Ketamine anesthesia enhances fear memory consolidation via noradrenergic activation in the basolateral amygdala

Morena and colleagues frame the study around clinical observations that trauma patients who receive ketamine during emergency care show exacerbated early post-traumatic stress reactions, a predictor of later post-traumatic stress disorder (PTSD). Ketamine, widely used as an intravenous anaesthetic, is primarily an NMDA receptor antagonist but also has sympathomimetic properties: it increases heart rate and blood pressure, blocks peripheral and central adrenaline/noradrenaline reuptake, and elevates firing of locus coeruleus (LC) noradrenergic neurons. Because noradrenergic activation—driven peripherally by adrenal adrenaline and centrally by the LC—promotes memory consolidation in the basolateral amygdala (BLA) during stress, the investigators hypothesised that ketamine’s sympathomimetic action might potentiate consolidation of traumatic memories when administered in the immediate post-trauma period. The study set out to test whether ketamine administered immediately after, versus several hours after, an aversive training enhances memory consolidation in rats, and to determine whether peripheral and/or central adrenergic mechanisms mediate any such effects. Specifically, the experiments probed the roles of the adrenal medulla, the LC, systemic β-adrenoceptor blockade, and local β-adrenoceptor blockade within the BLA in modulating ketamine’s effects on inhibitory avoidance memory retention.

Adult male Sprague–Dawley rats (n = 206) were used and maintained under standard laboratory conditions. Behavioural experiments were conducted during the light phase. The primary behavioural assay was a single-trial inhibitory avoidance task: rats entered a dark compartment and received a single inescapable footshock (0.35 mA for non-cannulated animals; higher intensities for cannulated groups). Retention was tested 48 hours later by measuring latency to re-enter the shock compartment (cut-off 600 s). Longer latencies were interpreted as stronger memory retention. Ketamine was administered intraperitoneally (i.p.) at an anaesthetic dose of 125 mg/kg either immediately after training or 3 h later to test time-dependence. Propranolol was given systemically (2 mg/kg i.p.) 30 min before training to probe β-adrenoceptor involvement, and in a separate experiment propranolol was infused bilaterally into the BLA (0.5 μg/0.2 μl per side) immediately after training. To test central noradrenergic contribution, the LC was temporarily inactivated by bilateral lidocaine infusions (4% wt/vol; 0.5 μl/side) immediately post-training. Peripheral noradrenergic signalling was tested via adrenal medullectomy (ADMX), with animals allowed five weeks’ recovery and supplied with 0.50% saline for one week post-surgery to mitigate corticosteroid deficiency effects. Stereotaxic surgery was used to implant bilateral guide cannulae 2 mm above either the BLA or LC; placements were later verified histologically and six rats were excluded for misplacement. Sleep-related measures of anaesthesia (onset and duration of loss of righting reflex) were recorded after ketamine injection to assess whether anaesthetic sleep parameters correlated with memory effects. The investigator conducting and analysing behavioural tests was blind to treatment condition. Statistical analyses included paired t-tests to confirm learning, unpaired t-tests, two-way ANOVAs with Tukey–Kramer post-hoc tests where appropriate, and Pearson correlations. Data are presented as mean ± SEM and p < 0.05 was considered significant.

