Function of the ventrolateral medulla in the control of the circulation

Uncovering Hemispheric Asymmetry and Directed Oscillatory Brain-Heart Interplay in Anxiety Processing: An fMRI Study

Brain-heart interactions (BHI) are critical for generating and processing emotions, including anxiety. Understanding specific neural correlates would be instrumental for greater comprehension and potential therapeutic interventions of anxiety disorders. While prior work has implicated the pontine structure as a central processor in cardiac regulation in anxiety, the distributed nature of anxiety processing across the cortex remains elusive. To address this, we performed a whole-brain-heart analysis using the full frequency directed transfer function to study resting-state spectral differences in BHI between high and low anxiety groups undergoing fMRI scans. Our findings revealed a hemispheric asymmetry in low-frequency interplay (0.05 Hz - 0.15 Hz) characterized by ascending BHI to the left insula and descending BHI from the right insula. Furthermore, we provide evidence supporting the "pacemaker hypothesis", highlighting the pons' function in regulating cardiac activity. Higher frequency interplay (0.2 Hz - 0.4Hz) demonstrate a preference for ascending interactions, particularly towards ventral prefrontal cortical activity in high anxiety groups, suggesting the heart's role in triggering a cognitive response to regulate anxiety. These findings highlight the impact of anxiety on BHI, contributing to a better understanding of its effect on the resting-state fMRI signal, with further implications for potential therapeutic interventions in treating anxiety disorders.

Control of defensive behavior by the nucleus of Darkschewitsch GABAergic neurons

The nucleus of Darkschewitsch (ND), mainly composed of GABAergic neurons, is widely recognized as a component of the eye-movement controlling system. However, the functional contribution of ND GABAergic neurons (NDGABA) in animal behavior is largely unknown. Here, we show that NDGABA neurons were selectively activated by different types of fear stimuli, such as predator odor and foot shock. Optogenetic and chemogenetic manipulations revealed that NDGABA neurons mediate freezing behavior. Moreover, using circuit-based optogenetic and neuroanatomical tracing methods, we identified an excitatory pathway from the lateral periaqueductal gray (lPAG) to the ND that induces freezing by exciting ND inhibitory outputs to the motor-related gigantocellular reticular nucleus, ventral part (GiV). Together, these findings indicate the NDGABA population as a novel hub for controlling defensive response by relaying fearful information from the lPAG to GiV, a mechanism critical for understanding how the freezing behavior is encoded in the mammalian brain.

Circadian rhythm influences naloxone induced morphine withdrawal and neuronal activity of lateral paragigantocellularis nucleus

Investigations have shown that the circadian rhythm can affect the mechanisms associated with drug dependence. In this regard, we sought to assess the negative consequence of morphine withdrawal syndrome on conditioned place aversion (CPA) and lateral paragigantocellularis (LPGi) neuronal activity in morphine-dependent rats during light (8:00-12:00) and dark (20:00-24:00) cycles. Male Wistar rats (250-300 g) were received 10 mg/kg morphine or its vehicle (Saline, 2 mL/kg/12 h, s.c.) in 13 consecutive days for behavioral assessment tests. Then, naloxone-induced conditioned place aversion and physical signs of withdrawal syndrome were evaluated during light and dark cycles. In contrast to the behavioral part, we performed in vivo extracellular single-unit recording for investigating the neural response of LPGi to naloxone in morphine-dependent rats on day 10 of morphine/saline exposure. Results showed that naloxone induced conditioned place aversion in both light and dark cycles, but the CPA score during the light cycle was larger. Moreover, the intensity of physical signs of morphine withdrawal syndrome was more severe during the light cycle (rest phase) compare to the dark one. In electrophysiological experiments, results indicated that naloxone evoked both excitatory and inhibitory responses in LPGi neurons and the incremental effect of naloxone on LPGi activity was stronger in the light cycle. Also, the neurons with the excitatory response exhibited higher baseline activity in the dark cycle, but the neurons with the inhibitory response showed higher baseline activity in the light cycle. Interestingly, the baseline firing rate of neurons recorded in the light cycle was significantly different in response (excitatory/inhibitory) -dependent manner. We concluded that naloxone-induced changes in LPGi cellular activity and behaviors of morphine-dependent rats can be affected by circadian rhythm and the internal clock.

Chiari I malformation with neurogenic hypertension after suboccipital decompression

Patients with Chiari I malformations have a spectrum of symptoms related to craniocervical structural compression. Neurogenic hypertension has been reported in some patients with this disorder, with resolution of hypertension after suboccipital decompressions in some cases. The pathophysiology of neurogenic hypertension in these patients is thought to be related to ventrolateral medullary compression. The important role of the ventrolateral medulla in blood pressure control has been studied extensively. In this report, we present the case of a pediatric patient diagnosed with a Chiari I malformation who developed hypertension after a suboccipital decompression. We believe that this case demonstrates the important role of the ventrolateral medulla in neurogenic hypertension.

Role of neuronal nitric oxide synthase on cardiovascular functions in physiological and pathophysiological states

This review describes and summarizes the role of neuronal nitric oxide synthase (nNOS) on the central nervous system, particularly on brain regions such as the ventrolateral medulla (VLM) and the periaqueductal gray matter (PAG), and on blood vessels and the heart that are involved in the regulation and control of the cardiovascular system (CVS). Furthermore, we shall also review the functional aspects of nNOS during several physiological, pathophysiological, and clinical conditions such as exercise, pain, cerebral vascular accidents or stroke and hypertension. For example, during stroke, a cascade of molecular, neurochemical, and cellular changes occur that affect the nervous system as elicited by generation of free radicals and nitric oxide (NO) from vulnerable neurons, peroxide formation, superoxides, apoptosis, and the differential activation of three isoforms of nitric oxide synthases (NOSs), and can exert profound effects on the CVS. Neuronal NOS is one of the three isoforms of NOSs, the others being endothelial (eNOS) and inducible (iNOS) enzymes. Neuronal NOS is a critical homeostatic component of the CVS and plays an important role in regulation of different systems and disease process including nociception. The functional and physiological roles of NO and nNOS are described at the beginning of this review. We also elaborate the structure, gene, domain, and regulation of the nNOS protein. Both inhibitory and excitatory role of nNOS on the sympathetic autonomic nervous system (SANS) and parasympathetic autonomic nervous system (PANS) as mediated via different neurotransmitters/signal transduction processes will be explored, particularly its effects on the CVS. Because the VLM plays a crucial function in cardiovascular homeostatic mechanisms, the neuroanatomy and cardiovascular regulation of the VLM will be discussed in conjunction with the actions of nNOS. Thereafter, we shall discuss the up-to-date developments that are related to the interaction between nNOS and cardiovascular diseases such as hypertension and stroke. Finally, we shall focus on the role of nNOS, particularly within the PAG in cardiovascular regulation and neurotransmission during different types of pain stimulus. Overall, this review focuses on our current understanding of the nNOS protein, and provides further insights on how nNOS modulates, regulates, and controls cardiovascular function during both physiological activity such as exercise, and pathophysiological conditions such as stroke and hypertension.

Clinical Importance of the Posterior Inferior Cerebellar Artery: A Review of the Literature

The posterior inferior cerebellar artery (PICA), with its unique anatomical complexity, is of great clinical importance and involved in many diseases including aneurysm, ischemic stroke, neurovascular compression syndrome (NVCS), arteriovenous malformation (AVM), and brain tumor. However, a comprehensive systematic review of the importance of the PICA is currently lacking. In this study, we perform a literature review of PICA by searching all the associated papers in the PUBMED database hoping to provide a better understanding of the artery. The PICA has tortuous and variable course and territory, divided into 5 segments. Various aneurysms involving PICA were not uncommon, of which the treatment is challenging. The PICA infarct typically manifests lateral medullary syndrome (LMS) and is more likely to cause mass effects. The PICA frequently compresses the medulla and the cranial nerves resulting in various neurovascular compression syndromes (NVCS). Arteriovenous malformation (AVM) fed by PICA are associated with aneurysm and dissection which have high risk of rupture and worse outcome. PICA injured by head trauma can cause fatal SAH. VA terminating in PICA probably cause Bow hunter's syndrome (BHS). The PICA supplies many brain tumors and can be used in intracerebellar chemotherapy. The PICA can be exposed and injured during surgeries especially in telovelar approach, and it also plays an important role in bypass surgeries, hinting the surgical importance of PICA. In conclusion, PICA is very important in clinical practice.

