Rostral ventrolateral medullary neurons projecting to locus coeruleus have cardiorespiratory inputs

The anatomical and functional relationship between the P3 and autonomic components of the orienting response

Many psychophysiologists have noted the striking similarities between the antecedent conditions for the P3 component of the event-related potential and the orienting response: both are typically elicited by salient, unexpected, novel, task-relevant, and other motivationally significant stimuli. Although the close coupling of the P3 and orienting response has been well documented, the neural basis and functional role of this relationship is still poorly understood. Here we propose that the simultaneous occurrence of the P3 and autonomic components of the orienting response reflects the co-activation of the locus coeruleus-norepinephrine system and the peripheral sympathetic nervous system by their common major afferent: the rostral ventrolateral medulla, a key sympathoexcitatory region. A comparison of the functional significance of the locus coeruleus-norepinephrine system and the peripheral sympathetic nervous system suggests that the P3 and orienting response reflect complementary cognitive and physical contributions to the mobilization for action following motivationally significant stimuli.

Afferent and efferent connections of C1 cells with spinal cord or hypothalamic projections in mice

The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.

Hypotensive and vasorelaxant effects induced by the ethanolic extract of the Mimosa caesalpiniifolia Benth. (Mimosaceae) inflorescences in normotensive rats

ETHNOPHARMACOLOGICAL RELEVANCE

Caatinga is highly influenced by its seasonality. This species is endemic in the northeastern region, which is rich in plants with pharmacological potential. Many of these plants are used by the population and some of them have confirmed pharmacological properties. Mimosa caesalpiniifolia Benth. (Mimosaceae) is a native plant from northeastern Brazil׳s caatinga, popularly known as sabiá and cascudo. The tea from the inflorescence of this species is used by the population of the semi-arid for the treatment of hypertension, and the utilization of the plant bark for the staunching of bleedings and wound washing in order to prevent inflammation; also, the ingestion of the bark infusion is used in the treatment of bronchitis. However, its pharmacological effects and mechanisms of action have not yet been studied. The aim of the present study was to determine the effect of the ethanolic extract of M. caesalpiniifolia on the cardiovascular system in rats.

MATERIAL AND METHODS

In a study for the assessment of the hypotensive effect of the extract, the polyethylene catheters were inserted in the aorta artery and inferior vena cava for the measurement of the arterial pressure and heart rate. When intragastric administration was performed, only one catheter was implanted in the abdominal aorta. In studies for the vasorelaxant activity, mesenteric arterial rings (1-2mm) were used: they were kept in Tyrode׳s solution (95% O2 and 5% CO2) and submitted to tension of 0.75 g/f for 1h. The results were expressed as mean ± S.E.M., significant to the values of p<0.05.

RESULTS

The administration of the doses through venous pathway (6.25; 12.5 and 25mg/kg, i.v.) promoted hypotension followed by bradycardia in the higher doses. The pre-treatment with atropine (2mg/kg, i.v.) interrupted both the hypotension and the bradycardia; with hexamethonium, hypotension was reverted and bradycardia was attenuated. While the administration of tea/flowers (25mg/kg i.v.) also promoted a following section of hypotension, a slight increase in heart rate was observed. When administered orally, MC-EtOH/flowers (100mg/kg, v.o.) promoted a decrease in the arterial pressure from 90 min on, without a significant alteration in the heart rate in relation to the control. In the in vitro study, a pharmacological trial was performed with the extracts obtained from parts of the species M. caesalpiifolia (leaves, bark, fruit and inflorescences). Among all extracts tested, the ethanolic extract from the inflorescences (MC-EtOH/flowers) presented higher vasorelaxant potency in relation to the other parts of the plant. Henceforth, MC-EtOH/flowers was used in the sequence. In mesenteric preparations pre-contracted with phenylephrine (10(-5)M), the MC-EtOH/flowers (0.1-750 µg/ml) promoted vasorelaxant effect regardless of the vascular endothelium. MC-EtOH/flowers inhibited the contractions induced by the cumulative addition of phenylephrine (10(-9)-10(-5)mol/l) or CaCl2 (10(-6)-3 × 10(-2)M), in a concentration-dependent way. In contractions induced by S(-)Bay K 8644, a Cav-L activator, the MC-EtOH/flowers promoted concentration-dependent relaxation, corroborating previous results.

