Long-term plasticity in hippocampus: Mechanisms of hypoxic adaptation and long-term depression of synaptic strength

Abstract

Throughout evolution the mammalian brain has acquired many and varied strategies to protect itself against a variety of environmental insults, as well as mechanisms that mold neural function and architecture in response to normal sensory information. To date, most research on these mechanisms has focused on separate implications of these plasticities, even though they are, in all likelihood, interrelated. This thesis addresses questions in both these areas in an attempt to elucidate mechanisms underlying each form of plasticity, toward the goal of identifying potential commonalities.;I hypothesized that hypoxia-triggered gene activation and protein synthesis might be necessary for inducible neuroprotection. My results demonstrate that hypoxia-induced neuroprotection does require gene transcription and mRNA translation to new proteins, but that this gene regulation is not triggered by the activation of glutamate receptors, because it can still be induced by hypoxia when excitatory ionotropic glutamatergic receptors are blocked pharmacologically. This led me to hypothesize that some direct sensor of oxygen tension might play an important inductive role. In peripheral systems, a heme-containing sensor has been identified which can activate genes that code for erythropoeitin in response to locally induced hypoxic environments. I tested the hypothesis that a similar heme protein might be directly sensing the changed oxygen tension and being reconfigured into an active (deoxygenated) state that then triggers neuroprotective gene expression. I confirmed this hypothesis by demonstrating that carbon monoxide, which locks heme in an oxygenated (inactive) state, prevents the induction of the neuroprotection.;The hypotheses I tested, therefore, were that an inhibitor of NO-sensitive guanylyl cyclase (NOGC) would block the induction of long-term depression (LTD) and that NO-mediated activation of cyclic GMP protein kinase (PKG), a major target for the increased cGMP after NOGC activation, would lead to enhanced LTD. Using the selective inhibitor KN-62, I also tested the role of the Ca{dollar}\sp{lcub}2+{rcub}{dollar}-activated enzyme Ca{dollar}\sp{lcub}2+{rcub}{dollar}/calmodulin-dependent protein kinase II, an enzyme already implicated in the induction of LTP, in the events triggering LTD.;My results demonstrate that NOGC is required for the induction of de novo LTD, as opposed to the NOADPRT cascade, and that its site of action is most likely the presynaptic terminal. In contrast, I showed that NOGC only partially contributes to depotentiation and not at all to heterosynaptic LTD, indicating that there are both NOGC-dependent and independent components to the multiple LTD phenomena. Furthermore, both an NO donor and a direct activator of PKG enhance the magnitude of LTD elicited by a submaximal stimulus, confirming my hypothesis that NOGC-generated cyclic GMP, through activation of PKG, is probably a biochemical pathway in the induction of de novo, homosynaptic LTD. Finally, I also found that a selective inhibitor of calcium/calmodulin-dependent protein kinase II (CaMKII), KN-62, does block the induction of both de novo LTD and depotentiation, suggesting that activation of this Ca{dollar}\sp{lcub}2+{rcub}{dollar}-triggered enzyme plays separate roles in the induction of both LTP and LTD, perhaps at different synaptic loci. (Abstract shortened by UMI.).