Time course and pattern of optic fiber regeneration following tectal lobe removal in the goldfish

Following single tectal lobe removal in the adult goldfish, Carassius auratus, the pattern of regeneration of the optic fibers which had previously projected to that tectum was examined at 1, 2, 4, 6, 8, 10, and 12 weeks postoperative using ³H-proline radioautography. We found that regenerating optic fibers grew across the midline through the transverse, minor, horizontal, and posterior commissures to innervate the remaining tectum. At early postoperative times innervation of the tectum was continuous, while later, the regenerating fibers segregated into discrete patches in the superficial layers of the tectum. In addition, regenerating fibers also grew into non-optic centers/pathways such as the habenula, the fasciculus retroflexus, the forebrain, the torus semicircularis, the valvula and corpus cerebelli, the hypothalamus, and the medulla. While optic fibers were no longer apparent in the habenula and the fasciculus retroflexus after 2 weeks postoperative, all other structures were still occupied by the fibers at 12 weeks postoperative. Since most of the innervated pathways were either tectal efferent pathways, which should contain degenerating debris and proliferating glial cells after the tectal removal, or pathways closely associated with traumatized areas, we suggest that degenerating axonal debris and proliferating glia may play an important role in guiding regenerating fibers in this system.     

Anatomical evidence for the influence of degenerating pathways on regenerating optic fibers following surgical manipulations in the visual system of the goldfish

We have used [³H]proline radioautography to trace regenerating optic fibers in the goldfish following: (1) the removal of the right tectal lobe and the right eye, and (2) the removal of both tectal lobes. Our results indicate that following the removal of the right tectal lobe and the right eye, both the denervated tectal efferent pathways, and the denervated visual pathways and terminal zones of the enucleated eye were penetrated by the regenerating optic fibers. In addition, following bilateral lobectomy, the denervated tectal efferent pathways were bilaterally penetrated by the regenerating fibers. Since, in both types of operations, these denervated pathways and terminal zones should undergo degeneration, our results support the suggestion that the presence of degenerating axonal debris and proliferating glia may play an important role in guiding regenerating optic fibers in the visual system of the goldfish.     

Deployment of optic nerve fibers is determined by positional markers in the frog''s tectum

In postmetamorphic Xenopus laevis, portions of the optic tectum were removed and reimplanted in various orientations. The optic nerve fiber projection to the reimplant was mapped electrophysiologically at various post-operative times. In most cases, the projection which regenerated into the reimplant was rotated by an amount equal to the surgical rotation of the reimplant. These results demonstrate the existence of a set of positional markers in the tectum which determines the polarity of the visuotectal projection. The polarity of this set of tectal markers is stable under the conditions of our experiments. We term these positional markers tectal locus specificities.     

Retinal fibers alter tectal positional markers during the expansion of the retinal projection in goldfish

Although widely accepted, the theory, that neurones carry immutable cytochemical markers which specify their synaptic connections, is not consistent with plastic reorganizations. Half retinal fish were therefore tested for changed markers following expansion. Optic nerve crush at the time of the half retinal ablation resulted in regeneration of a normal, restricted projection; but nerve crush following expansion (many months later) resulted in reestablishment of the expanded projection, assessed both by electrophysiological mapping and by radioautography. Since this implied changed markers, the half retina and tectum were tested independently using the ipsilateral tectum and eye as controls. In normal fish, removal of one tectum and deflection of the corresponding optic tract toward the remaining tectum resulted in regeneration of a positionally normal but ipsilateral map. In experimental fish, after the half retina had expanded its projection to the contralateral tectum, its optic tract was deflected to the control tectum. After 40 days it had regenerated a normal, restricted map indicating that the retinal markers had not changed. Such restricted projections did not expand in the presence of the normal projection even after a year or more. Similarly, the optic tract from the normal eye was deflected to cause innervation of the tectum containing the expanded half retinal projection. After 40 days, the projection regenerated from the normal eye was similar to the expanded half retinal projection. Areas of the normal retina corresponding to the missing areas of the half retina were not represented. Tectal markers had been altered by the half retinal fibers. In a final group, tecta were denervated and tested at various intervals by innervation from ipsilateral half retinal eyes. After five months of denervation, the regenerating fibers were no longer restricted to the rostral tectum but formed an expanded projection initially. Apparently tectal markers are induced by the retinal fibers, changed during expansion, and disappear during long-term denervation.     

