Fluoxetine suppresses synaptically induced [Ca2+]i spikes and excitotoxicity in cultured rat hippocampal neurons

Fluoxetine is a widely used antidepressant with an action that is primarily attributed to the inhibition of serotonin re-uptake into the synaptic terminals of the central nervous system. Fluoxetine also has blocking effects on various ion channels, including Ca2+ channels. It remains unclear, however, how fluoxetine may affect synaptically induced [Ca2+]i spikes. We investigated the effects of fluoxetine on [Ca2+]i spikes, along with the subsequent neurotoxicity that is synaptically evoked by lowering extracellular Mg2+ in cultured rat hippocampal neurons. Fluoxetine inhibited the synaptically induced [Ca2+]i spikes in p-chloroamphetamine-treated and non-treated neurons, in a concentration-dependent manner. However, other selective serotonin reuptake inhibitors, such as paroxetine and citalopram, did not significantly affect the spikes. Pretreatment with fluoxetine for 5 min inhibited [Ca2+]i increases induced by glutamate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and N-methyl-d-aspartate. Fluoxetine also inhibited α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced currents. In addition, fluoxetine decreased the [Ca2+]i responses induced by the metabotrophic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine or the ryanodine receptor agonist caffeine. Fluoxetine inhibited [Ca2+]i responses induced by 20 mM KCl. Fluoxetine decreased the release of FM1-43 induced by electric field stimulation. Furthermore, fluoxetine inhibited 0.1 mM [Mg2+]o-induced cell death. Collectively, our results suggest that fluoxetine suppresses the spikes and protects neurons against excitotoxicity, particularly in cultured rat hippocampal neurons, presumably due to both direct inhibition of presynaptic glutamate release and postsynaptic glutamate receptor-mediated [Ca2+]i signaling. In addition to an indirect inhibitory effect via 5-HT levels, these data suggest a new, possibly direct inhibitory action of fluoxetine on synaptically induced [Ca2+]i spikes and neuronal cell death.

► We used cultured neurons to focus effect of fluoxetine on glutamatergic transmission.
► Fluoxetine inhibits synaptically induced [Ca2+]i spikes in PCA-treated or control cells.
► Fluoxetine inhibits glutamate release and glutamate receptor-mediated Ca2+ signaling.
► Fluoxetine protects neurons against 0.1 mM [Mg2+]o-induced excitotoxicity.
► Fluoxetine directly inhibits synaptically induced [Ca2+]i spikes and death.