Influence of the hypothalamic paraventricular nucleus on cardiovascular neurones in the rostral ventrolateral medulla of the rat

Inactivation of the paraventricular nucleus attenuates the cardiogenic sympathetic afferent reflex in the spontaneously hypertensive rat

BACKGROUND

Myocardial ischemia causes the release of bradykinin, which stimulates cardiac afferents, causing sympathetic excitation and chest pain. Glutamatergic activation of the paraventricular hypothalamic nucleus (PVN) in the spontaneously hypertensive rat (SHR) drives elevated basal sympathetic activity. Thus, we tested the hypothesis that inactivation of the PVN attenuates the elevated reflex response to epicardial bradykinin in the SHR and that ionotropic PVN glutamate receptors mediate the elevated reflex.

METHODS

We recorded the arterial pressure and renal sympathetic nerve activity (RSNA) response to epicardial bradykinin application in anesthetized SHR and Wistar Kyoto (WKY) rats before and after PVN microinjection of GABA A agonist muscimol or ionotropic glutamate receptor antagonist kynurenic acid.

RESULTS

Muscimol significantly decreased the arterial pressure response to bradykinin from 180.4 ± 5.8 to 119.5 ± 6.9 mmHg in the SHR and from 111.8 ± 7.0 to 84.2 ± 8.3 mmHg in the WKY and the RSNA response from 186.2 ± 7.1 to 142.7 ± 7.3% of baseline in the SHR and from 201.0 ± 11.5 to 160.2 ± 9.3% of baseline in the WKY. Kynurenic acid significantly decreased the arterial pressure response in the SHR from 164.5 ± 5.0 to 126.2 ± 7.7 mmHg and the RSNA response from 189.9 ± 13.7to 168.5 ± 12.7% of baseline but had no effect in the WKY.

CONCLUSION

These results suggest that tonic PVN activity is critical for the full manifestation of the CSAR in both the WKY and SHR. Glutamatergic PVN activity contributes to the augmented CSAR observed in the SHR.

Autonomic nervous system dysfunction throughout menopausal transition: A potential mechanism underpinning cardiovascular and cognitive alterations during female ageing

Cardiovascular diseases (CVD) and neurodegenerative disorders, such as Alzheimer's disease (AD), are highly prevalent conditions in middle-aged women that severely impair quality of life. Recent evidence suggests the existence of an intimate cross-talk between the heart and the brain, resulting from a complex network of neurohumoral circuits. From a pathophysiological perspective, the higher prevalence of AD in women may be explained, at least in part, by sex-related differences in the incidence/prevalence of CVD. Notably, the autonomic nervous system, the main heart-brain axis physiological orchestrator, has been suggested to play a role in the incidence of adverse cardiovascular events in middle-aged women because of decreases in oestrogen-related signalling during transition into menopause. Despite its overt relevance for public health, this hypothesis has not been thoroughly tested. Accordingly, in this review, we aim to provide up to date evidence supporting how changes in circulating oestrogen levels during transition to menopause may trigger autonomic dysfunction, thus promoting cardiovascular and cognitive decline in women. A main focus on the effects of oestrogen-mediated signalling at CNS structures related to autonomic regulation is provided, particularly on the role of oestrogens in sympathoexcitation. Improving the understanding of the contribution of the autonomic nervous system on the development, maintenance and/or progression of both cardiovascular and cognitive dysfunction during the transition to menopause should help improve the clinical management of elderly women, with the outcome being an improved life quality during the natural ageing process.

Hypothalamic paraventricular nucleus augments baroreflex sensitivity, role of angiotensin II

The hypothalamic paraventricular nucleus (PVN) is an important brain region involved in control of the cardiovascular system. Direct injection of angiotensin II (AngII) into the PVN produces a short or long pressor response. This study was performed in anesthetized rats to find whether the parvocellular part of the paraventricular nucleus (PVNp) affects the baroreflex. And if so, what is the effect of AngII injected into the PVNp on the baroreflex? Drugs were microinjected into the PVNp while blood pressure and heart rate were recorded continuously. We found that microinjection of AT1 and AT2 receptor antagonists into the PVNp region did not affect the baseline mean arterial pressure (MAP) and heart rate (HR) indicating that under normal conditions AngII may not provide tonic activity, at least in anaesthetized animals. Bilateral microinjections of a synaptic blocker (CoCl₂) into the PVNp attenuated the baroreflex gains in responses to loading and unloading of baroreceptors, indicating that PVNp is involved in the baroreflex rate component. Microinjection of AngII into the PVNp increased MAP and HR. However, AngII slightly attenuated the baroreflex rate component using its two receptors AT1 and AT2. Collectively, these findings suggest that the PVNp as a whole is involved in the baroreflex. But AngII attenuates the heart rate response of the baroreflex through AT1 and AT2 receptors.

Sodium Intake and Disease: Another Relationship to Consider

Sodium (Na+) is crucial for numerous homeostatic processes in the body and, consequentially, its levels are tightly regulated by multiple organ systems. Sodium is acquired from the diet, commonly in the form of NaCl (table salt), and substances that contain sodium taste salty and are innately palatable at concentrations that are advantageous to physiological homeostasis. The importance of sodium homeostasis is reflected by sodium appetite, an "all-hands-on-deck" response involving the brain, multiple peripheral organ systems, and endocrine factors, to increase sodium intake and replenish sodium levels in times of depletion. Visceral sensory information and endocrine signals are integrated by the brain to regulate sodium intake. Dysregulation of the systems involved can lead to sodium overconsumption, which numerous studies have considered causal for the development of diseases, such as hypertension. The purpose here is to consider the inverse-how disease impacts sodium intake, with a focus on stress-related and cardiometabolic diseases. Our proposition is that such diseases contribute to an increase in sodium intake, potentially eliciting a vicious cycle toward disease exacerbation. First, we describe the mechanism(s) that regulate each of these processes independently. Then, we highlight the points of overlap and integration of these processes. We propose that the analogous neural circuitry involved in regulating sodium intake and blood pressure, at least in part, underlies the reciprocal relationship between neural control of these functions. Finally, we conclude with a discussion on how stress-related and cardiometabolic diseases influence these circuitries to alter the consumption of sodium.

The Paraventricular Nucleus of the Hypothalamus in Control of Blood Pressure and Blood Pressure Variability

The paraventricular nucleus (PVN) is a highly organized structure of the hypothalamus that has a key role in regulating cardiovascular and osmotic homeostasis. Functionally, the PVN is divided into autonomic and neuroendocrine (neurosecretory) compartments, both equally important for maintaining blood pressure (BP) and body fluids in the physiological range. Neurosecretory magnocellular neurons (MCNs) of the PVN are the main source of the hormones vasopressin (VP), responsible for water conservation and hydromineral balance, and oxytocin (OT), involved in parturition and milk ejection during lactation. Further, neurosecretory parvocellular neurons (PCNs) take part in modulation of the hypothalamic–pituitary–adrenal axis and stress responses. Additionally, the PVN takes central place in autonomic adjustment of BP to environmental challenges and contributes to its variability (BPV), underpinning the PVN as an autonomic master controller of cardiovascular function. Autonomic PCNs of the PVN modulate sympathetic outflow toward heart, blood vessels and kidneys. These pre-autonomic neurons send projections to the vasomotor nucleus of rostral ventrolateral medulla and to intermediolateral column of the spinal cord, where postganglionic fibers toward target organs arise. Also, PVN PCNs synapse with NTS neurons which are the end-point of baroreceptor primary afferents, thus, enabling the PVN to modify the function of baroreflex. Neuroendocrine and autonomic parts of the PVN are segregated morphologically but they work in concert when the organism is exposed to environmental challenges via somatodendritically released VP and OT by MCNs. The purpose of this overview is to address both neuroendocrine and autonomic PVN roles in BP and BPV regulation.

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.

Another controller system for arterial pressure. AngII-vasopressin neural network of the parvocellular paraventricular nucleus may regulate arterial pressure during hypotension

Angiotensin II (AngII) immunoreactive cells, fibers and receptors, were found in the parvocelluar region of paraventricular nucleus (PVNp) and AngII receptors are present on vasopressinergic neurons. However, the mechanism by which vasopressin (AVP) and AngII may interact to regulate arterial pressure is not known. Thus, we tested the cardiovascular effects of blockade of the AngII receptors on AVP neurons and blockade of vasopressin V1a receptors on AngII neurons. We also explored whether the PVNp vasopressin plays a regulatory role during hypotension in anesthetized rat or not. Hypovolemic-hypotension was induced by gradual bleeding from femoral venous catheter. Either AngII or AVP injected into the PVNp produced pressor and tachycardia responses. The responses to AngII were blocked by V1a receptor antagonist. The responses to AVP were partially attenuated by AT1 antagonist and greatly attenuated by AT2 antagonist. Hemorrhage augmented the pressor response to AVP, indicating that during hemorrhage, sensitivity of PVNp to vasopressin was increased. By hemorrhagic-hypotension and bilateral blockade of V1a receptors of the PVNp, we found that vasopressinergic neurons of the PVNp regulate arterial pressure towards normal during hypotension. Taken together these findings and our previous findings about angII (Khanmoradi and Nasimi, 2017a) for the first time, we found that a mutual cooperative system of angiotensinergic and vasopressinergic neurons in the PVNp is a major regulatory controller of the cardiovascular system during hypotension.

