Cardiac Vagal Preganglionic Neurons Research Articles

Control of Heart Rate by Nucleus Ambiguus Cardiovagal Neurons

Heart rate is a well-established indicator of overall health and well-being, yet little is known about the neurons that control it. Although the heart has its own pacemaker, heart rate is largely regulated by the interplay between sympathetic activities (via sympathoexcitatory neurons) and parasympathetic activities (via cardiac vagal preganglionic neurons (CVNs)). The nucleus ambiguus (nAmb), a region in the medullary reticular formation of the brainstem, is a major center of parasympathetic cardiac control and houses the majority of CVNs, along with other neurons that control respiratory functions (bronchoconstriction, bronchosecretion) and upper airway and esophageal muscles. Because of the diversity of neuron types in the nAmb, it is challenging to isolate CVNs for study, which greatly limits what we know about their gene expression, synaptic circuitry, and specific roles in cardiac function. To address these issues, we characterized cardiovagal neurons based on transcriptome-wide mRNA expression, synaptic circuitry, and sufficiency for heart rate control. Our single-cell transcriptomics analysis of 235 nAmb neurons pointed to three molecularly distinct neuron subtypes (marked by the genes Crhr2, Vipr2, and Tbx3) in the nAmb, which we validated and anatomically mapped by in situ detection of marker transcripts. We then used marker-driven genetic technology to fluorescently label the axon projections of Tbx3 nAmb neurons and imaged them in optically cleared heart samples. Our results show that these neurons innervate multiple sites in the atria and ventricles, consistent with multiple roles in cardiac function. Similar synaptic circuitry studies of the Crhr2 nAmb and Vipr2 nAmb neurons showed that they innervate the esophagus and larynx, respectively, but do not innervate the heart. Furthermore, using an intersectional optogenetics approach to specifically manipulate the activity of nAmb neuron subtypes in vivo, we have shown that activating the Tbx3 nAmb neurons in both freely behaving and anesthetized mice dramatically and immediately decreases heart rate (from 430 bpm to 160 bpm) as determined via EKG readings, whereas activating other subtypes of nAmb neurons did not affect heart rate. This effect on heart rate was abolished by administration of atropine, a muscarinic receptor antagonist. Together, these studies not only highlight the impact of Tbx3 nAmb neurons on heart rate, but also reveal a distinct molecular, anatomical, and functional organization within the nAmb.

Mechanisms Contributing to the Generation of Mayer Waves.

Mayer waves may synchronize overlapping propriobulbar interneuronal microcircuits constituting the respiratory rhythm and pattern generator, sympathetic oscillators, and cardiac vagal preganglionic neurons. Initially described by Sir Sigmund Mayer in the year 1876 in the arterial pressure waveform of anesthetized rabbits, authors have since extensively observed these oscillations in recordings of hemodynamic variables, including arterial pressure waveform, peripheral resistance, and blood flow. Authors would later reveal the presence of these oscillations in sympathetic neural efferent discharge and brainstem and spinal zones corresponding with sympathetic oscillators. Mayer wave central tendency proves highly consistent within, though the specific frequency band varies extensively across, species. Striking resemblance of the Mayer wave central tendency to the species-specific baroreflex resonant frequency has led the majority of investigators to comfortably presume, and generate computational models premised upon, a baroreflex origin of these oscillations. Empirical interrogation of this conjecture has generated variable results and derivative interpretations. Sinoaortic denervation and effector sympathectomy variably reduces or abolishes spectral power contained within the Mayer wave frequency band. Refractorines of Mayer wave generation to barodeafferentation lends credence to the hypothesis these waves are chiefly generated by brainstem propriobulbar and spinal cord propriospinal interneuronal microcircuit oscillators and likely modulated by the baroreflex. The presence of these waves in unitary discharge of medullary lateral tegmental field and rostral ventrolateral medullary neurons (contemporaneously exhibiting fast sympathetic rhythms [2–6 and 10 Hz bands]) in spectral variability in vagotomized pentobarbital-anesthetized and unanesthetized midcollicular (i.e., intercollicular) decerebrate cats supports genesis of Mayer waves by supraspinal sympathetic microcircuit oscillators. Persistence of these waves following high cervical transection in vagotomized unanesthetized midcollicular decerebrate cats would seem to suggest spinal sympathetic microcircuit oscillators generate these waves. The widespread presence of Mayer waves in brainstem sympathetic-related and non-sympathetic-related cells would seem to betray a general tendency of neurons to oscillate at this frequency. We have thus presented an extensive and, hopefully cohesive, discourse evaluating, and evolving the interpretive consideration of, evidence seeking to illumine our understanding of origins of, and insight into mechanisms contributing to, the genesis of Mayer waves. We have predicated our arguments and conjectures in the substance and matter of empirical data, though we have occasionally waxed philosophical beyond these traditional confines in suggesting interpretations exceeding these limits. We believe our synthesis and interpretation of the relevant literature will fruitfully inspire future studies from the perspective of a more intimate appreciation and conceptualization of network mechanisms generating oscillatory variability in neuronal and neural outputs. Our evaluation of Mayer waves informs a novel set of disciplines we term quantum neurophysics extendable to describing subatomic reality. Beyond informing our appreciation of mechanisms generating sympathetic oscillations, Mayer waves may constitute an intrinsic property of neurons extant throughout the cerebrum, brainstem, and spinal cord or reflect an emergent property of interactions between arteriogenic and neuronal oscillations.