CONCLUSION

The tea of flowers of M. caesalpiniifolia promotes hypotension and tachycardia, whereas ethanolic extract (MC-EtOH) promotes hypotension and bradycardia involving the participation of the muscarinic and ganglionic pathways, as well as vasorelaxant action involving the Ca(2+) influx inhibition blockade.

Sleep-wake control of the upper airway by noradrenergic neurons, with and without intermittent hypoxia

Hypoglossal (XII) motoneurons innervate muscles of the tongue whose tonic and inspiratory modulated activity protects the upper airway from collapse in patients affected by the obstructive sleep apnea (OSA) syndrome. Both norepinephrine and serotonin provide wakefulness-related excitatory drives that maintain activity in XII motoneurons, with the noradrenergic system playing a particularly prominent role in rats. When noradrenergic and serotonergic drives are antagonized, no further decline of XII nerve activity occurs during pharmacologically induced rapid eye movement (REM) sleep-like state. This is the best evidence to date that, at least in this model, the entire REM sleep-related decline of upper airway muscle tone results from withdrawal of these two excitatory inputs. A major component of noradrenergic input to XII motoneurons originates from pontine noradrenergic neurons that have state-dependent patterns of activity, maximal during wakefulness, and minimal, or absent during REM sleep. Our data suggest that not all ventrolateral medullary catecholaminergic neurons follow this pattern, with adrenergic C1 neurons probably increasing their activity during REM sleep. When rats are subjected to chronic-intermittent hypoxia, noradrenergic drive to XII motoneurons is increased by mechanisms that include sprouting of noradrenergic terminals in the XII nucleus, and increased expression of α1-adrenoceptors; an outcome that may underlie the elevated baseline activity of upper airway muscles during wakefulness in OSA patients.

Evidence that adrenergic ventrolateral medullary cells are activated whereas precerebellar lateral reticular nucleus neurons are suppressed during REM sleep

Rapid eye movement sleep (REMS) is generated in the brainstem by a distributed network of neurochemically distinct neurons. In the pons, the main subtypes are cholinergic and glutamatergic REMS-on cells and aminergic REMS-off cells. Pontine REMS-on cells send axons to the ventrolateral medulla (VLM), but little is known about REMS-related activity of VLM cells. In urethane-anesthetized rats, dorsomedial pontine injections of carbachol trigger REMS-like episodes that include cortical and hippocampal activation and suppression of motoneuronal activity; the episodes last 4–8 min and can be elicited repeatedly. We used this model to determine whether VLM catecholaminergic cells are silenced during REMS, as is typical of most aminergic neurons studied to date, and to investigate other REMS-related cells in this region. In 18 anesthetized, paralyzed and artificially ventilated rats, we obtained extracellular recordings from VLM cells when REMS-like episodes were elicited by pontine carbachol injections (10 mM, 10 nl). One major group were the cells that were activated during the episodes (n = 10). Their baseline firing rate of 3.7±2.1 (SD) Hz increased to 9.7±2.1 Hz. Most were found in the adrenergic C1 region and at sites located less than 50 µm from dopamine β-hydroxylase-positive (DBH⁺) neurons. Another major group were the silenced or suppressed cells (n = 35). Most were localized in the lateral reticular nucleus (LRN) and distantly from any DBH⁺ cells. Their baseline firing rates were 6.8±4.4 Hz and 15.8±7.1 Hz, respectively, with the activity of the latter reduced to 7.4±3.8 Hz. We conclude that, in contrast to the pontine noradrenergic cells that are silenced during REMS, medullary adrenergic C1 neurons, many of which drive the sympathetic output, are activated. Our data also show that afferent input transmitted to the cerebellum through the LRN is attenuated during REMS. This may distort the spatial representation of body position during REMS.