Transcriptome Analysis Reveals Downregulation of Urocortin Expression in the Hypothalamo-Neurohypophysial System of Spontaneously Hypertensive Rats

The chronically increased blood pressure characteristic of essential hypertension represents an insidious and cumulative risk for cardiovascular disease. Essential hypertension is a multifactorial condition, with no known specific aetiology but a strong genetic component. The Spontaneously Hypertensive rat (SHR) shares many characteristics of human essential hypertension, and as such is a commonly used experimental model. The mammalian hypothalamo-neurohypophyseal system (HNS) plays a pivotal role in the regulation of blood pressure, volume and osmolality. In order to better understand the possible role of the HNS in hypertension, we have used microarray analysis to reveal differential regulation of genes in the HNS of the SHR compared to a control normotensive strain, the Wistar Kyoto rat (WKY). These results were validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). One of the genes identified and validated as being downregulated in SHR compared to WKY was that encoding the neuropeptide urocortin (Ucn). Immunohistochemical analyses revealed Ucn to be highly expressed within magnocellular neurons of the PVN and SON, with pronounced localisation in dendritic projections containing oxytocin and vasopressin. When Ucn was overexpressed in the PVN of the SHR by in vivo lentiviral mediated gene transfer, blood pressure was unaffected but there were significant, transient reductions in the VLF spectra of systolic blood pressure consistent with an action on autonomic balance. We suggest that Ucn may act, possibly via dendritic release, to subtly regulate neurohumoral aspects of arterial pressure control.

Medullary and Hypothalamic Functional Magnetic Imaging During Acute Hypoxia in Tracing Human Peripheral Chemoreflex Responses

[Figure: see text].

Sympathoexcitatory input from hypothalamic paraventricular nucleus neurons projecting to rostral ventrolateral medulla is enhanced after myocardial infarction

Elevated sympathetic vasomotor tone seen in heart failure (HF) may involve dysfunction of the hypothalamic paraventricular nucleus neurons that project to the rostral ventrolateral medulla (PVN-RVLM neurons). This study aimed to elucidate the role of PVN-RVLM neurons in the maintenance of resting renal sympathetic nerve activity (RSNA) after myocardial infarction (MI). In male rats, the left coronary artery was chronically ligated to induce MI. The rats received PVN microinjections of an adeno-associated viral (AAV) vector encoding archaerhodopsin T (ArchT) with the reporter yellow fluorescence protein (eYFP). The ArchT rats had abundant distributions of eYFP-labeled, PVN-derived axons in the RVLM. In anesthetized ArchT rats with MI (n = 12), optogenetic inhibition of the PVN-RVLM pathway achieved by 532-nm-wavelength laser illumination to the RVLM significantly decreased RSNA. This effect was not found in sham-operated ArchT rats (n = 6). Other rat groups received RVLM microinjections of a retrograde AAV vector encoding the red light-drivable halorhodopsin Jaws (Jaws) with the reporter green fluorescence protein (GFP) and showed expression of GFP-labeled cell bodies and dendrites in the PVN. Laser illumination of the PVN at a 635 nm wavelength elicited significant renal sympathoinhibition in Jaws rats with MI (n = 9) but not in sham-operated Jaws rats (n = 8). These results indicate that sympathoexcitatory input from PVN-RVLM neurons is enhanced after MI, suggesting that this monosynaptic pathway is part of the central nervous system circuitry that plays a critical role in generating an elevated sympathetic vasomotor tone commonly seen with HF.NEW & NOTEWORTHY Using optogenetics in rats, we report that sympathoexcitatory input from hypothalamic paraventricular nucleus neurons that project to the rostral ventrolateral medulla is enhanced after myocardial infarction. It is suggested that this monosynaptic pathway makes up a key part of central nervous system circuitry underlying sympathetic hyperactivation commonly seen in heart failure.

Cardiovascular responses produced by resistin injected into paraventricular nucleus mediated by the glutamatergic and CRFergic transmissions within rostral ventrolateral medulla

OBJECTIVES

Resistin, as a 12.5 kDa cysteine-rich polypeptide, is expressed in hypothalamus and regulates sympathetic nerve activity. It is associated with obesity, metabolic syndrome and cardiovascular diseases. In this study, we investigated the neural pathway of cardiovascular responses induced by injection of resistin into paraventricular nucleus (PVN) with rostral ventrolateral medulla (RVLM).

MATERIALS AND METHODS

Adult male rats were anesthetized with urethane (1.4 g/kg intraperitoneally). Resistin (3 µg/1 µl/rat) was first injected into PVN, and the glutamatergic, corticotrophin-releasing factor (CRF)-ergic and angiotensinogenic transmission was inhibited by injecting of their antagonist in RVLM. Arterial pressure (AP) and heart rate (HR) were monitored before and after the injection.

RESULTS

The results showed that resistin injection into PVN significantly increased AP and HR compared to control group and prior to its injection (P<0.05). Injection of AP5 ((2R)-amino-5-phosphonovaleric acid; (2R)-amino-5-phosphonopentanoate) (50 nM/rat), losartan (10 nM/rat) and astressin (50 nM/rat) into RVLM reduced cardiovascular responses produced by injected resistin into PVN. Injection of AP5+losartan or astressin+losartan or astressin+AP5 into RVLM could significantly reduce cardiovascular responses produced by resistin compared to before injection (P<0.05). Furthermore, the depressor responses generated by AP5+losartan injected into RVLM were significantly stronger than the depressor responses generated by AP5+astressin and/or astressin+losartan injected into RVLM (P<0.05).

CONCLUSION

It can be concluded that glutamatergic and CRFergic transmissions have crucial contribution to cardiovascular responses produced by resistin. The results provided new and potentially important insight regarding neural transmission when the plasma level of resistin increases; this reveals the role of resistin in cardiovascular responses such as metabolic syndrome and hypertension.

Sympathoexcitation by hypothalamic paraventricular nucleus neurons projecting to the rostral ventrolateral medulla

KEY POINTS

Causal relationships between central cardiovascular pathways and sympathetic vasomotor tone have not been evidenced. This study aimed to verify the sympathoexcitatory role of hypothalamic paraventricular nucleus neurons that project to the rostral ventrolateral medulla (PVN-RVLM neurons). By using optogenetic techniques, we demonstrated that stimulation of PVN-RVLM glutamatergic neurons increased renal sympathetic nerve activity and arterial pressure via, at least in part, stimulation of RVLM C1 neurons in rats. This monosynaptic pathway may function in acute sympathetic adjustments to stressors and/or be a component of chronic sympathetic hyperactivity in pathological conditions such as heart failure.

ABSTRACT

The rostral ventrolateral medulla (RVLM), which is known to play an important role in regulating sympathetic vasomotor tone, receives axonal projections from the hypothalamic paraventricular nucleus (PVN). However, no studies have proved that excitation of the PVN neurons that send axonal projections to the RVLM (PVN-RVLM neurons) causes sympathoexcitation. This study aimed to directly examine the sympathoexcitatory role of PVN-RVLM neurons. Male rats received microinjections into the PVN with an adeno-associated virus (AAV) vector that encoded a hybrid of channelrhodopsin-2/1 with the reporter tdTomato (ChIEF-tdTomato), or into the RVLM with a retrograde AAV vector that encoded a channelrhodopsin with green fluorescent protein (ChR2-GFP_retro ). Under anaesthesia with urethane and α-chloralose, photostimulation (473 nm wavelength) of PVN-RVLM neurons, achieved by laser illumination of either RVLM of ChIEF-tdTomato rats (n = 8) or PVN of ChR2-GFP_retro rats (n = 4), elicited significant renal sympathoexcitation. Immunofluorescence confocal microscopy showed that RVLM adrenergic C1 neurons of ChIEF-tdTomato rats were closely associated with tdTomato-labelled, PVN-derived axons that contained vesicular glutamate transporter 2. In another subset of anaesthetized ChIEF-tdTomato rats (n = 6), the renal sympathoexcitation elicited by photostimulation of the PVN was suppressed by administering ionotropic glutamate receptor blockers into the RVLM. These results demonstrate that excitation of PVN-RVLM glutamatergic neurons leads to sympathoexcitation via, at least in part, stimulation of RVLM C1 neurons.

