Role of serotonin in the development of the retinotectal projection

Serotonin (5-HT)-containing axons and 5-HT receptors are present in the hamster superior colliculus (SC) before birth and are coincident with the development of retinal ganglion cell axon terminals in this nucleus (retinotectal projection). Neural activity in the SC is suppressed by 5-HT presynaptically at receptors located on retinotectal axons. Elevation of 5-HT levels in the SC postnatally, by subcutaneous injection of 5,7-dihydroxytryptamine or by direct release of 5-HT within the SC, prevents refinement of the retinotectal terminal field and maintains it in an immature state. Retinal ganglion cell number and density remain normal in such animals. The persistence of a poorly differentiated retinotectal projection following 5-HT elevation may be a consequence of reduced retinotectal activity.