Baroreceptor inhibition of the locus coeruleus noradrenergic neurons

Chronic Pain-Associated Cardiovascular Disease: The Role of Sympathetic Nerve Activity

Chronic pain affects many people world-wide, and this number is continuously increasing. There is a clear link between chronic pain and the development of cardiovascular disease through activation of the sympathetic nervous system. The purpose of this review is to provide evidence from the literature that highlights the direct relationship between sympathetic nervous system dysfunction and chronic pain. We hypothesize that maladaptive changes within a common neural network regulating the sympathetic nervous system and pain perception contribute to sympathetic overactivation and cardiovascular disease in the setting of chronic pain. We review clinical evidence and highlight the basic neurocircuitry linking the sympathetic and nociceptive networks and the overlap between the neural networks controlling the two.

[Interactions between the brain and heart]

The brain and heart are closely interconnected. Physiologically, the brain influences the way the heart beats. An example for this physiological influence is the control of the heart rate via efferences of the autonomic nervous system. Clinical examples for this direction of interactions include cardiac complications after stroke as well as takotsubo cardiomyopathy; however, the heart and brain are reciprocally connected so that heart activity also influences the brain beyond its function as the generator of bloodflow supplying the brain. Examples for this are the perception of stimuli depending on the time of presentation during the heart cycle. Clinical examples of the direction of this interaction constitute stroke as a thromboembolic complication of atrial fibrillation as well as the correlation of atrial fibrillation and dementia. This review article gives an overview of the bidirectional interactions between the heart and brain, partly including the cardiovascular system, discusses their implications for the clinical routine and gives an outlook on current fields of research.

Head-Down Tilt Position, but Not the Duration of Bed Rest Affects Resting State Electrocortical Activity

Adverse cognitive and behavioral conditions and psychiatric disorders are considered a critical and unmitigated risk during future long-duration space missions (LDSM). Monitoring and mitigating crew health and performance risks during these missions will require tools and technologies that allow to reliably assess cognitive performance and mental well-being. Electroencephalography (EEG) has the potential to meet the technical requirements for the non-invasive and objective monitoring of neurobehavioral conditions during LDSM. Weightlessness is associated with fluid and brain shifts, and these effects could potentially challenge the interpretation of resting state EEG recordings. Head-down tilt bed rest (HDBR) provides a unique spaceflight analog to study these effects on Earth. Here, we present data from two long-duration HDBR experiments, which were used to systematically investigate the time course of resting state electrocortical activity during prolonged HDBR. EEG spectral power significantly reduced within the delta, theta, alpha, and beta frequency bands. Likewise, EEG source localization revealed significantly lower activity in a broad range of centroparietal and occipital areas within the alpha and beta frequency domains. These changes were observed shortly after the onset of HDBR, did not change throughout HDBR, and returned to baseline after the cessation of bed rest. EEG resting state functional connectivity was not affected by HDBR. The results provide evidence for a postural effect on resting state brain activity that persists throughout long-duration HDBR, indicating that immobilization and inactivity per se do not affect resting state electrocortical activity during HDBR. Our findings raise an important issue on the validity of EEG to identify the time course of changes in brain function during prolonged HBDR, and highlight the importance to maintain a consistent body posture during all testing sessions, including data collections at baseline and recovery.

Electrocortical Evidence for Impaired Affective Picture Processing after Long-Term Immobilization

The neurobehavioral risks associated with spaceflight are not well understood. In particular, little attention has been paid on the role of resilience, social processes and emotion regulation during long-duration spaceflight. Bed rest is a well-established spaceflight analogue that combines the adaptations associated with physical inactivity and semi-isolation and confinement. We here investigated the effects of 30 days of 6 degrees head-down tilt bed rest on affective picture processing using event-related potentials (ERP) in healthy men. Compared to a control group, bed rest participants showed significantly decreased P300 and LPP amplitudes to pleasant and unpleasant stimuli, especially in centroparietal regions, after 30 days of bed rest. Source localization revealed a bilateral lower activity in the posterior cingulate gyrus, insula and precuneus in the bed rest group in both ERP time frames for emotional, but not neutral stimuli.

Validation of the xylazine/ketamine anesthesia test as a predictor of the emetic potential of pharmacological compounds in rats

