The ipsilateral retinocollicular projection in the rabbit: an autoradiographic study of postnatal development and effects of unilateral enucleation

Connection from the dorsal column nuclei to the superior colliculus in the rat: Topographical organization and somatotopic specific plasticity in response to neonatal enucleation

Somatosensory stimuli from the body to deep and intermediate strata of the superior colliculus (SC) are relayed from the dorsal column nuclei (DCN), gracile (GrN) and cuneate (CuN). Electrophysiological studies have shown that the somatosensory representation in SC is arranged into a map-like pattern. However, there is a lack of studies confirming a morphological correlate of such an organization. On the other hand, after neonatal enucleation in rodents, somatosensory inputs ascend from their normal termination territory in intermediate and deep collicular strata to invade the more dorsally located visual strata. However, the origin of these reactive afferents has not been specified. By using anterograde (biotinylated dextran amine 10,000; BDA) and retrograde (Fluoro-Gold; FG) tracers, we studied separately the connection from GrN and CuN to the intact and neonatally deafferented SC. GrN-collicular afferents were found to terminate mainly within the periphery of the caudomedial SC quadrant, whereas CuN-collicular fibers innervated primarily the lateral part of the rostrolateral and caudolateral collicular quadrants, in a way consistent with previously described functional data. Retrograde tracing experiments using FG injected in SC confirmed this topographical arrangement. Injections of BDA in GrN or CuN of neonatally enucleated rats showed that reactive fibers reaching superficial strata are only those CuN-collicular fibers innervating the caudolateral SC quadrant, where the forelimb is represented. The present results provide an anatomical substrate for the known somatotopic organization of tactile representation in SC and further reinforce the previous proposal that the plastic reorganization of DCN-collicular afferents following neonatal enucleation constitutes a functional compensatory response.

Plastic reaction of the rat visual corticocollicular connection after contralateral retinal deafferentiation at the neonatal or adult stage: axonal growth versus reactive synaptogenesis

The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.

A comparison of contrast letter thresholds in unilateral eye enucleated subjects and binocular and monocular control subjects

We previously reported that unilaterally eye enucleated subjects show superior contrast letter acuity to normally sighted monocular viewing control subjects. We suggested that reorganization of the visual system in the enucleated subjects may compensate for their loss of binocularity. Here we measured contrast letter acuity in normally sighted binocular control subjects and compared these results to previously published results of eye enucleated subjects and monocular viewing control subjects. We found equivalent performance between enucleated subjects and binocular control subjects, suggesting that performance of enucleated subjects might be due to some form of neural summation.

Contrast letter thresholds in the non-affected eye of strabismic and unilateral eye enucleated subjects

To investigate the effects of visual disruption on contrast letter thresholds of the non-affected eye, subjects with one eye enucleated, strabismic subjects using the non-deviating eye and normal control subjects were asked to identify letters on eye charts and single letter cards which varied in contrast (between 4 and 96%) and size. At all contrast, contrast letter acuity of eye enucleated subjects was superior to both normal control subjects and strabismic subjects. Early onset strabismic subjects (onset < 24 months) showed inferior performance to normal control subjects at all contrasts of 25% and above. Late onset strabismic subjects showed normal performance at all contrasts, except for high contrast single letters, where performance was inferior to normal control subjects. Further, for all subjects groups, performance on letter charts was similar to performance on single letter cards. We conclude that disruption to the visual system caused by eye enucleation or strabismus is not equivalent. These differences may be due to intrinsic differences between the visual systems of eye enucleated subjects and strabismic subjects and/or to the profound differences in deprivation caused by the two conditions.

The development of optokinetic nystagmus in strabismic and monocularly enucleated subjects

Assymmetries of monocular optokinetic nystagmus (OKN) following anomalous visual experience are thought to be due to disruption at the cortical level. Visual disruption usually results from eye suture (in animals), unilateral dense and central cataracts or strabismus (in humans). Many form-deprived animals and humans frequently show a residual strabismus after lid opening (animals) or cataract extraction and optical correction (humans). We wanted to determine whether strabismus was unique in causing monocular asymmetries of OKN. Two independent observers rated eye movement videotapes of 20 normal subjects, the non-deviating eye of 25 unilateral strabismic subjects and 29 unilaterally eye-enucleated subjects, who were watching either a nasally directed square wave grating, a temporally directed square wave grating, or a blank field. Observers rated the proportion of trials where OKN occurred, the duration of OKN in each trial and the number of beats of OKN within each trial. Monocular OKN was symmetrical in normal subjects for the proportion and duration measures, but half the normal group showed small but significant asymmetries for the beats measure. Subjects in both enucleate and strabismic groups showed asymmetries of OKN favouring nasally directed stimulation, but only the early onset strabismics (as a group) showed asymmetries that were significantly greater (P less than 0.05) than the normal group. Asymmetry scores correlated significantly with age of diagnosis of strabismus for the strabismic group but not with age of enucleation for the enucleate group. The results are discussed in terms of binocular competition.

