Does witnessing experimental procedures produce stress in male rats?

The objective of this study was to test the hypotheses that male rats are stressed by being in the same room as animals subjected to common husbandry and experimental procedures and that the level of stress is affected by housing density. Two commonly used indices of stress, heart rate (HR) and mean arterial blood pressure (MAP), were determined by using radiotelemetry for 2 h before and 3 h after rats witnessed the following procedures: decapitation, decapitation and necropsy, cage change, restraint and subcutaneous injection, and restraint and tail-vein injection. In addition, home cage behaviors (sleeping, awake, moving, rearing, and grooming) were scored once each minute for 15 min before and 45 min after the procedures. Witnessing decapitation or decapitation and necropsy of 6 other rats induced small, but significant, increases in HR and MAP in animals housed alone, whereas responses in animals housed with one or three cagemates were more transient or not significant. Witnessing a routine cage change also induced small increases in HR and MAP in rats housed alone or with one cagemate, but HR and MAP decreased in rats housed four per cage. HR and MAP did not change in rats witnessing restraint and a subcutaneous injection of other rats, but these indices were transiently increased when rats witnessed animals being restrained in a rodent restrainer and given a tail-vein injection. Home cage behaviors were significantly altered only in rats witnessing decapitation and necropsy and then only in rats housed alone. We conclude that male Sprague-Dawley rats are not significantly stressed when present in the same room in which decapitation or other common experimental procedures are being performed, especially when the animals are housed with cagemates.