of motor activity. A characteristic cessation in the activity of the five future DLM motoneurons coincides in time with the retraction of their dendrites and their terminal arborizations, whereas their activation during ecdysis coincides with the onset of new outgrowth. Inducing advanced activity by stimulating the motoneurons selectively with ecdysis-like patterns results in significant outgrowth of their terminal arborizations. Therefore, steroids might act in concert with activity-dependent mechanisms during the postembryonic modifications of neuromuscular systems.

MAP2 phosphorylation and visual plasticity in Xenopus

Microtubule-associated protein 2 (MAP2) has been implicated in activity-dependent structural changes in dendrites. MAP2 regulates the assembly of cytoskeletal proteins such as microtubules and actin, and its function is phosphorylation-dependent. In hippocampus, MAP2 has been reported to be dephosphorylated by activation of the NMDA-type glutamate receptor, a key player in synaptic plasticity. In this work, we used a phospho-specific MAP2 antibody (Ab 305) that recognizes epitopes close to the microtubule-binding domain to investigate the possible role of MAP2 in the Xenopus visual system. The binocular system in Xenopus exhibits activity-dependent synapse rearrangement during a critical period of development. We have found that, in critical period animals, NMDA receptor activation leads to the dephosphorylation of MAP2 at sites recognized by Ab 305 in a dose-dependent manner. We compared the responses of MAP2 to NMDA treatment in animals with high binocular plasticity (critical period juveniles and dark-reared adults) and low plasticity (normal adults). Our results show that, in all groups, NMDA treatment induces the dephosphorylation of MAP2. Tecta from frogs with different degrees of plasticity show no differences in the baseline level of MAP2 phosphorylation or in the NMDA-induced MAP2 dephosphorylation response. These results suggest that activity may modify dendrite structure via the NMDA receptor--MAP2-cytoskeletal protein pathway, but this pathway does not seem to be a determinant of the degree of plasticity.

Adaptive axonal remodeling in the midbrain auditory space map

The auditory space map in the external nucleus of the inferior colliculus (ICX) of barn owls is highly plastic, especially during early life. When juvenile owls are reared with prismatic spectacles (prisms) that displace the visual field laterally, the auditory spatial tuning of neurons in the ICX adjusts adaptively to match the visual displacement. In the present study, we show that this functional plasticity is accompanied by axonal remodeling. The ICX receives auditory input from the central nucleus of the inferior colliculus (ICC) via topographic axonal projections. We used the anterograde tracer biocytin to study experience-dependent changes in the spatial pattern of axons projecting from the ICC to the ICX. The projection fields in normal adults were sparser and more restricted than those in normal juveniles. The projection fields in prism-reared adults were denser and broader than those in normal adults and contained substantially more bouton-laden axons that were appropriately positioned in the ICX to convey adaptive auditory spatial information. Quantitative comparison of results from juvenile and prism-reared owls indicated that prism experience led to topographically appropriate axonal sprouting and synaptogenesis. We conclude that this elaboration of axons represents the formation of an adaptive neuronal circuit. The density of axons and boutons in the normal projection zone was preserved in prism-reared owls. The coexistence of two different circuits encoding alternative maps of space may underlie the ability of prism-reared owls to readapt to normal conditions as adults.

The development of abnormal axon trajectories after rotation of one eye in Xenopus

The targeting of isthmotectal axons in the Xenopus binocular pathway is guided by both activity-dependent cues and activity-independent cues. Abnormal visual activity induced by unilateral eye rotation overrides activity-independent cues and causes isthmotectal axons to arborize at new locations during a critical period of development that ends approximately 3 months postmetamorphosis (PM). Horseradish peroxidase staining of isthmotectal axons reveals that they normally run rostrocaudally in the tectum; in contrast, those axons in animals with early eye rotation have circuitous trajectories. In this paper, by studying the trajectories and branching patterns of isthmotectal axons at different times after eye rotation, we aimed to investigate when and how activity cues determine the projection pattern of isthmotectal axons. As suggested by electrophysiological recording, isthmotectal axons initially grow normally and make arbors according to activity-independent cues despite the presence of abnormal visual input. Our findings demonstrate that the development of abnormal trajectories starts by 2 weeks PM in response to eye rotation and is a protracted process. It begins in the tectal regions in which the initial connections of isthmotectal axons are first formed according to activity-independent cues. At transitional stages (5 and 10 weeks), axons with arbors at two different locations are observed, with locations corresponding to the old and new termination sites, respectively. Later, at 10 weeks of age, the fainter horseradish peroxidase staining in arbors at old termination sites suggests that the older arbors are undergoing withdrawal.

Plasticity in the tectum of Xenopus laevis: binocular maps

Xenopus frogs exhibit dramatic changes in the binocular projections to the tectum during a critical period of development. Their eyes change position in the head, moving from lateral to dorsal and creating an increasing region of binocular overlap. There is a corresponding shift of binocular projections to the tectum that keeps the two eyes' maps in register with each other throughout this period. The ipsilateral input is relayed via the nucleus isthmi. Two factors bring the ipsilateral projection into register with the contralateral projection. First, chemoaffinity cues establish a crude topographic map beginning when the shift of eye position begins. Approximately 1 month later, visual cues bring the ipsilateral map into register with the contralateral map. The role of visual input is demonstrated by the ability of the axons that bring the ipsilateral eye's map to the tectum to reorganize in response to a surgical rotation of one eye and to come into register with the contralateral eye's map. This plasticity can be blocked by NMDA receptor antagonists during the critical period. In normal adults, reorganization is minimal. Eye rotation fails to induce reorganization of the ipsilateral map. However, plasticity persists indefinitely in animals that are reared in the dark, and plasticity can be restored in normally-reared animals by treatment with NMDA. The working model to explain this plasticity posits that correlated input from the two eyes triggers opening of NMDA receptor channels and initiates events that stabilize appropriately-located isthmotectal connections. Specific tests of this model are discussed.

