Association between flexibility activity and blood-pressure change among older adults in Japan: A 5-year longitudinal study

This longitudinal study examined the relationship between flexibility-activity and blood-pressure (BP) change among older adults in Japan. Our study included 452 older adults who took part in our survey in both 2012/2013 and 2017/2018. The seated systolic blood pressure (SBP) and diastolic BP (DBP) were measured both at baseline and at the 5 years follow-up. The frequencies of the different physical activities at baseline were assessed using a questionnaire. A generalized linear mixed model was used to estimate the non-standardized coefficient (B) of BP change associated with flexibility activity, after adjustments for sex, age, body mass index, smoking status, alcohol consumption, antihypertensive medication use, history of heart disease, walking time, and muscle-strengthening activity as a fixed-effect, and area of residence as a random-effect. Higher flexibility-activity frequency was significantly associated with reduced SBP (B = -0.77 [95% confidence intervals = -1.36, -0.18], p for linear trend = 0.01, p for quadratic trend = 0.85) and DBP (-0.33 [-0.71, 0.05], p for linear trend = 0.09, p for quadratic trend = 0.04). Engaging in flexibility activity for 5 days per week was significantly associated with a reduction in DBP (B = -4.16, 95% CI [-7.53, -0.79], p = 0.02) compared with that in the reference group (0 days per week). Interaction tests were not significant between basic variables (sex, age, BMI, and antihypertensive medication) and flexibility. In conclusion, higher flexibility activity frequency was associated with a reduction in BP in older adults. Future longitudinal and interventional studies should examine the effects of flexibility activity on cardiovascular disease prevention.

Impact of exercise on brain-bone marrow interactions in chronic stress: potential mechanisms preventing stress-induced hypertension

We examined the effect of chronic restraint stress and the counteractive effects of daily exercise on the molecular basis of the brain-bone marrow (BM) interactions, by especially focusing on the paraventricular nucleus (PVN) of the hypothalamus. Male Wistar rats were assigned into control, restraint stress, and stress + daily spontaneous exercise (SE) groups. BM and hypothalamic gene expression profiles were examined through the undertaking of RT-PCR and microarrays, respectively. The inflammatory blood cell population was investigated through flow cytometry. Through the use of immunohistochemistry, we examined the presence of BM-derived C-C chemokine receptor type 2 (CCR2)-expressing microglial cells in the rat PVN. The gene expression levels of BM inflammatory factors such as those of interleukin 1 beta and CCR2, and the inflammatory blood cell population were found to be significantly higher in both restrained groups compared with control group. Interestingly, chronic restraint stress alone activated the recruitment of BM-derived CCR2-expressing microglial cells into the PVN, whereas daily spontaneous exercise prevented it. A notable finding was that restraint stress upregulated relative gene expression of hypothalamic matrix metalloproteinase 3 (MMP3), which increases the permeability of the blood-brain barrier (BBB), and that exercise managed to normalize it. Moreover, relative expression of some hypothalamic genes directly involved in the facilitation of cell migration was downregulated by daily exercise. Our findings suggest that daily spontaneous exercise can reduce the numbers of BM-derived CCR2-expressing microglial cells into the PVN through the prevention of stress-induced changes in the hypothalamic gene expression.NEW & NOTEWORTHY Chronic restraint stress can upregulate MMP3 gene expression in the rat hypothalamus, whereas daily spontaneous exercise can prevent this stress-induced effect. Stress-induced BM-derived inflammatory cell recruitment into the rat PVN can be prevented by daily spontaneous exercise. Stress-induced increase of hypothalamic MMP3 gene expression may be responsible for BBB injury, thereby allowing for BM-derived inflammatory cells to be recruited and to accumulate in the rat PVN, and to be subsequently involved in the onset of stress-induced hypertension.

Conditional Regulation of Blood Pressure in Response to Emotional Stimuli by the Central Nucleus of the Amygdala in Rats

Humans and animals can determine whether a situation is favorable to them and act accordingly. For this, the autonomic tuning of the cardiovascular system to supply energy to active skeletal muscles through the circulatory system is as important as motor control. However, how the autonomic cardiovascular responses are regulated in dynamically changing environments and the neuronal mechanisms underlying these responses remain unclear. To resolve these issues, we recorded the blood pressure and heart rate of head-restrained rats during dynamically changing appetitive and aversive classical conditioning tasks. The rats displayed various associations between conditioned stimuli and unconditioned stimuli in appetitive (sucrose water), neutral (no outcome), and aversive (air puff) blocks. The blood pressure and heart rate in the appetitive block gradually increased in response to the reward-predicting cue and the response to the actual reward vigorously increased. The reward-predictive response was significantly higher than the responses obtained in the neutral and aversive condition blocks. To investigate whether the reward-predictive pressor response was caused by orofacial movements such as anticipatory licking behavior, we separately analyzed high- and low-licking trials. The conditioned pressor response was observed even in trials with low-licking behaviors. Blood pressure and heart rate responses to the air puff-predicting cue in the aversive block were not significantly different from the responses in the neutral block. The conditioned blood pressure response rapidly changed with condition block switching. Furthermore, to examine the contribution of the amygdala as an emotion center to these conditioned responses, we bilaterally microinjected a GABA_A receptor agonist, muscimol, into the central nucleus of the amygdala. Pharmacological inactivation of the central nucleus of the amygdala significantly decreased the reward-predictive pressor responses. These results suggest that the blood pressure is adaptively and rapidly regulated by emotional conditioned stimuli and that the central nucleus of the amygdala participates in regulating the pressor response in dynamically changing situations.

