Daily rhythms in heartbeat rate are intrinsic to the zebrafish heart

It is a well-established fact that different tissues within the body contain their own circadian clocks or pacemakers, where it is proposed that the clock controls the local, daily cell biology of that organ.^1,2 In mammals, these peripheral clocks work in concert with and are entrained by rhythmic signals arising from the suprachiasmatic nucleus (SCN) in the hypothalamus of the animal, among other systemic cues.² In the case of zebrafish, the circadian system appears to be highly decentralized with each tissue not only having an internal circadian clock, but also being directly light entrained.¹ Several years ago, we showed that the zebrafish heart contains its own circadian pacemaker at the gene expression level.¹ This is also the case in mammals, where the circadian clock controls approximately 10% of the genes expressed in the heart.³ However, heart rate itself is generally thought to be regulated by several well-described autonomic cues, neurotransmitters, and hormones. In this study, we report that, for larval zebrafish hearts, the daily change in heartbeat rate is not only clock-controlled in vivo, but that this rhythm also persists in vitro, indicating that the cardiac circadian clock itself can directly drive this major physiological oscillation.