Social Isolation Affects the Pattern of Cardiovascular Responses to Repetitive Acoustic Startle Stimuli

The aim of this study was to investigate the cardiovascular responses to repetitive alerting stimuli in rats subjected to intermittent social isolation, in comparison with animals housed in pairs. 2. Ten male Wistar rats were implanted with a blood pressure (BP) telemetric system and enrolled in a randomized cross-over study design. Rats were either isolated or housed in pairs for an 8 day period. At the end of each period, the animals were exposed to five acoustic stimuli (110 dB at 15 kHz, 80 ms after the impact, duration 700 ms) at 60 s intervals. For each stimulus, maximal BP and heart rate (HR) responses were calculated. BP variability was analysed in the frequency domain before the first stimulation using power spectral analysis. 3. Isolated animals showed more faster breathing (1.71 vs 1.42 Hz in the paired condition). The 0.4 Hz zone of the systolic BP spectral power was not significantly affected by isolation (1.11 vs 0.85 mmHg2). BP and HR resting levels of isolated rats (121 mmHg for the systolic BP and 290 b.p.m.) were similar to those of animals housed in pairs (119 mmHg for the systolic BP and 279 b.p.m.). 4. The first acoustic stimulus caused a brief rise in BP of a comparable amplitude in both conditions (24 +/- 2 mmHg). A biphasic HR response was also observed, but the delayed bradycardia was more marked during isolation (37 vs 6 b.p.m. decrease). BP and HR levels were restored within 20 s. 5. Interestingly, BP responses were progressively attenuated to become negligible at the fifth presentation (1 mmHg) in the isolated state, while during the paired state a substantial pressor response (13 mmHg) after each successive stimulus was maintained. Paired rats exhibited a constant HR profile across the trials, consisting in a slight HR increase (< 5 b.p.m.), concomitant with the BP elevation, followed by a delayed bradycardia (around 15 b.p.m.). HR profiles in the isolated condition differed markedly: HR rises increased in magnitude with the trial number, reaching 30 b.p.m. after the 5th trial. 6. Spontaneous BP and HR increases, as observed throughout the experiment were quantitatively small (6 mmHg and < 10 b.p.m.) in the two conditions. 7. These data indicate that isolation markedly affects the BP habituation profile to repetitive alerting stimuli. The BP response attenuation might unmask a tachycardic response. Alternatively, the amplified tachycardia following the latter stimuli might express sensitization to the conditioned fear resulting from isolation.