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

Pupilloconstriction to light can be mediated in rats through direct illumination of retinae previously transplanted to intracranial locations. Transplant-driven and normal pupillary light responses are stable under optimal testing conditions, and parameters describing the response can be quantified precisely. The present study demonstrates the interaction between transplant-driven and normal pupillary response patterns. When stimuli are presented concurrently to a transplanted retina and to the remaining eye in host rats, a greater degree of pupilloconstriction occurs than when either the transplanted or the host eye is illuminated independently. This suggests that transplant and host retinal inputs act in concert to determine pupil diameter. The second portion of this study investigates the pattern of retinal input to the pretectum to determine if a structural basis for such functional interactions may exist. Crossed and uncrossed retinal projections to the olivary pretectal nucleus occupy non-overlapping regions of this bilaterally represented nucleus in normal rats, with a greater number of optic axons directed to the contralateral olivary pretectal nucleus. Retinae transplanted to the midbrain of neonatal rats, from whom the contralateral eye had been removed, also project to the olivary pretectal nucleus at maturity. By contrast with the normal pattern of segregated retinal inputs, however, the terminal fields of transplant axons were found to overlap extensively with the retinal projection from the remaining host eye. In addition, the relative proportion of transplant axons directed to the ipsilateral and contralateral olivary pretectal nucleus varied among animals. The lack of spatial segregation between inputs from transplant and host sources and the relative proportion of ipsilateral and contralateral transplant axons together may represent a structural basis for the observed functional interactoin of these inputs to the neural circuit subserving pupilloconstriction to light. These features may also relate to the marked improvements in transplant-mediated responses that frequently occur when optic input from the remaining host eye is eliminated. The results presented here, together with our previous transplant studies, show that this preparation can be used to provide insight into more general questions as to the dynamic interactions that occur between converging sensory inputs in the generation of integrated output responses.