The xylazine/ketamine anesthesia test is widely used as a predictor of the emetic potential of pharmacological compounds in rats. An emetic reflex is usually triggered by the emetic center, which is populated with many different chemoreceptors. Inhibition of the α2 adrenergic receptor (α2 receptor) is involved in the initiation of the emetic reflex, and this is the key mechanism behind the xylazine/ketamine anesthesia test. In this study, we attempt to validate this test as a predictor of the emetic potential of pharmacological compounds. Furthermore, it was investigated whether an anti-emetic potential of pharmacological compounds could be assessed within this test as well. Rats were anesthetized with a combination of low doses of ketamine and xylazine, and subsequently treated with PDE4 inhibitor rolipram, α2 receptor antagonist yohimbine, α2 receptor agonist clonidine, tricyclic antidepressant imipramine, D2-receptor antagonist haloperidol, or 5-HT3 receptor antagonist (and anti-emetic drug) ondansetron. We were able to successfully reproduce the reduction in anesthesia time after rolipram or yohimbine treatment, as found in previous studies and has been suggested to be indicative of emetic properties of these treatments is humans. Furthermore, clonidine shortened anesthesia duration whereas imipramine and haloperidol lengthened anesthesia duration. Ondansetron was unable to rescue the reduction in duration of anesthesia induced by either rolipram or yohimbine. Altogether, the xylazine/ketamine anesthesia test is a reliable measure for α2 receptor antagonism. However, it may not be appropriate to assess emesis independent of this mechanism.

Does the body give the brain an attentional boost? Examining the relationship between attentional and cardiac gating

Studies on mind-body interactions have largely focused on how mental states modulate bodily physiological responses. Increasing evidence suggests that bodily states also modulate mental states. Here we investigated how both may be integrated in the brain at the resolution of a heartbeat, examining how phasic fluctuations of peripheral blood pressure and central attentional resources combine to influence cognition. We examined the effects of cardiac phase on the performance of two simultaneous tasks: a go/no-go letter detection task where targets were concurrently presented on background faces and a short-term memory face discrimination task. Short-term memory for the background face was better when the initial face was encoded during the systole rather than diastole phase and when it was paired with a target rather than a distractor. There was no significant interaction between cardiac phase and letter detection. These data suggest that peripheral blood pressure and central attention independently regulate cognitive performance.

The effects of breathing at a frequency of 0.1 Hz on affective state, the cardiovascular system, and adequacy of ventilation

The present study aimed to investigate changes induced by breathing at 0.1 Hz in affective state, cardiovascular activity, and adequacy of ventilation as well as the relation between changes in peripheral physiological processes and alteration of affect. Eighty-three participants were randomly assigned to one of three groups: Two groups doing paced breathing at 0.1 Hz, one with and the other without a cover story hiding the goal of the experiment, and, as a control, paced breathing at 0.28 Hz. We measured the effects of breathing at 0.1 Hz on affective state (unpleasant and pleasant arousals), respiratory sinus arrhythmia (RSA), sympathetic control of the heart (preejection period, PEP), and adequacy of ventilation as measured by partial pressure of end-tidal CO₂ (PetCO₂ ). The use of a cover story did not influence the effects of paced breathing on the study outcomes. In the 0.1 Hz groups, unpleasant arousal decreased only among men. Changes in RSA were not related to changes in affect. Respiratory frequency did not influence PEP. However, changes in PEP were inversely related to changes in pleasant arousal. PetCO₂ decreased in all conditions, and a larger drop in PetCO₂ predicted a greater decrease in unpleasant arousal. The results obtained corroborate previous findings showing that slow paced breathing may lead to moderate hyperventilation among untrained participants and suggest that hyperventilation during breathing at 0.1 Hz is not deep enough to produce an increase in affective arousal.

Coupling of respiration and attention via the locus coeruleus: Effects of meditation and pranayama

The locus coeruleus (LC) has established functions in both attention and respiration. Good attentional performance requires optimal levels of tonic LC activity, and must be matched to task consistently. LC neurons are chemosensitive, causing respiratory phrenic nerve firing to increase frequency with higher CO₂ levels, and as CO₂ level varies with the phase of respiration, tonic LC activity should exhibit fluctuations at respiratory frequency. Top-down modulation of tonic LC activity from brain areas involved in attentional regulation, intended to optimize LC firing to suit task requirements, may have respiratory consequences as well, as increases in LC activity influence phrenic nerve firing. We hypothesize that, due to the physiological and functional overlaps of attentional and respiratory functions of the LC, this small neuromodulatory nucleus is ideally situated to act as a mechanism of synchronization between respiratory and attentional systems, giving rise to a low-amplitude oscillation that enables attentional flexibility, but may also contribute to unintended destabilization of attention. Meditative and pranayama practices result in attentional, emotional, and physiological enhancements that may be partially due to the LC's pivotal role as the nexus in this coupled system. We present original findings of synchronization between respiration and LC activity (via fMRI and pupil dilation) and provide evidence of a relationship between respiratory phase modulation and attentional performance. We also present a mathematical dynamical systems model of respiratory-LC-attentional coupling, review candidate neurophysiological mechanisms of changes in coupling dynamics, and discuss implications for attentional theory, meditation, and pranayama, and possible therapeutic applications.