Effects of choline and other nicotinic agonists on the tectum of juvenile and adult Xenopus frogs: a patch-clamp study

We have used anatomical methods and whole-cell patch-clamp recording to assess the distribution of nicotinic receptors in the tectum of Xenopus frogs and to measure effects of nicotinic ligands (carbachol, cytisine and nicotine) on glutamatergic spontaneous miniature excitatory postsynaptic currents. Our results confirm that retinotectal axons account for the majority of nicotinic receptors in the tectum and that nicotinic agonists exert presynaptic effects that increase the rate of transmitter release on to tectal cells. The nicotinic blockers mecamylamine and methyllycaconitine reduced responses to carbachol and cytisine. A small percentage of cells also showed postsynaptic responses. We have assessed whether there are developmental changes in the frequency of occurrence of spontaneous miniature excitatory postsynaptic currents. The first three months post-metamorphosis fall within the critical period for the dramatic plasticity displayed by binocular inputs during development in Xenopus. During this period, visual activity governs the formation of orderly maps relayed from the ipsilateral eye via the cholinergic projection from the nucleus isthmi to the tectum. In this study, we have found that critical-period tecta (two to 12 weeks postmetamorphosis) tend to have higher spontaneous activity than do older tecta (two to 69 weeks postmetamorphosis), and that nicotinic agonists increase that activity in both groups, with the result that the peak rates in response to nicotinic agonists are higher during the critical period than later. We also investigated the possible role of choline as an agonist of nicotinic receptors in the tectum. We have found that choline, as well as carbachol and cytisine, can cause a reversible increase in the rate of miniature excitatory postsynaptic currents. This result may help to explain how the isthmotectal projection, which accounts for the overwhelming majority of cholinergic input to the tectum, can exert effects on retinotectal terminals even though there are no morphologically identifiable synapses between the two populations. We have examined the morphology of cells filled with biocytin during the patch-clamp experiments, and we find that cells with dendrites in the stratum zonale, a layer with particularly dense input from the contralateral nucleus isthmi, have higher spontaneous activity than cells with dendrites that do not extend into that layer. Nicotinic agonists increased the activity recorded in both classes of cells. In addition, four pretectal cells were identified. Nicotinic agonists increased the rate of spontaneous activity recorded in that population. The results indicate that retinotectal transmission in the superior colliculus can be increased presynaptically by activity of the cholinergic projections of the nucleus isthmi. This modulation may be the basis for observations that blocking of cholinergic input disrupts the formation of topographic retinotectal projections. Moreover, the ability of choline to activate these receptors suggests that this metabolite of acetylcholine may permit paracrine activation of presynaptic receptors even though the tectum contains high acetylcholinesterase activity.

Synchronizing retinal activity in both eyes disrupts binocular map development in the optic tectum

Spatiotemporal correlations in the pattern of spontaneous and evoked retinal ganglion cell (RGC) activity are believed to influence the topographic organization of connections throughout the developing visual system. We have tested this hypothesis by examining the effects of interfering with these potential activity cues during development on the functional organization of binocular maps in the Xenopus frog optic tectum. Paired recordings combined with cross-correlation analyses demonstrated that exposing normal frogs to a continuous 1 Hz of stroboscopic illumination synchronized the firing of all three classes of RGC projecting to the tectum and induced similar patterns of temporally correlated activity across both lobes of the nucleus. Embryonic and eye-rotated larval animals were reared until early adulthood under equivalent stroboscopic conditions. The maps formed by each RGC class in the contralateral tectum showed normal topography and stratification after strobe rearing, but with consistently enlarged multiunit receptive fields. Maps of the ipsilateral eye, formed by crossed isthmotectal axons, showed significant disorder and misalignment with direct visual input from the retina, and in the eye-rotated animals complete compensatory reorientation of these maps usually induced by this procedure failed to occur. These findings suggest that refinement of retinal arbors in the tectum and the ability of crossed isthmotectal arbors to establish binocular convergence with these retinal afferents are disrupted when they all fire together. Our data thus provide direct experimental evidence that spatiotemporal activity patterns within and between the two eyes regulate the precision of their developing connections.

Associative learning

This chapter reviews evidence demonstrating the essential role of the cerebellum and its associated circuitry in the learning and memory of classical conditioning of discrete behavioral responses (e.g., eyeblink, limb flexion, head turn). It now seems conclusive that the memory traces for this basic category of associative learning are formed and stored in the cerebellum. Lesion, neuronal recording, electrical microstimulation, and anatomical procedures have been used to identify the essential conditioned stimulus (CS) circuit, including the pontine mossy fiber projections to the cerebellum; the essential unconditioned stimulus (US) reinforcing or teaching circuit, including neurons in the inferior olive (dorsal accessory olive) projecting to the cerebellum as climbing fibers; and the essential conditioned response (CR) circuit, including the interpositus nucleus, its projection via the superior cerebellar peduncle to the magnocellular red nucleus, and rubral projections to premotor and motor nuclei. Each major component of the eyeblink CR circuit was reversibly inactivated both in trained animals and over the course of training. In all cases in trained animals, inactivation abolished the CR (and the UR as well when motor nuclei were inactivated). When animals were trained during inactivation (and not exhibiting CRs) and then tested without inactivation, animals with inactivation of the motor nuclei, red nucleus, and superior peduncle had fully learned, whereas animals with inactivation of a very localized region of the cerebellum (anterior interpositus and overlying cortex) had not learned at all. Consequently, the memory traces are formed and stored in the cerebellum. Several alternative possibilities are considered and ruled out. Both the cerebellar cortex and the interpositus nucleus are involved in the memory storage process, suggesting that a phenomenon-like long-term depression (LTD) is involved in the cerebellar cortex and long-term potentiation (LTP) is involved in the interpositus. The experimental findings reviewed in this chapter provide perhaps the first conclusive evidence for the localization of a basic form of memory storage to a particular brain region, namely the cerebellum, and indicate that the cerebellum is indeed a cognitive machine.

An anatomical basis for visual calibration of the auditory space map in the barn owl's midbrain

The map of auditory space in the external nucleus of the inferior colliculus (ICX) of the barn owl is calibrated by visual experience during development. ICX neurons are tuned for interaural time difference (ITD), the owl's primary cue for sound source azimuth, and are arranged into a map of ITD. When vision is altered by rearing owls with prismatic spectacles that shift the visual field in azimuth, ITD tuning in the ICX shifts adaptively. In contrast, ITD tuning remains unchanged in the lateral shell of the central nucleus of the inferior colliculus (ICCls), which provides the principal auditory input to the ICX, suggesting that the projection from the ICCls to the ICX is altered by prism-rearing. In this study, the topography of the ICCls-ICX projection was assessed in normal and prism-reared owls by retrograde labeling using biotinylated dextran amine. In juvenile owls at the age before prism attachment, and in normal adults, labeling patterns were consistent with a topographic projection, with each ICX site receiving input from a restricted region of the ICCls with similar ITD tuning. In prism-reared owls, labeling patterns were systematically altered: each ICX site received additional, abnormal input from a region of the ICCls where ITD tuning matched the shifted ITD tuning of the ICX neurons. These results indicate that anatomical reorganization of the ICCls-ICX projection contributes to the visual calibration of the ICX auditory space map.

