Exercise Training Differentially Affects Intrinsic Excitability of Autonomic and Neuroendocrine Neurons in the Hypothalamic Paraventricular Nucleus

Effects of physical training on hypothalamic neuronal activation and expressions of vasopressin and oxytocin in SHR after running until fatigue

To assess the influence of physical training on neuronal activation and hypothalamic expression of vasopressin and oxytocin in spontaneously hypertensive rats (SHR), untrained and trained normotensive rats and SHR were submitted to running until fatigue while internal body and tail temperatures were recorded. Hypothalamic c-Fos expression was evaluated in thermoregulatory centers such as the median preoptic nucleus (MnPO), medial preoptic nucleus (mPOA), paraventricular nucleus of the hypothalamus (PVN), and supraoptic nucleus (SON). The PVN and the SON were also investigated for vasopressin and oxytocin expressions. Although exercise training improved the workload performed by the animals, it was reduced in SHR and followed by increased internal body temperature due to tail vasodilation deficit. Physical training enhanced c-Fos expression in the MnPO, mPOA, and PVN of both strains, and these responses were attenuated in SHR. Vasopressin immunoreactivity in the PVN was also increased by physical training to a lesser extent in SHR. The already-reduced oxytocin expression in the PVN of SHR was increased in response to physical training. Within the SON, neuronal activation and the expressions of vasopressin and oxytocin were reduced by hypertension and unaffected by physical training. The data indicate that physical training counterbalances in part the negative effect of hypertension on hypothalamic neuronal activation elicited by exercise, as well as on the expression of vasopressin and oxytocin. These hypertension features seem to negatively influence the workload performed by SHR due to the hyperthermia derived from the inability of physical training to improve heat dissipation through skin vasodilation.

Exercise-induced neuroplasticity in autonomic nuclei restores the cardiac vagal tone and baroreflex dysfunction in aged hypertensive rats

Aging is accompanied by considerable deterioration of homeostatic systems, such as autonomic imbalance characterized by heightened sympathetic activity, lower parasympathetic tone, and depressed heart rate (HR) variability, which are aggravated by hypertension. Here, we hypothesized that these age-related deficits in aged hypertensive rats can be ameliorated by exercise training, with benefits to the cardiovascular system. Therefore, male 22-mo-old spontaneously hypertensive rats (SHRs) and age-matched Wistar Kyoto (WKY) submitted to moderate-intensity exercise training (T) or kept sedentary (S) for 8 wk were evaluated for hemodynamic/autonomic parameters, baroreflex sensitivity, cardiac sympathetic/parasympathetic tone and analysis of dopamine β-hydroxylase (DBH+) and oxytocin (OT+) pathways of autonomic brain nuclei. Aged SHR-S versus WKY-S exhibited elevated mean arterial pressure (MAP: +51%) and HR (+20%), augmented pressure/HR variability, no cardiac vagal tone, and depressed reflex control of the heart (HR range, -28%; gain, -49%). SHR-T exhibited a lower resting HR, a partial reduction in the MAP (-14%), in the pressure/HR variabilities, and restored parasympathetic modulation, with improvement of baroreceptor reflex control when compared with SHR-S. Exercise training increased the ascending DBH+ projections conveying peripheral information to the paraventricular nucleus of hypothalamus (PVN), augmented the expression of OT+ neurons, and reduced the density of DBH+ neurons in the rostral ventrolateral medulla (RVLM) of SHR-T. Data indicate that exercise training induces beneficial neuroplasticity in brain autonomic circuitry, and it is highly effective to restore the parasympathetic tone, and attenuation of age-related autonomic imbalance and baroreflex dysfunction, thus conferring long-term benefits for cardiovascular control in aged hypertensive individuals.NEW & NOTEWORTHY Exercise training reduces high blood pressure and cardiovascular autonomic modulation in aged hypertensive rats. The dysfunction in the baroreflex sensitivity and impaired parasympathetic tone to the heart of aged hypertensive rats are restored by exercise training. Exercise induces beneficial neuroplasticity in the brain nuclei involved with autonomic control of cardiovascular function of aged hypertensive rats.

Forebrain control of breathing: Anatomy and potential functions

The forebrain plays important roles in many critical functions, including the control of breathing. We propose that the forebrain is important for ensuring that breathing matches current and anticipated behavioral, emotional, and physiological needs. This review will summarize anatomical and functional evidence implicating forebrain regions in the control of breathing. These regions include the cerebral cortex, extended amygdala, hippocampus, hypothalamus, and thalamus. We will also point out areas where additional research is needed to better understand the specific roles of forebrain regions in the control of breathing.

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.

Exercise training rescues the electrical activity of liver-projecting DMNV neurones in response to oxytocin in spontaneously hypertensive rats

A neural circuit between the paraventricular nucleus of the hypothalamus (PVN) and the dorsal motor nucleus of the vagus (DMNV) constitutes part of an important parasympathetic autonomic pathway that controls hepatic glucose production. Intracerebroventricular injection of insulin activates oxytocinergic neurones in the PVN and elicits the release of oxytocin into the circulation, which plays an important role in the metabolism of glucose. Moreover, the central action of insulin can reduce the concentration of glucose in blood taken from the hepatic vein of Wistar rats via activation of vagal efferent nerves to the liver. This mechanism is impaired in sedentary spontaneously hypertensive rats (SHR). Because aerobic exercise increases vagal tone, partly mediated by increasing the oxytocinergic connections between the PVN and DMNV, we hypothesised that oxytocin (OT) might alter the excitability of liver-projecting DMNV neurones. Thus, we investigated the effects of OT on electrical properties of the liver-projecting DMNV neurones from Wistar, SHR subjected to 4 weeks of exercise training, as well sedentary controls, using whole cell patch-clamping. The results show that OT increased the resting membrane potential of DMNV neurones in Wistar rats, as well as the firing frequency of these cells, but not in sedentary SHR. However, in SHR subjected to 4 weeks of exercise training, the effects of OT on liver-projecting DMNV neurones of were similar to those seen in Wistar rats. These findings show that OT elicits similar changes in the electrophysiological properties of liver-projecting DMNV neurones of Wistar and exercise-trained but not sedentary SHR. These results indicate that exercise training can restore the sensitivity of liver-projecting DMNV neurones of exercise-trained SHR to OT.

Activated Oxytocin Neurons in the PVN-DVC Pathway in Asthmatic Rats

Asthma is a heterogeneous disease, and the central nervous system (CNS) also participates in the pathogenesis of asthma. We previously reported the amygdala might regulate asthmatic attacks via projecting to the paraventricular hypothalamic nucleus (PVN). The dorsal vagal complex (DVC) is a crucial region that modulates respiratory. This study aimed to observe the activity in both PVN and DVC and the connection between PVN and DVC in asthmatic rats. Immunohistochemistry was conducted to observe the changes in Fos and oxytocin (OT) expression. Retrograde tracing using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and double immunohistochemistry for OT and Fos was used to observe the HRP/OT/Fos positive neurons distribution in the PVN. The results showed that during an asthma attack, the Fos positive neurons increased in both PVN and DVC over time. The expression of OT positive neurons in PVN showed a similar trend in parallel to the c-Fos positive neurons in PVN. The HRP retrograde-labeled neurons were densely distributed in the medial and lateral subnucleus in the PVN. OT⁺/HRP⁺ and Fos⁺/OT⁺/HRP⁺ accounted for 18.14%, and 2.37% of HRP-labeled neurons, respectively. Our study showed PVN and DVC were activated and the expression of OT positive neurons in PVN were increased over time during an asthma attack. The existence of connection between PVN and DVC suggested the OT neurons in PVN might project to DVC which might be involved in the pathogenesis of asthma.