Long-term stimulation of cardiac vagal preganglionic neurons reduces blood pressure in the spontaneously hypertensive rat.

Arterial hypertension is associated with autonomic nervous system dysfunction. Different interventional strategies have been implemented in recent years for the reduction of sympathetic activity in patients with hypertension. However, the therapeutic benefit of increasing vagal tone in hypertensive patients remains largely unexplored. Here, we describe the effects of long-term activation of vagal neural pathways on arterial pressure, heart rate arterial pressure variability and spontaneous baroreflex sensitivity in spontaneously hypertensive rats (SHR) and normotensive Wistar rats. Brainstem vagal preganglionic neurons residing in the dorsal vagal motor nucleus (DVMN) were targeted with a lentiviral vector to induce the expression of an artificial G(s) protein-coupled receptor termed designer receptors exclusively activated by designer drugs (DREADD-Gs). The transduced neurons were activated daily by systemic administration of otherwise inert ligand clozapine-n-oxide. Arterial pressure measurements were recorded in conscious freely moving animals after 21 consecutive days of DVMN stimulation. Resting arterial pressure was significantly lower in SHRs expressing DREADD-Gs in the DVMN, compared with control SHRs expressing enhanced green fluorescent protein. No changes in arterial pressure were detected in Wistar rats expressing DREADD-Gs compared with rats expressing enhanced green fluorescent protein in the DVMN. Pharmacogenetic activation of DREADD-Gs-expressing DVMN neurons in SHRs was accompanied with increased baroreflex sensitivity and a paradoxical decrease in cardio-vagal components of heart rate and systolic arterial pressure variability in SHRs. These results suggest that long-term activation of vagal parasympathetic pathways is beneficial in restoring autonomic balance in an animal model of neurogenic hypertension and might be an effective therapeutic approach for the management of hypertension.

Effects of VPAC1 activation in nucleus ambiguus neurons

The pituitary adenylyl cyclase-activating polypeptide (PACAP) and its G protein-coupled receptors, PAC1, VPAC1 and VPAC2 form a system involved in a variety of biological processes. Although some sympathetic stimulatory effects of this system have been reported, its central cardiovascular regulatory properties are poorly characterized. VPAC1 receptors are expressed in the nucleus ambiguus (nAmb), a key center controlling cardiac parasympathetic tone. In this study, we report that selective VPAC1 activation in rhodamine-labeled cardiac vagal preganglionic neurons of the rat nAmb produces inositol 1,4,5-trisphosphate receptor-mediated Ca2+ mobilization, membrane depolarization and activation of P/Q-type Ca2+ channels. In vivo, this pathway converges onto transient reduction in heart rate of conscious rats. Therefore we demonstrate a VPAC1-dependent mechanism in the central parasympathetic regulation of the heart rate, adding to the complexity of PACAP-mediated cardiovascular modulation.

Tonically Active cAMP-Dependent Signaling in the Ventrolateral Medulla Regulates Sympathetic and Cardiac Vagal Outflows.

