The carotid body (CB) located in the bifurcation of the carotid arteries is the main peripheral chemoreceptor sensing arterial levels of PO₂, PCO₂, and pH. Also, changes in blood flow, temperature, osmolarity and glucose are able to elicit CB chemosensory excitation (Gonzalez et al., 1994; Pardal and López-Barneo, 2002; Iturriaga and Alcayaga, 2004; Iturriaga et al., 2007). The CB consists of clusters of chemoreceptor (glomus or type I) cells organized around the capillary network, synaptically connected to the nerve terminals of sensory neurons whose somata are in the petrosal ganglion, and surrounded by sustentacular glial (type II) cells. The most accepted model for chemoreception proposes that hypoxia closes K⁺ channels, leading to glomus cell depolarization, entry of Ca²⁺ and the release of excitatory transmitters (ACh and ATP), which in turn increases the discharge in the nerve endings of the chemosensory neurons (Iturriaga and Alcayaga, 2004; Iturriaga et al., 2007). In the last years, new exciting evidences have shown that the CB plays a crucial role in the pathogenesis of several human sympathetic-mediated diseases, including obstructive sleep apnea (OSA), congestive heart failure, resistant hypertension and insulin resistance (Koyama et al., 2000; Prabhakar et al., 2005; Schultz et al., 2007; Iturriaga et al., 2009; Abdala et al., 2012; Del Rio et al., 2013; Paton et al., 2013; Porzionato et al., 2013; Ribeiro et al., 2013). Accordingly, targeting the CB in several pathological conditions has been proposed to be a future promising therapeutic tool for the treatment of sympathetic-mediated diseases. Indeed, the selective ablation of the CB markedly improve rat survival in experimental heart failure (Del Rio et al., 2013), prevent the development of insulin resistance and hypertension in rats fed with high sucrose diet (Ribeiro et al., 2013) and reduced high blood pressure in neurogenic and resistant hypertension (McBryde et al., 2013; Paton et al., 2013).

The OSA syndrome elicited by repeated upper airways occlusion, is usually associated with daytime sleepiness, fatigue, and deficits in attention and executive function (Beebe and Gozal, 2002; Idiaquez et al., 2014). Furthermore, OSA is recognized as an independent risk factor for systemic hypertension (~50% of OSA patients develop diurnal hypertension, Somers et al., 2008; Calhoun, 2010), and is associated with stroke, pulmonary hypertension, coronary artery disease and atrial fibrillation (Fletcher, 2000; Parati et al., 2007; Somers et al., 2008; Dempsey et al., 2010). Indeed, several epidemiological studies have shown that OSA is an independent risk factor for the progression of the hypertension, showing a positive relationship between the apnea/hypopnea index (AHI) and high blood pressures (Young et al., 1993; Peppard et al., 2000; Eckert and Malhotra, 2008; Marin et al., 2012). Moreover, results obtained from the Wisconsin Sleep Cohort (an ongoing 21-years longitudinal study performed on 1500 Wisconsin state employees) showed that untreated OSA patients have a high mortality risk associated with AHI (Nieto et al., 2000; Young et al., 2008). According to the “Recommendations for the management of patients with obstructive sleep apnoea and hypertension” recently published by the European Union Cooperation in Scientific and Technological Research Action B26 on OSA, with the endorsement of the European Respiratory Society and the European Society of Hypertension (Parati et al., 2013) OSA is defined as “The combination of at least five obstructive breathing episodes per hour during sleep (apnoea, hypopnoea and respiratory effort related arousal events) and the following diagnostic criteria (A and/or B to be fulfilled). A: Excessive daytime sleepiness that is not better explained by other factors. B: Two or more of the following symptoms that are not better explained by other factors: Choking or gasping during sleep, recurrent awakenings from sleep, unrefreshing sleep, daytime fatigue and impaired concentration.” According to this study, the AHI defines the severity of OSA: mild OSA: AHI 5–15 events/h; moderate OSA: AHI 15–30 events/h and severe OSA: AHI > 30 events/h (Parati et al., 2013).