Changes in the topographically organized connections between the nucleus isthmi and the optic tectum after partial tectal ablation in adult goldfish

The projection of the nucleus isthmi to the ipsilateral optic tectum was examined in normal goldfish. This was compared to the projection in animals in which the entire visual field had been induced to compress onto a rostral half tectum by caudal tectal ablation. The isthmo-tectal projection was examined by making localized injections of horseradish peroxidase into the optic tecta and observing the patterns of labeled cells within the nucleus isthmi. The teleost nucleus isthmi consists of a cell sparse medulla covered by a cellular cortex, which is thick on the rostral, medial, and dorsal surfaces of the nucleus. Almost all isthmic cells projecting to the tectum were located in the area of thick cortex. In normal fish, rostral tectal injections labeled cells in the rostroventral portion of the thick cortex; injections midway in the rostrocaudal tectal axis labeled more caudodorsally located cells, and caudal tectal injections labeled cells a little further caudally in extreme dorsal cortex. The rostroventral to caudodorsal isthmic axis was therefore seen to project rostrocaudally along the tectum. This topography contrasts somewhat with the situation seen in amphibia where the rostrocaudal tectal axis receives projections from the rostrocaudal isthmic axis. In fish with half-tectal ablations, injections near the caudal edge of the half tectum (at a site that had originally been midtectal) labeled cells that had previously projected to caudal tectum. Rostral tectal injections in fish with compression of the visual field gave a normal pattern of labeled isthmic cells. The results indicate that a topographically ordered isthmo-tectal projection exists in goldfish that may be induced to compress onto a half tectum.     

A GABAergic tecto–tegmento–tectal pathway in pigeons

Previous studies have demonstrated that the optic tecta of the left and right brain halves reciprocally inhibit each other in birds. In mammals, the superior colliculus receives inhibitory γ‐aminobutyric acid (GABA)ergic input from the basal ganglia via both the ipsilateral and the contralateral substantia nigra pars reticulata (SNr). This contralateral SNr projection is important in intertectal inhibition. Because the basal ganglia are evolutionarily conserved, the tectal projections of the SNr may show a similar pattern in birds. Therefore, the SNr could be a relay station in an indirect tecto–tectal pathway constituting the neuronal substrate for the tecto–tectal inhibition. To test this hypothesis, we performed bilateral anterograde and retrograde tectal tracing combined with GABA immunohistochemistry in pigeons. Suprisingly, the SNr has only ipsilateral projections to the optic tectum, and these are non‐GABAergic. Inhibitory GABAergic input to the contralateral optic tectum arises instead from a nearby tegmental region that receives input from the ipsilateral optic tectum. Thus, a disynaptic pathway exists that possibly constitutes the anatomical substrate for the inhibitory tecto–tectal interaction. This pathway likely plays an important role in attentional switches between the laterally placed eyes of birds. J. Comp. Neurol. 524:2886–2913, 2016. © 2016 Wiley Periodicals, Inc.     