Differential impacts of brain stem oxidative stress and nitrosative stress on sympathetic vasomotor tone

Based on work-done in the rostral ventrolateral medulla (RVLM), this review presents four lessons learnt from studying the differential impacts of oxidative stress and nitrosative stress on sympathetic vasomotor tone and their clinical and therapeutic implications. The first lesson is that an increase in sympathetic vasomotor tone because of augmented oxidative stress in the RVLM is responsible for the generation of neurogenic hypertension. On the other hand, a shift from oxidative stress to nitrosative stress in the RVLM underpins the succession of increase to decrease in sympathetic vasomotor tone during the progression towards brain stem death. The second lesson is that, by having different cellular sources, regulatory mechanisms on synthesis and degradation, kinetics of chemical reactions, and downstream signaling pathways, reactive oxygen species and reactive nitrogen species should not be regarded as a singular moiety. The third lesson is that well-defined differential roles of oxidative stress and nitrosative stress with distinct regulatory mechanisms in the RVLM during neurogenic hypertension and brain stem death clearly denote that they are not interchangeable phenomena with unified cellular actions. Special attention must be paid to their beneficial or detrimental roles under a specific disease or a particular time-window of that disease. The fourth lesson is that, to be successful, future antioxidant therapies against neurogenic hypertension must take into consideration the much more complicated picture than that presented in this review on the generation, maintenance, regulation or modulation of the sympathetic vasomotor tone. The identification that the progression towards brain stem death entails a shift from oxidative stress to nitrosative stress in the RVLM may open a new vista for therapeutic intervention to slow down this transition.

Protective effects of estrogen against cardiovascular disease mediated via oxidative stress in the brain

During their reproductive years women produce significant levels of estrogens, predominantly in the form of estradiol, that are thought to play an important role in cardioprotection. Mechanisms underlying this action include both estrogen-mediated changes in gene expression, and post-transcriptional activation of protein signaling cascades in the heart and in neural centers controlling cardiovascular function, in particular, in the brainstem. There, specific neurons, especially those of the bulbar region play an important role in the neuronal control of the cardiovascular system because they control the outflow of sympathetic activity and parasympathetic activity as well as the reception of chemical and mechanical signals. In the present review, we discuss how estrogens exert their cardioprotective effect in part by modulating the actions of internally generated products of cellular oxidation such as reactive oxygen species (ROS) in brain stem neurons. The significance of this review is in integrating the literature of oxidative damage in the brain with the literature of neuroprotection by estrogen in order to better understand both the benefits and limitations of using this hormone to prevent cardiovascular disease.

Paraventricular Nucleus Modulates Excitatory Cardiovascular Reflexes during Electroacupuncture

The paraventricular nucleus (PVN) regulates sympathetic outflow and blood pressure. Somatic afferent stimulation activates neurons in the hypothalamic PVN. Parvocellular PVN neurons project to sympathoexcitatory cardiovascular regions of the rostral ventrolateral medulla (rVLM). Electroacupuncture (EA) stimulates the median nerve (P5-P6) to modulate sympathoexcitatory responses. We hypothesized that the PVN and its projections to the rVLM participate in the EA-modulation of sympathoexcitatory cardiovascular responses. Cats were anesthetized and ventilated. Heart rate and mean blood pressure were monitored. Application of bradykinin every 10-min on the gallbladder induced consistent pressor reflex responses. Thirty-min of bilateral EA stimulation at acupoints P5-P6 reduced the pressor responses for at least 60-min. Inhibition of the PVN with naloxone reversed the EA-inhibition. Responses of cardiovascular barosensitive rVLM neurons evoked by splanchnic nerve stimulation were reduced by EA and then restored with opioid receptor blockade in the PVN. EA at P5-P6 decreased splanchnic evoked activity of cardiovascular barosensitive PVN neurons that also project directly to the rVLM. PVN neurons labeled with retrograde tracer from rVLM were co-labeled with μ-opioid receptors and juxtaposed to endorphinergic fibers. Thus, the PVN and its projection to rVLM are important in processing acupuncture modulation of elevated blood pressure responses through a PVN opioid mechanism.

Maximal intermittent handgrip strategy: design and evaluation of an exercise protocol and a grip tool

Handgrip (HG) exercise has been prescribed as a lifestyle intervention to successfully reduce resting blood pressure (BP) among heterogeneous groups of participants. Current HG protocols have limited accessibility due to complicated exercise prescriptions and sophisticated required equipment. Therefore, this research describes the design and evaluation of the maximal intermittent (MINT) HG exercise strategy, consisting of both a novel exercise protocol (32×5 seconds maximal grip squeezes separated by 5 seconds of rest between sets) and an original grip tool. This research was a multistep progressive design that included 51 postmenopausal women as participants in three separate research studies. Part 1 of this research focuses on the MINT exercise protocol. A literature-informed rationale for the design of the protocol is described. This includes exercise intensity, work-to-rest ratio, and total exercise duration with reference to the unique physiology (mechanoreflex and metaboreflex) of postmenopausal women. Subsequent experimental analyses of acute responses to the MINT protocol revealed that women produced 50% of their maximum grip force with moderate cardiovascular responses (increases of systolic BP: 41.6 mmHg, diastolic BP: 20.1 mmHg, heart rate: 35.1 bpm) that remained far below the thresholds of concern identified by the American College of Sports Medicine. Part 2 of this research describes the creation of a novel grip tool, beginning with a mixed-methods assessment of participant opinions regarding two distinct in-laboratory grip tools, leading to the creation of four prototype MINT tools. Structured focus groups revealed a strong preference for MINT prototype 1 for all tool design features, including color, shape, size, and foam grip. Collectively, the result of this multistep research is a novel HG exercise strategy with enhanced accessibility by being easy to understand and simple to execute. The long-term training effectiveness of MINT as an exercise intervention for the reduction of resting BP has yet to be determined.

Central GPR109A Activation Mediates Glutamate-Dependent Pressor Response in Conscious Rats

G protein-coupled receptor 109A (GPR109A) activation by its ligand nicotinic acid (NA) in immune cells increases Ca(2+) levels, and Ca(2+) induces glutamate release and oxidative stress in central blood pressure (BP)-regulating nuclei, for example, the rostral ventrolateral medulla (RVLM), leading to sympathoexcitation. Despite NA's ability to reach the brain, the expression and function of its receptor GPR109A in the RVLM remain unknown. We hypothesized that NA activation of RVLM GPR109A causes Ca(2+)-dependent l-glutamate release and subsequently increases neuronal oxidative stress, sympathetic activity, and BP. To test this hypothesis, we adopted a multilevel approach, which included pharmacologic in vivo studies along with ex vivo and in vitro molecular studies in rat pheochromocytoma cell line (PC12) cells (which exhibit neuronal phenotype). We present the first evidence for GPR109A expression in the RVLM and in PC12 cells. Next, we showed that RVLM GPR109A activation (NA) caused pressor and bradycardic responses in conscious rats. The resemblance of these responses to those caused by intra-RVLM glutamate and their attenuation by NMDA receptor (NMDAR) blockade (2-amino-5-phosphonopentanoic acid) and enhancement by l-glutamate uptake inhibition (l-trans-pyrrolidine-2,4-dicarboxylic acid, PDC) supported our hypothesis. NA increased Ca(2+), glutamate, nitric oxide and reactive oxygen species (ROS) levels in PC12 cells and increased RVLM ROS levels. The inactive NA analog isonicotinic acid failed to replicate the cardiovascular and biochemical effects of NA. Further, GPR109A knockdown (siRNA) abrogated the biochemical effects of NA in PC12 cells. These novel findings yield new insight into the role of RVLM GPR109A in central BP control.