Exercise training increases GAD65 expression, restores the depressed GABAA receptor function within the PVN and reduces sympathetic modulation in hypertension

GABAergic inhibitory input within the paraventricular hypothalamic nucleus (PVN) plays a key role in restraining sympathetic outflow. Although experimental evidence has shown depressed GABA_A receptor function plus sympathoexcitation in hypertension and augmented GABA levels with reduced sympathetic activity after exercise training (T), the mechanisms underlying T‐induced effects remain unclear. Here we investigated in T and sedentary (S) SHR and WKY: (1) time‐course changes of hemodynamic parameters and PVN glutamic acid decarboxylase (GAD) isoforms’ expression; (2) arterial pressure (AP) and heart rate (HR) responses, sympathetic/parasympathetic modulation of heart and vessels and baroreflex sensitivity to GABA_A receptor blockade within the PVN. SHR‐S versus WKY‐S exhibited higher AP and HR, increased sympathetic reduced parasympathetic modulation, smaller baroreflex sensitivity, and reduced PVN GAD65 immunoreactivity. SHR‐T and WKY‐T showed prompt maintained increase (2–8 weeks) in GAD65 expression (responsible for GABA vesicular pool synthesis), which occurred simultaneously with HR reduction in SHR‐T and preceded MAP fall in SHR‐T and resting bradycardia in WKY‐T. There was no change in GAD67 expression (mainly involved with GABA metabolic pool). Resting HR in both groups and basal MAP in SHR were negatively correlated with PVN GAD65 expression. Normalized baroreflex sensitivity and autonomic control observed only in SHR‐T were due to recovery of GABA_A receptor function into the PVN since bicuculline administration abolished these effects. Data indicated that training augments in both groups the expression/activity of GABAergic neurotransmission within presympathetic PVN neurons and restores GABA_A receptors′ function specifically in the SHR, therefore strengthening GABAergic modulation of sympathetic outflow in hypertension.

Activity-Dependent Neuroplastic Changes in Autonomic Circuitry Modulating Cardiovascular Control: The Essential Role of Baroreceptors and Chemoreceptors Signaling

Aerobic exercise training improves the autonomic control of the circulation. Emerging evidence has shown that exercise induces neuroplastic adaptive changes in preautonomic circuitry controlling sympathetic/parasympathetic outflow to heart and vessels. The mechanisms underlying neuronal plasticity are, however, incompletely understood. Knowing that sinoaortic denervation blocks training-induced cardiovascular benefits, we investigate whether baroreceptors’ and chemoreceptors’ signaling are able to drive neuronal plasticity within medullary and supramedullary pathways controlling autonomic outflow. Male Wistar rats submitted to sinoaortic denervation (SAD) or dopamine β-hydroxylase-saporin lesion (DBHx) and respective controls (SHAM) were allocated to training (T) or sedentary (S) protocols for 8 weeks. After hemodynamic measurements at rest, rats were deeply anesthetized for brain harvesting. The density of DBH and oxytocin (OT) cell bodies and terminals were analyzed in brainstem and hypothalamic brain areas (double immunofluorescence reactions, optic and confocal microscopy). In SHAM rats training augmented the density of DBH+ neurons in the nucleus of solitary tract, increased the density of ascending NORergic projections and the number of DBH+ boutons contacting preautonomic OT+ neurons into paraventricular hypothalamic preautonomic nuclei, augmented the density of local OTergic neurons and enhanced the density of OT+ terminals targeting brainstem autonomic areas. These plastic changes occurred simultaneously with reduced sympathetic/increased parasympathetic activity, augmented baroreflex sensitivity and reduced resting heart rate. SAD reduced the density of both DBH+ fibers ascending from brainstem to paraventricular nucleus of hypothalamus and preautonomic OT+ neurons projecting to the brainstem, abrogated training-induced plastic changes and autonomic adaptive responses without changing the treadmill performance. Minor neuroplastic changes with preserved baroreflex sensitivity were observed in trained rats after partial selective disruption of ascending NORergic projections. Our data indicated that afferent inputs conveyed by arterial baroreceptors and chemoreceptors are the main stimuli to drive both inactivity-induced and activity-dependent neuroplasticity within the autonomic circuitry.

Exacerbated effects of prorenin on hypothalamic magnocellular neuronal activity and vasopressin plasma levels during salt-sensitive hypertension

Accumulating evidence supports that the brain renin-angiotensin system (RAS), including prorenin (PR) and its receptor (PRR), two newly discovered RAS players, contribute to sympathoexcitation in salt-sensitive hypertension. Still, whether PR also contributed to elevated circulating levels of neurohormones such as vasopressin (VP) during salt-sensitive hypertension, and if so, what are the precise underlying mechanisms, remains to be determined. To address these questions, we obtained patch-clamp recordings from hypothalamic magnocellular neurosecretory neurons (MNNs) that synthesize the neurohormones oxytocin and VP in acute hypothalamic slices obtained from sham and deoxycorticosterone acetate (DOCA)-salt-treated hypertensive rats. We found that focal application of PR markedly increased membrane excitability and firing responses in MNNs of DOCA-salt, compared with sham rats. This effect included a shorter latency to spike initiation and increased numbers of spikes in response to depolarizing stimuli and was mediated by a more robust inhibition of A-type K⁺ channels in DOCA-salt compared with sham rats. On the other hand, the afterhyperpolarizing potential mediated by the activation of Ca²⁺-dependent K⁺ channel was not affected by PR. mRNA expression of PRR, VP, and the Kv4.3 K⁺ channel subunit in the supraoptic nucleus of DOCA-salt hypertensive rats was increased compared with sham rats. Finally, we report a significant decrease of plasma VP levels in neuron-selective PRR knockdown mice treated with DOCA-salt, compared with wild-type DOCA-salt-treated mice. Together, these results support that activation of PRR contributes to increased excitability and firing discharge of MNNs and increased plasma levels of VP in DOCA-salt hypertension.NEW & NOTEWORTHY Our studies support that prorenin (PR) and its receptor (PRR) within the hypothalamus contribute to elevated plasma vasopressin levels in deoxycorticosterone acetate-salt hypertension, in part because of an exacerbated effect of PR on magnocellular neurosecretory neuron excitability; Moreover, our study implicates A-type K+ channels as key underlying molecular targets mediating these effects. Thus, PR/PRR stands as a novel therapeutic target for the treatment of neurohumoral activation in salt-sensitive hypertension.

Resistance training improves cardiovascular autonomic control and biochemical profile of rats exposed to Western diet in the perinatal period

INTRODUCTION AND OBJECTIVES

Consumption of a Western diet during the perinatal period is associated with development of cardiovascular disease. Resistance training (RT) has been used to treat cardiovascular disorders. The aim of this work was to assess the effect of RT on cardiometabolic disorders in rats exposed to a Western diet in the perinatal period.

METHODS

Female Wistar rats were fed with control or Western diet during pregnancy and lactation. The pups were divided into three groups: Control (C), Western Diet Sedentary (WDS) and Western Diet + RT (WDRT). At 60 days of age, all animals started the RT protocol (five times a week for four weeks). At the end, blood pressure was recorded for analysis of heart rate variability and baroreflex sensitivity (BRS). Blood samples were collected for biochemical analysis.

RESULTS

RT reduced blood pressure and vascular sympathetic modulation and increased BRS. There were improvements in biochemical profile, with reductions in fasting blood glucose, total cholesterol and low-density lipoprotein, and an increase in high-density lipoprotein.

CONCLUSION

RT led to beneficial adaptations in the cardiovascular system, mediated by changes in the mechanisms of autonomic control and biochemical profile of animals exposed to a Western diet in the perinatal period.

