Preclinical evaluation of reconsolidation blockade by clonidine as a potential novel treatment for posttraumatic stress disorder

Abstract

Exposure to traumatic events can lead to posttraumatic stress disorder (PTSD). Current PTSD treatments typically only produce partial improvement. Hence, there is a need for preclinical research to identify new candidate drugs and to develop novel therapeutic approaches. Animal studies have indicated that fear memories can be weakened by blocking restabilization after retrieval, a process known as reconsolidation. Furthermore, evidence suggests that there are important alterations of the noradrenergic system in PTSD, and hence it may be of interest to study drugs that target this pathway. Here, we investigated the efficacy of clonidine, an α₂-adrenoreceptor agonist, to block reconsolidation in an animal model of persistent traumatic memories. Using an auditory fear conditioning paradigm in rats, we tested the efficacy of clonidine to weaken fear memory retention when administered systemically after retrieval. We evaluated dosage, number of treatments, and specificity in reconsolidation blockade. We found that postretrieval administration of clonidine disrupts fear-related memories in a dose-dependent manner and that two treatments are sufficient for maximal memory impairment. Furthermore, we determined that this effect is long lasting and specific to reconsolidation processes as shown by the selectivity to affect reactivated memories and the absence of spontaneous recovery and of postreactivation short-term memory impairment. Our results demonstrate the efficacy of systemic administration of clonidine following retrieval to persistently disrupt fear memory retention through reconsolidation blockade. This study provides important preclinical parameters for future therapeutic strategies involving clonidine to block reconsolidation as a novel treatment for PTSD symptoms.

Figure 1

Postreactivation administration of clonidine impairs reconsolidation of auditory fear memories. (a) Schematic of the experimental design. Rats received a single systemic injection of clonidine or its vehicle immediately after a reactivation session, and were tested for postreactivation long-term memory 1 day (PR-LTM) and 1 week later (PR-LTM 2). A dose of (b) 50 μg/kg (n=20), (c) 100 μg/kg (n=25) and (d) 200 μg/kg (n=20) was effective at impairing memory reconsolidation compared with the vehicle group (respectively, n=16, n=25, and n=20) as shown by an impaired conditioned response (freezing) at both time points. Bars represent mean±SEM freezing to the tone. Markers represent the mean±SEM freezing before the onset of the tone. Statistical significance: ^*p<0.05, ^**p<0.01, ^***p<0.001.

Figure 2

Clonidine does not impair retention of nonreactivated fear memories. (a) Schematic of the experimental design. Rats received a single systemic injection of clonidine (100 μg/kg) or its vehicle without a memory reactivation session and were tested for long-term memory retention 1 day (LTM) and 1 week later (LTM 2). (b) Clonidine-treated rats (n=12) showed a similar conditioned response (freezing) to the vehicle group (n=12) when tested 24 h or 1 week after injection. Bars represent mean±SEM freezing to the tone. Markers represent the mean±SEM freezing before the onset of the tone.

Figure 3

Postreactivation administration of clonidine does not impair short-term fear memories. (a) Schematic of the experimental design. Rats received a single systemic injection of clonidine (100 μg/kg) or its vehicle immediately after a reactivation session and were tested 4 h later for postreactivation short-term memory (PR-STM) and 1 day later for postreactivation long-term memory (PR-LTM). (b) Clonidine-treated rats (n=12) showed a similar conditioned response (freezing) to the vehicle group (n=9) when tested 4 h after reactivation, but reduced freezing behavior 1 day after injection. Bars represent mean±SEM freezing to the tone. Markers represent the mean±SEM freezing before the onset of the tone. Statistical significance: ^***p<0.001.

Figure 4

Postreactivation administration of clonidine does not impair the ability to learn new fear memories. (a) Schematic of the experimental design. After receiving a postreactivation injection of clonidine (200 μg/kg) or vehicle, and being tested for memory retention 1 day (PR-LTM) and 1 week later (PR-LTM 2), rats were conditioned to fear a different tone using a different auditory fear protocol. (b) Rats that previously received clonidine (n=12) showed intact fear behavior (freezing) compared with the vehicle-treated animals (n=12) when tested 1 day (test 1) or 1 week later (test 2). Bars represent mean±SEM freezing to the tone. Markers represent the mean±SEM freezing before the onset of the tone.

Figure 5

Two postreactivation clonidine treatments are sufficient to maximally impair fear memory retention. (a) Schematic of the experimental design. Rats received a systemic injection of clonidine (100 μg/kg) or its vehicle immediately after a reactivation session for 3 consecutive days and were tested for postreactivation long-term memory 1 day (PR-LTM) and 1 week later (PR-LTM 2). (b) Clonidine-treated rats (n=16) showed an impaired conditioned response (freezing) as compared with the vehicle group (n=15) at each test session. Memory disruption was observed after the first clonidine treatment and reached its maximum after two treatments at day 3. Bars represent mean±SEM freezing to the tone. Markers represent the mean±SEM freezing before the onset of the tone. Statistical significance: ^*p<0.05, ^**p<0.01, ^***p<0.001.