The cyclic obstruction of the upper airways during OSA leads to intermittent hypoxia and hypercapnia, negative intrathoraxic pressure, sleep fragmentation, and micro-arousals (Somers et al., 2008; Dempsey et al., 2010). During the airway occlusion, the resulting hypoxia and hypercapnia stimulates the CB chemoreceptor eliciting reflex acute sympathetic, hypertensive and hyperventilatory responses (Gozal and Kheirandish-Gozal, 2008; Somers et al., 2008; Garvey et al., 2009; Dempsey et al., 2010). Among these disturbances, the chronic intermittent hypoxia (CIH) is considered the main factor for the development of diurnal hypertension (Lavie, 2003; Gozal and Kheirandish-Gozal, 2008; Lévy et al., 2008; Somers et al., 2008; Arnardottir et al., 2009; Dempsey et al., 2010). Although the link between OSA and hypertension is well proved, the mechanisms underlying the pathogenesis of the hypertension are not entirely known. The most accepted proposal states that CIH elicits systemic oxidative stress, inflammation, and sympathetic hyperactivity, which led to endothelial dysfunction and the hypertension (Lavie, 2003; Somers et al., 2008; Garvey et al., 2009; Ryan et al., 2009; Dempsey et al., 2010). Nevertheless, conclusions from studies performed in OSA patients are controversial, because OSA patients often present concomitant morbidities (i.e., obesity and metabolic alterations), which are confounding factors that increase the cardiovascular risk. Thus, animal model of CIH, which simulates the hypoxic-reoxygenation episodes and reproduce several cardiovascular pathologic features of OSA including sympathetic hyperactivity and hypertension, are the gold-standard model to study mechanisms involved in OSA (Fletcher et al., 1992; Peng et al., 2003, 2011; Iturriaga et al., 2005, 2009; Prabhakar et al., 2005; Schulz et al., 2008; Dematteis et al., 2009; Del Rio et al., 2010, 2011a, 2012; Dumitrascu et al., 2013).

OSA produces sympathetic hyperactivity, demonstrated by an increased muscle sympathetic neural activity to blood vessels (Carlson et al., 1993) and excessive accumulation of urinary catecholamines (Dimsdale et al., 1995). Similarly, animals exposed to CIH show enhanced sympathetic responses to hypoxia, and develop systemic hypertension (Fletcher et al., 1992; Greenberg et al., 1999; Dick et al., 2007; Feng et al., 2008; Huang et al., 2009; Zoccal et al., 2009; Del Rio et al., 2010; Marcus et al., 2010). The autonomic dysfunction is characterized by enhanced sympathetic outflow, a reduction of the efficiency of the cardiac baroreflex sensitivity and alterations of heart rate variability (HRV). Indeed, non-invasive spectral analysis of HRV shows an increased ratio of low (LF) to high frequency (HF) band power, with a relative predominance of the LF band and a reduced contribution of the HF band, suggesting preponderance of the sympathetic drive in patients with OSA (Narkiewicz et al., 1998a; Shiomi et al., 1996) and animals exposed to CIH (Lai et al., 2006; Rey et al., 2008; Del Rio et al., 2010). Furthermore, it has been shown that CIH elicits vagal withdrawal, attributed in part to neuronal loss in ambiguous nucleus (Yan et al., 2008). Therefore, it is likely that the enhanced sympathetic to parasympathetic balance along with the reduction of the baroreflex could contribute to impair HRV and the regulation of vasomotor tone of blood vessels finally eliciting systemic hypertension.

