Insights into the release mechanism of astrocytic glutamate evoking in neurons NMDA receptor-mediated slow depolarizing inward currents.

The gliotransmitter glutamate in different brain regions modulates neuronal excitability and synaptic transmission through a variety of mechanisms. Among the hallmarks of astrocytic glutamate release are the slow depolarizing inward currents (SICs) in neurons mediated by N-methyl-d-aspartate receptor activation. Different stimuli that evoke Ca²⁺ elevations in astrocytes induce neuronal SICs suggesting a Ca²⁺ -dependent exocytotic glutamate release mechanism of SIC generation. To gain new insights into this mechanism, we investigated the relationship between astrocytic Ca²⁺ elevations and neuronal SICs in mouse hippocampal slice preparations. Here we provide evidence that SICs, occurring either spontaneously or following a hypotonic challenge, are unchanged in the virtual absence of Ca²⁺ signal changes at astrocytic soma and processes, including spatially restricted Ca²⁺ microdomains. SICs are also unchanged in the presence of Bafilomycin A1 that after prolonged slice incubation depletes glutamate from astrocytic vesicles. We also found that hemichannels and TREK family channels-that recent studies proposed to mediate astrocytic glutamate release - are not involved in SIC generation. SICs are reduced by the volume-sensitive anion channel antagonists diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), quinine and fluoxetine, suggesting a possible contribution of these channels in SIC generation. Direct measurements of astrocytic glutamate release further confirm that hypotonicity-evoked gliotransmission is impaired following DIDS, quinine and fluoxetine while the exocytotic release of glutamate-that is proposed to mediate synaptic transmission modulation by astrocytes-remains unaffected. In conclusion, our data provide evidence that the release of glutamate generating SICs occurs independently on exocytotic Ca²⁺ -dependent glutamate release mechanism.

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