Purinergic signalling and TRPV1 receptors are associated with the carotid body plasticity induced by an apnoea‐like stimulus

Aerobic organisms rely on oxygen (O₂) as the final acceptor of the electrons from oxidative metabolism, which is essential to keep electron flow through the respiratory chain, adenosine triphosphate (ATP) synthesis, and therefore cell function. Consequently, a lack of O₂ is a life‐threatening condition to aerobic species. To lessen hypoxic cell damage, organisms increase O₂ uptake by means of hyperventilation triggered by the peripheral chemoreceptors. For instance, hypoxia, observed during longer apnoea, i.e. the interruption of breath, is an extremely deleterious situation that is mitigated by stimulation of peripheral chemoreceptors to re‐engage breathing (Costa et al. 2014).

Moreover, reduction in atmospheric O₂ partial pressure experienced, for example, in high altitudes leads to an acute and sustained reduction of the arterial partial pressure of O₂ ($P_{\mathrm{a}{\mathrm{O}}_2}$) leading to respiratory, cardiovascular and behavioural physiological reflex responses to maintain homeostasis (Costa et al. 2014). In contrast, in pathophysiological conditions, such as obstructive sleep apnoea in which humans have recurrent occlusion of the upper airways throughout sleep, a chronic and recurrent decrease in $P_{\mathrm{a}{\mathrm{O}}_2}$ is observed producing a critical impact in cardiorespiratory regulation (Caples et al. 2005). Given that the chronic reduction in $P_{\mathrm{a}{\mathrm{O}}_2}$ produced by intermittent sleep apnoea is associated with cardiorespiratory diseases, including neurogenic hypertension, it is important to search for new targets to treat apnoea and its harmful consequences. However, to better understand how apnoea may affect the progression of cardiovascular diseases it is necessary to discuss the physiological mechanisms involved in the detection of hypoxia.

The physiological mechanisms involved in the sensitivity to $P_{\mathrm{a}{\mathrm{O}}_2}$ reduction were initially investigated in dogs by Corneille Heymans in the early 20th century. Since then, several publications have evaluated the possible cellular and molecular mechanisms involved in the O₂ sensitivity. The peripheral chemoreceptors are cells highly specialized in sensing O₂ and are located in the carotid artery bifurcation in a chemosensitive organ called the carotid body. Carotid bodies have two distinct cell types: (a) the glomus cells or type I cells, i.e. the sentinel cells detecting the reduction in $P_{\mathrm{a}{\mathrm{O}}_2}$ in mammals, whose function is related to peripheral chemosensitivity, and (b) type II cells, whose role remains unclear. Peripheral chemoreceptors clusters are perfused by arterial blood through small blood vessels that originate in most species from the branches of the internal and external carotid, the occipital and the pharyngeal arteries (Kumar & Prabhakar, 2012). The sensory chemosensitivity of carotid bodies in response to a fall in $P_{\mathrm{a}{\mathrm{O}}_2}$ is related with the closure of K⁺ channels of glomus cells, leading to a Ca⁺²‐dependent release of acetylcholine, dopamine, adenosine, ATP, and other neurotransmitters on carotid sinus nerve (CSN) terminals. The CSN afferents establish contacts with brainstem regions involved in cardiorespiratory regulation (Kumar & Prabhakar, 2012).

A significant reduction in the arterial blood O₂ stimulates the carotid bodies and a series of the neural pathways are activated within the nucleus tractus solitarius (NTS), which in turn leads to an enhancement of respiratory and sympathetic activities. In brief, the NTS neurons send excitatory projections to the respiratory and pre‐sympathetic neurons in the rostral ventrolateral medulla and the nucleus ambiguous. Conscious rats exhibited tachypnoea, bradycardia and a significant increase in arterial pressure during acute peripheral chemoreceptor activation and these haemodynamic responses are abolished by carotid body removal (Costa et al. 2014).

In addition to acute stimulation of carotid bodies, chronic intermittent hypoxia (CIH) is largely used in studies on conscious animals, mimicking the periodic and repetitive reduction in the fraction of inspired O₂ observed in patients with obstructive sleep apnoea. Several studies showed the consequences of recurrent hypoxia in the cardiovascular system in rats, in particular the development of sympathetic overactivity and neurogenic hypertension. An interesting feature observed in ex vivo carotid body activity of rats previously exposed to 10 days of CIH (i.e. CIH preconditioning) is a sensory long‐term facilitation (sLTF), defined as a tonic enhancement of baseline activity of CSN, and an increase in its response to a hypoxic challenge (Peng et al. 2003). Regarding the functional consequence of this activity‐dependent plasticity in the carotid bodies of CIH‐preconditioned rats, these findings of Peng et al. (2003) may indicate the mechanism causing the persistent reflex activation of sympathetic nerve activity and hypertension observed in patients with periodic apnoea. For these reasons, mechanistic insights into the induction and maintenance phases of carotid body sLTF deserve investigation.

