The Effect Of Chronic Restraint Stress On Depression-Related Behaviour In The Forced Swim Test

AM Sadler, SJ Bailey. University of Bath, Bath, UK

Adolescent depression is a prevalent condition affecting up to 6% of 13-18 year olds (1). Antidepressant drugs clinically used to treat adult depression are associated with poor efficacy and increased suicidal behaviour when used in adolescents. As there are currently few animal models of adolescent depression, we are developing and validating a mouse model. Stress is consistently correlated with an increased risk of depression, both in adults and adolescents. Previously we have shown that 7 consecutive days of restraint stress (2 hours/day) increased both the time spent in and the distance travelled in the open arms of the elevated plus maze in both adult and juvenile mice (2). Here, we have investigated whether this apparently paradoxical anxiolytic effect of repeated stress is mirrored by changes in depression-related behaviour.Adult (9-10 weeks old) male BALB/cAnNCrl mice (Charles River, UK) were randomised into stressed and non-stressed groups (n=7-8/group for each timepoint). Stressed mice underwent either 3, 7 or 14 consecutive days of restraint (2 hours/day) in a restraint device. Non-stressed mice were gently handled each day. Blood samples were taken from the tail vein before and after restraint stress (3). Subsequently, mice were tested once in the forced swim test (6 min, 25°C). Test sessions were filmed, and time spent swimming, climbing and immobile in the last 4 minutes of the test were scored by an experimenter blind to treatment. The latency to the first period of immobility was also determined. Levels of corticosterone in the plasma at baseline and post stress were determined using an ELISA (IBL International). Behavioural data were analysed using unpaired t-tests, and corticosterone data were analysed using a two-way repeated measures mixed model analysis with Bonferroni correction for multiple comparisons (InVivoStat software).The 2h restraint was confirmed as stressful as plasma corticosterone was significantly elevated above baseline after 3 (390 ± 65ng/ml vs. 43 ± 4.8ng/ml, P<0.005), and 7 (360 ± 55ng/ml vs. 61 ± 13ng/ml, P<0.05) days stress. In the forced swim test, after 3 days stress, there was a trend for stressed mice to spend less time immobile (130 ± 11s (stressed) vs. 160 ± 12s (control), P=0.09) and more time swimming (110 ± 11s (stressed) vs. 77 ± 12s (control), P=0.09) than controls. After 7 days restraint stress there was no significant effect on the time spent immobile (160 ± 5.6s (stressed) vs. 150 ± 13s (control)), or swimming (79 ± 5.6s (stressed) vs. 90 ± 13s (control)). but there was a significant increase in the latency to first immobility (49 ± 6.7s (stressed) vs. 27 ± 3.5s (control), P<0.05). After 14 days stress, habituation of the corticosterone response was evident (150 ± 26ng/ml (stressed) vs. 57 ± 9.0ng/ml (control), P>0.05) and there was no change in the time spent immobile (140 ± 12s (stressed) vs. 150 ± 9.2s (control)), or swimming (100 ± 12s (stressed) vs. 90 ± 9.1s (control)).These data suggest that chronic restraint stress for 3 or 7 days induces an antidepressant-like effect in the forced swim test, consistent with our previous findings in the elevated plus maze (Sadler & Bailey, 2013a). These behavioural changes may reflect resilience on repeated exposure to stress, coupled with habituation of the corticosterone response. We are currently evaluating the depression-related behavioural effects of chronic restraint stress in juvenile mice.

(1) Masi G et al, Expert Opin Pharmacother, 11:375, 2010

(2) Sadler AM and Bailey SJ, J. Psychopharmacol, 27:A17, 2013

(3) Sadler AM and Bailey SJ, Lab Anim, 4:316, 2013