The brainstem network controlling blood pressure: an important role for pressor sites in the caudal medulla and cervical spinal cord

: Although medullary control of blood pressure (BP) has been extensively studied, the contribution of critical regions, such as pressor sites in the caudal medulla and upper cervical spinal cord and the lateral tegmental field, remains controversial and underappreciated. A series of pressor sites caudal to the caudal ventrolateral medulla (CVLM), including the caudal pressor area (CPA) and medullocervical pressor area, play an important role in control of BP. Activation and inhibition of these sites elicits pressor and depressor responses, respectively. Basal sympathetic tone is provided principally by the medullary lateral tegmental field and rostral ventrolateral medulla (RVLM). RVLM presympathetic neurons, which project to and drive preganglionic sympathetic somata in the intermediolateral cell column, are powerfully regulated by neurons in CVLM via tonic and phasic inhibition. The current state of knowledge is summarized thus: rostrocaudally organized columns of pressor sites caudal to CVLM extend to the upper cervical spinal cord; CPA pressor responses are RVLM-dependent; CPA mediates pressor responses by (first) inhibiting RVLM-projecting inhibitory CVLM units and (second) activating RVLM-projecting excitatory CVLM units; the chemoreflex is CPA-dependent; the baroreflex is CPA-independent; pressor responses to raphe obscurus stimulation are CPA-dependent; and medullocervical pressor area pressor responses are RVLM-independent, likely mediated by direct projections to the intermediolateral cell column. In this review, we seek to underscore and characterize the critical role played by the caudal medulla and upper cervical spinal cord in BP regulation and highlight important gaps in knowledge in interactions between the caudal medulla and other regions controlling BP, which may prove critical in revealing central mechanisms underlying pathophysiology of, and pharmacotherapeutic targets for, hypertension.

Effects of valproic acid on sympathetic activity and left ventricularmyocardial remodelling in rats during pressure overload

Background/aim: Pressure overload induces cardiac remodelling and results in heart failure. Enhanced sympathetic outflow participates in the development of cardiac remodelling for the duration of pressure overload as an independent factor. Valproic acid has recently been shown to reduce neuronal injury and have antiinflammatory and antiapoptotic effects as a histone deacetylase inhibitor. We speculate that the drug plays a specific role in alleviating cardiac remodelling by inhibiting sympathetic activity. Materials and methods: Surgery of partial abdominal aortic constriction was performed on male Sprague-Dawley rats. After 4 weeks, animal models of pressure overload were validated and then valproic acid (300 mg/kg) was administrated to rats once a day for the next 4 weeks. Experimental parameters were detected 4 weeks after validation. Results: The administration of valproic acid alleviated cardiomyocyte hypertrophy, myocardial interstitial fibrosis and left ventricular diastolic dysfunction caused by partial abdominal aortic constriction. Valproic acid reduced the levels of plasma and local norepinephrine, augmented concentrations of hypothalamic gamma-aminobutyric acid, and had no side effects on the hepatic and renal function of the animals. Conclusion: These results suggest that valproic acid may be a safe and effective therapeutic strategy for the inhibition of sympathetic outflow and cardiac remodelling.

Mapping and Analysis of the Connectome of Sympathetic Premotor Neurons in the Rostral Ventrolateral Medulla of the Rat Using a Volumetric Brain Atlas

Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) play a critical role in the generation of vasomotor sympathetic tone and are thought to receive convergent input from neurons at every level of the neuraxis; the factors that determine their ongoing activity remain unresolved. In this study we use a genetically restricted viral tracing strategy to definitively map their spatially diffuse connectome. We infected bulbospinal RVLM neurons with a recombinant rabies variant that drives reporter expression in monosynaptically connected input neurons and mapped their distribution using an MRI-based volumetric atlas and a novel image alignment and visualization tool that efficiently translates the positions of neurons captured in conventional photomicrographs to Cartesian coordinates. We identified prominent inputs from well-established neurohumoral and viscero-sympathetic sensory actuators, medullary autonomic and respiratory subnuclei, and supramedullary autonomic nuclei. The majority of inputs lay within the brainstem (88–94%), and included putative respiratory neurons in the pre-Bötzinger Complex and post-inspiratory complex that are therefore likely to underlie respiratory-sympathetic coupling. We also discovered a substantial and previously unrecognized input from the region immediately ventral to nucleus prepositus hypoglossi. In contrast, RVLM sympathetic premotor neurons were only sparsely innervated by suprapontine structures including the paraventricular nucleus, lateral hypothalamus, periaqueductal gray, and superior colliculus, and we found almost no evidence of direct inputs from the cortex or amygdala. Our approach can be used to quantify, standardize and share complete neuroanatomical datasets, and therefore provides researchers with a platform for presentation, analysis and independent reanalysis of connectomic data.

Central monoaminergic systems are a site of convergence of signals conveying the experience of exercise to brain circuits involved in cognition and emotional behavior

Physical activity can enhance cognitive function and increase resistance against deleterious effects of stress on mental health. Enhanced cognitive function and stress resistance produced by exercise are conserved among vertebrates, suggesting that ubiquitous mechanisms may underlie beneficial effects of exercise. In the current review, we summarize the beneficial effects of exercise on cognitive function and stress resistance and discuss central and peripheral signaling factors that may be critical for conferring the effects of physical activity to brain circuits involved in cognitive function and stress. Additionally, it is suggested that norepinephrine and serotonin, highly conserved monoamines that are sensitive to exercise and able to modulate behavior in multiple species, could represent a convergence between peripheral and central exercise signals that mediate the beneficial effects of exercise. Finally, we offer the novel hypothesis that thermoregulation during exercise could contribute to the emotional effects of exercise by activating a subset of temperature-sensitive serotonergic neurons in the dorsal raphe nucleus that convey anxiolytic and stress-protective signals to forebrain regions. Throughout the review, we discuss limitations to current approaches and offer strategies for future research in exercise neuroscience.

The identification and neurochemical characterization of central neurons that target parasympathetic preganglionic neurons involved in the regulation of choroidal blood flow in the rat eye using pseudorabies virus, immunolabeling and conventional pathway tracing methods

The choroidal blood vessels of the eye provide the main vascular support to the outer retina. These blood vessels are under parasympathetic vasodilatory control via input from the pterygopalatine ganglion (PPG), which in turn receives its preganglionic input from the superior salivatory nucleus (SSN) of the hindbrain. The present study characterized the central neurons projecting to the SSN neurons innervating choroidal PPG neurons, using pathway tracing and immunolabeling. In the initial set of studies, minute injections of the Bartha strain of the retrograde transneuronal tracer pseudorabies virus (PRV) were made into choroid in rats in which the superior cervical ganglia had been excised (to prevent labeling of sympathetic circuitry). Diverse neuronal populations beyond the choroidal part of ipsilateral SSN showed transneuronal labeling, which notably included the parvocellular part of the paraventricular nucleus of the hypothalamus (PVN), the periaqueductal gray, the raphe magnus (RaM), the B3 region of the pons, A5, the nucleus of the solitary tract (NTS), the rostral ventrolateral medulla (RVLM), and the intermediate reticular nucleus of the medulla. The PRV+ neurons were located in the parts of these cell groups that are responsive to systemic blood pressure signals and involved in systemic blood pressure regulation by the sympathetic nervous system. In a second set of studies using PRV labeling, conventional pathway tracing, and immunolabeling, we found that PVN neurons projecting to SSN tended to be oxytocinergic and glutamatergic, RaM neurons projecting to SSN were serotonergic, and NTS neurons projecting to SSN were glutamatergic. Our results suggest that blood pressure and volume signals that drive sympathetic constriction of the systemic vasculature may also drive parasympathetic vasodilation of the choroidal vasculature, and may thereby contribute to choroidal baroregulation during low blood pressure.

Central bombesin possibly induces S-nitrosylation of cyclooxygenase-1 in pre-sympathetic neurons of rat hypothalamic paraventricular nucleus

AIMS

Cyclooxygenase (COX) can be activated by nitric oxide-induced (NO-induced) conversion of cysteine thiol group of COX into S-nitrosothiol. We previously reported the involvement of brain COX/NO synthase (NOS) in centrally administered bombesin-, a stress-related neuropeptide, induced secretion of rat adrenal noradrenaline and adrenaline. To examine a possible involvement of the NO-induced modification of COX in bombesin-induced response, we investigated whether bombesin induces close proximity of COX-1 and neuronal NOS (nNOS) or S-nitroso-cysteine in pre-sympathetic spinally projecting neurons in the rat hypothalamic paraventricular nucleus (PVN), a regulatory center of adrenomedullary outflow.

MAIN METHODS

In twelve-week-old male Wistar rats, pre-sympathetic spinally projecting neurons in the PVN were labeled with a retrograde tracer Fluoro-Gold (FG). After intracerebroventricular administration of bombesin, we performed double immunohistochemical analysis for Fos and COX-1 or nNOS in FG-labeled PVN neurons. We also performed a fluorescent in situ proximity ligation assay (PLA) for visualizing of close proximity (<40 nm) of COX-1 with nNOS or S-nitroso-cysteine.

KEY FINDINGS

Bombesin significantly increased the number of Fos-immunoreactive cells in FG-labeled PVN neurons with COX-1 or nNOS immunoreactivity. 7-Nitroindazole, a selective nNOS inhibitor, abolished Fos-immunoreactivity induced by bombesin in COX-1-immunoreactive FG-labeled PVN neurons. Bombesin also induced PLA-positive signals indicating close proximity of COX-1/nNOS and COX-1/S-nitroso-cysteine in FG-labeled PVN neurons.