Evidence that the relative densities of afferents from both eyes control laminar distribution and binocular segregation of retinotectal projections in rats

In the superior colliculus of normal rodents the crossed retinal projection overlaps the uncrossed projection. The present study describes an abnormal laminar distribution and binocular segregation of the retinotectal afferents induced after the experimental enlargement of the uncrossed retinotectal pathway in pigmented rats. Intraocular injections of anterograde tracers were used to investigate the topographic and laminar organization of retinotectal projections in adult rats given unilateral optic tract lesions at birth. These lesions are known to increase the number of ipsilaterally projecting ganglion cells in the opposite retina. The uncrossed retinal projection to the remaining superior colliculus forms an abnormal band of terminal labeling at the superficial half of the stratum griseum superficiale, markedly different from the laminar distribution of this pathway in unoperated controls. This abnormal uncrossed projection has its maximum density at the rostrolateral quadrant of the tectum. Within this region, the crossed retinotectal projection retracts from the surface of the superior colliculus, leading to partial binocular segregation. The results suggest that both the laminar distribution and the experimental binocular segregation of retinotectal afferents depend on the balance of the densities of the converging pathways from both eyes in the superior colliculus.

Developmental changes in the pattern of retinal projections in pigmented and albino rabbits

The distribution of retinal axons and/or terminals in the retino-recipient nuclei of pigmented and albino rabbits varying in age from the 24th postconceptional day (24PCD) to adulthood was examined following unilateral intraocular injections of the enzyme horseradish peroxidase. Both in pigmented and albino rabbits contralateral retinal axons and/or terminals in the dorsal and ventral lateral geniculate nuclei (DLG and VLG), superior colliculi (SC), pretecta (PT) and accessory optic tract nuclei (AON) were already present on 24PCD. In the period 26-30PCD the contralateral projection occupied the entire volume of the DLG, VLG and SC. Although 32PCD (the day of birth) the proportions of the volumes of DLG and VLG occupied by the contralateral projections were slightly reduced, their volume continued to increase in absolute terms up to adulthood. In pigmented rabbits the ipsilateral projections to all retino-recipient nuclei were most dense and extensive on 26PCD. From 26PCD, the relative extent of the ipsilateral projections was gradually reduced, but a reduction in their absolute extent did not become evident until 32PCD. By 32PCD the ipsilateral projection to the AON had disappeared completely. The distribution of ipsilateral axons and/or terminals and the relative proportion of the nuclei occupied by the ipsilateral projection in all other retino-recipient nuclei had become adult-like by 34PCD. In albino rabbits only a sparse ipsilateral projection to the presumptive superficial collicular layers was present on 24PCD. In the remaining retino-recipient nuclei an ipsilateral projection was present on 26PCD. From 26PCD the relative extent and from 30PCD the absolute extent of ipsilateral retinal axons and/or terminals was gradually reduced. The relative extent of the ipsilateral projection had become almost adult-like by 34PCD. Throughout development ipsilateral projections in albinos were consistently less dense and less extensive than those in pigmented rabbits, and unlike in pigmented rabbits, the ipsilateral projections to the VLG and PT were only transient. The differences between the two strains in the pattern of retinofugal projections were further enhanced during the period of segregation of the ipsilateral and contralateral projections. Considering the fact that in both strains there is a partial correspondence between the period in which the spatial extent of the ipsilateral projections is reduced and the period of retinal ganglion cell (RGC) death, it is likely that RGC death plays a role in the process of segregation of the retinal afferents into ocular domains. However, our data suggest that other mechanism(s) also play an important role in the process.

Prenatal development of retinocollicular projections in the rabbit: an HRP study

The prenatal development of the rabbit's retinal projections to the superior colliculus (SC) was studied by using anterograde transport of horseradish peroxidase injected intraocularly. Fetuses aged embryonic day 21 (E21) to E29 and an adult rabbit were examined. Gestation in the rabbit is 30-31 days. On E21 contralaterally projecting retinal fibers invade across the entire SC. Their distribution is initially diffuse within the superficial laminae, but by E29 they have a distinct stratified appearance. Ipsilaterally projecting retinal fibers invade the rostral half of the SC on E21. By E23 they cover the entire SC and overlap the contralateral fibers both tangentially and radially. The ipsilateral fibers for the most part are sparsely distributed, but they form a dense focal distribution in the rostrolateral quarter of the SC. This focus straddles the stratum griseum superficiale/stratum opticum (SGS/SO) border. On E25 the ipsilateral fibers maintain their widespread distribution and focal rostrolateral concentration. By E27 they are excluded almost entirely from the caudal half of the SC and are reduced in density in the rostromedial quarter of the nucleus. On E29 the ipsilateral terminal field forms distinct patches and bands that are restricted to the rostrolateral quarter of the SC and are confined to the SGS/SO border. Thus, a few days before birth the pattern and location of the ipsilateral retinocollicular projection resemble those seen in the adult. The early widespread distribution of the ipsilaterally projecting retinal fibers to the SC and their eventual restriction in the fetal rabbit are consistent with the development of this projection in other mammalian orders.

