Background The current effective treatment options for posttraumatic stress disorder (PTSD) are limited, and therefore the need to explore new treatment strategies is critical. Pharmacological inhibition of the renin-angiotensin system is a common approach to treat hypertension, and emerging evidence highlights the importance of this pathway in stress and anxiety. A recent clinical study from our laboratory provides evidence supporting a role for the renin-angiotensin system in the regulation of the stress response in patients diagnosed with PTSD. Methods With an animal model of PTSD and the selective angiotensin receptor type 1 (AT1) antagonist losartan, we investigated the acute and long-term effects of AT1 receptor inhibition on fear memory and baseline anxiety. After losartan treatment, we performed classical Pavlovian fear conditioning pairing auditory cues with footshocks and examined extinction behavior, gene expression changes in the brain, as well as neuroendocrine and cardiovascular responses. Results After cued fear conditioning, both acute and 2-week administration of losartan enhanced the consolidation of extinction memory but had no effect on fear acquisition, baseline anxiety, blood pressure, and neuroendocrine stress measures. Gene expression changes in the brain were also altered in mice treated with losartan for 2 weeks, in particular reduced amygdala AT1 receptor and bed nucleus of the stria terminalis c-Fos messenger RNA levels. Conclusions These data suggest that AT1 receptor antagonism enhances the extinction of fear memory and therefore might be a beneficial therapy for PTSD patients who have impairments in extinction of aversive memories.
Hypertension is a common disorder with uncertain etiology. In the last several years, it has become evident that components of both the innate and adaptive immune system play an essential role in hypertension. Macrophages andT cells accumulate in the perivascular fat, the heart and the kidney of hypertensive patients, and in animals with experimental hypertension. Various immunosuppressive agents lower blood pressure and prevent end-organ damage. Mice lacking lymphocytes are protected against hypertension, and adoptive transfer of T cells, but not B cells in the animals restores their blood pressure response to stimuli such as angiotensin II or high salt. Recent studies have shown that mice lacking macrophages have blunted hypertension in response to angiotensin II and that genetic deletion of macrophages markedly reduces experimental hypertension. Dendritic cells have also been implicated in this disease. Many hypertensive stimuli have triggering effects on the central nervous system and signals arising from the circumventricular organ seem to promote inflammation. Studies have suggested that central signals activate macrophages andT cells, which home to the kidney and vasculature and release cytokines, including IL-6 and IL-17, which in turn cause renal and vascular dysfunction and lead to blood pressure ele-vation.These recent discoveries provide a new understanding of hypertension and provide novel therapeutic opportunities for treatment of this serious disease.
Background
Psychological stress is a significant risk factor for hypertension and also directly affects the immune system. We have previously reported that T lymphocytes are essential for development of hypertension and that the central nervous system contributes to peripheral T lymphocyte activation and vascular inflammation in this disease, however the role of T cell activation in stress-related hypertension remains unclear.
Methods
Wild-type (WT) and T cell deficient (RAG-1−/−) mice were subjected to daily episodes of stress and blood pressure was measured. Circulating T cell activation markers and vascular infiltration of immune cells were analyzed as well as stress hormone levels and gene expression changes in the brain. The effects angiotensin II infusion in the presence of chronic stress was also studied.
Results
Repeated daily stress contributed to acute elevations in blood pressure that were associated with increased activation of circulating T cells and increased vascular infiltration of T cells. Repeated stress increased blood pressure in WT but not RAG-1−/− mice. Adoptive transfer of T cells to RAG-1−/− mice restored blood pressure elevation in response to stress. Stress-related hypertension and vascular infiltration of T cells was markedly enhanced by angiotensin II. Moreover, angiotensin II infused mice exposed to chronic stress exhibited greater blood pressure reactivity to an episode of acute stress.
