Antidepressant, anxiolytic and procognitive effects of subacute and chronic ketamine in the chronic mild stress model of depression

Over the past decades, conventional antidepressant drugs have shown limited gains in overall efficacy and remain slow to act, typically requiring 4–6 weeks for clinical benefit. The discovery that a single infusion of the NMDA-receptor antagonist ketamine produces rapid antidepressant effects in some treatment‑resistant patients prompted extensive preclinical and clinical investigation. In animal work using the chronic mild stress (CMS) model of depression, which models anhedonia (decreased responsiveness to reward) and also produces anxiogenesis and cognitive deficits, prior studies have largely examined single or brief ketamine treatments given after withdrawal from stress and have reported rapid but short-lived antidepressant-like effects. Papp and colleagues designed the present study to address four gaps in that literature. They sought to determine whether repeated daily ketamine treatment could sustain antidepressant-like effects during ongoing stress, to characterise dose–response relationships, to compare the time of onset of ketamine effects with conventional antidepressant treatment (imipramine), and to test whether chronic or subacute ketamine could reverse CMS‑induced anxiety and cognitive deficits. Two experiments are reported: one exploring a range of ketamine doses over chronic administration, and a second comparing subacute versus chronic treatment with a single dose on behavioural endpoints including sucrose intake, elevated plus maze (EPM) performance and novel object recognition (NORT).

Male Wistar rats were acclimated for 1 month and housed singly under standard laboratory conditions. Animals were trained to consume 1% sucrose solution under 14 h food/water deprivation and baseline sucrose intake was measured across seven 1‑h tests. Rats were then matched by final baseline sucrose intake and allocated to control or CMS groups. The CMS regimen continued throughout drug treatment and comprised a weekly schedule of intermittent stressors applied individually and continuously (10–14 h each); these included two periods each of food or water deprivation, two periods of 45° cage tilt, two periods of intermittent illumination (lights switched every 2 h), two periods of soiled bedding, one period of paired housing, two periods of low‑intensity stroboscopic illumination (150 flashes/min) and three no‑stress periods. Control animals were housed separately and only experienced the 14 h deprivation before each sucrose test. Behavioural outcome measures were: sucrose intake (as an index of anhedonia), elevated plus maze (EPM) measures of anxiety (open arm entries and time), the novel object recognition test (NORT) for recognition memory (discrimination index = novel exploration / total exploration × 100), and locomotor activity measured with an infrared beam actophotometer (distance travelled in 15 min). NORT exploration was scored by an observer blinded to treatments. Drugs (ketamine and imipramine) were dissolved in 0.9% saline and administered intraperitoneally at 1 ml/kg. Experiment 1 tested chronic daily ketamine across a dose range (5, 10, 15, 30 mg/kg), imipramine (10 mg/kg) and vehicle in subgroups of control and CMS animals (n = 8 per subgroup). Drug treatment began after 2 weeks of CMS and continued for 5 weeks, with a further week of CMS during drug withdrawal; weekly sucrose tests were performed. EPM and locomotor testing occurred 24 h after the final injection. Experiment 2 compared subacute (5 days) versus chronic (5 weeks) treatment with ketamine 10 mg/kg or vehicle in matched cohorts (n = 8 per subgroup). In this experiment the NORT adaptation and test sessions were scheduled relative to the penultimate sucrose test so that the final sucrose test, EPM and NORT occurred after 3, 4 and 5 ketamine injections in the subacute cohort and in the corresponding weeks of the chronic cohort. Statistical analysis used ANOVA models appropriate to each design: three‑way ANOVA for sucrose intake (stress × drug × week), two‑way ANOVAs for EPM in experiment 1, and models adding the within‑subjects factor treatment duration (subacute/chronic) in experiment 2; significant interactions were followed by lower‑order ANOVAs and planned comparisons.

