INCREASED SEROTONINERGIC INNERVATION OF THE HAMSTERS SUPERIOR COLLICULUS ALTERS RETINOTECTAL PROJECTIONS

Anterograde tracing with horseradish peroxidase (HRP) was used to comp are the organization of retinotectal projections in normal adult hamst ers and in animals that sustained subcutaneous injections of the neuro toxin 5,7-dihydroxytryptamine (5,7-DHT) on the day of birth. Neonatal injection of this neurotoxin decreases the density of the serotoninerg ic (5-HT) innervation of the cerebral and cerebellar cortices, but inc reases the density of these fibers in the brainstem including the supe rior colliculus (SC). Analysis of tissue from the retinorecipient lami nae of the SC by high-pressure liquid chromatography indicated that th ese lesions increased the amount of 5-HT in the adult SC by 47%. The i ncreased serotoninergic innervation of SC was associated with a marked change in the distribution of the uncrossed retinotectal projection. In normal adult hamsters, fibers from the ipsilateral eye form dense c lusters in the lowermost stratum griseum superficiale (SGS) and stratu m opticum (SO). A small number of uncrossed fibers are also visible in the more caudal portions of these layers. In the animals that sustain ed neonatal 5,7-DHT injections, uncrossed retinotectal fibers formed a nearly continuous band in rostral SO and lower SGS, and numerous labe led fibers were present in the caudal SC, primarily in the SO. Neonata l treatment with 5,7-DHT also produced alterations in the crossed reti notectal pathway and in the crossed and uncrossed retinogeniculate pro jections. These results indicate that the 5-HT input to the developing brainstem may strongly influence the development of retinofugal proje ctions. (C) 1993 Wiley-Liss, Inc.