Development of the sympatho-vagal balance in the cardiovascular system in zebrafish (Danio rerio) characterized by power spectrum and classical signal analysis

The development of sympatho- vagal control of cardiac activity was analyzed in zebrafish ( Danio rerio) larvae from 2 to 15 days post fertilization ( d. p. f.) by pharmacological studies as well as by assessing short term heart rate variability. Changes in heart rate in response to cholinergic and adrenergic receptor stimulation or inhibition were investigated using in situ preparations and digital video- microscopic techniques. The data revealed that the heart responded to adrenergic stimulation starting at 4 d. p. f. and to cholinergic stimulation starting at 5 d. p. f. Atropine application resulted in an increase in heart rate beyond 12 d. p. f., while the inhibitory effect of cholinergic stimulation ceased at this time of development. Adrenergic inhibition ( propranolol) reduced heart rate for the first time at 5 d. p. f., but the reduction was only very small ( 3.8%). Between 5 and 12 d. p. f. propranolol application always resulted in a minor reduction in heart rate, but because the effect was so small it was not always significant. Because the presence of an adrenergic or cholinergic tone may influence the stability of heart rate, we analyzed short- term heart rate variability ( HRV). The frequency band width of heart rate variability revealed that HRV increased between 4 d. p. f. and 15 d. p. f. From 13 to 15 d. p. f. atropine reduced the frequency band width of HRV, whereas the combination of atropine and propranolol effectively reduced the frequency band width between 11 and 15 d. p. f. Classical power spectrum analysis using electrocardiograms is not possible in tiny zebrafish larvae and juveniles. It was therefore performed using optical methods, recording cardiac movement and cardiotachograms calculated from these measurements. Whereas heart movements contained frequency components characterizing HRV, the cardiotachogram did not show typical frequency spectra as known from other species.