Paradoxical actions of hydrogen peroxide on long-term potentiation in transgenic superoxide dismutase-1 mice

Abstract

Hydrogen peroxide (H2O2), a reactive oxygen species, is assumed to have a detrimental effect on neuronal plasticity. Indeed, H2O2 suppresses long-term potentiation (LTP) in hippocampal slices of normal rats and wild-type (wt) mice. Transgenic mice overexpressing superoxide dismutase (SOD) 1 (tg-SOD), which maintain high ambient H2O2, have also been shown to be impaired in their ability to express hippocampal LTP. Paradoxically, H2O2, at a concentration (50 microm) that blocks LTP in wt mice, actually enhanced LTP in slices of 2-month-old tg-SOD mice. H2O2-dependent LTP in tg-SOD was blocked by the protein phosphatase calcineurin inhibitor FK506, but not by rapamycin, an FK-binding protein 12 (FKBP12) inhibitor or by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), a serine-kinase inhibitor. Interestingly, wt and tg-SOD mice expressed similar levels of the antioxidant enzyme catalase and similar activity of glutathione peroxidase. An opposite situation was found in 2-year-old mice. Aged wt mice were impaired in LTP in a manner that could be reversed by the addition of H2O2. Surprisingly, aged tg-SOD mice exhibited larger LTP than that found in wt mice, but this was now reduced by 50 microm H2O2. Both young tg-SOD and aged control mice displayed altered protein phosphatase activity, compared with that of young controls; moreover, FK506 inhibited LTP in old tg-SOD as well as in old wt mice treated with H2O2. These data promoted a dual role for H2O2 in the regulation of LTP, and proposed that it is mediated by the protein phosphatase calcineurin.

Figure 1.

H₂O₂ reverses impaired LTP in tg-SOD mice. The experimental preparation shown schematically in the inset (top right) included a recording electrode placed in the stratum radiatum and two stimulating electrodes placed on either side such that two independent stimulation pathways could be used. A, LTP in hippocampal slices from 2-month-old wt mice was induced by TBS in the first pathway (arrow). Before TBS in the second pathway, the slice was perfused with 50 μm H₂O₂, which inhibited LTP. a-c, Representative traces of EPSPs at indicated times. B, A similar experiment shows that H₂O₂ markedly enhances potentiation in slices taken from tg-SOD mice, which otherwise express a low level of LTP. C, Paired-pulse potentiation is not different between genotypes. The ratio of the second EPSP slope to the first is plotted as a function of the interval between them. Right, Representative traces of the data summarized on the left.

Figure 2.

Calcineurin activity is required for H₂O₂-dependent LTP. A, EPSP slopes of 2-month-old tg-SOD mice treated with 50 μm H₂O₂ and 10 μm H7 or 20 μm FK506. Treatment with H₂O₂ restores LTP in response to TBS (Fig. 1), whereas adding FK506 (squares) but not H7 (circles) abolishes the effect of H₂O₂. B, Rapamycin does not mimic the effect of FK506 to inhibit H₂O₂-dependent LTP in tg-SOD slices. Adding rapamycin and 50 μm H₂O₂ results in LTP (squares), whereas untreated tg-SOD mice are impaired in LTP (circles) as in Figure 1 B.

Figure 3.

Tg-sod mice exhibit a complex redox environment. A, Tg-SOD mice (2 months old) were treated with BPN (bar), and TBS was applied to two pathways (arrows). BPN affected both the maintenance (squares) and the induction (circles) of LTP. B, Hippocampi from 2-month-old tg-SOD and wt mice were homogenized and measured for Gpx activity. Both genotypes demonstrated a similar level of Gpx-specific activity. C, Catalase immunoreactivity in 2-month-old tg-SOD and wt hippocampi. Homogenates from both genotypes were separated on SDS-PAGE and subjected to Western blot analysis. Similar levels of catalase were found for both genotypes. Also shown is the same blot stained with antibodies to neuron-specific enolase (NSE).

Figure 4.

Mouse hippocampal slice reaction to Xa/XO. A, Wt slices (2 months old) were perfused with 0.3 mg/ml Xa and 0.3 μl/ml XO (bar). TBS was applied to one pathway (arrow, squares), and a second pathway was not tetanized. B, A similar experiment, with 2-month-old tg-SOD slices. C, D, Wt and tg-SOD slices, respectively, were perfused with 0.3 mg/ml Xa and 0.5 μl/ml XO (bar), resulting in a transient decline in EPSPs, followed by a slow, small but significant increase in response above baseline in both cases.

Figure 5.

Slices from aged mice exhibit altered LTP. A, TBS was applied to hippocampal slices from wt mice, 2 years old (squares) and 2 months old (circles); the aged mice exhibited impaired short- and long-term potentiation. On the right is the input/output relationship for both age groups; at the top right are representative traces of the data summarized on the left. B, Results of the young tg-SOD slices are same as in Figure 1 B. Aged tg-SOD slices produced larger potentiation than their young counterparts.

Figure 6.

H₂O₂ reverses LTP phenotype in aged mice. A, TBS (arrows), applied to aged wt mice before (squares) and after (circles) perfusion with 50 μm H₂O₂ resulted in a large enhancement of both short- and long-term potentiation. B, In tg-SOD slices, under the same conditions, 50 μm H₂O₂ markedly attenuated LTP.

Figure 7.

Age and genotype affect endogenous H₂O₂ level and phosphatase activity. A, Hippocampal slices from young and old wt and tg-SOD mice were placed in HEPES-buffered medium in an imaging chamber. An H₂O₂-sensitive fluorescent dye (2,7-DCF) was added to the medium, and fluorescence was measured at 488 nm. Young tg-SOD mice exhibited slightly higher fluorescence than wt mice, whereas old wt mice exhibited significantly higher fluorescence than old tg-SOD mice. B, Phosphatase activity was measured for old and young hippocampal slices with or without 50 μm H₂O₂ (see Results for details).

Figure 8.

FK506 inhibits LTP in old wt mice treated with H₂O₂ and in old tg-SOD mice. A, Slices from 2-year-old wt mice perfused with 50 μm H₂O₂ alone (squares) expressed LTP, whereas slices perfused with H₂O₂ and 20 μm FK506 (circles) exhibited significantly lower LTP. B, Slices from 2-year-old tg-SOD mice exhibited LTP (squares), whereas the addition of 20 μm FK506 to the perfusion medium resulted in significantly lower LTP (circles).

Figure 9.

Schematic representation of the effect of ambient H₂O₂ levels on LTP. As described in Discussion, there is a threshold level of H₂O₂ necessary for optimal LTP. Deviating from that level in either direction will reduce the expression of LTP. Young wt mice maintain an ambient level of H₂O₂ that allows a transient, stimulus-induced flux of H₂O₂ to promote LTP. The exogenous addition of H₂O₂ drives the ambient level higher, inhibiting LTP. Tg-SOD mice maintain a high ambient level of H₂O₂ that induces a high level of anti-H₂O₂ cellular mechanisms, which in turn desensitize the cells to a transient H₂O₂ flux. Adding exogenous H₂O₂ increases the H₂O₂ level to a suprathreshold concentration. Old wt mice are under high ambient H₂O₂ levels because of mitochondrial leakage, and react in a similar manner. Old tg-SOD mice have an ambient H₂O₂ level that is the result of both transgene and age, resulting in a level of H₂O₂ on stimulation that remains high long enough to achieve the LTP threshold.