Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

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.     

Putative cholinergic projections from the nucleus isthmi and the nucleus reticularis mesencephali to the optic tectum in the goldfish (Carassius auratus)

The nucleus isthmi of fish and amphibians has reciprocal connections with the optic tectum, and biochemical studies suggested that it may provide a major cholinergic input to the tectum. In goldfish, we have combined immunohistochemical staining for choline acetyltransferase with retrograde labeling of nucleus isthmi neurons after tectal injections of horseradish peroxidase. Seven fish received tectal horseradish peroxidase injections, and brain tissue from these animals was subsequently processed for the simultaneous visualization of horseradish peroxidase and choline acetyltransferase. In many nucleus isthmi neurons the dense horseradish peroxidase label obscured the choline acetyltransferase reaction product but horseradish peroxidase and choline acetyltransferase were colocalized in 54 cells from nine nuclei isthmi. The somata of nucleus reticularis mesencephali neurons stained so intensely for choline acetyltransferase that we could not determine whether they were labelled also with horseradish peroxidase. However, the large choline acetyltransferase-immunoreactive axons of nucleus reticularis mesencephali neurons stained intensely enough for us to follow them rostrally; the axons are clustered together until the level of the rostral tectum where two groupings form: one travels into the tectum and the other travels rostroventrally to cross the midline and enter the contralateral diencephalic preoptic area. We conclude therefore that cholinergic neurons project to the optic tectum from the nucleus isthmi as well as nucleus reticularis mesencephali in goldfish.     

Retinal projection to a rotated tectal reimplant following long-term tectal denervation in adult goldfish

Following optic nerve crush, the precise termination sites of regenerating goldfish optic axons may be influenced by the presence or abscence of degenerating axonal debris from the previous projection. We investigated whether tectal polarity reversal can be induced in the absence of axonal debris The right optic tectum was denervated by contralateral eye removal. One year later, when no debris was present, a piece of the right tectum was rotated and innervation by the right eye was induced by removal of the left tectum. The new ipsilateral projection to the rotated region was correspondingly rotated. It is concluded that retention of tectal polarity is not dependent upon degenerating axonal debris.     

Mode of growth of retinal axons within the tectum of Xenopus tadpoles, and implications in the ordered neuronal connection between the retina and the tectum

Retinal axons of Xenopus tadpoles at various stages of larval development were filled with horseradish peroxidase (HRP), and their trajectories and the patterns of branching within the tectum were analyzed in wholemount preparations. To clarify temporal and spatial modes of growth of retinal axons during larval development, special attention was directed to labeling a restricted regional population of retinal axons with HRP, following reported procedures (H. Fujisawa, K. Watanabe, N. Tani, and Y. Ibata, Brain Res. 206:9-20, 1981; 206:21-26, 1981; H. Fujisawa, Dev. Growth Differ 26:545-553, 1984). In developing tadpoles, individual retinal axons arrived at the tectum, without clear sprouting. Axonal sprouting first began when growing tips of each retinal axon had arrived at the vicinity of its site of normal innervation within the tectum. Thus, the terminals of the newly added retinal axons were retinotopically aligned within the tectum. The retinotopic alignment of the terminals may be due to an active choice of topographically appropriate tectal regions by growth cones of individual retinal axons. The stereotyped alignment of the newly added retinal axons was followed by widespread axonal branching and preferential selection of those branches. Each retinal axon was sequentially bifurcated within the tectum, and old branches that had inevitably been left at ectopic parts of the tectum (owing to tectal growth) were retracted or degenerated in the following larval development. The above mode of axonal growth provides an adequate explanation of cellular mechanisms of terminal shifting of retinal axons within the tectum during development of retinotectal projection. Selection of appropriate branches may also lead to a reduction in the size of terminal arborization of retinal axons, resulting in a refinement in targeting.     

