Isolation, tactile startle and resting blood pressure in Long-Evans rats.

The effect of social isolation, consisting of individual housing in the vivarium under standard conditions and imposed on male rats older than 90 days, was assessed on tactile startle reactivity, nociception, resting heart rate and arterial blood pressure, and on intercorrelations among these variables. Tactile startle was affected by stimulus intensity, repeated trials and time of testing relative to the light/dark cycle, but not by social isolation from 1 to 29 days. Hot-plate latency was not affected by individual housing from 12 to 43 days. Blood pressure and heart rate were negatively correlated with isolation from 9 to 58 days, showing a significant decline over increasing days of isolation. Although not affected directly by isolation, tactile startle was affected secondarily by differences in blood pressure produced by individual housing. Rats with lower resting pressures responded to low-intensity startle stimuli that were not effective in rats with higher pressures, indicating a threshold difference and suggesting the hypothesis that blood pressure acts to modulate sensory reactivity.