Central Angiotensin Type 2 Receptor (AT 2 R) Stimulation Promotes Enhanced Extinction of Fear Learning Independent of Cardiovascular Measures

Angiotensin II receptor subtypes (AT 1 R and AT 2 R) are found in regions of the brain critical to the expression and extinction of conditioned fear, however the neurobiological role(s) remain unknown. We propose that brain angiotensin receptors contribute to modulating inhibitory and excitatory conditioned fear circuits, that maybe important in the pathology of posttraumatic stress disorder (PTSD). The objective of this study was to investigate the role of brain AT 2 R activation in fear memory. To study learned fear, C57BL/6 J mice (n= 6–10/group) underwent classical Pavlovian fear conditioning, pairing auditory cues with foot shocks. The neural circuitry underlying the fear response is highly conserved across mammalian species, making rodent models a valuable tool in the study of fear learning disorders, such as PTSD. The percentage of time spent freezing in response to a conditioned stimulus is used to quantify the ability to learn and remember fearful associations. Using this model, the expression of learned fear is tested 24 hours after conditioning, and memory retention one day later. Following the acquisition of fear, AT 2 R mRNA expression was elevated in the central amygdala (CeA) ( t (22) = 2.5; p<0.05), a critical region involved in the consolidation of fear memory. To further evaluate the functional role of brain AT 2 R, we administered (intra‐cerebral ventricle ‐ ICV) the highly selective AT 2 R agonist Compound 21 (C21). A single ICV injection of C21 at either 0.06ug/ul or 0.1ug/ul was administered prior to fear expression testing. Acute C21 (0.1ug/ul) treated mice showed a significant reduction in both fear expression (% freezing) (saline – 62.5% vs C21 ‐ 41.2%) ( t (27) = 2.8; p<0.05) and fear memory retention (saline – 52.9% vs C21 – 22.6%) ( t (28) = 3.8; p<0.05) following the acquisition of fear. Similarly, mice receiving C21 ICV for 2 weeks showed a trend for a reduction in fear memory retention (saline – 57.2% vs C21 – 39.6%) ( t (14) = 1.0; p=0.06 and this was independent of behavioral measures of anxiety as determined by open field testing. Moreover, C21 did not affect blood pressure or heart rate in telemetry‐implanted mice (Saline‐144 ± 3 SBP mmHg, HR 507 ± 13 bpm, n=5; C21 ‐ 136 ± 3 SBP mmHg; HR 518 bpm ± 26, n=4). These data suggest that activation of central AT 2 receptors promote the extinction of learned fear. Further studies are required to determine the neurobiological mechanism(s), which may involve changes in cerebral vascular blood flow and/or modulation of neuronal excitability and plasticity. Support or Funding Information NIH R00 HL107675‐03 AHA 15CSA24340001