Transplanted raphe and hippocampal fetal neurons do not displace afferent inputs to the dorsal hippocampus from serotonergic neurons in the median raphe nucleus of the rat.

To determine if fetal transplants can substitute for or suppress intrinsic serotonergic (5-HT) innervation, we studied the relationship between transplanted and the endogenous raphe neurons projecting to the hippocampus. Fetal raphe transplants produced a 5-HT hyperinnervation of dorsal hippocampus in adult rats. Yet, transplants of fetal raphe tissue did not affect the number of median raphe nucleus (MRN) neurons, approximately 300, which retrogradely transported HRP from the hippocampus. This provides evidence that transplanted 5-HT neurons can co-exist with intrinsic 5-HT nerve terminals in the target area for at least one month. In the second part of this study, fetal hippocampal tissue was transplanted into the host hippocampus. Intrinsic 5-HT immunoreactive fibers innervated the transplanted fetal tissue. Nevertheless, the number of MRN neurons innervating the host tissue as revealed by HRP retrograde transport remained unchanged. Changes in the innervation pattern and 5-HT level in the dorsal hippocampus occur following transplantation of fetal tissue. These changes are discussed and suggest that both the target tissue and afferent neurons readjust to accommodate extrinsic transplanted tissue.