Quantitative ultrastructural investigations on synaptogenesis in the cerebellum and the optic tectum of light-reared and dark-reared rainbow trout (Salmo gairdneri Rich.).

The morphogenetic differentiation of synapses in the cerebellum and the optic tectum of dark-and light- reared rainbow trout was investigated at critical stages of development. During normal differentiation the cerebellum is characterized by the appearance of 'indented', spinelike synapses. This type of synapses increases with age and prevails from day 60 on. At the same time the number of 'flat' synapses decreases. In the cerebellum the highest synaptic density (123 +/- 12 synapses/1,000 micrometer2) is reached 30 days after hatching when the larvae begin to swim. The optic tectum is characterized by a preponderance of flat synapses in early postnatal and adult life; maximal synaptic density (66 +/- 5 synapses/1,000 micrometer2) is reached 60 days after hatching when the larvae have reached optimal visual acuity. Light deprivation causes a considerable and significant reduction in the number of synapses per unit area in the cerebellum and the optic tectum. The length of synaptic contacts do not change. If light-deprived, the density of synaptic vesicles decreases significantly in the optic tectum of a 25-day-old trout (74 +/- 3 instead of 132 +/- 7 vesicles/micrometer2). In the cerebellum this effect is absent.