Neonatally elevated serotonin levels alter terminal arbors of individual retinal ganglion cells in superior colliculus of hamsters.

Previous studies from this laboratory showed that sprouting of serotoninergic (5-HT) axons in the hamster's superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth (postnatal day 0 [P-0]), resulted in an increased terminal distribution of the uncrossed retinocollicular projection that was not associated with any changes in the number or distribution of ipsilaterally projecting retinal ganglion cells. The present study was undertaken to determine what effect this manipulation had on the terminal arbors of such axons. Retinocollicular axons of normal and 5,7-DHT-treated animals were anterogradely labeled with small intraretinal injections of the lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) on P-16. After tissue processing on P-19, single retinocollicular axon arbors were reconstructed by using confocal microscopy. Quantitative analysis indicated that arbors from 5,7-DHT-treated hamsters had significantly greater total fiber lengths, areas, and volumes than those from normal animals. There were no differences between axons from the two groups in number of branch points, distribution of relative branch lengths, and numbers of bouton-like swellings. These results support the hypothesis that increased SC concentrations of 5-HT alter development of the uncrossed retinocollicular pathway such that a greater territory is covered by individual terminal arbors but that the number of synaptic contacts per arbor remains constant. This may explain, at least in part, the abnormally widespread distribution of the aggregate ipsilateral projection.