Acute right ventricular pressure overload compromises left ventricular function by altering septal strain and rotation

While right ventricular (RV) dysfunction has long been known to affect the performance of left ventricle (LV), the mechanisms remain poorly defined. Recently, speckle-tracking echocardiography has demonstrated that preservation of strain and rotational dynamics is crucial to both LV systolic and diastolic function. We hypothesized that alteration in septal strain and rotational dynamics of the LV occurs during acute RV pressure overload (RVPO) and leads to decreased cardiac performance. Seven anesthetized pigs underwent median sternotomy and placement of intraventricular pressure-volume conductance catheters. Two-dimensional echocardiographic images and LV pressure-volume loops were acquired for offline analysis at baseline and after banding of the pulmonary artery to achieve RVPO (>50 mmHg) induced RV dysfunction. RVPO resulted in a significant decrease (P < 0.05) in LV end-systolic elastance (50%), systolic change in pressure over change in time (19%), end-diastolic volume (22%), and cardiac output (37%) that correlated with decrease in LV global circumferential strain (58%), LV apical rotation (28%), peak untwisting (reverse rotation) rate (27%), and prolonged time to peak rotation (17%), while basal rotation was not significantly altered. RVPO reduced septal radial and circumferential strain, while no other segment of the LV midpapillary wall was affected. RVPO decreased septal radial strain on LV side by 27% and induced a negative radial strain from 28 ± 5 to -16 ± 2% on the RV side of the septum. The septal circumferential strain on both LV and RV side decreased by 46 and 50%, respectively, following RVPO (P < 0.05). Our results suggest that acute RVPO impairs LV performance by primarily altering septal strain and apical rotation.

Systemic administration of leptin potentiates the response of neurons in the nucleus of the solitary tract to chemoreceptor activation in the rat

Leptin microinjections into the nucleus of the solitary tract (NTS) have been shown to elicit sympathoexcitatory responses, and potentiate the cardiovascular responses to activation of the chemoreflex. In this study, experiments were done in Sprague-Dawley rats initially to provide a detailed mapping within the NTS complex of cells containing immunoreactivity to the long form of the leptin receptor (Ob-Rb). In a second series, this NTS region containing Ob-Rb immunoreactive cells was explored for single units antidromically activated by stimulation of pressor sites in the rostral ventrolateral medulla (RVLM). These antidromically identified neurons were then tested for their response to intra-carotid injections of leptin (50-100 ng/0.1 ml), and to activation of peripheral chemoreceptors following an injection of potassium cyanide (KCN) (80 μg/0.1 ml) into the carotid artery. Cells containing Ob-Rb-like immunoreactivity were found predominantly in the caudal NTS: within the medial, commissural and gelatinous (sub-postremal area) subnuclei of the NTS complex. Of 73 single units tested in these NTS regions, 48 were antidromically activated by stimulation of RVLM pressor sites and 25 of these single units responded with an increase in discharge rate after intra-carotid injections of leptin. In addition, 17 of these leptin responsive neurons were excited by the intra-carotid injections of KCN (80 μg/0.1 ml). Furthermore, the excitatory response of these single units to KCN was potentiated (59-83%) immediately following the leptin injection. These data indicate that leptin responsive neurons in NTS mediate chemoreceptor afferent information to pressor sites in the RVLM, and suggest that leptin may act as a facilitator on neuronal circuits within the NTS that potentiates the sympathoexcitatory responses elicited during the reflex activation of arterial chemoreceptors.

Leptin signaling in the nucleus of the solitary tract alters the cardiovascular responses to activation of the chemoreceptor reflex

Circulating levels of leptin are elevated in individuals suffering from chronic intermittent hypoxia (CIH). Systemic and central administration of leptin elicits increases in sympathetic nervous activity (SNA), arterial pressure (AP), and heart rate (HR), and it attenuates the baroreceptor reflex, cardiovascular responses that are similar to those observed during CIH as a result of activation of chemoreceptors by the systemic hypoxia. Therefore, experiments were done in anesthetized Wistar rats to investigate the effects of leptin in nucleus of the solitary tract (NTS) on AP and HR responses, and renal SNA (RSNA) responses during activation of NTS neurons and the chemoreceptor reflex. Microinjection of leptin (5-100 ng; 20 nl) into caudal NTS pressor sites (l-glutamate; l-Glu; 0.25 M; 10 nl) elicited dose-related increases in AP, HR, and RSNA. Leptin microinjections (5 ng; 20 nl) into these sites potentiated the increase in AP and HR elicited by l-Glu. Additionally, bilateral injections of leptin (5 ng; 100 nl) into NTS potentiated the increase in AP and attenuated the bradycardia to systemic activation of the chemoreflex. In the Zucker obese rat, leptin injections into NTS neither elicited cardiovascular responses nor altered the cardiovascular responses to activation of the chemoreflex. Taken together, these data indicate that leptin exerts a modulatory effect on neuronal circuits within NTS that control cardiovascular responses elicited during the reflex activation of arterial chemoreceptors and suggest that increased AP and SNA observed in individuals with CIH may be due, in part, by leptin's effects on the chemoreflex at the level of NTS.

Potential autonomic risk factors for chronic TMD: descriptive data and empirically identified domains from the OPPERA case-control study

Several case-control studies have been conducted that examine the association between autonomic variables and persistent pain conditions; however, there is a surprising dearth of published studies in this area that have focused on temporomandibular disorders (TMD). The current study presents autonomic findings from the baseline case-control study of the OPPERA (Orofacial Pain: Prospective Evaluation and Risk Assessment) cooperative agreement. Measures of arterial blood pressure, heart rate, heart rate variability, and indirect measures of baroreflex sensitivity were assessed at rest and in response to a physical (orthostatic) and psychological (Stroop) stressors in 1,633 TMD-free controls and 185 TMD cases. In bivariate and demographically adjusted analyses, greater odds of TMD case status were associated with elevated heart rates, reduced heart rate variability, and reduced surrogate measures of baroreflex sensitivity across all experimental procedures. Principal component analysis was undertaken to identify latent constructs revealing 5 components. These findings provide evidence of associations between autonomic factors and TMD. Future prospective analyses in the OPPERA cohort will determine if the presence of these autonomic factors predicts increased risk for developing new onset TMD.

PERSPECTIVE

This article reports autonomic findings from the OPPERA Study, a large prospective cohort study designed to discover causal determinants of TMD pain. Findings indicate statistically significant differences between TMD cases and controls across multiple autonomic constructs at rest and during both physical and psychologically challenging conditions. Future analyses will determine whether these autonomic factors increase risk for new onset TMD.

Brain stem oxidative stress and its associated signaling in the regulation of sympathetic vasomotor tone

There is now compelling evidence from studies in humans and animals that overexcitation of the sympathetic nervous system plays an important role in the pathogenesis of cardiovascular diseases. An excellent example is neurogenic hypertension, in which central sympathetic overactivation is involved in the development, staging, and progression of the disease, and one of the underlying mechanisms involves oxidative stress in key brain stem sites that are engaged in the regulation of sympathetic vasomotor tone. Using the rostral ventrolateral medulla (RVLM) and nucleus tractus solitarii (NTS) as two illustrative brain stem neural substrates, this article provides an overview of the impact of reactive oxygen species and antioxidants on RVLM and NTS in the pathogenesis of neurogenic hypertension. This is followed by a discussion of the redox-sensitive signaling pathways, including several kinases, ion channels, and transcription factors that underpin the augmentation in sympathetic vasomotor tone. In addition, the emerging view that brain stem oxidative stress is also causally related to a reduction in sympathetic vasomotor tone and hypotension during brain stem death, methamphetamine intoxication, and temporal lobe status epilepticus will be presented, along with the causal contribution of the oxidant peroxynitrite formed by a reaction between nitric oxide synthase II (NOS II)-derived nitric oxide and superoxide. Also discussed as a reasonable future research direction is dissection of the cellular mechanisms and signaling cascades that may underlie the contributory role of nitric oxide generated by different NOS isoforms in the differential effects of oxidative stress in the RVLM or NTS on sympathetic vasomotor tone.

Discharge activities of neurons in the nucleus paragigantocellularis during the development of morphine tolerance and dependence: a single unit study in chronically implanted rats

The nucleus paragigantocellularis (PGi) has been proposed to play a role in opiate dependence/withdrawal. In the present study, we examined the discharge activity of PGi neurons before and after the development of morphine tolerance/dependence in rats. A multi-wire electrode was chronically implanted in the PGi, which allowed us to monitor the effects of both acute and chronic morphine treatments on the activity of PGi neurons recorded from the same site. We found that acute morphine excited, inhibited or had no effect on 36%, 35% or 29% of PGi neurons (N=556), respectively. After 3 days of continuous morphine infusion, which led to morphine tolerance/dependence, the firing rates of both excitatory and inhibitory PGi neurons returned to pre-morphine treatment levels, indicating that the PGi neurons developed tolerance to both excitatory and inhibitory effects of morphine. Naltrexone-precipitated withdrawal from chronic morphine treatment also induced heterogeneous responses in the PGi. On a site-by-site basis, most of the sites that showed excitatory response to acute morphine exhibited inhibitory response during withdrawal, and all the sites that showed inhibitory response to acute morphine exhibited excitatory response during withdrawal. Correlation analysis further quantitatively showed that PGi neurons' responses to acute morphine and that during withdrawal were inversely correlated with a correlation coefficient of 0.73, suggesting that adaptations in the PGi during the development of morphine dependence share common neural mechanisms with the acute effect of morphine. These results provide new insights into the role of the PGi in the development of morphine tolerance/dependence.

