Effects of social defeat on acute cardiovascular response in salt-sensitive and salt-resistant rats

We studied the effects of social stress (SS) and a high salt diet on systolic blood pressure (SBP) and heart rate (HR): S/JR male rats (which exhibit marked elevations in SBP when placed on a high sodium diet) and R/JR male rats (which are resistant to the BP-elevating effects of a high sodium diet) were maintained on a low sodium diet (0.3% NaCl) or placed on a high sodium diet (8% NaCl). Within each dietary condition independent groups were either exposed to SS, by placement in the cage of a trained fighter male (Long-Evans breed) for 25 min, or exposed to no stress. The dietary regimen was imposed for 10 days with stress exposures on Days 1, 2, 3, 5, 7, and 9, with SBP and HR measured indirectly by tail plethysmography 3 min following exposure to SS. SS produced an acute decrease in SBP (20-30 mm Hg) in S/JR rats on the second and subsequent exposures, but did not affect HR. SS did not affect SBP of R/JR rats, but did produce a significant elevation of HR. Maintenance on the high sodium diet increased SBP in S/JR, but not R/JR, rats when it was measured on the eighth (no stress) day, but SS obscured the effects of diet on SBP on days when rats were stressed. Following exposure to attacks, defeated SS rats displayed an upright submissive posture relatively late during the first stress exposure when no change in SBP was observed after SS in S/JRs, but displayed the submissive posture immediately and with long duration on the second and subsequent exposures when a marked decrement in SBP was seen.