A morphometric study of cultured rat cerebral synapses exposed to different cationic media.

Quantitative techniques have been applied to compare the effects of high-K+, Mg2+ and Li+ media on the ultrastructure of cultured synapses alongside Na+-incubated controls. The explant cultures were prepared from 18-day-old embryonic rat cerebral cortices and maintained for 19 days in vitro. K+ -Stimulation for 25 min resulted in an increase in the mean perimeter and area of presynaptic terminals. Of these, the perimeter increase was the more pronounced, as indicated by a decrease in the value of the two-dimensional form factor. Reductions were also observed in the number of synaptic vesicles per presynaptic terminal, in the vesicle-terminal area ratio and in the synaptic vesicle density in an area adjacent to the presynaptic membrane, the latter two parameters being in positive linear correlation. The frequency of presynaptic cisternal/vacuolar profiles increased, and the synaptic curvature shifted in a positive direction. Synaptic length did not change following K+-exposure. Qualitative assessment indicated the presence of a network subjacent to the post-synaptic thickening and swelling of the postsynaptic ending after K+-stimulation. Incubation and fixation in Mg2+-media of two concentrations resulted in an increase in the number and area ratio of synaptic vesicles per terminal, and an elevation in the synaptic vesicle density in the higher Mg2+ concentration medium. Li+-treatment reduced the number of synaptic vesicles per terminal, the vesicle-terminal area ratio, and the vesicle density in the vicinity of the presynaptic membrane, while the synaptic curvature shifted in the positive direction. These changes were less pronounced than those characteristic of synapses in the K+ medium.