Individually ventilated cages cause chronic low-grade hypoxia impacting mice hematologically and behaviorally

Abstract

Use of individually ventilated caging (IVC) systems for mouse-based laboratory investigation has dramatically increased. We found that without mice present, intra-cage oxygen concentration was comparable (21%) between IVC housing and ambient environment caging (AEC) that used wire top lids. However, when mice were housed 4-to-a-cage for 1week, intra-cage oxygen dropped to 20.5% in IVC housing as compared to 21% for AEC housing. IVC intra-cage humidity was also elevated relative to AEC housing. Mice raised in IVC housing as compared to mice raised in AEC housing had higher RBC mass, hematocrit and hemoglobin concentrations. They also had elevated platelet counts but lower white blood cell counts. IVC mice, relative to AEC mice, had increased saccharin preference and increased fluid consumption but similar locomotion, food intake, social exploration and novel object recognition when tested in an AEC environment. Taken together, these data indicate that ventilated caging systems can have a 0.5% reduction from ambient oxygen concentration that is coupled to mouse red blood cell indices indicative of chronic exposure to a hypoxia. Importantly, IVC housing can impact behavioral testing for depressive-like behavior.

Figure 1

IVC mice show an increased preference for saccharin and consume more total fluid than AEC mice. (A) Results are expressed as percent saccharin consumed, means ± s.e.m., n = 12. Bars without a common superscript are significantly different (81.89 ± 3.60 v. 90.80 ± 1.24, AEC v. IVC; F(1,22) = 5.98, P = 0.02). (B) Results are expressed as total fluid consumed in grams, means ± SEM: n = 12. Bars without a common superscript are significantly different (9.11 ± 0.68 v. 11.07 ± 0.49, AEC v. IVC; F(1,22) = 6.02, P = 0.02).

Figure 1

IVC mice show an increased preference for saccharin and consume more total fluid than AEC mice. (A) Results are expressed as percent saccharin consumed, means ± s.e.m., n = 12. Bars without a common superscript are significantly different (81.89 ± 3.60 v. 90.80 ± 1.24, AEC v. IVC; F(1,22) = 5.98, P = 0.02). (B) Results are expressed as total fluid consumed in grams, means ± SEM: n = 12. Bars without a common superscript are significantly different (9.11 ± 0.68 v. 11.07 ± 0.49, AEC v. IVC; F(1,22) = 6.02, P = 0.02).

Figure 2

IVC mice recover more rapidly than AEC mice from LPS-induced loss of locomotor activity (LMA). Results are expressed as percent baseline (pre-LPS) LMA, means ± s.e.m., n = 6. Main effects: time (F(3,77) = 13.08, P < 0.01), housing type (F(1,77) = 3.69, P = 0.06); time × housing type interaction (F(3,77) = 3.31, P = 0.02). Bars within individual time points without common superscript letters are significantly different (P < 0.05). 2 h time point: P < 0.05, AEC + Saline v. AEC + LPS, IVC + LPS (108.712 ± 10.100 v. 23.911 ± 12.702, 12.088 ± 9.093, respectively); IVC + Saline v. AEC + LPS, IVC + LPS (92.242 ± 4.401 v. 23.911 ± 12.702, 12.088 ± 9.093, respectively). 4 h time point: P < 0.05, AEC + Saline v. AEC + LPS, IVC + LPS (120.219 ± 11.883 v. 47.083 ± 17.265, 37.702 ± 10.455, respectively); IVC + Saline v. IVC + LPS (103.703 ± 5.407 v. 37.702 ± 10.455). 8 h time point: P < 0.05, AEC + Saline v. AEC + LPS, IVC + LPS (132.792 ± 17.631 v. 52.404 ± 23.524, 24.057, respectively); IVC + Saline v. IVC + LPS (91.945 ± 11.038 v. 24.057 ± 10.648, respectively). 10 h time point: P < 0.05, AEC + Saline v. AEC + LPS (139.290 ± 9.015 v. 62.530 ± 16.733); IVC + Saline v. AEC + LPS (134.529 ± 11.157 v. 62.530 ± 16.733).