Intraocular grafts of nucleus raphe dorsalis provide cografted spinal cord with a serotonergic innervation

Nucleus raphe dorsalis was sequentially cografted with spinal cord to the anterior chamber of the eye of adult rats. After maturation in oculo, the double grafts were examined histologically and immunohistologically utilizing an antiserum to serotonin (5HT). No 5HT-like immunoreactivity could be detected in grafts of single spinal cord or in irides of nongrafted eyes. In single raphe grafts, positive cell bodies and a dense terminal plexus were found. 5HT-positive fibers also penetrated into the irides forming dense networks in the walls of blood vessels and elsewhere in the irides. When spinal cord was cografted to maturated nucleus raphe dorsalis grafts, the raphe graft provided the spinal cord graft with a serotonergic innervation. Interestingly, when the grafting procedure was done in reversed order, i.e., the spinal cord was grafted first and left to mature before immature raphe grafts were added to the eye, the average density of 5HT-positive nerve terminals in the spinal cord part was clearly higher. It is concluded that immature as well as mature isolated spinal cord tissue can become innervated by adjacent central 5HT-neurons. Thus isolated replicas of descending serotonergic pathways to the spinal cord, suitable for electrophysiological analysis, are obtained in oculo.