Post-weaning Social Isolated Flinders Sensitive Line Rats Display Bio-Behavioural Manifestations Resistant to Fluoxetine: A Model of Treatment-Resistant Depression

Abstract

Treatment-resistant depression (TRD) complicates the management of major depression (MD). The underlying biology of TRD involves interplay between genetic propensity and chronic and/or early life adversity. By combining a genetic animal model of MD and post-weaning social isolation rearing (SIR), we sought to produce an animal that displays more severe depressive- and social anxiety-like manifestations resistant to standard antidepressant treatment. Flinders Sensitive Line (FSL) pups were social or isolation reared from weaning [postnatal day (PND) 21], receiving fluoxetine (FLX) from PND 63 (10 mg/kg × 14 days), and compared to Sprague Dawley (SD) controls. Depressive-, anxiety-like, and social behaviour were assessed from PND 72 in the forced swim test (FST) and social interaction test (SIT). Post-mortem cortico-hippocampal norepinephrine (NE), serotonin (5-HT), and dopamine (DA), as well as plasma interleukin 6 (IL-6), tumour necrosis factor alpha (TNF-α), corticosterone (CORT), and dopamine-beta-hydroxylase (DBH) levels were assayed. FSL rats displayed significant cortico-hippocampal monoamine disturbances, and depressive- and social anxiety-like behaviour, the latter two reversed by FLX. SIR-exposed FSL rats exhibited significant immobility in the FST and social impairment which were, respectively, worsened by or resistant to FLX. In SIR-exposed FSL rats, FLX significantly raised depleted NE and 5-HT, significantly decreased DBH and caused a large effect size increase in DA and decrease in CORT and TNF-α. Concluding, SIR-exposed FSL rats display depressive- and social anxiety-like symptoms that are resistant to, or worsened by, FLX, with reduced plasma DBH and suppressed cortico-hippocampal 5-HT, NE and DA, all variably altered by FLX. Exposure of a genetic animal model of MD to post-weaning SIR results in a more intractable depressive-like phenotype as well as changes in TRD-related biomarkers, that are resistant to traditional antidepressant treatment. Given the relative absence of validated animal models of TRD, these findings are especially promising and warrant study, especially further predictive validation.

Keywords: bipolar disorder; gene-environment model; inflammation; monoamines; risk and resilience; social anxiety.

Figure 1

Study design.

Figure 2

(A) Mean cumulative weight in SD, FSL, and FSL-SIR rats following treatment with SAL or FLX. ^xxxp < 0.0001 vs. SD-SAL; ^###p < 0.0001, ^##p < 0.01 vs. FSL-SAL; ^∧∧∧p < 0.0001 vs. FSL-FLX. (B) Rate of weight gain during the treatment period. ^###p < 0.0001 vs. FSL-SAL. Data are represented as the mean of 12 animals. Data were analysed using two-way ANOVA followed by Bonferroni post hoc test. Data are presented as mean ± SEM. Precise p–values are presented in the text. FSL, Flinders' Sensitive Line; FLX, fluoxetine; SAL, saline; SD, Sprague-Dawley; SIR, social isolation rearing.

Figure 3

(A) Locomotor activity measured as distance travelled in the open field test (OFT). ^xp < 0.05 vs. SD-SAL; ^aad = 1.5 vs. SD-SAL, ^ccd = 0.9 vs. FSL-SAL; ^bbd = 1.0 vs. FSL-SIR-SAL. (B-D) Immobility and swimming and climbing behaviours as measured in the forced swim test (FST). (B) Immobility (s). ^xxxp < 0.0001, ^xxp < 0.01 vs. SD-SAL; ^##p < 0.01 vs. FSL-SAL; ^∧∧p < 0.01 vs. FSL-FLX; ^ad = 1.1 vs. FSL-SAL. (C) Swimming (s). ^xxxp < 0.0001, ^xp < 0.05 vs. SD-SAL; ^###p < 0.0001, ^##p < 0.01, ^#p < 0.05 vs. FSL-SAL; ^bd = 1.5 vs. FSL-FLX. (D) Climbing (s). ^xxp < 0.01 vs. SD-SAL; ^cd = 1.2 vs. SD-SAL, ^fd = 1.0 vs. FSL-SAL, ^ed = 1.0 vs. FSL-FLX. Data were analysed using two-way ANOVA followed by Bonferroni post hoc test and Cohen's d analysis. Data are presented as mean ± SEM. Detailed p–values in the text. FSL, Flinders' Sensitive Line; FLX, fluoxetine; SAL, saline; SD, Sprague-Dawley; SIR, social isolation rearing.