Effects of exercise training on dendritic morphology in the cardiorespiratory and locomotor centers of the mature rat brain

It has been shown that dendritic branching in neural cardiorespiratory and locomotor centers can be attenuated with exercise training (ET) initiated immediately after weaning. The purpose of this study was to determine whether neuroplastic changes occur within cardiorespiratory and locomotor centers due to ET after maturation. Male Sprague-Dawley rats (21 days old, n=28) were individually housed in standard cages. At 91 days of age, animals were divided into two groups: untrained (UN; n=14) and trained (TR; n=14). The TR group exercised spontaneously for 50 days on running wheels. ET indexes were obtained, including maximal O2 consumption, percent body fat, resting heart rate, and heart weight-to-body weight ratios. The brain was processed with a modified Golgi-Cox procedure. Impregnated neurons from the periaqueductal gray (PAG), posterior hypothalamic area (PH), nucleus of the tractus solitarius (NTS), cuneiform nucleus (CnF), rostral ventrolateral medulla, nucleus cuneatus, and cerebral cortex were examined. Neurons were traced and analyzed using the Sholl concentric ring analysis of dendritic branching. The mean total number of dendritic intersections with the concentric rings per neuron per animal were compared between UN and TR groups. There were significant differences between UN and TR groups in the PH, PAG, CnF, and NTS in the total number of intersections per animal. In some areas, the effect size was smaller when ET was initiated in mature animals, possibly related to their relatively reduced activity levels. In conclusion, the adult rat brain remains dynamic and adapts to chronic ET. However, some brain areas appear to be more affected if ET is initiated in early postnatal development.

Vasopressin and sympathetic systems mediate the cardiovascular effects of the GABAergic system in the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BST) is an important part of the limbic system. It has been shown that chemical stimulation of the BST elicited cardiovascular depressive and bradycardic responses. It was also demonstrated that GABA is present in the BST, though its role in cardiovascular control is not yet understood. This study was performed to find the effects of GABA receptor subtypes in the BST on cardiovascular responses and to find the possible mechanisms that mediate these responses in urethane-anesthetized rats. Microinjection of muscimol (500 pmol/100 nl), a GABA(A) agonist, into the BST produced a weak unsignificant decrease in the mean arterial pressure (MAP) and heart rate (HR). Injection of bicuculline methiodide (BMI, 100 pmol/100 nl), a GABA(A) antagonist, caused a significant increase in the MAP (41.3+/-5.1 mmHg) as well as in the HR (33.2+/-5.6 beats/min). Injection of two doses (500 and 1000 pmol/100 nl) of phaclofen, a GABA(B) antagonist, produced no significant change in either MAP or HR. Administration (i.v.) of the muscarinic receptor blocker, homatropine methyl bromide had no effect on the magnitude of mean arterial pressure or heart rate responses to BMI. This suggests that the parasympathetic system is not involved in these responses. However, administration (i.v.) of the nicotinic receptor blocker, hexamethonium bromide had no effect on the magnitude of mean arterial pressure response but abolished heart rate response to BMI. This suggests that the sympathetic system is involved in the bradycardic effect of GABA. On the other hand, administration (i.v.) of a selective vasopressin V(1) receptor antagonist abolished the pressor effect of BMI, which indicates that the GABAergic system of the BST decreases the arterial pressure via tonic inhibition of vasopressin release. In summary, we demonstrated, for the first time, that GABA exerts its influence in the BST through the activation of GABA(A), but not GABA(B), receptors that, in turn, tonically inhibit vasopressin release and sympathetic outflow to the heart.

Right atrial stretch alters fore- and hind-brain expression of c-fos and inhibits the rapid onset of salt appetite

The inflation of an intravascular balloon positioned at the superior vena cava and right atrial junction (SVC-RAJ) reduces sodium or water intake induced by various experimental procedures (e.g. sodium depletion; hypovolaemia). In the present study we investigated if the stretch induced by a balloon at this site inhibits a rapid onset salt appetite, and if this procedure modifies the pattern of immunohistochemical labelling for Fos protein (Fos-ir) in the brain. Male Sprague-Dawley rats with SVC-RAJ balloons received a combined treatment of furosemide (Furo; 10 mg (kg bw)(-1)) plus a low dose of the angiotensin-converting enzyme inhibitor captopril (Cap; 5 mg (kg bw)(-1)). Balloon inflation greatly decreased the intake of 0.3 m NaCl for as long as the balloon was inflated. Balloon inflation over a 3 h period following Furo-Cap treatment decreased Fos-ir in the organum vasculosum of the lamina terminalis and the subfornical organ and increased Fos-ir in the lateral parabrachial nucleus and caudal ventrolateral medulla. The effect of balloon inflation was specific for sodium intake because it did not affect the drinking of diluted sweetened condensed milk. Balloon inflation and deflation also did not acutely change mean arterial pressure. These results suggest that activity in forebrain circumventricular organs and in hindbrain putative body fluid/cardiovascular regulatory regions is affected by loading low pressure mechanoreceptors at the SVC-RAJ, a manipulation that also attenuates salt appetite.

Sildenafil acts on the central nervous system increasing sympathetic activity

Sildenafil induces vasodilation and is used for treating erectile dysfunction. Although its influence on resting heart function appears to be minimal, recent studies suggest that sildenafil can increase sympathetic activity. We therefore tested whether sildenafil injected into the central nervous system alters the autonomic control of the cardiovascular system in conscious rats. The effect of sildenafil citrate injected into the lateral cerebral ventricle was evaluated in conscious rats by means of the recording of lumbar sympathetic nerve activity (LSNA), spectral analysis of systolic arterial pressure and heart rate variability, spontaneous baroreflex sensitivity, and baroreflex control of LSNA. Intracerebroventricular (ICV, 100 microg /5 microl) administration of sildenafil caused remarkable tachycardia without significant change in basal arterial pressure and was associated with a conspicuous increase (47 +/- 14%) in LSNA. Spectral analysis demonstrated that systolic arterial pressure oscillations in the low frequency (LF) range were increased (from 6.3 +/- 1.5 to 12.8 +/- 3.8 mmHg(2)), whereas the high frequency (HF) range was not affected by ICV administration of sildenafil. Sildenafil increased pulse interval oscillations at LF and decreased them at HF. The LF-HF ratio increased from 0.04 +/- 0.01 to 0.17 +/- 0.06. Spontaneous baroreflex sensitivity measured by the sequence method and the baroreflex relationship between mean arterial pressure and LSNA were not affected by ICV administration of sildenafil. In conclusion, sildenafil elicited an increase in sympathetic nerve activity that is not baroreflex mediated, suggesting that this drug is able to elicit an autonomic imbalance of central origin. This finding may have implications for understanding the cardiovascular outcomes associated with the clinical use of this drug.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.

Glutamatergic systems in the bed nucleus of the stria terminalis, effects on cardiovascular system

The bed nucleus of the stria terminalis (BST) is a part of the limbic system. Two studies have shown that microinjection of L: -glutamate in the BST elicited cardiovascular depressive and bradycardic responses, but in one study, both pressor and depressor responses were observed in the chemical stimulation of BST by glutamate in the urethane-anesthetized rats. Also, the roles of glutamate receptor subtypes have not been investigated yet. The aim of this study was to find the effects of glutamate and its receptors on the blood pressure and heart rate in the BST of urethane-anesthetized rats. The drugs (50 nl) were microinjected into the BST of anaesthetized rats. The blood pressure and heart rate were recorded throughout each experiment. The average changes in the mean arterial pressure and heart rate at different intervals were compared both within each case group and between the case and the control groups, using repeated measures ANOVA. Microinjection of L: -glutamate (0.25 M) into the BST resulted in the decrease of the mean arterial pressure (-18.85 +/- 3.84 mmHg) and heart rate (-18 +/- 4 beats/min). Injection of AP5, antagonist of glutamate NMDA receptor (2.5 , 5 mM) and CNQX, antagonist of glutamate AMPA receptor (0.5, 1 mM) had no significant effect on the mean arterial pressure and heart rate. Either Ap5 or CNQX, when co-injected with glutamate, abolished the depressor and bradycardic effects of glutamate, suggesting that simultaneous activation of both glutamate receptors is necessary for the effect of glutamate system to emerge.