Implication of the Central Nucleus of the Amygdala in Cardiovascular Regulation and Limiting Maximum Exercise Performance During High-intensity Exercise in Rats

To date, the mechanism of central fatigue during high-intensity exercise has remained unclear. Here we elucidate the central mechanisms of cardiovascular regulation during high-intensity exercise with a focus on the hypothesis that amygdala activation acts to limit maximum exercise performance. In the first of three experiments, we probed the involvement of the central nucleus of the amygdala (CeA) in such regulation. Wistar rats were subjected to a maximum exercise test and their total running time and cardiovascular responses were compared before and after bilateral CeA lesions. Next, probing the role of central pathways, we tested whether high-intensity exercise activated neurons in CeA and/or the hypothalamic paraventricular nucleus (PVN) that project to the nucleus tractus solitarius (NTS). Finally, to understand the potential autonomic mechanisms affecting maximum exercise performance, we measured the cardiovascular responses in anesthetized rats to electrical microstimulation of the CeA, PVN, or both. We have found that (1) CeA lesions resulted in an increase in the total exercise time and the time at which an abrupt increase in arterial pressure appeared, indicating an apparent suppression of fatigue. (2) We confirmed that high-intensity exercise activated both the PVN-NTS and CeA-NTS pathways. Moreover, we discovered that (3) while stimulation of the CeA or PVN alone both induced pressor responses, their simultaneous stimulation also increased muscle vascular resistance. These results are evidence that cardiovascular responses during high-intensity exercise are affected by CeA activation, which acts to limit maximum exercise performance, and may implicate autonomic control modulating the PVN-NTS pathway via the CeA.

Estradiol-dependent gene expression profile in the amygdala of young ovariectomized spontaneously hypertensive rats

Estrogen plays a role in cardiovascular functions, emotional health, and energy homeostasis via estrogen receptors expressed in the brain. The comorbid relationship between rising blood pressure, a decline in mood and motivation, and body weight gain after menopause, when estrogen levels drop, suggests that the same brain area(s) contributes to protection from all of these postmenopausal disorders. The amygdala, a major limbic system nucleus known to express high estrogen receptor levels, is involved in the regulation of such physiological and psychological responses. We hypothesized that elevated estrogen levels contribute to premenopausal characteristics by activating specific genes and pathways in the amygdala. We examined the effect of 1-month estradiol treatment on the gene expression profile in the amygdala of ovariectomized young adult female spontaneously hypertensive rats. Estradiol substitution significantly decreased blood pressure, prevented body weight gain, and enhanced the voluntary physical activity of ovariectomized rats. In the amygdala of ovariectomized rats, estradiol treatment downregulated the expression of genes associated with estrogen signaling, cholinergic synapse, dopaminergic synapse, and long-term depression pathways. These findings indicate that the transcriptomic characteristics of the amygdala may be involved in estrogen-dependent regulation of blood pressure, physical activity motivation, and body weight control in young adult female spontaneously hypertensive rats.

Connectome analysis of male world-class gymnasts using probabilistic multishell, multitissue constrained spherical deconvolution tracking

In athletes, long‐term intensive training has been shown to increase unparalleled athletic ability and might induce brain plasticity. We evaluated the structural connectome of world‐class gymnasts (WCGs), as mapped by diffusion‐weighted magnetic resonance imaging probabilistic tractography and a multishell, multitissue constrained spherical deconvolution method to increase the precision of tractography at the tissue interfaces. The connectome was mapped in 10 Japanese male WCGs and in 10 age‐matched male controls. Network‐based statistic identified subnetworks with increased connectivity density in WCGs, involving the sensorimotor, default mode, attentional, visual, and limbic areas. It also revealed a significant association between the structural connectivity of some brain structures with functions closely related to the gymnastic skills and the D‐score, which is used as an index of the gymnasts' specific physical abilities for each apparatus. Furthermore, graph theory analysis demonstrated the characteristics of brain anatomical topology in the WCGs. They displayed significantly increased global connectivity strength with decreased characteristic path length at the global level and higher nodal strength and degree in the sensorimotor, default mode, attention, and limbic/subcortical areas at the local level as compared with controls. Together, these findings extend the current understanding of neural mechanisms that distinguish WCGs from controls and suggest brain anatomical network plasticity in WCGs resulting from long‐term intensive training. Future studies should assess the contribution of genetic or early‐life environmental factors in the brain network organization of WCGs. Furthermore, the indices of brain topology (i.e., connection density and graph theory indices) could become markers for the objective evaluation of gymnastic performance.

Regional brain gray matter volume in world-class artistic gymnasts

The relationship between long-term intensive training and brain plasticity in gymnasts has recently been reported. However, the relationship between abilities in different gymnastic events and brain structural changes has not been explored. This study aimed to evaluate the correlation between world-class gymnasts (WCGs)' specific abilities in different gymnastics events and their gray matter (GM) volume. Ten right-handed Japanese male WCGs and 10 right-handed gender- and age-matched controls with no history of gymnastic training participated in this study. Whole brain three-dimensional T1-weighted images (magnetization-prepared rapid gradient-echo sequence) with 0.90 mm3 voxels were obtained using a 3 T-MRI scanner from each subject. Volume-based morphometry (VolBM) was used to compare GM volume differences between WCGs and controls. We then explored the correlation between specific gymnastic abilities using different gymnastic apparatuses, and GM volume. Significantly higher GM volumes (false discovery rate-corrected p < 0.05) in the inferior parietal lobule, middle temporal gyrus, precentral gyrus, rostral middle frontal gyrus, and superior frontal gyrus were demonstrated in WCGs, compared with controls using VolBM. Moreover, significant positive correlations were observed between brain regions and the difficulty scores for each gymnastic event, for example, rings and inferior parietal lobule and parallel bars and rostral middle frontal gyrus. These results may reflect the neural basis of an outstanding gymnastic ability resulting from brain plasticity in areas associated with spatial perception, vision, working memory, and motor control.

Bidirectional cardiovascular responses evoked by microstimulation of the amygdala in rats

Although the amygdala is known as a negative emotion center for coordinating defensive behaviors, its functions in autonomic control remain unclear. To resolve this issue, we examined effects on cardiovascular responses induced by stimulation and lesions of the amygdala in anesthetized and free-moving rats. Electrical microstimulation of the central nucleus of the amygdala (CeA) induced a gradual increase in arterial pressure (AP) and heart rate (HR), whereas stimulation of adjacent nuclei evoked a phasic AP decrease. The gain of the baroreceptor reflex was not altered by CeA stimulation, suggesting that CeA activity increases both AP and HR by resetting baroreceptor reflex function. Disinhibition of GABAergic input by amygdalar microinjection of the GABAA receptor antagonist induced robust increases in AP and HR. Furthermore, bilateral electrolytic lesions of CeA evoked consistent AP increases over the light/dark cycle. These results suggest that the amygdala exerts 'bidirectional' autonomic control over the cardiovascular system.