Distribution of visual callosal neurons in normal and strabismic cats

It has been suggested that synchronous activation of cortical loci in the two cerebral hemispheres during development leads to the stabilization of juvenile callosal connections in some areas of the visual cortex. One way in which loci in opposite hemispheres can be synchronously activated is if they receive signals generated by the same stimulus viewed through different eyes. These ideas lead to the prediction that shifts in the cortical representation of the visual field caused by misalignment of the visual axes (strabismus) should change the width of the callosal zone in the striate cortex. We tested this prediction by using quantitative techniques to compare the tangential distribution of callosal neurons in the striate cortex of strabismic cats to that in normally reared cats. Animals were rendered strabismic surgically at 8-10 days of age and were allowed to survive a minimum of 18 weeks, at which time multiple intracortical injections of the tracer horseradish peroxidase (HRP) were used to reveal the distribution of callosally projecting cells in the contralateral striate cortex. HRP-labeled cells were counted in coronal sections, and data from four animals with divergent strabismus (exotropia) and four with convergent strabismus (esotropia) were compared to those from four normally reared animals. Although our data from strabismic cats do not differ markedly from those reported previously, we find that the distribution of callosal cells in the striate cortex of these cats does not differ significantly from that in our normally reared control cats. These results do not bear out the prediction that surgically shifting the visual axes leads to stabilization of juvenile callosal axons in anomalous places within the striate cortex.

Experience-dependent mechanism of binocular map plasticity in Xenopus: incongruent connections are masked by retinal input

When one eye in larval Xenopus is rotated to change the orientation of its visual map on the contralateral tectum, the map from the other eye to the same tectum can subsequently alter its orientation to match the rotated input. This plasticity re-establishes binocular congruence between the two maps and involves an experience-dependent reorganization of functional connections in the crossed isthmotectal projection, which delivers the ipsilateral eye's map to the tectum. Here, recordings were made at the same tectal sites in 8 eye-rotated frogs immediately before and after removing the rotated retina. All had congruent binocular maps prior to enucleation, with matching receptive fields in the two eyes at each recording site. In 5 frogs, eye removal unmasked additional, incongruent receptive fields in the ipsilateral eye, recorded from previously undetectable crossed isthmotectal connections occupying their original positions in the tectum. This result suggests that the plasticity also involves functional suppression of binocularly-incongruent inputs from the ipsilateral eye by retinotectal afferents.

Ultrastructure and GABA immunoreactivity in layers 8 and 9 of the optic tectum of Xenopus laevis

This study presents an ultrastructural analysis of layers 8 and 9 in the optic tectum of Xenopus laevis. Retinotectal axons were labelled with horseradish peroxidase and tectal cells were labelled with antibody to GABA. Four distinct axonal and dendritic structures were identified. GABA-negative axon terminals formed asymmetric synapses and were categorized as type a-1 (which included retinotectal axons), characterized by medium size synaptic vesicles and pale mitochondria, and type a-2 (non-retinotectal) with large vesicles and dense mitochondria. GABA-negative dendrites (type d) contained dense mitochondria, microtubules in the dendritic shafts, and dendritic spines devoid of microtubules. GABA-positive structures contained small synaptic vesicles and dense mitochondria. Some dendrites (type D) were not only postsynaptic but were also presynaptic elements, as defined by the presence of vesicles and distinct synaptic clefts with symmetric specializations. GABA-positive presynaptic structures were mostly located in vesicle-filled, bulbous extensions of dendritic shafts and usually terminated onto dendritic spines. Some type D dendrites were the middle element in serial synapses, with input from either GABA-positive or GABA-negative structures and output to GABA-negative structures. Retinotectal terminals were identified as one of the synaptic inputs to GABA-positive processes. Glia were characterized by granular cytoplasm and large mitochondria, often displaying a crystalline matrix structure. These results indicate that GABA-positive neurons are a prominent component of circuitry in the superficial layers of the tectum of Xenopus and that, as in mammals, they participate in serial synaptic arrangements in which retinotectal axons are the first element. These arrangements are consistent with complex processing of visual input to the tectum and a central role for inhibitory processes in the shaping of tectal responses.

Acceleration by NMDA treatment of visually induced map reorganization in juvenile Xenopus after larval eye rotation

Each tectal lobe of Xenopus frogs receives two topographic maps, one via the ipsilateral eye and one via the contralateral eye. The alignment of the ipsilateral map with the contralateral map depends upon binocular visual input during a critical period that extends from late tadpole to early juvenile stages. Rotation of one eye during the critical period leads to reorganization of the ipsilateral map, which eventually comes back into alignment with the contralateral map despite the abnormal eye position. The ipsilateral eye's map initially develops as if there had been no alteration in eye position; there is a delay of 4-6 weeks before reorganization can be detected by electrophysiological mapping. In this paper, the possible role of the NMDA receptor in the delay in reorganization is addressed. The degree of NMDA receptor activation may need to be above some threshold level to trigger reorganization. If NMDA receptor activation normally is below that level until after the first month postmetamorphosis, then exogenous NMDA might boost the process sufficiently to start the reorganization process sooner than usual. In order to test this possibility, the left eye of tadpoles was rotated and NMDA was applied to the right tectal lobe for 3-5 weeks, starting at 1 week postmetamorphosis. Electrophysiological mapping demonstrated that reorganization takes place more rapidly than in untreated frogs or frogs treated with vehicle only. This result is consistent with the interpretation that the activation of the NMDA receptor is a rate-limiting step in the activity-dependent matching of binocular maps in Xenopus tectum.

Position, guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography.