The ventrolateral medulla contains presympathetic and vagal preganglionic neurons that control vasomotor and cardiac vagal tone, respectively. G protein-coupled receptors influence the activity of these neurons. Gα s activates adenylyl cyclases, which drive cyclic adenosine monophosphate (cAMP)-dependent targets: protein kinase A (PKA), the exchange protein activated by cAMP (EPAC), and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. The aim was to determine the cardiovascular effects of activating and inhibiting these targets at presympathetic and cardiac vagal preganglionic neurons. Urethane-anesthetized rats were instrumented to measure splanchnic sympathetic nerve activity (sSNA), arterial pressure (AP), heart rate (HR), as well as baroreceptor and somatosympathetic reflex function, or were spinally transected and instrumented to measure HR, AP, and cardiac baroreflex function. All drugs were injected bilaterally. In the rostral ventrolateral medulla (RVLM), Sp-cAMPs and 8-Br-cAMP, which activate PKA, as well as 8-pCPT, which activates EPAC, increased sSNA, AP, and HR. Sp-cAMPs also facilitated the reflexes tested. Sp-cAMPs also increased cardiac vagal drive and facilitated cardiac baroreflex sensitivity. Blockade of PKA, using Rp-cAMPs or H-89 in the RVLM, increased sSNA, AP, and HR and increased HR when cardiac vagal preganglionic neurons were targeted. Brefeldin A, which inhibits EPAC, and ZD7288, which inhibits HCN channels, each alone had no effect. Cumulative, sequential blockade of all three inhibitors resulted in sympathoinhibition. The major findings indicate that Gα s-linked receptors in the ventral medulla can be recruited to drive both sympathetic and parasympathetic outflows and that tonically active PKA-dependent signaling contributes to the maintenance of both sympathetic vasomotor and cardiac vagal tone.

Low-intensity aerobic training (T) blocks age-induced decrease of cardiac vagal preganglionic neurons in Spontaneously Hypertensive rats (SHR)

Low-intensity aerobic training (T) blocks age-induced decrease of cardiac vagal preganglionic neurons in Spontaneously Hypertensive rats (SHR)

Cardiovagal and somatic sensory nerve functions in healthy subjects.

Heart rate response to deep breathing (HRDB), which depends on the integrity of cardiac vagal preganglionic neurons and efferent fibers, and the function of sural nerve fibers are both associated with an age-related decline process. The aim of this study was to determine whether the effects attributed to aging on cardiovagal and sural nerve function decline are associated. HRDB and sural sensory nerve action potential (SNAP) amplitude, latency, and conduction velocity (SCV) were measured in one hundred healthy asymptomatic subjects (aged 14-92 years, 41 women). Multiple and simple linear regressions were used to analyze the relationships between the variables. There were significant linear relationships between sural SNAP amplitude and HRDB with age. There was also a significant linear relationship between sural SNAP amplitude and HRDB (correlation coefficient 0.46, p<0.0001), but the model explained only 21.5% of the variability in HRDB. Cardiovagal function assessed by HRDB is associated with sural SNAP amplitude in healthy subjects. Age-related decline only partially explained the variability seen in the association. Other genetic and environmental factors may also play a role.

Role of ionotropic GABA, glutamate and glycine receptors in the tonic and reflex control of cardiac vagal outflow in the rat

BackgroundCardiac vagal preganglionic neurons (CVPN) are responsible for the tonic, reflex and respiratory modulation of heart rate (HR). Although CVPN receive GABAergic and glutamatergic inputs, likely involved in respiratory and reflex modulation of HR respectively, little else is known regarding the functions controlled by ionotropic inputs. Activation of g-protein coupled receptors (GPCR) alters these inputs, but the functional consequence is largely unknown. The present study aimed to delineate how ionotropic GABAergic, glycinergic and glutamatergic inputs contribute to the tonic and reflex control of HR and in particular determine which receptor subtypes were involved. Furthermore, we wished to establish how activation of the 5-HT1A GPCR affects tonic and reflex control of HR and what ionotropic interactions this might involve.ResultsMicroinjection of the GABAA antagonist picrotoxin into CVPN decreased HR but did not affect baroreflex bradycardia. The glycine antagonist strychnine did not alter HR or baroreflex bradycardia. Combined microinjection of the NMDA antagonist, MK801, and AMPA antagonist, CNQX, into CVPN evoked a small bradycardia and abolished baroreflex bradycardia. MK801 attenuated whereas CNQX abolished baroreceptor bradycardia. Control intravenous injections of the 5-HT1A agonist 8-OH-DPAT evoked a small bradycardia and potentiated baroreflex bradycardia. These effects were still observed following microinjection of picrotoxin but not strychnine into CVPN.ConclusionsWe conclude that activation of GABAA receptors set the level of HR whereas AMPA to a greater extent than NMDA receptors elicit baroreflex changes in HR. Furthermore, activation of 5-HT1A receptors evokes bradycardia and enhances baroreflex changes in HR due to interactions with glycinergic neurons involving strychnine receptors. This study provides reference for future studies investigating how diseases alter neurochemical inputs to CVPN.

