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.