Angiotensin type 1 receptor inhibition enhances the extinction of fear memory

Abstract

Background: The current effective treatment options for posttraumatic stress disorder (PTSD) are limited, and therefore the need to explore new treatment strategies is critical. Pharmacological inhibition of the renin-angiotensin system is a common approach to treat hypertension, and emerging evidence highlights the importance of this pathway in stress and anxiety. A recent clinical study from our laboratory provides evidence supporting a role for the renin-angiotensin system in the regulation of the stress response in patients diagnosed with PTSD.

Methods: With an animal model of PTSD and the selective angiotensin receptor type 1 (AT1) antagonist losartan, we investigated the acute and long-term effects of AT1 receptor inhibition on fear memory and baseline anxiety. After losartan treatment, we performed classical Pavlovian fear conditioning pairing auditory cues with footshocks and examined extinction behavior, gene expression changes in the brain, as well as neuroendocrine and cardiovascular responses.

Results: After cued fear conditioning, both acute and 2-week administration of losartan enhanced the consolidation of extinction memory but had no effect on fear acquisition, baseline anxiety, blood pressure, and neuroendocrine stress measures. Gene expression changes in the brain were also altered in mice treated with losartan for 2 weeks, in particular reduced amygdala AT1 receptor and bed nucleus of the stria terminalis c-Fos messenger RNA levels.

Conclusions: These data suggest that AT1 receptor antagonism enhances the extinction of fear memory and therefore might be a beneficial therapy for PTSD patients who have impairments in extinction of aversive memories.

Keywords: Angiotensin receptor type 1 (AT(1)); PTSD; cardiovascular disease; fear memory; renin-angiotensin; stress.

Conflict of interest statement

CONFLICT OF INTEREST:

All authors report no biomedical financial interests or potential conflicts of interest.

Published by Elsevier Inc.

Figures

Figure 1. Angiotensin type 1 (AT1R) receptor inhibition enhances the extinction of learned fear

Prior to losartan treatment (pre-drug) acquired fear during cued fear conditioning with five tone-shock pairings in pre-vehicle (n = 10) and pre-losartan (n = 11) groups (A). 24hr following fear acquisition, losartan (1 mg/kg or 10 mg/kg i.p) was given prior to extinction training (B, E; n=20–25/group). 24 hr following extinction training, and in the absence of drug, mice were tested for extinction retention of learned fear, expressed as total average freezing and in blocks of 5 CS tones (C,D,F,G). *P < 0.05

Figure 2. Acute administration of losartan (10 mg/kg, i.p.) does not affect baseline blood pressure or anxiety measures

Minute analysis (every other minute) blood pressure measured by radiotelemetry following acute administration of saline (n = 4) or losartan (10 mg/kg, i.p n = 4) (A). Arrows represent time point of injection and start extinction training testing at minute 40 following injection of drug or vehicle. Distance travelled during open-field testing (B) and percent time in open arms of elevated plus maze test of vehicle (C) (n = 6–8/group).

Figure 3. Chronic inhibition of angiotensin type 1 (AT1R) receptor enhances extinction of learned fear

Following two weeks of losartan (10 mg/kg/day) (n = 17) or vehicle (n = 13) infusion, fear acquisition expressed as percent freezing during cued fear conditioning with five tone-shock pairings (A). Extinction training/fear expression was tested and total average freezing within the session (B) and represented in blocks of 5 CS tones (C) 24 hr following fear acquisition. Extinction retention of learned fear, expressed as total average freezing and in blocks of 5 CS tones are shown in panels (D) and (E) (n = 18–20). * P < 0.05

Figure 4. AT1 receptor mRNA gene expression following extinction training in losartan treated mice

AT1 receptor messenger RNA (mRNA) levels in the amygdala (A.) Prefrontal Cortex (B.) and Bed nucleus stria terminalis of vehicle (n = 7) or losartan infused mice (n = 7) (A). Example of coronal brain section with black circles designating the surrounding regions of the amygdala isolated for reverse transcriptase quantitative polymerase chain reaction (B) (reprinted from Paxinos and Franklin (26) with permission from Elsevier, copyright 2006). *P < 0.05

Figure 5. Messenger RNA (mRNA) levels in the bed nucleus stria terminalis (BNST) and prefrontal cortex (PFC) of vehicle or losartan infused mice following extinction training in losartan treated mice

qPCR-determined quantitative levels (fold changes) of mRNAs encoding BNST (A–C) and PFC (D–F) (n = 7–8) *P < 0.05.

Figure 6. Effect of chronic losartan on central and peripheral indices of stress activation

Plasma corticosterone levels (A) and plasma renin activity (B) in mice treated with vehicle or losartan for 2 weeks following acute restraint stress (n =9–10/group). Induction of c-Fos protein quantified as number of positive cell counts per section of paraventricular nucleus (C) (n = 5–7/group). Representative images showing reduced cFos protein induction in losartan versus vehicle animals exposed to acute restraint stress (D–E). *P < 0.05