Carotid body activity – are we eating our way into the ventilatory pathway?

Maintenance of the partial pressure of arterial O2 and CO2 blood gases ( and , respectively) is coordinated through two distinct, but interacting chemoreflex feedback loops. From a traditional perspective, the central respiratory chemoreceptors respond slowly to increases in (i.e. hypercapnia) within the brainstem where central chemoreceptors reside. The peripheral chemoreceptors (e.g. carotid bodies (CBs) and aortic body) respond more quickly to increases in (i.e. hypercapnia) and/or decreases in (i.e. hypoxia). It is difficult to separate the contribution of these chemoreceptors to ventilation in humans, but the animal model provides a unique setting to tightly control the blood gas stimulus delivered to both central and peripheral chemoreceptors. In more recent years, it has become apparent that the chemoreflex control of breathing is sensitive not only to alterations in arterial blood gases, but also to other physiological entities such as leptin, which was elegantly explored recently in an article in this issue of The Journal of Physiology by Ribeiro et al. (2018). Leptin, a hormone within the body responsible for food intake regulation and energy expenditure, has been shown to elicit powerful ventilatory effects mediated via central nervous system pathways. For example, a study by Inyushkina and colleagues (2009) used rats to study the effects of leptin microinjections within the solitary tract nucleus and observed rapid alterations in ventilation. However, the role of leptin on peripheral vs. central chemoreceptor activity remains unclear. Indeed, a recent study by Olea et al. (2015) focused on the role of leptin and the leptin receptor ObRb in CB cells. The current study by Ribeiro and colleagues (2018) investigated the effects of leptin on CB ventilatory control and energy metabolism in both obese and normal rats. It is worth noting that both rats and humans express leptin receptors within the CB. The study conducted by Ribeiro et al. (2018) demonstrated that in the high fat (HF) rat model the respiratory response to leptin administration was blunted. Similarly, research conducted by Polotsky et al. (2001) found that in obese female mice, an increased amount of leptin was required to maintain a regular ventilatory response, but this may have been due to a decline in progesterone and oestradiol sex hormones. The latter finding highlights the critical importance of conducting future studies that focus on the potential sex differences in the role of leptin on the control of breathing. The study by Ribeiro and colleagues used 3-month-old Wistar rats, which were divided into two separate groups and fed predetermined diets for 3 weeks. The control group was fed a standard diet consisting of 14.53% protein, 55.06% carbohydrates and 10.00% fat. The HF diet group was prescribed an elevated fat (23.10%), elevated protein (34.90%) and reduced carbohydrate (25.90%) diet. The HF group was designed to replicate a diet that would result in obesity, insulin resistance, and hypertension after long-term adherence. The primary findings were the following: (1) leptin administration increased minute ventilation in the control rats, but this effect was blunted in the HF diet rats suffering from metabolic syndrome, (2) an increase in body weight influenced ventilation due to lung restriction, resulting in an elevated ventilatory drive and thus respiratory rate, and (3) leptin administration altered carotid sinus nerve (CSN) discharge in the control rats, but not in the HF rats. The study by Ribeiro et al. (2018) was commendable, but there were some aspects of the investigation that warrant further comment. The current study only used male rats; however, other studies have previously demonstrated that leptin levels are higher in female rats when fed both regular and HF diets, compared to male rats on the same diet with similar weight, and therefore female rats could potentially suffer from a greater degree of respiratory depression. This could be explained through interactions between increased leptin for similar ventilatory values, an increased leptin sensitivity related to ventilatory drive, or differences in fat type and distribution (Polotsky et al. 2001). Human studies have shown that differences in fat distribution between males and females results in higher leptin levels in females compared to males, but in rats, it is unclear if this relationship plays a role in basal leptin differences between males and females (Polotsky et al. 2001). Circulating leptin in the blood has been shown to influence ventilation in a similar manner to hypoxia by mechanisms related to the CB phosphorylation. The current study exposed control and HF diet rats to bilateral occlusions of the carotid artery after leptin bolus injections in either the external carotid artery or through the femoral vein. It was seen that leptin increased basal carotid sinus nerve (CSN) chemosensory activity by up to 111.50% in control rats, but this was not observed in the HF rat group. Importantly, leptin administration increased the release of adenosine by 186.9% in the control group, but in contrast, it did not modify adenosine release in HF rats. These data highlight the importance of leptin in the control of breathing, since adenosine is one of the key mediators involved in the response to hypoxia in animals. The findings in this study are further supported by other reports. For example, a study by Olea et al. (2015) concluded that leptin did not alter CB-related catecholamine release. However, in contrast with the findings by Ribeiro et al. (2018), Olea and colleagues (2015) suggested that leptin injections were ineffective, likely because leptin mediates ventilation via central mechanisms. Due to the relationship between these recently published reports, the role of leptin on CB catecholamine release is further solidified, but it should still be explored in future investigations, especially with comparisons between different injection sites, and employing a similar experimental approach as Ribeiro and colleagues. The authors of the current study should be praised for their use of multiple injection sites and various leptin doses, and for comparing between normoxic and hypoxic environments, and future work should aim to employ similar experimental designs, and importantly, to determine the role of leptin on chemosensitivity in both the human and rat model. A consideration for future work is that although type I CB cells are found in both human and rat, human CB cells are both positive and negative for leptin and its receptors, whereas data from rat CB cells suggest that they are all positive. Rats also have tyrosine hydroxylase-positive cells in addition to the presence of leptin receptors, whereas only 35.1% of these cells were found in humans (Porzionato et al. 2011). Another notable difference is that rats have leptin receptors in almost every type I cell, but humans only have them in a subpopulation, although human receptors seem to evoke more powerful ventilatory responses. Additionally, leptin replacements in obese mice have been consistently found to increase basal ventilation and the ventilatory response to CO2, but in humans this relationship is more unclear (Porzionato et al. 2011). Due to the differences between cell types, receptors and ventilatory responses between animals and humans, the applicability of the results from Ribeiro et al. (2018) remains debateable. The relationship between leptin, ventilation and obesity is an area of high research interest, and the role of leptin on both central and peripheral chemoreceptor activity and interaction requires further investigation. The landmark study by Ribeiro and colleagues (2018) determined the effects of leptin on carotid body activity using sophisticated methodological techniques. However, more research is required in order to determine the contribution of both carotid body and aortic body activity to the ventilatory response to leptin, and whether there are sex differences in the ventilatory response to leptin, and ultimately, future studies in this area should aim to bridge the gap between the animal and human model. In addition, due to the increasing prevalence of obesity in developed countries, which could potentially lead to long-term alterations in circulating leptin and leptin sensitivity, future related research should focus on sympathetic nervous system over-activity and carotid body discharge. Furthermore, obesity-associated physiological changes could result in insulin resistance and hypertension, and elucidating the leptin-related mechanisms outlined by the current study could further our understanding on the detrimental effects of HF diets. None All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. We would like to thank Mr Michael M. Tymko (UBC–Okanagan Campus) for guidance and insightful feedback.