Selective retinal reinnervation of a surgically created tectal island in goldfish. II. Electron microscopic analysis

In the preceding study (Edwards et al., '85), we showed that regenerating optic axons reestablish a topographically restricted projection to a caudal tectal island created by surgical removal of a 1-mm-wide strip of caudal tectum in goldfish. In the present ultrastructural study, we evaluated the dependence of this axonal outgrowth on the presence of tectal target tissue caudal to the gap. Axon counts in the lesion zone were compared between cases with complete caudal tectal ablation and cases with ablation sparing a caudal tectal island (with and without optic nerve crush). During the postoperative interval of 20-50 days (early period), up to about 1,000 unmyelinated axons with features characteristic of optic axons were present in numerous small subpial bundles in both preparations. In the subsequent interval of 50-110 days (middle period), less than 200 axons were counted caudal to simple half-tecta, whereas 4,000-14,000 myelinated and unmyelinated axons were present in a few large bundles which crossed the lesion zone of tectal island cases. In this period, optic terminals could be demonstrated in the tectal island using the anterograde horseradish peroxidase method. At 170-300 days after surgery (late period), bridging bundles contained between 2,000 and 6,000 largely myelinated axons. We conclude that caudal tectal tissue is not necessary for the initial outgrowth of a small number of axons beyond a rostral half-tectum. The target is essential, however, for the maintenance of these axon fascicles and for the subsequent massive outgrowth of axons to the island. The contributions of glial guidance, diffuse exploratory outgrowth, and target-produced trophic factors to the formation of an initially exuberant projection to the island are discussed. A process of selective axon collateral withdrawal is proposed to account for the decrease in axon numbers within bridging bundles in the late period and for the late restriction in the retinal origin of the island projection indicated by results in the preceding study (Edwards et al., '85).     

Tests of the regenerative capacity of tectal efferent axons in the frog, Rana pipiens

Experiments were designed to determine if neurons of the ranid optic tectum, a major target of the optic nerve, possess the same regenerative potential as optic axons. Normal tectal efferent (TE) projections were reexamined by using the anterograde transport of 3H-proline and autoradiography (n = 18), bulk-filling damaged TE axons with horseradish peroxidase (HRP; n = 18) and anterogradely transporting wheat germ agglutinin-HRP (n = 8) to label TE axons. Results were similar to reports that used degeneration methods (Rubinson: Brain Behav. Evol. 1:529-561, '68; Lazar: Acta. Biol. Hung. 20:171-183, '69). Following a brainstem hemisection just caudal to the nucleus isthmi (1-20 weeks), the ipsilateral descending TE pathway was autoradiographically examined (n = 20). While all other TE projections appeared normal, there was no detectable ipsilateral descending projection beyond the lesion site. Ascending TE axons were cut at the anterior tectal border by hemisecting the left diencephalon (LDH)--a lesion that also cuts optic axons projecting to the left tectum. There was no indication of TE axonal regeneration with the aid of autoradiography or HRP histochemistry 1-30 weeks postlesion (n = 48) even when the medial diencephalon was intentionally left intact (n = 4). However, in all four cases examined, optic axons regenerated following the same LDH where TE axonal regeneration failed (also see Stelzner, Lyon, and Strauss: Anat. Rec. 205:191A-192A, '83). Local effects of LDH should be similar for both the cut optic and cut TE axons. Other factors were tested that may contribute to the lack of TE axonal regeneration. Our results indicate that optic regeneration itself (n = 8), postaxotomy retrograde cell death of TE neurons (n = 6), deafferentation of the tectum of optic axons, and potential sprouting within tectal targets by intact contralateral TE axons (n = 10) are not critical factors aborting TE axonal regeneration. TE axons filled with HRP at chronic periods after LDH (n = 4) terminate anomalously near the LDH border. Many of these endings are similar to reactive endings or terminal clubs seen after axonal injury in the mammalian CNS. Our results suggest that this disparity in regenerative ability of optic and TE axons may be related to a difference in the responsive ability of these cell types to initiate or maintain axonal elongation after axotomy within the amphibian CNS environment.     

Bilateral tectal innervation by regenerating optic nerve fibers in goldfish: a radioautographic, electrophysiological and behavioral study.

