ABSTRACT: Obstructive sleep apnea (OSA) is a sleep-related breathing disorder that has becoming a more prevalent clinical condition in recent years. This clinical condition is responsible for a wide range of comorbidities, in particular systemic hypertension (HTN). OSA’s etiopathogenesis includes four clinical features, namely, chronic intermittent hypoxia (CIH), hypercapnia, increased intrathoracic pressure and sleep fragmentation. Among them, CIH is a well-established factor in the development of HTN. OSA is a major cause of resistant HTN, which means that the correct therapeutic strategy to control this particular type of HTN is still undefined. In fact, the antihypertensive effect of continuous positive airway pressure - CPAP (OSA’s gold standard therapeutic procedure) is moderate, and even traditional antihypertensive drugs show some lack of efficacy to control OSA-induced HTN, urging to find new suitable antihypertensive drugs. Therefore, the investigation for novel therapeutic targets is a hot topic in the field of OSA. Animal models of OSA, reproducing its major feature, CIH, have been used to investigate the mechanisms of HTN associated to CIH, in order to seek new therapeutic approaches, as this is the case of the present dissertation. The general goal of this thesis is to find a novel therapeutic target to HTN induced by CIH, with the use of an animal model. Most of the pharmacological approaches for HTN in CIH consisted in the abrogation of the sympathetic and/or the renin-angiotensin-aldosterone systems (RAAS), with some positive results, but still with weak outcomes. Indeed, previous results from our group showed that carvedilol (an α-and β -adrenergic blocker) had no antihypertensive effect in an animal model of HTN induced by CIH. These facts led us to deduce that other signaling pathways directly stimulated by CIH at cellular level should be investigated and pharmacologically manipulated to infer its effect over BP. We believe that transcription factors, as they are involved in long-term adaptive regulations, are plausible mechanistic bridges between the immediate cyclical intermittent hypoxia cycles and the long-term deleterious effects of CIH-OSA, as HTN. With this in mind, four major arguments converge to sustain the assumption that the transcription factor aryl hydrocarbon receptor (AHR) might be a putative mechanistic link between CIH and HTN: 1) Hypoxia inducible factor 1α (HIF-1α), a hypoxic signaling pathway known to be triggered by CIH, and AHR share the same binding partner to assist their canonical activation routes, suggesting a potential crosstalk/interplay between HIF-1 and AHR; 2) AHR has an important role in the regulation of blood pressure; 3) Some of the molecular mechanisms leading to HTN are common for AHR and CIH (RAAS activation, oxidative stress, endothelial dysfunction, regulation of central cardiovascular areas (for instance, in brainstem); 4) CIH, as a chronic inflammatory state, can lead to the overproduction of certain metabolites known to be AHR ligands (e.g. kynurenine metabolites). Thus, the working hypothesis of this thesis is the following: CIH upregulates the AHR circuitry, and consequently contributes to the development of systemic HTN. This thesis aims to investigate, for the first time, the effect of CIH on the AHR circuitry and the consequences of its pharmacological manipulation on HTN induced by CIH. In order to pursue this objective, male Wistar Han rats, aged 8-12 weeks, were exposed to a mild CIH paradigm (5.6 cycles/hour, 10.5 hours/day, from 9.30am to 8pm). We initiated our experiments by exposing groups of rats to a time-course of CIH. Groups of rats were exposed to 14, 21 and 60 days of CIH, and for each time-point, a corresponding control/normoxic (Nx) group was also used (n=5-8/group). At the end of the exposure, the animals were sacrificed and blood and several organs (renal cortex, renal medulla, liver, visceral adipose tissue, spleen and hippocampus) were collected. From those organs, we extracted total ribonucleic acids (RNA) to perform quantitative polymerase chain reaction analysis (qPCR) for several genes, namely genes involved in the AHR pathway (Ahr, Cyp1a1, Cyp1b1, Cyp1a2, Arnt), hypoxia inducible factor (HIF) family (Hif1a, Epas1, Hif3a, Vegfa), NF-kB pathway (Rela, Nfkb2, Il6, Il1b), RAAS members (Ren,Ace, Agtr1) and epithelial-mesenchymal transition and fibrosis markers (Fn1, Vim, Cdh1, Col1a1, Acta2). Moreover, considering that CIH is an oxidative stress disorder, we analyzed, by HighPerformance Liquid Chromatography with Fluorescence Detection (HPLC-FD), the dynamic of cysteine (Cys), an important antioxidant thiol, in the renal tissues of rats exposed to the CIH time-course. We also quantified several tryptophan metabolites by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in plasma and urine of several time-points (1, 7, 14, 21 and 60 days) and in kidney of CIH-exposed rats (21 days). Following these experiments, we modulated pharmacologically the AHR pathway, with an AHR antagonist, CH-223191 (5 mg/kg, once a day, by oral gavage in 1 ml of vegetable oil). We used the antagonist in two major sets of experiments. One set was designed to investigate particularly the preventive role of this compound on CIH-induced HTN, while the other aimed to determine its capacity to revert this outcome. BP and HR measurements were determined by radiotelemetry, at 8am (active period, lights-off) and 6pm (inactive period, lights-on). Following 21 days of CIH exposure, RAAS genes were overexpressed in the kidney, as well as some HIF family genes (Hif1a, Epas1 and Hif3a). Rela mRNA expression was upregulated after 21 days and Nkfb2 after 60 days of CIH, in the renal cortex. Visceral adipose tissue mRNA expressions of Nfkb2 and Il6 were increased, reflecting the inflammatory effects of CIH on the adipose tissue. In the same time-point (21 days), expression of epithelial-mesenchymal transition genes was increased (Fn1 and Vim overexpressions) in the kidney. Kidney cysteine dynamic was altered with hypoxia. Over time, we found an overall decrease in total cysteine content and in the protein bound fraction, and a U-shaped variation in their redox couple (reduced/oxidized), suggesting that kidney is susceptible to the oxidative damage triggered by CIH.In addition, we observed, in the kidney, an elevated kynurenine:tryptophan ratio (KTR), revealing a pro-kynurenine state, which may lead to AHR activation. In contrast, we found increased plasma levels of several serotonin-related metabolites. The AHR antagonist, CH-223191, was able to prevent and revert partially the HTN during the active phase of the animals (lights-off period). However, during the inactive phase (lights-on, simultaneous to the hypoxic cycles), the antihypertensive effect of the AHR antagonist was absent. Also, the compound did not recover the loss of the dipping profile of BP and HR originated by CIH. In conclusion, the AHR pathway seems to be overactivated in the kidney, upon CIH exposure, and its pharmacological modulation was able to counteract the increased BP, showing a significant antihypertensive during the active period. These results stimulate further studies to clarify the full potential of this pathway as a novel therapeutic target to HTN induced by CIH/OSA, as well as their mechanisms of action.