Prophylactic Ketamine Attenuates Learned Fear

This saline-controlled rodent study evaluates whether ketamine (30 mg/kg) is effective in reducing fear or preventing fear reactivation using a contextual fear conditioning (CFC) paradigm. The study found ketamine as most useful in the clinic if administered in a prophylactic manner a week prior to a stressor to protect against increased fear responses to aversive stimuli.

Abstract

“Ketamine has been reported to be an efficacious antidepressant for major depressive disorder and posttraumatic stress disorder. Most recently, ketamine has also been shown to be prophylactic against stress-induced depressive-like behavior in mice. It remains unknown, however, when ketamine should be administered relative to a stressor in order to maximize its antidepressant and/or prophylactic effects. Moreover, it is unknown whether ketamine can be prophylactic against subsequent stressors. We systematically administered ketamine at different time points relative to a fear experience, in order to determine when ketamine is most effective at reducing fear expression or preventing fear reactivation. Using a contextual fear conditioning (CFC) paradigm, mice were administered a single dose of saline or ketamine (30 mg/kg) at varying time points before or after CFC. Mice administered prophylactic ketamine 1 week, but not 1 month or 1 h before CFC, exhibited reduced freezing behavior when compared with mice administered saline. In contrast, ketamine administration following CFC or during extinction did not alter subsequent fear expression. However, ketamine administered before reinstatement increased the number of rearing bouts in an open field, possibly suggesting an increase in attentiveness. These data indicate that ketamine can buffer a fear response when given a week before as prophylactic, but not when given immediately before or after a stress-inducing episode. Thus, ketamine may be most useful in the clinic if administered in a prophylactic manner 1 week before a stressor, in order to protect against heightened fear responses to aversive stimuli.“

Authors: Josephine C. McGowan, Christina T. LaGamma, Sean C. Lim, Melina Tsitsiklis, Yuval Neria, Rebecca A. Brachman & Christine A. Denny

Summary

INTRODUCTION

Posttraumatic stress disorder (PTSD) is an illness characterized by persistent, vivid re-experiencing of a traumatic event, hyperarousal, and avoidance of stimuli associated with the trauma.

Currently, clinicians use pharmacology, psychotherapy, or a combination of both methods to reduce the symptomology of PTSD, including selective serotonin reuptake inhibitors (SSRIs), benzodiazepines, and anti-adrenergic agents. However, consistent therapies have not been established for trauma victims.

Researchers are pursuing potential strategies to prevent the onset of PTSD and other psychiatric illnesses, such as major depressive disorder. Ketamine, an antagonist of the glutamate N-methyl-D-aspartate (NMDA) receptor and an activator of AMPA receptors, has been found to have several properties that highlight its candidacy as a preventative treatment.

Ketamine has been found to be efficacious in the treatment of chronic PTSD, but the exact timing of when it should be administered remains unknown.

In this study, mice administered prophylactic ketamine 1 week, but not 1 month, before CFC exhibited reduced freezing behavior when compared with mice administered saline. Ketamine administration following CFC or before extinction did not alter subsequent fear expression.

129S6/SvEvTac mice were housed four to five per cage in a 12 h (06:00 – 18:00 h) light – dark colony room at 22 °C.

Drugs

A single injection of ketamine (30 mg/kg) was administered once during the course of each experiment.

Contextual Fear Conditioning

Mice were placed in a conditioning chamber and received 3 shocks 180, 240, and 300 s later. They were removed 15 s following the last shock.

Extinction (E)

Two extinction protocols were piloted, and the protocol from Trouche et al (2013) was chosen as the most consistent and robust. Each group of mice was subject to two extinction trials per day, and freezing was scored for each trial.

Statistical Analysis

All data were analyzed using StatView 5.0 software or Prism 5.0a. ANOVA was used for all analyses, and Fisher’s protected least significant difference post-hoc analysis was used where appropriate.

An Injection of Ketamine 1 Week Before CFC Decreases Fear Expression

To determine if ketamine could buffer fear expression during CFC, mice were injected with ketamine 1 week before CFC and were trained using a three-shock CFC protocol. Ketamine-injected mice showed significantly decreased levels of fear during extinction training.

An Injection of Ketamine 1 Month Before CFC Does Not Alter Fear Expression

Ketamine may be administered at a defined interval before a stressor to buffer against fear expression, but not before a shock or during extinction.