Following unilateral enucleation and optic nerve crush in goldfish, the remaining nerve regenerates and innervates both optic tecta. Approximately 5% of the nerve fibers reach the ipsilateral optic tectum (IOT) via the ipsilateral tract at the chiasma. Comparable debris in both tracts was not sufficient to result in an IOT projection since when both nerves were crushed simultaneously the usual pattern was seen, i.e., each nerve innervated a contralateral optic tectum (COT). When the arrival of one nerve at the chiasma was delayed by staggering the nerve crushes, the nerve that first arrived at the chiasma partially innervated the Iot. In most instances the entire IOT was innervated, however, the stratigraphic distribution of fibers in the various tectal lamina was atypical. Electrophysiological analysis indicated that fibers from each area of the retina innervated the IOT visuotopically. The COT was ablated in order to determine whether the IOT projection could mediate behavior. All fish failed to respond to changes in illumination as measured by respiration and failed to swim with or against the stripes in an optomotor drum. Thus, the IOT input, possibly because of its sparseness, could not be shown to be behaviorally functional.     

Visual recovery in goldfish following unilateral optic tectum ablation: Evidence of competition between optic axons for tectal targets

Anatomical studies suggest that regenerating optic axons which invade the ipsilateral lobe of the optic tectum following ablation of the contralateral lobe compete with resident optic axons for synaptic sites on tectal neurons. Invader optic axons are initially uniformly distributed over the entire tectal lobe. With time, the invader and resident optic axons progressively segregate so that the invaders are localized in bands or islands separated by areas that are innervated mainly by the residents. When the resident optic axons are destroyed by ablating the eye opposite to the experimental eye, the invader axons remain continuously distributed and the segregation process apparently does not occur. We investigated the relationship between the segregation process and the recovery of visual function by the invader axons. Visual recovery was measured with a behavioral method in which the index of vision was the occurrence of a branchial suppression response to a moving spot of red light that was classically conditioned to an electric shock stimulus. The minimum time to reappearance of vision following ablation of the contralateral lobe of the tectum in two-eye fish was similar to the reported time of onset of the segregation process. Visual recovery occurred sooner when the opposite eye was removed. The restored vision in both groups disappeared following subsequent ablation of the remaining lobe of the tectum. These results suggest that the goldfish optic tectum normally contains no free synaptic sites for anomalous optic afferents and that the invader axons must compete for targets with the resident optic afferents. The invader axons can apparently remain unconnected or non-functional for several weeks following their arrival in the ipsilateral tectal lobe.     

Connections of eye-saccade-related areas within mesencephalic reticular formation with the optic tectum in goldfish

Physiological studies demonstrate that separate sites within the mesencephalic reticular formation (MRF) can evoke eye saccades with different preferred directions. Furthermore, anatomical research suggests that a tectoreticulotectal circuit organized in accordance with the tectal eye movement map is present. However, whether the reticulotectal projection shifts with the gaze map present in the MRF is unknown. We explored this question in goldfish, by injecting biotin dextran amine within MRF sites that evoked upward, downward, oblique, and horizontal eye saccades. Then, we analyzed the labeling in the optic tectum. The main findings can be summarized as follows. 1) The MRF and the optic tectum were connected by separate axons of the tectobulbar tract. 2) The MRF was reciprocally connected mainly with the ipsilateral tectal lobe, but also with the contralateral one. 3) The MRF received projections chiefly from neurons located within intermediate and deep tectal layers. In addition, the MRF projections terminated primarily within the intermediate tectal layer. 4) The distribution of labeled neurons in the tectum shifted with the different MRF sites in a manner consistent with the tectal motor map. The area containing these cells was targeted by a high-density reticulotectal projection. In addition to this high-density topographic projection, there was a low-density one spread throughout the tectum. 5) Occasionally, boutons were observed adjacent to tectal labeled neurons. We conclude that the organization of the reticulotectal circuit is consistent with the functional topography of the MRF and that the MRF participates in a tectoreticulotectal feedback circuit.     