Feedback from the heart: Emotional learning and memory is controlled by cardiac cycle, interoceptive accuracy and personality

Feedback processing is critical to trial-and-error learning. Here, we examined whether interoceptive signals concerning the state of cardiovascular arousal influence the processing of reinforcing feedback during the learning of 'emotional' face-name pairs, with subsequent effects on retrieval. Participants (N=29) engaged in a learning task of face-name pairs (fearful, neutral, happy faces). Correct and incorrect learning decisions were reinforced by auditory feedback, which was delivered either at cardiac systole (on the heartbeat, when baroreceptors signal the contraction of the heart to the brain), or at diastole (between heartbeats during baroreceptor quiescence). We discovered a cardiac influence on feedback processing that enhanced the learning of fearful faces in people with heightened interoceptive ability. Individuals with enhanced accuracy on a heartbeat counting task learned fearful face-name pairs better when feedback was given at systole than at diastole. This effect was not present for neutral and happy faces. At retrieval, we also observed related effects of personality: First, individuals scoring higher for extraversion showed poorer retrieval accuracy. These individuals additionally manifested lower resting heart rate and lower state anxiety, suggesting that attenuated levels of cardiovascular arousal in extraverts underlies poorer performance. Second, higher extraversion scores predicted higher emotional intensity ratings of fearful faces reinforced at systole. Third, individuals scoring higher for neuroticism showed higher retrieval confidence for fearful faces reinforced at diastole. Our results show that cardiac signals shape feedback processing to influence learning of fearful faces, an effect underpinned by personality differences linked to psychophysiological arousal.

Imaging Posture Veils Neural Signals

Whereas modern brain imaging often demands holding body positions incongruent with everyday life, posture governs both neural activity and cognitive performance. Humans commonly perform while upright; yet, many neuroimaging methodologies require participants to remain motionless and adhere to non-ecological comportments within a confined space. This inconsistency between ecological postures and imaging constraints undermines the transferability and generalizability of many a neuroimaging assay. Here we highlight the influence of posture on brain function and behavior. Specifically, we challenge the tacit assumption that brain processes and cognitive performance are comparable across a spectrum of positions. We provide an integrative synthesis regarding the increasingly prominent influence of imaging postures on autonomic function, mental capacity, sensory thresholds, and neural activity. Arguing that neuroimagers and cognitive scientists could benefit from considering the influence posture wields on both general functioning and brain activity, we examine existing imaging technologies and the potential of portable and versatile imaging devices (e.g., functional near infrared spectroscopy). Finally, we discuss ways that accounting for posture may help unveil the complex brain processes of everyday cognition.

Autonomic dysfunction in mild cognitive impairment: evidence from power spectral analysis of heart rate variability in a cross-sectional case-control study

BACKGROUND

Mild cognitive impairment (MCI) is set to become a major health problem with the exponential ageing of the world's population. The association between MCI and autonomic dysfunction, supported by indirect evidence and rich with clinical implications in terms of progression to dementia and increased risk of mortality and falls, has never been specifically demonstrated.

AIM

To conduct a comprehensive assessment of autonomic function in subjects with MCI by means of power spectral analysis (PSA) of heart rate variability (HRV) at rest and during provocative manoeuvres.

METHODS

This cross-sectional study involved 80 older outpatients (aged ≥ 65) consecutively referred to a geriatric unit and diagnosed with MCI or normal cognition (controls) based on neuropsychological testing. PSA was performed on 5-minute electrocardiographic recordings under three conditions--supine rest with free breathing (baseline), supine rest with paced breathing at 12 breaths/minute (parasympathetic stimulation), and active standing (orthosympathetic stimulation)--with particular focus on the changes from baseline to stimulation of indices of sympathovagal balance: normalized low frequency (LFn) and high frequency (HFn) powers and the LF/HF ratio. Blood pressure (BP) was measured at baseline and during standing. Given its exploratory nature in a clinical population the study included subjects on medications with a potential to affect HRV.

RESULTS

There were no significant differences in HRV indices between the two groups at baseline. MCI subjects exhibited smaller physiological changes in all three HRV indices during active standing, consistently with a dysfunction of the orthosympathetic system. Systolic BP after 10 minutes of standing was lower in MCI subjects, suggesting dysautonomia-related orthostatic BP dysregulation.

CONCLUSIONS

Our study is novel in providing evidence of autonomic dysfunction in MCI. This is associated with orthostatic BP dysregulation and the ongoing follow-up of the study population will determine its prognostic relevance as a predictor of adverse health outcomes.

