Non-synaptic exocytosis enhanced in rat cerebellar granule neurons cultured under survival-promoting conditions

Cultured rat cerebellar granule neurons (CGNs) are often used to analyze activity-dependent neuronal selection occurring during brain development. The CGNs survive long only when the culture medium contains a depolarizing agent. However, it is argued whether the depolarization critical for survival is of presynaptic or postsynaptic compartment. Since CGNs form no synapses among them, it is generally assumed that the latter would be the case. But it is possible that the depolarization would induce exocytosis of survival-promoting substances whether or not CGNs form synapses. Here we directly examined the exocytotic activities of CGNs under survival-promoting and survival-limiting conditions by electron microscopy to support this possibility. CGNs possessed clusters of synaptic vesicle-like vesicles (SVVs) in neuritic varicosities. CGNs cultured in high-KCl medium had significantly smaller SVV clusters than those cultured in low-KCl medium. The number of SVVs increased when the high KCl-cultured CGNs were transferred to low-KCl medium, indicating a sustained high rate of exocytosis in high-KCl medium. The majority of the varicosities containing SVVs were not apposed to definite postsynaptic structures, indicating that exocytosis occurs from a non-synaptic surface. Fluorescence Ca(2+) imaging revealed that the high KCl-cultured CGNs had spots of high Ca(2+) along their neurites, corresponding to the varicosities.