L.S. Jen

Enlargement of uncrossed retinal projections in the albino rat: additive effects of neonatal eye removal and thalamectomy

The present study in the albino rat investigates the effects of neonatal unilateral eye removal and/or thalamectomy upon the number of ipsilaterally projecting retinal ganglion cells (IPRGCs), using retrograde fluorescent labelling and electron microscopy. The results show that neonatal unilateral eye removal and thalamectomy result in a significant increase in the number of aberrant IPRGCs as compared to that observed in normal rats, and the effects of the two different neonatal lesions appear to be additive. These findings strongly suggest that there are at least two subpopulations of IPRGCs, which normally do not exist in mature albino rats, but which can be preserved into adulthood by neonatal enucleation or thalamectomy. The data also suggest that about 70% of the entire population of IPRGCs, most of which normally exist only transiently in neonatal retinas, can be retained in rats which receive both unilateral eye removal and thalamectomy at the neonatal stage.

Displaced retinal ganglion cells in normal rats and rats with one eye enucleated at birth

The displaced retinal ganglion cells in normal rats and rats with left eye enucleated at birth were studied using the fluorescent dye Fast blue as neuronal marker. The dye was introduced to the right optic tract of the rats 7 days before perfusion. Frozen coronal sections of the brains and retinal whole-mounts from all rats were examined under ultraviolet illumination. In the normal rat, the displaced cells constituted 0.91% and 2.07% of the entire population of cells projecting to the contralateral and ipsilateral side of the brain, respectively. While the contralaterally projecting displaced cells were widely distributed in the retina, the ipsilateral ones concentrated mainly in the temporal retinal crescent in all rats studied. However, an increase in the number of the ipsilaterally projecting displaced cells was detected in unilaterally enucleated rats as compared to that observed in normal animals.

The connections between the parabigeminal nucleus and the superior colliculus in the Golden hamster

The present study in hamsters shows that the horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) injected into the eye can be transported transneuronally in both anterograde and retrograde directions to the parabigeminal nuclei (PBN). Moreover, the patterns of labelling in the PBN are similar to those observed after introduction of horseradish peroxidase (HRP) into the superior colliculus. These results suggest that the WGA-HRP is a reliable pathway tracer for studying second-order connections at least in the visual system in mammals.

Development of parvalbumin immunoreactive neurons in normal and intracranially transplanted retinas in the rat

SummaryRetinas from embryonic day 14 Sprague-Dawley rats were transplanted to the midbrain or cerebral cortex of newborn (P0) rats of which the right eye was enucleated at the time of transplantation. Parvalbumin immunoreactive (PV-I) neurons were studied in the developing retinal transplants, and in the remaining retina of the host, as well as in normal retinas. PV-I neurons were identifiable in retinas of normal and host rats from postnatal day 5 (P5) onward, with the PV-I somata primarily in the inner half of the inner nuclear layer and in the ganglion cell layer. An adult-like distribution of PV-I neurons was attained at P35, as judged by cell packing density, intensity of immunostaining, laminar distribution and soma size of subpopulations of PV-I cells. A similar time course of development and distribution of PV-I somata was observed in the retinal transplants, except for some minor differences such as a slight delay in PV-I cells achieving their final distribution. These findings provide evidence that PV-I neurons can survive, differentiate and mature according to pre-determined programmes intrinsic to the retinal tissue following transplantation to a new and foreign environment.

The cholinergic system in retinal transplants in rats.

Retinas from embryonic day 13 or 14 Sprague-Dawley albino rats were transplanted to the brainstem of newborn rats with unilateral eye enucleation at birth. Two months after the transplantation, the activity and distribution of acetylcholinesterase and choline acetyltransferase were studied using histochemical and immunocytochemical methods respectively. Results obtained showed that the staining patterns of these two cholinergic enzymes in the retinal transplants were essentially the same as those observed in the retinas of normal rats and in the control retinas of the recipient animals. The similarities in the distribution of these two cholinergic enzymes in these retinas suggest that the cholinergic system in the retinal transplants is likely to be functional.

Elimination of transient dendritic spines in ipsilaterally projecting retinal ganglion cells in rats with neonatal unilateral thalamotomy.

Using the DiI and intracellular Lucifer Yellow labeling techniques in the rat, we have demonstrated that the unilateral neonatal thalamotomy does not result in retention of transient dendritic spines of ipsilaterally projecting retinal ganglion cells (IPRGCs), although the thalamotomy is known to retain the normally transient IPRGCs (Chan et al., Dev. Brain Res., 49 (1989) 265-274). These results suggest that the process of elimination or retraction of transient dendritic spines occurs in retinal ganglion cells during development regardless of whether they make connections with appropriate or inappropriate loci in the visual targets, and/or a decrease in interactions with neighboring retinal ganglion cells.

Preservation of the entire population of normally transient ipsilaterally projecting retinal ganglion cells by neonatal lesions in the rat

SummaryThe number of ipsilaterally projecting retinal ganglion cells (IPRGCs) in developing normal rats and rats which received unilateral thalamic lesion and monocular enucleation at birth was studied using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) as a retrograde neuronal marker. The results showed that the number of IPRGCs labelled in day-21 rats which received lesions at birth was comparable to the highest number of IPRGCs observed in normal rats on day 0 (day of birth). These results suggest that the entire population of IPRGCs which had their axons already grown in or near to their target structures on day 0 can be rescued by neonatal lesions.

Cytochrome oxidase activity in the rat retina following unilateral thalamic lesion.

The present study has investigated the cytochrome oxidase (CO) activity in retinas of normal rats and rats which received central lesions at birth or young adult stage. The results show that a thalamic lesion which injured the retinal ganglion cell axons in young adult rats led to severe loss of CO activity in the ganglion, inner and outer plexiform layers in the retina contralateral to the lesion as compared to those of normal rats. In contrast, distinct CO-reactive bands and cells were clearly observed in corresponding laminae in retinas in which almost the entire population of retinal ganglion cells was eliminated by a neonatal thalamic lesion. These results indicate that retinal ganglion cells contribute significantly to the CO activity observed in the inner retinal laminae under normal but not under abnormal conditions, and suggest that considerable changes in the activity of the remaining neurons and possibly reorganization of neural circuitry within the retina in rats which received neonatal lesions has taken place, as revealed by CO histochemistry.

Cytochrome oxidase activity in retinas transplanted to the brainstem in rats

Fetal retinas were transplanted to the brainstem of newborn rats and their morphological features were examined using the cytochrome oxidase histochemical method at maturity. The results showed that the inner segments of photoreceptors, outer and inner plexiform layers as well as ganglion cells with large somata were moderately to darkly stained for cytochrome oxidase. This pattern is basically the same as that observed in the normal retina, suggesting that cytochrome oxidase can be used not only for revealing spatial but also functional organization of retinal transplants.

The cells of origin of the aberrant recrossed retinotectal projection in the rat

The cells of origin of the aberrant recrossed retinotectal projection induced by neonatal unilateral eye enucleation and collicular lesion in rats were studied using horseradish peroxidase and fluorescent dyes as pathway tracers. The results showed that labelled ganglion cells which gave rise to the recrossed retinotectal projection were distributed in almost the entire area of the remaining retina. However, more labelled cells were observed in the nasal than in the temporal half of the retina, and very few cells contributing to the recrossed projection were located in the lower temporal retinal crescent where cells of the uncrossed retinotectal fibers were heavily concentrated. In addition, a small number of recrossed retinotectal fibers were found to arise from ganglion cells which simultaneously sent out uncrossed retinotectal fibers terminating in the same remaining colliculus.