ORGANIZATION OF REGENERATING SEROTONERGIC FIBERS IN THE HIPPOCAMPAL-FORMATION

To evaluate the capacity of fiber outgrowth of serotonergic and dopami nergic neurons from the dorsal raphe tissue, the following three exper iments were performed; (1) fetal mesencephalic raphe tissue was transp lanted into the ventricle near the denervated hippocampal formation of adult rats, (2) fetal mesencephalic raphe and neonatal hippocampal ti ssues were transplanted into the anterior eye chamber of adult rats, a nd (3) fetal mesencephalic raphe tissue was explanted together with th e neonatal hippocampal tissue. The extent of the fiber outgrowth was e xamined immunohistochemically using serotonin and tyrosine hydroxylase (TH) antisera. Three months after transplantation into the host brain , serotonin-immunoreactive (ir) fibers from raphe graft were densely d istributed throughout the graft and in the host hippocampal formation, and TH-ir fibers were restricted to an area near the somata of TH-ir neurons. In particular, hyperinnervation of serotonin-ir fibers was ob served in the molecular layer of the dentate gyrus. Two months after i ntraocular transplantation, mesencephalic raphe tissue contained a lar ge number of serotonin- and TH-ir neurons and fibers. The distribution pattern of outgrowing serotonin-ir fibers in the hippocampal tissue w as similar to that observed following intraventricular transplantation . Two weeks after explantation, the raphe tissue contained numerous se rotonin-ir neurons and their fibers. These fibers extended into the hi ppocampal tissue in the same manner as the intraventricular and intrao cular transplants. These results indicate that the intrinsic factors o f hippocampal tissue influence the organization of serotonergic fibers in the hippocampal formation.