C1 neurons excite locus coeruleus and A5 noradrenergic neurons along with sympathetic outflow in rats

C1 neurons activate sympathetic tone and stimulate the hypothalamic–pituitary–adrenal axis in circumstances such as pain, hypoxia or hypotension. They also innervate pontine noradrenergic cell groups, including the locus coeruleus (LC) and A5. Activation of C1 neurons reportedly inhibits LC neurons; however, because these neurons are glutamatergic and have excitatory effects elsewhere, we re-examined the effect of C1 activation on pontine noradrenergic neurons (LC and A5) using a more selective method. Using a lentivirus that expresses channelrhodopsin2 (ChR2) under the control of the artificial promoter PRSx8, we restricted ChR2 expression to C1 neurons (67%), retrotrapezoid nucleus neurons (20%) and cholinergic neurons (13%). The LC contained ChR2-positive terminals that formed asymmetric synapses and were immunoreactive for vesicular glutamate transporter type 2. Low-frequency photostimulation of ChR2-expressing neurons activated LC (38 of 65; 58%) and A5 neurons (11 of 16; 69%) and sympathetic nerve discharge. Locus coeruleus and A5 inhibition was not seen unless preceded by excitation. Locus coeruleus activation was eliminated by intracerebroventricular kynurenic acid. Stimulation of ChR2-expressing neurons at 20 Hz produced modest increases in LC and A5 neuronal discharge. In additional rats, the retrotrapezoid nucleus region was destroyed with substance P–saporin prior to lentivirus injection into the rostral ventrolateral medulla, increasing the proportion of C1 ChR2-expressing neurons (83%). Photostimulation in these rats activated the same proportion of LC and A5 neurons as in control rats but produced no effect on sympathetic nerve discharge owing to the destruction of bulbospinal C1 neurons. In conclusion, low-frequency stimulation of C1 neurons activates pontine noradrenergic neurons and sympathetic nerve discharge, possibly via the release of glutamate from monosynaptic C1 inputs.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

CNS arousal mechanisms bearing on sex and other biologically regulated behaviors

It now seems possible to move beyond analyzing only the mechanisms for specific sexual behaviors to the analysis of 'generalized arousal' that underlies all motivated behaviors. Our science has advanced sufficiently to attack mechanisms linking specific motivations to these general arousal mechanisms that intrinsically activate all biologically-regulated behaviors including ingestive behaviors. Learning from the well-developed reproductive behavior paradigm, we know that sex hormone effects on hypothalamic neurons have been studied to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been worked out, and several functional genomic regulations have been discovered. Here we focus for the first time on three chemical systems that signal 'generalized arousal' and which impact hormone-dependent hypothalamic neurons of importance to sexual arousal: histamine, norepinephrine and enkephalin. Progress in linking generalized arousal to specific motivational mechanisms is reviewed.

Generalized arousal of mammalian central nervous system

A fundamental capacity of the mammalian CNS is becoming amenable to study with the techniques of functional genomics. Emphasized in this review are ascending connections from the medullary reticular formation and descending connections from the paraventricular nucleus of the hypothalamus. In particular, sex hormone effects on neurons allow us to relate generalized arousal to a specific form of arousal which is required for reproductive behaviors.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Does the brain noradrenaline network mediate the effects of the CO2 challenge?

The inhalation of carbon dioxide (CO2) is commonly used in patients and volunteers as a means of producing anxiety or panic. It is generally believed that patients with panic disorder are more vulnerable to the effects of CO2 than patients with other anxiety disorders or healthy volunteers and there is speculation and debate as to the mechanism for this apparent sensitivity. Recent work from our group has shown that a single inhalation of 35% CO2 activates the hypothalamic-pituitary-adrenocortical (HPA) axis, increases blood pressure (BP) and increases subjective fear responses in healthy volunteers. Correlation analyses reveal a relationship between the changes in BP and the cortisol increase. These findings led us to postulate that a common mechanism may mediate these and the subjective responses to inhalation of CO2. We propose that the noradrenergic system, particularly the locus coeruleus (LC), but including the A1 and A2 cell groups, may be a key mediator of these responses. This article examines the evidence and discusses the results of studies from our laboratory in relation to a neuroanatomical model centring on the LC.

Activation of nitric oxide-producing neurons in the brain stem during cardiac sympathoexcitatory reflexes in the cat