Monosynaptic glutamatergic activation of locus coeruleus and other lower brainstem noradrenergic neurons by the C1 cells in mice

The C1 neurons, located in the rostral ventrolateral medulla (VLM), are activated by pain, hypotension, hypoglycemia, hypoxia, and infection, as well as by psychological stress. Prior work has highlighted the ability of these neurons to increase sympathetic tone, hence peripheral catecholamine release, probably via their direct excitatory projections to sympathetic preganglionic neurons. In this study, we use channelrhodopsin-2 (ChR2) optogenetics to test whether the C1 cells are also capable of broadly activating the brain's noradrenergic system. We selectively expressed ChR2(H134R) in rostral VLM catecholaminergic neurons by injecting Cre-dependent adeno-associated viral vectors into the brain of adult dopamine-β-hydroxylase (DβH)(Cre/0) mice. Most ChR2-expressing VLM neurons (75%) were immunoreactive for phenylethanolamine N-methyl transferease, thus were C1 cells, and most of the ChR2-positive axonal varicosities were immunoreactive for vesicular glutamate transporter-2 (78%). We produced light microscopic evidence that the axons of rostral VLM (RVLM) catecholaminergic neurons contact locus coeruleus, A1, and A2 noradrenergic neurons, and ultrastructural evidence that these contacts represent asymmetric synapses. Using optogenetics in tissue slices, we show that RVLM catecholaminergic neurons activate the locus coeruleus as well as A1 and A2 noradrenergic neurons monosynaptically by releasing glutamate. In conclusion, activation of RVLM catecholaminergic neurons, predominantly C1 cells, by somatic or psychological stresses has the potential to increase the firing of both peripheral and central noradrenergic neurons.

What the heart forgets: Cardiac timing influences memory for words and is modulated by metacognition and interoceptive sensitivity

Mental functions are influenced by states of physiological arousal. Afferent neural activity from arterial baroreceptors at systole conveys the strength and timing of individual heartbeats to the brain. We presented words under limited attentional resources time-locked to different phases of the cardiac cycle, to test a hypothesis that natural baroreceptor stimulation influences detection and subsequent memory of words. We show memory for words presented around systole was decreased relative to words at diastole. The deleterious memory effect of systole was greater for words detected with low confidence and amplified in individuals with low interoceptive sensitivity, as indexed using a heartbeat counting task. Our observations highlight an important cardiovascular channel through which autonomic arousal impacts a cognitive function, an effect mitigated by metacognition (perceptual confidence) and interoceptive sensitivity.

Activation and inhibition of neurons in the hippocampal ventral subiculum by norepinephrine and locus coeruleus stimulation

The ventral subiculum (vSub) has been implicated in a wide range of neurocognitive functions, including responses to fear, stress, and anxiety. The vSub receives dense noradrenergic (NE) inputs from the locus coeruleus (LC), and the LC-NE system is heavily implicated in attention and is known to be activated by stressors. However, the way in which the neurons in the vSub respond to activation of the LC-NE is not well understood. In this study, the direct LC innervation of the vSub was investigated. The effect of norepinephrine (NE) on single vSub neurons was examined using microiontophoresis combined with electrophysiological recordings in anesthetized rats, and this response compared with the effect of electrical stimulation of the LC. Iontophoretic NE inhibited all vSub neurons tested, whereas LC stimulation inhibited 16% and activated 38% of neurons. Inhibition was mediated primarily by alpha-2 receptors, whereas activation was mediated by beta-adrenergic receptors. Furthermore, this effect was not mediated via the LC-basolateral amygdala (BLA) pathway, because BLA inactivation did not block LC stimulation-evoked activation of the vSub. These results indicate that the LC-NE system is a potent modulator of vSub activity. Based on these findings, stress-induced activation of the LC-NE system is expected to evoke inhibition and activation in the vSub, both of which may contribute to stress adaptation, whereas an imbalance of this system may lead to pathological stress responses in mental disorders.

On the feasibility of using motor imagery EEG-based brain-computer interface in chronic tetraplegics for assistive robotic arm control: a clinical test and long-term post-trial follow-up

STUDY DESIGN

Survey and long-term clinical post-trial follow-up (interviews/correspondence) on nine chronic, post spinal cord injury (SCI) tetraplegics.

OBJECTIVE

To assess feasibility of the use of Electroencephalography-based Brain-Computer Interface (EEG-BCI) for reaching/grasping assistance in tetraplegics, through a robotic arm.

SETTINGS

Physical and (neuromuscular) Rehabilitation Medicine, Cardiology, Neurosurgery Clinic Divisions of TEHBA and UMPCD, in collaboration with 'Brain2Robot' (composed of the European Commission-funded Marie Curie Excellence Team by the same name, hosted by Fraunhofer Institute-FIRST), in the second part of 2008.

METHODS

Enrolled patients underwent EEG-BCI preliminary training and robot control sessions. Statistics entailed multiple linear regressions and cluster analysis. A follow-up-custom questionnaire based-including patients' perception of their EEG-BCI control capacity was continued up to 14 months after initial experiments.

RESULTS

EEG-BCI performance/calibration-phase classification accuracy averaged 81.0%; feedback training sessions averaged 70.5% accuracy for 7 subjects who completed at least one feedback training session; 7 (77.7%) of 9 subjects reported having felt control of the cursor; and 3 (33.3%) subjects felt that they were also controlling the robot through their movement imagination. No significant side effects occurred. BCI performance was positively correlated with beta (13-30 Hz) EEG spectral power density (coefficient 0.432, standardized coefficient 0.745, P-value=0.025); another possible influence was sensory AIS score (range: 0 min to 224 max, coefficient -0.177, standardized coefficient -0.512, P=0.089).

