EFFECTS OF NEONATAL AND ADULT ENUCLEATION ON THE SYNAPTIC ORGANIZATION OF THE SEROTONINERGIC PROJECTION TO THE SUPERFICIAL GRAY LAYER OF THE HAMSTERS SUPERIOR COLLICULUS

Previous experiments from this laboratory have demonstrated that eithe r neonatal or adult enucleation results in a marked increase in the de nsity of the serotoninergic (5-HT) projection to the superficial layer s of the hamster's superior colliculus (SC). The present study was und ertaken to determine whether this alteration in the density of the 5-H T projections to SC in hamsters that sustained eye removals was associ ated with any change in the synaptic organization of this pathway. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles m ade synaptic contacts in the stratum griseum superficiale (SGS). Of th ese, 65% were axoaxonic, 30% were axodendritic, and 5% were axosomatic . In contrast, 22% of 400 5-HT-positive profiles in the neonatally enu cleated hamsters and 17.5% of 400 such profiles in hamsters that susta ined 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-po sitive profiles that make synaptic contacts in SC, but neither manipul ation significantly changes the distribution of these contacts among d ifferent types of postsynaptic targets.