Conclusions
These data demonstrate that stress-dependent hypertension triggers an inflammatory response that raises blood pressure at baseline and augments the hypertension caused by angiotensin II. These data provide insight as to how psychological stress contributes to hypertension.
Reactive oxygen species and the NADPH oxidases contribute to hypertension via mechanisms that remain undefined. Reactive oxygen species produced in the central nervous system have been proposed to promote sympathetic outflow, inflammation, and hypertension, but the contribution of the NADPH oxidases to these processes in chronic hypertension is uncertain. We therefore sought to identify how NADPH oxidases in the subfornical organ (SFO) of the brain regulate blood pressure and vascular inflammation during sustained hypertension. We produced mice with loxP sites flanking the coding region of the NADPH oxidase docking subunit p22. SFO-targeted injections of an adenovirus encoding cre-recombinase markedly diminished p22, Nox2, and Nox4 mRNA in the SFO, as compared with a control adenovirus encoding red-fluorescent protein injection. Increased superoxide production in the SFO by chronic angiotensin II infusion (490 ng/kg min ×2 weeks) was blunted in adenovirus encoding cre-recombinase-treated mice, as detected by dihydroethidium fluorescence. Deletion of p22 in the SFO eliminated the hypertensive response observed at 2 weeks of angiotensin II infusion compared with control adenovirus encoding red-fluorescent protein-treated mice (mean arterial pressures=97±15 versus 154±6 mm Hg, respectively; P=0.0001). Angiotensin II infusion also promoted marked vascular inflammation, as characterized by accumulation of activated T-cells and other leukocytes, and this was prevented by deletion of the SFO p22. These experiments definitively identify the NADPH oxidases in the SFO as a critical determinant of the blood pressure and vascular inflammatory responses to chronic angiotensin II, and further support a role of reactive oxygen species in central nervous system signaling in hypertension.
A prominent pathology textbook used in the United States includes an image illustrating the renal histopathology caused by malignant hypertension. The legend describes striking “onion skin” changes of a renal arteriole in the center of this figure. Curiously, a sea of mononuclear inflammatory cells surrounding this arteriole is overlooked both in the figure legend and in the related text. Moreover, nothing regarding inflammation or immune reactions is discussed. This lack of attention to inflammatory cells is, however, not surprising. While many experimental studies have implicated inflammation in hypertension, these have largely been performed in experimental animals and there is no proof that inflammation contributes to human hypertension. In fact, some anti-inflammatory or immune suppressing drugs (non-steroidal anti-inflammatory drugs and cyclosporine for example) paradoxically cause hypertension in humans, likely via off-target effects. Often the term “inflammation” is used in the context of cardiovascular disease as a catchall referring to non-specific phenomena such as elevation of C-reactive protein or the presence of macrophages in a tissue. Most clinicians and investigators find this vague and difficult to understand. Even more puzzling is that many studies now implicate the adaptive immune response, and in particular, lymphocytes in hypertension and vascular disease. Traditionally, bacterial, viral or tumor antigens activate this arm of immune defense. As such, it has been hard to imagine how adaptive immunity could be involved in a disease like hypertension. In this article, we will attempt to address some of these puzzling questions. We will briefly review components of the innate and adaptive immune response, discuss data from many groups, including our own, that suggest that common forms of hypertension are immune mediated, and provide a working hypothesis of how signals from the central nervous system trigger an immune response that causes hypertension.
Rationale
We have previously found that T lymphocytes are essential for development of angiotensin II-induced hypertension however the mechanisms responsible for T cell activation in hypertension remain undefined.
Objective
To study the roles of the central nervous system and pressure elevation in T cell activation and vascular inflammation caused by angiotensin II.