Regulation of sympathetic vasomotor activity by the hypothalamic paraventricular nucleus in normotensive and hypertensive states

The hypothalamic paraventricular nucleus (PVN) is a unique and important brain region involved in the control of cardiovascular, neuroendocrine, and other physiological functions pertinent to homeostasis. The PVN is a major source of excitatory drive to the spinal sympathetic outflow via both direct and indirect projections. In this review, we discuss the role of the PVN in the regulation of sympathetic output in normal physiological conditions and in hypertension. In normal healthy animals, the PVN presympathetic neurons do not appear to have a major role in sustaining resting sympathetic vasomotor activity or in regulating sympathetic responses to short-term homeostatic challenges such as acute hypotension or hypoxia. Their role is, however, much more significant during longer-term challenges, such as sustained water deprivation, chronic intermittent hypoxia, and pregnancy. The PVN also appears to have a major role in generating the increased sympathetic vasomotor activity that is characteristic of multiple forms of hypertension. Recent studies in the spontaneously hypertensive rat model have shown that impaired inhibitory and enhanced excitatory synaptic inputs to PVN presympathetic neurons are the basis for the heightened sympathetic outflow in hypertension. We discuss the molecular mechanisms underlying the presynaptic and postsynaptic alterations in GABAergic and glutamatergic inputs to PVN presympathetic neurons in hypertension. In addition, we discuss the ability of exercise training to correct sympathetic hyperactivity by restoring blood-brain barrier integrity, reducing angiotensin II availability, and decreasing oxidative stress and inflammation in the PVN.

Hypoxia activates a neuropeptidergic pathway from the paraventricular nucleus of the hypothalamus to the nucleus tractus solitarii

The paraventricular nucleus of the hypothalamus (PVN) contributes to both autonomic and neuroendocrine function. PVN lesion or inhibition blunts cardiorespiratory responses to peripheral chemoreflex activation, suggesting that the PVN is required for full expression of these effects. However, the role of efferent projections to cardiorespiratory nuclei and the neurotransmitters/neuromodulators that are involved is unclear. The PVN sends dense projections to the nucleus tractus solitarii (nTS), a region that displays neuronal activation following hypoxia. We hypothesized that acute hypoxia activates nTS-projecting PVN neurons. Using a combination of retrograde tracing and immunohistochemistry, we determined whether hypoxia activates PVN neurons that project to the nTS and examined the phenotype of these neurons. Conscious rats underwent 2 h normoxia (21% O₂, n = 5) or hypoxia (10% O₂, n = 6). Hypoxia significantly increased Fos immunoreactivity in nTS-projecting neurons, primarily in the caudal PVN. The majority of activated nTS-projecting neurons contained corticotropin-releasing hormone (CRH). In the nTS, fibers expressing the CRH receptor corticotropin-releasing factor receptor 2 (CRFR2) were colocalized with oxytocin (OT) fibers and were closely associated with hypoxia-activated nTS neurons. A separate group of animals that received a microinjection of adeno-associated virus type 2-hSyn-green fluorescent protein (GFP) into the PVN exhibited GFP-expressing fibers in the nTS; a proportion of these fibers displayed OT immunoreactivity. Thus, nTS CRFR2s appear to be located on the fibers of PVN OT neurons that project to the nTS. Taken together, our findings suggest that PVN CRH projections to the nTS may modulate nTS neuronal activation, possibly via OTergic mechanisms, and thus contribute to chemoreflex cardiorespiratory responses.

Exercise training abrogates age-dependent loss of hypothalamic oxytocinergic circuitry and maintains high parasympathetic activity

Neuroanatomical studies associating neuronal tract tracing and immunohistochemistry identified reciprocal (ascending noradrenergic/descending oxytocinergic, OTergic) connections between brainstem cardiovascular nuclei and the paraventricular hypothalamic nucleus (PVN). Previous functional studies indicated that exercise training (T) augmented the expression/activity of OTergic pathway and improve the autonomic control of the heart. Knowing that ageing is associated with autonomic dysfunction and sinoaortic denervation blocked T-induced beneficial effects, we hypothesized that T was able to reduce age-dependent impairment by improving the afferent signaling to PVN and augmenting OTergic modulation of cardiovascular control. We evaluated the combined effects of T and age on plastic remodeling of ascending dopamine β-hydroxylase (DBH+) and descending OT+ pathways and correlated them with cardiovascular parameters. Male Wistar rats were submitted to T or kept sedentary for 8 weeks. After evaluating arterial pressure, heart rate (HR), their variabilities and spectral components in conscious rats at rest, brains were harvested to analyze the plastic remodeling of brain autonomic nuclei (immunofluorescence + confocal microscopy). The density of DBH+ neurons within the nucleus of solitary tract (NTS) and caudal ventrolateral medulla, the number of DBH+ terminals overlapping OT+ neurons in PVN preautonomic nuclei, as well as the density of OT+ neurons and their projections to NTS and dorsal motor nucleus of the vagus were markedly reduced in S rats during 8-weeks of inactivity In contrast, these effects were completely blocked by T and reversed to a large augmentation of DBH+ and OT+ densities in both cell bodies and terminals within autonomic nuclei and target areas. All plastic changes observed correlated positively with parasympathetic activity to the heart (HF-PI, but not with LF-PI) and negatively with resting HR. Data indicate that T, by increasing beneficial neuroplastic adaptive changes within brainstem-PVN reciprocal network, abrogates age-dependent deleterious remodeling and augments parasympathetic modulation of the heart, therefore improving autonomic function. This article is protected by copyright. All rights reserved.

The effects of exercise on hypothalamic neurodegeneration of Alzheimer's disease mouse model

Alzheimer’s disease is a neurodegenerative disorder that affects the central nervous system. In this study, we characterized and examined the early metabolic changes in the triple transgenic mouse AD model (3xtg-AD), and their relationship with the hypothalamus, a key regulator of metabolism in the central nervous system. We observed that the 3xtg-AD model exhibited significantly higher oxygen consumption as well as food intake before reported amyloid plaque formation, indicating that metabolic abnormalities occurred at early onset in the 3xtg-AD model compared with their counterparts. Analysis of gene expression in the hypothalamus indicated increased mRNA expression of inflammation- and apoptosis-related genes, as well as decreased gene expression of Agouti-related protein (AgRP) and Melanocortin 4 receptor (MC4R) at 12 weeks of age. Immunofluorescence analysis revealed that pro-opiomelanocortin (POMC) and NPY-expressing neurons decreased at 24 weeks in the 3xtg-AD model. Four weeks of voluntary exercise were sufficient to reverse the gene expression of inflammation and apoptotic markers in the hypothalamus, six weeks of exercise improved glucose metabolism, moreover, 8 weeks of voluntary exercise training attenuated apoptosis and augmented POMC and NPY-expressing neuronal populations in the hypothalamus compared to the control group. Our results indicated that early onset of metabolic abnormalities may contribute to the pathology of AD, which is associated with increased inflammation as well as decreased neuronal population and key neuropeptides in the hypothalamus. Furthermore, early intervention by voluntary exercise normalized hypothalamic inflammation and neurodegeneration as well as glucose metabolism in the 3xtg-AD model. The data, taken as a whole, suggests a hypothalamic-mediated mechanism where exercise prevents the progression of dementia and of Alzheimer’s disease.

Oxytocin mediates the beneficial effects of the exercise training on breast cancer

NEW FINDINGS

What is the central question of this study? We hypothesized that potential anti-tumour effects of exercise training might be mediated by oxytocin and explored the underlying mechanisms in a mouse model of breast cancer. What is the main finding and its importance? Interval exercise training, by inducing oxytocin secretion, may reduce the activity of the PI3K/Akt and ERK pathways, and consequently, results in a smaller tumour volume in a mouse model of breast cancer. Exercise training can affect the growth of breast tumours. We hypothesized that exercise training might reduce breast tumour growth by inducing oxytocin (OT) secretion and its related signalling pathways, such as PI3K/Akt and ERK. Therefore, 56 BALB/c mice were equally divided into seven groups to study the effects of OT and atosiban (an oxytocin receptor antagonist) together with interval exercise training on mammary tumour growth, as well as tumour-related signalling pathways, including PI3K/Akt and ERK. Animal weight, OT plasma concentration, tumour weight and volume were measured at the end of the study. PI3K/Akt and ERK were evaluated by Western blot and qPCR assays. The results showed that OT plasma concentration was significantly increased in trained animals. The volume and weight of tumours were decreased significantly after both exercise training and OT administration. The expression of genes involved in tumour cell proliferation, such as PI3KR2, Akt and mTOR, was notably lower in the exercise-trained and OT-treated groups. Furthermore, the expression of genes involved in cell apoptosis, such as caspase-3 and Bax, was significantly increased in the tumour tissues. In addition, Western blot results showed that phosphorylated Akt and ERK were significantly decreased in the exercise training and OT groups compared with the tumour group. Interestingly, atosiban reversed these effects. These results indicated that interval exercise training, acting via OT secretion, may reduce PI3K/Akt and ERK axis activities, and consequently, decrease tumour volume and weight in a mouse model of breast cancer.