Autonomic control of cardiorespiratory interactions in fish, amphibians and reptiles

Control of the heart rate and cardiorespiratory interactions (CRI) is predominantly parasympathetic in all jawed vertebrates, with the sympathetic nervous system having some influence in tetrapods. Respiratory sinus arrhythmia (RSA) has been described as a solely mammalian phenomenon but respiration-related beat-to-beat control of the heart has been described in fish and reptiles. Though they are both important, the relative roles of feed-forward central control and peripheral reflexes in generating CRI vary between groups of fishes and probably between other vertebrates. CRI may relate to two locations for the vagal preganglionic neurons (VPN) and in particular cardiac VPN in the brainstem. This has been described in representatives from all vertebrate groups, though the proportion in each location is variable. Air-breathing fishes, amphibians and reptiles breathe discontinuously and the onset of a bout of breathing is characteristically accompanied by an immediate increase in heart rate plus, in the latter two groups, a left-right shunting of blood through the pulmonary circuit. Both the increase in heart rate and opening of a sphincter on the pulmonary artery are due to withdrawal of vagal tone. An increase in heart rate following a meal in snakes is related to withdrawal of vagal tone plus a non-adrenergic-non-cholinergic effect that may be due to humoral factors released by the gut. Histamine is one candidate for this role.

The role of the vagus nerve in the generation of cardiorespiratory interactions in a neotropical fish, the pacu, Piaractus mesopotamicus

The role of the vagus nerve in determining heart rate (f(H)) and cardiorespiratory interactions was investigated in a neotropical fish, Piaractus mesopotamicus. During progressive hypoxia f(H) initially increased, establishing a 1:1 ratio with ventilation rate (f(R)). Subsequently there was a hypoxic bradycardia. Injection of atropine abolished a normoxic inhibitory tonus on the heart and the f(H) adjustments during progressive hypoxia, confirming that they are imposed by efferent parasympathetic inputs via the vagus nerve. Efferent activity recorded from the cardiac vagus in lightly anesthetized normoxic fish included occasional bursts of activity related to spontaneous changes in ventilation amplitude, which increased the cardiac interval. Restricting the flow of aerated water irrigating the gills resulted in increased respiratory effort and bursts of respiration-related activity in the cardiac vagus that seemed to cause f(H) to couple with f(R). Cell bodies of cardiac vagal pre-ganglionic neurons were located in two distinct groups within the dorsal vagal motor column having an overlapping distribution with respiratory motor-neurons. A small proportion of cardiac vagal pre-ganglionic neurons (2%) was in scattered positions in the ventrolateral medulla. This division of cardiac vagal pre-ganglionic neurons into distinct motor groups may relate to their functional roles in determining cardiorespiratory interactions.

The basis of vagal efferent control of heart rate in a neotropical fish,the pacu,Piaractus mesopotamicus

