Repeated ketamine exposure induces an enduring resilient phenotype in adolescent and adult rats

Abstract

Background: Major depressive disorder afflicts up to 10% of adolescents. However, nearly 50% of those afflicted are considered nonresponsive to available treatments. Ketamine, a noncompetitive N-methyl-D-aspartate receptor antagonist has shown potential as a rapid-acting and long-lasting treatment for major depressive disorder in adults. Thus, the effectiveness and functional consequences of ketamine exposure during adolescence were explored.

Methods: Adolescent male rats (postnatal day [PD] 35) received two ketamine (0, 5, 10, or 20 mg/kg) injections, 4 hours apart, after exposure to day 1 of the forced swim test (FST). The next day, rats were reexposed to the FST to assess ketamine-induced antidepressant-like responses. Separate groups were exposed to chronic unpredictable stress to confirm findings from the FST. After these initial experiments, adolescent naive rats were exposed to either 1 or 15 consecutive days (PD35-49) of ketamine (20 mg/kg) twice daily. Ketamine's influence on behavioral reactivity to rewarding (i.e., sucrose preference) and aversive (i.e., elevated plus-maze, FST) circumstances was then assessed 2 months after treatment. To control for age-dependent effects, adult rats (PD75-89) were exposed to identical experimental conditions.

Results: Ketamine (20 mg/kg) reversed the chronic unpredictable stress-induced depression-like behaviors in the FST. Repeated ketamine exposure resulted in anxiolytic- and antidepressant-like responses 2 months after drug exposure. None of the ketamine doses used were capable of inducing drug-seeking behaviors as measured by place preference conditioning.

Conclusions: Repeated ketamine exposure induces enduring resilient-like responses regardless of age of exposure. These findings point to ketamine, and its repeated exposure, as a potentially useful antidepressant during adolescence.

Keywords: Adolescence; anxiety; depression; ketamine; rats; resilience; stress.

Figure 1

Effects of ketamine (0, 5, 10, or 20 mg/kg) on behavioral despair as measured in the forced swim test (FST) in adolescent (PD35) male rats 24 hours after 1-day (i.e., two injections, 4 hours apart) of ketamine (n=8/dose). (A) Ketamine significantly increased latency to become immobile regardless of dose (p<0.05, respectively). (B) Only the 20 mg/kg ketamine dose significantly reduced total immobility (p<0.05). (C) Ketamine (10 and 20 mg/kg) significantly reduced immobility and increased swimming counts, whereas only 20 mg/kg significantly increased climbing counts when compared to controls. Data are presented as latencies to become immobile and total immobility (in seconds) and as cumulative 5-second intervals of immobility, swimming, and climbing counts (mean ± SEM). *Significantly different from saline-treated controls (p<0.05).

Figure 2

Effects of chronic unpredictable stress (CUS) and ketamine's (20 mg/kg) antidepressant efficacy in adolescent rats (n=8–12/group). (A) Exposure to CUS reduced weight gain across days. (B) Levels of circulating serum corticosterone (CORT) were assessed in control (i.e., no stress) and CUS-exposed adolescent rats 72 hours after receiving saline or ketamine (20 mg/kg), and in controls exposed to stress for 5 minutes immediately before blood collection. Serum CORT levels were significantly elevated by acute as well as CUS exposure, and were not affected by ketamine treatment (p<0.05). (C) Exposure to CUS significantly reduced adolescent rats' preference for sucrose (p<0.05). (D) No changes in total liquid intake were observed. (E) CUS significantly reduced rats' latency to become immobile (p<0.05), whereas a single ketamine injection (20 mg/kg) reversed the CUS-induced deficit back to control levels (p>0.05). (F) Exposure to CUS significantly increased total immobility (p<0.05) while a single ketamine injection (20 mg/kg) reduced total immobility back to control levels (p>0.05). Data are presented as latencies to become immobile and total immobility (in seconds; mean ± SEM). CUS + SAL, chronic unpredictable stress and saline; CUS + KET, chronic unpredictable stress and ketamine. *Significantly different from saline-treated controls (p<0.05). ^εSignificantly different from CUS + ketamine-treated group (p<0.05).

Figure 3

Immediate effects of a single injection of ketamine (20 mg/kg) on distance traveled in adolescent and adult rats. (A) Adolescent rats treated with ketamine (n=19) had significantly increased locomotor activity during the first 15 minutes immediately after a single ketamine exposure when compared to saline-treated adolescent controls (n=20; p<0.05). Their total distance traveled was also significantly higher than controls (p<0.05). (B) Ketamine treated adults (n=19) exhibit no differences in distance traveled across time or total distance traveled compared to their respective saline-treated controls (n=20). Data are represented as mean distance traveled (mean ± SEM, in cm). *Significantly different from saline-treated controls (p<0.05).

Figure 4

Repeated exposure to ketamine (20 mg/kg, twice daily) disrupts normal weight gain and average daily food intake of adolescent- and adult-treated rats (n=10/group). (A) Repeated ketamine from postnatal days (PD) 35–49 reduced weight gain on days 9 and 11 of treatment (p<0.05) in adolescent rats. Average daily chow intake during and 15 days after treatment was significantly reduced in ketamine-treated adolescents. (B) Adults (PD75+) repeatedly treated with ketamine also show reduced weight gain on days 4, 6, and 8–10 (p<0.05). Ketamine-treated adults daily intake was also significantly reduced when compared to saline-treated controls (p<0.05). Data are represented as body weight in grams and average daily intake across days in grams (mean ± SEM). Shaded area indicates ketamine treatment days. *Significantly different from saline-treated controls (p<0.05).

Figure 5

Effects of 1 and 15 days of ketamine exposure (20 mg/kg; twice daily) on anxiety-like behavior. (A–B) One day of ketamine treatment did not produce changes in anxiety-like behaviors as measured in the elevated-plus maze (EPM) regardless of age of exposure (n=10/group). (C–D) Conversely, repeated exposure to ketamine (20 mg/kg), twice daily, induced significant increases in time spent in the open arms of the EPM in both adolescent-and adult-treated rats (p<0.05; n=10/group). Data are presented as percent time spent (mean ±DSEM) in the open arms of the EPM. *Significantly different from saline-treated controls (p<0.05).

Figure 6

Lasting effects of repeated (15 days) exposure to ketamine (20 mg/kg, twice daily) on behavioral despair using the forced swim test (FST) paradigm, 2 months after drug exposure, in adolescent (A–C) and adult (D–F) rats. Adolescent (PD35–49; n=12/group) rats show significantly increased latencies to immobility (A), lower total immobility (B), decreased immobility as well as higher swimming counts (C) when compared with saline-treated rats 2 months after drug exposure (p<0.05). Similarly treated adult rats (PD 75–89; n=11–12/group) also exhibited significantly increased latencies to immobility (D), lower total immobility (E), decreased immobility and increased swimming counts (F) 2 months after drug treatment (p<0.05). Data are presented as latencies to become immobile and total immobility (in seconds) and as cumulative 5-second intervals of swimming, climbing, and immobile counts (mean ± SEM). *Significantly different from saline-treated rats (p<0.05).