Intracardiac neuromodulation of pacemaker rate in the adult zebrafish heart in vitro

Heart rate (HR) in vertebrates is determined by the discharge frequency of cardiac pacemaker cells, which are innervated by the autonomic nervous system. The portion of this system located within the heart, the intracardiac nervous system (ICNS), transmits impulses to cardiac effectors for control of cardiac output. Central autonomic neurons modulate the heart through sympathetic (acceleratory) and parasympathetic (inhibitory) axons in the vagosympathetic trunks. In the classical model of cardiac control, the balance between sympathetic and parasympathetic efferent signals determines HR. Recent evidence of spontaneous neural activity within the ICNS in isolated hearts suggests a local regulatory element in cardiac control. We examined the potential for ICNS modulation of HR in the isolated zebrafish heart, using atropine to block parasympathetic drive to the pacemaker and timolol to block sympathetic drive. Atropine (3, 10, and 30 µM) evoked tachycardia of similar magnitude at 22 and 28°C. Timolol (3, 10, and 30 µM) caused bradycardia at both temperatures, with greater proportional HR responses to all doses at the higher temperature. Our results suggest that both sympathetic and parasympathetic outputs from the ICNS influence the pacemaker, with the degree of sympathetic drive being temperature-sensitive. Intrinsic HR, obtained after dual antagonist application, was not significantly different from HR before drug application so the effects of sympathetic and parasympathetic drive on pacemaker rate appeared to be evenly balanced. We propose that the ICNS in the isolated zebrafish heart is capable of modulating HR in the absence of extracardiac autonomic inputs.NEW & NOTEWORTHY In this study, we show that the intracardiac nervous system, a key integrative pathway for autonomic control of the heart, can modulate pacemaker rate in the absence of extracardiac innervation in the isolated adult zebrafish heart. Pharmacological blockade of muscarinic and β-adrenergic receptors reveals opposing inhibitory and excitatory influences on heart rate, and these effects are eliminated when intracardiac nicotinic synaptic transmission is disrupted with hexamethonium.