Corticosterone exposure augments sensitivity to the behavioral and neuroplastic effects of fluoxetine in C57BL/6 mice

Abstract

Both genetic background and pre-existing stress play critical roles in the effects of antidepressant drugs. The current studies showed this principal by demonstrating that exposure to the stress hormone corticosterone (CORT) allowed behavioral and neurogenic effects to emerge following chronic treatment with fluoxetine of C57BL/6 mice, a strain ordinarily resistant to these effects. Adult male mice were implanted subcutaneously with 21-day slow-release CORT pellets (10 mg) or placebo and then co-treated with 5 mg/kg fluoxetine (b.i.d., i.p.) or saline for 14 days. Animals were then assessed for approach behavior in the novelty-induced hypophagia (NIH) test, hippocampal cell proliferation, corticosteroid receptor expression, and CORT plasma levels. Co-treatment of CORT with fluoxetine significantly reduced approach behavior in the novel environment of the NIH test and increased hippocampal cell proliferation whereas fluoxetine given alone was ineffective. CORT given alone did not alter approach behavior in the novel environment and caused a smaller increase of cell proliferation. The CORT effect was blocked by adrenalectomy and was likely due to increased adrenal feedback. Cell proliferation in CORT-treated animals was associated with reduced mineralocorticoid, but not glucocorticoid, receptor mRNA expression. Although the pellets were advertised to release CORT for 21 days, plasma CORT levels were increased at 1 day after implantation but were not sustained when measured at 7 days or longer intervals. Nevertheless, the transient CORT increase was sufficient to induce long-lasting behavioral and molecular changes when followed by fluoxetine treatment. These studies warrant further investigation into the role of glucocorticoids and environmental stress as adjunctive facilitators of the response to antidepressants, especially for treatment-resistant patients.

Keywords: Anxiety; Corticosterone; Fluoxetine; Neurogenesis; Stress; Treatment resistance.

Fig. 1

Effects of 10 mg CORT pellet and fluoxetine treatment on weight gain. SAL = Saline, FLX = Fluoxetine, CORT = Corticosterone. Arrow denotes start of fluoxetine treatment. (A) Weight change over time in each treatment group. Placebo treated animals gained weight over time. CORT/SAL animals displayed inhibited weight gain whereas CORT/FLX animals showed normal weight gain after beginning FLX treatment. Symbols represent significant differences compared to day 1: a (PLACEBO/FLX, p < 0.05) b (PLACEBO/SAL, p < 0.01) c (CORT/FLX, p < 0.01) (B) Overall weight change (from day 1 to day 21) showed that fluoxetine treatment increased weight gain in both placebo and CORT treated animals (n = 9–10 per group). Data is depicted as mean ± SEM. ##p < 0.01 within placebo or CORT treated groups.

Fig. 2

Effects of 10 mg CORT pellet and fluoxetine treatment in the novelty induced hypophagia test. SAL = Saline, FLX = Fluoxetine, CORT = Corticosterone. A) Fluoxetine significantly reduced approach latency in the novel arena in animals exposed to CORT, but not placebo. B) There was no effect of treatment on amount consumed. C) In the home cage, CORT exposure significantly reduced latency to approach and D) increased the amount of food consumed (n = 8–10 per group). Data is depicted as mean ± SEM. #p < 0.05 within CORT treated groups, *p < 0.05 between placebo and CORT treated groups.

Fig. 3

Effects of 10 mg CORT pellet and fluoxetine treatment on hippocampal cell proliferation, CORT plasma levels, and hippocampal corticosteroid receptor expression. SAL = Saline, FLX = Fluoxetine, CORT = Corticosterone. A) Values are expressed as the number of BrdU-positive cells per 10,000 7-AAD events. Intact animals exhibited a significant increase in hippocampal cell proliferation after treatment with CORT. This effect was further augmented in CORT-exposed animals treated with fluoxetine. Fluoxetine had no effect in placebo treated animals (n = 14–18 per group). B) CORT treatment significantly reduced plasma CORT levels. Fluoxetine had no additive effect on CORT levels (n = 15–19 per group). C) CORT had no effect on hippocampal GR mRNA expression but (D) reduced hippocampal MR mRNA expression (n = 15–19 per group). Data is depicted as mean ± SEM. ###p < 0.001 within CORT treated groups, ***p < 0.001, *p < 0.05 between placebo and CORT treated groups.

Fig. 4

Effects of adrenalectomy or low dose CORT exposure on hippocampal cell proliferation. SAL = Saline, FLX = Fluoxetine, CORT = Corticosterone. A) Values are expressed as the number of BrdU-positive cells per 10,000 7-AAD events. In adrenalectomized animals exposed to 10 mg CORT pellets, fluoxetine produced an increase in proliferation (n = 7–9 per group. B). In intact animals exposed to 2.5 mg CORT pellets, hippocampal cell proliferation was increased, but not further augmented by fluoxetine (n = 9–10 per group). Data is depicted as mean ± SEM. #p < 0.05 within CORT treated groups, *p < 0.05 between placebo and CORT treated groups.

Fig. 5

Effects of CORT pellet on plasma CORT levels over time. Adrenalectomized animals implanted with 10 mg CORT pellets displayed significantly reduced CORT plasma levels after 7 days of treatment (n = 5–6 per group). Data is depicted as mean ± SEM. Absolute mean values are: 247.2 ± 43.92, 20.8 ± 7.02, 34.7 ± 24.44 and 37.6 ± 16.32 ng/ml ****p < 0.0001, ***p < 0.001 compared to day 1.