Blood pressure variability in acute ischemic stroke depends on hemispheric stroke location

The relationship between blood pressure (BP) variability and stroke location was examined in 85 patients admitted with acute ischemic stroke. The patients were divided into three groups according to stroke location: right hemisphere (32 patients), left hemisphere (30 patients) and non-localized (23 patients). BP upon admission was 147.94/76.53 +/- 20.72/13.70 mmHg in the right hemisphere group, 151.81/76.10 +/- 25.69/16.23 mmHg in the left hemisphere and 155.23/83.41 +/- 30.45/15.74 in the non-localized group. The left hemisphere group had significantly (p < 0.01) greater variations in systolic and diastolic BP between days 2 and 3 and in systolic BP between days 3 and 4 after stroke compared with the other groups. BP in the left hemisphere group was less stable than in the other two groups. Non-localized patients without pre-existing hypertension had a significantly lower and more stable BP during the week following stroke than non-localized patients with pre-existing hypertension. Non-localized patients with pre-existing hypertension had the highest BP and showed no improvement during the week. Systolic BP tended to be higher and less stable in left hemisphere patients than in right hemisphere, whereas among non-localized ischemic stroke patients BP was higher in those who had a prior diagnosis of hypertension.

Medullary monoamines and NMDA-receptor regulation of cardiovascular responses during peripheral nociceptive stimuli

We have previously reported that AMPA-receptor blockade within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during mechanical, but not during thermal stimulation [Gray, T., Lewis III, E., Maher, T.J., Ally, A., 2001. AMPA-receptor blockade within the RVLM modulates cardiovascular responses via glutamate during peripheral stimuli. Pharmacol. Res. 43, 47-54]. In this study, we examined the role of NMDA-receptor blockade within the RVLM on cardiovascular responses and release of biogenic monoamines (serotonin [5HT], dopamine [DA], and norepinephrine [NE]) during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Both mechanical and thermal stimulation have been shown to activate peripheral Adelta and C-fiber polymodal nociceptors. Noxious mechanical stimuli were induced by applying a pinch to alternate hindpaw for 5s while the noxious thermal stimuli involved immersion of the metatarsus of alternate hindpaw in a water bath at a temperature of 52 degrees C for 5 s. Mechanical stimulation increased mean arterial pressure (MAP), heart rate (HR), extracellular fluid 5HT, and DA concentrations (n=10). However, extracellular levels of NE were decreased within the RVLM. Furthermore, NMDA-receptor blockade with a competitive antagonist, AP-7 (200 nM), within the RVLM attenuated the cardiovascular responses and changes in 5HT and DA, but had no effect on NE levels. The thermal stimulation elicited similar increases in MAP and HR, however, extracellular levels of 5HT or DA did not change. Concentrations of NE were decreased during a thermal stimulation similar to the levels observed following mechanical stimuli. In contrast to mechanical stimuli, bilateral administration of AP-7 (200-1 mM) into the RVLM had no effect on cardiovascular responses, 5HT, DA or NE concentrations during a thermal stimulation. These results show that NMDA receptors within the RVLM most likely play a role in modulating cardiovascular responses by altering 5HT and DA concentrations within the RVLM during mechanical but not thermal nociception. Overall, the present study delineates the NMDA-receptor mediated central integrative mechanisms within the RVLM that coordinate processing of sensory impulses arising from peripheral noxious stimulation.

Medullary noradrenergic neurons release norepinephrine in the medial amygdala in females in response to mating stimulation sufficient for pseudopregnancy

In the female rat, stimuli from the uterine cervix and vagina are carried to the brain areas involved in the mating-induced pseudopregnancy (PSP) response via the ventral noradrenergic bundle. Noradrenergic neurons projecting through this tract synapse in many forebrain areas including the amygdala, and neurons in the posterodorsal medial amygdala (MePD) are activated following mating. The goal of this experiment was to investigate whether norepinephrine (NE) is released into the MePD after mating using microdialysis and to determine the origin of this release. Ovariectomized estrogen- and progesterone-treated rats were implanted unilaterally with guide cannulae aimed at the MePD. Females were placed with males until they received 15 intromissions (15I), 5 intromissions (5I) or 15 mounts-without-intromission (MO). Dialysate samples collected every 20 min for 2 h before to 3 h after mating were analyzed for NE using HPLC with electrochemical detection. A significant increase in mean NE release in the MePD was seen at 80 min after mating onset in females receiving 15I, and no increase was seen in animals receiving 5I or MO. The time of peak NE release varied in 15I animals from 60 to 160 min after mating. Mean baseline levels of NE did not differ between groups. The retrograde tracer FluoroGold (FG), administered through the probe after cessation of dialysis sampling, was observed within identified noradrenergic cells primarily within the A1 and A2 cell groups. Infusion of anti-dopamine-beta-hydroxylase-saporin (DBH-SAP) into the MePD lesioned noradrenergic neurons located in the A1 and A2 cell groups. Because high levels of NE release occurred in the MePD only after the females received a number of intromissions sufficient to induce PSP, these results suggest that NE release within the MePD may be important for the establishment of PSP.

Cardiovascular effects and c-Fos expression in the rat hindbrain in response to innocuous stomach distension

The present work was planned to study the effects of non-noxious gastric distension on hemodynamic variables and on cardiovascular hindbrain areas detected by means of c-Fos immunoreactivity, to determine the afferent and central mechanisms involved. In anesthetized rats, innocuous stomach distension increased arterial blood pressure and heart rate and induced c-Fos immunoreactivity within nucleus tractus solitarii, nucleus ambiguus, ventrolateral medulla and lateral reticular nucleus. Bilateral vagotomy abolished the pressor response and c-Fos immunoreactivity in nucleus ambiguus and ventrolateral medulla. Also, c-Fos immunoreactivity was significantly decreased in nucleus tractus solitarii and lateral reticular nucleus. After bilateral splanchnicotomy the pressor and tachycardic responses caused by gastric distension were reduced. c-Fos immunoreactivity in nucleus tractus solitarii, lateral reticular nucleus and nucleus ambiguus was reduced in comparison to the intact rats. In ventrolateral medulla a preferential localization of c-Fos immunoreactivity was found within its caudal portion. It was shown that such gastric distension, known to activate low threshold mechanoreceptors, induced cardiovascular effects via both vagal and splanchnic afferents and involving their central convergence and interaction in modulating the baroreceptor buffer system.

Differential responsiveness of c-Fos expression in the rat medulla oblongata to different treadmill running speeds

Expression of the inducible transcription factor c-Fos was mapped in the rat medulla oblongata to identify the brain areas respond to different running speeds. Rats were subjected to 30 min of running, either at high speed, low speed or just sitting on a treadmill (control). Blood lactate levels were measured to confirm the physiological impact of different exercise intensities. The number of c-Fos-ir cells was counted and their spatial distributions were mapped through the rostral to the caudal level in the medulla. A statistically significant exercise intensity-dependent induction of c-Fos was observed in the nucleus of the solitary tract (NTS) and caudal ventrolateral medulla (CVL) in the medulla. Further, c-Fos induction was more predominant in the caudal part of each nucleus. The present data clearly show that different running speeds cause differential activation of each nucleus in the medulla, and in particular, the caudal parts of the NTS and the CVL are the most responsive to speed changes. The present study identifies brain areas newly found to be responsive to changes in running speed. These findings are likely to be particularly helpful in studies of specific neural circuits and their functions in response to different running speeds.