The role of the parasympathetic nervous system, operating via the vagus nerve, in determining heart rate (f(H)) and cardiorespiratory interactions was investigated in the neotropical fish Piaractus mesopotamicus. Motor nuclei of branches of cranial nerves VII, IX and X, supplying respiratory muscles and the heart, have an overlapping distribution in the brainstem, while the Vth motor nucleus is more rostrally located. Respiration-related efferent activity in the cardiac vagus appeared to entrain the heart to ventilation. Peripheral stimulation of the cardiac vagus with short bursts of electrical stimuli entrained the heart at a ratio of 1:1 over a range of frequencies, both below and sometimes above the intrinsic heart rate. Alternatively, at higher bursting frequencies the induced f(H) was slower than the applied stimulus, being recruited by a whole number fraction (1:2 to 1:6) of the stimulus frequency. These effects indicate that respiration-related changes in f(H) in pacu are under direct, beat-to-beat vagal control. Central burst stimulation of respiratory branches of cranial nerves VII, IX and X also entrained the heart, which implies that cardiorespiratory interactions can be generated reflexly. Central stimulation of the Vth cranial nerve was without effect on heart rate, possibly because its central projections do not overlap with cardiac vagal preganglionic neurons in the brainstem. However, bursts of activity recorded from the cardiac vagus were concurrent with bursts in this nerve, suggesting that cardiorespiratory interactions can arise within the CNS, possibly by irradiation from a central respiratory pattern generator, when respiratory drive is high.

5-HT1A/7receptor agonist excites cardiac vagal neurons via inhibition of both GABAergic and glycinergic inputs

To study the synaptic mechanisms involved in the 5-hydroxytryptamine1A/7 (5-HT1A/7) receptor-mediated reflex control of cardiac vagal preganglionic neurons (CVPN). CVPN were retrogradely labeled and identified in brain stem slices of newborn rats, and their synaptic activity was examined using whole-cell patch-clamp. 8-Hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), an agonist of 5-HT1A/7 receptors, had no effect on the glutamatergic inputs of CVPN. In contrast, it significantly decreased the frequency and the amplitude of both the GABAergic and the glycinergic spontaneous inhibitory postsynaptic currents (sIPSC). 8-OH-DPAT also caused significant amplitude decrease of the GABAergic currents evoked by stimulation of the nucleus tractus solitarius. Both the frequency inhibition and the amplitude inhibition of the GABAergic and the glycinergic sIPSC by 8-OH-DPAT had dose-dependent tendencies and could be reversed by WAY-100635, an antagonist of 5-HT1A/7 receptors. In the pre-existence of tetrodotoxin, 8-OH-DPAT had no effect on the GABAergic or the glycinergic miniature inhibitory postsynaptic currents, and had no effect on the GABAergic or the glycinergic currents evoked by exogenous GABA or glycine. The 5-HT1A/7 receptor agonist excites CVPN indirectly via the inhibition of both the GABAergic and glycinergic inputs. These findings have at least in part revealed the synaptic mechanisms involved in the 5-HT1A/7 receptor-mediated reflex control of cardiac vagal nerves in intact animals.

NO Differentially Regulates Neurotransmission to Premotor Cardiac Vagal Neurons in the Nucleus Ambiguus

NO is involved in the neural control of heart rate, and NO synthase expressing neurons and terminals have been localized in the nucleus ambiguus where parasympathetic cardiac vagal preganglionic neurons are located; however, little is known about the mechanisms by which NO alters the activity of premotor cardiac vagal neurons. This study examines whether the NO donor sodium nitroprusside ([SNP] 100 micromol/L) and precursor, l-arginine (10 mmol/L), modulate excitatory and inhibitory synaptic neurotransmission to cardiac vagal preganglionic neurons. Glutamatergic, GABAergic, and glycinergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. Both SNP, as well as l-arginine, increased the frequency of GABAergic neurotransmission to cardiac vagal preganglionic neurons but decreased the amplitude of GABAergic inhibitory postsynaptic currents. In contrast, both l-arginine and SNP inhibited the frequency of glutamatergic and glycinergic synaptic events in cardiac vagal preganglionic neurons. SNP and l-arginine also decreased glycinergic inhibitory postsynaptic current amplitude, and this response persisted in the presence of tetrodotoxin. Inclusion of the NO synthase inhibitor 7-nitroindazole (100 mumol/L) prevented the l-arginine-evoked responses. These results demonstrate that NO differentially regulates excitatory and inhibitory neurotransmission, facilitating GABAergic and diminishing glutamatergic and glycinergic neurotransmission to cardiac vagal neurons.

