Quantitative aspects of the synaptic organization of the superior colliculus in control and dark-reared rabbits.

Fine structural and quantitative aspects of the synaptic organization of the superior colliculus of the rabbit were studied. The synaptic density, the frequency of the distinct types of terminals, the length of the contact zones, the percentage of serial contacts and the number of synaptic vesicles per terminal were estimated in control, dark-reared and dark-reared-recovered animals. The ultrastructure of the synaptic terminals and the complex interconnectivity correspond to that described for other species. The depth distribution of terminals with round synaptic vesicles agrees quite well with the distribution of cortical and retinal afferents found in lesion experiments. Dark-rearing has little effect on the quantitative organization of the synaptic terminals. In contrast with previous observations in the visual cortex of the same animals37 no changes in the density of synaptic vesicles is observed in the superior colliculus after long term dark-rearing.