Effects of neonatal and adult enucleation on the synaptic organization of the serotoninergic projection to the superficial gray layer of the hamster's superior colliculus

Previous experiments from this laboratory have demonstrated that either neonatal or adult enucleation results in a marked increase in the density of the serotoninergic (5-HT) projection to the superficial layers of the hamster's superior colliculus (SC). The present study was undertaken to determine whether this alteration in the density of the 5-HT projections to SC in hamsters that sustained eye removals was associated with any change in the synaptic organization of this pathway. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles made synaptic contacts in the stratum griseum superficiale (SGS). Of these, 65% were axoaxonic, 30% were axodendritic, and 5% were axosomatic. In contrast, 22% of 400 5-HT-positive profiles in the neonatally enucleated hamsters and 17.5% of 400 such profiles in hamsters that sustained eye removals in adulthood made synaptic contacts. In the neonatal enucleates, 66% of the contacts were axoaxonic, 33% were axodendritic, and 1% were axosomatic. In the adult enucleates, 73% of the contacts were axoaxonic, 26% were axodendritic, and 1% were axosomatic. Thus, both neonatal and adult enucleation increase the percentage of 5-HT-positive profiles that make synaptic contacts in SC, but neither manipulation significantly changes the distribution of these contacts among different types of postsynaptic targets.