Interaction between defects in ventilatory and thermoregulatory control in mice lacking 5-HT neurons

Abstract

We have previously shown that mice with near-complete absence of 5-HT neurons (Lmx1bf/f/p) display a blunted hypercapnic ventilatory response (HCVR) and impaired cold-induced thermogenesis, but have normal baseline ventilation (), core body temperature (TCore) and hypoxic ventilatory responses (HVR) at warm ambient temperatures (TAmb; 30 degrees C). These results suggest that 5-HT neurons are an important site for integration of ventilatory, metabolic and temperature control. To better define this integrative role, we now determine how a moderate cold stress (TAmb of 25 degrees C) influences ventilatory control in adult Lmx1bf/f/p mice. During whole animal plethysmographic recordings at 25 degreesC, baseline , metabolic rate , and TCore of Lmx1bf/f/p mice were reduced (P < 0.001) compared to wild type (WT) mice. Additionally, the HCVR was reduced in Lmx1bf/f/p mice during normoxic (-33.1%) and hyperoxic (-40.9%) hypercapnia. However, in Lmx1bf/f/p mice was equal to that in WT mice while breathing 10% CO2, indicating that non-5-HT neurons may play a dominant role during extreme hypercapnia. Additionally, ventilation was decreased during hypoxia in Lmx1bf/f/p mice compared to WT mice at 25 degrees C due to decreased TCore. These data suggest that a moderate cold stress in Lmx1bf/f/p mice leads to further dysfunction in ventilatory control resulting from failure to adequately maintain TCore. We conclude that 5-HT neurons contribute to the hypercapnic ventilatory response under physiologic, more than during extreme levels of CO2, and that mild cold stress further compromises ventilatory control in Lmx1bf/f/p mice as a result of defective thermogenesis.

Figure 1. The normoxic hypercapnic ventilatory response and body temperature are reduced in Lmx1b^f/f/p mice at 25°C

A, Minute ventilation (V̇_E), B, change in ventilation from baseline, C, respiratory frequency (fR), D, tidal volume (V_T), E, core temperature (T_Core) and F, oxygen consumption (V̇_O2) at rest and breathing 0%, 3%, 5%, 7% and 10% inspired CO₂ in normoxia (F_IO2 = 0.21) at room temperature (RT: 25°C; A–D, F) or thermoneutral (TN: 30°C (Hodges, M. R. et al., 2008b)) in WT (n=12) and Lmx1b^f/f/p mice (n=10). Two-way ANOVA (genotype and condition, or ambient temperature as factors) or unpaired t-test, * denotes P<0.05 for WT versus Lmx1b^f/f/p, ^# denotes P<0.05 for 10% CO₂ versus baseline. Data are mean ± SEM.

Figure 2. The hyperoxic hypercapnic ventilatory response and body temperature are reduced in Lmx1b^f/f/p mice at 25°C

A, Minute ventilation (V̇_E), B, change in ventilation from baseline, C, respiratory frequency (fR), D, tidal volume (V_T), and E, core temperature (T_Core) at rest and breathing 0%, 3%, 5%, 7% and 10% inspired CO₂ in hyperoxia (F_IO2 = 0.5) at room temperature (RT: 25°C) or thermoneutral (TN: 30°C (Hodges, M. R. et al., 2008b)) in WT (n=12) and Lmx1b^f/f/p mice (n=10). Two-way ANOVA (genotype and condition, or ambient temperature as factors) or unpaired t-test, * denotes P<0.05 for WT versus Lmx1b^f/f/p, ^# denotes P<0.05 for 10% CO₂ versus baseline. Data are mean ± SEM.

Figure 3. Lmx1b^f/f/p mice have a blunted hypoxic ventilatory response at 25°C

A, Minute ventilation (V̇_E), B, respiratory frequency (fR), C, tidal volume (V_T), D, V̇_E/V̇_O2 ratio, E, V̇_O2, and F, core temperature (T_Core) breathing room air (RA) and during minutes 2–10 of a 10-minute hypoxia challenge (F_IO2 = 0.1) in WT (n=9) and Lmx1b^f/f/p mice (n=7). Two-way ANOVA (genotype and time as factors) and unpaired t-test, * denotes P<0.05 for WT versus Lmx1b^f/f/p mice. Data are mean ± SEM.

Figure 4. The ventilation to oxygen consumption ratio is reduced at 25°C due to a reduced body temperature in Lmx1b^f/f/p mice

A, Minute ventilation (V_E; % control), B, V̇_O2, C, V̇_E/V_O2 ratio (% control), and D, core temperature (T_Core) in WT (solid symbols) and Lmx1b^f/f/p (open symbols) mice at T_Amb of 25°C (squares) and 30°C (triangles (Hodges, M. R. et al., 2008b)). Note that both genotypes shift ventilatory and metabolic strategies during hypoxia at different T_Amb, and that the V̇_E/V_O2 ratio is reduced under cool conditions. Two-way ANOVA (genotype and time, or T_Amb as factors) and unpaired t-test, * denotes P<0.05 for WT versus Lmx1b^f/f/p mice, # denotes P<0.05 for 25°C versus 30°C. Data are mean ± SEM.