New insights on brain stem death: From bedside to bench

As much as brain stem death is currently the clinical definition of death in many countries and is a phenomenon of paramount medical importance, there is a dearth of information on its mechanistic underpinnings. A majority of the clinical studies are concerned only with methods to determine brain stem death. Whereas a vast amount of information is available on the cellular and molecular mechanisms of cell death, rarely are these studies directed specifically towards the understanding of brain stem death. This review presents a framework for translational research on brain stem death that is based on systematically coordinated clinical and laboratory efforts that center on this phenomenon. It begins with the identification of a novel clinical marker from patients that is related specifically to brain stem death. After realizing that this "life-and-death" signal is related to the functional integrity of the brain stem, its origin is traced to the rostral ventrolateral medulla (RVLM). Subsequent laboratory studies on this neural substrate in animal models of brain stem death provide credence to the notion that both "pro-life" and "pro-death" programs are at work during the progression towards death. Those programs (mitochondrial functions, nitric oxide, peroxynitrite, superoxide anion, coenzyme Q10, heat shock proteins and ubiquitin-proteasome system) hitherto identified from the RVLM are presented, along with their cellular and molecular mechanisms. It is proposed that outcome of the interplay between the "pro-life" and "pro-death" programs (dying) in this neural substrate determines the final fate of the individual (being dead). Thus, identification of additional programs in the RVLM and delineation of their regulatory mechanisms should shed new lights on future directions for clinical management of life-and-death.

Neuroplastic adaptations to exercise: neuronal remodeling in cardiorespiratory and locomotor areas

Neuronal activity has been shown to be attenuated in cardiorespiratory and locomotor centers of the brain in response to a single bout of exercise in trained (TR) vs. untrained (UN) animals, but the mechanisms remain obscure. Based on this finding, dendritic branching patterns of seven brain areas associated with cardiorespiratory and locomotor activity were examined in TR and UN animals. Twenty-eight male Sprague-Dawley rats were kept in individual cages and divided into TR and UN. TR were provided with a running wheel and exercised spontaneously. After 85 or 120 days, exercise training indexes were obtained, including maximal oxygen consumption, percent body fat, resting heart rate, and heart weight-to-body weight ratios. The brain was removed and processed according to a modified Golgi-Cox procedure. Impregnated neurons from seven brain areas were examined in coronal sections: the periaqueductal gray, posterior hypothalamic area, nucleus of the tractus solitarius, rostral ventrolateral medulla, cuneiform nucleus, nucleus cuneatus, and cerebral cortex. Neurons were traced using a camera lucida technique and analyzed using the Sholl analysis of dendritic branching. t-tests were conducted to compare the mean number of intersections per neuron by grouping inner rings and outer rings and also comparing the total number of intersections per animal. There were significant differences between groups in the posterior hypothalamic area, periaqueductal gray, cuneiform nucleus, and nucleus of the tractus solitarius in the inner rings, outer rings, and the total number of intersections per animal. Our results show that dendritic fields of neurons in important cardiorespiratory and locomotor centers of the brain are attenuated in TR animals.

nNOS-containing neurons in the hypothalamus and medulla project to the RVLM

Nitric oxide (NO) within the brain is known to have an important influence on sympathetic nerve activity (SNA). NO is found in the paraventricular nucleus (PVN), caudal ventrolateral medulla (CVLM) and the nucleus tractus solitarius (NTS), regions that project to the rostral ventrolateral medulla (RVLM), an area that is critical in the regulation of SNA. The aim of the present study was to determine whether neurons in the PVN, NTS and CVLM that project to the RVLM contain the neuronal isoform of nitric oxide synthase (nNOS) and are, therefore, capable of producing NO. Under pentobarbitone general anaesthesia, the retrogradely-transported tracer, rhodamine-tagged microspheres, were microinjected into the RVLM of rats (n = 6). Two weeks later, the animals were re-anaesthetised, perfused with para-formaldehyde and the brains were removed. Hypothalamic and medullary sections were processed for nNOS immunohistochemistry and the RVLM-projecting neurons were identified using fluorescence microscopy. We found nNOS-containing neurons were present throughout the PVN, CVLM and NTS and that these were intermingled with neurons that projected to the RVLM. Of the neurons in the PVN and CVLM that projected to the RVLM, approximately 12 +/- 1% and 8 +/- 3%, respectively, contained nNOS. In the NTS only 1 +/- 1% of the neurons were double-labeled. This study highlights anatomical pathways emanating from the PVN and CVLM, in particular, which may contribute to the effects on SNA elicited by NO within the brain.

Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli

We have previously reported that nitric oxide (NO) within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during thermal, but not during mechanical nociceptive stimulation (Ishide. T., Maher, T.J., Ally, A. 2003. Role of nitric oxide in the ventrolateral medulla on cardiovascular responses and glutamate neurotransmission during mechanical and thermal stimuli. Pharmacol. Res. 47, 59-68). In this study, we examined the role of nitric oxide within the dorsolateral periaqueductal gray matter (PAG), a higher center integrating nociceptive reflexes, on cardiovascular responses and glutamate release during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Two types of stimuli were studied, both activating peripheral A(delta) and C fiber polymodal nociceptors. Noxious mechanical stimulus was given by applying a bilateral hindpaw pinch for 5 s. Mechanical stimulation of a hindlimb increased mean arterial pressure (MAP), heart rate (HR), and extracellular fluid glutamate within PAG by 20+/-3 mmHg, 37+/-6 bpm, and 1.7+/-0.3 ng/5 microl, respectively (n=10). Bilateral microdialysis of L-arginine (1.0 microM), a NO precursor, into the PAG significantly attenuated MAP, HR, and glutamate increases during a mechanical stimulation. Subsequent administration of N(G)-methyl-L-arginine (L-NMMA) (1.0 microM), a NO synthase inhibitor, into the PAG blocked the ability of NO within PAG to modulate the cardiovascular responses to mechanical stimulus. The noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in water-bath at a temperature of 52 degrees C for 5 s. Similar increases were observed following thermal stimulation: 35+/-5 mmHg, 40+/-6 bpm, and 1.14+/-0.4 ng/5 microl (n=10). L-Arginine attenuated both cardiovascular responses and glutamate increase during thermal nociception. These results demonstrate that NO within the dorsolateral PAG plays a role in modulating cardiovascular responses by altering glutamate concentrations during both thermal and mechanical nociception.

Direct projections from caudal ventrolateral medullary depressor sites to the subfornical organ

Experiments were performed in the male Wistar rat to investigate the projections from cardiovascular responsive sites in the ventrolateral medulla (VLM) to the subfornical organ (SFO). Unilateral iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHA-L) were made into either caudal VLM (CVLM) sites at which microinjection of l-glutamate (10 nl; 0.25 M) elicited decreases in mean arterial pressure or into rostral VLM (RVLM) sites at which l-glutamate microinjection elicited increases in arterial pressure. After a survival period of 7-10 days, transverse sections of the forebrain and brainstem were processed for PHA-L immunoreactivity. After injections of PHA-L into the CVLM, axonal and presumptive terminal labeling was found bilaterally throughout the rostrocaudal extent of the SFO, although most of the projections were observed within the rostral half of the nucleus. Within the SFO, labeling was found primarily in the lateral aspects of the nucleus, often in close proximity to blood vessels. In addition, CVLM injections resulted in labeling within the organum vasculosum of the laminae terminalis (OVLT) and within the ventral and dorsal components of the median preoptic nucleus (MnPO) bilaterally, but with an ipsilateral predominance. In contrast, PHA-L injections into the RVLM did not result in axonal labeling in the SFO or OVLT, although a few labeled axons were found to course through the region of the ventral component of MnPO. These data have demonstrated that neurons within the cardiovascular responsive region of the CVLM send direct axonal projections to the SFO and other structures of the laminae terminalis, and suggest that the CVLM may function in the modulation of the activity of neurons of circumventricular organs to intra- and extracellular signals of body fluid balance.