In a recent study published in this issue of The Journal of Physiology, Roy et al. (2018) documented a new mechanism from acute carotid body sLTF, which was not associated with CIH preconditioning. The authors hypothesized that sLTF can be induced in carotid bodies from naive rats, i.e. without CIH preconditioning. Furthermore, in this paper, it was postulated that ATP released by glomus cells during hypoxia and hypercapnia activate P2X2/3 purinergic receptors and that the activation of P2X receptors leads to TRPV1 (transient receptor potential vanilloid type 1) phosphorylation increasing the ionic currents in the carotid sinus nerve (CSN). They also evaluated whether or not the purinergic and TRPV1 receptors are involved in the induction and maintenance phases of carotid body sLTF in the apnoea‐like stimuli (acute intermittent hypoxia with concurrent hypercapnia (AIH‐Hc)). Roy et al. (2018) used the well‐accepted ex‐vivo carotid body – CSN preparation, which keeps the perfusion pressure, temperature and pH in the carotid bifurcation comparable with that in whole animals. They also used an in‐vivo anaesthetized rat preparation to evaluate the role of TRPV1 receptors in sympathetic sLTF. Using these elegant approaches, the innovation of this study was to show that AIH‐Hc leads to a robust carotid body sLTF and increases the carotid body sensitivity to hypoxia and temperature in naive rats.

Taking into consideration that this activity‐dependent plasticity in the carotid bodies during and after the apnoea‐like stimuli was observed in the absence of CIH preconditioning, Roy et al. (2018) suggested key mechanisms involved in this phenomenon. In the following experimental protocol, they reported that both non‐selective P2X receptor antagonist PPADS (pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulfonic acid) and the selective P2X2/3 inhibitor TNP‐ATP (2,4.6, trinitrophenol‐ATP) abolished sLTF. These findings indicate that ATP activation of P2X2/3 receptors is important in the maintenance of sLTF induced by apnoea‐like stimuli. In addition, using hyperoxia to silence glomus cells, the activation of postsynaptic purinergic receptors (from CSN terminals) stimulated carotid body activity. These data allowed the authors to suggest that sLTF induced by apnoea‐like stimuli depends on the participation of postsynaptic P2X receptor activation. This hypothesis was confirmed by the exogenous application of ATP which produced sLTF. To determine if TRPV1 receptors are also involved in the AIH‐Hc‐evoked sLTF, the authors used a TRPV1 antagonist (AMG9810) which halved the carotid body sLTF. Interestingly, Roy et al. (2018) evaluated the role of TRPV1 receptors using in vivo approaches in response to hyperoxia and hypoxia. This set of experiments showed that in their anaesthetized rat preparations the maintenance of carotid body sLTF is dependent on TRPV1 receptors only under normoxic conditions. The authors suggested that these findings may contribute to understanding of the link between the obstructive sleep apnoea, sympathetic overactivity and neurogenic hypertension.

Besides the mechanisms involved in the maintenance of sLTF (P2X2/3 and TRPV1 receptors), another relevant contribution of the study by Roy et al. (2018) is related to the mechanisms involved in the induction of this carotid body plasticity during apnoea‐like stimuli. It is known that 5‐hydroxytryptamine (5‐HT) and angiotensin type 1 (AT‐1) receptors evoke carotid body sLTF (Prabhakar et al. 2015). The application of the anti‐hypertensive drugs ketanserin (a 5‐HT receptor antagonist) and losartan (an AT‐1 receptor blocker) demonstrated that 5‐HT2 and AT‐1 receptors are involved in the induction, but not in the maintenance of sLTF (Roy et al. 2018). On the other hand, inhibition of protein kinase C (PKC) showed that the phosphorylation mediated by this protein is a key factor in both induction and maintenance of sLTF (Roy et al. 2018).

In conclusion, the set of results obtained by Roy et al. (2018) has advanced our knowledge of the plasticity induced by apnoea‐like stimuli due to activation of P2X and TRPV1 receptors in the CSN terminals from naive rats. Given that upregulation of the carotid body activity may lead to cardiovascular dysfunction, the pharmacological control of purinergic signalling and TRPV1 receptors may be a potential therapeutic target in the control of neurogenic hypertension associated with intermittent apnoea.

Additional information

Competing interests

None declared.

Author contributions

Both authors wrote and approved the final version of the manuscript. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Funding

The authors are supported by scholarships from Fundação de Amparo à Pesquisa e Desenvolvimento do Estado de São Paulo (FAPESP; grant no. 2017/09878‐0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).