Guidance and topographic stabilization of nasal chick retinal axons on target-derived components in vitro

We studied mechanisms underlying the generation of topographic order within the developing chick retinotectal connection by combining the recently introduced stripe assay with a novel membrane protein fractionation technique. Our experiments show a preference of temporal and nasal retinal fibers for growing on cell membranes prepared from their proper target area. In addition, membrane preparations from posterior tectum were found to prolong substantially the survival of nasal neurites in vitro. We conclude that tropic as well as trophic interactions contribute to the generation of topographic maps during embryogenesis, in our case to the homing of nasal axons within the posterior tectum.

The Critical Period for Experience-dependent Plasticity in a System of Binocular Visual Connections in Xenopus laevis: Its Extension by Dark-rearing

Following surgical rotation of an eye, the Xenopus 'intertectal' system is capable of a vision-dependent alteration of its connectivity, that restores spatial registration of binocular maps on the optic tectum. In the preceding paper (Keating and Grant, Eur. J. Neurosci., 4, 27 - 36, 1992), we reported that this capacity undergoes a progressive, age-dependent restriction during a critical period around the time of metamorphosis, so that rotations produced in animals aged >/=3 months postmetamorphosis normally evoke no such alteration of the system. Here we examine whether this restriction is rigidly age-dependent or whether vision can influence its profile. We report that in animals dark-reared from embryonic stage 35 through the critical period to 3 months, 1 year or even 2 years after metamorphosis, rotations instituted at those ages now result in intertectal reorganization if a period of normal vision is allowed after the operation. Similarly, intertectal alteration was also seen in animals eye-rotated at larval stage 58, then dark-reared just for the duration of the critical period, and subsequently returned, at 3 months of age, to a normal visual environment. We conclude, therefore, that the normal developmental restriction in the plasticity of the Xenopus intertectal system is not strictly age-dependent, but that vision contributes to the process by activating the underlying plasticity mechanisms.

The Critical Period for Experience-dependent Plasticity in a System of Binocular Visual Connections in Xenopus laevis: Its Temporal Profile and Relation to Normal Developmental Requirements

A commissural system of 'intertectal' connections in Xenopus mediates the registration of binocular visual maps at the midbrain optic tectum. Following surgical eye rotation in larval animals, the system can completely alter its pattern of connectivity to restore binocular visual registration at the tectum. This experimentally induced plasticity is known to require visual experience and thought to be subject to an age-related restriction: eye rotation in adult animals is reported to provoke no subsequent intertectal alteration. In this paper we describe the detailed age-dependence of this plasticity. One eye was rotated in 238 animals of various developmental stages between mid-larval and adult life. At each age, different animals received rotations of different sizes, ranging from 20 to 180 degrees. The pattern of intertectal connectivity was mapped electrophysiologically 1 - 2 years postoperatively. A 'critical' period was defined around the time of metamorphosis: the vast majority of animals receiving a rotation in larval life (up to approximately 2 weeks before metamorphic climax) showed altered intertectal connections, whereas none of the animals operated upon at 3 months or more postmetamorphosis displayed the plasticity. At intervening ages, altered intertectal connections were found only in response to progressively smaller eye rotations. The profile of this critical period was further shown to mirror temporal features of the changes in eye position that occur in Xenopus as natural consequences of head growth, and which themselves impose a normal developmental requirement for intertectal plasticity. We conclude that the capacity of the Xenopus intertectal system for plasticity in response to abnormal experience undergoes a progressive age-dependent decline, and that the profile of this decline is delimited by normal requirements.

Activity-dependent development of spinal cord motor neurons

Patterned neuronal activity in early postnatal life can regulate the acquisition of the mature morphological and electrophysiological properties of neurons. Many properties of motor neurons are developmentally regulated and may be influenced by epigenetic factors. The pattern of activation of motor neurons can regulate axon terminal morphology and synaptic efficacy at the neuromuscular junction. Motor neuron morphology and synaptic connections can also be modified by exposure to specific hormones in the early postnatal period. The acquisition of mature physiological and anatomical properties is paralleled by the acquisition of specific molecular properties. Recent experiments using molecular markers for motor neuron differentiation indicate that motor neurons undergo activity-dependent development during a circumscribed period in early postnatal life. Normal motor neuron differentiation requires a normal pattern of neuronal activity in early postnatal life. Differentiation also requires activation of the NMDA receptor over the same time period. The activity-dependent development of morphological, electrophysiological and molecular properties of motor neurons is similar to activity-dependent development in the vertebrate visual system. The neuromuscular system may provide an accessible system for characterizing the molecules subserving the translation of patterned neuronal activity into mature neuronal phenotype.

Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. III. Modifications following early eye rotation

In the frog, Xenopus laevis, a system of intertectal connections underlies the visual projection from an eye to its ipsilateral tectal lobe and is involved in the topographic representation of binocular visual space. Rotation of one eye in early life may be followed by a radical rearrangement of the connections in this system. The modified pattern which later emerges is that which keeps the visual projection through the ipsilateral eye in topographic registration with the direct visual projection from the contralateral eye to the same tectal lobe. This plasticity requires visual experience. In this paper we describe the time-course and sequence of events by which this plasticity is effected. Following rotation of one eye in larval animals or in animals undergoing metamorphic climax, the earliest evidence of intertectal modification was found 3-4 weeks after metamorphosis. With increasing intervals after metamorphosis an increasing proportion of animals displayed modified intertectal systems. At intermediate intervals many animals showed partial modifications, which were interpreted as transitional stages in the modification process. Analysis of these transitional stages indicated that the sequence of events involved in the elaboration of a modified intertectal system following the experimental alteration of eye alignment exhibits features in common with rearrangements of the system that occur during normal development in response to growth-related alterations in eye alignment.