SIGNIFICANCE

Centrally administered bombesin possibly induces S-nitrosylation of COX-1 through close proximity of COX-1 and nNOS in pre-sympathetic spinally projecting PVN neurons, thereby activating COX-1 during the bombesin-induced activation of central adrenomedullary outflow in the rat.

Estrogen replacement modulates voltage-gated potassium channels in rat presympathetic paraventricular nucleus neurons

Background

The hypothalamic paraventricular nucleus (PVN) is an important site in the regulation of the autonomic nervous system. Specifically, PVN neurons projecting to the rostral ventrolateral medulla (PVN-RVLM) play a regulatory role in the determination of the sympathetic outflow in the cardiovascular system. In the PVN-RVLM neurons, the estrogen receptor β is expressed. However, to date, the effects of estrogen on PVN-RVLM neurons have not been reported. The present study investigated estrogen-mediated modulation of two voltage-gated potassium channel (Kv) subunits, Kv4.2 and Kv4.3, that are expressed predominantly in PVN neurons and the functional current of Kv4.2 and Kv4.3, the transient outward potassium current (I_A).

Results

Single-cell real-time RT-PCR analysis showed that 17β-estradiol (E2) replacement (once daily for 4 days) selectively down-regulated Kv4.2 mRNA levels in the PVN-RVLM neurons of ovariectomized female rats. There was no change in Kv4.3 levels. Whole-cell patch-clamp recordings demonstrated that E2 also diminished I_A densities. Interestingly, these effects were most apparent in the dorsal cap parvocellular subdivision of the PVN. E2 also shortened a delay in the excitation of the PVN-RVLM neurons.

Conclusions

These findings demonstrate that E2 exerts an inhibitory effect on the functions of I_A, potentially by selectively down-regulating Kv4.2 but not Kv4.3 in PVN-RVLM neurons distributed in a specific parvocellular subdivision.

Purinergic and glutamatergic interactions in the hypothalamic paraventricular nucleus modulate sympathetic outflow

P2X receptors are expressed on ventrolateral medulla projecting paraventricular nucleus (PVN) neurons. Here, we investigate the role of adenosine 5'-triphosphate (ATP) in modulating sympathetic nerve activity (SNA) at the level of the PVN. We used an in situ arterially perfused rat preparation to determine the effect of P2 receptor activation and the putative interaction between purinergic and glutamatergic neurotransmitter systems within the PVN on lumbar SNA (LSNA). Unilateral microinjection of ATP into the PVN induced a dose-related increase in the LSNA (1 nmol: 38 ± 6 %, 2.5 nmol: 72 ± 7 %, 5 nmol: 96 ±13 %). This increase was significantly attenuated by blockade of P2 receptors (pyridoxalphosphate-6-azophenyl-20,40-disulphonic acid, PPADS) and glutamate receptors (kynurenic acid, KYN) or a combination of both. The increase in LSNA elicited by L-glutamate microinjection into the PVN was not affected by a previous injection of PPADS. Selective blockade of non-N-methyl-D-aspartate receptors (6-cyano-7-nitroquinoxaline-2,3-dione disodium salt, CNQX), but not N-methyl-D-aspartate receptors (NMDA) receptors (DL-2-amino-5-phosphonopentanoic acid, AP5), attenuated the ATP-induced sympathoexcitatory effects at the PVN level. Taken together, our data show that purinergic neurotransmission within the PVN is involved in the control of SNA via P2 receptor activation. Moreover, we show an interaction between P2 receptors and non-NMDA glutamate receptors in the PVN suggesting that these functional interactions might be important in the regulation of sympathetic outflow.

Tonic γ-aminobutyric acid-ergic activity in the hypothalamic arcuate nucleus is attenuated in the spontaneously hypertensive rat

We tested the hypothesis that tonic γ-aminobutyric acid-ergic activity in the hypothalamic arcuate nucleus (ARCN) modulates blood pressure control and attenuation of this inhibitory activity contributes to hypertension in the spontaneously hypertensive rats (SHR). Mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA) were recorded in urethane-anesthetized, artificially ventilated, adult male SHR and Wistar-Kyoto rats (WKY). Microinjections of gabazine into the ARCN elicited significantly smaller increases in MAP, HR, and GSNA in baroreceptor-intact SHR compared with baroreceptor-intact WKY. Attenuation of the responses to gabazine in SHR persisted, despite lowering of their baseline MAP to levels of WKY or barodenervation. Microinjections of N-methyl-d-aspartic acid (NMDA) into the ARCN elicited decreases in MAP and GSNA and increases in HR in baroreceptor-intact WKY. However, after microinjections of gabazine into the ARCN, microinjections of NMDA into the same nucleus elicited pressor responses in baroreceptor-intact WKY. In barodenervated WKY, increases in MAP and GSNA were elicited by ARCN stimulation by NMDA and the increases in HR were exaggerated. In baroreceptor-intact SHR, ARCN stimulation by NMDA elicited increases in MAP, GSNA, and HR which persisted, despite lowering of baseline MAP or barodenervation. Increases in MAP and GSNA elicited by ARCN stimulation by NMDA in barodenervated SHR were significantly greater than corresponding increases in barodenervated WKY. These results indicated that attenuated γ-aminobutyric acid-ergic activity in the ARCN and impaired baroreflex function may contribute to increases in blood pressure and sympathetic nerve activity after ARCN stimulation by NMDA and elevation of baseline blood pressure in SHR.

Rostroventrolateral medulla neurons with commissural projections provide input to sympathetic premotor neurons: anatomical and functional evidence

The activity of neurons in the rostral ventrolateral medulla (RVLM) is critical for the generation of vasomotor sympathetic tone. Multiple pre-sympathetic pathways converge on spinally projecting RVLM neurons, but the origin and circumstances in which such inputs are active are poorly understood. We have previously shown that input from the contralateral brainstem contributes to the baseline activity of this population: in the current study we investigate the distribution, phenotype and functional properties of RVLM neurons with commissural projections in the rat. We firstly used retrograde transport of fluorescent microspheres to identify neurons that project to the contralateral RVLM. Labelled neurons were prominent in a longitudinal column that extended over 1 mm caudal from the facial nucleus and contained hybridisation products indicating enkephalin (27%), GABA (15%) and adrenaline (3%) synthesis and included 6% of bulbospinal neurons identified by transport of cholera toxin B. Anterograde transport of fluorescent dextran-conjugate from the contralateral RVLM revealed extensive inputs throughout the RVLM that frequently terminated in close apposition with catecholaminergic and bulbospinal neurons. In urethane-anaesthetised rats we verified that 28/37 neurons antidromically activated by electrical stimulation of the contralateral pressor region were spontaneously active, of which 13 had activity locked to central respiratory drive and 15 displayed ongoing tonic discharge. In six tonically active neurons sympathoexcitatory roles were indicated by spike-triggered averages of splanchnic sympathetic nerve activity. We conclude that neurons in the RVLM project to the contralateral brainstem, form synapses with sympathetic premotor neurons, and have functional properties consistent with sympthoexcitatory function.

Astrocytes modulate a postsynaptic NMDA-GABAA-receptor crosstalk in hypothalamic neurosecretory neurons

A dynamic balance between the excitatory and inhibitory neurotransmitters glutamate and GABA is critical for maintaining proper neuronal activity in the brain. This balance is partly achieved via presynaptic interactions between glutamatergic and GABA(A)ergic synapses converging into the same targets. Here, we show that in hypothalamic magnocellular neurosecretory neurons (MNCs), a direct crosstalk between postsynaptic NMDA receptors (NMDARs) and GABA(A) receptors (GABA(A)Rs) contributes to the excitatory/inhibitory balance in this system. We found that activation of NMDARs by endogenous glutamate levels controlled by astrocyte glutamate transporters, evokes a transient and reversible potentiation of postsynaptic GABA(A)Rs. This inter-receptor crosstalk is calcium-dependent and involves a kinase-dependent phosphorylation mechanism, but does not require nitric oxide as an intermediary signal. Finally, we found the NMDAR-GABA(A)R crosstalk to be blunted in rats with heart failure, a pathological condition in which the hypothalamic glutamate-GABA balance is tipped toward an excitatory predominance. Together, our findings support a novel form of glutamate-GABA interactions in MNCs, which involves crosstalk between NMDA and GABA(A) postsynaptic receptors, whose strength is controlled by the activity of local astrocytes. We propose this inter-receptor crosstalk to act as a compensatory, counterbalancing mechanism to dampen glutamate-mediated overexcitation. Finally, we propose that an uncoupling between NMDARs and GABA(A)Rs may contribute to exacerbated neuronal activity and, consequently, sympathohumoral activation in such disease conditions as heart failure.