Topographic projections between the nucleus isthmi and the optic tectum in a teleost,Navodon Modestus

Following horseradish peroxidase injections into the optic tectum of a teleost,Navodon modestus, reciprocal and topographic projections between the nucleus isthmi and the ipsilateral optic tectum were determined. The isthmo-tectal fibers diverge to the optic tectum while maintaining the spatial arrangements of the isthmic cells from which the fibers originate. The tecto-isthmic projections also keep the spatial arrangements in the optic tectum. The tectal fibers converge near the nucleus isthmi and terminate in the non-cellular portion of the nucleus. The reciprocal topography is apparent in the combined results of 9 experiments with one tectal injection in each region. No labeled cells and fibers were found in the contralateral nucleus isthmi.     

Some efferent connections of the superficial pretectum in the goldfish

The diencephalic nuclei known as the lateral geniculate and rotundus in goldfish appear to belong to the pretectal region. They do not project to the telencephalon. A combination of degeneration and horseradish tract-tracing experiments show that that the latter nucleus projects caudally to the so-called mammillary body and to a precerebellar cell group-nucleus lateralis valvulae.     

Evidence for centripetally shifting terminals on the tectum of postmetamorphic Rana pipiens

In larval frogs the retina and tectum grow in topologically dissimilar patterns: new cells are added as peripheral annuli in the retina and as caudal crescents in the tectum. Retinotopy is maintained by the continual caudalward shifting of the terminals of the optic axons. After metamorphosis the pattern of growth changes. The retina continues to add new ganglion cells peripherally, but there is no neurogenesis in the tectum. To maintain retinotopy in postmetamorphic frogs, the terminals of the optic axons must continually shift toward the central tectum. We tested the proposal of centripetally shifting axons by making punctate injections of horseradish peroxidase (HRP) in the tectum of adult Rana pipiens and observing the patterns of filled cells in the contralateral retina, as was done in the goldfish (Easter and Stuermer, '84). Punctate applications of HRP in the tectum should be taken up: 1) by fascicles, and label a partial anulus of cells, 2) by terminals, and label a cluster of cells in the corresponding retinotopic site, and 3) by the extrafascicular axonal segments, and label a band of cells connecting the partial annulus to the cluster. If the terminals have shifted centripetally, the band of cells labeled through their extrafascicular segments should have a spoke-like orientation, with the center of the retina as the hub. As the tectal site moves from rostral to caudal, this band of cells should move, pendulum-like, from temporal to nasal retina. In general, the patterns of HRP-filled retinal cells we observed were consistent with our predictions. In addition, HRP taken up by the oldest (rostral) tectal axons produced more complex patterns of filled cells that indicated that these axons had shifted both caudally before metamorphosis and centripetally after.     

Retinotopic analysis of fiber pathways in the regenerating retinotectal system of the adult newt cynops pyrrhogaster

Retinotopic analysis of the pathways of regenerating retinal fibers within the optic tract and in the tectum of an adult newt was performed by selective labeling of the retinal fibers with horseradish peroxidase. At the tenth week of regeneration, all the regenerating retinal fibers from different retinal quadrants had terminal arbors nearly at the parts of the tectum innervated normally by those quadrants. The pathways for individual retinal fibers, however, were greatly disorganized within the optic tract and did not show any retinotopic ordered geography. The most rostral segregation of pathways of regenerating fibers was observed at the diencephalo-tectal junction. THe temporal retinal fibers invaded the tectum directly, while the dorsal, ventral and nasal retinal fibers generally shifted toward the dorsomedial or the lateral direction, as if they traced the dorsomedial or the lateral tracts formed in normal newt. The direction of the shifting of fiber pathways, however, did not depend on the origins of retinal fibers within retinal circumference, but depended on the location of fibers with in the optic tract. As a result, a large number of regenerating fibers reached their normal sites of innervation within the tectum via anomalous routes. These mis-routed fibers did not form branches or terminal arbors at ectopic parts within the tectum.     