Optic fiber segregation in goldfish with two eyes innervating one tectal lobe

An ipsilateral retinotectal projection was induced by ablating one tectal lobe. Radioautography indicated that the ipsilateral projection initially spread out continuously over the remaining tectal lobe. With time, the continuous projection progressively changed into a rostrocaudally oriented banded projection that was comprised of high and low silver grain density bands. The undamaged native projection from the contralateral eye also became transformed into high and low density bands. The results indicate that foreign fibers displace portions of the native projection. Complete segregation of the two projections was found at any time point examined. Low density bands did not represent spillover of label from a high density band since cobalt-filled optic fibers were found in low density bands. Quantitative analysis indicated that the contralateral projection occupied more tectal area than the ipsilateral projection. Area occupied by a projection, band width and band frequency showed appreciable between-fish variability. Correlations between band width and per cent of area occupied by a projection approached unity, indicating that large projections were associated with wider bands. A model is proposed to account for induced banding in lower vertebrates.     

Neuron types in the zebrafish optic tectum labeled by an id2b transgene

The larval zebrafish optic tectum has emerged as a prominent model for understanding how neural circuits control visually guided behaviors. Further advances in this area will require tools to monitor and manipulate tectal neurons with cell type specificity. Here, we characterize the morphology and neurotransmitter phenotype of tectal neurons labeled by an id2b:gal4 transgene. Whole-brain imaging of stable transgenic id2b:gal4 larvae revealed labeling in a subset of neurons in optic tectum, cerebellum, and hindbrain. Genetic mosaic labeling of single neurons within the id2b:gal4 expression pattern enabled us to characterize three tectal neuron types with distinct morphologies and connectivities. The first is a neuron type previously identified in the optic tectum of other teleost fish: the tectal pyramidal neuron (PyrN). PyrNs are local interneurons that form two stratified dendritic arbors and one stratified axonal arbor in the tectal neuropil. The second tectal neuron type labeled by the id2b:gal4 transgene is a projection neuron that forms a stratified dendritic arbor in the tectal neuropil and an axon that exits tectum to form a topographic projection to torus longitudinalis (TL). A third neuron type labeled is a projection neuron with a nonstratified dendritic arbor and a descending axonal projection to tegmentum. These findings establish the id2b:gal4 transgenic as a useful tool for future studies aimed at elucidating the functional role of tectum, TL, and tegmentum in visually guided behaviors.     

Tectal mosaic: organization of the descending tectal projections in comparison to the ascending tectofugal pathway in the pigeon

The optic tectum of vertebrates is an essential relay station for visuomotor behavior and is characterized by a set of connections that comprises topographically ordered input from the eyes and an output that reaches premotor hindbrain regions. In the avian tectofugal system, different ascending cell classes have recently been identified based on their dendritic and axonal projection patterns, although comparable information about the descending cells is missing. By means of retrograde tracing, the present study describes the detailed morphology of tectal output neurons that constitute the descending tectobulbar and tectopontine pathways in pigeons. Descending cells were more numerous in the dorsal tectum and differed in terms of 1) their relative amount of ipsi- vs. contralateral projections, 2) the location of the efferent cell bodies within different tectal layers, and 3) their differential access to visual input via dendritic ramifications within the outer retinorecipient laminae. Thus, the descending tectal system is constituted by different cell classes presumably processing diverse aspects of the visual environment in a visual field-dependent manner. We demonstrate, based on a careful morphological analysis and on double-labeling experiments, that the descending pathways are largely separated from the ascending projections even when they arise from the same layers. These data support the concept that the tectum is arranged as a mosaic of multiple cell types with diverse input functions at the same location of the tectal map. Such an arrangement would enable the tectal projections onto diverse areas to be both retinotopically organized and functionally specific.     

Chronic application of NMDA decreases the NMDA sensitivity of the evoked tectal potential in the frog.