Impact of heart-specific disruption of the circadian clock on systemic glucose metabolism in mice

The daily rhythm of glucose metabolism is governed by the circadian clock, which consists of cell-autonomous clock machineries residing in nearly every tissue in the body. Disruption of these clock machineries either environmentally or genetically induces the dysregulation of glucose metabolism. Although the roles of clock machineries in the regulation of glucose metabolism have been uncovered in major metabolic tissues, such as the pancreas, liver, and skeletal muscle, it remains unknown whether clock function in non-major metabolic tissues also affects systemic glucose metabolism. Here, we tested the hypothesis that disruption of the clock machinery in the heart might also affect systemic glucose metabolism, because heart function is known to be associated with glucose tolerance. We examined glucose and insulin tolerance as well as heart phenotypes in mice with heart-specific deletion of Bmal1, a core clock gene. Bmal1 deletion in the heart not only decreased heart function but also led to systemic insulin resistance. Moreover, hyperglycemia was induced with age. Furthermore, heart-specific Bmal1-deficient mice exhibited decreased insulin-induced phosphorylation of Akt in the liver, thus indicating that Bmal1 deletion in the heart causes hepatic insulin resistance. Our findings revealed an unexpected effect of the function of clock machinery in a non-major metabolic tissue, the heart, on systemic glucose metabolism in mammals.

Trpv4 involvement in the sex differences in blood pressure regulation in spontaneously hypertensive rats

Arterial pressure (AP) is lower in premenopausal women than in men of a similar age. Premenopausal women exhibit a lower sympathetic activity and a greater baroreceptor reflex; however, mechanisms controlling sex differences in blood pressure regulation are not well understood. We hypothesized that different neuronal functions in the cardiovascular centers of the brains of men and women may contribute to the sex difference in cardiovascular homeostasis. Our previous studies on male spontaneously hypertensive rats (SHRs) and their normotensive counterparts, Wistar Kyoto (WKY) rats, revealed that the gene-expression profile of the nucleus tractus solitarius (NTS), a region of the medulla oblongata that is pivotal for regulating the set point of AP, is strongly associated with AP. Thus, we hypothesized that gene-expression profiles in the rat NTS are related to sex differences in AP regulation. Because female SHRs clearly exhibit lower AP than their male counterparts of a similar age, we investigated whether SHR NTS exhibits sex differences in gene expression by using microarray and RT-qPCR experiments. The transcript for transient receptor potential cation channel subfamily V member 4 ( Trpv4) was found to be upregulated in SHR NTS in females compared with that in males. The channel was expressed in neurons and glial cells within NTS. The TRPV4 agonist 4-alpha-phorbol-12,13-didecanoate (4α-PDD) decreased blood pressure when injected into NTS of rats. These findings suggest that altered TRPV4 expression might be involved in the sex differences in blood pressure regulation.

Evidence for a histaminergic input from the ventral tuberomammillary nucleus to the solitary tract nucleus involved in arterial pressure regulation

The tuberomammillary nucleus (TMN) of the posterior hypothalamus has a high density of histaminergic neurons, the projection fibers of which are present in many areas of the brain, including the nucleus tractus solitarius (NTS), which controls arterial pressure (AP). In this study, we investigated whether the TMN–NTS pathway is involved in central cardiovascular regulation. Bicuculline, a gamma‐aminobutyric acid type A (GABA_A) receptor antagonist, was microinjected into the ventral TMN of anesthetized rats and its effects on AP and heart rate (HR) were observed. We also evaluated the effect of cetirizine, an H₁ receptor antagonist, microinjected into the NTS on cardiovascular responses induced by electrical stimulation of the TMN. Both AP and HR increased following bicuculline microinjection into the ventral TMN. Similar pressor and tachycardic responses were observed after electrical stimulation of the ventral TMN. Microinjection of cetirizine into the NTS partially inhibited the pressor response but had no effect on HR. Finally, the treadmill test was associated with a high level of c‐Fos expression in both ventral TMN and NTS neurons. These results suggest that the TMN–NTS pathway is involved in regulation of AP, presumably under a high‐arousal phase, such as that during exercise.

Combined long-term caffeine intake and exercise inhibits the development of diabetic nephropathy in OLETF rats

This study was performed to examine the effects of long-term caffeine-intake, with and without exercise, on the progression of diabetic nephropathy (DN) in an obese diabetic rat model. Thirty-two male Otsuka Long-Evans Tokushima fatty (OLETF) rats were assigned to sedentary (OLETF-Sed), exercise (OLETF-Ex), caffeine-intake (OLETF-Caf), and combined (OLETF-Caf + Ex) groups. Caffeine-intake groups were fed rat chow containing caffeine (90.7 ± 4.7 mg/kg/day). The OLETF-Ex and OLETF-Caf + Ex groups were able to run voluntarily at any time using a rotatory wheel. Body weight (BW) and blood pressure (BP) were measured weekly from 24 to 29 wk of age. Pre- and posttreatment serum glucose, insulin, and creatinine concentrations were measured, and a 24 h urine sample was collected for measurement of creatinine clearance (Ccr) and albumin excretion (UE_Alb). After treatment, the kidneys were removed for morphological analysis. The OLETF-Caf and OLETF-Caf + Ex groups exhibited no BP increase during the study. Both the caffeine-intake groups exhibited a significant increase in urine volume (UV), electrolyte excretion, and Ccr, and decreased UE_Alb, following treatment. Furthermore, no structural damage was observed in the kidneys of rats from either caffeine-intake group, whereas the OLETF-Sed and OLETF-Ex groups exhibited DN progression. This study demonstrates that caffeine-intake alone and/or combined with exercise significantly decreases BW and improves glucose intolerance, without the progression of DN. Further research should be performed to examine whether the quantities of caffeine contained in a normal human daily intake also have a protective effect against kidney damage.NEW & NOTEWORTHY The present study showed that caffeine administration alone and/or combined with exercise results in an improvement of diabetic nephropathy (DN), including an increase in creatinine clearance and urinary Na excretion, a decrease in urinary protein excretion, and in renal morphological findings. To our knowledge, there are no other studies showing that caffeine administration inhibits DN progression.