CONCLUSION

Limited but real potential for self-assistance in chronic tetraplegics by EEG-BCI-actuated mechatronic devices was found, which was mainly related to spectral density in the beta range positively (increasing therewith) and to AIS sensory score negatively.

Cardiovascular baroreceptor activity and selective inhibition of monoamine oxidase

Cardiovascular baroreceptor responsiveness of conscious rats treated with selective inhibitors of monoamine oxidase (MAO) types A and B was determined by measurement of blood pressure (BP) and heart rate (HR) responses to intravenous injection of phenylephrine and sodium nitroprusside. Treatment with selegiline (1 or 5 mg/kg p.o. daily for 7 days) did not significantly modify resting levels of BP and HR, lower or upper HR plateau levels, or HR/BP gain. Treatment with clorgyline (2 mg/kg p.o. daily for 7 days) increased HR/BP gain but also did not modify resting BP or HR, or lower and upper plateau levels of HR. The results are compatible with an effect of MAO-A inhibition to modify monoamine levels in medullary areas participating in CNS control of blood pressure.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

Dynorphin and stress-related peptides in rat locus coeruleus: contribution of amygdalar efferents

The interaction between the stress axis and endogenous opioid systems has gained substantial attention, because it is increasingly recognized that stress alters individual sensitivity to opiates. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC). We have previously described interactions of several opioid peptides [e.g., proopiomelanocortin, enkephalin (ENK)] with the stress-related peptide corticotropin-releasing factor (CRF) in the LC. To examine further the interactions among dynorphin (DYN), ENK, and CRF in the LC, sections were processed for detection of DYN and CRF or DYN and ENK in rat brain. DYN- and CRF-containing axon terminals overlapped noradrenergic dendrites in this region. Dual immunoelectron microscopy showed coexistence of DYN and CRF; 35% of axon terminals containing DYN were also immunoreactive for CRF. In contrast, few axon terminals contained both DYN and ENK. A potential DYN/CRF afferent is the central nucleus of the amygdala (CeA). Dual in situ hybridization showed that, in CeA neurons, 31% of DYN mRNA-positive cells colocalized with CRF mRNA, whereas 53% of CRF mRNA-containing cells colocalized with DYN mRNA. Finally, to determine whether limbic DYN afferents target the LC, the CeA was electrolytically lesioned. Light-level densitometry of DYN labeling in the LC showed a significant decrease in immunoreactivity on the side of the lesion. Taken together, these data indicate that DYN- and CRF-labeled axon terminals, most likely arising from amygdalar sources, are positioned dually to affect LC function, whereas DYN and ENK function in parallel.

Systemic effect of cannabinoids on the spontaneous firing rate of locus coeruleus neurons in rats

Previous reports have described modulation of noradrenergic activity by cannabinoid receptors. The aim of the present research was to examine the effect of two synthetic cannabinoid CB1/CB2 receptor agonists, R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)-methyl]pyrrolol-[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone (WIN 55212-2) and (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol (CP 55940), on the spontaneous activity of locus coeruleus noradrenergic neurons by single-unit extracellular recordings in vivo and in vitro. In anaesthetized rats, intravenous administrations of WIN 55212-2 (31.3-500 microg/kg) or CP 55940 (31.3-500 microg/kg) increased the firing rate of locus coeruleus neurons in a dose-dependent manner. The stimulatory effect of WIN 55212-2 was blocked by pretreatment with the cannabinoid CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR 141716A; 2 mg/kg). Paradoxically, local administration of WIN 55212-2 (8.3-31.3 pmol) into the locus coeruleus and intracerebroventricular injections of WIN 55212-2 (10-20 microg) or CP 55940 (20-40 microg) failed to change the spontaneous firing rate of locus coeruleus neurons. Likewise, in rat brain slice preparations perfusion with WIN 55212-2 (10 microM) or CP 55940 (10-30 microM) did not specifically affect the spontaneous firing rate of locus coeruleus cells. Therefore, we conclude that synthetic cannabinoids increase the spontaneous firing activity of noradrenergic neurons in the rat locus coeruleus through cannabinoid CB1 receptors. This stimulation appears to be indirectly induced via a receptor mechanism probably located at the peripheral level.