The activity-dependent mechanism that refines the topography of the retinotectal projection in frogs is mediated by the NMDA receptor. Earlier studies found that chronic treatment of the optic tectum with the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-AP5) desegregated eye-specific stripes in three-eyed frogs, while chronic treatment with NMDA sharpened stripe borders (Cline et al., 1987; Cline and Constantine-Paton, 1990). We now report that this same chronic treatment with NMDA decreases the electro-physiologically measured sensitivity of the optic tectum to applied NMDA: acute application of a given concentration of NMDA decreased the evoked tectal potential to a lesser extent in animals chronically treated with NMDA than it did in normal and sham-treated animals. This is observed as a shift to the right in the NMDA dose-response curves for both the positive and negative postsynaptic components of the evoked tectal response. We believe that this decreased NMDA receptor effectiveness further restricts the intermingling of axon branches from the two eyes by limiting synapse stabilization to areas where afferent activity is most correlated. This would account for the anatomical sharpening of stripe borders (i.e., increased afferent segregation). Quantitative autoradiographic analysis of 3H-glutamate binding to NMDA receptors indicated that binding densities within the tectum did not differ between control groups and NMDA chronically treated groups. We suggest that in the experimental animals the response to NMDA may be decreased by a change in the effectiveness of individual NMDA receptors rather than by decreases in receptor number. This experimentally induced change may be analogous to naturally occurring decreases in receptor function that correlate with the end of some periods of visual plasticity in mammals.     

A critical period in the development of tectal neurons in the chick, as revealed by early enucleation

To further study the existence of a critical trophic period in the development of the chick optic tectum¹⁷, during which the presence of retinal synapses is essential to the continued growth of tectal neurons, we have unilaterally enucleated embryos between stages 14–20 and allowed survival until stages 35–43. If the critical trophic period is between stages 40–44, as previously reported¹⁷, then we reasoned that early removal of the eye might not have any effect on tectal development until the critical period. We assessed tectal neuron survival by staining for degeneration in the efferent projections of tectal neurons. In early enucleates, degeneration was present from stages 37–43, and the severity of the degeneration was much reduced in comparison to animals enucleated during the critical period.These findings substantiate the proposition that there is a critical period late in chick tectal development. However, because the degeneration in tectal projections is less intense than in animals enucleated during the critical period, we suggest that the early enucleation has permitted axons from the remaining eye to be routed to the deafferented tectum, where they may help to sustain a portion of the tectal neurons through the critical period. Moreover, the somewhat earlier appearance of degeneration in tectal efferent pathways of early enucleates suggests that a subtle trophic relationship between retina and tectum may exist prior to stage 40, even though this relationship is not revealed when enucleations are performed later, as between stages 35–40 (ref. 17).     

Tangential neuronal migration in the avian tectum: cell type identification and mapping of regional differences with quail/chick homotopic transplants.

This paper is a sequel to a previous report, using quail/chick chimeras with partial tectal transplants, in which a tangential invasion of host (chick) tectal territories by cells originating in the quail graft was demonstrated. The cells displaying this secondary tangential migration appeared restricted to two strata (stratum griseum centrale (SGC) and stratum griseum et fibrosum superficiale (SGFS)). Here we describe the morphology of the tangentially displaced neurons, as well as their overall distribution in the host tectal lobe, by means of an antibody that specifically recognizes quail cells, staining them in a Golgi-like manner. Neurons that migrated into the SGC are identified as multipolar projection neurons, typical of this stratum. The majority of cells that migrated into the SGFS correspond to horizontal neurons, as was also corroborated by observations in Golgi-impregnated material. These horizontal cells are concentrated in laminae b, d and f, where their processes form well delimited axonal plexuses. In confirmation of previous results, SGC neurons have a limited range of migration, whereas SGFS cells translocate across much longer distances. In reconstructions of appropriate cases, a remarkable polarity was noted. Significant invasion of chick tectum by quail cells mostly occurred in the rostral half of the host tectum. The long-range migration of superficial horizontal cells frequently reached, but did not cross, the rostral tectal boundary. Conversely, tangential migration in the caudal half of the host tectum was scarce and coincided with a typical arrangement of quail-derived radial columns interdigited with chick-derived columns. These findings are discussed in relation to existing data on immature neuronal populations, molecular marker distribution and polarity of the avian optic tectum.     