A reduction in SK channels contributes to increased activity of hypothalamic magnocellular neurons during heart failure

KEY POINTS

Small conductance Ca2+ -activated K+ (SK) channels play an important role in regulating the excitability of magnocellular neurosecretory cells (MNCs). Although an increased SK channel function contributes to adaptive physiological responses, it remains unknown whether changes in SK channel function/expression contribute to exacerbated MNC activity under disease conditions. We show that the input-output function of MNCs in heart failure (HF) rats is enhanced. Moreover, the SK channel blocker apamin enhanced the input-output function in sham, although not in HF rats. We found that both the after-hyperpolarizing potential magnitude and the underlying apamin-sensitive IAHP are blunted in MNCs from HF rats. The magnitude of spike-induced increases in intracellular Ca2+ levels was not affected in MNCs of HF rats. We found a diminished expression of SK2/SK3 channel subunit mRNA expression in the supraoptic nucleus of HF rats. Our studies suggest that a reduction in SK channel expression, but not changes in Ca2+ -mediated activation of SK channels, contributes to exacerbated MNC activity in HF rats.

ABSTRACT

Small conductance Ca2+ -activated K+ channels (SK) play an important role in regulating the activity of magnocellular neurosecretory cells (MNCs) and hormone release from the posterior pituitary. Moreover, enhanced SK activity contributes to the adaptive responses of MNCs to physiological challenge, such as lactation. Nevertheless, whether changes in SK function/expression contribute to exacerbated MNC activity during diseases such as heart failure (HF) remains unknown. In the present study, we used a combination of patch clamp electrophysiology, confocal Ca2+ imaging and molecular biology in a rat model of ischaemic HF. We found that the input-output function of MNCs was enhanced in HF compared to sham rats. Moreover, although the SK blocker apamin (200 nm) strengthened the input-output function in sham rats, it failed to have an effect in HF rats. The magnitude of the after-hyperpolarizing potential (AHP) following a train of spikes and the underlying apamin-sensitive IAHP were blunted in MNCs from HF rats. However, spike-induced increases in intracellular Ca2+ were not affected in the MNCs of HF rats. Real-time PCR measurements of SK channel subunits mRNA in supraoptic nucleus punches revealed a diminished expression of SK2/SK3 subunits in HF compared to sham rats. Together, our studies demonstrate that MNCs from HF rats exhibit increased membrane excitability and an enhanced input-output function, and also that a reduction in SK channel-mediated, apamin-sensitive AHP is a critical contributing mechanism. Moreover, our results suggest that the reduced AHP is related to a down-regulation of SK2/SK3 channel subunit expression but not the result of a blunted activity-dependent intracellular Ca2+ increase following a burst of action potentials.

An increased extrasynaptic NMDA tone inhibits A-type K+ current and increases excitability of hypothalamic neurosecretory neurons in hypertensive rats

KEY POINTS

A functional coupling between extrasynaptic NMDA receptors (eNMDARs) and the A-type K+ current (IA ) influences homeostatic firing responses of magnocellular neurosecretory cells (MNCs) to a physiological challenge. However, whether an altered eNMDAR-IA coupling also contributes to exacerbated MNC activity and neurohumoral activation during disease states is unknown. We show that activation of eNMDARs by exogenously applied NMDA inhibited IA in MNCs obtained from sham, but not in MNCs from renovascular hypertensive (RVH) rats. Neither the magnitude of the exogenously evoked NMDA current nor the expression of NMDAR subunits were altered in RVH rats. Conversely, we found that a larger endogenous glutamate tone, which was not due to blunted glutamate transport activity, led to the sustained activation of eNMDARs that tonically inhibited IA , contributing in turn to higher firing activity in RVH rats. Our studies show that exacerbated activation of eNMDARs by endogenous glutamate contributes to tonic inhibition of IA and enhanced MNC excitability in RVH rats.

ABSTRACT

We recently showed that a functional coupling between extrasynaptic NMDA receptors (eNMDARs) and the A-type K+ current (IA ) influences the firing activity of hypothalamic magnocellular neurosecretory neurons (MNCs), as well as homeostatic adaptive responses to a physiological challenge. Here, we aimed to determine whether changes in the eNMDAR-IA coupling also contributed to exacerbated MNC activity during disease states. We used a combination of patch-clamp electrophysiology and real-time PCR in MNCs in sham and renovascular hypertensive (RVH) rats. Activation of eNMDARs by exogenously applied NMDA inhibited IA in sham rats, but this effect was largely blunted in RVH rats. The blunted response was not due to changes in eNMDAR expression and/or function, since neither NMDA current magnitude or reversal potential, nor the levels of NR1-NR2A-D subunit expression were altered in RVH rats. Conversely, we found a larger endogenous glutamate tone, resulting in the sustained activation of eNMDARs that tonically inhibited IA and contributed also to higher ongoing firing activity in RVH rats. The enhanced endogenous glutamate tone in RVH rats was not due to blunted glutamate transporter activity. Rather, a higher transporter activity was observed, which possibly acted as a compensatory mechanism in the face of the elevated endogenous tone. In summary, our studies indicate that an elevated endogenous glutamate tone results in an exacerbated activation of eNMDARs, which in turn contributes to diminished IA magnitude and increased firing activity of MNCs from hypertensive rats.

A-type K+ channels contribute to the prorenin increase of firing activity in hypothalamic vasopressin neurosecretory neurons

Recent studies have supported an important contribution of prorenin (PR) and its receptor (PRR) to the regulation of hypothalamic, sympathetic, and neurosecretory outflows to the cardiovascular system, including systemic release of vasopressin (VP), both under physiological and cardiovascular disease conditions. Still, the identification of precise cellular mechanisms and neuronal/molecular targets remain unknown. We have recently shown that PRR is expressed in VP neurons and that their activation increases neuronal activity. However, the underlying ionic channel mechanisms are undefined. Here, we performed patch-clamp electrophysiology from identified VP neurons in acute hypothalamic slices obtained from enhanced green fluorescent protein-VP transgenic rats. Voltage-clamp recordings showed that PR inhibited the magnitude of A-type K⁺ current (I_A; ~50% at -25 mV), a subthreshold voltage-dependent current that restrains VP firing activity. PR also increased the inactivation rate of I_A and shifted the steady-state voltage-dependent inactivation function toward more hyperpolarized membrane potential (~7 mV shift), thus resulting in less channel availability to be activated at any given membrane potential. PR also inhibited a sustained component of I_A ("window" current). PR-mediated changes in action potential waveform and increased firing activity were occluded when I_A was blocked by 4-aminopyridine. Finally, PR failed to increase superoxide production within the supraoptic nucleus/paraventricular nucleus, and PR excitatory effects persisted in slices treated with the SOD mimetic tempol. Taken together, these experiments indicated that PR excitatory effects on vasopressin neurons involve inhibition of I_A, due, in part, to increases in its voltage-dependent inactivation properties. Moreover, our results indicate that PR effects did not involve an increase in oxidative stress.NEW & NOTEWORTHY Here, we demonstrate that prorenin/the prorenin receptor is an important signaling unit for the regulation of vasopressin firing activity and, thus, systemic hormonal release. We identified A-type K⁺ channels as key molecular targets mediating prorenin stimulation of vasopressin neuronal activity, thus standing as a potential therapeutic target for neurohumoral activation in cardiovascular disease.