Mechanism of anomalous retinal correspondence: maintenance of binocularity with alteration of receptive-field position in the lateral suprasylvian (LS) visual area of strabismic cats

We have examined the effects of rearing kittens with a unilateral convergent strabismus, induced surgically at 3 weeks of age, on the binocularity (ocular dominance) and receptive-field position of neurons in the motion-sensitive lateral suprasylvian (LS) area of cat extrastriate cortex. Data were compared to those obtained from area 17 in the same animals, and from the two areas of cortex in normal adult cats. Interocular alignment of the operated cats was assessed in alert adults using corneal reflex photography and during recording from the positions of retinal landmarks under paralysis. The strabismus magnitude in each operated cat was calculated by comparison with equivalent data from the normal animals. Strabismus always caused a major loss of binocularity in area 17. The remaining binocular neurons had receptive-field (RF) pairs arising from positions of normal correspondence in the two retinae and would thus have been responsive to different regions of visual space through the misaligned eyes in the alert animal. In area LS, the effects were dependent on the strabismus magnitude. In the group of four cats with pronounced strabismus (18-30 deg crossed), a loss of binocularity occurred in area LS equivalent in severity to that in area 17. The majority of the remaining binocular LS neurons possessed RF pairs in normal retinal correspondence and would thus, in the alert animal, have received spatially disparate visual input through the two eyes. This also occurred in three other cats with more moderate strabismus (11-15 deg crossed), although only a small breakdown in the binocularity of area LS was apparent. The group of cats with mild strabismus (less than or equal to 10 deg crossed) had normal proportions of binocular neurons in area LS. In three of these cats, the maintenance of binocularity was accompanied by shifts in RF position, with visual inputs arising from anomalous retinal locations. These shifts compensated, in part, for the strabismus angle present in each cat, so that most of the binocular LS neurons would have received inputs from regions of visual correspondence through the misaligned eyes when the animal was alert. Similar mechanisms could afford a basis for the binocular visual compensations that occur in humans with small-angle strabismus of early onset. If so, anomalous retinal correspondence in such individuals would have as a locus areas of extrastriate cortex with a role in motion perception, and would involve alterations to the neural substrate underlying normal binocular vision.

Chronic effects of NMDA and APV on tectal output in Xenopus laevis

In the South African clawed-toed frog Xenopus laevis, visual experience plays a crucial role in the formation of matching binocular maps in the tectum. The ipsilateral eye's projection, relayed through the crossed isthmotectal projection, displays marked plasticity in response to altered visual input during a critical period of development. This plasticity and the events responsible for the end of the critical period are mediated by N-methyl-D-aspartate (NMDA) receptor function. We have previously reported that chronic blockade of tectal NMDA receptors with the NMDA antagonist 5-amino-phosphonovaleric acid (APV) prevents plasticity of the crossed isthmotectal projection during the critical period, while chronic treatment with NMDA restores this plasticity after the end of the critical period. These results raise the question of whether the effects on plasticity are due to changes in electrical responsiveness of the treated tissue. In this study, we have quantitatively assessed the actions of APV and NMDA on certain aspects of tectal cell activity in Xenopus during and after the critical period by recording the output of the nucleus isthmi cells that are activated by the tectum after three weeks of treatment. We have found that chronic APV treatment does not alter tectal output, as indicated by the firing of isthmotectal axons, during the critical period and that chronic NMDA treatment increases tectal output in postcritical period Xenopus. Tectal output does not differ between normal Xenopus during and after the end of the critical period.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of the crossed isthmotectal projection in Xenopus frogs

The nucleus isthmi (NI) of frogs is a relay for input from the eye to the ipsilateral tectum; each NI receives retinotopic input from one tectum and sends retinotopic output to both tecta. The crossed isthmotectal projection in Xenopus displays tremendous plasticity during development. Physiological and anatomical studies have suggested that the location at which a developing isthmotectal axon will terminate is determined by the correlation of its visually evoked activity with the activity of nearby retinotectal terminals. What structures could mediate such communication? We have examined quantitatively the ultrastructural characteristics of crossed isthmotectal axons and synapses in order to determine whether retinotectal axons communicate directly with isthmotectal axons via axo-axonic synapses or whether the communication is indirect, e.g., via common postsynaptic dendrites. Our results support the conclusion that isthmotectal axons interact with retinotectal axons indirectly and that tectal cell dendrites are the critical site of interaction.

Plasticity in the ipsilateral visuotectal projection persists after lesions of one nucleus isthmi in Xenopus

Visual input has a profound effect on the development of binocular maps in the tectum of the frog Xenopus laevis. Input from the ipsilateral eye, which is relayed to the tectum via the opposite nucleus isthmi, is normally in register with the retinotectal map from the contralateral eye. However, if one eye is rotated during larval stages while the other eye is left in normal orientation, then the resulting mismatched visual input induces the crossed isthmotectal axons to change their trajectories and to establish a reoriented ipsilateral visuotectal map in register with the contralateral retinotectal map. The major cue which aligns the two maps is the correlation of visually-evoked activity from the two eyes. This experiment was designed to determine whether the uncrossed isthmotectal projection is necessary to organize the map transmitted by the crossed isthmotectal axons. Each NI receives a topographic map from the tectum on the same side of the brain and therefore carries the same topographic information as the retinotectal projection, and each NI transmits that map not only to the opposite tectum but also back to the same tectum from which it received its input. Thus, the uncrossed isthmotectal axons provide each tectum with a map which is essentially topographically identical to the retinotecal map but which is slightly delayed temporally. The uncrossed isthmotectal axons therefore could provide topographic cues to the guide the alignment of the crossed isthmotectal axons as they establish the ipsilateral visuotectal map. In order to determine whether the uncrossed isthmotectal projection is an important source of topographic cues for the crossed isthmotectal axons, the right nucleus isthmi was ablated and one eye was rotated by 90 degrees-150 degrees in midlarval tadpoles.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between isthmotectal fibers and other tectopetal systems in the leopard frog

We studied the relationship of isthmotectal input to other tectal afferent fiber systems in three ways. 1) Using horseradish peroxidase (HRP) histochemistry, we determined the nonretinal inputs to the superficial tectum. In different sets of animals we a) applied HRP to the tectal surface; b) inserted HRP crystals into the tectum; c) injected small volumes of HRP solutions into the superficial tectum. N. isthmi accounts for more than 65% of the nonretinal extrinsic input in the superficial tectal layers. One set of fibers from the contralateral n. isthmi projects to the most superficial layer. Fibers from posterior thalamus and tegmentum project to both superficial and deeper layers in the tectum, but not to the most superficial layer. The ipsilaterally projecting isthmotectal fibers terminate in the deeper superficial layers. 2) We investigated the relationship between retinofugal and contralaterally projecting isthmotectal pathways. We orthogradely labelled n. isthmi fibers by unilateral HRP injections into n. isthmi, and we also labelled retinal fibers by injecting tritiated l-proline into both eyes. In such animals contralaterally projecting isthmotectal fibers cross in the dorsal posterior region of the optic chiasm. From the chiasm to the tectum isthmotectal fibers and retinofugal fibers are admixed. 3) We determined whether other fiber systems cross with contralaterally projecting isthmotectal fibers. We cut the posterior part of the optic chiasm and applied HRP crystals to the cut. Only n. isthmi and retina are retrogradely labelled.

