STRUCTURAL PLASTICITY OF OPTIC SYNAPSES IN THE RAT SUPRACHIASMATIC NUCLEUS - ADAPTATION TO LONG-TERM INFLUENCE OF LIGHT AND DARKNESS

Synapses of optic afferents (optic synapses) in the suprachiasmatic nu cleus of hooded rats were morphometrically evaluated after exposing th e animals to 12 h, 14 days, 2 months, and 8 months of constant light ( light rats) and darkness (dark rats). Compared with dark rats, optic s ynapses from light rats have larger boutons with larger mitochondria, more clear vesicles, fewer dense-core vesicles and front-line vesicles , smaller presynaptic dense projections, a smaller amount of postsynap tic density material, a smaller relative number of Gray-type I (asymme tric) junctions, a greater relative number of Gray-type II (symmetric) junctions, as well as more and larger mitochondria in the postsynapti c dendrites. Junctions of optic synapses are mostly straight, but the small number of positively curved contacts are more flattened in light rats than in dark rats. An age-related increase in the size of presyn aptic dense projections was also observed. There are no changes in the sizes of clear and dense-core vesicles, in the size of synaptic junct ions and their numerical density in area, and in the unspecific contac t area between pre- and postsynaptic elements. The changes in optic bo utons are characteristic for activated and relatively disused synapses with a slow, tonic firing rate. It appears that (1) the amount of pos tsynaptic density material is proportional to the strength of Gray-typ e I synapses, and that (2) some excitatory optic synapses become inhib itory after long-term activity, whereas some inhibitory synapses turn into excitatory contacts after long-term disuse.