Use of dopamine-β-hydroxylase-deficient mice to determine the role of norepinephrine in the mechanism of action of antidepressant drugs

Norepinephrine (NE) is thought to play an important role in the pathophysiology of depression, and in the mechanism of action of antidepressant compounds. Previously, we created mice that are unable to synthesize NE and epinephrine due to targeted disruption of the dopamine-β-hydroxylase gene (Dbh). To specifically test the role of NE in mediating behavioral changes elicited by antidepressants, these mice were examined in the forced swim test. There was no difference in baseline immobility scores in the forced swim test between Dbh^+/- mice, which have normal levels of NE, and Dbh^-/- mice. However, the Dbh^-/- mice failed to demonstrate antidepressant-like behavioral effects following the administration of several classes of antidepressants. These included the NE reuptake inhibitors desipramine and reboxetine, the monoamine oxidase inhibitor pargyline, and the atypical antidepressant bupropion. In addition, desipramine significantly reduced immobility in the Dbh^-/- mice following pretreatment with the synthetic NE precursor L-threo-3,4-dihydroxyphenylserine, but not saline. Biochemical studies showed that there was no significant difference in the regional brain levels of NE transporter immunoreactivity or monoamine oxidase activity, the primary targets for most of the compounds examined. Taken together, these data show that the use of mice that lack endogenous NE may be an important strategy for unraveling the role of NE in tests sensitive to the effects of various psychotherapeutic agents.