Beta‐adrenergic‐related cardiac interoception in Pavlovian fear conditioning

Abstract

Background Memory formation is enhanced by stress and b-Adrenergic signaling is largely involved in consolidation and reconsolidation processes. Current FDA-approved drugs for the treatment of post-traumatic stress disorder (PTSD) are limited in number and in efficacy. Pharmacological agents that modulate b-Adrenergic activity, such as propranolol, have shown some potential clinical utility in treating PTSD. However, the specific mechanisms of b-Adrenergic activity on fear memory and cardiac interoception are unknown. In the current study we investigated the effects of the beta-agonist isoproterenol (ISO) on short- and long-term memory in mice. We hypothesized that acute ISO would enhance short- and long-term memory and increase behavioral displays of anxiety. Methods Male C57Bl/6J mice completed a Pavlovian auditory fear conditioning protocol assessing fear expression. We evaluated the effect of two doses of acute peripherally administered (i.p) isoproterenol (0.1 mg/kg, 0.01 mg/kg) versus saline (i.p) on freezing behavior during fear conditioning. In fear expression studies, mice were conditioned with 5 auditory tones (CS's) each paired with a foot shock (UCS). Twenty-four hours later, mice received injections 20 minutes prior to a fear expression test involving presentation of 4 CS's. Long-term memory was assessed 24 hours later with another 4-CS presentation. Open field testing and elevated plus maze (EPM) were also used to assess the effects of .1 mg/kg ISO on generalized measures of anxiety. Results In fear expression studies, 0.1 mg/kg ISO mice showed significantly greater freezing (M=71.9%, SD=30.5) to 4-CS presentation compared to saline mice (M=58.1%, SD=32.5), F(1,6)=11.81, p=.01. We found no significant effects of .01 mg/kg on fear expression. In open field studies, ISO mice displayed significantly less locomotion compared to saline, as shown across multiple parameters including total center entries and distance travelled in center. An unpaired t-test showed that ISO mice made significantly less center entries in a 30-minute test (M=27.2, SD=10.94) than saline mice (M=48.3, SD=11.47), t(6)=3.90, p=.001. For EPM testing, ISO injected mice displayed increased percentage of time in closed arms versus open arms (M=85.9% time, SD= 10.03) compared to saline mice (M=67.1 % time, SD=16.32), t(10)=2.41, p=.04. Conclusion: Overall, these results suggest that acute peripheral administration of isoproterenol enhances short-term expression of fear responses independent of long-term responses in mice. Together, these studies help elucidate physiological mechanisms underlying fear learning, namely, receptor-mediated b-adrenergic responses, cardioceptive influences on fear expression, and afferent feedback as part of an internal context cue in fear memory.