Comparison of Ca2+ Responses to Stimulation of Glutamate Receptors in the Rat Cerebral Cortex after Hypobaric Hypoxia of Different Severities

Hypobaric Preconditioning Modifies Group I mGluRs Signaling in Brain Cortex

The study assessed involvement of Ca(2+) signaling mediated by the metabotropic glutamate receptors mGluR1/5 in brain tolerance induced by hypoxic preconditioning. Acute slices of rat piriform cortex were tested 1 day after exposure of adult rats to mild hypobaric hypoxia for 2 h at a pressure of 480 hPa once a day for three consecutive days. We detected 44.1 ± 11.6 % suppression of in vitro anoxia-induced increases of intracellular Ca(2+) levels and a fivefold increase in Ca(2+) transients evoked by selective mGluR1/5 agonist, DHPG. Western blot analysis of cortical homogenates demonstrated a 11 ± 4 % decrease in mGluR1 immunoreactivity (IR), and in the nuclei-enriched fraction a 12 ± 3 % increase in IR of phospholipase Cβ1 (PLCβ1), which is a major mediator of mGluR1/5 signaling. Immunocytochemical analysis of the cortex revealed increase in the mGluR1/5 and PLCβ1 IR in perikarya, and a decrease in IR of the neuronal inositol trisphosphate receptors (IP3Rs). We suggest that enhanced expression of mGluR5 and PLCβ1 and potentiation of Ca(2+) signaling may represent pro-survival upregulation of Ca(2+)-dependent genomic processes, while decrease in mGluR1 and IP3R IR may be attributed to a feedback mechanism preventing excessive intracellular Ca(2+) release.

Short-term episodes of hypoxia induce posthypoxic hyperexcitability and selective death of GABAergic hippocampal neurons

We have previously developed a rat hippocampal neuronal cell model for the registration of the preconditioning effect and posthypoxic hyperexcitability (Turovskaya et al., 2011). Repeated episodes of short-term hypoxia are reported to suppress the amplitude of Ca(2+) response to NMDA in majority of neurons, reflecting the effect of preconditioning in the culture. In addition, exposure to hypoxia causes posthypoxic hyperexcitability: this is characterized by the onset of spontaneous synchronous Ca(2+) transients in a population of neurons in a neural network during the period of reoxygenation after each hypoxic episode. The nature of this phenomenon is unknown, although it has been observed that there always exists a minority of neurons in which there is no effect of hypoxic preconditioning. In this small population of neurons, the amplitude of Ca(2+) response to NMDA is not suppressed, but rather increases after each episode of hypoxia. Here we report the type of these neurons and their role in the generation of posthypoxic hyperexcitability. We compared the effect of short-term hypoxia on the amplitude of the Ca(2+) response to NMDA and the Ca(2+) transient generation in two populations of neurons - inhibitory GABAergic and excitatory glutamatergic. We have demonstrated that the neurons in which the preconditioning effect was not observed are GABAergic. Moreover at the instant moment of the posthypoxic synchronous Ca(2+)-transient generation (during reoxygenation) there is a global increase of [Ca(2+)]i and subsequent apoptosis in some GABAergic neurons. Anti-inflammatory cytokine interleukin-10 prevents the development of posthypoxic hyperexcitability, inhibiting the spontaneous synchronous Ca(2+) transients. At the same time, interleukin-10 protects GABAergic neurons from death, by restoring the effect of hypoxic preconditioning in them. Activation of one of the signaling pathways initiated by interleukin-10 appears to be necessary for the development of hypoxic preconditioning in GABAergic neurons. Overall our results indicate that short-term episodes of hypoxia can damage GABAergic neurons and weaken the inhibitory action of GABAergic neurons in a neural network. Activation of PI3K-dependent survival signaling pathways in neurons of this type is a possible strategy to protect these cells against hypoxia.