Effect of neuropeptide-Y on tectal field potentials in the toad

Field potentials (FP) recorded from the surface of toad's optic tectum (OT) to electrical stimulation of the contralateral optic nerve (ON) show initial positive deflections P^*, followed by negative wave N and positive wave P; the former result from axonal inputs, the latter two resemble excitatory and inhibitory postsynaptic processes, respectively. Electrostimulation of the pretectum ipsilaterally to the recorded OT — preceeding ON-stimulation — strongly attenuates the N wave, suggesting pretectotectal inhibitory influences. The N wave of the tectal FP evoked by ON-stimulation is reduced, too, after application of the neuropeptide-Y (NPY) to the tectal surface. Previous authors have shown that frog's pretectum contains NPY immunoreactive pretectotectal projecting cells.     

Neural substrata underlying tectal eye movement codification in goldfish

The optic tectum encodes orienting eye saccades in a spatially ordered map. To investigate whether the functional properties of each tectal site are related to a particular pattern of connectivity with downward structures in the brainstem, two sets of experiments were carried out. First, biotinylated dextran amine (BDA) was injected at different tectal sites along the anteroposterior axis. Electrical stimulation at these sites evoked saccades whose horizontal component amplitudes increased with the distance to the rostral pole. In the second experiment, BDA and fluoro-ruby (FR) were injected at different tectal sites along the mediolateral axis. Electrical stimulation here evoked saccades with different upward and downward directions, but similar horizontal component amplitudes. A major finding of the first experiment was that a topographic link of the tectum exists with the mesencephalic reticular formation, but that such a connection was absent or very attenuated for the rhombencephalic reticular formation. In the second set of experiments, the clusters of BDA and FR boutons left by the mediolateral tectal sites were separated in the rostral mesencephalon, at the level of the nucleus of the medial longitudinal fasciculus, but overlapped in the caudal mesencephalon and rhombencephalon. These data provide evidence that decodification of tectal motor commands is based, at least in part, on the connectivity of each tectal locus on downward structures with the brainstem.     

Intraocular colchicine inhibits competition between resident and foreign optic axons for functional connections in the doubly innervated goldfish optic tectum

After unilateral optic tectum ablation in the goldfish, regenerating optic axons grow into the optic layers of the remaining ipsilateral tectal lobe and regain visual function. The terminal arbors of the foreign fibers are initially diffusely distributed among the resident optic axons, but within two months the axon terminals from each retina are seen to segregate into irregular ocular dominance patches. Visual recovery is delayed until after segregation. This suggests that the foreign fibers compete with the residents for tectal targets and that the segregation of axon terminations is an anatomical characteristic of the process. Here we investigate whether inhibiting axonal transport in the resident fibers inhibits competition with foreign fibers. The eye contralateral to the intact tectal lobe received a single injection of 0.1 μg colchicine, which does not block vision with the intact eye. We measured visual function using a classical conditioning technique. Segregation of axon terminations was examined shortly following visual recovery by autoradiography. The no-drug control fish showed reappearance of vision with the experimental eye at 9 weeks postoperatively and ocular dominance patches were well developed. Colchicine administered to the intact eye (resident fibers) several weeks postsurgery decreased the time to reappearance of vision with the experimental eye by several weeks. Autoradiography revealed some signs of axonal segregation but the labeled foreign axons were mainly continuously distributed. Administration of colchicine at the time of tectum ablation, or of lumicolchicine at two weeks postoperatively produced normal visual recovery times. Fast axonal transport of³H-labeled protein was inhibited by 1.0 and 0.5 μg but not by 0.1 μg of colchicine or by 1.0 μg of lumicolchicine. Previous studies showed that while 0.1 μg of colchicine does not block vision it is sufficient to inhibit axonal regeneration following optic nerve crush. We conclude that two retinas can functionally innervate one tectum without forming conspicuous ocular dominance columns, and that the ability of residents to compete with the in-growing foreign axons is very sensitive to inhibition of axoplasmic transport or other processes that are inhibited by intraocular colchicine.