Long-term moderate exercise accelerates the recovery of stress-evoked cardiovascular responses

Psychological stress is an important global health problem. It is well documented that stress increases the incidences of various cardiovascular disorders. Regular exercise is known to reduce resting blood pressure (BP) and heart rate (HR). This study was designed to clarify the effects of long-term exercise on stress-evoked cardiovascular responses and to emphasize post-stress recovery effects. Male Wistar rats underwent 8 weeks of moderate treadmill training, with cardiovascular responses, autonomic nervous system activities and local Fos reactivity changes in the cardiovascular regulation center were monitored before, during and after immobilization stress. A spectral analysis of cardiovascular parameters was used to examine autonomic nervous activities. We found that long-term exercise (i) lowered resting BP, HR and sympathetic activity, but increased resting parasympathetic activity and baroreflex sensitivity (BRS); (ii) accelerated post-stress recovery of stress-evoked cardiovascular and sympathetic responses along with increased BRS and (iii) accelerated post-stress recovery of stress-evoked neuron activations in the paraventricular nucleus, but delayed it in the nucleus of the tractus solitarius. We conclude that, in rats, long-term exercise accelerated recovery of stress-evoked cardiovascular responses differentially altering hypothalamic and medullar neuron activities.

Neural control of circulation and exercise: a translational approach disclosing interactions between central command, arterial baroreflex, and muscle metaboreflex

The last 100 years witnessed a rapid and progressive development of the body of knowledge concerning the neural control of the cardiovascular system in health and disease. The understanding of the complexity and the relevance of the neuroregulatory system continues to evolve and as a result raises new questions. The purpose of this review is to articulate results from studies involving experimental models in animals as well as in humans concerning the interaction between the neural mechanisms mediating the hemodynamic responses during exercise. The review describes the arterial baroreflex, the pivotal mechanism controlling mean arterial blood pressure and its fluctuations along with the two main activation mechanisms to exercise: central command (parallel activation of central somatomotor and autonomic descending pathways) and the muscle metaboreflex, the metabolic component of exercise pressor reflex (feedback from ergoreceptors within contracting skeletal muscles). In addition, the role of the cardiopulmonary baroreceptors in modulating the resetting of arterial baroreflex is identified, and the mechanisms in the central nervous system involved with the resetting of baroreflex function during dynamic exercise are also described. Approaching a very relevant clinical condition, the review also presents the concept that the impaired arterial baroreflex function is an integral component of the metaboreflex-mediated exaggerated sympathetic tone in subjects with heart failure. This increased sympathetic activity has a major role in causing the depressed ventricular function observed during submaximal dynamic exercise in these patients. The potential contribution of a metaboreflex arising from respiratory muscles is also considered.

Optimizing cardiovascular benefits of exercise: a review of rodent models

Although research unanimously maintains that exercise can ward off cardiovascular disease (CVD), the optimal type, duration, intensity, and combination of forms are yet not clear. In our review of existing rodent-based studies on exercise and cardiovascular health, we attempt to find the optimal forms, intensities, and durations of exercise. Using Scopus and Medline, a literature review of English language comparative journal studies of cardiovascular benefits and exercise was performed. This review examines the existing literature on rodent models of aerobic, anaerobic, and power exercise and compares the benefits of various training forms, intensities, and durations. The rodent studies reviewed in this article correlate with reports on human subjects that suggest regular aerobic exercise can improve cardiac and vascular structure and function, as well as lipid profiles, and reduce the risk of CVD. Findings demonstrate an abundance of rodent-based aerobic studies, but a lack of anaerobic and power forms of exercise, as well as comparisons of these three components of exercise. Thus, further studies must be conducted to determine a truly optimal regimen for cardiovascular health.

Involvement of N-methyl-D-aspartate glutamate receptor and nitric oxide in cardiovascular responses to dynamic exercise in rats

Dynamic exercise evokes sustained cardiovascular responses, which are characterized by arterial pressure and heart rate increases. Although it is well accepted that there is central nervous system mediation of cardiovascular adjustments during exercise, information on the role of neural pathways and signaling mechanisms is limited. It has been reported that glutamate, by acting on NMDA receptors, evokes the release of nitric oxide through activation of neuronal nitric oxide synthase (nNOS) in the brain. In the present study, we tested the hypothesis that NMDA receptors and nNOS are involved in cardiovascular responses evoked by an acute bout of exercise on a rodent treadmill. Moreover, we investigated possible central sites mediating control of responses to exercise through the NMDA receptor-nitric oxide pathway. Intraperitoneal administration of the selective NMDA glutamate receptor antagonist dizocilpine maleate (MK-801) reduced both the arterial pressure and heart rate increase evoked by dynamic exercise. Intraperitoneal treatment with the preferential nNOS inhibitor 7-nitroindazole reduced exercise-evoked tachycardiac response without affecting the pressor response. Moreover, treadmill running increased NO formation in the medial prefrontal cortex (MPFC), bed nucleus of the stria teminalis (BNST) and periaqueductal gray (PAG), and this effect was inhibited by systemic pretreatment with MK-801. Our findings demonstrate that NMDA receptors and nNOS mediate the tachycardiac response to dynamic exercise, possibly through an NMDA receptor-NO signaling mechanism. However, NMDA receptors, but not nNOS, mediate the exercise-evoked pressor response. The present results also provide evidence that MPFC, BNST and PAG may modulate physiological adjustments during dynamic exercise through NMDA receptor-NO signaling.

Peptide and lipid modulation of glutamatergic afferent synaptic transmission in the solitary tract nucleus

The brainstem nucleus of the solitary tract (NTS) holds the first central neurons in major homeostatic reflex pathways. These homeostatic reflexes regulate and coordinate multiple organ systems from gastrointestinal to cardiopulmonary functions. The core of many of these pathways arise from cranial visceral afferent neurons that enter the brain as the solitary tract (ST) with more than two-thirds arising from the gastrointestinal system. About one quarter of ST afferents have myelinated axons but the majority are classed as unmyelinated C-fibers. All ST afferents release the fast neurotransmitter glutamate with remarkably similar, high-probability release characteristics. Second order NTS neurons receive surprisingly limited primary afferent information with one or two individual inputs converging on single second order NTS neurons. A- and C-fiber afferents never mix at NTS second order neurons. Many transmitters modify the basic glutamatergic excitatory postsynaptic current often by reducing glutamate release or interrupting terminal depolarization. Thus, a distinguishing feature of ST transmission is presynaptic expression of G-protein coupled receptors for peptides common to peripheral or forebrain (e.g., hypothalamus) neuron sources. Presynaptic receptors for angiotensin (AT1), vasopressin (V1a), oxytocin, opioid (MOR), ghrelin (GHSR1), and cholecystokinin differentially control glutamate release on particular subsets of neurons with most other ST afferents unaffected. Lastly, lipid-like signals are transduced by two key ST presynaptic receptors, the transient receptor potential vanilloid type 1 and the cannabinoid receptor that oppositely control glutamate release. Increasing evidence suggests that peripheral nervous signaling mechanisms are repurposed at central terminals to control excitation and are major sites of signal integration of peripheral and central inputs particularly from the hypothalamus.

