We propose to examine the role of adrenergic signaling in synaptic plasticity, learning and memory using mouse molecular genetics. Specifically, we have created mice that are unable to synthesize norepinephrine (NE) and epinephrine due to a targeted disruption of the dopamine B-hydroxylase (Dbh) gene. Homozygotes (Dbh-/-) completely lack NE; however NE can be restored rapidly in vivo and in vitro using the synthetic amino acid precursor of NE (DOPS). This model has several advantages over prior pharmacologic approaches, including completeness of effect, specificity for NE, and reversibility. Prior studies using various techniques have often generated conflicting results with regard to the roles of NE in synaptic plasticity, learning and memory. Some studies have suggested a role for NE in the formation of emotional (aversive) memories. To test this possibility, we have begun to characterize the ability of Dbh-/- mice to learn and remember an aversive event using fear conditioning. Preliminary results indicate a specific deficit in the consolidation of contextual but not cued memory, suggesting hippocampal function may be altered in the absence of NE. For this reason we have begun to examine synaptic plasticity in the hippocampus. Preliminary results from these studies suggest that the late phase of long-term potentiation in region CAl is deficient. Because other studies have suggested a critical role of synaptic plasticity in region CAl for learning and memory, we propose to examine whether intracellular signaling pathways implicated in learning and memory are altered in region CAl following stimuli that elicit the late phase of LTP in vitro, and following fear conditioning in vivo. Finally, we will test whether compensation for the absence of NE occurs during development, and whether dopamine released from the adrenergic terminals of Dbh-/- mice can substitute at least partially for NE. These goals will be achieved through the use of a second mouse model (Th-/-/Dat-Th+/-) that should lack DA as well as NE in the adrenergic neurons specifically. Some of these mice will be raised with NE present (by supplying L-DOPA pre- and postnatally). L-DOPA will then be withdrawn in half prior to using the mice in the above studies.
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