3H‐D‐aspartate release from cerebellar granule neurons is differentially regulated by glutamate‐ and K+‐stimulation

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Abstract
Neurotransmitter release in response to either 55 mM K+ or 25 μM glutamate as well as its dependency on Ca2+ from different sources was compared in cultured glutamatergic cerebellar granule cells from rat brain. The intracellular Ca+ concentration was monitored at the single cell level in neurites as well as cell bodies employing the fluorescent Ca2+ indicator fura‐2. Transmitter release was assayed using 3H‐D‐aspartate to label the exogenously accessable glutamate pools, which in these neurons is believed to also include the transmitter pool. In an attempt to distinguish whether transmitter release was dependent on an intact cytoskeleton or not, the colchicine‐like drug Nocodazole, which also blocks transport of vesicles, was used. K+‐stimulated transmitter release consisted for the major part (around 70%) of a Ca2+ dependent, Nocodazole sensitive release component and this K+‐induced release appeared to be almost exclusively dependent on N‐type Ca2+ channels. In contrast, 50% of the glutamate‐induced Ca2+‐dependent release was triggered by Ca2+ from a Dantrolene sensitive intracellular Ca2+ pool. Since these neurons undergo a pronounced maturational change in which neurotransmitter vesicles become increasingly prominent, the Ca2+ responses and transmitter release evoked by the two different stimuli were investigated as a function of the culture period. K+ and glutamate were found to increase intracellular [Ca2+] differentlly. In 1‐day‐old cultures K+ elicited a small albeit significant increase in [Ca2+]i while glutamate was completely without effect. In 7‐day‐old neurons both agents induced a large increase in [Ca2+]i. However, K+ induced equal Ca2+ responses in cell bodies and neurites, whereas glutamate induced an almost 4 times lower Ca2+ response in neurites compared to cell bodies.These findings suggest that K+ and glutamate may recruit transmitter and release‐triggering‐Ca2+ from different pools. This could be of pivotal importance for the understanding of glutamate homeostasis during physiological as well as pathological conditions.

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