The pupillary light response: functional and anatomical interaction among inputs to the pretectum from transplanted retinae and host eyes

Retinal transplantation--advantages of intact fetal sheets

Retinal transplantation aims to prevent blindness and to restore eyesight, i.e., to rescue photoreceptors or to replace damaged photoreceptors with the hope of reestablishing neural circuitry. Retinal donor tissue has been transplanted as dissociated cells or intact sheets. A promising experimental paradigm is the subretinal transplantation of sheets of fetal retina with or without its attached retinal pigment epithelium (RPE) into recipient rats with retinal degeneration. As long as healthy RPE either from the host or from the graft is present, such transplants can develop lamination resembling a normal retina. Different methods have been used to demonstrate transplant/host connectivity. In two different rat retinal degeneration models, visually evoked responses can be demonstrated in an area of the superior colliculus corresponding to the placement of the transplant in the retina. In summary, sheets of fetal retina can morphologically repair an area of a degenerated retina, and there is evidence to suggest that transplants form synaptic connections with the host and restore visual responses in blind rats.

Cell transplantation as a treatment for retinal disease

It has been shown that photoreceptor degeneration can be limited in experimental animals by transplantation of fresh RPE to the subretinal space. There is also evidence that retinal cell transplants can be used to reconstruct retinal circuitry in dystrophic animals. Here we describe and review recent developments that highlight the necessary steps that should be taken prior to embarking on clinical trials in humans.

Retinal degeneration and transplantation in the Royal College of Surgeons rat

The Royal College of Surgeons rat provides a valuable animal model for examining the ontogeny of inherited or acquired photoreceptor degeneration and for assessing various treatment paradigms. Here we describe a sequence of events in which photoreceptor loss induces secondary changes that ultimately result in a progressive loss of retinal ganglion cells. The functional consequences of photoreceptor loss are described and compared with those observed in dystrophic animals that received grafts of pigment epithelial cells at an early stage in the degenerative process. The results of this work suggest that transplantation might slow or halt the progress of photoreceptor loss in certain human retinal degenerative conditions, provided suitable safeguards have been put in place.

Parameters of transplant-mediated pupilloconstriction in rats with unilateral olivary pretectal lesions

Embryonic retina, transplanted to the midbrain of neonatal rats, innervates the host brain and mediates a pupilloconstrictor response in the host eye. We hypothesise that there is a dynamic interaction between normal host and transplant photic input to the pupilloconstrictor centres. This study aims to characterise the nature of these interactions and determine the impact of experimental lesions on this reflex system. Pupillary diameter in normal rats is regulated by integration of intensity levels of the light delivered to the two eyes. The integration occurs at the primary input nucleus, the olivary pretectal nucleus, and at the output nucleus, the Edinger-Westphal nucleus. We have examined the pattern of integration of inputs delivered through the host eye and a retinal transplant placed over the midbrain at birth. Restriction of the site of integration to one olivary pretectal nucleus by ablating the contralateral nucleus causes a substantial enhancement of the transplant-mediated response and a major reduction in the host eye-mediated response. Although the pattern of change is quite similar to that seen between the two eyes of a normal animal after a similar lesion, the magnitude of improvement of the transplant response is much greater. The integration that occurs between transplant and host inputs is dynamic, and the efficacy of the transplant can be modulated by the competing host input. These results have implications for the use of neural transplants in degenerative diseases and might also offer insight into the limited functional recovery that occurs after neurological injury.

The pupillary light response: assessment of function mediated by intracranial retinal transplants

We have adapted a pupillometry measurement system to test the functional efficacy of retinae previously transplanted over the midbrain of neonatal rats in mediating a pupillary light reflex in the host eye. This has permitted us to examine factors influencing various parameters of the response, and to study transplant-mediated responses in comparison with responses mediated by way of the normal consensual pathway. Despite the unusual location of these transplanted retinae and the absence of supportive tissues normally associated with retinae in situ, it is clear that pupilloconstriction in the host eye can be elicited by transplant illumination. Under the optimal conditions here defined, response parameters for individual animals were stable with repeated testing over extended periods. When considered as individual cases, response amplitude, constriction rate and response latency were intensity dependent, although responses elicited by transplant illumination were less sensitive than normal, typically by 2-3 log units. Large-amplitude transplant-mediated pupillary responses could, however, be elicited repeatedly throughout long trains of stimuli, unlike normal responses, which rapidly failed to recover to baseline under similar test conditions. Finally, even though some cellular elements of the visual cycle are absent in transplanted retinae, pupilloconstriction in the host eye could be elicited repeatedly by transplant illumination as long as two years after transplantation took place. These observations indicate the applicability of this preparation as an assay for the effects of experimental manipulations on information processing and response plasticity in the visual system, and as a tool for examining, in general, the necessary conditions for optimal function of grafts that work by synthesizing and relaying neural signals.