Long-term depression in the adult hippocampus in vivo involves activation of extracellular signal-regulated kinase and phosphorylation of Elk-1.

Protein kinase cascades likely play a critical role in the signaling events that underlie synaptic plasticity and memory. The extracellular signal-regulated kinase (ERK) cascade is suited well for such a role because its targets include regulators of gene expression. Here we report that the ERK cascade is recruited during long-term depression (LTD) of synaptic strength in area CA1 of the adult hippocampus in vivo and selectively impacts on phosphorylation of the nuclear transcription factor Elk-1. Using a combination of in vivo electrophysiology, biochemistry, pharmacology, and immunohistochemistry, we found the following: (1) ERK phosphorylation, including phosphorylation of nuclear ERK, and ERK phosphotransferase activity are increased markedly, albeit transiently, after the induction of NMDA receptor-dependent LTD at the commissural input to area CA1 pyramidal cells in the hippocampus of anesthetized adult rats; (2) LTD-inducing paired-pulse stimulation fails to produce lasting LTD in the presence of the ERK kinase inhibitor SL327, which suggests that ERK activation is necessary for the persistence of LTD; and (3) ERK activation during LTD results in increased phosphorylation of Elk-1 but not of the transcription factor cAMP response element-binding protein. Our findings indicate that the ERK cascade transduces signals from the synapse to the nucleus during LTD in hippocampal area CA1 in vivo, as it does during long-term potentiation in area CA1, but that the pattern of coupling of the ERK cascade to transcriptional regulators differs between the two forms of synaptic plasticity.

Synaptic localization of a functional NADPH oxidase in the mouse hippocampus.

Superoxide has been shown to be critical for hippocampal long-term potentiation (LTP) and hippocampus-dependent memory function. A possible source for the generation of superoxide during these processes is NADPH oxidase. The active oxidase consists of two membrane proteins, gp91phox and p22phox, and four cytosolic proteins, p40phox, p47phox, p67phox, and Rac. Upon stimulation, the cytosolic proteins translocate to the membrane to form a complex with the membrane components, which results in production of superoxide. Here, we determined the presence, localization, and functionality of a NADPH oxidase in mouse hippocampus by examining the NADPH oxidase proteins as well as the production of superoxide. All of the NADPH oxidase proteins were present in hippocampal homogenates and enriched in synaptoneurosome preparations. Immunocytochemical analysis of cultured hippocampal neurons indicated that all NADPH oxidase proteins were localized in neuronal cell bodies as well as dendrites. Furthermore, double labeling analysis using antibodies to p67phox and the presynaptic marker synaptophysin suggest a close association of the NADPH oxidase subunits with synaptic sites. Finally, stimulation of hippocampal slices with phorbol esters triggered translocation of the cytoplasmic NADPH oxidase proteins to the membrane and an increase in superoxide production that was blocked by inhibitors of NADPH oxidase. Taken together, our data suggest that NADPH oxidase is present in mouse hippocampus and might be the source of superoxide production required for LTP and memory function.