Thinking on your back: Solving anagrams faster when supine than when standing

There is potentially less locus coeruleus-noradrenergic system activity when lying down than when standing, an effect expected to develop via a difference in baroreceptor load. Furthermore, there is evidence implying that locus coeruleus-noradrenergic system activity impairs attempts to solve anagrams. Consistent with these ideas, we found that subjects solved anagrams significantly faster when supine than when standing. With anagrams characterized as insight problems, our finding suggests that insight may be influenced by body posture.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Differential amino acid transmission in the locus coeruleus of Wistar Kyoto and spontaneously hypertensive rats

In addition to differences in their blood pressure, Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) are known to differ in their emotional behaviour. The neurochemistry underlying these differences is not well understood. In the present study the release rates of the two main regulatory amino acids in the locus coeruleus, glutamate and gamma-aminobutyric acid (GABA), were monitored in WKY rats and SHR to investigate whether basal and/or challenged neurotransmission differs between these strains. The strains differed in their basal blood pressure (WKY 102+/-2 mmHg, SHR 140+/-4 mmHg), as well as in their emotional behaviour, since WKY rats displayed enhanced anxiety-related behaviour in the open field test (time in centre: WKY 197+/-40 s/30 min, SHR 741+/-93 s/30 min). Basal glutamate and GABA release rates did not differ between WKY rats and SHR. A rise in blood pressure induced by intravenous infusion of noradrenaline for 10 min enhanced GABA release in WKY rats by 60%, while no effect was observed in SHR. Glutamate release did not respond to experimental hypertension in both strains. Intravenous infusion of sodium nitroprusside led to a fall in blood pressure, which was less pronounced and was of shorter duration in WKY rats than in SHR. The depressor response had no effect on amino acid release in the locus coeruleus of both strains. Mild stress induced by noise or tail pinch led to slight rises in arterial blood pressure (10 mmHg and 20 mmHg respectively), which were similar in WKY rats and SHR. Tail pinch enhanced the release rates of glutamate and GABA in the locus coeruleus of WKY rats and SHR; however, no strain differences were noted. Noise stress did not significantly influence amino acid release. These findings demonstrate that SHR and WKY rats differ in GABAergic neurotransmission, which is revealed in response to specific cardiovascular challenges, but not to mild stressors. The observed lack of GABA response to blood pressure elevation in SHR may reflect a disturbed mechanism counteracting high blood pressure, possibly contributing to hypertension in this strain.

Repeated restraint stress-induced increase in baroreceptor reflex sensitivity: role of corticotropin-releasing factor

The effect of central administration of a corticotropin-releasing factor antagonist on repeated restraint stress-induced changes in the baroreceptor reflex response to phenylephrine was examined in male Wistar-Kyoto rats. Rats were instrumented with intracerebroventricular guide cannula and femoral arterial and venous catheters. On each of 5 consecutive days, two groups received either a central infusion of saline or an infusion of the corticotropin-releasing factor antagonist, astressin. One saline- and one astressin-treated group experienced 20 min of restraint stress 10 min after each infusion. The other saline- and astressin-treated groups served as non-stressed controls. Twenty-four hours later, each rat received 3 doses of phenylephrine which produced equivalent increases in mean arterial pressure in each of the 4 treatment groups. Reflex bradycardia was significantly greater in the saline-treated/repeated restraint group than in the saline-treated/no restraint group. This effect of repeated restraint on the baroreceptor reflex was attenuated by administration of astressin prior to each session of restraint. A single 20 min session of restraint stress failed to alter baroreceptor reflex sensitivity. However, repeated central infusions of exogenous CRF failed to alter BRR sensitivity. In a separate experiment, astressin failed to attenuate the increases in mean arterial pressure and heart rate which occurred during each session of restraint stress and, in fact, diminished habituation of the blood pressure response in the last session. The results suggest that repeated stress increases thesensitivity of the baroreceptor reflex and that corticotropin-releasing factor has a role in this stress-induced change.

Environmental influences on viscero(noci)ceptive brain activities: the effects of sheltering

Visceral disorders are always accompanied by pain and/or a sense of ill-being that entails people to isolate themselves both physically and socially. By analogy with what happens in human beings, we have transferred to the rat the question of whether a protective, dark and quiet environment would influence the brain activities induced by visceral chemically-induced (cyclophosphamide [CP], 100 mg/kg/ip) adverse conditions of life. CP is an antitumoral drug that induces severe side effects (cystitis, headache, nausea, photophobia, phonophobia) and produces a strong state of ill-being in human beings. Brain activities were quantified using the expression of the Fos protein, a molecular marker of neuronal activity. The results compare data from groups of paired animals having been offered a shelter or not. Data were collected 4 h after the injection of CP, i. e., when cystitis was fully developed. Sheltered and unsheltered groups did not differ in bladder pathology. Intentional sheltering was shown to attenuate the expression of the CP-related Fos-Li activity within the locus coeruleus (LC) without affecting that of the structures known preferentially to process nociceptive inputs of bladder origin (dorsal vagal complex, ventrocaudal bulbar reticular formation, nucleus centralis of amygdala, dorsolateral portion of bed nucleus of stria terminalis). The LC levels of tyrosine hydroxylase and galanin neuronal contents were not affected. The LC belongs to the emotional activation system and can respond to a wide range of somatosensory and viscerosensory stimuli. Our hypothesis is that the LC would be processing the nervous activities that accompany the sense of ill-being coming from adverse conditions of life, including visceral disorders, and that voluntary isolation, by reducing its activity, would enable animals to minimize their level of distress.