Systemic leukotriene B4 receptor antagonism lowers arterial blood pressure and improves autonomic function in the spontaneously hypertensive rat

KEY POINTS

Evidence indicates an association between hypertension and chronic systemic inflammation in both human hypertension and experimental animal models. Previous studies in the spontaneously hypertensive rat (SHR) support a role for leukotriene B₄ (LTB₄ ), a potent chemoattractant involved in the inflammatory response, but its mode of action is poorly understood. In the SHR, we observed an increase in T cells and macrophages in the brainstem; in addition, gene expression profiling data showed that LTB₄ production, degradation and downstream signalling in the brainstem of the SHR are dynamically regulated during hypertension. When LTB₄ receptor 1 (BLT1) receptors were blocked with CP-105,696, arterial pressure was reduced in the SHR compared to the normotensive control and this reduction was associated with a significant decrease in systolic blood pressure (BP) indicators. These data provide new evidence for the role of LTB₄ as an important neuro-immune pathway in the development of hypertension and therefore may serve as a novel therapeutic target for the treatment of neurogenic hypertension.

ABSTRACT

Accumulating evidence indicates an association between hypertension and chronic systemic inflammation in both human hypertension and experimental animal models. Previous studies in the spontaneously hypertensive rat (SHR) support a role for leukotriene B₄ (LTB₄ ), a potent chemoattractant involved in the inflammatory response. However, the mechanism for LTB₄ -mediated inflammation in hypertension is poorly understood. Here we report in the SHR, increased brainstem infiltration of T cells and macrophages plus gene expression profiling data showing that LTB₄ production, degradation and downstream signalling in the brainstem of the SHR are dynamically regulated during hypertension. Chronic blockade of the LTB₄ receptor 1 (BLT1) receptor with CP-105,696, reduced arterial pressure in the SHR compared to the normotensive control and this reduction was associated with a significant decrease in low and high frequency spectra of systolic blood pressure, and an increase in spontaneous baroreceptor reflex gain (sBRG). These data provide new evidence for the role of LTB₄ as an important neuro-immune pathway in the development of hypertension and therefore may serve as a novel therapeutic target for the treatment of neurogenic hypertension.

Altered neurotrophic factors' expression profiles in the nucleus of the solitary tract of spontaneously hypertensive rats

AIM

Our previous findings suggest that the nucleus of the solitary tract (NTS), a pivotal region for regulating the set point of arterial pressure, exhibits abnormal inflammation in pre-hypertensive and spontaneously hypertensive rats (SHRs), with elevated anti-apoptotic and low apoptotic factor levels compared with that of normotensive Wistar-Kyoto (WKY) rats. Whether this chronic condition affects neuronal growth and plasticity in the NTS remains unknown. To unveil the characteristics of the neurodevelopmental environment in the NTS of SHRs, we investigated the expression of neurotrophic factors transcripts in SHRs.

METHODS

RT(2) Profiler PCR Array targeting rat neurotrophins and their receptors was used to screen for differentially expressed transcripts in the NTS of SHRs compared to that of WKY rats. Protein expression and physiological functions of some of the differentially expressed transcripts were also studied.

RESULTS

Gene and protein expressions of glial cell line-derived neurotrophic factor family receptor alpha-3 (Gfrα-3) factor were both upregulated in the NTS of adult SHRs. Gene expressions of corticotropin-releasing hormone-binding protein (Crhbp), interleukin-10 receptor alpha (Il-10ra) and hypocretin (Hcrt) were downregulated in the NTS of adult SHRs. The Gfrα-3 transcript was increased and the Hcrt transcript was decreased in the NTS of young pre-hypertensive SHRs, suggesting that these profiles are not secondary to hypertension. Moreover, microinjection in the NTS of hypocretin-1 decreased blood pressure in adult SHRs.

CONCLUSION

These results suggest that altered neurotrophic factors transcript profiles may affect the normal development and function of neuronal circuitry that regulates cardiovascular autonomic activity, thereby resulting in manifestations of neurogenic hypertension in SHRs.

Spaceflight affects postnatal development of the aortic wall in rats

We investigated effect of microgravity environment during spaceflight on postnatal development of the rheological properties of the aorta in rats. The neonate rats were randomly divided at 7 days of age into the spaceflight, asynchronous ground control, and vivarium control groups (8 pups for one dam). The spaceflight group rats at 9 days of age were exposed to microgravity environment for 16 days. A longitudinal wall strip of the proximal descending thoracic aorta was subjected to stress-strain and stress-relaxation tests. Wall tensile force was significantly smaller in the spaceflight group than in the two control groups, whereas there were no significant differences in wall stress or incremental elastic modulus at each strain among the three groups. Wall thickness and number of smooth muscle fibers were significantly smaller in the spaceflight group than in the two control groups, but there were no significant differences in amounts of either the elastin or collagen fibers among the three groups. The decreased thickness was mainly caused by the decreased number of smooth muscle cells. Plastic deformation was observed only in the spaceflight group in the stress-strain test. A microgravity environment during spaceflight could affect postnatal development of the morphological and rheological properties of the aorta.

Blood pressure variability, impaired autonomic function and vascular senescence in aged spontaneously hypertensive rats are ameliorated by angiotensin blockade

OBJECTIVE

Elderly hypertensive patients are characterized by blood pressure (BP) variability, impaired autonomic function, and vascular endothelial dysfunction and stiffness. However, the mechanisms causing these conditions are unclear. The present study examined the effect of angiotensin receptor blockers (ARBs) on aged spontaneously hypertensive rats (SHR).

METHODS

We surgically implanted telemetry devices in SHR and WKY at the age of 15 weeks (Young) and 80 weeks (Aged). Aged SHR were orally administered either olmesartan or valsartan once daily at 19:00 h (at the beginning of the dark period (active phase)) for 4 weeks to examine the effects on BP variability, impaired autonomic function, and vascular senescence.

