Sympatho-vagal balance and intrinsic sinoatrial node activity in larval zebrafish quantified by heart rate variability

The larval zebrafish is a powerful in vivo model for studying cardiac disease. Alterations in sympatho-vagal balance and intrinsic sinoatrial node activity, key contributors to cardiac dysfunction, can be assessed using heart rate variability (HRV) indices. However, a validated method for HRV quantification in larval zebrafish has not been established. Here, we address this unmet need by (a) validating that beat intervals (BI) can be accurately extracted from fluorescence-based heart imaging, (b) adapting HRV indices from humans to larval zebrafish at 6 days post-fertilization (dpf), and (c) quantifying HRV indices in response to altered or absent sympatho-vagal balance. We show that: (a) BI and HRV extracted from fluorescence imaging and electrophysiological measurements are comparable; (b) The majority of the power spectral density (PSD) of BI sequences in larval zebrafish, which quantify the power of a signal that is distributed across different frequencies, resides within the high-frequency range (0.458–1.564 Hz); (c) Sympathetic activity is higher than vagal activity and predominantly modulates the PSD of BI sequences in high and very low frequency bands (0.0033–0.079 Hz), whereas vagal activity mainly affects the low frequency range (0.079–0.458 Hz); and (d) Intrinsic pacemaker mechanisms contribute to the low- and high-frequency regimes of the PSD of BI sequences. Thus, even at the larval stage a synergy between sympatho-vagal and intrinsic sinoatrial node activity exists, with dominant sympathetic activity. Moreover, our methodology for BI measurement and drug treatment in this model could serve as a basis for high-throughput drug screening for cardiac arrhythmias.