LONG-TERM IMPAIRMENT OF AUTONOMIC CIRCADIAN-RHYTHMS AFTER BRIEF INTERMITTENT SOCIAL STRESS

This experiment was designed to examine the short- and long-term behav ioral, cardiovascular, and thermoregulatory responses to brief intermi ttent agonistic confrontations in rats. The experimental procedure inv olves resident-intruder confrontations consisting of a 10-min period d uring which both animals are separated in the home cage of the residen t, followed by a brief physical encounter leading to defeat of the int ruder and a 10-min period, when the intruder was alone in the home cag e of the resident. These 30-min-long confrontations were repeated on 5 consecutive days. Before the first confrontation with a resident, an intruder rat's telemetered heart rate and core temperature show a stab le circadian rhythm that is entrained by the light cycle. Acutely, the confrontations produce immediate and large tachycardia and hypertherm ia in intruders. A decrease in amplitude of the circadian rhythms for heart rate and core temperature, as detected by cosinor analysis, pers ists for at least 10 days after the last of five daily brief confronta tions with the resident. The defensive upright posture is nearly absen t during the first exploration of the resident's home cage, but is dis played by the intruder for one-third of the available time before and after the fifth defeat. Intermittent brief social stress is sufficient to induce profound changes in defensive behavior and long-lasting dep ression of circadian rhythmicity that persist for weeks.