RESULTS

Aging and hypertension in SHR additively caused the following: increased low frequency (LF) power of systolic BP, a decreased spontaneous baroreceptor reflex gain (sBRG), increased BP variability, increased urinary norepinephrine excretion, increased vascular senescence-related beta-galactosidase positive cells and oxidative stress. Treatment with olmesartan or valsartan significantly ameliorated these changes in aged SHR. However, olmesartan ameliorated these changes in aged SHR better than valsartan. The reductions in BP caused by olmesartan in aged SHR were sustained longer than reductions by valsartan. This result indicates longer-lasting inhibition of the AT1 receptor by olmesartan than by valsartan.

CONCLUSION

ARBs ameliorated autonomic dysfunction, BP variability, and vascular senescence in aged SHR. Olmesartan ameliorated the aging-related disorders better than valsartan and was associated with longer-lasting AT1 receptor inhibition by olmesartan. Thus, the magnitude of improvement of these aging-related abnormalities differs for ARBs.

Activation of histamine H1 receptors in the nucleus tractus solitarii attenuates cardiac baroreceptor reflex function in rats

AIM

The nucleus tractus solitarii (NTS) is a central brainstem structure that plays an important role in regulating cardiovascular homeostasis. In this study, we examined whether H1 receptors in the NTS can control the baroreceptor reflex function by modulating synaptic transmission.

METHODS

Cardiac baroreceptor reflex function was assessed before and after the microinjection of 2-pyridylethylamine (10-25 nmol), a histamine H1 receptor-specific agonist, into the NTS of urethane-anaesthetized Wistar rats. The cardiovascular responses induced by l-glutamate microinjection into the NTS were also examined before and after the NTS administration of 2-pyridylethylamine.

RESULTS

Nucleus tractus solitarii microinjections of 2-pyridylethylamine significantly inhibited the gain of the cardiac baroreceptor reflex and bradycardiac/depressor responses induced by l-glutamate microinjection into the NTS. These findings suggest that histamine H1 receptors regulate the cardiac baroreceptor reflex in a post-synaptic manner to inhibit barosensitive NTS neurons.

CONCLUSION

Taken together with our previous findings, the present results provide further evidence that histamine may play a role within the NTS in regulating cardiovascular homeostasis.

Brain inflammation in neurogenic hypertension

One likely mechanism of essential hypertension (EH) is increased sympathoexcitation due to abnormal functions in the cardiovascular center of the brain. Recent findings obtained using experimental animal models of EH have shown that abnormal inflammation in the cardiovascular center may contribute to the onset of hypertension. Inflammatory molecules such as cytokines and reactive oxygen species released from the inflamed vasculature and glial cells in the medulla oblongata and hypothalamus might directly or indirectly affect neuronal functions. This in turn could increase sympathetic nerve activity and consequently arterial pressure. Abnormal inflammatory responses in the brain could also be central mechanisms underlying angiotensin II-related EH. In this review, we present the current understanding of EH mechanisms with regard to inflammatory responses in the cardiovascular center.

Novel mechanism for disrupted circadian blood pressure rhythm in a rat model of metabolic syndrome--the critical role of angiotensin II

BACKGROUND

This study was performed to determine the characteristics and mechanism of hypertension in SHR/NDmcr-cp(+/+) rats (SHRcp), a new model of metabolic syndrome, with a focus on the autonomic nervous system, aldosterone, and angiotensin II.

METHODS AND RESULTS

We measured arterial blood pressure (BP) in SHRcp by radiotelemetry combined with spectral analysis using a fast Fourier transformation algorithm and examined the effect of azilsartan, an AT1 receptor blocker. Compared with control Wistar-Kyoto rats (WKY) and SHR, SHRcp exhibited a nondipper-type hypertension and displayed increased urinary norepinephrine excretion and increased urinary and plasma aldosterone levels. Compared with WKY and SHR, SHRcp were characterized by an increase in the low-frequency power (LF) of systolic BP and a decrease in spontaneous baroreflex gain (sBRG), indicating autonomic dysfunction. Thus, SHRcp are regarded as a useful model of human hypertension with metabolic syndrome. Oral administration of azilsartan once daily persistently lowered BP during the light period (inactive phase) and the dark period (active phase) in SHRcp more than in WKY and SHR. Thus, angiotensin II seems to be involved in the mechanism of disrupted diurnal BP rhythm in SHRcp. Azilsartan significantly reduced urinary norepinephrine and aldosterone excretion and significantly increased urinary sodium excretion in SHRcp. Furthermore, azilsartan significantly reduced LF of systolic BP and significantly increased sBRG in SHRcp.

CONCLUSIONS

These results strongly suggest that impairment of autonomic function and increased aldosterone in SHRcp mediate the effect of angiotensin II on circadian blood pressure rhythms.

Excessive leukotriene B4 in nucleus tractus solitarii is prohypertensive in spontaneously hypertensive rats

Inflammation within the brain stem microvasculature has been associated with chronic cardiovascular diseases. We found that the expression of several enzymes involved in arachidonic acid-leukotriene B4 (LTB4) production was altered in nucleus tractus solitarii (NTS) of spontaneously hypertensive rat (SHR). LTB4 produced from arachidonic acid by 5-lipoxygenase is a potent chemoattractant of leukocytes. Leukotriene B4-12-hydroxydehydrogenase (LTB4-12-HD), which degrades LTB4, was downregulated in SHR rats compared with that in Wistar-Kyoto rats. Quantitative real-time PCR revealed that LTB4-12-HD was reduced by 63% and 58% in the NTS of adult SHR and prehypertensive SHR, respectively, compared with that in age-matched Wistar-Kyoto rats (n=6). 5-lipoxygenase gene expression was upregulated in the NTS of SHR (≈50%; n=6). LTB4 levels were increased in the NTS of the SHR, (17%; n=10, P<0.05). LTB4 receptors BLT1 (but not BLT2) were expressed on astroglia in the NTS but not neurons or vessels. Microinjection of LTB4 into the NTS of Wistar-Kyoto rats increased both leukocyte adherence and arterial pressure for over 4 days (peak: +15 mm Hg; P<0.01). In contrast, blockade of NTS BLT1 receptors lowered blood pressure in the SHR (peak: -13 mm Hg; P<0.05) but not in Wistar-Kyoto rats. Thus, excessive amounts of LTB4 in NTS of SHR, possibly as a result of upregulation of 5-lipoxygenase and downregulation of LTB4-12-HD, can induce inflammation. Because blockade of NTS BLT1 receptors lowered arterial pressure in the SHR, their endogenous activity may contribute to the hypertensive state of this rodent model. Thus, inflammatory reactions in the brain stem are causally associated with neurogenic hypertension.

