Releasing the breaks on breathing frequency

1.

The volume of air breathed into the human body each minute is determined by how much and how often we breathe. Yet, we are only beginning to make sense of the intricacies of the network that controls, and the players that fine tune and adjust this not‐so‐simple process. In an article in this issue of Experimental Physiology, Souza and Machado (2023) investigated the contribution of A_2A receptors in cardiorespiratory function at rest during normoxia and following 1 day of sustained hypoxia. In this study, the authors have employed state of the art methods, measuring invasive blood pressure and breathing in freely behaving conscious mice. Adenosine and the purinergic family of receptors are widely expressed throughout the body. Adenosine, an endogenous nucleoside, acts on its pleiotropic G‐protein‐coupled receptors that are expressed on the cell surface. Adenosine receptors are a well‐known therapeutic target. Caffeine citrate, one of the most used neonatal drugs, is a non‐selective antagonist of adenosine receptors, but multiple mechanisms of action are thought to contribute to its beneficial effects that ultimately reduce pauses in breathing. Adenosine A_2A receptor signalling has been implicated in the modulation of breathing at multiple levels of the respiratory network and its expression has been identified in the carotid body, retrotrapezoid nucleus, Bötzinger complex, caudal raphe neurons, gigantocellularis nucleus and rostral ventrolateral medulla. Centrally, A_2A receptors are expressed in glia and neurons and contribute to neural development and control of neurotransmitter release.

Souza and Machado (2023) have utilised A_2A knockout (KO) mice to examine the functional contribution of A_2A receptors to the respiratory phenotype. One of their main findings was that A_2A KO mice presented with higher respiratory frequency compared to the Balb/c wild‐type (WT) controls, but both A_2A KO mice and WT controls had similar pH and associated biochemical concentrations. Mechanisms that modulate our pattern of breathing change with age. Age‐related changes are illustrated in work published by Mayer et al. (2006), who investigated the time‐dependent influence of adenosine on breathing. They report that administration of a specific A_2A receptor agonist brought about a decrease in frequency and/or apnoea in young rats, but this phenotype was not apparent in adult rats. Interestingly, the study by Souza and Machado was performed on 6‐ to 8‐week‐old mice, at a juvenile stage, where the absence of A_2A receptors at this developmental stage releases the breaks on breathing frequency. Would assessment of respiratory pattern in older A_2A KO mice reveal similar results?

Adenosine availability often increases during hypoxia as a result of enhanced precursor availability and hypoxia‐induced repression of enzymes which break it down. While adenosine has been shown to support anti‐inflammatory response in immune cells, A_2A receptor inhibition has been demonstrated to be neuroprotective with potential for use in treating seizures and the prevention of hypoxic‐induced injury. Souza and Machado have revealed that the hypoxia‐induced increase in breathing frequency observed in the WT is absent in the A_2A KO mice, following exposure to 1 day of sustained hypoxia. So, does A_2A receptor agonism restrain the rate at which we breathe? Monteiro et al. (2011), investigated the role of purinergic signalling in anaesthetised rats and reported that adenosine administration increases respiratory rate and volume in a dose‐dependent manner and that this effect is primarily mediated through peripheral A_2A receptors both in adult (12 weeks) and aged (24 months) animals, though they also show that when the A_2A antagonist SCH 58261 is administered on its own, it has dose‐dependent excitatory effects in both adult and aged rats. There is evidence for adenosine and its receptor as an acute facilitator of breathing peripherally but also as a delayed and longer‐lasting respiratory depressor when acting centrally, through its facilitation of GABAergic signalling, though there is much to be resolved in terms of regional adenosine regulation and the functional role of A_2A receptors in discrete regions of the respiratory network.

Adenosine and its signalling pathways also play their part in the regulation of the cardiovascular system. Adenosine itself is used therapeutically to slow the heart and stimulate coronary vasodilation in the case of paroxysmal supraventricular tachycardia. In a seminal paper by Ledent et al. (1997), the authors report an increase in blood pressure in adult A_2A KO mice (14 weeks) bred on a background of CD1 mice; their measurements of blood pressure were higher despite taken after 4 h under halothane anaesthesia. Blood pressure data reported in Souza and Machado do not support a role for A_2A receptors in the maintenance of normal blood pressure or in hypoxic‐induced changes to blood pressure (6–8 weeks). While there are a number of confounding factors that preclude a direct comparison, the age difference between the cohorts may be worthy of investigation. However, in agreement with Souza and Machado, experiments using the A_2A receptor antagonist SCH58261 in adult rats have reported no changes in blood pressure (Monteiro et al., 2011). Many questions remain not only about the role of A_2A receptors in the systemic vasculature but also about its role in the RVLM where it has been associated with modulating evoked blood pressure responses. All study designs have their limitations, and while pharmacological approaches are frequently limited by the lack of concentration/dose response curves, lack of subtype receptor specificity and potential off‐site targets, genetic knockout models are limited by their congenital nature and adaptive responses. It has been reported that in A_2A KO mice A_2B receptor subtypes are upregulated and vice versa in A_2B KO mice.

Therapeutic administration of drugs that modulate the purinergic signalling are generally delivered systemically, and therefore understanding their systemic effects is important, yet we have much to learn about the role of adenosine in discrete cells and tissues and how purinergic signalling alters cardiorespiratory control in health and disease. In a recent study by Aguiar et al. (2020), the authors put forward evidence showing improved stamina and power with the A_2A antagonist SCH‐58261 and posit that potential disruption of A_2A signalling improves dopaminergic signalling with increased availability of D2/D3 receptors, causing wakefulness and psychostimulation. Performance enhancing effects of A_2A signalling in experiments involving conscious animals may further temper our mechanistic conclusions.

AUTHOR CONTRIBUTIONS

FMD devised the concept of the article, wrote the manuscript and approved the final version.

CONFLICT OF INTEREST

The author has no conflicts of interest.

McDonald, F. (2023). Releasing the breaks on breathing frequency. Experimental Physiology, 108, 1370–1371. 10.1113/EP091467