The normal organization of the lateral posterior nucleusin the golden hamster and its reorganization after neonatal superior colliculus lesions

We have studied the normal organization of the hamster lateral posterior nucleus and its reorganization after neonatal superior colliculus lesions. First, we divided the lateral posterior nucleus into rostrolateral, rostromedial and caudal subdivisions and determined the normal distributions of terminals contributed to each division by the ipsilateral and contralateral superior colliculi, the ipsilateral posterior neocortex and the contralateral retina. Since the rostrolateral subdivision receives most of the projections from the ipsilateral superior colliculus, our studies concentrated on this region. The rostrolateral subdivision contains synaptic clusters formed primarily by medium-sized M-terminals synapsing around a central dendrite. Electron microscopic observations showed that the majority of M-terminals are from the ipsilateral colliculus, although a few are contributed by the contralateral colliculus and retina. Therefore, after an ipsilateral neonatal colliculus lesion the synaptic clusters must develop in the absence of their major input. The next step was to examine the distributions of the remaining afferents to the rostrolateral subdivision in adult animals which had received ipsilateral neonatal colliculus lesions. In these cases, the normally restricted projection fields of the contralateral colliculus and the retina expand until they share a border in the rostrolateral subdivision. In contrast the cortical projection, which normally extends throughout the lateral posterior nucleus, is reduced in the region containing retinal terminals. At the ultrastructural level, we found morphologically normal synaptic clusters and showed the M-terminals now occupying the clusters are contributed by the remaining colliculus and the retina. The results suggested that the afferents to the lateral posterior nucleus normally compete for synaptic space and that this competition continues after a neonatal colliculus lesion. In our final experiments, we performed various combinations of neonatal lesions (bilateral superior colliculus, superior colliculus and retina, superior colliculus and cortex) and found that the remaining afferents expand their terminal fields still further in the absence of two inputs.