In addition, OSA syndrome is also associated with endothelial dysfunction and vascular remodeling (Ip et al., 2004; Patt et al., 2010). OSA patients show an increased intima-media thickness (Minoguchi et al., 2005; Monneret et al., 2012) and a reduced nitric oxide-mediated vasodilatation (Kato et al., 2000). Similarly, some studies found that CIH reduced acetylcholine (ACh)-mediated vasodilation in rats (Tahawi et al., 2001; Dopp et al., 2011), but other reported a normal endothelial function in hypertensive CIH-treated rats (Julien et al., 2003; Lefebvre et al., 2006). Indeed, Lefebvre et al. (2006) found that CIH had no effect on the ACh-mediated vasodilatation of carotid, aortic and mesenteric beds, as well as on the contractile responses induced by noradrenaline and angiotensin II (Ang II) in arteries from CIH-rats compared to the arteries from control rats. However, they found that the contraction induced by endothelin-1 (ET-1) was higher in arteries from CIH-rats. More recently, Philippi et al. (2010) studied the time-course of the alteration of the endothelium dependent vasodilation in rats exposed to CIH. They found that CIH produces functional and structural changes in skeletal muscle arteries within the first 2 weeks of CIH, and those alterations were accompanied by systemic oxidative stress. Friedman et al. (2014) found that ROS generation during CIH activates NFATc3, which in turn increase the vascular response to ET-1. The administration of Tempol, a superoxide dismutase (SOD) mimetic, during CIH prevents the increased NFATc3 activity in the arteries from CIH-exposed mice, supporting that ROS is an important upstream signal in the CIH-induced NFATc3. Together, the available information suggest that vascular beds are affected by exposure to CIH, and that enhanced contractile responsiveness to vasoactive molecules such as ET-1 is critically dependent on ROS formation.

Patients recently diagnosed with OSA, present potentiated pressor and ventilatory responses to hypoxia (Narkiewicz et al., 1998a,b, 1999), suggesting that the peripheral hypoxic chemoreflex were enhanced by CIH. Fletcher et al. (1992) were the first to obtain evidences that the CB is involved in the hypertension induced by CIH. They found that the bilateral CB denervation prevented the development of hypertension in rats exposed to CIH for 35 days. Despite this seminal observation, the proposal that the CB contributes to the progression of the cardiovascular pathologies associated to OSA was not seriously considered. However, in the last decade a growing body of new evidences have support the proposal that the CB contributes to the progression of the CIH-induced hypertension (See for reviews: Prabhakar et al., 2005; Smith and Pacchia, 2007; Weiss et al., 2007; Somers et al., 2008; Garvey et al., 2009; Iturriaga et al., 2009; Dempsey et al., 2010). Recordings of rat and cat CB chemosensory discharges in situ and in vitro have demonstrate that CIH selectively increases basal chemosensory discharges in normoxia, and potentiates chemosensory and ventilatory responses to acute hypoxia (Peng et al., 2003, 2004; Rey et al., 2004, 2006; Prabhakar et al., 2005; Iturriaga et al., 2009; Del Rio et al., 2010, 2012). In addition, CIH induces plasticity of the CB chemosensory activity manifested as long-term facilitation. Indeed, Peng et al. (2003) found that chemosensory baseline discharges increased when the CB was excited by repetitive acute intermittent hypoxia in rats exposed to CIH. They reported that following 10 episodes of 12% O₂ lasting for 15 s, interspersed with 5 min of 95% O₂, the baseline chemosensory discharge increased with each episode of hypoxia, which persist for 60 min following the end of the hypoxic stimulus.

The mechanisms underlying the enhanced CB chemosensory reactivity to hypoxia induced by CIH are not entirely known (Iturriaga et al., 2009). Oxidative stress (Peng et al., 2003, 2009; Del Rio et al., 2010, 2012; Marcus et al., 2010), ET-1 (Rey et al., 2006, 2007; Pawar et al., 2009), Ang II (Lam et al., 2008, 2012; Fung, 2014), and pro-inflammatory cytokines (Iturriaga et al., 2009; Del Rio et al., 2011a, 2012; Lam et al., 2012) have been associated with the CB chemosensory potentiation. However, the primary molecular target responsible for the increased chemoreceptor discharge remains unknown. Recently, we studied the effects evoked by CIH on TASK K⁺ channel activity and the depolarization induced by acute hypoxia in CB glomus cells from adult rats exposed to CIH (Ortiz et al., 2013). We measured membrane potential, single channel and macroscopic currents in the presence of TEA and 4-aminopyridine in CB chemoreceptor cells isolated from adult rats exposed to CIH for 7 days. CIH treatment did not change the resting membrane potential, but the hypoxic-evoked depolarization increased by 2-fold. Moreover, the hypoxic inhibition of the open probability of the TASK-K⁺ channel was larger and faster in glomus cells from CIH-treated rats. This novel effect of CIH may contribute to explain the potentiation of CB oxygen chemoreception.