Immediate, but not delayed, ketamine enhances 48-h aversive memory. Replicating earlier findings, rats given ketamine (125 mg/kg i.p.) immediately after inhibitory avoidance training showed significantly longer 48-h retention latencies than vehicle controls (unpaired t14 = 2.51, P = 0.03). When ketamine was administered 3 h after training there was no significant difference in retention compared with vehicle (t14 = 1.47, P = 0.16). Training latencies did not differ between treatment groups in either timing condition, indicating no pre-existing behavioural differences. Adrenal medullary catecholamines are required for ketamine’s effect. In animals subjected to adrenal medullectomy (ADMX), the memory-enhancing effect of immediate post-training ketamine was absent. Two-way ANOVA on retention latencies showed main effects of ketamine (F1,35 = 9.54, P = 0.004) and ADMX (F1,35 = 12.86, P = 0.001) with a significant interaction (F1,35 = 7.49, P = 0.01). Post-hoc tests indicated that sham-operated rats treated with ketamine had higher retention latencies than both sham+vehicle and ADMX+ketamine groups (P < 0.001), while vehicle-treated sham and ADMX rats did not differ. Systemic β-adrenoceptor blockade prevents ketamine’s memory potentiation. Propranolol (2 mg/kg i.p.) given before training blocked the memory-promoting effect of immediate ketamine. Two-way ANOVA for retention latencies revealed a propranolol main effect (F1,42 = 9.05, P = 0.004) and a ketamine × propranolol interaction (F1,42 = 11.42, P = 0.002); the ketamine main effect alone was not significant (F1,42 = 2.87, P = 0.10). Post-hoc comparisons showed ketamine alone increased retention relative to vehicle and to ketamine+propranolol (P < 0.01 and P < 0.001, respectively). Propranolol alone did not differ from vehicle. An intact LC is necessary for the effect. Temporary LC inactivation with lidocaine abolished ketamine’s enhancement of retention. Two-way ANOVA showed significant effects of ketamine (F1,43 = 4.84, P = 0.03) and LC lesion (F1,43 = 5.25, P = 0.03); post-hoc tests indicated sham-lesioned rats given ketamine had higher retention than both sham+vehicle and LC-lesioned+ketamine groups (P < 0.05). Sham-lesioned and LC-lesioned vehicle groups did not differ. β-Adrenoceptor activation within the BLA mediates ketamine’s effect. Bilateral intra-BLA propranolol infusion immediately after training prevented the ketamine-induced enhancement of retention. Two-way ANOVA revealed main effects of ketamine (F1,38 = 6.11, P = 0.02) and propranolol (F1,38 = 7.63, P = 0.009) and a ketamine × propranolol interaction (F1,38 = 4.58, P = 0.04). Post-hoc tests showed ketamine alone elevated retention relative to vehicle (P < 0.05) and to ketamine+intra-BLA propranolol (P < 0.01); propranolol alone did not differ from vehicle. Ketamine-induced anaesthesia parameters do not account for memory effects. Measures of ketamine-induced sleep onset and duration did not differ between experimental conditions within each experiment (all reported unpaired t-tests non-significant), and Pearson correlations between sleep measures and 48-h retention latencies were non-significant. This supports that the memory-enhancing effects are not explained by differences in anaesthetic sleep properties.

Morena and colleagues interpret the data as evidence that ketamine administered immediately after a stressful event enhances consolidation of aversive memory through combined peripheral and central sympathomimetic actions that culminate in β-adrenoceptor activation within the BLA. The investigators note that ketamine’s inhibition of noradrenaline reuptake and its capacity to raise LC firing can increase noradrenergic signalling both peripherally and centrally, and that peripheral adrenaline can drive central noradrenaline release via vagal pathways to brainstem nuclei. Consistent with this model, elimination of adrenal catecholamine release (via ADMX), interruption of central noradrenergic signalling (temporary LC inactivation), systemic β-adrenoceptor blockade, or selective β-adrenoceptor blockade in the BLA each prevented ketamine’s enhancement of 48-h inhibitory avoidance retention. The authors stress the time-dependence of the effect: ketamine enhanced memory only when given immediately, not 3 h, after training, aligning with the short-lived noradrenergic surge in the BLA following stress (reported as less than 1 hour). They also address and discount the alternative explanation that changes in sleep induced by anaesthesia accounted for the results, since sleep onset/duration did not differ across treatment conditions and did not correlate with retention. Positioning their findings relative to prior work, the investigators acknowledge discrepant reports in the literature—especially those using sub-anaesthetic ketamine doses or different timings, species, strains and paradigms—but note that clinical reports often associate peri-trauma ketamine with increased acute PTSD symptoms. They propose a dual, time-dependent profile for ketamine: when given soon after trauma as an anaesthetic it may facilitate maladaptive memory consolidation and raise PTSD risk, whereas ketamine administered at different times or at sub-anaesthetic doses can have therapeutic effects in established PTSD and depression. Translationally, the authors suggest that combining ketamine with agents that reduce noradrenergic activation, such as β-blockers or α2-adrenoceptor agonists (for example dexmedetomidine), might mitigate ketamine’s memory-enhancing effects and lower the risk of trauma-related disorders in emergency care settings. They present these implications cautiously as a basis for future clinical studies.

The study identifies a mechanism by which ketamine anaesthesia, when administered immediately after a traumatic experience, enhances consolidation of fear memory: a time-dependent, combined peripheral-central potentiation of the sympathetic/noradrenergic system that engages β-adrenoceptors in the basolateral amygdala. These findings provide a rationale for clinical investigations aimed at managing peri-trauma ketamine use—potentially by co-administering β-blockers or α2-adrenoceptor agonists—to reduce the risk of developing trauma-related disorders.