The Involvement of the Serotonergic System in Ketamine and Fluoxetine Combination-induced Cognitive Impairments in Mice

Abstract

Background: Glutamatergic N-methyl-D-aspartate (NMDA) receptors play vital roles in memory formation. Changes in the activity of these receptors influence memory processes. Ketamine is a noncompetitive NMDA receptor antagonist drug with promising mood-altering and pain-reducing effects in low doses. These effects are believed to be related to altered serotonergic transmission.

Methods: The present study investigated the involvement of the serotonergic system in low-dose ketamine administrations' effects on memory acquisition, consolidation, and retrieval processes. Sixty-four male BALB/c mice were used in this experiment and separated into 8t groups. Mice were treated subchronically with a selective serotonin reuptake inhibitor, fluoxetine, and a serotonin depletion agent, p-chlorophenylalanine (pCPA). A serotonin antagonist, methiothepin, and ketamine were acutely administered 60 minutes before or after the behavioral tests. A passive avoidance (PA) test measured emotional memory acquisition, consolidation, and retrieval processes. Hippocampi malondialdehyde (MDA) levels were analyzed, and histopathological examinations were performed.

Results: Ketamine alone did not significantly affect memory encoding processes in the PA test, while the ketamine-fluoxetine combination disrupted memory consolidation. Fluoxetine negatively affected the memory acquisition process, which was normalized during the consolidation and retrieval trials. Drug applications did not significantly alter hippocampal MDA levels. In all ketamine-applied groups, histopathologic alterations were evident.

Conclusion: Low-dose ketamine administration induces neurodegeneration, and it also impairs memory functions when combined with fluoxetine, indicating increased serotonergic transmission may be involved in the memory-impairing and neurotoxic effects of ketamine.

Figure 1.

Memory acquisition trial, step-through latencies in the passive avoidance test. VEH, vehicle; KET, ketamine (20 mg/kg); FLU, fluoxetine (20 mg/kg, 7 days); MET, methiothepin (0.1 mg/kg); pCPA, para-chlorophenylalanine (150 mg/kg, 4 days); KF, ketamine–fluoxetine combination; KM, ketamine–methiothepin combination; and KP, ketamine–pCPA combination. Each column represents the mean ± SEM of 7-8 mice. ^aP < .01 vs. VEH group, ^bP < .05 vs. VEH group, ^cP < .05 vs. KET group. One-way ANOVA followed by a post hoc Tukey test was used for statistical analysis.

Figure 2.

Memory consolidation trial, step-through latencies in the passive avoidance test. VEH, vehicle; KET, ketamine (20 mg/kg); FLU, fluoxetine (20 mg/kg, 7 days); MET, methiothepin (0.1 mg/kg); pCPA, para-chlorophenylalanine (150 mg/kg, 4 days); KF, ketamine–fluoxetine combination; KM, ketamine–methiothepin combination; KP, ketamine–pCPA combination. Each column represents the mean ± SEM of 7-8 mice. ^aP < .05 vs. VEH group, ^bP < .01 vs. KET group, ^cP < .05 vs. FLU group, and ^dP < .01 vs. VEH group. One-way ANOVA followed by a post hoc Tukey test was used for statistical analysis.

Figure 3.

Memory retrieval trial, step-through latencies in the passive avoidance test. VEH, vehicle; KET, ketamine (20 mg/kg); FLU, fluoxetine (20 mg/kg, 7 days); MET, methiothepin (0.1 mg/kg); pCPA, para-chlorophenylalanine (150 mg/kg, 4 days); KF, ketamine–fluoxetine combination; KM, ketamine–methiothepin combination; KP, ketamine–pCPA combination. Each column represents the mean ± SEM of 7-8 mice. One-way ANOVA was used for statistical analysis.

Figure 4.

A–H: Histopathologic micrographs of groups. Normal histologic appearance of the brain in VEH (A), FLU (C), MET (D), and pCPA (E) groups. Histopathologic appearances in KET (B), KF (F), KM (G), and KP (H) groups. Arrowhead: perineural and perivascular edema, arrow: necrosis, asterisk: satellitosis. Staining: hematoxylin and eosin, Bar: 50 µm (A, C, D, E), 20 µm (B, F, G, H).

Figure 5.

Hippocampal MDA levels of the mice. VEH, vehicle; KET, ketamine (20 mg/kg); FLU, fluoxetine (20 mg/kg, 7 days); MET, methiothepin (0.1 mg/kg); pCPA, para-chlorophenylalanine (150 mg/kg, 4 days); KF, ketamine–fluoxetine combination; KM, ketamine–methiothepin combination; KP, ketamine–pCPA combination. Each column represents the mean ± SEM of 7-8 mice’s hippocampal MDA levels. One-way ANOVA was used for statistical analysis.