Sympathetic control of cardiac output by noradrenaline: quasi‐synaptic quantal release or interstitial diffusion and spillover?

The heart is highly innervated by sympathetic and parasympathetic nerves that are essential for regulation of heart rate, contractility and relaxation speed to maintain cardiac output during blood pressure regulation and its increase during exercise. The main underlying mechanism was discovered one century ago by the German physiologist Otto Loewi with elegant experiments on frog hearts showing for the first time that nerve impulse transmission was through chemical and not electrical communication (Loewi, 1922). During his experiments on the “Vagusstoff” (vagus substance), which was later identified as acetylcholine, responsible for slowing the heart rate, he also discovered that the coronary fluid collected during stimulation of the “accelerator nerve” could be used to increase the heart rate when perfused into another heart (Loewi, 1922). Today we know that this is due to a “spillover” of neuronally released noradrenaline (norepinephrine) into the bloodstream, resulting from a capacity overload of the noradrenaline reuptake transporter uptake-1 during the high sympathetic stimulation. In classical synaptic communication, neurotransmitters are typically released in a “quantal” manner by fusion of individual vesicles into a very narrow synaptic cleft. By using a quantal transmission, the presynaptic vesicles are not used up all at once and repetitive stimulation allows fine tuning of postsynaptic function. Until recently, narrow synaptic clefts and networks of sympathetic neurons have not been described in the heart, and it was generally believed that sympathetic nerves discharge noradrenaline into the myocardial interstitium where it diffuses and can “spill over” into the capillaries and the coronary fluid (Thackeray & Bengel, 2016). However, there is increasing anatomical evidence for a “quasi-synaptic” communication with narrow synaptic clefts between sympathetic nerves and cardiomyocytes (Zaglia & Mongillo, 2017), which is now also supported with the first functional data by Prando at al. in this issue of The Journal of Physiology (Prando et al. 2018). The authors describe cardiac sympathetic nerves in the heart with “pearl-necklace” morphology indicative of sympathetic varicosities. Electron microscopy showed synaptic structures with vesicles in a pre-docked state at the presynaptic side and a very small (<100 nm) intermembrane space towards the “postsynaptic” cardiomyocyte. Because of the structural similarity to the neuromuscular junction, the authors proposed a “quasi-synaptic” coupling of cardiac sympathetic neurons and cardiomyocytes, which would require less energy for noradrenaline synthesis, release and reuptake than a mechanism depending on interstitial diffusion with “spillover” (Zaglia & Mongillo, 2017). To estimate the effectiveness of the proposed mechanism in vitro, the authors have used elegant co-culture experiments with isolated sympathetic nerves and neonatal rat cardiomyocytes transfected with a FRET-based cAMP sensor to show neuronally induced localized cAMP elevation selectively in cardiomyocytes with direct connection to a nerve (Prando et al. 2018). A competitive β-blocker binding assay allowed the estimation of local noradrenaline concentration in the “synaptic cleft” in the range of 100 nmol/l. However, when applying this concentration globally the response was 10 times larger, which is due to low density local action of the “quasi-synaptic” coupling. To study the proposed mechanism in vivo, the authors used an optogenetic method for light-induced release of noradrenaline from cardiac sympathetic neurons, which was shown before to increase the heart rate (Wengrowski et al. 2015). In the current report, the authors proved that one single 10 ms-long light-induced noradrenaline release event is already able to accelerate the subsequent heart beat with a delay of only 170 ms (Prando et al. 2018). This rapid signal transduction supports the idea of a quantal “quasi-synaptic” communication rather than slow noradrenaline interstitial diffusion. Interestingly, application of low doses of the β-blockers propranolol or metoprolol to mice decreased the resting heart rate but did not completely block the light-induced neurogenic heart rate acceleration. This suggests that the resting heart rate is controlled by basal low noradrenaline release and neuronal stimulation results in high noradrenaline concentration, which could be due to a “quasi-synaptic” local release mechanism but also due to a saturation of the uptake-1 transporter leading to noradrenaline diffusion and spillover. Future competitiveβ-blocker binding assays and block of uptake-1 by desipramine will be suited to discriminate between the “quasi-synaptic” and the “diffusion and spillover” mechanism in the intact heart. The newly proposed concept of a “quasi-synaptic” communication has the potential to open a new research field and to improve our understanding of brain and heart interaction. Such a mechanism would enable temporal precise neuronal control of heart rate and contractility on a beat-to-beat basis by quantal noradrenaline release. Furthermore it will be interesting to transfer known synaptic plasticity concepts by investigation of presynaptic (vesicle content, number of released vesicles, role of α2A adrenoceptors), intrasynaptic (activity of degrading enzymes or reuptake transporters) or postsynaptic (β-receptor internalization, local β1 and β2 receptor distribution) mechanisms. The discussed concept is also important from a clinical point of view, as patients with cardiac disease often respond poorly to adrenergic stimulation but show high interstitial noradrenaline and “spillover” (Prando et al. 2018). It still remains to be proved that in the intact heart in vivo all ventricular cardiomyocytes (>100 μm long, 20 μm diameter) and all sinus node pacemaker cells are functionally coupled to sufficient numbers of sympathetic varicosities. Thus, it is not understood if and how localized “quasi-synaptic” β-adrenergic stimulation and spatially restricted downstream signalling would be able to affect all cardiomyocytes homogeneously to their full extent. Importantly, this is in clear contrast to the mechanism at the neuromuscular junction, in which an evoked