Norepinephrine-deficient mice lack responses to antidepressant drugs, including selective serotonin reuptake inhibitors

Abstract

Mice unable to synthesize norepinephrine (NE) and epinephrine due to targeted disruption of the dopamine beta-hydroxylase gene, Dbh, were used to critically test roles for NE in mediating acute behavioral changes elicited by different classes of antidepressants. To this end, we used the tail suspension test, one of the most widely used paradigms for assessing antidepressant activity and depression-related behaviors in normal and genetically modified mice. Dbh(-/-) mice failed to respond to the behavioral effects of various antidepressants, including the NE reuptake inhibitors desipramine and reboxetine, the monoamine oxidase inhibitor pargyline, and the atypical antidepressant bupropion, even though they did not differ in baseline immobility from Dbh(+/-) mice, which have normal levels of NE. Surprisingly, the effects of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine, sertraline, and paroxetine were also absent or severely attenuated in the Dbh(-/-) mice. In contrast, citalopram (the most selective SSRI) was equally effective at reducing immobility in mice with and without NE. Restoration of NE by using L-threo-3,4-dihydroxyphenylserine reinstated the behavioral effects of both desipramine and paroxetine in Dbh(-/-) mice, thus demonstrating that the reduced sensitivity to antidepressants is related to NE function, as opposed to developmental abnormalities resulting from chronic NE deficiency. Microdialysis studies demonstrated that the ability of fluoxetine to increase hippocampal serotonin was blocked in Dbh(-/-) mice, whereas citalopram's effect was only partially attenuated. These data show that NE plays an important role in mediating acute behavioral and neurochemical actions of many antidepressants, including most SSRIs.

Fig. 1.

The effects of antidepressants on immobility in the TST. An ANOVA revealed a significant genotype × dose interaction for the tricyclic antidepressant desipramine [F(2, 53) = 7.80, P < 0.01] (A), for the selective NE reuptake inhibitor reboxetine [F(2, 48) = 5.83, P < 0.01] (B), and for the atypical antidepressant bupropion (20 mg/kg i.p.) and the monoamine oxidase inhibitor pargyline (75 mg/kg i.p. given twice) [F(2, 113) = 4.65, P = 0.01] (C). The SSRI sertraline reduced immobility in control but not in Dbh^–/– mice, according to a significant genotype × dose interaction [F(3, 68) = 4.64, P < 0.01] (D). The pattern of effects in Dbh^–/– mice varied with the different SSRIs, fluoxetine, paroxetine, and citalopram [F(6, 211) = 8.97, P < 0.01] (E). n = 9–38 mice per group. *, Groups that differed significantly from saline-treated mice of the corresponding genotype (P < 0.05); #, groups differed significantly from the opposite genotype tested at the same dose (P < 0.05), according to Fisher's probable least-squares difference test. There were no differences in immobility of mice treated with saline between genotypes in all experiments. For Figs. 1, 2, 3, 4, data are represented as mean values, with vertical lines indicating 1 SEM.

Fig. 2.

The effects of the selective NE reuptake inhibitor reboxetine (10 mg/kg i.p.) and the SSRI citalopram (5 mg/kg i.p.) given individually and in combination on immobility in the TST. An ANOVA revealed a significant interaction between drug treatment and genotype [F(3, 72) = 4.22, P < 0.01]. n = 9–10 mice per group. *, Groups that differed significantly from saline-treated mice of the corresponding genotype (P < 0.05); #, groups differed significantly from the opposite genotype tested at the same dose (P < 0.05), according to Fisher's probable least-squares difference test.

Fig. 3.

Restoring NE in Dbh^–/– mice by means of pretreatment with l-DOPS reinstates behavioral changes induced by desipramine and paroxetine in the TST. There was a significant three-way genotype × drug treatment × l-DOPS pretreatment interaction [F(1, 160) = 24.22, P < 0.001], according to ANOVA. n = 9–20 mice per group. *, Groups that differed significantly from saline-treated mice of the corresponding genotype (P < 0.05); #, groups differed significantly from the opposite genotype tested at the same dose (P < 0.05); $, groups that differed from relevant drug-treated control (P < 0.05), according to Fisher's probable least-squares difference test.

Fig. 4.

Effects of citalopram and fluoxetine on extracellular 5-HT concentrations in Dbh^+/– and Dbh^–/– mice. Baseline values were determined from the average of four samples preceding drug injection or infusion. All infusions were begun at time = 0 and all injections were performed at time = 0. (A) Effects of systemic administration of 20 mg/kg i.p. fluoxetine in Dbh^–/– (n = 5; baseline = 6.15 ± 0.20 fmol/12-μl sample) and Dbh^+/– mice (n = 5; baseline = 4.34 ± 0.23 fmol/12-μl sample). ANOVA indicated a significant genotype × time interaction (F(9, 72) = 4.69, P = 0.001). (B) Local perfusion of 1 μM fluoxetine on extracellular 5-HT levels in the ventral hippocampus did not produce significantly different effects between Dbh^–/– (n = 5; baseline = 2.39 ± 0.08 fmol/12-μl sample) and control mice (n = 5; baseline = 1.77 ± 0.04 fmol/12-μl sample); P > 0.05. (C) Effects of systemic administration of 20 mg/kg i.p. citalopram in Dbh^–/– (n = 5; baseline = 3.32 ± 0.18 fmol/12-μl sample) and Dbh^+/– mice (n = 7; baseline = 4.75 ± 0.08 fmol/12-μl sample). ANOVA indicated a significant genotype × time interaction (F(9, 90) = 4.60, P < 0.001). Data are expressed as mean (fmol/12-μl sample) + 1 SEM.