Transcriptome of the NTS in exercise-trained spontaneously hypertensive rats: implications for NTS function and plasticity in regulating blood pressure

The nucleus tractus solitarii (NTS) controls the cardiovascular system during exercise, and alteration of its function may underlie exercise-induced cardiovascular adaptation. To understand the molecular basis of the NTS's plasticity in regulating blood pressure (BP) and its potential contribution to the antihypertensive effects, we characterized the gene expression profiles at the level of the NTS after long-term daily wheel running in spontaneously hypertensive rats (SHRs). Genome-wide microarray analysis was performed to screen for differentially expressed genes in the NTS between exercise-trained (12 wk) and control SHRs. Pathway analysis using the Kyoto Encyclopedia of Genes and Genomes database revealed that daily exercise altered the expression levels of NTS genes that are functionally associated with metabolic pathways (5 genes), neuroactive ligand-receptor interactions (4 genes), cell adhesion molecules (3 genes), and cytokine-cytokine receptor interactions (3 genes). One of the genes that belonged to the neuroactive ligand-receptor interactions category was histamine receptor H(1). Since we confirmed that the pressor response induced by activation of this receptor is increased after long-term daily exercise, it is suggested that functional plasticity in the histaminergic system may mediate the facilitation of blood pressure control in response to exercise but may not be involved in the lowered basal BP level found in exercise-trained SHRs. Since abnormal inflammatory states in the NTS are known to be prohypertensive in SHRs, altered gene expression of the inflammatory molecules identified in this study may be related to the antihypertensive effects in exercise-trained SHRs, although such speculation awaits functional validation.

Transcriptome of the NTS in exercise-trained spontaneously hypertensive rats: implications for NTS function and plasticity in regulating blood pressure

The nucleus tractus solitarii (NTS) controls the cardiovascular system during exercise, and alteration of its function may underlie exercise-induced cardiovascular adaptation. To understand the molecular basis of the NTS's plasticity in regulating blood pressure (BP) and its potential contribution to the antihypertensive effects, we characterized the gene expression profiles at the level of the NTS after long-term daily wheel running in spontaneously hypertensive rats (SHRs). Genome-wide microarray analysis was performed to screen for differentially expressed genes in the NTS between exercise-trained (12 wk) and control SHRs. Pathway analysis using the Kyoto Encyclopedia of Genes and Genomes database revealed that daily exercise altered the expression levels of NTS genes that are functionally associated with metabolic pathways (5 genes), neuroactive ligand-receptor interactions (4 genes), cell adhesion molecules (3 genes), and cytokine-cytokine receptor interactions (3 genes). One of the genes that belonged to the neuroactive ligand-receptor interactions category was histamine receptor H(1). Since we confirmed that the pressor response induced by activation of this receptor is increased after long-term daily exercise, it is suggested that functional plasticity in the histaminergic system may mediate the facilitation of blood pressure control in response to exercise but may not be involved in the lowered basal BP level found in exercise-trained SHRs. Since abnormal inflammatory states in the NTS are known to be prohypertensive in SHRs, altered gene expression of the inflammatory molecules identified in this study may be related to the antihypertensive effects in exercise-trained SHRs, although such speculation awaits functional validation.

Upregulation of junctional adhesion molecule-A is a putative prognostic marker of hypertension

Aims

Establishing biochemical markers of pre-hypertension and early hypertension could help earlier diagnostics and therapeutic intervention. We assess dynamics of junctional adhesion molecule-A (JAM-A) expression in rat models of hypertension and test whether JAM-A expression could be driven by angiotensin (ANG) II and whether JAM-A contributes to the progression of hypertension. We also compare JAM-A expression in normo- and hypertensive humans.

Methods and results

In pre-hypertensive and spontaneously hypertensive rats (SHRs), JAM-A protein was overexpressed in the brainstem microvasculature, lung, liver, kidney, spleen, and heart. JAM-A upregulation at early and late stages was even greater in the stroke-prone SHR. However, JAM-A was not upregulated in leucocytes and platelets of SHRs. In Goldblatt 2K-1C hypertensive rats, JAM-A expression was augmented before any increase in blood pressure, and similarly JAM-A upregulation preceded hypertension caused by peripheral and central ANG II infusions. In SHRs, ANG II type 1 (AT₁) receptor antagonism reduced JAM-A expression, but the vasodilator hydralazine did not. Body-wide downregulation of JAM-A with Vivo-morpholinos in juvenile SHRs delayed the progression of hypertension. In the human saphenous vein, JAM-A mRNA was elevated in hypertensive patients with untreated hypertension compared with normotensive patients but reduced in patients treated with renin–angiotensin system antagonists.

Conclusion

Body-wide upregulation of JAM-A in genetic and induced models of hypertension in the rat precedes the stable elevation of arterial pressure. JAM-A upregulation may be triggered by AT₁ receptor-mediated signalling. An association of JAM-A with hypertension and sensitivity to blockers of ANG II signalling were also evident in humans. We suggest a prognostic and possibly a pathogenic role of JAM-A in arterial hypertension.

Adiponectin, adiponectin receptors, and epigenetic regulation of adipogenesis

We studied the molecular mechanism of obesity-induced insulin resistance and adipogenesis. Plasma adiponectin and adiponectin receptor (AdipoR1) in muscle are down-regulated in obesity. Analysis of muscle-specific AdipoR1 knockout mice revealed the pivotal role of adiponectin/AdipoR1 in the regulation of mitochondrial biogenesis via AMPK- and SIRT1-mediated PGC-1α activation as well as Ca(2+)-dependent up-regulation of PGC-1α expression. Reduced adiponectin/AdipoR1 signals in muscle in obesity appear to cause PGC-1α inactivation as well as down-regulation and consequently impaired mitochondrial biogenesis and insulin resistance. In the epigenetic analysis of adipogenesis, we demonstrated that adipocyte-specific formaldehyde-assisted isolation of regulatory elements (FAIRE) peaks are associated with genes up-regulated by adipogenesis, whereas preadipocyte-specific FAIRE peaks are associated with genes down-regulated by adipogenesis. Computational motif analyses of adipocyte-specific FAIRE peaks confirmed PPARγ and CCAAT-enhancer binding proteins (C/EBPs) on the top list, consistent with their crucial roles in adipogenic transcription, and also revealed NFIA and NFIB to be important regulators of proper adipocyte differentiation.