Our previous studies have shown that selective inhibition of nitric oxide in the brain reduces pressor responses to activation of cardiac sympathetic afferents, thus suggesting that nitric oxide is involved in central regulation of cardiac-cardiovascular sympathoexcitatory reflexes. Central neural regions in which nitric oxide-producing neurons are activated during these reflexes have not been well characterized. In the present study, we located nitric oxide-producing neurons in the brain stem activated by the input from cardiac sympathetic afferents by detecting colocalization of c-Fos immunoreactivity with nitric oxide synthesizing neurons. Expression of c-Fos has been used as a marker of activated neurons. Nitric oxide-producing neurons were identified by histochemical labeling of nicotine adenine dinucleotide phosphate-diaphorase (NADPH-d). In anesthetized cats with bilateral barodenervation and cervical vagotomy, bradykinin (1-10 microg in 0.1 ml; n=6) was applied to the anterior surface of the left ventricle six times every 20 minutes. Repetitive application of bradykinin consistently increased blood pressure, while the vehicle for bradykinin (0.9% saline, n=5) produced no responses. A substantial fraction (6-27%) of NADPH-d positive neurons displayed Fos immunoreactivity in the nucleus of the solitary tract, caudal and rostral ventral lateral medulla, lateral tegmental field, locus coeruleus and parabrachial nucleus in the bradykinin-treated cats. However, either no or rare (1-4%) double-labeled cells were found in these regions in control animals. Thus, nitric oxide-producing neurons are activated in several regions in the brain stem during stimulation of cardiac sympathetic afferents by bradykinin. Our data suggest that nitric oxide functions as a neurotransmitter/modulator in these areas to regulate the cardiac sympathoexcitatory reflexes.

Central pathways in the pons and midbrain involved in cardiac sympathoexcitatory reflexes in cats

Activation of cardiac sympathetic afferents elicits pain and excitatory cardiovascular reflexes including acute hypertension and tachyarrhythmias. Our previous studies have shown that specific regions in the medulla, such as the nucleus of solitary tract and ventrolateral medulla, are involved in central regulation of cardiac sympathoexcitatory reflexes. However, the contributions of supramedullary nuclei to these reflexes have not been characterized. In the present study, we located activated neurons in the pons and midbrain induced by inputs from cardiac sympathetic afferents by detecting their c-Fos immunoreactivity. In anesthetized cats with bilateral carotid denervation and cervical vagotomy, epicardial application of bradykinin (1-10 microg, in 0.1 ml; n=7) was performed on the anterior surface of the left ventricle six times, every 20 min. Repetitive application of bradykinin caused consistent excitatory cardiovascular reflexes characterized by increases in blood pressure and heart rate. No responses were evoked by the vehicle for bradykinin (0.9% saline, n=7). Compared to control cats, c-Fos immunoreactive cells were significantly increased (P<0.05) in the rostral pons, caudal and intermediate midbrain in the bradykinin-treated cats. The specific areas activated include the parabrachial nucleus, Kölliker-Fuse nucleus, locus coeruleus, dorsal nucleus of raphe, and dorsal, lateral and ventrolateral periaqueductal gray. From these results we suggest that cardiovascular-related regions in the pons and midbrain form part of a long loop in central integration of cardiac sympathoexcitatory reflexes.

Optical imaging of the ventral medullary surface across sleep-wake states

We hypothesized that spontaneous activity declines over widespread areas of the cat ventral medullary surface (VMS) during rapid eye movement (REM) sleep. We assessed neural and hemodynamic activity, measured as changes in reflected 660- and 560-nm wavelength light, from the VMS during sleep and waking states in five adult, unrestrained cats and in two control cats. Relative to quiet sleep, overall activity declined, and variability, assessed by standard deviation, increased by 25% during REM sleep. Variability in activity during waking also increased by 45% over quiet sleep, but mean activity was unchanged. REM sleep onset was preceded by a reduction in the hemodynamic signal from 5 to 60 s before neural activity decline. The activity decline during REM sleep, previously noted in the goat rostral VMS, extends to intermediate VMS areas of the cat and differs from most neural sites, such as the cortex, hippocampus, and thalamus, which increase activity during REM sleep. The activity decline during REM sleep has the potential to modify VMS responsiveness to baroreceptor and chemoreceptor challenges during the REM state.

Catechol activation in rat rostral ventrolateral medulla after systemic isocapnic metabolic acidosis