The sympathetic hyperactivity induced by CIH is likely to be the result of the enhanced CB chemosensory drive, but we cannot preclude excitatory effects of CIH on other structures of the chemorefelex pathway. Indeed, the same molecules that are involved in the enhanced CB chemosensitivity (e.g., Ang II, ET-1, and NO) could act at multiple sites to contribute to CIH-induced arterial blood pressure rise (e.g., higher CNS centers, peripheral arteries vessels). The chemosensory petrosal neurons that innervate the CB glomus cells project to the NTS in the brainstem, which is the main integrative nucleus for visceral inputs. The NTS send projections to the RVLM that contain the pre-sympathetic neurons projecting to the pre-ganglionar neurons in the spinal cord. RVLM neurons participate in the control of BP, and in the CB-mediated activation of the sympathetic responses (Guyenet et al., 2010). It has been shown that CIH increased the expression of the neuronal activation markers c-Fos, and FosB/ΔFos in the NTS and RVLM. Indeed, Greenberg et al. (1999) found that CIH-exposure of rats for 30 days increased c-fos labeling in the NTS and the RVLM. More recently, several studies reported that CIH increases FosB/ΔFosB in the subfornical organ, the median preoptic nucleus, the paraventricular nucleus, the NTS and the RVLM (Knight et al., 2011; Cunningham et al., 2012; Bathina et al., 2013). Thus, other structures outside the brainstem might contribute to intermittent hypoxia-induced hypertension (e.g., paraventricular nucleus of the hypothalamus, as shown by Sharpe et al., 2013). The available evidences strongly suggests that oxidative stress is the key mediator of the enhanced CB chemosensory responses to hypoxia and the hypertension induced by CIH, but the actions of the oxidative stress on the BP regulation in rats exposed to CIH may occur in multiple sites of the chemoreflex pathway, including the NTS, RVLM, and/or the arterial blood vessels. Indeed, it has been proposed that superoxide anions in the brainstem contribute to elevate the arterial blood pressure in rat models of neurogenic hypertension such as the stroke-prone spontaneously hypertensive rat (Kishi et al., 2004) and Ang II induced hypertension (Chan and Chan, 2012). Although it is well known that oxidative stress, produced by Ang II and NADPH activation, in the brainstem elicits sympathetic activation, the role played by the oxidative stress induced by CIH in the progression of the hypertension is less known. In addition, Marcus et al. (2012) found that CIH impairs the vasodilatory responses in small arteries isolated from the skeletal muscle circulation in rats, an effect blocked by losartan, a Ang II type 1 receptor blocker. Intermittent hypoxia also caused an increase in the ratio of Ang II type 1 receptors (responsible for vasoconstriction and trophic effects) to Ang II type 2 receptors (responsible for vasodilation and anti-trophic properties) in peripheral arteries. On the other hand, oxidative stress has also been involved in the impaired vasodilatation in response to ACh in rats exposed to CIH. Indeed, the treatment of CIH-exposed rats with Tempol restores the normal vascular function (Phillips et al., 2006). Moreover, Dopp et al. (2011) reported that concomitant treatment with allopurinol, a xanthine oxidase inhibitor, attenuated the impairment of ACh induced vasodilatation in gracillis arteries of rats exposed to CIH for 14 days.

The pathophysiological mechanisms involved in the development of hypertension in OSA are not fully understood. It is widely accepted that the CIH-induced oxidative stress contributes to enhance the CB chemosensory reactivity to oxygen and to the progression of the hypertension (Figure 1). Several studies have shown that concomitant administration of antioxidants, SOD mimetic, anti-inflammatory agents, ETA, and AT-1 receptor blockers, all of them reducing the levels of ROS formation and/or blocking the downstream signaling pathways induced by CIH, effectively prevents the enhanced CB chemosensory as well as the development of the hypertension. In addition, results showing that ablation of the CBs before the exposure to CIH significantly prevent the development of the hypertension strongly suggest a main role of the CB in the progression of the hypertension following CIH. However, the effect of the oxidative stress on the arterial blood pressure in rats exposed to CIH may also occur in multiple sites of the chemoreflex pathway, including the CB, the central cardiorespiratory centers and/or the arterial vessels. Thus, understanding how the oxidative stress and the molecules activated by CIH may interact at the CB and systemic levels would provide insights into the generation of the cardiovascular complications of OSA.