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.

Efferent projections of the nucleus of the solitary tract to peri-locus coeruleus dendrites in rat brain: evidence for a monosynaptic pathway

Locus coeruleus (LC) neurons respond to autonomic influences, are activated by physiological stressors, and discharge in parallel with peripheral sympathetic nerves. The circuitry underlying modulation of LC activity by physiological manipulations (i.e., hemodynamic stress, hypovolumia) remains unclear. Specifically, monosynaptic projections from primary baroreceptor centers to the LC have been suggested by electrophysiological studies but have not been unequivocally established. Light microscopic anterograde tract-tracing studies have previously shown that neurons originating in the nucleus of the solitary tract (NTS) project to a region of the rostrodorsal pontine tegmentum, which contains noradrenergic dendrites of the LC; however, it is not known whether these NTS efferents specifically target LC dendrites. Therefore, we combined peroxidase labeling of biotinylated dextran amine (BDA) or Phaseolus vulgaris-leucoagglutinin (PHA-L) from the NTS with gold-silver labeling for tyrosine hydroxylase (TH) in the rostrolateral peri-LC region. Injections placed into neighboring nuclei (nucleus gracilis, hypoglossal nucleus) served as controls. Only injections centered in the NTS produced anterograde labeling in peri-LC regions containing TH processes. By electron microscopy, BDA- or PHA-L-labeled axon terminals originating from the NTS contained small, clear, and some large dense-core vesicles and formed heterogeneous synaptic contacts characteristic of both excitatory- and inhibitory-type transmitters. Approximately 19% of the BDA and PHA-L axon terminals examined originating from the commissural portion of the NTS formed synaptic specializations with dendrites exhibiting TH immunoreactivity in the peri-LC. These results demonstrate that neurons projecting from the cardiovascular-related portion of the NTS target noradrenergic dendrites, indicating that barosensitive NTS neurons may directly modulate the activity of LC neurons and may serve to integrate autonomic responses in brain by influencing the widespread noradrenergic projections of the LC. In addition, these findings demonstrate that extranuclear dendrites are an important termination site for afferents to the LC.

Nitrergic stimulation of the locus coeruleus modulates blood pressure and heart rate in the anaesthetized rat

To investigate whether nitric oxide is involved in the cardiovascular responses mediated via the locus coeruleus, the effects of microinjections of L-arginine and L-glutamate into the locus coeruleus on blood pressure and heart rate were investigated in sodium pentobarbitone-anaesthetized rats. Unilateral microinjection of L-arginine (25, 50 nmol) elicited dose-related depressor (-17 +/- 4, -25 +/- 4 mmHg) and bradycardic (13 +/- 3, 24 +/- 6 b.p.m.) effects. Furthermore, these effects were attenuated by prior local microinjection of N(G)-nitro-L-arginine (40 nmol). Peripheral muscarinic receptor blockade with atropine methyl nitrate (1 mg/kg, i.v.) attenuated the bradycardic but not the depressor responses to L-arginine. L-Glutamate (2 nmol) microinjections also mediated depressor (-27 +/- 6 mmHg) and bradycardic (53 +/- 23 b.p.m.) effects that were attenuated by microinjections of dizocilpine maleate (1 nmol) into the locus coeruleus. In addition, pretreatment with N(G)-nitro-L-arginine (40 nmol) also significantly attenuated the depressor response elicited by L-glutamate. These results suggest that nitrergic and glutamatergic pathways are operative within the locus coeruleus to modulate cardiovascular function, and also that a functional interaction may exist between the nitrergic and glutamatergic systems within the rat locus coeruleus.

Projections of medullary and pontine noradrenergic neurons to the horizontal limb of the nucleus of diagonal band in the rat

Recent investigations in the rat have implicated a noradrenergic innervation to the horizontal nucleus of the diagonal band of Broca as a critical link in a neural circuit that conveys baroreceptor information centrally to inhibit the firing of vasopressin-secreting neurons in the hypothalamic supraoptic nucleus. In this study we used small intra-diagonal band injections of a retrograde tracer, rhodamine latex microspheres, in combination with tyrosine hydroxylase histochemistry to identify brainstem noradrenergic cells contributing to this innervation. In three cases where tracer injections were limited to the horizontal limb of the diagonal band, we observed 20-50 double-labelled neurons ipsilaterally in the dorsal part of the locus coeruleus (A6) and the caudal nucleus tractus solitarius (A2), and bilaterally in the caudal ventrolateral medulla (A1). Double-labelled neurons were also noted in the ventral tegmental area (dopaminergic A10 cell group). Although all major brainstem noradrenergic cell groups contribute fibers to the horizontal limb of the nucleus of diagonal band, data from physiological studies suggest that the noradrenergic A2 neurons in the nucleus tractus solitarius are the most likely pathway through which it receives this baroreceptor information.

