# Time of day dependent changes in embryonic heart rate are detectable after maturation of rhythmic circadian gene expression in the eye, but before the heart in Xenopus laevis tadpoles cultured in LD

## TL;DR

This study shows that heart rate rhythms in Xenopus laevis tadpoles develop after circadian genes in the eye but before the heart matures.

## Contribution

The study reveals a time-of-day-dependent heart rate rhythm that emerges before circadian gene rhythms in the heart.

## Key findings

- Circadian genes per1 and nr1d1 show rhythmic expression in the eye before the heart.
- Heart rate rhythms appear after 3.2 days post fertilization in light-dark cycles.
- Heart rate may be regulated by light or the pineal gland before heart gene rhythms mature.

## Abstract

We systematically characterized onset of expression of circadian genes in the embryonic eye and heart of
Xenopus
laevis
tadpoles. We found that
period1 (per1)
and
nr1d1
(
rev-erbα)
were the first circadian genes to display significant 24-hour rhythms in the developing eye and heart in a 12-hour light-dark cycle (LD). Rhythmic expression of both oscillator and output genes were present in the eye by 2.75 days post fertilization (dpf), but not in 15 dpf hearts. Surprisingly, rhythmic oscillation of heart rate occurred after 3.2 dpf suggesting that heart rate may be controlled directly by light or indirectly by the pineal in LD.