Symptoms of anxiety and mood disturbance alter cardiac and peripheral autonomic control in patients with metabolic syndrome

Previous investigations show that metabolic syndrome (MetSyn) causes sympathetic hyperactivation. Symptoms of anxiety and mood disturbance (AMd) provoke sympatho-vagal imbalance. We hypothesized that AMd would alter even further the autonomic function in patients with MetSyn. Twenty-six never-treated patients with MetSyn (ATP-III) were allocated to two groups, according to the levels of anxiety and mood disturbance: (1) with AMd (MetSyn + AMd, n = 15), and (2) without AMd (MetSyn, n = 11). Ten healthy control subjects were also studied (C, n = 10). AMd was determined using quantitative questionnaires. Muscle sympathetic nerve activity (MSNA, microneurography), blood pressure (oscillometric beat-to-beat basis), and heart rate (ECG) were measured during a baseline 10-min period. Spectral analysis of RR interval and systolic arterial pressure were analyzed, and the power of low (LF) and high (HF) frequency bands were determined. Sympatho-vagal balance was obtained by LF/HF ratio. Spontaneous baroreflex sensitivity (BRS) was evaluated by calculation of α-index. MSNA was greater in patients with MetSyn + AMd compared with MetSyn and C. Patients with MetSyn + AMd showed higher LF and lower HF power compared with MetSyn and C. In addition, LF/HF balance was higher in MetSyn + AMd than in MetSyn and C groups. BRS was decreased in MetSyn + AMd compared with MetSyn and C groups. Anxiety and mood disturbance alter autonomic function in patients with MetSyn. This autonomic dysfunction may contribute to the increased cardiovascular risk observed in patients with mood alterations.

Exercise training normalizes an increased neuronal excitability of NTS-projecting neurons of the hypothalamic paraventricular nucleus in hypertensive rats

Elevated sympathetic outflow and altered autonomic reflexes, including impaired baroreflex function, are common findings observed in hypertensive disorders. Although a growing body of evidence supports a contribution of preautonomic neurons in the hypothalamic paraventricular nucleus (PVN) to altered autonomic control during hypertension, the precise underlying mechanisms remain unknown. Here, we aimed to determine whether the intrinsic excitability and repetitive firing properties of preautonomic PVN neurons that innervate the nucleus tractus solitarii (PVN-NTS neurons) were altered in spontaneously hypertensive rats (SHR). Moreover, given that exercise training is known to improve and/or correct autonomic deficits in hypertensive conditions, we evaluated whether exercise is an efficient behavioral approach to correct altered neuronal excitability in hypertensive rats. Patch-clamp recordings were obtained from retrogradely labeled PVN-NTS neurons in hypothalamic slices obtained from sedentary (S) and trained (T) Wistar-Kyoto (WKY) and SHR rats. Our results indicate an increased excitability of PVN-NTS neurons in SHR-S rats, reflected by an enhanced input-output function in response to depolarizing stimuli, a hyperpolarizing shift in Na(+) spike threshold, and smaller hyperpolarizing afterpotentials. Importantly, we found exercise training in SHR rats to restore all these parameters back to those levels observed in WKY-S rats. In several cases, exercise evoked opposing effects in WKY-S rats compared with SHR-S rats, suggesting that exercise effects on PVN-NTS neurons are state dependent. Taken together, our results suggest that elevated preautonomic PVN-NTS neuronal excitability may contribute to altered autonomic control in SHR rats and that exercise training efficiently corrects these abnormalities.

Metabolic context regulates distinct hypothalamic transcriptional responses to antiaging interventions

The hypothalamus is an essential relay in the neural circuitry underlying energy metabolism that needs to continually adapt to changes in the energetic environment. The neuroendocrine control of food intake and energy expenditure is associated with, and likely dependent upon, hypothalamic plasticity. Severe disturbances in energy metabolism, such as those that occur in obesity, are therefore likely to be associated with disruption of hypothalamic transcriptomic plasticity. In this paper, we investigated the effects of two well-characterized antiaging interventions, caloric restriction and voluntary wheel running, in two distinct physiological paradigms, that is, diabetic (db/db) and nondiabetic wild-type (C57/Bl/6) animals to investigate the contextual sensitivity of hypothalamic transcriptomic responses. We found that, both quantitatively and qualitatively, caloric restriction and physical exercise were associated with distinct transcriptional signatures that differed significantly between diabetic and non-diabetic mice. This suggests that challenges to metabolic homeostasis regulate distinct hypothalamic gene sets in diabetic and non-diabetic animals. A greater understanding of how genetic background contributes to hypothalamic response mechanisms could pave the way for the development of more nuanced therapeutics for the treatment of metabolic disorders that occur in diverse physiological backgrounds.

Afferent signaling drives oxytocinergic preautonomic neurons and mediates training-induced plasticity

We showed previously that oxytocinergic (OTergic) projections from the hypothalamic paraventricular nucleus (PVN) to the dorsal brain stem mediate training-induced heart rate (HR) adjustments and that beneficial effects of training are blocked by sinoaortic denervation (SAD; Exp Physiol 94: 630-640; 1103-1113, 2009). We sought now to determine the combined effect of training and SAD on PVN OTergic neurons in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. Rats underwent SAD or sham surgery and were trained (55% of maximal capacity) or kept sedentary for 3 mo. After hemodynamic measurements were taken at rest, rats were deeply anesthetized. Fresh brains were frozen and sliced to isolate the PVN; samples were processed for OT expression (real-time PCR) and fixed brains were processed for OT immunofluorescence. In sham rats, training improved treadmill performance and increased the gain of baroreflex control of HR. Training reduced resting HR (-8%) in both groups, with a fall in blood pressure (-10%) only in SHR rats. These changes were accompanied by marked increases in PVN OT mRNA expression (3.9- and 2.2-fold in WKY and SHR rats, respectively) and peptide density in PVN OTergic neurons (2.6-fold in both groups), with significant correlations between OT content and training-induced resting bradycardia. SAD abolished PVN OT mRNA expression and markedly reduced PVN OT density in WKY and SHR. Training had no effect on HR, PVN OT mRNA, or OT content following SAD. The chronic absence of inputs from baroreceptors and chemoreceptors uncovers the pivotal role of afferent signaling in driving both the plasticity and activity of PVN OTergic neurons, as well as the beneficial effects of training on cardiovascular control.

Daily voluntary exercise alters the cardiovascular response to hemorrhage in conscious male rats

The present study tested the hypothesis that voluntary wheel-exercised rats would better tolerate severe hemorrhage (HEM) compared to age matched sedentary (SED) controls. Conscious rats housed with (EX, n = 8) or without (SED, n = 8) a running wheel for 6 weeks underwent a 30% total blood volume HEM over 15 min and were euthanized 90 min later and brain tissue was processed for Fos-like immunoreactivity (FLI). Both EX and SED groups displayed typical responses to HEM (initial tachycardia followed by decreased HR and MAP) but at the end of HEM, mean arterial pressure (93 ± 6 vs 58 ± 3 mm Hg) and heart rate (316 ± 17 vs. 247 ± 22 bpm,) were higher in the EX vs. SED animals and 60 min following the end of HEM, HR remained significantly elevated in the EX vs SED animals. The altered HR response to HEM in the EX animals was linked to a significant difference in sympatho-vagal drive identified by heart rate variability analysis and an augmented baroreflex response to hypotension tested in a separate group of animals (n = 4-5/group). In many of the brain regions analyzed, EX rats displayed lower levels of FLI compared to SED rats. Significantly lower levels of FLI in the EX vs SED rats were identified in the middle and caudal external lateral subnucleus of the lateral parabrachial nucleus and the dorsal cap of the hypothalamic paraventricular nucleus. These results suggest that enhanced tolerance to HEM following daily exercise may result from an EX-induced reduction in excitation or exaggerated inhibition in central circuits involved in autonomic control.

