Mild Transient Hypercapnia as a Novel Fear Conditioning Stimulus Allowing Re-Exposure during Sleep

Introduction: Studies suggest that sleep plays a role in traumatic memories and that treatment of sleep disorders may help\ud alleviate symptoms of posttraumatic stress disorder. Fear-conditioning paradigms in rodents are used to investigate causal\ud mechanisms of fear acquisition and the relationship between sleep and posttraumatic behaviors. We developed a novel\ud conditioning stimulus (CS) that evoked fear and was subsequently used to study re-exposure to the CS during sleep.\ud Methods: Experiment 1 assessed physiological responses to a conditioned stimulus (mild transient hypercapnia, mtHC; 3.0%\ud CO2; n = 17)+footshock for the purpose of establishing a novel CS in male FVB/J mice. Responses to the novel CS were\ud compared to tone+footshock (n = 18) and control groups of tone alone (n = 17) and mild transient hypercapnia alone\ud (n = 10). A second proof of principle experiment re-exposed animals during sleep to mild transient hypercapnia or air\ud (control) to study sleep processes related to the CS.\ud Results: Footshock elicited a response of acute tachycardia (30–40 bpm) and increased plasma epinephrine. When tone\ud predicted footshock it elicited mild hypertension (1–2 mmHg) and a three-fold increase in plasma epinephrine. When mtHC\ud predicted footshock it also induced mild hypertension, but additionally elicited a conditioned bradycardia and a smaller\ud increase in plasma epinephrine. The overall mean 24 hour sleep–wake profile was unaffected immediately after fear\ud conditioning.\ud Discussion: Our study demonstrates the efficacy of mtHC as a conditioning stimulus that is perceptible but innocuous\ud (relative to tone) and applicable during sleep. This novel model will allow future studies to explore sleep-dependent\ud mechanisms underlying maladaptive fear responses, as well as elucidate the moderators of the relationship between fear\ud responses and sleep