The goal of this proposal is to better understand the molecular mechanisms by which the retrieval of declarative/episodic memory occurs. The proposal focuses on the time-limited role that norepinephrine (NE) and ?1-adrenergic signaling play in the hippocampus during memory retrieval.
The first aim will determine in which hippocampal cell(s) ?1 signaling acts to promote memory retrieval. Contextual fear, which depends to NE and ?1 signaling, will be used to assess hippocampus-dependent memory. Cued (tone) fear, which does not depend on NE or ?1 signaling, will be used to test whether manipulations affect performance or memory. Imaging of immediate-early gene induction in mice will be employed as a marker of neuronal activity following exposure to salient and neutral contexts, ?1 signaling in the mice will be manipulated pharmacologically and genetically so that it is blocked or activated either, systemically, only in the dorsal hippocampus, or in specific subfields of the hippocampus. Because ?1 signaling activates the cAMP / protein kinase A pathway, the second aim will determine whether this pathway is required for memory retrieval. Dorsal hippocampal infusions of agents that block this pathway will be performed in control mice, and infusions of agents that stimulate this pathway will be performed in mutant mice that lack ?1 signaling. The relationship between NE and activation of extracellular signal-regulated kinase and phosphatidylinositol 3- kinase, which are also required for retrieval, will be determined. Further, one of the most prominent physiological effects of ?1 signaling in the hippocampus is reduction of the slow afterhyperpolarization (sAHP) that mediates accommodation of firing.
The third aim will determine whether calcium influx through voltage-dependent calcium channels influences retrieval, and whether pharmacologic block of the sAHP rescues retrieval in mutant mice lacking ?1 signaling.
This aim will also determine whether there is a transient reduction in the sAHP that can be observed in brain slices of mice after fear conditioning but not pseudoconditioning and, if so, whether the reduction depends on ?1 signaling. Finally, a key hypothesis explaining why NE is required for some but not all memory retrieval will be tested in the fourth aim. Relevance: Dysfunction of adrenergic signaling may contribute to symptoms of depression and post- traumatic stress disorder that include difficulties with memory retrieval in the former and unwanted, intrusive retrieval of traumatic memories in the latter. Results from this proposal may also be relevant to potential cognitive side effects that might arise when treating heart failure, hypertension and performance anxiety with drugs that block ? receptors. Finally, results should also be relevant to understanding how dysregulation of neuronal Ca++ homeostasis in the elderly may lead to memory deficits.
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