Neurogenesis and cell death in the isthmic nuclei of the frog Limnodynastes dorsalis

In Limnodynastes dorsalis, neurogenesis of the isthmic nuclei was determined by 3H-thymidine autoradiography. The nuclei developed in a rostroventral to caudodorsal direction. Peak neurogenesis occurred in midlarval life, resulting in the formation of a substantial portion of each nucleus. Cell death was evidenced by the presence of pyknotic cells within the nucleus in all except the earliest stages. Peak cell death occurred around metamorphic climax. It was during this period that a rim neuropil appeared that segregates within the nucleus the contralateral and ipsilateral components of the tecto-isthmo-tectal pathways underlying binocular vision. It is also a time just prior to the onset of electrophysiologically detectable ipsilateral visual responses. However, no systematic cell death was found. Therefore the role of cell death in the formation of the isthmic nuclei remains obscure, although it may be involved in the establishment of tecto-isthmo-tectal connections.

Effects of intraocular tetrodotoxin on the development of the retinocollicular pathway in the Syrian hamster

The developing uncrossed retinocollicular projection in the Syrian hamster undergoes a characteristic set of changes during the first 2 postnatal weeks. The retinal fibres, which initially project across the whole superior colliculus, withdraw from the caudal part and their terminals become clustered into deep, discrete clumps rostrally. Coincident with these afferent changes, there is substantial retinal ganglion cell death. To examine whether neuronal activity plays a role in these changes, we made daily injections of the sodium channel blocker tetrodotoxin (TTX) into one or both eyes from postnatal day 2 or 4 up to day 12. Following TTX treatment, the uncrossed terminals retracted on schedule from the caudal and superficial parts of the superior colliculus and came to lie, as normal, in the deep layers rostrally. Within the rostral superior colliculus, however, the uncrossed terminals from TTX-injected eyes lost their characteristic patchy distribution and were arranged diffusely. When only one eye received TTX injections, this inhibiting effect on terminal segregation was seen only in the projections from the TTX-treated eye. The effect of TTX treatment on terminal segregation was much less severe than that of unilateral enucleation, after which uncrossed terminals persis throughout the entire superior colliculus. TTX injections appeared to have little effect on overall ganglion cell death since the total number of ganglion cells in the crossed projection from TTX-treated eyes was similar to that in normal eyes. However, the relative distribution of uncrossed cells in temporal and nasal retina was altered. In eyes that received TTX injections, the proportion of uncrossed cells in nasal retina was about 1.6 times that in normal animals and was close to the proportion seen in unilaterally enucleated animals. This increase in the treated eye occurred whether one or both eyes were injected with TTX. We conclude that neuronal activity plays a role in the segregation of uncrossed terminals into discrete clumps in rostral colliculus and in the preferential elimination of uncrossed cells from the nasal retina. The inactive uncrossed projections from TTX-treated eyes showed the greatest degree of disruption. The extent of the disruption was similar whether the crossed input from the other eye was active or inactive. This suggests that the activity-drive interactions between ganglion cells within one eye are more significant than those between the two eyes in shaping the final form of the uncrossed retinocollicular projection.

Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. II. Abnormalities following early visual deprivation

During normal metamorphic and post-metamorphic growth of the frog, Xenopus laevis, there is a major and orderly remodelling of the pattern of neuronal connections in the intertectal system. These changes preserve the spatial registration of binocular visual inputs to each optic tectum in the face of continuous changes in relative eye alignment (Grant and Keating 1989). We suggested that visual experience might be utilised by the intertectal system to effect the maturational remodelling of its connections, with particular involvement in maintaining binocular visual registration. To investigate this suggestion we studied the development of the intertectal system in animals that had been reared in total darkness from before the onset of function in the system. Visual deprivation did not affect the developmental ocular migration that normally occurs in Xenopus, nor did it affect the maturation of the contralateral visuotectal projection. Abnormalities were, however, observed in the ipsilateral visuotectal projection of all dark-reared animals studied, reflecting perturbation of the underlying intertectal system. The abnormalities included disorder and deficits in the projection, which became more marked with age. Quantitative analyses of the spatial registration of binocular visual inputs to the optic tectum revealed that, in all dark-reared animals studied, registration was both significantly poorer and systematically shifted compared to normal controls. Analysis of maturational changes in the pattern of intertectal connections in visually-deprived animals led to the conclusion that intrinsic developmental processes generate an initially well-organised intertectal system and programme much of its continuous expansion with age. Visual experience, however, is necessary for the large scale and orderly remodelling of the system which, during normal maturation, preserves binocular visual registration despite changes in interocular alignment.

Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. I. Normal maturational changes in response to changing binocular geometry

During metamorphic and post-metamorphic life in the frog. Xenopus laevis, growth-related changes in skull shape produce radical alterations in the spatial relationship between the two eyes. These changes in binocular visual geometry were measured using optical techniques. Between the onset of metamorphic climax at stage 60 and adulthood (2 or more years post-metamorphosis) each eye migrates nasally by 55 degrees and dorsally by 50 degrees with respect to the major body axes of the animal. As a result the nasotemporal extent of the binocular visual field increases from 30 degrees to 162 degrees between these ages. Electrophysiological methods were used to determine changes in the neural representation of the binocular visual field at the paired midbrain optic tecta and in the tectal projection of pairs of corresponding retinal loci at various developmental points between these ages. The proportion of each tectal surface devoted to the representation of the binocular visual field increases from 11% at stage 60 to 77% at adulthood. Retinal correspondence, and hence the tectal projection of corresponding retinal loci, undergoes radical alteration during this period. In normal adults an intertectal system of connections selectively links the tectal projection of corresponding retinal loci and thus provides a neuronal mechanism for integrating binocular visual information in the optic tecta. Electrophysiological methods were used to determine how the intertectal system accommodates the developmental challenge posed by the enlarging binocular visual field and changing retinal correspondence. Between stage 60 and adulthood the ipsilateral visuotectal projection which is the product of the intertectal system, increases in size as the binocular visual field and its tectal representation enlarges. Moreover, throughout this period, it provides a mechanism for integrating binocular visual information in the optic tecta by maintaining its spatial registration with the contralateral visuotectal projection from the other eye. Analysis of the pattern of functional intertectal connections reveals that during the course of normal maturation this system undergoes continuous processes of expansion and of orderly and major remodelling.