An Injection of Ketamine 24 h Before CFC Does Not Alter Fear Expression

We injected ketamine 24 h before CFC training, and found that it changed the behavior of the mice on the last day of extinction.

Ketamine administration 1 h before CFC resulted in increased immobility before the shock presentation, but decreased travel during the shock presentations. Ketamine-injected mice froze comparably during extinction.

An Injection of Ketamine Following CFC or Before Extinction Does Not Alter Fear Expression

Ketamine injections did not alter fear extinction behavior following encoding, and ketamine injections 1 week before extinction did not alter fear expression during extinction.

An Injection of Ketamine 1 Week Before Reinstatement Increases Attentiveness, But Does Not Decrease Fear Expression

Ketamine injections 1 week before one-shock reinstatement of a stressor did not alter subsequent fear expression or alter behavior in other stressful paradigms. However, ketamine injections 1 week before reinstatement increased attentiveness as measured by the number of rearing bouts in an OF.

We next wanted to determine if ketamine’s effects on attentiveness were influenced by the strength of the reinstatement experience. A single injection of ketamine 1 week before three-shock reinstatement did not alter fear expression, or influence depressive-like or exploratory behavior in the FST and OF, respectively.

An Injection of Ketamine Following Reinstatement Decreases Fear Following Three-shock CFC, but not One-shock CFC

We administered ketamine following a one-shock reinstatement experience to determine whether it would influence subsequent fear expression.

Ketamine buffered fear expression during the first re-exposure and transiently decreased freezing throughout extinction, but did not alter subsequent fear expression 24 hours after three-shock reinstatement.

DISCUSSION

Here we show that ketamine is most effective when administered 1 week before the start of CFC to buffer against a heightened fear response during the first re-exposure to the aversive context. However, ketamine administered at various other time points does not facilitate extinction or alter subsequent behavior.

Ketamine prophylactically administered 1 week before CFC training protects against depressive-like behavior and fear expression. However, ketamine may impair the acquisition of fear or memory retrieval when administered at the 1-week time point.

Ketamine’s prophylactic efficacy is dependent on when it is administered before a fear-inducing stimulus, and it is unknown whether there is an intermediate time point between 24 h and 1 week that also allows for prophylactic efficacy.

Ketamine administered 1 h before CFC resulted in increased immobility and a blunted response to the shocks. Ketamine did not affect time immobile or the response to the shocks when administered 24 h before CFC.

Ketamine has been found to affect locomotor activity and memory, and it is possible that the increased freezing observed during CFC could indicate ketamine’s effect on heightening the memory of the aversive context.

In this study, we administered ketamine to rats 1 h after CFC, 1 week after CFC, or 1 h before extinction training, and found that ketamine was not effective in reducing the fear response.

Ketamine did not alter fear expression 1 week before a one-shock reinstatement paradigm, contrary to the effect of prophylactic ketamine 1 week before CFC. This suggests that previous stressors influence subsequent stressor responses.

Although ketamine administration did not affect locomotion in the OF, it increased the number of rearing bouts. This behavior has been suggested as a measure of attentive behavior, though it remains to be determined whether this behavior is a positive outcome.

Ketamine had no effect on fear expression or depressive-like behavior in the FST, but increased rearing behavior was observed. These data suggest that the severity of the stressor experienced may play a critical role in effectiveness of therapy.

Administering ketamine 1 h after a relatively weak reinstatement did not have effects on fear expression or extinction, but did decrease subsequent fear expression during extinction, indicating that ketamine may indeed be having an effect on memory consolidation. Ketamine’s effects on memory consolidation may be similar to those of propranolol, a beta-blocker, which is used to treat PTSD. However, ketamine may be more effective in conditions that robustly activate the fear system.

Ketamine may have a prophylactic effect on fear memory. Previous studies have shown that NMDA receptor antagonists have an effect on fear behavior, but the current study has shown that ketamine blocks both fear acquisition and expression.

This study suggests that ketamine may be most effective in buffering fear responsive when administered as a prophylactic 1 week before a stressor. However, the use of CFC as a measure of a deleterious fear response merits further discussion.

Ketamine may be most effective when administered in a vaccine-like manner, but only within a particular time window before the stressor. Ketamine treatment could also enhance attentiveness to one’s surroundings as long as 2 weeks following administration.

ACKNOWLEDGMENTS

We thank the Barnard Noyce Teacher Scholars Program, Columbia University Summer Undergraduate Research Fellowship, Columbia University Summer Program for Under-Represented Students, and the Brain & Behavior Research Foundation for support.