Noradrenaline release in the locus coeruleus of conscious rats is triggered by drugs, stress and blood pressure changes

The in vivo release of noradrenaline (NA) in the locus coeruleus (LC) of conscious rats was enhanced by local superfusion of pargyline, idazoxan, bicuculline, AMPA as well as by experimentally induced hypotension. Noise stress considerably enhanced NA release in the LC and this response was promoted after local alpha2-adrenoceptor blockade by idazoxan. Air jet stress and noise stress elicited comparable increases in NA release in the LC and the simultaneously superfused amygdala. The NA responses in both areas did not change during a second exposure to each of the stressors. It is concluded that NA release at the somatodendritic level of LC neurons is triggered by high LC activity and most likely serves to limit LC activation to excitatory stimuli by feedback inhibition via alpha2-adrenoceptors.

Rostral dorsolateral pontine neurons with sympathetic nerve-related activity

Spike-triggered averaging, arterial pulse-triggered analysis, and coherence analysis were used to classify rostral dorsolateral pontine (RDLP) neurons into groups whose naturally occurring discharges were correlated to only the 10-Hz rhythm (n = 29), to only the cardiac-related rhythm (n = 15), and to both rhythms (n = 15) in inferior cardiac sympathetic nerve discharge (SND) of urethan-anesthetized cats. Most of the neurons with activity correlated to only the cardiac-related rhythm were located medial to the other two groups of neurons. The firing rates of most RDLP neurons with activity correlated to only the 10-Hz rhythm (9 of 12) or both rhythms (7 of 8) were decreased during baroreceptor reflex-induced inhibition of SND produced by aortic obstruction; thus, they are presumed to be sympathoexcitatory. The firing rates of four of seven RDLP neurons with activity correlated to only the cardiac-related rhythm increased during baroreceptor reflex activation; thus, they may be sympathoinhibitory. We conclude that the RDLP contains a functionally heterogeneous population of neurons with sympathetic nerve-related activity. These neurons could not be antidromically activated by stimulation of the thoracic spinal cord.

Neurons in rostral ventrolateral medulla mediate vestibular inhibition of locus coeruleus in rats

The effects of caloric vestibular stimulation on the central noradrenergic neurons system were examined in the rat. In urethane-anesthetized rats, caloric stimulation inhibited the spontaneous activity of noradrenergic locus coeruleus neurons and increased systemic blood pressure. Electrical and chemical lesions in the ventrolateral medulla attenuated both the locus coeruleus inhibition and the blood pressure increase in response to caloric stimulation. Neither the neuronal inhibition nor the pressor effect was attenuated by any deafferentation of the forebrain or baroreceptors, or lesioning of the nucleus tractus solitarius. These findings indicate that the caloric stimulation-induced locus coeruleus inhibition is mediated by neurons in the ventrolateral medulla, and that these neurons also mediate the vestibulo-pressor responses. The locus coeruleus inhibition via the ventrolateral medulla is, however, considered to be independent of ventrolateral medulla-mediated systemic pressor effect. Collectively these findings suggest that the ventrolateral medulla is the major origin of inhibitory vestibular input to the noradrenergic neurons of the locus coeruleus, and that the ventrolateral medulla plays an important role in the vestibulo-autonomic response.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Release of excitatory and inhibitory amino acids from the locus coeruleus of conscious rats by cardiovascular stimuli and various forms of acute stress

The release of amino acids in the locus coeruleus (LC) of conscious, freely moving rats was studied in time periods of 3 min by use of push-pull superfusion under basal conditions and during application of various experimental stimuli known to influence the activity of the LC-noradrenergic system. Tail pinch for 3 min led immediately to a pronounced tetrodotoxin-sensitive increase in the release rates of the excitatory amino acids (EAA) glutamate (Glu) and aspartate (Asp) and to moderate increases in GABA and taurine (Tau) outflow. Immobilization stress for 9 min elevated the release of the EAA Glu and Asp, as well as that of the inhibitory amino acid GABA to a similar extent. A fall of blood pressure (BP) by nitroprusside or haemorrhage slightly enhanced the release rates of Glu and Asp. Noradrenaline-induced rise in BP, as well as hypervolaemia increased the release rate of GABA, but did not influence the release rates of Glu, Asp, Tau and arginine (Arg). The results provide direct evidence that the amino acid release pattern in the LC of conscious rats differs in response to various stimuli, according to the modality of the stimulus. A functional significance of excitatory and inhibitory amino acids in the regulation of LC activity during stress and haemodynamic changes is suggested.