Experiment 1: Two weeks of CMS induced a reliable decrease in sucrose intake. A three‑way stress × drug × weeks interaction was reported [F(30,504) = 2.28, P < 0.001]. Imipramine (10 mg/kg) progressively increased sucrose intake in CMS animals with a visually apparent onset at 2 weeks and statistical significance by week 3 [F(6,84) = 4.33, P < 0.001]. Ketamine showed a dose‑dependent profile. At 5 mg/kg there was a significant effect [F(6,84) = 4.53, P < 0.001] but only apparent at the fifth week of treatment. Ketamine 10 mg/kg produced a faster effect, with an increase visible at week 1 and significant by week 2 [F(6,84) = 4.07, P < 0.001]; an analysis restricted to baseline and week 1 data also showed a significant week‑1 effect [F(1,14) = 5.12, P < 0.05]. Higher doses (15 and 30 mg/kg) did not restore sucrose intake (15 mg/kg: F(6,84) = 1.44, NS; 30 mg/kg: F(6,84) = 0.89, NS); at 30 mg/kg ketamine reduced sucrose intake in control animals. In the EPM, significant drug effects on open arm measures were found overall [F(5,84) = 3.12, P < 0.02; F(5,84) = 4.17, P < 0.002], and these were significant within the CMS group [F(5,42) = 4.77, P < 0.002; F(5,42) = 4.89, P < 0.001], with drug‑treated CMS animals showing increased open arm scores relative to CMS vehicle. Closed‑arm entries (locomotion) and actophotometer beam breaks did not show significant CMS or drug effects (actophotometer stress: F(1,84) = 3.09, P = 0.083). Experiment 2: Chronic ketamine (10 mg/kg) again induced a time‑dependent recovery of sucrose intake in CMS animals, with a three‑way stress × drug × weeks interaction [F(6,168) = 2.76, P < 0.02], and strong stress × weeks and drug × weeks effects in the ketamine subgroups [F(6,84) = 9.63, P < 0.001; F(6,84) = 8.35, P < 0.001]. CMS‑ketamine intake was significantly higher than CMS‑vehicle from week 1 onward and was not different from control‑ketamine; the effect persisted one week after drug withdrawal. In the subacute cohort there was no clear effect at day 3 (the 3‑day test was not significant [F(1,28) = 0.42, NS]) and the CMS‑ketamine group consumed less sucrose than control‑ketamine at that point [F(1,14) = 5.48, P < 0.05], consistent with an onset around 7 days rather than within 1–2 days. EPM findings in experiment 2 mirrored experiment 1: CMS decreased open arm entries/time and ketamine 10 mg/kg fully reversed these decreases in both subacute and chronic cohorts, with a significant stress × drug interaction [open arm entries: F(1,56) = 9.54, P < 0.005; open arm time: F(1,56) = 8.57, P < 0.005], and no significant interaction with cohort (subacute versus chronic). Closed arm entries were slightly decreased by ketamine [F(1,56) = 4.52, P < 0.05] but overall locomotor confounds were minimal. In the NORT, CMS abolished object discrimination, reducing the discrimination index to near 50%, and ketamine 10 mg/kg fully restored discrimination after both subacute (5‑day) and chronic (5‑week) treatment [stress: F(1,56) = 10.45, P < 0.002; stress × drug interaction: F(1,56) = 12.17, P < 0.001], with no significant cohort interaction. Ketamine induced a small decrease in total exploration time [F(1,56) = 6.31, P < 0.02], marginally lower in the chronic cohort, but this did not account for the restoration of discrimination. Overall, daily ketamine produced sustained antidepressant‑like, anxiolytic and procognitive effects in the CMS model, with an apparent optimal dose of 10 mg/kg and limited locomotor confounding.

Papp and colleagues interpret their findings as demonstrating that daily ketamine treatment can sustain antidepressant‑like effects in the CMS model during continued stress exposure, with an optimal dose of 10 mg/kg and an onset of action faster than imipramine but slower than some reports of single‑dose ketamine effects. Both experiments showed that ketamine reversed CMS‑induced anhedonia (sucrose intake), anxiety‑like behaviour in the EPM and cognitive impairment in the NORT; the procognitive and anxiolytic effects were evident after brief treatment, whereas the antianhedonic effect emerged more slowly (approximately 1 week at 10 mg/kg, and later at 5 mg/kg). The authors note methodological differences from prior preclinical studies that may account for the later onset observed here: previous studies typically tested after single injections and in a stress‑free interval, whereas the present protocol maintained CMS throughout drug administration and testing and did not assess outcomes within hours of the first dose. They also highlight the dose dependency observed, confirming 10 mg/kg as optimal in rats, with reduced efficacy at 15–30 mg/kg; this loss of efficacy was not readily explained by sedation, since locomotor measures were largely unaffected. The report emphasises the surprising finding of a procognitive effect, given many reports of ketamine‑induced cognitive deficits, and cites prior animal and clinical observations that cognition can improve following repeated or delayed assessments post‑ketamine. The authors discuss potential neural circuitries that might mediate differential domains of effect—speculative suggestions include prefrontal–amygdala pathways for anxiolysis, multisynaptic routes to nucleus accumbens for anhedonia, and prefrontal–ventral hippocampal projections for cognitive changes—but they label such mechanisms as tentative and in need of further research. Finally, the investigators place their results in clinical context: while ketamine shows rapid antidepressant efficacy, clinical effects are transient, and there are concerns about maintenance, delivery, side effects and abuse potential. Papp and colleagues argue that their paradigm of prolonged daily treatment under ongoing stress, combined with a battery assessing multiple functional domains, may offer greater translational value for studying strategies to sustain therapeutic benefit and for evaluating alternative glutamatergic agents. They acknowledge limitations inherent in differences from prior studies (notably the lack of early‑hours assessment after the first dose and the NORT being tested at a single ketamine dose) and call for further work to map circuitry and to test generalisability across doses and models.