Collateral axonal projections from hypothalamic hypocretin neurons to cardiovascular sites in nucleus ambiguus and nucleus tractus solitarius

Hypocretin-1 (hcrt-1)-containing axons have been shown to have an extensive distribution within the central nervous system, although the total number of hypothalamic hcrt-1 neurons has been shown to be small. This suggests that hcrt-1 neurons may innervate central structures with similar function through collateral axonal projections. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether hypothalamic hcrt-1-containing neurons send collateral axonal projections to cardiovascular sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb) in the rat. Fluorogold- (FG) and/or rhodamine (Rd)-labeled latex microspheres were microinjected into either the NTS or Amb at sites that elicited bardycardia responses (L-glutamate; 0.25 M; 10 nl). After a survival period of 10-15 days, the rats were sacrificed and tissue sections of the hypothalamus were processed immunohistochemically for the identification of hcrt-1-containing cell bodies. After injection of the tract-tracers into the NTS or Amb, retrogradely labeled neurons were observed within several hypothalamic regions; the paraventricular hypothalamic nucleus, lateral hypothalamic area, perifornical hypothalamic area, and posterior hypothalamus, bilaterally, but with an ipsilateral predominance. In addition, after NTS injections, retrogradely labeled neurons were found within the ipsilateral caudal arcuate nucleus. Of the total number (1107+/-97) of hcrt-1-immunoreactive neurons found bilaterally within the lateral and perifornical hypothalamic nuclei, 7.9+/-1.4% were found to be retrogradely labeled from the NTS, 16.4+/-1.8% from the Amb, and 3.1+/-0.5% from both medullary sites. Hcrt-1 neurons projecting to the NTS were found mainly in and around the perifornical hypothalamic region, with a smaller number in the caudal lateral hypothalamic area. On the other hand, those innervating the Amb were primarily observed within the caudal lateral hypothalamic area, with a smaller number in the perifornical hypothalamic area. Neurons with collateral axonal projections to NTS and Amb were observed within two specific hypothalamic areas: one group of neurons was found in the perifornical hypothalamic area, and the other was observed in the lateral hypothalamic region just dorsal to the retrochiasmatic component of the supraoptic nucleus. These data indicate that axons from hcrt-1 neurons bifurcate to innervate functionally similar cardiovascular-responsive sites in the NTS and Amb. Although the function of these hcrt-1-containing hypothalamic-medullary pathways is not known, they likely represent the anatomical substrate by which the lateral hypothalamic hcrt-1 neurons simultaneously coordinate autonomic-cardiovascular responses to different behaviors.

Medullary and spinal cord projections from cardiovascular responsive sites in the rostral ventromedial medulla

The rostral ventromedial medulla (RVMM) is a sympathoexcitatory area. However, little is known about its efferent projections. In this study, biotinylated dextran amine (BDA) or Phaseolus vulgaris leucoagglutinin (PHA-L) were used to investigate the medullary and spinal cord projections from pressor sites in RVMM. Initially, RVMM was systematically explored in urethane-anesthetized rats using microinjection of L-glutamate for sites that elicited increases in arterial pressure. A pressor area was identified that included the rostral magnocellular reticular and rostral lateral paragigantocellular reticular nuclei. In the second series of experiments, BDA or PHA-L was iontophoretically injected into RVMM pressor sites. Anterograde labeling was observed throughout the brainstem and spinal cord, bilaterally, but with an ipsilateral predominance. Dense labeling was observed within the nucleus of the solitary tract (NTS); the greatest density of labeling was observed in the caudal dorsolateral, medial, and ventrolateral subnuclei. Additionally, light to moderately dense labeling was found within the nucleus substantia gelatinosus and commissural nucleus. In the nucleus ambiguus/ventrolateral medullary (Amb/VLM) region, the density of labeling was greatest in caudal regions. Within Amb, most of the labeling was localized to its external formation. Anterograde labeling was also found throughout the spinal cord. In the thoracolumbar segments, dense axonal labeling was observed within the dorsolateral funiculus. These labeled axons innervated the intermediolateral nucleus and the central autonomic area. Taken together, these data suggest that RVMM neurons elicit increases in sympathetic activity by likely providing a direct excitatory input to spinal sympathetic preganglionic neurons, and by a direct inhibitory input to medullary cardioinhibitory and depressor areas.

Effects of Angiotensin II Type 1 Receptor Antagonist on Pressor Responses to Pulsatile Compression of the Rostral Ventrolateral Medulla in Rats

The rostral ventrolateral medulla (RVLM) is known to be a major center regulating sympathetic and cardiovascular activities. A possible association between neurovascular compression of the RVLM and essential hypertension has been indicated. The present study was performed to determine the role of angiotensin II (AngII) in the pressor and sympathetic responses to pulsatile compression of the RVLM. To determine the role of glutamate and AngII in the RVLM, L-glutamate (Glu) 2 nmol or AngII 100 pmol was injected into the RVLM with or without RVLM pretreatment of kynurenate (Glu receptor antagonist) 3nmol, candesartan (AngII type 1 (AT1) receptor antagonist) 2 nmol, or PD123319 (AngII type 2 (AT2) receptor antagonist) 1 nmol in anesthetized Wistar rats. In addition, to determine the role of glutamate and AngII in the pressor and sympathetic effects to the RVLM compression, kynurenate, candesartan, or PD123319 was locally injected before pulsatile compression of the RVLM. Finally, to determine the effects of peripherally administered AngII antagonists in these pressor and sympathetic excitatory responses, candesartan 0.25 micromol or PD123319 0.05 micromol was intravenously injected before pulsatile compression of the RVLM. Glu injected into the RVLM significantly increased mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SNA), and these effects were reduced by RVLM pretreatment with kynurenate, but were unaffected by candesartan or PD123319. AngII injected into the RVLM and pulsatile compression of the RVLM also increased MAP and SNA. However, in contrast with Glu injections, these effects were reduced by RVLM pretreatment with candesartan or kynurenate, but were unaffected by PD123319. Pressor and sympathetic excitatory responses to RVLM compression were reduced by intravenous pretreatment with candesartan but not with PD123319. These results indicate that, upon pulsatile compression of the RVLM, AngII may activate RVLM neurons via AT1 receptors and stimulate Glu release to thereby elicit sympathetic activation and pressor effects. Candesartan may exert its hypotensive effect at least in part by affecting the RVLM neurons to reduce sympathetic outflow induced by pulsatile compression of the RVLM.

Vascular Compression of Rostral Medulla Oblongata: Prospective MR Imaging Study in Hypertensive and Normotensive Subjects

PURPOSE

To prospectively evaluate prevalence of neurovascular contacts (NVCs) at the rostral medulla oblongata in normotensive and hypertensive subjects.

MATERIALS AND METHODS

Forty-three patients with severe essential hypertension and 45 normotensive subjects were matched for age, sex, and body mass index. Magnetic resonance (MR) imaging included transverse and coronal T2-weighted turbo spin-echo (section thickness, 3.0 mm), transverse three-dimensional (3D) time-of-flight MR angiographic (section thickness, 0.8 mm), and 3D constructive interference in steady state (CISS) (section thickness, 1.0 mm) sequences. All MR images were reviewed by two radiologists who were blinded to the hypertensive status of subjects. Presence and degree of NVC at rostral medulla and left/right rostral ventrolateral medulla (RVLM) were evaluated together with conspicuity of anatomic structures on MR images. Differences in prevalence of NVC among normotensive and hypertensive subjects were tested for statistical significance (P <.05) by using nonparametric tests.

RESULTS

Among hypertensive patients, 34 (79%) of 43 showed NVC of rostral medulla at any location, and 14 (33%) of 43 had NVC at the left RVLM. In controls (normotensive subjects), 35 (78%) of 45 showed NVC of rostral medulla, and 17 (38%) of 45 had NVC at left RVLM. Prevalence of NVC was not significantly different between both groups at any location of rostral medulla. Compared with T2-weighted turbo spin-echo and 3D time-of-flight MR imaging sequences, 3D CISS offered better contrast resolution of neural and vascular structures and superior delineation of outer vascular contours.

CONCLUSION

Vascular compression of the rostral medulla oblongata is a frequent finding in both hypertensive and normotensive subjects. Results of this study do not support NVC at left RVLM as an etiologic factor in essential hypertension.

Hypotensive effects of neuromedin U microinjected into the cardiovascular-related region of the rat nucleus tractus solitarius

Neuromedin U (NMU) is a brain-gut peptide with potent contractile effects on the uterus and smooth muscle. Intracerebroventricular injection of NMU reportedly decreased food intake and body-weight gain in the rat. We evaluated the effects of NMU delivered by microinjection into the rat nucleus tractus solitarius (NTS) on cardiovascular responses. At the concentrations used (5, 10 or 50 pmol), the intra-NTS injection of NMU in artificial cerebrospinal fluid produced a significant reduction in both the mean arterial pressure and heart rate. The hypotensive responses were dose dependent. Our findings suggest that NMU may act as a neurotransmitter or neuromodulative substance that causes excitation of neurons in the NTS and that it may play a role in cardiovascular regulation in vivo.