Effect of barodenervation on cardiovascular responses elicited from the hypothalamic arcuate nucleus of the rat

We have previously reported that chemical stimulation of the hypothalamic arcuate nucleus (ARCN) in the rat elicited increases as well as decreases in blood pressure (BP) and sympathetic nerve activity (SNA). The type of response elicited from the ARCN (i.e., increase or decrease in BP and SNA) depended on the level of baroreceptor activity which, in turn, was determined by baseline BP in rats with intact baroreceptors. Based on this information, it was hypothesized that baroreceptor unloading may play a role in the type of response elicited from the ARCN. Therefore, the effect of barodenervation on the ARCN-induced cardiovascular and sympathetic responses and the neurotransmitters in the hypothalamic paraventricular nucleus (PVN) mediating the excitatory responses elicited from the ARCN were investigated in urethane-anesthetized adult male Wistar rats. Bilateral barodenervation converted decreases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA) elicited by chemical stimulation of the ARCN with microinjections of N-methyl-D-aspartic acid to increases in MAP and GSNA and exaggerated the increases in heart rate (HR). Combined microinjections of NBQX and D-AP7 (ionotropic glutamate receptor antagonists) into the PVN in barodenervated rats converted increases in MAP and GSNA elicited by the ARCN stimulation to decreases in MAP and GSNA and attenuated increases in HR. Microinjections of SHU9119 (a melanocortin 3/4 receptor antagonist) into the PVN in barodenervated rats attenuated increases in MAP, GSNA and HR elicited by the ARCN stimulation. ARCN neurons projecting to the PVN were immunoreactive for proopiomelanocortin, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH). It was concluded that increases in MAP and GSNA and exaggeration of tachycardia elicited by the ARCN stimulation in barodenervated rats may be mediated via release of alpha-MSH and/or ACTH and glutamate from the ARCN neurons projecting to the PVN.

Role of the hypothalamic arcuate nucleus in cardiovascular regulation

Recently the hypothalamic arcuate nucleus (Arc) has been implicated in cardiovascular regulation. Both pressor and depressor responses can be elicited by the chemical stimulation of the Arc. The direction of cardiovascular responses (increase or decrease) elicited from the Arc depends on the baseline blood pressure. The pressor responses are mediated via increase in sympathetic nerve activity and involve activation of the spinal ionotropic glutamate receptors. Arc-stimulation elicits tachycardic responses which are mediated via inhibition of vagal input and excitation of sympathetic input to the heart. The pathways within the brain mediating the pressor and tachycardic responses elicited from the Arc have not been delineated. The depressor responses to the Arc-stimulation are mediated via the hypothalamic paraventricular nucleus (PVN). Gamma aminobutyric acid type A receptors, neuropeptide Y1 receptors, and opiate receptors in the PVN mediate the depressor responses elicited from the Arc. Some circulating hormones (e.g., leptin and insulin) may reach the Arc via the leaky blood-brain barrier and elicit their cardiovascular effects. Although the Arc is involved in mediating the cardiovascular responses to intravenously injected angiotensin II and angiotensin-(1-12), these effects may not be due to leakage of these peptides across the blood-brain barrier in the Arc; instead, circulating angiotensins may act on neurons in the SFO and mediate cardiovascular actions via the projections of SFO neurons to the Arc. Cardiovascular responses elicited by acupuncture have been reported to be mediated by direct and indirect projections of the Arc to the RVLM.

Cardiovascular responses to chemical stimulation of the hypothalamic arcuate nucleus in the rat: role of the hypothalamic paraventricular nucleus

The mechanism of cardiovascular responses to chemical stimulation of the hypothalamic arcuate nucleus (ARCN) was studied in urethane-anesthetized adult male Wistar rats. At the baseline mean arterial pressure (BLMAP) close to normal, ARCN stimulation elicited decreases in MAP and sympathetic nerve activity (SNA). The decreases in MAP elicited by ARCN stimulation were attenuated by either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or beta-endorphin receptor blockade in the ipsilateral hypothalamic paraventricular nucleus (PVN). Combined blockade of GABA-A, NPY1 and opioid receptors in the ipsilateral PVN converted the decreases in MAP and SNA to increases in these variables. Conversion of inhibitory effects on the MAP and SNA to excitatory effects following ARCN stimulation was also observed when the BLMAP was decreased to below normal levels by an infusion of sodium nitroprusside. The pressor and tachycardic responses to ARCN stimulation at below normal BLMAP were attenuated by blockade of melanocortin 3/4 (MC3/4) receptors in the ipsilateral PVN. Unilateral blockade of GABA-A receptors in the ARCN increased the BLMAP and heart rate (HR) revealing tonic inhibition of the excitatory neurons in the ARCN. ARCN stimulation elicited tachycardia regardless of the level of BLMAP. ARCN neurons projecting to the PVN were immunoreactive for glutamic acid decarboxylase 67 (GAD67), NPY, and beta-endorphin. These results indicated that: 1) at normal BLMAP, decreases in MAP and SNA induced by ARCN stimulation were mediated via GABA-A, NPY1 and opioid receptors in the PVN, 2) lowering of BLMAP converted decreases in MAP following ARCN stimulation to increases in MAP, and 3) at below normal BLMAP, increases in MAP and HR induced by ARCN stimulation were mediated via MC3/4 receptors in the PVN. These results provide a base for future studies to explore the role of ARCN in cardiovascular diseases.

Centrally administered bombesin activates COX-containing spinally projecting neurons of the PVN in anesthetized rats

The paraventricular nucleus (PVN) of the hypothalamus has a heterogenous structure containing different types of output neurons that project to the median eminence, posterior pituitary, brain stem autonomic centers and sympathetic preganglionic neurons in the spinal cord. Presympathetic neurons in the PVN send mono- and poly-synaptic projections to the spinal cord. In the present study using urethane-anesthetized rats, we examined the effects of centrally administered bombesin (a homologue of the mammalian gastrin-releasing peptide) on the mono-synaptic spinally projecting PVN neurons pre-labeled with a retrograde tracer Fluoro-Gold (FG) injected into T8 level of the spinal cord, with regard to the immunoreactivity for cyclooxygenase (COX) isozymes (COX-1/COX-2) and Fos (a marker of neuronal activation). FG-labeled spinally projecting neurons were abundantly observed in the dorsal cap, ventral part and posterior part of the PVN. The immunoreactivity of each COX-1 and COX-2 was detected in FG-labeled spinally projecting PVN neurons in the vehicle (10 μl of saline/animal, i.c.v.)-treated group, while bombesin (1 nmol/animal, i.c.v.) had no effect on the number of these immunoreactive neurons for each COX isozyme with labeling of FG. On the other hand, the peptide significantly increased the number of double-immunoreactive neurons for Fos and COX-1/COX-2 with FG-labeling in the PVN (except triple-labeled neurons for FG, COX-2 and Fos in the dorsal cap of the PVN), as compared to those of vehicle-treated group. These results suggest that centrally administered bombesin activates spinally projecting PVN neurons containing COX-1 and COX-2 in rats.

Central neuromodulatory pathways regulating sympathetic activity in hypertension

The classical neurotransmitters, glutamate and GABA, mediate fast (milliseconds) synaptic transmission and modulate its effectiveness through slow (seconds to minutes) signaling processes. Angiotensinergic pathways, from the lamina terminalis to the paraventricular nucleus (PVN)/supraoptic nucleus and rostral ventrolateral medulla (RVLM), are activated by stimuli such as circulating angiotensin type II (Ang II), cerebrospinal fluid (CSF) sodium ion concentration ([Na(+)]), and possibly plasma aldosterone, leading to sympathoexcitation, largely by decreasing GABA and increasing glutamate release. The aldosterone-endogenous ouabain (EO) pathway is a much slower neuromodulatory pathway. Aldosterone enhances EO release, and the latter increases chronic activity in angiotensinergic pathways by, e.g., increasing expression for Ang I receptor (AT(1)R) and NADPH oxidase subunits in the PVN. Blockade of this pathway does not affect the initial sympathoexcitatory and pressor responses but to a large extent, prevents chronic responses to CSF [Na(+)] or Ang II. Recruitment of these two neuromodulatory pathways allows the central nervous system (CNS) to shift gears to rapidly cause and sustain sympathetic hyperactivity in an efficient manner. Decreased GABA release, increased glutamate release, and enhanced AT(1)R activation in, e.g., the PVN and RVLM contribute to the elevated blood pressure in a number of hypertension models. In Dahl S rats and spontaneous hypertensive rats, high salt activates the CNS aldosterone-EO pathway, and the salt-induced hypertension can be prevented/reversed by specific CNS blockade of any of the steps in the cascade from aldosterone synthase to AT(1)R. Further studies are needed to advance our understanding of how and where in the brain these rapid, slow, and very slow CNS pathways are activated and interact in models of hypertension and other disease states associated with chronic sympathetic hyperactivity.