The catechol signal recorded using in vivo voltammetry within the rat rostral ventrolateral medulla (RVLM) can be interpreted as a catechol-specific index of the integrated activity of RVLM adrenergic barosensitive bulbospinal and nonbulbospinal neurons. To test the hypothesis that systemic acidosis leads to the activation of RVLM adrenergic neurons, the RVLM catechol signal was observed in rats after mild systemic acidosis (pH 7.20-7.25 for 30 min) induced by 1 M HCl under halothane anesthesia, controlled mechanical ventilation, and continuous infusion of Ringer lactate. Particular attention was paid to ensure that changes in mean arterial pressure (MAP) were <15 mmHg during HCl challenge. Saline administration was not associated with any significant change in all considered variables (n = 5). Mild isocapnic systemic acidosis was associated with an increase in catechol signal (n = 5), irrespective of carotid sinus nerve section (n = 5). In keeping with the aim of the study, there were minor (<15 mmHg) but significant changes in MAP among saline, intact, and deafferented groups. Changes in heart rate were not significant. In conclusion, a catechol activation is observed in the RVLM when arterial pressure is maintained during isocapnic systemic metabolic acidosis. This catechol activation appears primarily centrally mediated. Therefore, adrenergic RVLM neurons may relay inputs from the central respiratory generator to the sympathetic system and/or act as chemosensors for H+ next to the surface of the ventrolateral medulla.

Antagonist precipitated clonidine withdrawal in rat: effects on locus coeruleus neurons, sympathetic nerves and cardiovascular parameters

The goal of the present study was to examine the effect of clonidine withdrawal on the neural control of blood pressure. Rats were treated for 7-13 days with clonidine via osmotic minipumps (200 microg kg(-1) day(-1), s.c.). Controls received saline or were sham operated. Withdrawal was precipitated by the alpha2-adrenergic receptor (alpha2-AR) antagonist atipamezole. Most experiments were done under halothane anesthesia. Chronic treatment with clonidine did not change mean arterial pressure (MAP) or heart rate (HR) but raised femoral artery resistance and the activity of locus coeruleus neurons slightly. Atipamezole given to rats treated chronically with clonidine produced the following effects: no change in MAP, severe tachycardia, sustained increase in splanchnic sympathetic nerve discharge (SND; +75 +/- 13%), transient increase in lumbar SND (+23 +/- 7%), ON-OFF activity pattern in the locus coeruleus (LC). The ON phase of LC activity was synchronized with upswings of SND and with small changes in MAP. A second alpha2-AR antagonist, methoxyidazoxan, produced effects identical to those of atipamezole. Atipamezole given to control rats produced no effect on MAP, HR, SND or LC activity. Atipamezole reversed the hypotension, sympathoinhibition and bradycardia produced by acute administration of clonidine. In awake rats treated chronically with clonidine, atipamezole did not change MAP but produced arterial pressure lability and tachycardia. In conclusion, under anesthesia, selective alpha2-AR antagonists elicit a clonidine withdrawal syndrome that displays autonomic characteristics reminiscent of the spontaneous withdrawal syndrome found in awake rats. The most prominent features of this syndrome are tachycardia, sympathoactivation, lack of hypertension and an oscillating activity pattern of brainstem neurons leading to abrupt changes in SND and in MAP.

Monosynaptic projections from the nucleus tractus solitarii to C1 adrenergic neurons in the rostral ventrolateral medulla: comparison with input from the caudal ventrolateral medulla

The rostral ventrolateral medulla (RVL) contains reticulospinal adrenergic (C1) neurons that are thought to be sympathoexcitatory and that form the medullary efferent limb of the baroreceptor reflex pathway. The RVL receives direct projections from two important autonomic regions, the caudal ventrolateral medulla (CVL) and the nucleus tractus solitarii with immunocytochemical identification of C1 adrenergic neurons in the RVL to compare the morphology of afferent input from these two autonomic regions into the RVL. NTS (n = 203) and CVL (n = 380) efferent terminals had similar morphology and vesicular content, but CVL efferent terminals were slightly larger than NTS efferent terminals. Overall, efferent terminals from either region were equally likely to contact adrenergic neurons in the RVL (21% for NTS, 25% for CVL). Although efferents from both regions formed both symmetric and asymmetric synapses, NTS efferent terminals were statistically more likely to form asymmetric synapses than CVL efferent terminals. CVL efferent terminals were more likely to contact adrenergic somata than were NTS efferents, which usually contacted dendrites. These findings 1) support the hypothesis that a portion of NTS efferents to the RVL may be involved in sympathoexcitatory, e.g., chemoreceptor, reflexes (via asymmetric synapses), whereas those from the CVL mediate sympathoinhibition (via symmetric synapses); and 2) provide an anatomical substrate for differential postsynaptic modulation of C1 neurons by projections from the NTS and CVL. With their more frequent somatic localization, CVL inhibitory inputs may be more influential than excitatory NTS inputs in determining the discharge of RVL neurons.

