Adult neurogenesis with 5-HT expression in lesioned goldfish spinal cord

In contrast to mammals, spontaneous nerve regeneration occurs in the teleost spinal cord. In the present study, we examined whether neurogenesis is involved in posttraumatic regeneration in the goldfish spinal cord. In intact fish, many spinal cells positive for both a monoclonal neuronal marker (Hu) and bromodeoxyuridine (BrdU) were observed 24 h after i.p. injection of BrdU, suggesting that constant neurogenesis occurs in the goldfish spinal cord. After hemisection of the spinal cord, the number of spinal cells positive for Hu and BrdU was significantly increased around the lesion site. The number of Hu- and BrdU-positive cells reached the maximum level 7 days after hemisection. In intact fish, spinal cells positive for both Hu and BrdU were also observed 5 weeks after BrdU injection, suggesting that newborn neurons survive for a long time. Six weeks after hemisection, the number of surviving Hu- and BrdU-positive cells at the lesion site was significantly increased as compared with that in intact fish, and some of them were also positive for 5-HT. A retrograde tract tracing study showed that the 5-HT+ neurons were close to the regenerated axons passing through the lesion site. These results suggest that adult neurogenesis occurs in the goldfish spinal cord, and that neurogenesis is activated by spinal cord lesion. The newly produced neurons survive a long time at the lesion site, and might participate in the repair of injured tissue and in the regeneration of descending long axons beyond the lesion site.