Methods and Results
To prevent the central actions of angiotensin II we created anteroventral third cerebral ventricle (AV3V) lesions in mice. The elevation in blood pressure in response to angiotensin II was virtually eliminated by AV3V lesions, as was activation of circulating T cells and the vascular infiltration of leukocytes. In contrast, AV3V lesioning did not prevent the hypertension and T cell activation caused by the peripheral acting agonist norepinephrine. To determine if T cell activation and vascular inflammation are due to central influences or are mediated by blood pressure elevation, we administered hydralazine (250 mg/L) in the drinking water. Hydralazine prevented the hypertension, and abrogated the increase in circulating activated T cells and vascular infiltration of leukocytes caused by angiotensin II.
Conclusions
We conclude that the central and pressor effects of angiotensin II are critical for T cell activation and development of vascular inflammation. These findings also support a feed forward mechanism in which modest degrees of blood pressure elevation lead to T cell activation, which in turn promotes inflammation and further raises blood pressure, leading to severe hypertension.
Brief summary
We have previously shown that T cells are important for the development of hypertension and others have shown that CNS lesions such as AV3V disruption prevent hypertension. We examined the relationship between central actions of angiotensin II, T cell activation and hypertension by determining how AV3V lesions affect T cell activation and hypertensive responses to angiotensin II and norepinephrine. Our data are compatible with a scenario in which modest degrees of pressure elevation, mediated either directly by norepinephrine or via central actions of angiotensin II, promote an inflammatory response that leads to severe hypertension. These studies provide new insight into how the central nervous system contributes to systemic inflammation in hypertension.
In recent years a major research effort has focused on the role of inflammation, and in particular adaptive immunity, in the genesis of hypertension. Hypertension stimulates the accumulation of inflammatory cells including macrophages and T lymphocytes in peripheral tissues important in blood pressure control, such as the kidney and vasculature. Angiotensin II modulates blood pressure via actions on the central nervous system (CNS) and the adaptive immune system. Recent work suggests that the central actions of angiotensin II via the circumventricular organs lead to activation of circulating T-cells and vascular inflammation. The neuroimmune system plays an essential role in the pathogenesis of hypertension and further understanding of this relationship could lead to the development of new treatment strategies.
Chronic exposure to interferon (IFN)-alpha, an innate immune cytokine, produces high rates of behavioral disturbances, including depression and fatigue. These effects may be mediated by the actions of IFN-alpha on dopamine (DA) metabolism in the basal ganglia. Diminished conversion of phenylalanine (Phen) to tyrosine (Tyr), the primary amino acid precursor of DA, has been associated with inflammation, and may reflect decreased activity of the enzyme phenylalanine-hydroxylase (PAH). This study investigated the peripheral Phen/Tyr ratio in relation to cerebrospinal fluid (CSF) concentrations of DA and its metabolites in subjects treated with IFN-alpha plus ribavirin for hepatitis C and controls awaiting IFN-alpha therapy. Plasma Phen/Tyr ratios were significantly increased in IFN-alpha-treated subjects (n=. 25) compared to controls (n=. 9), and were negatively correlated with CSF DA (r=. -0.59, df. =. 15, p < . 0.05) and its metabolite, homovanillic acid (r=. -0.67, df. =. 15, p < . 0.01), and positively correlated with fatigue (r=. 0.44, df. =. 23, p < . .05) in IFN-alpha-treated patients but not controls. Given the role of tetrahydrobiopterin (BH4) in the PAH conversion of Phen to Tyr, CSF concentrations of BH4 and its inactive oxidized form, dihydrobiopterin (BH2), were examined along with CSF interleukin (IL)-6 in a subset of patients. BH2 concentrations were significantly increased in IFN-alpha-treated patients (n=. 12) compared to controls (n=. 7), and decreased CSF BH4 concentrations correlated with increased CSF IL-6 (r=. -0.57, df. =. 12, p < . 0.05). These results indicate that IFN-alpha is associated with decreased peripheral conversion of Phen to Tyr, which in turn is associated with reduced DA in the brain as well as fatigue. These alterations may be related to oxidation of BH4 secondary to IFN-alpha-induced activation of a CNS inflammatory response.