A comparative study of pre-sympathetic and Bötzinger neurons in the rostral ventrolateral medulla (RVLM) of the rat

The aim of this study was to investigate the degree of functional and anatomical overlap between two major neuronal subpopulations in the rostral ventrolateral medulla: pre-sympathetic (sympathoexcitatory) neurons, and expiratory neurons of the Bötzinger complex. Extracellular recordings were made with dye-filled microelectrodes in pentobarbital anesthetized, paralyzed and artificially ventilated adult Wistar rats. Tests applied included stimulation of baroreceptor afferents, activation of peripheral chemoreceptors and lung stretch receptors, changes in central respiratory drive with hyper- or hypoventilation, nociceptive stimulation, and antidromic stimulation from the T2 segment of the spinal cord or medulla oblongata at obex level. The two groups of neurons showed different patterns of spontaneous activity and generally different responses to these stimuli. The recording positions showed some overlap, but the majority of Bötzinger neurons were dorsolateral to pre-sympathetic neurons. There was a large overlap between the location of pre-sympathetic neurons and the lateral part of the C1 adrenergic group, but only a small overlap between these adrenergic neurons and Bötzinger neurons. These results indicate that the anatomically adjacent pre-sympathetic and Bötzinger expiratory neurons form two functionally distinct neuronal subpopulations.

Barosensitive neurons in the rostral ventrolateral medulla of the rat in vivo: morphological properties and relationship to C1 adrenergic neurons

The aim of this study, conducted in anaesthetized rats, was to examine the morphology of barosensitive neurons in the rostral ventrolateral medulla and their immunoreactivity for a catecholamine synthesizing enzyme, tyrosine hydroxylase. Thirty neurons displaying inhibitory postsynaptic potentials following stimulation of the aortic depressor nerve were intracellularly labelled with Lucifer Yellow or Neurobiotin. Some of these neurons could be excited antidromically from the second thoracic segment of the spinal cord, with conduction velocities of spinal axons ranging from 1.9 to 7.2 m/s. The filled somas were found immediately caudal to the facial nucleus and ventral or ventromedial to compact formation of the nucleus ambiguus. Some dendrites reached the ventral medullary surface. Axons usually projected dorsomedially and then made a sharp rostral and/or caudal turn. The caudally projecting axon could, in some cases, be followed to the first cervical segment of the spinal cord. Seven cells issued fine axon collaterals on the ipsilateral side. These were identified mainly in two areas: in the rostral ventrolateral medulla (or immediately dorsomedial to that region), and within the dorsal vagal complex. Seven of 27 examined cells (26%) were tyrosine hydroxylase-immunoreactive and were classified as C1 adrenergic neurons. No clear relationship was found between the presence or absence of adrenergic phenotype and the morphology of filled cells. However, the amplitude of aortic nerve-evoked inhibitory postsynaptic potentials was significantly larger in tyrosine hydroxylase-positive neurons. Possible reasons for the low percentage of barosensitive cells with tyrosine hydroxylase immunoreactivity found in this study, in comparison with previously published estimates, are discussed. This is the first study describing the morphology of neurons in this part of the medulla identified as barosensitive in vivo, and directly demonstrating adrenergic phenotype in a subset of these neurons.

Abdominal vagal stimulation excites bulbospinal barosensitive neurons in the rostral ventrolateral medulla

We used extracellular recordings to examine the central pathway whereby electrical stimulation of abdominal vagal afferents elevates arterial pressure in the rabbit. Bulbospinal neurons in the rostral ventrolateral medulla were identified by antidromic activation from the dorsolateral funiculus of the thoracic spinal cord. Their barosensitivity was assessed by their response to intravenous phenylephrine and by their cardiac cycle-related rhythmicity. We used peristimulus histogram procedures to assess the effect of electrical stimulation of abdominal vagal afferents on the discharge rate of these neurons. Electrical stimulation (one to three pulses) activated 98 of 123 neurons tested (80%), had no effect on 22 neurons (18%) and inhibited the remaining three neurons. Latency to peak excitation was 228 +/- 3 ms, indicating that the conduction velocity of the vagal afferents was about 0.6 m/s, in the unmyelinated fibre range. Lower oesophageal distension with a balloon excited 22 of 48 neurons (46%), inhibited 12 neurons (25%) and had no effect on the remaining 14 cells (29%). Vagally induced excitation was reduced by aortic depressor nerve stimulation in nine of 13 neurons. Lightly touching the animal's back and legs had no effect on 56 of 60 neurons. Nociceptive stimuli failed to affect 47 of 60 neurons tested. No excitation was seen with electrical stimulation of the sciatic or central ear nerves. Our study identifies a robust excitatory input from the abdominal vagus to bulbospinal barosensitive neurons in the rostral ventrolateral medulla. Relevant physiological stimuli include lower oesophageal distension. The pathways may be relevant to cardiovascular changes which accompany upper gastrointestinal function.

