Acclimation to a thermoneutral environment abolishes age-associated alterations in heart rate and heart rate variability in conscious, unrestrained mice

Abstract

Mice are among the most widely used translational models of cardiovascular aging and offer a method to quickly assess lifespan changes in a controlled environment. The standard laboratory temperature (20-22 °C), however, imposes a cold stress on mice that causes an increase in sympathetic nervous system-mediated activation of brown adipose tissue (BAT) to maintain a core body temperature of 36-37 °C. Thus, while physiologic data obtained recapitulate human physiology to a certain degree, interpretations of previous research in mice may have been contaminated by a cold stress, due to housing mice below their thermoneutral zone (30 °C). The purpose of this investigation was to examine how chronic sympathetic stimulation evoked by acclimation to 20 °C might obscure interpretation of changes in autonomic modulation of heart rate (HR) and heart rate variability (HRV) that accompany advancing age. HR and HRV before and after administration of a dual-autonomic blockade were measured via in-vivo ECG in young (3 months) and aged (30 months) male C57BL/6 telemetry-implanted mice following temperature acclimation for 3 days at 30 °C or 20 °C. Mean basal and intrinsic HR of both young and aged mice became markedly reduced at 30 °C compared to 20 °C. In both age groups, HRV parameters in time, frequency, and non-linear domains displayed increased variability at 30 °C compared to 20 °C under basal conditions. Importantly, age-associated declines in HRV observed at 20 °C were ameliorated when mice were studied at their thermoneutral ambient temperature of 30 °C. Thus, an accurate understanding of autonomic modulation of cardiovascular functions in mice of advanced age requires that they are housed in a metabolically neutral environment.

Keywords: Aging; Cardiac autonomic modulation; Heart rate; Heart rate variability; Thermoneutrality.

Fig. 1

Basal-state, intrinsic-state, and Δ (basal state- intrinsic state) comparison values at 20 °C and 30 °C in young and old mice of A mean heart rate (bpm), B time-domain parameter SDNN, C frequency-domain parameters categories (power spectral density), D non-linear domain parameters multi-scale entropy and Beta-slope, and E representative ECG tracings from young and old mice at 20 °C and 30 °C, respectively. Data is reported as means with standard error bars. Linear mixed effects models were used to examine age, temperature, and drug effects and interactions between effects while accounting for uneven group sizes and repeated measures

Fig. 1

Basal-state, intrinsic-state, and Δ (basal state- intrinsic state) comparison values at 20 °C and 30 °C in young and old mice of A mean heart rate (bpm), B time-domain parameter SDNN, C frequency-domain parameters categories (power spectral density), D non-linear domain parameters multi-scale entropy and Beta-slope, and E representative ECG tracings from young and old mice at 20 °C and 30 °C, respectively. Data is reported as means with standard error bars. Linear mixed effects models were used to examine age, temperature, and drug effects and interactions between effects while accounting for uneven group sizes and repeated measures

Fig. 2

Poincare plots, in which the interbeat interval (NN) is plotted against the subsequent interbeat interval (NN+1) in a representative young and old mouse under basal conditions or after administration of a double-autonomic blockade

Fig. 3

Power spectral density plots from a representative old animal at A 20 °C and B 30 °C. To account for marked differences in HR between ambient temperatures, different cut-off frequencies for different temperatures were used to categorize data (Behar et al. 2018): High-frequency power spectral density (HF.PSD) was defined as 1.50–5.0 Hz at 20 °C and 0.75–3.0 Hz at 30 °C. Low-frequency power spectral density (LF.PSD) was defined as 0.5–1.5 Hz at 20 °C and 0.2–0.75 Hz at 30 °C. Very low frequency power spectral density (VLF.PSD) was defined as 0–0.5 Hz at 20 °C and 0 to 0.2 Hz at 30 °C

Fig. 4

Age and ambient temperature dependence of the relationship of Mean NN to SDNN. A The NN vs SDNN relationship in both age groups in the basal and intrinsic states during acclimation at 20 °C and 30 °C. B Power-law functions (lnNN-lnSDNN) for the data in Panel A