We tested the influence of continuous high-K+ treatment on acetylcholine (ACh) release and choline acetyltransferase (ChAT) activity on septal cell culture, and septal and hippocampal cell co-culture obtained from rat embryos. Continuous 9 mM K+ treatment did not affect ACh release and ChAT activity in septal culture, but increased ACh release in co-culture without affecting ChAT activity. A slight increase in extracellular K + concentration, therefore, induced neuronal excitation. Continuous 55 mM K+ treatment increased ACh release in septal culture. This effect was due to direct excitation of septal neurons. In co-culture, 55 mM K+ treatment increased both ACh release and ChAT activity. These results indicate that hippocampal neurons are indispensable for the depolarization-induced increase in ChAT activity in the early stage of developing septal cholinergic neurons.
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