Age-related alterations in cardiac response to emotional stress: relations to behavioral reactivity in the rat.

Age and behavioral characteristics are considered as risk factors for disturbances of the cardiac rhythm. Emotional stress may be a disseminating factor. Therefore, cardiac responsiveness and behavioral reactivity and their relation as a function of age have been studied in the rat. Young (3-month-old) and young adult (5-month-old) rats display a relative deceleratory cardiac response with bradyarrhythmias in the initial phase of response to emotional stress evoked by stimuli associated with a previous painful experience. The behavioral response is immobility. The immobility response is also displayed by aged (21-month-old) and senescent (33-month-old) rats but the initial bradycardiac heart response to emotional stress is absent, and the incidence of repetitive extrasystoles is increasing with age. An inverse correlation between behavioral reactivity to novel stimuli and the bradycardiac heart responsiveness is observed in young and young adult individual animals. The behavioral reactivity of the aged and senescent rats is diminished, but the correlation with cardiac reactivity remains preserved. It is suggested that the behaviorally coupled inhibitory influences on the heart are diminishing during aging either due to impairments in the descending cholinergic (vagal) system or secondarily, due to a decrease in the central "drive" of this system.