Retinotopic analysis of fiber pathways in amphibians. I. The adult newt Cynops pyrrhogaster

The possibility that pathways of retinal fibers within the optic tract and the tectum of the adult newt are retinotopic was examined by selective labeling of the retinal fibers with horseradish peroxidase.Within the optic tract fibers from the ventral, temporal and dorsal retinal quadrants were ordered from the dorsal to ventral edges of the optic tract. The nasal retinal fibers exhibited two different pathways. The fibers from the dorsonasal retina ran along the ventral edge of the optic tract, while the fibers from the ventronasal retina ran along the dorsal edge of the optic tract. Segregation of pathways within the optic tract was incomplete between the nasal and other retinal fibers. The dorsonasal retinal fibers were mixed completely with the dorsal retinal fibers, and the ventronasal retinal fibers were mixed partly with the ventral retinal fibers.Both the dorsal and dorsonasal retinal fibers preferentially entered the lateral tract, and finally projected onto the ventrolateral parts of the middle tectum and of the caudal tectum, respectively. The ventral and ventronasal retinal fibers entered the dorsomedial tract, and projected onto the dorsomedial parts of the middle tectum and of the caudal tectum, respectively. The temporal retinal fibers invaded the nasal tectum directly.Most dorsal, ventral, and nasal retinal fibers ran along the sub-tracts as far as to the level of their terminals, then sharply turned in a direction toward the tectum.     

Monocular visual discrimination in the goldfish after unilateral ablation of the optic tectum

Goldfish with unilateral ablation of the optic tectum and trained to the ablated side failed to learn or to transfer interocularly a differential classically conditioned color discrimination. The direct ipsilateral retinal projection to the intact/naive side is therefore shown to contribute little to engram formation. Evidence is also presented demonstrating that the absence of one tectum does not affect the learning of a differential bar discrimination task when trained via the intact brain side.     

Staining of regenerated optic arbors in goldfish tectum: progressive changes in immature arbors and a comparison of mature regenerated arbors with normal arbors

Individual optic arbors, normal and regenerated, were stained via anterograde transport of HRP and viewed in tectal whole mounts. Camera lucida drawings were made of 119 normal optic arbors and of 242 regenerated arbors from fish 2 weeks to 14 months postcrush. These arbors were analyzed for axonal trajectory, spatial extent in the horizontal plane, degree of branching, number of branch endings, average depth, and degree of stratification. Normal optic arbors ranged in size from roughly 100 to 400 microns across in a continuous distribution, had an average of 20 branch endings with average of fifth-order branching, and were highly stratified into one of three planes within the major optic lamina (SO-SFGS). Small arbors arising from fine-caliber axons terminated in the most superficial plane of SO-SFGS; large arbors from coarse axons terminated in the superficial and middle planes; and medium arbors from medium-caliber axons terminated in the middle and deep planes of SO-SFGS, as well as deeper in the central gray and deep white layers. Arbors from central tectum tended to be much more tightly stratified than those in the periphery. No other differences between central and peripheral arbors were noted. Mature regenerated arbors (five months or more postcrush) were normal in their number of branch endings, order of branching, and depth of termination. Their branches covered a wider area of tectum, partially because of their early branching and abnormal trajectories of branches. Axonal trajectories were often abnormal with U-turns and tortuos paths. Fine-, medium-, and coarse-caliber axons were again present and gave rise to small, medium, and large arbors at roughly the same depths as in the normals. There was frequently a lack of stratification in the medium and large arbors, which spanned much greater depths than normal. Overall, however, regenerates reestablished nearly normal morphology except for axonal trajectory and stratification. Early in regeneration, the arbors went through a series of changes. At 2 weeks postcrush, regenerated axons had grown branches over a wider-than-normal extent of tectum, though they were sparsely branched and often tipped with growth cones. At 3 weeks, the branches were more numerous and covered a still wider extent (average of five times normal), many covering more than half the tectal length or width. At 4-5 weeks smaller arbors predominated, although a few enlarged arbors were present for up to 8 weeks. Additional small changes occurred beyond 8 weeks as the arbors became progressively more normal in appearance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Visual system of the channel catfish (Ictalurus punctatus): III. Fiber order in the optic nerve and optic tract