Misidentification of cardiac vagal pre-ganglionic neurons after injections of retrograde tracer into the pericardial space in the rat

We evaluated whether pericardial injections of the retrograde tracers cholera toxin subunit B (CTb) or Fast Blue (FB) reliably labelled cardiac vagal pre-ganglionic neurons. Injections of CTb into the pericardial space of the rat labelled neurons in both the external and compact formations of the nucleus ambiguus. Most labelled neurons were found in the compact formation of the nucleus ambiguus, and the majority of these, and only these, expressed immunoreactivity for calcitonin gene-related peptide. This distribution of labelled neurons and their immunohistochemical properties is characteristic of oesophageal motoneurons. Examination of the oesophagus following intra-pericardial CTb applications revealed strong labelling of motor end plates within the skeletal muscle of the thoracic but not the abdominal oesophagus. When a second retrograde tracer, FB, was injected into the abdominal oesophagus, labelled somata were found adjacent to CTb-labelled neurons in the compact formation of the nucleus ambiguus. No co-localisation of tracers was found, but identical proportions of calcitonin gene-related peptide (CGRP) immunoreactivity were observed in both groups of neurons. FB injected into the pericardial space labelled intra-cardiac neurons but not brainstem neurons. We conclude that intra-pericardial, and perhaps sub-epicardial, injections of some retrograde tracers are likely to label a subset of oesophageal, as well as cardiac, vagal motor neurons in the brainstem.

Immunohistochemical characterization of cardiac vagal preganglionic neurons in the rat

Cardiac vagal preganglionic neurons (CVN) control cardiac activity by negative chronotropic, dromotropic and inotropic effects. We attempted to characterize the distribution and neuronal properties of the CVN by using double labeling with the retrograde tracer cholera toxin B subunit (CTb) and immunohistochemistry for choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP) or nitric oxide synthase (NOS). Injection of CTb into the sinoatrial ganglia resulted in many retrogradely labeled of neurons in the dorsal motor nucleus of the vagus (DMV), the compact (AmC), semicompact (AmS), loose (AmL), external (AmE) formations of the nucleus ambiguus, and the intermediate zone (IZ) between DMV and the nucleus ambiguus. Almost all CTb-labeled neurons showed ChAT immunoreactivity in the DMV, AmC, AmS, AmL and IZ, but most of the CTb-labeled neurons showed no ChAT immunoreactivity in the AmE. Most of the CTb-labeled neurons were double-labeled with CGRP immunoreactivity in the AmC, AmS and AmL, but a few double-labeled neurons were found in the DMV, IZ and AmE. A few CTb-labeled neurons were double-labeled with NOS immunoreactivity only in the DMV. No TH-immunoreactive neurons were found among the CVN. These results indicate that there are four kinds of neurons among the CVN: non-cholinergic CVN in the AmE, cholinergic and CGRP-containing CVN in the AmC, AmS and AmL, and cholinergic or cholinergic and NOS-containing CVN in the DMV.

Effect of upper airway negative pressure and lung inflation on laryngeal motor unit activity in rabbit.

Distortion of the upper airway by negative transmural pressure (UANP) causes reflex vagal bradycardia. This requires activation of cardiac vagal preganglionic neurons, which exhibit postinspiratory (PI) discharge. We hypothesized that UANP would also stimulate cranial respiratory motoneurons with PI activity. We recorded 32 respiratory modulated motor units from the recurrent laryngeal nerve of seven decerebrate paralyzed rabbits and recorded their responses to UANP and to withholding lung inflation using a phrenic-triggered ventilator. The phasic inspiratory (n = 17) and PI (n = 5) neurons detected were stimulated by -10 cmH(2)O UANP and by withdrawal of lung inflation (P < 0.05, Friedman's ANOVA). Expiratory-inspiratory units (n = 10) were tonically active but transiently inhibited in postinspiration; this inhibition was more pronounced and prolonged during UANP stimuli and during no-inflation tests (P < 0.05). We conclude that, in addition to increasing inspiratory activity in the recurrent laryngeal nerve, UANP also stimulates units with PI activity.

