Development of Heart Rate Circadian Rhythm in Chickens

In chick embryos, various instantaneous heart rate (IHR) fluctuations (e.g. HR variability, HR irregularities) have been found and developmental patterns of mean heart rate (MHR) have been elucidated. IHR changes have also measured in newly hatched and young chickens in order to investigate the developmental patterns of MHR and any potential diurnal HR rhythmicity such as a circadian rhythm. The present report reviews measurements of embryonic and hatchling HR fluctuations and examine development of HR circadian rhythms during the late stages of incubation and after hatching. Embryonic HR was continuously measured during the last half period of incubation. After hatching, continuous measurements of HR were made for two weeks at the longest in eight chickens ranging in age from 0 to 47 days. During the embryonic development, circadian rhythm of HR was found to be absent until hatching. After hatching, distinct circadian rhythm of HR appeared with increasing daily MHR from day 0 to day 7. During the second week of post-hatch life, the daily scope of MHR (diurnal maximum-nocturnal minimum) reached a maximum level ranging from 200 to 300bpm with constant baseline of nocturnal MHR. In young chicks, the nocturnal HR baseline was the same as that during the fortnight of posthatch life and daily scope of MHR decreased to 100^150bpm.