A neuronal lactate uptake inhibitor slows recovery of extracellular ion concentration changes in the hippocampal CA3 region by affecting energy metabolism.

Astrocyte derived lactate supports pathologically enhanced neuronal metabolism but its role under physiological conditions is still a matter of debate. Here, we determined the contribution of astrocytic neuronal lactate shuttle to maintenance of ion homeostasis and energy metabolism. We tested for the effects of α-cyano-4-hydroxycinnamic acid (4-CIN), which could interfere with energy metabolism by blocking monocarboxylate-transporter 2 (MCT2) mediated neuronal lactate uptake, on evoked potentials, stimulus induced changes in K⁺, Na⁺, Ca²⁺ and oxygen concentrations as well as on changes in flavin adenine dinucleotide (FAD) autofluorescence in the hippocampal area CA3. MCT2 blockade by 4-CIN reduced synaptically evoked but not antidromic population spikes. This effect was dependent on the activation of K_ATP channels indicating reduced neuronal ATP synthesis. By contrast, lactate receptor activation by 3,5-dihydroxybenzoic acid (3,5-DHBA) resulted in increased antidromic and orthodromic population spikes suggesting that 4-CIN effects are not mediated by lactate accumulation and subsequent activation of lactate receptors. Recovery kinetics of all ion transients were prolonged and baseline K⁺ concentration became elevated by blockade of lactate uptake. Lactate contributed to oxidative metabolism as both baseline respiration and stimulus induced changes in pO₂ were decreased, while FAD fluorescence increased likely due to a reduced conversion of FAD into FADH₂. These data suggest that lactate shuttle contributes to regulation of ion homeostatsis and synaptic signaling even in the presence of ample glucose.

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