Effects of three different housing systems on activity and physiological parameters in male Spontaneously Hypertensive rats and Sprague Dawley rats

Abstract: Laboratory rats spend a majority of their lives in their home-cages. The cage often used for rats, Makrolon type IV, only gives them limited opportunities for physical activity. The welfare of the rats would increase by housing them in larger cages that allow an increased amount of physical activity. The aim of this study was to evaluate the effect of larger, alternative cages, on parameters that are affected by physical activity. We compared 24 male Sprague Dawley (SD) rats and 24 male Spontaneously Hypertensive rats (SH) which were housed strain-wise in two commercially available housing systems with controls in standard cages. The SH rat is an animal model that develops an increased blood pressure and insulin resistance. The commercially available housing systems were; ScantainerNOVO (NOV) (floor space approximately 2240 cm2, four cages connected to each other with floor space of approximately 8970 cm2, height 32.5 cm) and Enriched Rat Cage System (ERC) (floor space approximately 6020 cm2, height 46 cm), and the standard cage was Makrolon type IV cage (ST) (floor space 2240 cm2, height 18 cm). Eight rats per strain were kept in the NOV-cages and the ERC-systems, while rats were kept in pairs in the ST-cages. Every week the rats´ body weight, blood pressure and food consumption were recorded. During week 3 and 10 the rats performed an exercise test on an inclined treadmill, a so-called laddermill, where the rats climbed at three different intensities (7 m/min, 9 m/min and 11 m/min) followed by an endurance test. Blood samples were collected for analysis of blood lactate concentration. The rats' muscle strength was assessed in a so-called inclined plane test. After ten weeks the rats were euthanized, heart and adrenals were collected and weighed, and blood samples were taken for analysis of corticosterone- and insulin concentration. A temporary reduction in body weight gain in week 2 was seen in SD rats kept in NOV-cages, compared to those in ST-cages. The majority of rats succeeded in both exercise tests on the laddermill. In exercise test 2, SD rats kept in NOV-cages had a lower blood lactate level compared those kept in ST-cages. Both rat strains kept in ST-cages and SH rats kept in ERC-system had an increased lactate level in exercise test 2 compared to test 1. All rats kept in NOV-cages had a significantly increased endurance capacity in the exercise test 2. During week 10, SD rats kept in NOV-cages had an increased systolic blood pressure compared to those in ST-cages. The SH rats had a greater insulin concentration and relative heart weight compared to SD rats but no difference was seen between cage types. There was a positive correlation between body weight and insulin concentration in SH rats. Rats kept in ST-cages had the poorest performance in the inclined plane test, only 56% of the rats succeeded in holding on at an inclination exceeding 55°, compared to 88% of the rats in NOV-cages and 88 % of the rats in the ERC-system. In a summary the rats kept in NOV-cages showed the clearest signs of an increased home cage activity based on the results obtained from the exercise tests. No effects on other physiological parameters were detected, which indicates that the activity level of the rats in their home-cages was not intense enough to result in a change in these parameters. The SH rats maintained their increased systolic blood pressure and insulin resistance regardless of cage type. In addition, they had no differences in body-, heart-, or adrenal weight between the different housing systems. This shows that it is possible to keep SH rats in larger cages without affecting the animal model. The study shows that rats increase their spontaneous activity in their home cages but not enough to create a distinct training effect. However, increased home cage activity is still relevant for animal welfare and does not have to interfere with scientific results.