METYRAPONE AND FLUOXETINE SUPPRESS ENDURING BEHAVIOURAL BUT NOT CARDIAC EFFECTS OF SUB-CHRONIC STRESS IN RATS Running title: Sustained bradycardia after sub-chronic stress

In humans, chronic stressors have long been recognized as potential causes for cardiac dysregulation. Despite this, the underlying mechanistic links responsible for this association are still poorly understood. The purpose of this study was to determine whether exposure to a paradigm of sub-chronic stress can provoke enduring changes on the heart rate of experimental rats and, if so, to reveal the autonomic and neural mechanisms that mediate these effects. The study was conducted on adult male Sprague-Dawley rats instrumented for telemetric recording of heart rate and locomotor activity. Animals were submitted to a sub-chronic stress protocol, consisting of a 1-h foot shock session on five consecutive days. Heart rate and locomotor activity were recorded continuously for three days before and for six days after the sub-chronic stress period. Sub-chronic foot shock produced significant and enduring reduction in heart rate both during the dark/active (Δ=-23±3 bpm) and light/inactive (Δ=-20±3 bpm) phases of the circadian cycle, and a reduction in locomotor activity during the dark/active phase (Δ=-54±6 cph). The bradycardic effect of sub-chronic stress was not related to a reduced locomotion. Selective sympathetic (atenolol) and vagal (methyl-scopolamine) blockades were performed to reveal which autonomic component was responsible for this effect. We found that the fall in heart rate persisted after sub-chronic stress in animals treated with atenolol (active phase Δ=-16±3 bpm, inactive phase Δ=-19±2 bpm), whereas vagal blockade with scopolamine transiently prevented this effect, suggesting that the bradycardia following sub-chronic stress was predominately vagally mediated. Fluoxetine (selective serotonin reuptake inhibitor) and metyrapone (inhibitor of corticosterone synthesis) treatments did not affect heart rate changes but prevented the reduction in locomotion. We conclude that sub-chronic stress exposure in rats reduces heart rate via a rebound in vagal activation and that this effect is serotonin-and corticosterone-independent.