Effects of Chronic Stress on Norepinephrine Clearance in the CA3 Region of the Hippocampus
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The pathology between chronic stress, deregulation of serotonergic (5-HT) and noradrenergic (NE) neurotransmission and the onset of psychiatric disorders is poorly understood. Using in vivo chronoamperometry our previous findings showed 5-HT clearance is dramatically slower in chronically stressed wild-type mice, and that this reduction in clearance does not result from the loss of serotonin transporter (SERT) expression or function. NE transporters (NET) express a low affinity and high capacity for 5-HT uptake, and so can regulate 5-HT neurotransmission when high concentration of 5-HT are present in the extracellular fluid (EFC). Due to this relation, we hypothesized the reduced 5-HT clearance in stressed mice results from a downregulation in NET function and/or expression. To test this hypothesis, in vivo chronoamperometry was used to measure the clearance rate (µM/sec) of locally applied NE from ECF in the CA3 region of the hippocampus of anesthetized mice. Additionally, quantitative autoradiography was completed to establish if overall NET expression in the CA3 region had decreased. The forced swim test (FST, 10 minutes daily for 14 consecutive days) was administered as the stress paradigm, while unhandled mice served as controls. All experiments were conducted 24 hours after the last FST, or at matched times for non-stressed mice. Results show chronically stressed mice clear exogenously (externally) applied NE at a significantly faster rate than non-stressed controls, yet overall NET expression does not differ between the two conditions. Given the data, it is possible NET plasmallemmal expression has been upregulated in chronically stressed mice although overall protein expression has remained unchanged. Additionally, the reciprocal effects of chronic stress on 5-HT and NE clearance may be in part due to another transporter system with a low affinity for 5-HT being compromised instead of NET. In order to further elucidate the effects of chronic stress on serotonergic and noradrenergic transmission individually and in relation to their regulatory effects on each other, future studies will include administering selective norepinephrine reuptake inhibitor (SNRI) and/or a cocktail mixture (SSRI, selective serotonin reuptake inhibitor and SNRI) to index the functional tone of NETs in vivo in stressed and non-stressed conditions.