Development of the nucleus isthmi in Xenopus, II: Branching patterns of contralaterally projecting isthmotectal axons during maturation of binocular maps

The tectum of Xenopus frogs receives input from both eyes. The contralateral eye's projection reaches the tectum directly, via the optic nerve, and the ipsilateral eye's projection reaches the tectum indirectly, via the nucleus isthmi. Under normal conditions, the topography of the ipsilateral map relayed from the nucleus isthmi is in register with the topography of the retinotectal map from the contralateral eye. During development, the process of aligning the two maps is complicated by the dramatic changes in binocular overlap of the two eyes' visual fields which take place during late tadpole and juvenile stages. The goal of this study is to determine the branching patterns of contralaterally projecting isthmotectal axons before, during, and after the period of rapid eye migration. Isthmotectal axons were filled by anterograde transport of horseradish peroxidase (HRP) from the nucleus isthmi. The results show that crossed isthmotectal axons enter the entire extent of the tectum before binocular overlap begins to increase. Therefore, binocular overlap is not necessary for the initial isthmotectal projection to span the tectum. The density of isthmotectal branches rises dramatically at the same time that the eyes begin to shift. During the period when eye migration is most rapid, many isthmotectal axons form arbors which resemble adult arbors but which extend over greater proportions of the tectal surface. The axons appear to be directed toward appropriate mediolateral positions as they enter the tectum. Their trajectories are roughly rostocaudal, with relatively little change along the mediolateral dimension. These data, when combined with available physiological data, suggest that mediolateral order is initially established by vision-independent mechanisms but can be altered by vision-dependent mechanisms. Rostrocaudal order becomes discernable only at the time when binocular visual cues become available and appears to be established primarily on the basis of the activity of the retinotectal and isthmotectal axons.

Connections between the nucleus isthmi and the tectum in larval and post-metamorphic axolotls

The nucleus isthmi (NI) is the primary relay for the frog's ipsilateral visuotectal projection. Using electrophysiological methods, ipsilateral visuotectal activity has been recorded in thyroxine-treated, postmetamorphic axolotls but not in larval axolotls. In order to determine whether changes in isthmotectal projections are responsible for this change in electrophysiological responsiveness, we have investigated the connections between the tectum and the NI using horseradish peroxidase. Our results indicate that the axolotl's isthmotectal pathways are strikingly similar to those of the frog NI, and that the NI sends bilateral projections to the tecta in both larval and thyroxine-treated, postmetamorphic axolotls. Thus, the anatomical connections underlying the ipsilateral visuotectal projection are present during larval stages, despite the lack of electrophysiological evidence for the larval ipsilateral visuotectal projection. We hypothesize that thyroxine-induced metamorphosis produces changes in the terminal arborizations of the crossed isthmotectal projection which allow them to be detected by presynaptic electrophysiological techniques.

The ultrastructural organization of the isthmic nucleus in Xenopus

The isthmic nucleus (IN) of the frog brain forms a linkage, relaying visual information from one tectum to the other. It receives afferent input from the tectum of the same side and projects bilaterally to both tecta. The ultrastructural features of the tecto-isthmic synaptic connections were studied in young postmetamorphic Xenopus frogs. Most synaptic profiles in the isthmic nucleus have spheroidal vesicles and an asymmetric zone of apposition. Frequently, synaptic glomeruli consisting of up to 8 terminal boutons surrounding a shaft dendrite were observed. The synaptic density in the rostral IN was slightly higher than in the middle or caudal portions. Partial deafferentation by transection of the tecto-isthmic pathway or total deafferentation by removal of the tectum was followed by a widespread degeneration of terminals in the ipsilateral IN. In the former case, the density of synapses in the IN decreased initially by about 64%, and then increased by 30 days after operation to about 50% of the normal synaptic density. After tectal removal, all the terminal boutons in the isthmic neuropil degenerated by 3 days after operation. These studies, along with recent findings, indicate that most, if not all, of the afferent fibres to IN are of tectal origin.

A projection from the mesencephalic tegmentum to the nucleus isthmi in the frogs, Rana pipiens and Acris crepitans

The nucleus isthmi is a prominent part of the frog's visual system. Each nucleus isthmi receives input from the ipsilateral tectum and sends output to both tecta. Until now, no non-tectal inputs to the nucleus isthmi of amphibians have been demonstrated. Anterograde and retrograde tracing with horseradish peroxidase in Rana pipiens and Acris crepitans now reveal that a diffuse group of cells in the mesencephalic tegmentum projects to the caudal region of the contralateral nucleus isthmi. These cells are primarily within the nucleus anterodorsalis tegmenti. This same group of tegmental cells may also project to the caudal region of the ipsilateral nucleus isthmi. A similar investigation of the brain of another frog, Xenopus laevis, has not revealed any evidence of this tegmento-isthmic projection.

Visual experience and the maturation of the ipsilateral visuotectal projection in Xenopus laevis

The effect of visual deprivation upon the maturation of the ipsilateral visuotectal projection has been studied in Xenopus laevis. This topographically ordered projection is polysynaptic. The first stage involves the retinal projection to the contralateral optic tectum. The tectum projects to the nucleus isthmi on the same side. The final stage is the crossed isthmotectal projection from the nucleus isthmi to the tectum ipsilateral to the eye. The topographic precision of connections at various points in this polysynaptic pathway has been investigated by quantifying single-unit and multi-unit receptive field sizes in the contralateral and ipsilateral visuotectal projections. Observations have been made on normal animals of different ages to plot the normal maturational course of events. The effects of visual deprivation on this maturational process has been studied. Between one week and one year after metamorphosis there is an increase in the precision of connections in both the contralateral and ipsilateral visuotectal projections. Visual deprivation had no effect upon the parameters of the contralateral visuotectal projection. Ipsilateral visuotectal single units in dark-reared animals had normal receptive field sizes. Ipsilateral multi-unit receptive fields in dark-reared animals were considerably larger than in normal animals. It was concluded that the effects of visual deprivation are limited to effects on the crossed isthmotectal component of the intertectal system. In this component, however, visual experience seems to play an important role in the normal development and modification of connections. It is suggested that visual experience is utilized to accommodate changes in the system required to respond to normal changes in interocular geometry that take place with development in Xenopus.