Figure 4

Social interactive behaviour as measured in the social interaction test (SIT). (A) Social (amicable) behaviour. ^xxxp < 0.0001, ^xp < 0.05 vs. SD-SAL; ^##p < 0.01 vs. FSL-SAL. (B) Asocial/socially anxious-like behaviour. ^xxp < 0.01, ^xp < 0.05 vs. SD-SAL; ^∧∧p < 0.01, ^∧p < 0.05 vs. FSL-FLX; ^ad = 1.7, ^hd = 1.1 vs. FSL-SAL. Data were analysed using two-way ANOVA followed by Bonferroni post hoc test and Cohen's d analysis and Student's t-test with Welch's correction. Data are presented as mean ± SEM. Detailed p–values in the text. FSL, Flinders' Sensitive Line; FLX, fluoxetine; SAL, saline; SD, Sprague-Dawley; SIR, social isolation rearing.

Figure 5

Monoamine levels in the frontal cortex (A,C,E) and hippocampus (B,D,F). (A) NE. ^xxxp < 0.0001 vs. SD-SAL; ^###p < 0.0001 vs. FSL-SAL; ^∧∧∧p < 0.0001 vs. FSL-FLX; ^°°°p < 0.0001 vs. FSL-SIR-SAL. (B) NE. ^xxxp < 0.0001, ^xxp < 0.01 vs. SD-SAL; ^##p < 0.01 vs. FSL-SAL; ^∧∧p < 0.01 vs. FSL-FLX; ^°°°p < 0.0001 vs. FSL-SIR-SAL. (C) 5-HT. ^xxp < 0.01 vs. SD-SAL; ^##p < 0.01 vs. FSL-SAL; ^∧∧∧p < 0.0001 vs. FSL-FLX; ^°°p < 0.01 vs. FSL-SIR-SAL. (D) 5-HT. ^xxxp < 0.0001, ^xxp < 0.01 vs. SD-SAL; ^#p < 0.05 vs. FSL-SAL; ^∧∧p < 0.01 vs. FSL-FLX, ^°°p < 0.01 vs. FSL-SIR-SAL. ^bd = 1.0 vs. SD-SAL. (E) DA. ^zd = 0.8 vs. SD-SAL; ^cd = 1.0 vs. FSL-SAL, ^ed = 3.5, ^fd = 2.0 vs. FSL-FLX, ^gd = 2.4 vs. FSL-SIR-SAL. (F) DA. ^∧p < 0.05 vs. FSL-FLX. ^jd = 1.3 vs. SD-SAL, ^kd = 1.5 vs. FSL-FLX; ^md = 1.1 vs. FSL-SIR-SAL. Data were analysed using two-way ANOVA followed by Bonferroni post hoc test and Cohen's d analysis. Data are presented as mean ± SEM. Detailed p–values in the text. DA, dopamine; FSL, Flinders' Sensitive Line; FLX, fluoxetine; NE, norepinephrine; SAL, saline; 5-HT, serotonin; SIR, social isolation rearing; SD, Sprague-Dawley.

Figure 6

Plasma biochemistry. (A) DBH. ^xp < 0.05 vs. SD-SAL; ^td = 1.9, ^rd = 2.1 vs. FSL-SAL; ^vd = 1.8, ^ud = 2.0 vs. FSL-FLX. (B) CORT. ^xp < 0.05 vs. SD-SAL; ^#p < 0.05 vs. FSL-SAL; ^yd = 0.8 vs. SD-SAL; ^zd = 0.8 vs. FSL-SAL; ^vd = 0.8 vs. FSL-FLX. (C) IL-6. No significant effects between cohorts were observed. (D) TNF-α. ^yd = 1.1 vs. SD-SAL; ^wd = 0.8 vs. FSL-SAL; ^zd = 0.9 vs. FSL-FLX. Data were analysed using two-way ANOVA followed by Bonferroni post hoc test and Cohen's d analysis. Data are presented as mean ± SEM. Detailed p–values in the text. CORT, corticosterone; DBH, dopamine-beta-hydroxylase; FSL, Flinders' Sensitive Line; FLX, fluoxetine; IL-6, interleukin 6; SAL, saline; SD, Sprague-Dawley; SIR, social isolation rearing; TNF-α, tumour necrosis factor-alpha.