Central mechanisms involved in pilocarpine-induced pressor response

Pilocarpine (cholinergic muscarinic agonist) injected peripherally may act centrally to produce pressor responses; in the present study, using c-fos immunoreactive expression, we investigated the forebrain and brainstem areas activated by pressor doses of intravenous (i.v.) pilocarpine. In addition, the importance of vasopressin secretion and/or sympathetic activation and the effects of lesions in the anteroventral third ventricle (AV3V) region in awake rats were also investigated. In male Holtzman rats, pilocarpine (0.04 to 4μmol/kg b.w.) i.v. induced transitory hypotension followed by long lasting hypertension. Sympathetic blockade with prazosin (1mg/kg b.w.) i.v. or AV3V lesions (1 day) almost abolished the pressor response to i.v. pilocarpine (2μmol/kg b.w.), whereas the vasopressin antagonist (10μg/kg b.w.) i.v. reduced the response to pilocarpine. Pilocarpine (2 and 4μmol/kg b.w.) i.v. increased the number of c-fos immunoreactive cells in the subfornical organ, paraventricular and supraoptic nuclei of the hypothalamus, organ vasculosum of the lamina terminalis, median preoptic nucleus, nucleus of the solitary tract and caudal and rostral ventrolateral medulla. These data suggest that i.v. pilocarpine activates specific forebrain and brainstem mechanisms increasing sympathetic activity and vasopressin secretion to induce pressor response.

Role of small conductance calcium-activated potassium channels expressed in PVN in regulating sympathetic nerve activity and arterial blood pressure in rats

Small conductance Ca(2+)-activated K(+) (SK) channels regulate membrane properties of rostral ventrolateral medulla (RVLM) projecting hypothalamic paraventricular nucleus (PVN) neurons and inhibition of SK channels increases in vitro excitability. Here, we determined in vivo the role of PVN SK channels in regulating sympathetic nerve activity (SNA) and mean arterial pressure (MAP). In anesthetized rats, bilateral PVN microinjection of SK channel blocker with peptide apamin (0, 0.125, 1.25, 3.75, 12.5, and 25 pmol) increased splanchnic SNA (SSNA), renal SNA (RSNA), MAP, and heart rate (HR) in a dose-dependent manner. Maximum increases in SSNA, RSNA, MAP, and HR elicited by apamin (12.5 pmol, n = 7) were 330 ± 40% (P < 0.01), 271 ± 40% (P < 0.01), 29 ± 4 mmHg (P < 0.01), and 34 ± 9 beats/min (P < 0.01), respectively. PVN injection of the nonpeptide SK channel blocker UCL1684 (250 pmol, n = 7) significantly increased SSNA (P < 0.05), RSNA (P < 0.05), MAP (P < 0.05), and HR (P < 0.05). Neither apamin injected outside the PVN (12.5 pmol, n = 6) nor peripheral administration of the same dose of apamin (12.5 pmol, n = 5) evoked any significant changes in the recorded variables. PVN-injected SK channel enhancer 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO, 5 nmol, n = 4) or N-cyclohexyl-N-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-4-pyrimidin]amine (CyPPA, 5 nmol, n = 6) did not significantly alter the SSNA, RSNA, MAP, and HR. Western blot and RT-PCR analysis of punched PVN tissue showed abundant expression of SK1-3 channels. We conclude that SK channels expressed in the PVN play an important role in the regulation of sympathetic outflow and cardiovascular function.

Oxidative stress in the rostral ventrolateral medulla modulates excitatory and inhibitory inputs in spontaneously hypertensive rats

OBJECTIVES

The rostral ventrolateral medulla (RVLM) of the brainstem and the paraventricular nucleus (PVN) of the hypothalamus play crucial roles in central cardiovascular regulation. In hypertensive rats, an imbalance of excitatory and inhibitory inputs to the RVLM enhances central sympathetic outflow. Increased reactive oxygen species (ROS) in the RVLM also contribute to sympathoexcitation, leading to hypertension. The aim of the present study was to elucidate whether ROS in the RVLM modulate synaptic transmission via excitatory and inhibitory amino acids and influence the excitatory inputs to the RVLM from the PVN in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

We transfected adenovirus vectors encoding the manganese superoxide dismutase (AdMnSOD) gene to scavenge ROS in the RVLM both in Wistar-Kyoto rats and SHRs. The decreases in blood pressure and renal sympathetic nerve activity (RSNA) evoked by injecting kynurenic acid, a glutamate receptor blocker, into the RVLM were attenuated, and the increases in blood pressure and RSNA evoked by injecting bicuculline, a γ-amino butyric acid (GABA) receptor blocker, into the RVLM were enhanced in AdMnSOD-transfected SHRs compared with adenovirus vectors encoding the β-galactosidase (AdLacZ) gene-transfected SHRs. Furthermore, the increases in blood pressure and RSNA evoked by injecting bicuculline into the PVN were attenuated in AdMnSOD-transfected SHRs compared with AdLacZ-transfected SHRs.

CONCLUSION

These findings suggest that ROS in the RVLM enhance glutamatergic excitatory inputs and attenuate GABAergic inhibitory inputs to the RVLM, thereby increasing sympathoexcitatory input to the RVLM from the PVN in SHRs.

The hypothalamic-neurohypophyseal system: from genome to physiology

The elucidation of the genomes of a large number of mammalian species has produced a huge amount of data on which to base physiological studies. These endeavours have also produced surprises, not least of which has been the revelation that the number of protein coding genes needed to make a mammal is only 22 333 (give or take). However, this small number belies an unanticipated complexity that has only recently been revealed as a result of genomic studies. This complexity is evident at a number of levels: (i) cis-regulatory sequences; (ii) noncoding and antisense mRNAs, most of which have no known function; (iii) alternative splicing that results in the generation of multiple, subtly different mature mRNAs from the precursor transcript encoded by a single gene; and (iv) post-translational processing and modification. In this review, we examine the steps being taken to decipher genome complexity in the context of gene expression, regulation and function in the hypothalamic-neurohypophyseal system (HNS). Five unique stories explain: (i) the use of transcriptomics to identify genes involved in the response to physiological (dehydration) and pathological (hypertension) cues; (ii) the use of mass spectrometry for single-cell level identification of biological active peptides in the HNS, and to measure in vitro release; (iii) the use of transgenic lines that express fusion transgenes enabling (by cross-breeding) the generation of double transgenic lines that can be used to study vasopressin (AVP) and oxytocin (OXT) neurones in the HNS, as well as their neuroanatomy, electrophysiology and activation upon exposure to any given stimulus; (iv) the use of viral vectors to demonstrate that somato-dendritically released AVP plays an important role in cardiovascular homeostasis by binding to V1a receptors on local somata and dendrites; and (v) the use of virally-mediated optogenetics to dissect the role of OXT and AVP in the modulation of a wide variety of behaviours.

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.

Switching control of sympathetic activity from forebrain to hindbrain in chronic dehydration

We investigated the mechanisms responsible for increased blood pressure and sympathetic nerve activity (SNA) caused by 2-3 days dehydration (DH) both in vivo and in situ preparations. In euhydrated (EH) rats, systemic application of the AT(1) receptor antagonist Losartan and subsequent pre-collicular transection (to remove the hypothalamus) significantly reduced thoracic (t)SNA. In contrast, in DH rats, Losartan, followed by pre-collicular and pontine transections, failed to reduce tSNA, whereas transection at the medulla-spinal cord junction massively reduced tSNA. In DH but not EH rats, selective inhibition of the commissural nucleus tractus solitarii (cNTS) significantly reduced tSNA. Comparable data were obtained in both in situ and in vivo (anaesthetized/conscious) rats and suggest that following chronic dehydration, the control of tSNA transfers from supra-brainstem structures (e.g. hypothalamus) to the medulla oblongata, particularly the cNTS. As microarray analysis revealed up-regulation of AP1 transcription factor JunD in the dehydrated cNTS, we tested the hypothesis that AP1 transcription factor activity is responsible for dehydration-induced functional plasticity. When AP1 activity was blocked in the cNTS using a viral vector expressing a dominant negative FosB, cNTS inactivation was ineffective. However, tSNA was decreased after pre-collicular transection, a response similar to that seen in EH rats. Thus, the dehydration-induced switch in control of tSNA from hypothalamus to cNTS seems to be mediated via activation of AP1 transcription factors in the cNTS. If AP1 activity is blocked in the cNTS during dehydration, sympathetic activity control reverts back to forebrain regions. This unique reciprocating neural structure-switching plasticity between brain centres emphasizes the multiple mechanisms available for the adaptive response to dehydration.

Imbalanced K+ and Ca2+ subthreshold interactions contribute to increased hypothalamic presympathetic neuronal excitability in hypertensive rats

Despite the importance of brain-mediated sympathetic activation in the morbidity and mortality of patients with high blood pressure, the precise cellular mechanisms involved remain largely unknown. We show that an imbalanced interaction between two opposing currents mediated by potassium (I(A)) and calcium (I(T)) channels occurs in sympathetic-related hypothalamic neurons in hypertensive rats. We show that this imbalance contributes to enhanced membrane excitability and firing activity in this neuronal population. Knowledge of how these opposing ion channels interact in normal and disease states increases our understanding of underlying brain mechanisms contributing to the high blood pressure condition.

