Abstract
To study the cardiac function and its development of newborn mice, I first measured the heart rate (HR-PZT) of free-moving newborn C57BL/6 mice using a piezoelectric transducer (PZT) sensor, from immediately after birth (postnatal day 0: P0) to P14, and compared the HR-PZT with the heart rate (HR) obtained by ECG (HR-ECG). The HR-PZT increased age-dependently from 323±38 b/m at P0 (n=5) to 674±56 b/m at P14 (n=9), while the HR-ECG measured immediately after attaching ECG electrodes was over 57 b/m lower than the HR-PZT in the mice aged P3 to P14 (P<.0001-.0022, n=6-15). During 5 minutes of HR-ECG measurement, the HR-ECG did not change and remained lower than the HR-PZT in mice aged P3 to P8, while it recovered to a value comparable to the HR-PZT in the mice aged P9 to P14 and it slowly but significantly decreased in the mice aged P0.5 to P2. The present study demonstrates that the PZT system is the non-invasive and practical method for the measurement of HR in free-moving newborn mice in contrast to attaching ECG electrodes which is invasive and influences HR. Furthermore, I found that all mice aged P0 to P14 restrained by attaching ECG electrodes suddenly stopped their motion after several seconds of struggle just as tonic immobility at death feigning. Further study is needed to elucidate the mechanisms of the diverse changes in HR-ECG induced by attaching ECG electrodes to mice and the death-mimicking-like behavior in a neonatal period, during which the central nervous system dramatically develops. [J Physiol Sci. 2007;57 Suppl:S136]