In channel catfish the ganglion cell axons leave the retina via a ring of approximately 13 separate optic papillae. Each papilla serves an area of retina extending from the central zone of the retina to the periphery. Papillae located at a dorsal position in the ring serve exclusively dorsal retina. Ventrally located papillae, however, have an exaggerated peripheral retinal representation, so that they serve mostly ventral retina but also some areas of peripheral retina dorsal to the nasal and temporal poles. The ganglion cell axon bundles departing from the retina via individual papillae were labelled with horseradish peroxidase, and sections of the optic pathway were examined to reveal the topographic organization of the fibers. The topographic order of the optic nerve was dissimilar to that of cichlids and goldfish. Fibers from individual papillae remained together throughout the optic nerve. Close to the optic nerve head, the papillae were arranged as a continuum around the U-shaped optic nerve, without the discontinuity in the representation of the ventral retina seen in other fish. Fibers associated with the dorsal papillae were located at the tip of the caudolateral arm of the U, and fibers from ventral papillae were on the rostromedial arm. Fibers from nasally and temporally located papillae were found on the base of the U. By the level of the optic chiasm the U shape had flattened out but retained the relative ordering of the papillae. Rotation of the nerve as it became the optic tract brought the representation of the ventral papillae to the dorsal pole of the tract, and the dorsal papillae to the ventral tract. It was only in the optic tract that rearrangement of fibers became apparent. As described above, the axons of some ganglion cells in dorsal, peripheral retina left the retina and travelled through the optic nerve with axons from extreme ventral retina. In the optic tract, these dorsal fibers joined the main body of fibers from the dorsal retina. The significance of these observations for theories of fiber rearrangement is discussed.     

Regenerating retinal axons of goldfish respond to a repellent guiding component on caudal tectal membranes of adult fish and embryonic chick.

On a substrate of rostral/caudal tectal membrane stripes of adult fish, regenerating temporal retinal axons avoid the caudal membranes. Thus they behave like embryonic chick axons on chick E9 membranes. The caudal membranes of adult fish contain a repellent component that, as has previously been shown in the chick, is inactivated by the enzyme PI-PLC. Fish axons respond not only to their own but also to the repellent component of embryonic chick membranes. Fish and more so chick E9 caudal membranes have an outgrowth reducing effect on fish axons that is also abolished by PI-PLC treatment and is weaker on chick E16 membranes. Thus adult fish tecta express a guiding component for retinal axons related to that in the embryonic chick.     

Tectal projection neurons to the retinopetal nucleus in the filefish

Following horseradish peroxidase (HRP) injections into the preoptic retinopetal nucleus (PRN), neurons in the ipsilateral optic tectum were labeled retrogradely. Labeled neurons exhibited a 'Golgi-like' appearance, somata of these neurons were pyriform or round, and most of them were located in the stratum album centrale (SAC) or the stratum periventriculare (SPV). These neurons had a long apical dendrite, which ramified in the upper-half of SGC into horizontally arborized dendritic fields. The main trunk of the apical dendrites also gave off several branches in the stratum fibrosum et griseum superficiale (SFGS) and reached the stratum opticum (SO). These neurons resemble the 'large pyriform neurons' of Vanegas et al. (Vanegas, H., Laufer, M. and Amat, J., The optic tectum of a perciform teleost. I. General configuration and cytoarchitecture, J. Comp. Neurol., 154 (1974) 43-60) except that in the tecto-PRN neurons the axons originates from the apical dendritic shaft at or near the level of the SAC. Judging from their dendritic patterns, the tectal neurons projecting indirectly to the retina may receive non-retinal inputs besides the retinal input.     

