AUGMENTATION OF SEROTONIN IN THE DEVELOPING SUPERIOR COLLICULUS ALTERS THE NORMAL DEVELOPMENT OF THE UNCROSSED RETINOTECTAL PROJECTION

A previous study from this laboratory showed that sprouting of seroton inergic axons in the hamster's superior colliculus (SC) induced by a s ingle subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at b irth (postnatal day 0; P-0) resulted in an abnormal terminal distribut ion of the uncrossed retinotectal projection. The present study provid ed further evidence to support the role of increased 5-HT levels withi n the SC in this phenomenon. Slow-release polymer (ELVAX) chips impreg nated with serotonin (5-HT) were placed over the SC on either P-l or P -3, and retinotectal projections were assessed via anterograde transpo rt of horseradish peroxidase when animals reached P > 18. Analysis of ELVAX chips indicated that they released 5-HT in amounts of greater th an or equal to 1 pmole/hour for at least 12 days. Assessment of the SC of treated hamsters indicated significantly elevated 5-HT concentrati ons as late as P-12, but not on P-16. Implantation of 5-HT chips, but not control chips, resulted in abnormalities in the uncrossed retinote ctal projection similar to those observed in the 5,7-DHT-treated anima ls. The patches that normally develop in the rostral part of the strat um opticum were not present, and uncrossed axons were distributed dens ely in this layer and in the lower portion of the stratum griseum supe rficiale throughout the rostrocaudal and mediolateral extents of the S C. Quantitative analysis of these changes indicated significant differ ences between the organization of the uncrossed retinotectal projectio ns of 5-HT-treated animals vs. either blank-implant treated Or complet ely untreated animals but not between 5-HT-treated hamsters and animal s that received neonatal 5,7-DHT injections. All of these results supp ort the conclusion that increased SC concentrations of 5-HT altered re tinotectal development. J. Comp. Neurol. 393:84-92, 1998. (C) 1998 Wil ey-Liss, Inc.