Effects of very early monocular and binocular enucleation on primary visual centers in the tammar wallaby (Macropus eugenii)

The role of retinal afferents and their binocular interactions in the development of mammalian primary visual centers has been studied in the marsupial wallaby. Monocular and binocular enucleation was performed prior to any retinal innervation of the visual centers. After monocular enucleation retinal projections were traced by horseradish peroxidase histochemistry and compared with those in normal animals and those during development. The topography of retinal projections to the superior colliculus and the dorsal lateral geniculate nucleus after monocular enucleation was determined by making retinal lesions and tracing the remaining projections with horseradish peroxidase. The position and nature of the filling defects in terminal label were compared with controls with similarly placed lesions. The superior colliculus and dorsal lateral geniculate nucleus ipsilateral to the remaining eye were shrunken. Projections to the ipsilateral superior colliculus, ipsilateral accessory optic nuclei, and ipsilateral suprachiasmatic nucleus, although enlarged, never approached the density contralaterally, as was also the case during normal development. The expanded projection in the ipsilateral superior colliculus came primarily from temporal and ventral retina. In the dorsal lateral geniculate nucleus, terminal bands and cellular laminae, although not identical to normal, did develop. During normal development overlap of afferents from the two eyes occurs in the binocular region. The decrease in volume of the nucleus ipsilateral to the remaining eye after monocular enucleation suggests that the monocular region disappears in the absence of appropriate input and the binocular region survives. Contralaterally there was no decrease in volume, compatible with this idea. The topography of retinal projections supports this interpretation. It was normal contralaterally while ipsilaterally it was appropriate for the normal binocular region. There was an expansion of the projection along the lines of projection in what would normally be binocular regions of the nucleus, where retinal afferents failed to segregate in the absence of binocular competition. After binocular enucleation the alpha and beta segments of the dorsal lateral geniculate nucleus were still recognizable but cell-sparse zones were absent, as was the characteristic orientation of primary dendrites of geniculocortical cells. There are rigid developmental constraints operating on the innervation of territory by retinal afferents from the two eyes, and many features of the mature pattern arise without binocular interactions during development.

The origin and terminal arbors of individual uncrossed retinogeniculate fibers in rabbits

The distribution and morphology of individual uncrossed retinogeniculate fibers in both normal and monocularly enucleated adult Dutch-belted rabbits were studied using horseradish peroxidase and wheat germ agglutinin conjugated to horseradish peroxidase as neuronal markers. The results showed that the uncrossed retinogeniculate fibers were distributed almost entirely in the ipsilateral segment of the dorsal nucleus of the lateral geniculate body, and the extent of terminal distribution of the fibers observed in rabbits with one eye enucleated during the young adult stage was essentially the same as that in the normal rabbit. Most of the uncrossed retinogeniculate fibers appeared to arise as collateral branches of optic tract fibers which were apparently destined for the pretectum or the superior colliculus. The uncrossed retinogeniculate fibers labeled by the wheat germ agglutinin conjugated to horseradish peroxidase passed through the dorsal nucleus of the lateral geniculate body without any branching until they reached the ipsilateral segment. There they could be divided into several morphological types although the possibility that they may represent different classes of a continuum cannot be ruled out.

Retinofugal projections in hedgehog-tenrecs (Echinops telfairi and Setifer setosus)

Using the autoradiographic tracing technique the retinal projections were studied in the tenrecs, Echinops telfairi and Setifer setosus (insectivora, tenrecidae). Bilateral projections were found to the n. suprachiasmaticus, the anterior hypothalamic area, the dorsal and ventral lateral geniculate bodies, the pretectal olivary nucleus and the superior colliculus. The contralateral projections were usually more intense than the ipsilateral ones except the retinohypothalamic connections. A partial segregation of the projection fields from both eyes was present in the dorsal and ventral lateral geniculate bodies. In the superior colliculus retinal fibers predominantly involved the stratum zonale and the upper portion of the stratum griseum superficiale on both sides. The projections to the deeper portion of the colliculi were rather faint, particularly on the ipsilateral side. Target areas receiving contralateral projections exclusively were the periamygdaloid area (labeled only in Setifer), the terminal accessory nuclei including the n. tractus optici and the inferior colliculus. The data are compared with other species. The most striking finding may concern the projection to the medial terminal nucleus being quite prominent in marsupials and most eutherian mammals (including the erinaceomorphous hedgehogs), but greatly reduced in tenrecs and primates.