Semester

Summer

Date of Graduation

2008

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Physiology, Pharmacology & Neuroscience

Committee Chair

James M. O'Donnell.

Abstract

The relationship between the ability of repeated antidepressant treatment to cause down-regulation of the norepinephrine and serotonin transporters (NET, SERT) and produce antidepressant-like effects on behavior was determined. Treatment of rats with 15 mg/kg/day desipramine reduced NET expression, measured by 3H-nisoxetine binding and SDS-PAGE/immunoblotting, in cerebral cortex and hippocampus and reduced the time of immobility in the forced-swim test. The antidepressant-like effect on forced-swim behavior was evident two days following discontinuation of desipramine treatment when plasma and brain levels of desipramine and its major metabolite desmethyldesipramine were not detectable. Reduced NET expression resulted in decreased norepinephrine (NE) uptake, measured in vitro, and increased noradrenergic neurotransmission, measured in vivo using microdialysis. Fourteen-day treatment of rats with 20 mg/kg/day protriptyline or 7.5 mg/kg/day sertraline reduced NET and SERT expression, measured by 3H-nisoxetine/ 3H-citalopram binding and SDSPAGE/immunoblotting, in cerebral cortex and hippocampus and reduced the time of immobility in the forced-swim test. Six-week, but not 2-week, treatment with 20 mg/kg/day reboxetine caused down-regulation of NET and an antidepressant-like effect in the forced-swim test. Antidepressant-induced NET and SERT down-regulation was not due to the reduction of gene transcription, as determined using quantitative, real-time RT-PCR.;Since studies using cell cultures have revealed a role of protein kinase C (PKC) in NET regulation, experiments were carried out to assess the importance of this mechanism in brain tissues and determine its role in the mediation of antidepressant-like effects on behavior. It was found that the PKC activators beta-PMA and bryostatin-1 reduced NE uptake in cerebrocortical slices; this was due to decreased Vmax and unchanged Km values. Further, bilaterally intracerebroventricular (ICV) administration of beta-PMA produced a significant antidepressant-like effect on forced-swim behavior, which was reversed by co-administration of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a PKC inhibitor.;Overall, antidepressant-induced changes in NET/SERT expression paralleled those in the antidepressant-like effects on behavior. The antidepressant-like effects of desipramine and sertraline were blocked by inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine (AMPT) and inhibition of serotonin (5-HT) synthesis with parachlorophenylalanine (PCPA), respectively, suggesting that increased noradrenergic or serotonergic neurotransmission is an important mechanism underlying antidepressant activity. In addition, the results, which are consistent with clinical data in terms of time-course response, suggest an important role of NET and SERT regulation in the long-term behavioral effects of antidepressant drugs and that enhanced noradrenergic or serotonergic neurotransmission is necessary, but not sufficient, for its antidepressant actions. Understanding the mechanisms underlying NET and SERT regulation in vivo may suggest novel pharmacological targets for treating depression. Future studies may focus on identifying the role of PKC signaling in NET regulation, given that this signaling pathway appears to be an important mediator contributing to the long-term behavioral effects of antidepressant treatment.

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