Directionally specific changes in arterial pressure induce differential patterns of fos expression in discrete areas of the rat brainstem: a double-labeling study for Fos and catecholamines

Although the nucleus tractus solitarii (NTS) has been established as the primary site of synaptic integration for the baroreceptor reflex, the higher-order pathways responsive to, and mediating, changes in vasomotor tone are not well characterized. We used immunohistochemistry to determine the distribution of cells expressing the Fos protein following pharmacologically induced, directionally specific changes in arterial pressure. The goal of this investigation was to determine if this immediate early gene product is differentially expressed in neurons of the rat brainstem following increased (pressor) versus decreased (depressor) arterial blood pressure (AP). Because brainstem catecholaminergic (CA) cell groups have been implicated in cardiovascular regulation, a double-labeling immunohistochemical procedure was used to examine the distribution of Fos in CA cells. Animals received continuous intravenous infusion of either a vasoconstrictor (l-phenylephrine hydrochloride), a vasodilator (sodium nitroprusside), or physiological saline. Extensive Fos-like immunoreactivity (FLI) was induced in both the pressor and depressor conditions in the NTS, caudal ventrolateral medulla (CVLM), rostral ventrolateral medulla (RVLM), A5, locus coeruleus (LC), Kolliker-Fuse, and parabrachial nucleus (PBN). These regions have all been implicated in central cardiovascular regulation. There were differences in the anatomical distribution of Fos-positive cells along the rostrocaudal axis of CVLM in the pressor and depressor conditions. Specifically, increased AP induced significantly more FLI cells within the rostral aspects of CVLM, whereas decreased AP resulted in a significantly greater number of FLI cells within the caudal CVLM. This result suggests that selective vasomotor responses differentially engaged discrete subsets of neurons within this brainstem region. Overall, approximately 50% of CA-immunoreactive cells were also FLI (CA-FLI) in the A1, A5, and A7 regions. Interestingly, increased AP produced significantly more CA-FLI double-labeled cells within the caudal than rostral A1 compared with depressor and control groups. Additionally, increased AP yielded significantly less CA-FLI double-labeled cells within the caudal A2 region. This suggests that CA barosensitive neurons in the CVLM/A1 and NTS/A2 regions are functionally segregated along the rostrocaudal axis of these structures. While twice as many PNMT-FLI double-labeled neurons were found in the C1-C3 regions following vasomotor changes versus saline control, there were no differences in the numbers or anatomical locations of labeled cells between pressor versus depressor groups. The results of this study indicate that (1) tonic changes in AP induce robust Fos expression in brainstem cardiovascular areas and (2) neurons responsive to specific directional changes in arterial pressure are segregated in some brainstem regions.

Spatially and temporally differentiated patterns of c-fos expression in brainstem catecholaminergic cell groups induced by cardiovascular challenges in the rat