Nitric oxide inhibition in paraventricular nucleus on cardiovascular and autonomic modulation after exercise training in unanesthetized rats

It is well known that regular physical exercise alter cardiac function and autonomic modulation of heart rate variability (HRV). The paraventricular nucleus of hypothalamus (PVN) is an important site of integration for autonomic and cardiovascular responses, where nitric oxide (NO) plays an important role. The aim of our study was to evaluate the cardiovascular parameters and autonomic modulation by means of spectral analysis after nitric oxide synthase (NOS) inhibition in the PVN in conscious sedentary (S) or swimming trained (ST) rats. After swimming training protocol, adult male Wistar rats, instrumented with guide cannulas to PVN and femoral artery and vein catheters were submitted to mean arterial pressure (MAP) and heart rate (HR) recording. At baseline, the physical training induced a resting bradycardia (S: 374±5, ST: 346±1bpm) and promoted adaptations in HRV characterized by an increase in high-frequency oscillations (HF; 26.43±6.91 to 88.96±2.44) and a decrease in low-frequency oscillations (LF; 73.57±6.91 to 11.04±2.44) in normalized units. The microinjection of N(ω)-nitro-l-arginine methyl ester (l-NAME) in the PVN of sedentary and trained rats promoted increase in MAP and HR. l-NAME in the PVN did not significantly alter the spectral parameters of HRV of sedentary animals, however in the trained rats increased LF oscillations (11.04±2.44 to 27.62±6.97) and decreased HF oscillations (88.96±2.44 to 72.38±6.97) in normalized units compared with baseline. Our results suggest that NO in the PVN may collaborate to cardiac autonomic modulation after exercise training.

Exercise-induced neuronal plasticity in central autonomic networks: role in cardiovascular control

It is now well established that brain plasticity is an inherent property not only of the developing but also of the adult brain. Numerous beneficial effects of exercise, including improved memory, cognitive function and neuroprotection, have been shown to involve an important neuroplastic component. However, whether major adaptive cardiovascular adjustments during exercise, needed to ensure proper blood perfusion of peripheral tissues, also require brain neuroplasticity, is presently unknown. This review will critically evaluate current knowledge on proposed mechanisms that are likely to underlie the continuous resetting of baroreflex control of heart rate during/after exercise and following exercise training. Accumulating evidence indicates that not only somatosensory afferents (conveyed by skeletal muscle receptors, baroreceptors and/or cardiopulmonary receptors) but also projections arising from central command neurons (in particular, peptidergic hypothalamic pre-autonomic neurons) converge into the nucleus tractus solitarii (NTS) in the dorsal brainstem, to co-ordinate complex cardiovascular adaptations during dynamic exercise. This review focuses in particular on a reciprocally interconnected network between the NTS and the hypothalamic paraventricular nucleus (PVN), which is proposed to act as a pivotal anatomical and functional substrate underlying integrative feedforward and feedback cardiovascular adjustments during exercise. Recent findings supporting neuroplastic adaptive changes within the NTS-PVN reciprocal network (e.g. remodelling of afferent inputs, structural and functional neuronal plasticity and changes in neurotransmitter content) will be discussed within the context of their role as important underlying cellular mechanisms supporting the tonic activation and improved efficacy of these central pathways in response to circulatory demand at rest and during exercise, both in sedentary and in trained individuals. We hope this review will stimulate more comprehensive studies aimed at understanding cellular and molecular mechanisms within CNS neuronal networks that contribute to exercise-induced neuroplasticity and cardiovascular adjustments.

Influence of sedentary versus physically active conditions on regulation of plasma renin activity and vasopressin

Physical inactivity is an independent risk factor for cardiovascular disease. Sedentary animals compared to physically active controls exhibit enhanced sympathoexcitatory responses, including arterial baroreflex-mediated sympathoexcitation. Hypotension-induced sympathoexcitation is also associated with the release of vasoactive hormones. We hypothesized that sedentary conditions may enhance release of the vasoactive hormones AVP and ANG II. To test this hypothesis, the humoral response to hypotension was examined in conscious rats after 9-12 wk of sedentary conditions or "normally active" conditions. Normally active conditions were produced by allowing rats access to running wheels in their home cages. Running distance peaked after 4 wk (4.5 +/- 0.7 km/day), and the total distance run after 9 wk was 174 +/- 23 km (n = 25). Similar levels of hypotension were induced in conscious sedentary or physically active animals with the arterial vasodilator, diazoxide (25 mg/kg iv). Control experiments used a saline injection of equivalent volume. Plasma samples were collected and assayed for plasma AVP concentration and plasma renin activity (PRA). Sedentary conditions significantly enhanced resting and hypotension-induced PRA relative to normal physical activity. In contrast, resting and hypotension-induced AVP levels were not statistically different between groups. These data suggest that baroreflex-mediated activation of the renin-angiotensin system, but not AVP secretion, is enhanced by sedentary conditions. We speculate that augmented activation of the renin-angiotensin system may be related to enhanced sympathetic outflow observed in sedentary animals and may contribute to increased risk of cardiovascular disease in the sedentary population.

The NTS and integration of cardiovascular control during exercise in normotensive and hypertensive individuals

Due to upward resetting of baroreceptors, tachycardia coexists with increased pressure during dynamic exercise. This review critically evaluates current knowledge of proposed mechanisms to explain the continuous resetting of baroreflex control of heart rate and sympathetic nerve activity during and after exercise and exercise training. Of interest is the exercise-induced upward resetting that occurs in hypertensive and normotensive individuals. Accumulated evidence indicates that not only somatosensory afferents, but also inputs from central command projecting to the nucleus tractus solitarius (NTS) in the dorsal brainstem may mediate inhibition of excitatory neurotransmission on barosensitive neurons. Specific coordinated activation of vasopressinergic and oxytocinergic projections to the NTS is essential to tonically maintain baroreflex sensitivity and to adjust heart rate and cardiac output to circulatory demand at rest and during exercise in both sedentary and trained individuals. These findings reinforce the paramount importance of the NTS in integration of cardiovascular control during exercise.

Exercise training and sympathetic nervous system activity: evidence for physical activity dependent neural plasticity

1. It has been generally accepted that regular physical activity is associated with beneficial effects on the cardiovascular system. In fact, the idea that exercise maintains cardiovascular health is evident by the direct links between a sedentary lifestyle and the risk of cardiovascular and other disease states. 2. Cardiovascular diseases, such as hypertension and heart failure, are often associated with sympathetic nervous system (SNS) overactivity. Conversely, exercise has been shown to reduce hypertension and decrease elevated SNS activity. In addition, there is evidence that exercise may reduce resting blood pressure and sympathetic outflow in normal individuals. 3. Although somewhat controversial in humans, evidence from animal studies also indicates that exercise training reduces baroreflex-mediated and other forms of sympathoexcitation in normal individuals. Collectively, these data are consistent with the hypothesis that physical activity may decrease, and physical inactivity may increase, the incidence of cardiovascular disease via alterations in SNS activity. Despite the important clinical implications of this possibility, the mechanisms by which exercise alters control of SNS activity remain to be fully elucidated. 4. Recent evidence suggests that central nervous system (CNS) plasticity occurs under a variety of conditions, including varying levels of physical activity. The purpose of the present brief review is to provide evidence that changes within the CNS contribute importantly to altered regulation of the SNS observed following exercise training. The primary hypothesis is that physical activity versus inactivity produces plasticity within neural networks that regulate SNS activity. This hypothesis is supported by published and preliminary data that suggest that exercise training may reduce sympathoexcitation by reducing activation of neurons within cardiovascular regions of the brain. These mechanisms are likely to be important in disease states of sympathetic overactivity and in normal healthy individuals whose risk of cardiovascular disease is reduced by leading an active versus sedentary lifestyle.

