Does cage size affect heart rate and blood pressure of male rats at rest or after procedures that induce stress-like responses?

The objective of this study was to compare cardiovascular function and home-cage behavior of male Sprague-Dawley rats under resting conditions or after common husbandry and experimental procedures when the animals were housed with 3 other rats in cages with 920 or 1250 cm2 of floor space. Heart rate (HR), mean arterial blood pressure (MAP), and movement in the cage were monitored continuously by using radiotelemetry. The cardiovascular data are reported for a 1-h period in the morning before any human interventions, for the entire 12-h dark period, and before and after the following acute husbandry and experimental procedures: routine cage change; simulated cage change; restraint and subcutaneous (s.c.) injection; transport to another room and s.c. injection; restraint and tail-vein injection; witnessing tail-vein injection of a cage mate; witnessing a simulated decapitation; witnessing decapitation of six rats; witnessing decapitation and necropsy of six rats; exposure to the odors of urine and feces from stressed rats; exposure to the odor of dried rat blood; and being handled and weighed. Home-cage behaviors (sleeping, awake, moving, rearing, and grooming) also were scored before and after the acute procedures. Undisturbed HR in the morning was not different between the two housing groups; but undisturbed morning MAP and undisturbed nocturnal HR, MAP, and activity were significantly reduced (P<0.05) in rats housed in smaller cages. When animals were subjected to acute procedures, floor space induced a prolonged and significant effect (P<0.05) only after a cage change, with HR being reduced in rats housed in small cages. Sleeping behavior was significantly reduced (P<0.05) after many of the acute procedures, but cage size only slightly affected this behavior. We conclude that housing male rats in groups of four at 60% of the recommended cage floor space per rat does not produce crowding stress relative to that of rats housed at 80% of the recommendations. In addition, this study, coupled with our previous results, suggests that close interaction between group-housed rats appears to be more important to stress reduction than is providing increased floor space per animal.