Hypertension induced by angiotensin II and a high salt diet involves reduced SK current and increased excitability of RVLM projecting PVN neurons

Although evidence indicates that activation of presympathetic paraventricular nucleus (PVN) neurons contributes to the pathogenesis of salt-sensitive hypertension, the underlying cellular mechanisms are not fully understood. Recent evidence indicates that small conductance Ca(2+)-activated K(+) (SK) channels play a significant role in regulating the excitability of a key group of sympathetic regulatory PVN neurons, those with axonal projections to the rostral ventrolateral medulla (RVLM; i.e., PVN-RVLM neurons). In the present study, rats consuming a high salt (2% NaCl) diet were made hypertensive by systemic infusion of angiotensin II (AngII), and whole cell patch-clamp recordings were made in brain slice from retrogradely labeled PVN-RVLM neurons. To determine if the amplitude of SK current was altered in neurons from hypertensive rats, voltage-clamp recordings were performed to isolate SK current. Results indicate that SK current amplitude (P < 0.05) and density (P < 0.01) were significantly smaller in the hypertensive group. To investigate the impact of this on intrinsic excitability, current-clamp recordings were performed in separate groups of PVN-RVLM neurons. Results indicate that the frequency of spikes evoked by current injection was significantly higher in the hypertensive group (P < 0.05-0.01). Whereas bath application of the SK channel blocker apamin significantly increased discharge of neurons from normotensive rats (P < 0.05-0.01), no effect was observed in the hypertensive group. In response to ramp current injections, subthreshold depolarizing input resistance was greater in the hypertensive group compared with the normotensive group (P < 0.05). Blockade of SK channels increased depolarizing input resistance in normotensive controls (P < 0.05) but had no effect in the hypertensive group. On termination of current pulses, a medium afterhyperpolarization potential (mAHP) was observed in most neurons of the normotensive group. In the hypertensive group, the mAHP was either small or absent. In the latter case, an afterdepolarization potential (ADP) was observed that was unaffected by apamin. Apamin treatment in the normotensive group blocked the mAHP and revealed an ADP resembling that seen in the hypertensive group. We conclude that diminished SK current likely underlies the absence of mAHPs in PVN-RVLM neurons from hypertensive rats. Both the ADP and greater depolarizing input resistance likely contribute to increased excitability of PVN-RVLM neurons from rats with AngII-Salt hypertension.

Modulation of cardiorespiratory function mediated by the paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) coordinates autonomic and neuroendocrine systems to maintain homeostasis and to respond to stress. Neuroanatomic and neurophysiologic experiments have provided insight into the mechanisms by which the PVN acts. The PVN projects directly to the spinal cord and brainstem and, specifically, to sites that control cardio-respiratory function: the intermediolateral cell columns and phrenic motor nuclei in the spinal cord and rostral ventrolateral medulla (RVLM) and the rostral nuclei in the ventral respiratory column (rVRC) in the brainstem. Activation of the PVN increases ventilation (both tidal volume and frequency) and blood pressure (both heart rate and sympathetic nerve activity). Excitatory and inhibitory neurotransmitters including glutamate and GABA converge in the PVN to influence its neuronal activity. However, a tonic GABAergic input to the PVN directly modulates excitatory outflow from the PVN. Further, even within the PVN, microinjection of GABA(A) receptor blockers increases glutamate release suggesting an indirect mechanism by which GABA control contributes to PVN functions. PVN activity alters blood pressure and ventilation during various stresses, such as maternal separation, chronic intermittent hypoxia (CIH), dehydration and hemorrhage. Among the several PVN neurotransmitters and neurohormones, vasopressin and oxytocin modulate ventilation and blood pressure. Here, we review our data indicating that increases in vasopressin and vasopressin type 1A (V(1A)) receptor signalling in the RVLM and rVRC are mechanisms increasing blood pressure and ventilation after exposure to CIH. That blockade of V(1A) receptors in the medulla normalizes baseline blood pressure as well as blunts PVN-evoked blood pressure and ventilatory responses in CIH-conditioned animals indicate the role of vasopressin in cardiorespiratory control. In summary, morphological and functional studies have found that the PVN integrates sensory input and projects to the sympathetic and respiratory control systems with descending projections to the medulla and spinal cord.

Reduction in synaptic GABA release contributes to target-selective elevation of PVN neuronal activity in rats with myocardial infarction

Neuronal activity in the paraventricular nucleus (PVN) is known to be elevated in rats with heart failure. However, the type of neurons involved and the underlying synaptic mechanisms remain unknown. Here we examined spontaneous firing activity and synaptic currents in presympathetic PVN neurons in rats with myocardial infarction (MI), using slice patch clamp combined with the retrograde labeling technique. In PVN neurons projecting to the rostral ventrolateral medulla (PVN-RVLM), MI induced a significant increase in basal firing rate (1.79 to 3.02 Hz, P < 0.05) and a reduction in the frequency of spontaneous (P < 0.05) and miniature (P < 0.01) inhibitory postsynaptic currents (IPSCs). In addition, MI induced an increase in the paired-pulse ratio of evoked IPSCs (P < 0.05). Bicuculline, a GABA(A) receptor antagonist, increased the firing rate of PVN-RVLM neurons in sham-operated (1.21 to 2.74 Hz, P < 0.05) but not MI (P > 0.05) rats. In contrast, in PVN neurons projecting to the intermediolateral horn of the spinal cord (PVN-IML), MI did not induce any significant changes in the basal firing rate and the properties of spontaneous and miniature IPSCs. The properties of spontaneous excitatory postsynaptic currents (EPSCs) were not altered in either neuron group. In conclusion, our results indicate that MI induces an elevation of firing activity in PVN-RVLM but not in PVN-IML neurons and that the elevated firing rate is largely due to a decrease in GABA release. These results provide evidence for a novel target-selective synaptic plasticity in the PVN that is associated with the sympathetic hyperactivity commonly seen in heart failure.

Paraventricular hypothalamic nucleus: axonal projections to the brainstem

The paraventricular hypothalamic nucleus (PVH) contains many neurons that innervate the brainstem, but information regarding their target sites remains incomplete. Here we labeled neurons in the rat PVH with an anterograde axonal tracer, Phaseolus vulgaris leucoagglutinin (PHAL), and studied their descending projections in reference to specific neuronal subpopulations throughout the brainstem. While many of their target sites were identified previously, numerous new observations were made. Major findings include: 1) In the midbrain, the PVH projects lightly to the ventral tegmental area, Edinger-Westphal nucleus, ventrolateral periaqueductal gray matter, reticular formation, pedunculopontine tegmental nucleus, and dorsal raphe nucleus. 2) In the dorsal pons, the PVH projects heavily to the pre-locus coeruleus, yet very little to the catecholamine neurons in the locus coeruleus, and selectively targets the viscerosensory subregions of the parabrachial nucleus. 3) In the ventral medulla, the superior salivatory nucleus, retrotrapezoid nucleus, compact and external formations of the nucleus ambiguous, A1 and caudal C1 catecholamine neurons, and caudal pressor area receive dense axonal projections, generally exceeding the PVH projection to the rostral C1 region. 4) The medial nucleus of the solitary tract (including A2 noradrenergic and aldosterone-sensitive neurons) receives the most extensive projections of the PVH, substantially more than the dorsal vagal nucleus or area postrema. Our findings suggest that the PVH may modulate a range of homeostatic functions, including cerebral and ocular blood flow, corneal and nasal hydration, ingestive behavior, sodium intake, and glucose metabolism, as well as cardiovascular, gastrointestinal, and respiratory activities.

Altered balance of gamma-aminobutyric acidergic and glutamatergic afferent inputs in rostral ventrolateral medulla-projecting neurons in the paraventricular nucleus of the hypothalamus of renovascular hypertensive rats

An imbalance of excitatory and inhibitory functions has been shown to contribute to numerous pathological disorders. Accumulating evidence supports the idea that a change in hypothalamic gamma-aminobutyric acid (GABA)-ergic inhibitory and glutamatergic excitatory synaptic functions contributes to exacerbated neurohumoral drive in prevalent cardiovascular disorders, including hypertension. However, the precise underlying mechanisms and neuronal substrates are still not fully elucidated. In the present study, we combined quantitative immunohistochemistry with neuronal tract tracing to determine whether plastic remodeling of afferent GABAergic and glutamatergic inputs into identified RVLM-projecting neurons of the hypothalamic paraventricular nucleus (PVN-RVLM) contributes to an imbalanced excitatory/inhibitory function in renovascular hypertensive rats (RVH). Our results indicate that both GABAergic and glutamatergic innervation densities increased in oxytocin-positive, PVN-RVLM (OT-PVN-RVLM) neurons in RVH rats. Despite this concomitant increase, time-dependent and compartment-specific differences in the reorganization of these inputs resulted in an altered balance of excitatory/inhibitory inputs in somatic and dendritic compartments. A net predominance of excitatory over inhibitory inputs was found in OT-PVN-RVLM proximal dendrites. Our results indicate that, along with previously described changes in neurotransmitter release probability and postsynaptic receptor function, remodeling of GABAergic and glutamatergic afferent inputs contributes as an underlying mechanism to the altered excitatory/inhibitory balance in the PVN of hypertensive rats.

