Does the early exuberant retinal projection to the superior colliculus in the neonatal rat develop synaptic connections?

The retinal projections of newborn rats are more extensive than those of the adult and retract to form the adult pattern during the first week after birth. In this study, the retinal projections from one eye of newborn hooded rats were labeled by anterograde transport of horseradish peroxidase (HRP) following intravitreal injection of the tracer, and HRP-labeled axons and terminals were then sought by electron microscopy in coronal sections of the superior colliculus cut at 3 levels: rostral, middle and caudal. Labeled presynaptic profiles were present in the superior colliculus on the side contralateral to the injection at all rostro-caudal levels and in the rostral portion of the ipsilateral superior colliculus. But in approximately 37 sections analyzed in detail from 11 animals, only one example was found of an unequivocally presynaptic profile in the middle or caudal regions of the ipsilateral superior colliculus. Thus it would appear that in the newborn hooded rat, the exuberant ipsilateral projections to the middle and caudal parts of the superior colliculus form no or very few transient synaptic connections.