The host serotonin projection to tectal grafts in young rats: an immunohistochemical study.

It has been shown previously that embryonic tectal tissue grafted to the midbrain of newborn rats is innervated by fibers from a variety of host regions, including the raphe nuclei. The present study examined the distribution within tectal transplants of axons arising from these serotonin-containing neurons in the host brain stem. Fetal tectal tissue was transplanted to the midbrain of young host rats aged 0 to 21 days. After 7 or more weeks, the host serotonin projection to the grafts was examined immunohistochemically using a monoclonal antibody to serotonin. In most cases, visualization of serotonin axons was enhanced by treating the animals with 5, 7-dihydroxytryptamine. Tectal transplants were found lying on or embedded within the host midbrain. In the newborn hosts, serotonin innervation was densest and most frequently encountered in the nonembedded grafts. Fibers with serotonin-like immunoreactivity were also seen in the tectal tissue grafted into older hosts. The pattern of innervation of serotonin fibers varied from one graft to another and did not correlate with any obvious morphologic or histochemical features within the grafts. This is in contrast to the distribution of host retinal and cortical axons, which have been shown to consistently project to specific and recognizable regions within the graft neuropil.