Integration of metabolic and cardiovascular diurnal rhythms by circadian clock

Understanding how the 24-hour blood-pressure rhythm is programmed has been one of the most challenging questions in cardiovascular research. The 24-hour blood-pressure rhythm is primarily driven by the circadian clock system, in which the master circadian pacemaker within the suprachiasmatic nuclei of the hypothalamus is first entrained to the light/dark cycle and then transmits synchronizing signals to the peripheral clocks common to most tissues, including the heart and blood vessels. However, the circadian system is more complex than this basic hierarchical structure, as indicated by the discovery that peripheral clocks are either influenced to some degree or fully driven by temporal changes in energy homeostasis, independent of the light entrainment pathway. Through various comparative genomic approaches and through studies exploiting mouse genetics and transgenics, we now appreciate that cardiovascular tissues possess a large number of metabolic genes whose expression cycle and reciprocally affect the transcriptional control of major circadian clock genes. These findings indicate that metabolic cycles can directly or indirectly affect the diurnal rhythm of cardiovascular function. Here, we discuss a framework for understanding how the 24-hour blood-pressure rhythm is driven by the circadian system that integrates cardiovascular and metabolic function.

Acute reductions in blood flow restricted to the dorsomedial medulla induce a pressor response in rats

OBJECTIVES

The brainstem nucleus of the solitary tract (nucleus tractus solitarii, NTS) is a pivotal region for regulating the set-point of arterial pressure, the mechanisms of which are not fully understood. Based on evidence that the NTS exhibits O2-sensing mechanisms, we examined whether a localized disturbance of blood supply, resulting in hypoxia in the NTS, would lead to an acute increase in arterial pressure.

METHODS

Male Wistar rats were used. Cardiovascular parameters were measured before and after specific branches of superficial dorsal medullary veins were occluded; we assumed these were drainage vessels from the NTS and would produce stagnant hypoxia. Hypoxyprobe-1, a marker for detecting cellular hypoxia in the post-mortem tissue, was used to reveal whether vessel occlusion induced hypoxia within the NTS.

RESULTS

Following vessel occlusion, blood flow in the dorsal surface of the medulla oblongata including the NTS region showed an approximately 60% decrease and was associated with hypoxia in neurons located predominantly in the caudal part of the NTS as revealed using hypoxyprobe-1. Arterial pressure increased and this response was pronounced significantly in both magnitude and duration when baroreceptor reflex afferents were sectioned.

CONCLUSION

These results suggest that localized hypoxia in the NTS increases arterial pressure. We suggest this represents a protective mechanism whereby the elevated systemic pressure is a compensatory mechanism to enhance cerebral perfusion. Whether this physiological mechanism has any relevance to neurogenic hypertension is discussed.

Histamine receptor H1 in the nucleus tractus solitarii regulates arterial pressure and heart rate in rats

Axons of histamine (HA)-containing neurons are known to project from the posterior hypothalamus to many areas of the brain, including the nucleus tractus solitarii (NTS), a central brain structure that plays an important role in regulating arterial pressure. However, the functional significance of NTS HA is still not fully established. In this study, we microinjected HA or 2-pyridylethylamine, a HA-receptor H(1)-specific agonist, into the NTS of urethane-anesthetized Wister rats to identify the potential functions of NTS HA on cardiovascular regulation. When HA or H(1)-receptor-specific agonist was bilaterally microinjected into the NTS, mean arterial pressure (MAP) and heart rate (HR) were significantly increased, whereas pretreatment with the H(1)-receptor-specific antagonist cetirizine into the NTS significantly inhibited the cardiovascular responses. The maximal responses of MAP and HR changes induced by HA or H(1)-receptor-specific agonist were dose dependent. We also confirmed gene expression of HA receptors in the NTS and that the expression level of H(1) mRNA was higher than that of the other subtypes. In addition, we found that H(1) receptors are mainly expressed in neurons of the NTS. These findings suggested that HA within the NTS may play a role in regulating cardiovascular homeostasis via activation of H(1) receptors expressed in the NTS neurons.

Contributions of vascular inflammation in the brainstem for neurogenic hypertension

Essential hypertension is idiopathic although it is accepted as a complex polygenic trait with underlying genetic components, which remain unknown. Our supposition is that primary hypertension involves activation of the sympathetic nervous system. One pivotal region controlling arterial pressure set point is nucleus tractus solitarii (NTS). We recently identified that pro-inflammatory molecules, such as junctional adhesion molecule-1, were over expressed in endothelial cells of the microvasculature supplying the NTS in an animal model of human hypertension (the spontaneously hypertensive rat: SHR) compared to normotensive Wistar Kyoto (WKY) rats. We have also shown endogenous leukocyte accumulation inside capillaries within the NTS of SHR but not WKY rats. Despite the inflammatory state in the NTS of SHR, transcripts of some inflammatory molecules such as chemokine (C-C motif) ligand 5 (Ccl5), and its receptors, chemokine (C-C motif) receptor 1 and 3 were down-regulated in the NTS of SHR compared to WKY rats. This may be compensatory to avoid further strong inflammatory activity. More importantly, we found that down-regulation of Ccl5 in the NTS of SHR may be pro-hypertensive since microinjection of Ccl5 into the NTS of SHR decreased arterial pressure but was less effective in WKY rats. Leukocyte accumulation of the NTS microvasculature may also induce an increase in vascular resistance and hypoperfusion within the NTS; the latter may trigger release of pro-inflammatory molecules which via paracrine signaling may affect central neural cardiovascular activity conducive to neurogenic hypertension. All told, we suggest that vascular inflammation within the brainstem contributes to neurogenic hypertension by multiple pathways.