Visual system of the channel catfish (Ictalurus punctatus): II. The morphology associated with the multiple optic papillae and retinal ganglion cell distribution

The retinal organization associated with the multiple optic papillae of the catfish Ictalurus punctatus was examined. In each retina from ten to 17 papillae form an oval ring (which is wider dorsoventrally than mediolaterally). The dorsalmost papilla in this ring lies at the center of the retina. In addition, up to seven small papillae lie within the ring. Bundles of fibers leave the neural retina via the papillae. These bundles remain separate as they pass through the distal portions of the neural retina and then merge before passing through the choroid. Bundles running through dorsal papillae receive fibers from a roughly wedge-shaped retinal area; bundles running through ventral papillae receive fibers from a small area of central retina and a disproportionately large area of peripheral retina. A band of high ganglion cell density was observed extending between the nasal and temporal poles of the retina. No correlation was found between the retinal areas contributing fibers to the bundles of axons emerging from individual papillae and the areas of high cell density. Furthermore, no correlation was found between the average area of the retinal ganglion cells and the ganglion cell density. From HRP preparations and axon counts we estimate that each retina of 95-mm catfish contains about 50,000 ganglion cells.     

Corticotectal terminals in the superior colliculus of the rabbit: a light- and electron microscopic analysis using horseradish peroxidase (HRP)-tetramethylbenzidine (TMB)

The projection from the striate cortex to the superior colliculus was studied light- and electron microscopically by means of anterogradely transported horseradish peroxidase and tetramethylbenzidine histochemistry. Labeled boutons were found in the stratum zonale (SZ) and in the stratum griseum superficiale (SGS), not in stratum opticum (SO). There are two maxima of frequency of labeled boutons, one in middle SGS at about 500 microns depth, and a smaller one in upper SGS at about 200 microns depth. Such a bimodal distribution of corticotectal terminals has not been described in any species before. Labeled myelinated axons were found in SGS and SO with a maximal frequency in middle SGS at about 400 microns depth. The myelinated axons in SZ, which are commonly considered to be of cortical origin, were not labeled. The labeled cortical terminals contained numerous round synaptic vesicles and predominantly dark mitochondria. They formed usually asymmetrical synapses and contacted dendrites, some of which contained synaptic vesicles. Occasionally, labeled boutons were observed which definitely did not belong to the type that is generally considered to be of cortical origin.     

The connections between the olfactory bulb and the brain in the goldfish

We have investigated olfactory bulb connections in the goldfish by using both horseradish peroxidase applied to olfactory tract lesions and wheat germ agglutinin-conjugated peroxidase administered to the olfactory bulb. The projections from the bulb pass to the brain in two major bundles: the lateral and the medial olfactory tracts. The lateral tract innervates the posterior terminal field in the area dorsalis. The medial tract divides into two rami. The dorsolateral ramus is the most substantial olfactory bundle in the brain and innervates several targets in both the area dorsalis and the area ventralis. The ventromedial ramus appears to innervate targets in the area ventralis exclusively. In addition, fibers of the medial olfactory tract (both rami) innervate the preoptic nucleus as well as targets in the diencephalon and, possibly, in the mesencephalon as well. Olfactory fibers from the dorsolateral and ventro-medial rami cross the midline in the dorsal and ventral olfactory decussations, respectively. Between these two decussations is a dense olfactory plexus which has not previously been reported, and which may serve as a nexus allowing interchange of fibers between the two olfactory rami. The terminal nerve in the goldfish has two parts. The major part of the nerve projects to the ventral nucleus (Vv) and is far more extensive than has previously been reported. A much less substantial component of the terminal nerve projects to the two retinae. There are four large groups of cells in the telencephalon which project to the olfactory bulb. Of these two, the dorsal and the anterolateral groups have not been described in previous studies of the goldfish. We also report weakly labeled bulbopetal cells in the nucleus preglomerulosus and in the locus ceruleus.