Brainstem catecholaminergic neurons have been implicated as mediating adaptive autonomic and neuroendocrine responses to cardiovascular challenges. To clarify the nature of this involvement, immuno- and hybridization histochemical methods were used to follow c-fos expression in these neurons in response to acute stimuli that differentially affect blood pressure and volume. From low basal levels, hypotensive hemorrhage (15%) provoked a progressive increase in the number and distribution of Fos-immunoreactive (ir) nuclei in the nucleus of the solitary tract (NTS), the A1 and C1 cell groups of the ventrolateral medulla, and in the pontine A5, locus coeruleus, and lateral parabrachial cell groups peaking at 2.0-2.5 hours after the challenge. Fos-ir ventrolateral medullary neurons, subsets of which were identified as projecting to the paraventricular hypothalamic nucleus or spinal cord, were predominantly aminergic, whereas most of those in the NTS were not. Infusion of an angiotensin II antagonist blunted hemorrhage-induced Fos expression in the area postrema, and attenuated that seen elsewhere in the medulla and pons. Nitroprusside-induced isovolemic hypotension yielded a pattern of c-fos induction similar to that seen following hemorrhage, except in the area postrema and the A1 cell group, where the response was muted or lacking. Phenylephrine-induced hypertension stimulated a restricted pattern of c-fos expression, largely limited to induced hypertension stimulated a restricted pattern of c-fos expression, largely limited to non-aminergic neurons, whose distribution in the NTS conformed to the termination patterns of primary baroreceptor afferents, and in the ventrolateral medulla overlapped in part with those of vagal cardiomotor and depressor neurons. These findings underscore the importance of brainstem catecholaminergic neurons in effecting integrated homeostatic responses to cardiovascular challenges and their ability to responding strategically to specific modalities of cardiovascular information. They also foster testable predictions as to effector neuron populations that might be recruited to respond to perturbations in individual circulatory parameters.

Baroreceptor inhibition of the locus coeruleus noradrenergic neurons

The locus coeruleus is involved in the regulation of blood pressure. The present study was undertaken to address the question of how the blood pressure, in turn, changes the activity of the locus coeruleus neurons via the action of baroreceptors. In chloralose- and urethane-anesthetized rats, the central cut end of the aortic depressor nerve, which does not contain chemoreceptor afferents in this species, was stimulated electrically to excite baroreceptor afferents after bilateral vagotomy and sectioning of the carotid sinus nerve. Single train-pulse stimulation of the aortic depressor nerve provoked the inhibition of ongoing activity in 48% of locus coeruleus neurons tested, but 30% of them responded by excitation with subsequent inhibition. However, when the train-pulse stimulation was repeated with a frequency of 5 Hz, which is close to that of the heartbeat in the rat, all neurons were markedly inhibited. Another series of experiments was conducted in vagotomized and carotid sinus nerve-sectioned rats with intact aortic depressor nerves. When blood pressure was elevated by an intravenous injection of a pressor agent, methoxamine or angiotensin II, or by rapid blood loading, ongoing activities of all locus coeruleus neurons tested were suppressed. In contrast, intravenously injected nitroprusside, a depressor agent, increased the activity of locus coeruleus neurons. In rats with all nerves preserved, rapid blood loading markedly inhibited the activity of these neurons. Such inhibition was partially but significantly attenuated by bilateral sectioning of the aortic depressor nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

The medullary rostral ventrolateral pressor region: an electrophysiological study in decerebrate rat

Decerebrate rat was used to study the discharge pattern of barosensitive neurons. This preparation avoids general anesthesia and suppresses painful sensations of the animal. Twenty-eight spontaneously active units were recorded in the rostral ventrolateral region of the medulla (RVLM). Six units had projections to the spinal cord (bulbospinal) and 22 were not antidromically activated by spinal cord stimulation (NAA). Transient hypertension induced by intravenous injection of noradrenaline depressed the activity of 21 units, and increased activity of 7, regardless of axonal destination. Unlike anesthetized rat, the decerebrate rat possesses spontaneously active neurons with excitatory response to hypertension.

Projections from inspiratory neurons of the ventral respiratory group to the subretrofacial nucleus of the cat

Arterial blood pressure and the activity of many sympathetic nerves are known to be affected by changes in central respiratory activity. The central neurons responsible for this respiratory modulation are unknown. In the present study we have labelled inspiratory neurons (n = 24) in the rostral ventral respiratory group and Bötzinger complex in the medulla oblongata of the cat using intracellular injection of biocytin. The filled neurons were examined to see if they had axonal projections to the subretrofacial nucleus, an important brainstem nucleus in the tonic and reflex control of blood pressure. The subretrofacial nucleus was identified histologically as a cluster of neurons in the rostral ventrolateral medulla, some of which are tyrosine hydroxylase immunoreactive. Varicose axons arising from labelled inspiratory neurons were mostly found dorsal to this cluster, within the area corresponding to the Bötzinger complex. A small number of axon varicosities were seen in the subretrofacial nucleus. The results suggest that a part of the respiratory modulation of sympathetic nerve activity may be due to a direct synaptic input from inspiratory neurons of the ventral respiratory group to neurons of the subretrofacial nucleus.