Increased vasopressin transmission from the paraventricular nucleus to the rostral medulla augments cardiorespiratory outflow in chronic intermittent hypoxia-conditioned rats

A co-morbidity of sleep apnoea is hypertension associated with elevated sympathetic nerve activity (SNA) which may result from conditioning to chronic intermittent hypoxia (CIH). Our hypothesis is that SNA depends on input to the rostral ventrolateral medulla (RVLM) from neurons in the paraventricular nucleus (PVN) that release arginine vasopressin (AVP) and specifically, that increased SNA evoked by CIH depends on this excitatory input. In two sets of neuroanatomical experiments, we determined if AVP neurons project from the PVN to the RVLM and if arginine vasopressin (V(1A)) receptor expression increases in the RVLM after CIH conditioning (8 h per day for 10 days). In the first set, cholera toxin beta subunit (CT-beta) was microinjected into the RVLM to retrogradely label the PVN neurons. Immunohistochemical staining demonstrated that 14.6% of CT-beta-labelled PVN neurons were double-labelled with AVP. In the second set, sections of the medulla were immunolabelled for V(1A) receptors, and the V(1A) receptor-expressing cell count was significantly greater in the RVLM (P < 0.01) and in the neighbouring rostral ventral respiratory column (rVRC) from CIH- than from room air (RA)-conditioned rats. In a series of physiological experiments, we determined if blocking V(1A) receptors in the medulla would normalize blood pressure in CIH-conditioned animals and attenuate its response to disinhibition of PVN. Blood pressure (BP), heart rate (HR), diaphragm (D(EMG)) and genioglossus muscle (GG(EMG)) activity were recorded in anaesthetized, ventilated and vagotomized rats. The PVN was disinhibited by microinjecting a GABA(A) receptor antagonist, bicuculline (BIC, 0.1 nmol), before and after blocking V(1A) receptors within the RVLM and rVRC with SR49059 (0.2 nmol). In RA-conditioned rats, disinhibition of the PVN increased BP, HR, minute D(EMG) and GG(EMG) activity and these increases were attenuated after blocking V(1A) receptors. In CIH-conditioned rats, a significantly greater dose of blocker (0.4 nmol) was required to blunt these physiological responses (P < 0.05). Further, this dose normalized the baseline BP. In summary, AVP released by a subset of PVN neurons modulates cardiorespiratory output via V(1A) receptors in the RVLM and rVRC, and increased SNA in CIH-conditioned animals depends on up-regulation of V(1A) receptors in the RVLM.

In vivo discharge properties of hypothalamic paraventricular nucleus neurons with axonal projections to the rostral ventrolateral medulla

The hypothalamic paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) are key components of a neural network that generates and regulates sympathetic nerve activity (SNA). Although each region has been extensively studied, little is presently known about the in vivo discharge properties of individual PVN neurons that directly innervate the RVLM. Here extracellular recording was performed in anesthetized rats, and antidromic stimulation was used to identify single PVN neurons with axonal projections to the RVLM (n = 94). Neurons were divided into two groups that had either unbranched axons terminating in the RVLM (i.e., PVN-RVLM neurons, n = 65) or collateralized axons targeting both the RVLM and spinal cord [i.e., PVN-RVLM/intermediolateral cell column (IML) neurons, n = 29]. Many PVN-RVLM (32/65, 49%) and PVN-RVLM/IML (17/29, 59%) neurons were spontaneously active. The average firing frequency was not different across groups. Spike-triggered averaging revealed that spontaneous discharge of most neurons was temporally correlated with renal SNA (PVN-RVLM: 12/21, 57%; PVN-RVLM/IML: 6/9, 67%). Time histograms triggered by the electrocardiogram (ECG) R-wave indicated that discharge of most cells was also cardiac rhythmic (PVN-RVLM: 25/32, 78%; PVN-RVLM/IML: 10/17, 59%). Raising and lowering arterial blood pressure to increase and decrease arterial baroreceptor input caused a corresponding decrease and increase in firing frequency among cells of both groups (PVN-RVLM: 9/13, 69%; PVN-RVLM/IML: 4/4, 100%). These results indicate that PVN-RVLM and PVN-RVLM/IML neurons are both capable of contributing to basal sympathetic activity and its baroreflex modulation.

Regulation of tonic GABA inhibitory function, presympathetic neuronal activity and sympathetic outflow from the paraventricular nucleus by astroglial GABA transporters

Neuronal activity in the hypothalamic paraventricular nucleus (PVN), as well as sympathetic outflow from the PVN, is basally restrained by a GABAergic inhibitory tone. We recently showed that two complementary GABA(A) receptor-mediated modalities underlie inhibition of PVN neuronal activity: a synaptic, quantal inhibitory modality (IPSCs, I(phasic)) and a sustained, non-inactivating modality (I(tonic)). Here, we investigated the role of neuronal and/or glial GABA transporters (GATs) in modulating these inhibitory modalities, and assessed their impact on the activity of RVLM-projecting PVN neurons (PVN-RVLM neurons), and on PVN influence of renal sympathetic nerve activity (RSNA). Patch-clamp recordings were obtained from retrogradely labelled PVN-RVLM neurons in a slice preparation. The non-selective GAT blocker nipecotic acid (100-300 microM) caused a large increase in GABA(A)I(tonic), and reduced IPSC frequency. These effects were replicated by beta-alanine (100 microM), but not by SKF 89976A (30 microM), relatively selective blockers of GAT3 and GAT1 isoforms, respectively. Similar effects were evoked by the gliotoxin L-alpha-aminodipic acid (2 mM). GAT blockade attenuated the firing activity of PVN-RVLM neurons. Moreover, PVN microinjections of nipecotic acid in the whole animal diminished ongoing RSNA. A robust GAT3 immunoreactivity was observed in the PVN, which partially colocalized with the glial marker GFAP. Altogether, our results indicate that by modulating ambient GABA levels and the efficacy of GABA(A)I(tonic), PVN GATs, of a likely glial location, contribute to setting a basal tone of PVN-RVLM firing activity, and PVN-driven RSNA.

Excitability of paraventricular nucleus neurones that project to the rostral ventrolateral medulla is regulated by small-conductance Ca2+-activated K+ channels

Whole cell patch-clamp recordings were performed in brain slices to investigate mechanisms regulating the excitability of paraventricular nucleus (PVN) neurones that project directly to the rostral ventrolateral medulla (RVLM) (PVN-RVLM neurones) of rats. In voltage-clamp recordings, step depolarization elicited a calcium-dependent outward tail current that reversed near E(K). The current was nearly abolished by apamin and by UCL1684, suggesting mediation by small-conductance Ca(2+)-activated K(+) (SK) channels. In current-clamp recordings, depolarizing step current injections evoked action potentials that underwent spike-frequency adaptation (SFA). SK channel blockade with apamin or UCL1684 increased the spike frequency without changing the rate of SFA. Upon termination of step current injection, a prominent medium after-hyperpolarization potential (mAHP) was observed. SK channel blockade abolished the mAHP and revealed an after-depolarization potential (ADP). In response to ramp current injections, the rate of sub-threshold depolarization was increased during SK channel blockade, indicating that depolarizing input resistance was increased. Miniature EPSC frequency, amplitude, and decay kinetics were unaltered by bath application of apamin, suggesting that SK channel blockade likely increased excitability by a postsynaptic action. We conclude that although SK channels play little role in generating SFA in PVN-RVLM neurones, their activation nevertheless does dampen excitability. The mechanism appears to involve activation of a mAHP that opposes a prominent ADP that would otherwise facilitate firing.

Neurochemistry of the paraventricular nucleus of the hypothalamus: implications for cardiovascular regulation

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and endocrine homeostasis. The PVN integrates specific afferent stimuli to produce an appropriate differential sympathetic output. The neural circuitry and some of the neurochemical substrates within this circuitry are discussed. The PVN has at least three neural circuits to alter sympathetic activity and cardiovascular regulation. These pathways innervate the vasculature and organs such as the heart, kidney and adrenal medulla. The basal level of sympathetic tone at any given time is dependent upon excitatory and inhibitory inputs. Under normal circumstances the sympathetic nervous system is tonically inhibited. This inhibition is dependent upon GABA and nitric oxide such that nitric oxide potentiates local GABAergic synaptic inputs onto the neurones in the PVN. Excitatory neurotransmitters such as glutamate and angiotensin II modify the tonic inhibitory activity. The neurotransmitters oxytocin, vasopressin and dopamine have been shown to affect cardiovascular function. These neurotransmitters are found in neurones of the PVN and within the spinal cord. Oxytocin and vasopressin terminal fibres are closely associated with sympathetic preganglionic neurones (SPNs). Sympathetic preganglionic neurones have been shown to express receptors for oxytocin, vasopressin and dopamine. Oxytocin causes cardioacceleratory and pressor effects that are greatest in the upper thoracic cord while vasopressin cause these effects but more significant in the lower thoracic cord. Dopaminergic effects on the cardiovascular system include inhibitory or excitatory actions attributed to a direct PVN influence or via interneuronal connections to sympathetic preganglionic neurones.