Differential effects of vasopressinergic and oxytocinergic pre-autonomic neurons on circulatory control: reflex mechanisms and changes during exercise

1. The role of vasopressinergic and oxytocinergic (VPergic and OTergic, respectively) projections to the brain stem in the modulation of heart rate control is discussed on the basis of both changes in the peptide content of the dorsal brain stem (DBS) and functional effects following reflex- and exercise-induced activation in the presence and/or absence of receptor blockade within the nucleus tractus solitarius (NTS) and/or peripheral autonomic block. 2. Experimental data showed a dual effect of NTS VPergic projections on reflex control: (i) to maintain tonically the reflex sensitivity; and (ii) to reset reflex bradycardia towards higher heart rate values when transiently activated. The VPergic drive causes less sympathetic inhibition during pressure increases, mainly by reducing peripheral information going to NTS second-order neurons. Treadmill running increases the vasopressin content within the DBS. This activates NTS V(1) receptors to cause a significant improvement of exercise tachycardia in both sedentary and trained rats. 3. The OTergic drive to DBS areas (NTS/dorsal motor nucleus of the vagus) is also tonic for baroreceptor reflex control: it improves reflex bradycardia by facilitating vagal outflow to the heart. An acute bout of exercise increases DBS oxytocin (OT) content in trained rats, causing a significant blunting of exercise tachycardia only in this group. In both sedentary and trained groups, basal heart rate varies inversely with DBS OT content, the resting bradycardia of trained rats being associated with higher OT content. 4. Specific coordinated activation of VPergic and OTergic suprabulbar pathways is essential to adjust heart rate and cardiac output to circulatory demand at rest and during exercise in both sedentary and trained individuals.

Training-induced pressure fall in spontaneously hypertensive rats is associated with reduced angiotensinogen mRNA expression within the nucleus tractus solitarii

Knowing that exercise training reduces arterial pressure in hypertensive individuals and that pressure fall is accompanied by blockade of brain renin-angiotensin system, we sought to investigate whether training (T) affects central renin-angiotensin system. Spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto controls (WKY) were submitted to training or kept sedentary (S) for 3 months. After functional recordings, brain was removed and processed for autoradiography (brain stem sequential slices hybridized with (35)S-oligodeoxynucleotide probes for angiotensinogen [Aogen] and angiotensin II type 1 [AT(1A)] receptors). Resting arterial pressure and heart rate were higher in SHR(S) (177+/-2 mm Hg, 357+/-12 bpm versus 121+/-1 mm Hg, 320+/-9 bpm in WKY(S); P<0.05). Training was equally effective to enhance treadmill performance and to cause resting bradycardia (-10%) in both groups. Training-induced blood pressure fall (-6.3%) was observed only in SHR(T). In SHR(S) (versus WKY(S)) AT(1A) and Aogen mRNA expression were significantly increased within the NTS and area postrema (average of +67% and +41% for AT(1A) and Aogen, respectively; P<0.05) but unchanged in the gracilis nucleus. Training did not change AT(1A) expression but reduced NTS and area postrema Aogen mRNA densities specifically in SHR(T) (P<0.05 versus SHR(S), with values within the range of WKY groups). In SHRs, NTS Aogen mRNA expression was correlated with resting pressure (y=5.95x +41; r=0.55; P<0.05), with no significant correlation in the WKY group. Concurrent training-induced reductions of both Aogen mRNA expression in brain stem cardiovascular-controlling areas and mean arterial pressure only in SHRs suggest that training is as efficient as the renin-angiotensin blockers to reduce brain renin-angiotensin system overactivity and to decrease arterial pressure.

Exercise training fails to modify arterial baroreflex sensitivity in ovariectomized female rats

In men, exercise training attenuates age-related reduction in baroreflex sensitivity, which is related to cardiovascular health. It is unknown, however, if this holds true for post-menopausal women. We examined the effects of exercise training on baroreceptor-heart rate (HR) reflex sensitivity in ovariectomized (OVX) and sham-operated (SO) Wistar-Kyoto rats. At the age of 8 weeks, OVX and SO rats were assigned to either sedentary or exercise-trained group. Exercise training was performed on a treadmill 5 days per week. At the age of 20 weeks, baroreflex sensitivity in response to increases in blood pressure (BRSinc) and decreases in blood pressure (BRSdec) were evaluated by injections of phenylephrine and sodium nitroprusside, respectively. Both BRSinc and BRSdec were significantly reduced in sedentary OVX rats compared with sedentary SO rats. Exercise training decreased resting HR and BRSdec, but had no effect on BRSinc in SO rats. In OVX rats, exercise training decreased resting HR but modified neither BRSdec nor BRSinc. We conclude that withdrawal of female sex hormones in normotensive female rats is associated with reduced baroreflex sensitivity in response to both increase and decrease in blood pressure and that exercise training fails to modulate the decline of BRSinc associated with withdrawal of female sex hormones. To maintain high level of BRSinc in post-menopausal women, hormone replacement therapy may be needed.

Exercise training-induced remodeling of paraventricular nucleus (nor)adrenergic innervation in normotensive and hypertensive rats

Activation of oxytocin (OT)ergic projections from the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarii contributes to cardiovascular adjustments during exercise training (EXT). Moreover, a deficit in this central OTergic pathway is associated with altered cardiovascular function in hypertension. Since PVN catecholaminergic inputs, known to be activated during EXT, modulate PVN cardiovascular-related functions, we aimed here to determine whether remodeling of PVN (nor)adrenergic innervation occurs during EXT and whether this phenomenon is affected by hypertension. Confocal immunofluorescence microscopy and tract tracing were used to quantify changes in (nor)adrenergic innervation density in PVN subnuclei and in identified dorsal vagal complex (DVC) projecting neurons (PVN-DVC) in EXT normotensive [Wistar-Kyoto rat (WKY)] and hypertensive [spontaneously hypertensive rat (SHR)] rats. In WKY, EXT increased the density of PVN dopamine β-hydroxylase immunoreactivity (DBHir) (160%). Furthermore, the number and density of DBHir boutons overlapping PVN-DVC OTergic neurons were also increased during EXT (130%), effects that were blunted in SHR. Conversely, while DBHir in the medial parvocellular subnucleus (an area enriched in corticotropin-releasing hormone neurons) was not changed by EXT in WKY, a diminished DBHir was observed in trained SHR. Overall, these data support the concept that the PVN (nor)adrenergic innervation undergoes plastic remodeling during EXT, an effect that is differentially affected during hypertension. The functional implications of PVN (nor)adrenergic remodeling in relation to the central peptidergic control of cardiovascular function during EXT are discussed.

Exercise training prevents arterial baroreflex dysfunction in rats treated with central angiotensin II

Angiotensin II (Ang II)-induced arterial baroreflex dysfunction is associated with superoxide generation in the brain. Exercise training (EX) improves baroreflex function and decreases oxidative stress in cardiovascular diseases linked to elevated central Ang II. The aim of this study was to determine whether previous EX prevents baroreflex impairment caused by central administration of exogenous Ang II via an Ang II-superoxide mechanism. Four groups of rats were used: non-EX artificial cerebrospinal fluid infused, non-EX Ang II infused, EX artificial cerebrospinal fluid infused, and EX Ang II infused. Rats were treadmill trained for 3 to 4 weeks and subjected to intracerebroventricular infusion of Ang II over the last 3 days of EX. Twenty-four hours after the end of EX, the arterial baroreflex was assessed in anesthetized rats. Compared with non-EX artificial cerebrospinal fluid-infused rats, Ang II significantly decreased baroreflex sensitivity (maximum gain: 3.0+/-0.2% of maximum per millimeter of mercury versus 1.6+/-0.1% of maximum per millimeter of mercury; P<0.01), which was abolished by acute intracerebroventricular infusion of the Ang II type 1 receptor antagonist losartan and the reduced nicotinamide-adenine dinucleotide phosphate oxidase inhibitor apocynin. EX prevented the decrease in baroreflex sensitivity and downregulated Ang II type 1 receptor and NADPH oxidase subunit protein expression in the paraventricular nucleus of Ang II-infused rats. Finally, EX decreased superoxide production in the paraventricular nucleus of Ang II-infused rats. These results indicate that EX improves arterial baroreflex function in conditions of high brain Ang II, which is mediated by the central Ang II type 1 receptor and associated with a reduction in central oxidative stress.