Trophic and outgrowth-promoting effects of K(+)-induced depolarization on developing thalamic cells in organotypic culture.

The aim of this study was to investigate how different levels of K(+)-induced depolarization affect the survival and growth of isolated, cultured thalamic explants from mice aged embryonic day 13 to postnatal day 2. K+ was added to explants in serum-free culture medium. After culture for three days, explants were sectioned and Nissl-stained or photographed under phase contrast for quantification of neurite outgrowth. Viable and pyknotic cells were counted in sectioned material. The results revealed that, with no added K+, both viability and neurite outgrowth decreased as the age of the thalamic explant increased: most cells survived in embryonic day 13 explants, most died in postnatal day 2 explants. Adding K+ had an age- and dose-dependent effect on viability and neurite outgrowth. The greatest viability-promoting effect of adding K+ was at embryonic day 19: adding 5 mM K+ rescued the majority of these cells, although there was no effect on neurite outgrowth at this age (i.e., enhanced viability did not necessarily produce increased outgrowth). This same dose of K+ had its greatest effect on neurite outgrowth at embryonic day 17. No dose of added K+ had a stimulatory effect on viability and neurite outgrowth after embryonic day 19. The highest dose of K+ used here (50 mM) inhibited thalamic cell survival. We suggest that the survival and growth of the prenatal thalamus can occur without external influences. This intrinsic control may use an autocrine mechanism that becomes increasingly reliant on neural activity for its maintenance as it ages. After birth, when thalamic cells may switch their dependence to cortex-derived growth factors, this intrinsic control may become ineffective.