Cardioacceleratory responses to hypocretin-1 injections into rostral ventromedial medulla

Intracisternal injections of hypocretin-1 (hcrt-1) have been shown to elicit sympathoexciatory responses. However, the location of central sites that may mediate these cardiovascular effects have not been clearly elucidated. This study was done in male Wistar rats to investigate the effects of microinjections of hcrt-1 into the rostral ventromedial medulla (RVMM) on mean arterial pressure (MAP), heart rate (HR) and the arterial baroreflex. An initial series of experiments was done to provide a detailed mapping of the location of hcrt-1- and hcrt-1 receptors (hcrtR-1)-like immunoreactivity (i.r.) in the RVMM region. Hcrt-1 and hcrtR-1 ir were found throughout the RVMM region, but primarily within the magnocellular reticular nucleus and the adjacent nucleus paragigantocellularis lateralis. In the second series, this region containing hcrt-1 and hcrtR-1 ir was explored for sites that elicited changes in MAP and HR in the anaesthetized rat. Microinjection of hcrt-1 (0.5-2.5 pmol) into the region of magnocellular reticular nucleus elicited a dose-dependent increase in HR, with little or no change in MAP. Administration (i.v.) of the muscarinic receptor antagonist atropine methyl bromide significantly attenuated ( approximately 62%) the HR response whereas, the total autonomic blockade abolished the HR response. Finally, unilateral or bilateral microinjection of hcrt-1 into the magnocellular reticular nucleus significantly attenuated the reflex bradycardia resulting from the activation of the baroreflex following the increase in MAP from an iv injection of phenylephrine. These data suggest that hcrt-1 in the RVMM region activates neuronal circuits that both inhibit vagal activity and increase sympathetic activity to the heart, and that it alters the excitability of central circuits that reflexly control the circulation.

Cardiovascular responses to hypocretin-1 in nucleus ambiguus of the ovariectomized female rat

Experiments were done to investigate the effect of chronic estrogen (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus ambiguus (Amb). Microinjections of hcrt-1 (0.5-2.5 pmol) into the external formation of Amb (Ambe) in the urethane anaesthetized, E treated OVX animal or OVX only animal, elicited a dose-related decrease in heart rate (HR). On the other hand, hcrt-1 injections into Ambe did not elicit consistent changes in mean arterial pressure (MAP). The HR response was mediated by vagal excitation as ipsilateral vagotomy abolished the bradycardia response. The bradycardia responses were consistently of greater magnitude and longer duration in the OVX+E animals compared to the OVX only female animals. Finally, it was found that the reflex bradycardia to activation of arterial baroreceptors, as a result of increasing systemic arterial pressure with phenylephrine, was only significantly potentiated in the OVX+E animals. These data suggest that hcrt-1 in the Ambe of the female elicits an increase in vagal cardiomotor neuronal activity to the heart, and that the circulating level of E alters not only the sensitivity of Ambe neurons to hcrt-1 but also the sensitivity of these neurons during activation of baroreceptor afferent inputs.

Mendelian hypertension with brachydactyly as a molecular genetic lesson in regulatory physiology

Mendelian forms of hypertension have delivered a treasure trove of novel genes. To date, the molecular mechanisms of five such syndromes have been largely clarified, including glucocorticoid-remediable aldosteronism, Liddle's syndrome, apparent mineralocorticoid excess, an activating mutation of the mineralocorticoid receptor, and pseudohypoaldosteronism type 2. Each of these conditions features salt sensitivity with increased sodium and volume reabsorption by the kidney and low plasma renin activity. None of the gene loci for these syndromes has been convincingly linked to hypertension in the general population. We are investigating kindreds who have autosomal-dominant hypertension and brachydactyly. Affected persons invariably have both anomalies. The hypertension is severe and results in death at about age 50 years from stroke. The condition resembles essential hypertension, because renin, aldosterone, and norepinephrine responses are normal and no salt sensitivity is present. The response to antihypertensive drugs is general. Another feature is diminished baroreflex sensitivity with markedly impaired blood pressure buffering. Furthermore, the ventrolateral medulla may be compromised in these patients, because neurovascular anomalies are a regular finding. We mapped the gene(s) for this disease to chromosome 12p and narrowed the chromosomal region by studying more affected families. Interestingly, the same locus was recently mapped in Chinese families with essential hypertension. Our 3-centimorgan region contains genes encoding a phosphodiesterase, an ATP-dependent potassium channel, and its regulator the sulfonylurea receptor 2. Screening of the coding regions revealed that none of these candidate genes harbor obvious mutations; however, other genetic mechanisms may nevertheless compromise their function. Our study underscores the importance of regulatory physiology to the understanding of a complex genetic syndrome.

Prolonged inhibition of rostral ventral lateral medullary premotor sympathetic neurons by electroacupuncture in cats

We have shown that electroacupuncture (EA) at the Neiguan-Jianshi (N-J) acupoints over the median nerve reduces myocardial ischemia by modulating the pressor response induced by application of bradykinin on the gallbladder. The present study was designed to investigate the neural substrate underlying the prolonged modulatory effect of EA on visceral afferent input into the rostral ventral lateral medulla (rVLM). Experiments were performed on ventilated anesthetized cats. Neuronal activity was recorded while either stimulating the splanchnic nerve or applying EA at the N-J acupoints. Thirty-three cells responsive to splanchnic nerve and median nerve stimulation were antidromically driven from the intermediolateral columns, T(2)-T(4), indicating their function as premotor sympathetic neurons. These neurons also received baroreceptor input demonstrating that they were cardiovascular sympathoexcitatory cells. Arterial pulse-triggered averaging and coherence analysis demonstrated a correlation between cardiac-related discharge activity with 2.8+/-0.3 Hz rhythms and arterial blood pressure. Stimulation (2 Hz, 1-4 mA, 0.5 ms) of the splanchnic nerve for 30 s evoked excitatory responses. These neuronal responses were reduced during and after 30-min stimulation of EA at the Neiguan-Jianshi acupoints. These splanchnic nerve-induced excitatory responses in neurons subjected to 30 min of EA were reduced by 68%. Iontophoresis of naloxone promptly reversed the EA-induced inhibitory effect by 52%. Neuronal activity in the rVLM induced by splanchnic nerve stimulation was reduced for 50 (or more) min after termination of EA in 7 of 12 rVLM neurons. Our results indicate that rVLM premotor sympathetic cardiovascular neurons receive convergent input from the gallbladder through the splanchnic nerve and N-J acupoints through the median nerves. Through an opioid mechanism, EA inhibits splanchnic nerve-induced excitatory responses of these rVLM neurons. Many of these neurons receiving convergent visceral and somatic input exhibit long-lasting inhibition by EA.

Estrogen alters the bradycardia response to hypocretin-1 in the nucleus tractus solitarius of the ovariectomized female

Experiments were performed to investigate the effect of 17beta-estradiol (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus tractus solitarius (NTS). In an initial series of experiments, the distribution of hcrt-1-like immunoreactivity within the region of the NTS was mapped in both OVX only and OVX+E animals. Hcrt-1 immunoreactivity was found throughout the NTS region in both groups of females, predominantly within the caudal interstitial, commissural, medial and lateral subnuclei of the NTS. The relative density of hcrt-1 immunoreactivity in all NTS subnuclei was similar in both female groups. Microinjections of hcrt-1 (0.5-10 pmol) into the caudal lateral and medial subnuclei of the NTS complex of the alpha-chloralose of the urethane-anaesthetized E-treated OVX rat elicited a dose-related decrease in heart rate (HR). On the other hand, although a dose-response effect on arterial pressure was evident, significant arterial pressure responses were observed only at the higher dose of hcrt-1 (>2.5 pmol). In the OVX only female rat, microinjection of hcrt-1 into similar NTS sites elicited a bradycardia and depressor response only at the highest dose of hcrt-1, and these responses were significantly smaller in magnitude than those elicited in the OVX+E animal. In addition, in the OVX only animals, a few sites within the caudal commissural subnucleus of the NTS complex were found at which hcrt-1 elicited tachycardia and pressor responses. Finally, it was found that the reflex bradycardia to the activation of arterial baroreceptors as a result of increasing systemic arterial pressure with phenylephrine (2-4 microg/kg) was significantly potentiated in the OVX+E animals only. These data suggest that hcrt-1 in the NTS of the female activates a neuronal circuit that controls the circulation and that the circulating level of E alters the sensitivity of these cardiovascular circuits to hcrt-1.