Hypoxia causes depression of synaptic plasticity, hyperexcitation of neuronal networks, and the death of specific populations of neurons. However, brief episodes of hypoxia can promote the adaptation of cells. Hypoxic preconditioning is well manifested in glutamatergic neurons, while this adaptive mechanism is virtually suppressed in GABAergic neurons. Here, we show that brain-derived neurotrophic factor (BDNF) overexpression in neurons enhances the preconditioning effect of brief episodes of hypoxia. The amplitudes of the NMDAR-and AMPARmediated Ca 2? responses of glutamatergic and GABAergic neurons gradually decreased after repetitive brief hypoxia/ reoxygenation cycles in cell cultures transduced with the (AAV)-Syn-BDNF-EGFP virus construct. In contrast, the amplitudes of the responses of GABAergic neurons increased in non-transduced cultures after preconditioning. The decrease of the amplitudes in GABAergic neurons indicated the activation of mechanisms of hypoxic preconditioning. Preconditioning suppressed apoptotic or necrotic cell death. This effect was most pronounced in cultures with BDNF overexpression. Knockdown of BDNF abolished the effect of preconditioning and promoted the death of GABAergic neurons. Moreover, the expression of the anti-apoptotic genes Stat3, Socs3, and Bcl-xl substantially increased 24 h after hypoxic episodes in the transduced cultures compared to controls. The expression of genes encoding the pro-inflammatory cytokines IL-10 and IL-6 also increased. In turn, the expression of pro-apoptotic (Bax, Casp-3, and Fas) and proinflammatory (IL-1b and TNFa) genes decreased after hypoxic episodes in cultures with BDNF overexpression. Inhibition of vesicular BDNF release abolished its protective action targeting inhibition of the oxygen-glucose deprivation (OGD)-induced [Ca 2? ] i increase in GABAergic and glutamatergic neurons, thus promoting their death. Bafilomycin A1, Brefeldin A, and tetanus toxin suppressed vesicular release (including BDNF) and shifted the gene expression profile towards excitotoxicity, inflammation, and apoptosis. These inhibitors of vesicular release abolished the protective effects of hypoxic preconditioning in glutamatergic neurons 24 h after hypoxia/reoxygenation cycles. This finding indicates a significant contribution of vesicular BDNF release to the development of the mechanisms of hypoxic preconditioning. Thus, our results demonstrate that BDNF plays a pivotal role in the activation and enhancement of the preconditioning effect of brief episodes of hypoxia and promotes tolerance of the most vulnerable populations of GABAergic neurons to hypoxia/ischemia.

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Norepinephrine is one of the key neurotransmitters in the hippocampus, but its role in the functioning of the neuroglial networks remains unclear. Here we show that norepinephrine suppresses NH4Cl‐induced oscillations of the intracellular Ca2+ concentration ([Ca2+]i) in hippocampal neurons. We found that the inhibitory effect of norepinephrine against ammonium‐induced [Ca2+]i oscillations is mediated by activation of alpha‐2 adrenergic receptors. Furthermore, UK 14,304, an agonist of alpha‐2 adrenergic receptors, evokes a biphasic [Ca2+]i elevation in a minor population of astrocytes. This elevation consists of an initial fast, peak‐shaped [Ca2+]i rise, mediated by Giβγ subunit and subsequent PLC‐induced mobilization of Ca2+ from internal stores, and a plateau phase, mediated by a Ca2+ influx from the extracellular medium through store‐operated and TRPC3 channels. We show the correlation between the Ca2+ response in astrocytes and suppression of [Ca2+]i oscillations in neurons. The inhibitory effect of UK 14,304 is abolished in the presence of gallein, an inhibitor of Gβγ‐signaling. In turn, application of the agonist in the presence of the PLC inhibitor decreases the frequency and amplitude of [Ca2+]i oscillations in neurons but does not suppress them. The same effect is observed in the presence of bicuculline, a GABA(A) receptor antagonist. We demonstrate that UK 14,304 application increases the frequency and amplitude of slow outward chloride currents in neurons, indicating the release of GABA by astrocytes. Thus, our findings indicate that the activation of astrocytic alpha‐2 adrenergic receptors stimulates GABA release from astrocytes via Giβγ subunit‐associated signaling pathway, contributing to the suppression of neuronal activity.

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