Metabolic cycles are linked to the cardiovascular diurnal rhythm in rats with essential hypertension

Background

The loss of diurnal rhythm in blood pressure (BP) is an important predictor of end-organ damage in hypertensive and diabetic patients. Recent evidence has suggested that two major physiological circadian rhythms, the metabolic and cardiovascular rhythms, are subject to regulation by overlapping molecular pathways, indicating that dysregulation of metabolic cycles could desynchronize the normal diurnal rhythm of BP with the daily light/dark cycle. However, little is known about the impact of changes in metabolic cycles on BP diurnal rhythm.

Methodology/Principal Findings

To test the hypothesis that feeding-fasting cycles could affect the diurnal pattern of BP, we used spontaneously hypertensive rats (SHR) which develop essential hypertension with disrupted diurnal BP rhythms and examined whether abnormal BP rhythms in SHR were caused by alteration in the daily feeding rhythm. We found that SHR exhibit attenuated feeding rhythm which accompanies disrupted rhythms in metabolic gene expression not only in metabolic tissues but also in cardiovascular tissues. More importantly, the correction of abnormal feeding rhythms in SHR restored the daily BP rhythm and was accompanied by changes in the timing of expression of key circadian and metabolic genes in cardiovascular tissues.

Conclusions/Significance

These results indicate that the metabolic cycle is an important determinant of the cardiovascular diurnal rhythm and that disrupted BP rhythms in hypertensive patients can be normalized by manipulating feeding cycles.

IL-6 microinjected in the nucleus tractus solitarii attenuates cardiac baroreceptor reflex function in rats

Recent gene array and molecular studies have suggested that an abnormal gene expression profile of interleukin-6 (IL-6) in the nucleus tractus solitarii (NTS), a pivotal region for regulating arterial pressure, may be related to the development of neurogenic hypertension. However, the precise functional role of IL-6 in the NTS remains unknown. In the present study, we have tested whether IL-6 affects cardiovascular control at the level of the NTS. IL-6 (1, 10, and 100 fmol) was microinjected in the NTS of Wistar rats (280-350 g) under urethane anesthesia. Although the baseline levels of arterial pressure and heart rate did not change following IL-6 injections, the cardiac baroreflex in response to increased arterial pressure was dose-dependently attenuated. In addition, IL-6 (100 fmol) microinjections also attenuated l-glutamate-induced bradycardia at the level of the NTS. Immunohistochemical detection of IL-6 in naïve rats demonstrated that it was predominantly observed in neurons within the brain stem, including the NTS. These findings suggest that IL-6 within the NTS may play an important role for regulating cardiovascular control via modulation of input signals from baroreceptor afferents. Whether the abnormal gene expression of IL-6 in the NTS is associated in a causal way with hypertension remains to be resolved.

The potency of different serotonergic agonists in counteracting opioid evoked cardiorespiratory disturbances

Serotonin receptor (5-HTR) agonists that target 5-HT(4(a))R and 5-HT(1A)R can reverse mu-opioid receptor (mu-OR)-evoked respiratory depression. Here, we have tested whether such rescuing by serotonin agonists also applies to the cardiovascular system. In working heart-brainstem preparations in situ, we have recorded phrenic nerve activity, thoracic sympathetic chain activity (SCA), vascular resistance and heart rate (HR) and in conscious rats, diaphragmatic electromyogram, arterial blood pressure (BP) and HR via radio-telemetry. In addition, the distribution of 5-HT(4(a))R and 5-HT(1A)R in ponto-medullary cardiorespiratory networks was identified using histochemistry. Systemic administration of the mu-OR agonist fentanyl in situ decreased HR, vascular resistance, SCA and phrenic nerve activity. Subsequent application of the 5-HT(1A)R agonist 8-OH-DPAT further enhanced bradycardia, but partially compensated the decrease in vascular resistance, sympathetic activity and restored breathing. By contrast, the 5-HT(4(a))R agonist RS67333 further decreased vascular resistance, HR and sympathetic activity, but partially rescued breathing. In conscious rats, administration of remifentanyl caused severe respiratory depression, a decrease in mean BP accompanied by pronounced bradyarrhythmia. 8-OH-DPAT restored breathing and prevented the bradyarrhythmia; however, BP and HR remained below baseline. In contrast, RS67333 further suppressed cardiovascular functions in vivo and only partially recovered breathing in some cases. The better recovery of mu-OR cardiorespiratory disturbance by 5-HT(1A)R than 5-HT(4(a))R is supported by the finding that 5-HT(1A)R was more densely expressed in key brainstem nuclei for cardiorespiratory control compared with 5-HT(4(a))R. We conclude that during treatment of severe pain, 5-HT(1A)R agonists may provide a useful tool to counteract opioid-mediated cardiorespiratory disturbances.

Chronic inhibition of standing behaviour alters baroreceptor reflex function in rats

AIM

To investigate whether daily orthostatic stress during development is an important factor affecting arterial baroreceptor reflex function, we examined the effect of chronic inhibition of upright standing behaviour on the baroreceptor reflex function in rats.

METHODS

Upright standing behaviour was chronically inhibited during the developmental period between 3 and 8 weeks of age in Sprague-Dawley rats and heart rate (HR) and aortic nerve activity in response to increased and decreased mean arterial pressure (MAP) was measured after the treatment period.

RESULTS

The baroreceptor cardiac gain in the rats grown without standing behaviour was significantly lower than the control rats grown in a normal commercial cage (1.0 +/- 0.1 beats min(-1) mmHg(-1) vs. 1.6 +/- 0.2 beats min(-1) mmHg(-1), P < 0.05). The range of HR change in the MAP-HR functional curve was also lowered by chronic inhibition of orthostatic behaviour (56.2 +/- 5.9 beats min(-1)) compared with that of the control rats (76.8 +/- 6.9 beats min(-1), P < 0.05). However the aortic afferent function remained normal after the treatment period, indicating that the attenuated baroreceptor reflex function may be due to other mechanisms involving functional alterations in the cardiovascular centres, efferents and/or peripheral organs. Body weight and adrenal weight were not affected by the inhibition of orthostatic behaviour, suggesting that the animals were not exposed to specific stress by this treatment.

CONCLUSION

These results indicate that active haemodynamic changes induced by orthostatic behaviour are an important factor for setting the basal level of reflex function during development. Moreover, our experimental model may be useful for studying mechanisms of attenuated baroreceptor reflex observed after exposure to a chronic inactive condition.