Distribution of cholinergic and non-cholinergic cardiac vagal preganglionic neurons in the rat medulla

Distribution of cholinergic and non-cholinergic cardiac vagal preganglionic neurons in the rat medulla

Anatomic and physiologic evaluation of a link between the nucleus reticularis gigantocellularis and nucleus ambiguus in the rat

Localized microinjection of wheat germ agglutinin-conjugated horseradish peroxidase into the nucleus ambiguus (NA) of Sprague-Dawley rats retrogradely labeled neurons that were bilaterally located within the confine of the nucleus reticularis gigantocellularis (NRGC). These cells conformed with the Golgi description of cytoarchitectonic configurations and size distributions for neurons in this reticular nucleus. Furthermore, field potentials antidromically evoked at the NA by stimulating the vagus nerve were augmented by concurrent activations of bilateral NRGC sites that were themselves capable of eliciting hypotension and bradycardia. We conclude that the gigantocellular reticular nucleus may at least employ the cardiac vagal preganglionic neurons in the NA to exert its actions on the heart.

Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla

Projections from the nucleus tractus solitarii (NTS) to autonomic control regions of the ventrolateral medulla, particularly the nucleus reticularis rostroventrolateralis (RVL), which serves as a tonic vasomotor center, were analyzed in rat by anterograde, retrograde, and combined axonal transport techniques. Autonomic portions of the NTS, including its commissural, dorsal, intermediate, interstitial, ventral, and ventrolateral subnuclei directly project to RVL as well as to other regions of the ventrolateral medulla. The projections are organized topographically. Rostrally, a small cluster of neurons in the intermediate third of NTS, the subnucleus centralis, and neurons in proximity to the solitary tract selectively innervate neurons in the retrofacial nucleus and nucleus ambiguus. Neurons generally located in more caudal and lateral sites in the NTS innervate the caudal ventrolateral medulla (CVL). The RVL, CVL, and nucleus retroambiguus are interconnected. A combined retrograde and anterograde transport technique was developed so as to prove that projections from the NTS to the ventrolateral medulla specifically innervate the region of RVL containing neurons projecting to the thoracic spinal cord or the region of the nucleus containing vagal preganglionic neurons. When the retrograde tracer, fast blue, was injected into the thoracic spinal cord, and wheat germ agglutinin-conjugate horseradish peroxidase (HRP) was injected into the NTS, anterogradely labeled terminals from the NTS surrounded the retrogradely labeled neurons in the RVL and in the nucleus retroambiguus in the caudal medulla. Among the bulbospinal neurons in the RVL innervated by the NTS were adrenaline-synthesizing neurons of the C1 group. When fast blue was applied to the cervical vagus, and HRP was injected into the NTS, anterogradely labeled terminals from the NTS surrounded retrogradely labeled neurons in the rostral dorsal motor nucleus of the vagus, the region of the nucleus ambiguus, the retrofacial nucleus, and the dorsal portion of the RVL, a region previously shown to contain cardiac vagal preganglionic neurons. This combined anterograde and retrograde transport technique provides a useful method for tracing disynaptic connections in the brain. These data suggest that the RVL is part of a complex of visceral output regions in the ventrolateral medulla, all of which receive afferent projections from autonomic portions of the NTS. Bulbospinal neurons in the RVL, in particular the C1 adrenaline neurons, may provide a portion of the anatomic substrate of the baroreceptor and other visceral reflexes.

Medullary origin of vagal preganglionic axons to the heart of the cat

It is apparent from the literature that a controversy exists concerning the site of origin of cardiac vagal preganglionic axons. Physiological studies have suggested that the location of these neurons may be different in different species and there has been disagreement between physiological and anatomical findings in the same species. We now present anatomical and neurophysiological studies suggesting that in the cat cardiac vagal preganglionic neurons are located in two medullary regions: the areas of the dorsal motor nucleus of the vagus (DMV) and of the nucleus ambiguus (AMB). This suggestion is based on the following observations. Firstly, after application of horseradish peroxidase to the right cardiac branches of the vagus nerve, labeled neurons were found primarily in the regions of the DMV and AMB. Additional scattered neurons were found in the reticular formation between these two nuclei. Secondly, following injections of tritiated amino acids into either the DMV or AMB, labeled vagal fibers were found in the atrial myocardium. Finally, electrical stimulation of the right cardiac branches of the vagus nerve antidromically activated neurons in the DMV and AMB regions with latencies corresponding to conduction velocities of B-fibers. In addition, these neurons were orthodromically excited by electrical stimulation of the carotid sinus and aortic depressor nerves.