A species difference between Rana and Xenopus in the occurrence of intertectal neuronal plasticity

In anuran amphibians, a system of neuronal connections links the two optic tecta and is involved in projections of the binocular visual field to the optic tecta. Electrophysiological studies have shown that in the frog, Xenopus laevis, the pattern of connections may be modified by procedures such as larval rotation of one eye. This modification appears to be effected by visual experience. Workers in other laboratories, however, found no evidence of such a modification in the related frog Rana pipiens. This difference in results may have been due to different rearing conditions and different recording techniques or may reflect a true species difference, in this respect, between Rana and Xenopus. In the present experiments, an attempt was made to distinguish between these possibilities by performing eye rotations in Rana and Xenopus, rearing them as identically as possible and recording from them under identical conditions. It was found that while Xenopus displayed the modification of intertectal connections, Rana did not. It is concluded that the different responses of the intertectal systems to larval eye rotation in Xenopus and Rana reflect a species difference.

The role of visual experience in the formation of binocular projections in frogs

Many parts of the visual system contain topographic maps of the visual field. In such structures, the binocular portion of the visual field is generally represented by overlapping, matching projections relayed from the two eyes. One of the developmental factors which helps to bring the maps from the two eyes into register is visual input. The role of visual input is especially dramatic in the frog, Xenopus laevis. In tadpoles of this species, the eyes initially face laterally and have essentially no binocular overlap. At metamorphosis, the eyes begin to move rostrodorsally; eventually, their visual fields have a 170 degree region of binocular overlap. Despite this major change in binocular overlap, the maps from the ipsilateral and contralateral eyes to the optic tectum normally remain in register throughout development. This coordination of the two projections is disrupted by visual deprivation. In dark-reared Xenopus, the contralateral projection is nearly normal but the ipsilateral map is highly disorganized. The impact of visual input on the ipsilateral map also is shown by the effect of early rotation of one eye. Examination of the tectal lobe contralateral to the rotated eye reveals that both the contralateral and the ipsilateral maps to that tectum are rotated, even though the ipsilateral map originates from the normal eye. Thus, the ipsilateral map has changed orientation to remain in register with the contralateral map. Similarly, the two maps on the other tectal lobe are in register; in this case, both projections are normally oriented even though the ipsilateral map is from the rotated eye. The discovery that the ipsilateral eye's map reaches the tectum indirectly, via a relay in the nucleus isthmi, has made it possible to study the anatomical changes underlying visually dependent plasticity. Retrograde and anterograde tracing with horseradish peroxidase have shown that eye rotation causes isthmotectal axons to follow abnormal trajectories. An axon's route first goes toward the tectal site where it normally would arborize but then changes direction to reach a new tectal site. Such rearrangements bring the isthmotectal axons into proximity with retinotectal axons which have the same receptive fields. Anterograde horseradish peroxidase filling has also been used to study the trajectories and arborizations of developing isthmotectal axons. The results show that the axons enter the tectum before the onset of eye migration but do not begin to branch profusely until eye movement begins to create a zone of binocular space.(ABSTRACT TRUNCATED AT 400 WORDS)

Intertectal neuronal plasticity in Xenopus laevis: persistence despite catecholamine depletion

In normal Xenopus, the tectum receives a direct projection from the contralateral retina and an indirect projection, via the intertectal system, from the ipsilateral eye. The two maps of binocular visual space, at each tectum, are in register. If one eye is rotated during larval development, the ipsilateral visuotectal projection compensates by changing its orientation. Rearrangement of the intertectal system brings the ipsilateral map back into register with the contralateral map. We sought to determine whether this intertectal plasticity required normal levels of brain monoamines. Animals received an eye rotation between stages 55-63 of larval life and were then placed in one of 3 groups. A first control group received no further treatment. A second control group was given intraventricular injections of ascorbate vehicle. The experimental group was given intraventricular injections of 6-hydroxydopamine in ascorbate vehicle. Two to 3 months after metamorphosis, visuotectal projections were mapped electrophysiologically and the brains were assayed for monoamines. Intertectal plasticity occurred in all 3 groups of animals, including animals in which brain catecholamine levels were severely reduced. We conclude that normal levels of brain catecholamines are not required for this form of neural plasticity.

The development of the nucleus isthmi in Xenopus laevis. I. Cell genesis and the formation of connections with the tectum

The nucleus isthmi (NI) of the amphibian relays visual input from one tectum to the other tectum and thus brings a visual map from the eye to the ipsilateral tectum. This isthmotectal visual map develops slowly; it is first detected electrophysiologically at stages 60-62, the age at which the eyes begin their dorsalward migration and the region of binocular overlap beings to increase in extent. During this critical period of life, normal binocular visual input is required for establishment of normal topographic isthmotectal projections. In this study, we have used anatomical methods to trace cell birth, cell death, and formation of connections by the nucleus isthmi during the critical period. Tritiated thymidine labelling demonstrates that cells in the nucleus isthmi are generated throughout most of tadpole life (stages 29-62). Most cells conform to an orderly ventrodorsal gradient starting from stage 29 and extending to stages 56; later cells are inserted at apparently random locations in the nucleus. We have re-examined the hypothesis of Tay and Straznicky ('80) that the order of cell genesis in the NI and tectum could help establish proper isthmotectal connections, and we find that a timing mechanisms does not explain the two-dimensional topography of the isthmotectal map but that timing may aid in proper mediolateral positioning of isthmotectal axons at the points where they first enter the tectum. Horseradish peroxidase labelling was used to investigate whether anatomical projections from tectum to NI and from NI to tectum are present prior to the onset of eye migration. The results show that there are tectoisthmotectal projections by stage 52. Moreover, isthmotectal axons grow into as yet monocular tectal regions prior to the onset of eye migration. At stage 60, when binocular overlap begins, isthmotectal axons are visible throughout the tectum but are densely branched only at the rostral tectal margin, the location where they are predicted to occur on the basis of electrophysiological maps.