Substance P nerve terminals synapse upon negative chronotropic vagal motoneurons

Molecularly defined circuits for cardiovascular and cardiopulmonary control

The sympathetic and parasympathetic nervous systems powerfully regulate internal organs¹, but the molecular and functional diversity of their constituent neurons and circuits remains largely unknown. Here we use retrograde neuronal tracing, single-cell RNA sequencing, optogenetics, and physiological experiments to dissect the cardiac parasympathetic control circuit in mice. We show that cardiac-innervating neurons in the brainstem nucleus ambiguus (Amb) are comprised of two molecularly, anatomically, and functionally distinct subtypes. One we call ACV (ambiguus cardiovascular) neurons (~35 neurons per Amb), define the classical cardiac parasympathetic circuit. They selectively innervate a subset of cardiac parasympathetic ganglion neurons and mediate the baroreceptor reflex, slowing heart rate and atrioventricular node conduction in response to increased blood pressure. The other, ACP (ambiguus cardiopulmonary) neurons (~15 neurons per Amb) innervate cardiac ganglion neurons intermingled with and functionally indistinguishable from those innervated by ACV neurons, but surprisingly also innervate most or all lung parasympathetic ganglion neurons; clonal labeling shows individual ACP neurons innervate both organs. ACP neurons mediate the dive reflex, the simultaneous bradycardia and bronchoconstriction that follows water immersion. Thus, parasympathetic control of the heart is organized into two parallel circuits, one that selectively controls cardiac function (ACV circuit) and another that coordinates cardiac and pulmonary function (ACP circuit). This new understanding of cardiac control has implications for treating cardiac and pulmonary diseases and for elucidating the control and coordination circuits of other organs.

The mammalian diving response: an enigmatic reflex to preserve life?

The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

Substance P Modulation of Hypoglossal Motoneuron Excitability During Development: Changing Balance Between Conductances

Although Substance P (SP) acts primarily through neurokinin 1 (NK1) receptors to increase the excitability of virtually all motoneurons (MNs) tested, the ontogeny of this transmitter system is not known for any MN pool. Hypoglossal (XII) MNs innervate tongue protruder muscles and participate in several behaviors that must be functional from birth including swallowing, suckling and breathing. We used immunohistochemistry, Western immunoblotting, and whole cell recording of XII MNs in brain stem slices from rats ranging in age from postnatal day zero (P0) to P23 to explore developmental changes in: NK1 receptor expression; currents evoked by SPNK1 (an NK1-selective SP receptor agonist) and; the efficacy of transduction pathways transforming ligand binding into channel modulation. Despite developmental reductions in XII MN NK1 receptor expression, SPNK1 current density remained constant at 6.1 ± 1.0 (SE) pA/pF. SPNK1 activated at least two conductances. Activation of a pH-insensitive Na+ conductance dominated in neonates (P0–P5), but its contribution fell from ∼80 to ∼55% in juveniles (P14–P23). SPNK1 also inhibited a pH-sensitive, two-pore domain K+ (TASK)-like K+ current. Its contribution increased developmentally. First, the density of this pH-sensitive K+ current doubled between P0 and P23. Second, SPNK1 did not affect this current in neonates, but reduced it by 20% at P7–P10 and 80% in juveniles. In addition, potentiation of repetitive firing was greatest in juveniles. These data establish that despite apparent reductions in NK1 receptor density, SP remains an important modulator of XII MN excitability throughout postnatal development due, in part, to increased expression of a pH-sensitive, TASK-like conductance.

Age-related loss of cardiac vagal preganglionic neurones in spontaneously hypertensive rats

Despite the findings that impaired vagal control of the heart rate occurs in human hypertension, leading to greater cardiovascular risk, the mechanism of this impairment is as yet unknown. Observations in humans and experiments in the spontaneously hypertensive rat (SHR) suggested that such impairment may be related to an anomaly in central vagal neurones. We therefore set out to determine whether the numbers and distribution of cardiac-projecting vagal preganglionic neurones in the medulla of adult (12 week) hypertensive SHR are different from those in young (4 week) prehypertensive SHR and in age-matched Wistar-Kyoto (WKY) rats of two age groups. The number of vagal neurones, identified by labelling with the fluorescent tracer DiI applied to the heart, was essentially similar in the three areas of the medulla analysed (dorsal vagal nucleus, nucleus ambiguus and intermediate reticular zone) in young SHR and young or adult WKY rats. In contrast, fewer vagal neurones were labelled in adult SHR compared with young SHR or WKY rats. This difference was due to highly significant reductions in vagal neurones in the dorsal vagal nucleus and nucleus ambiguus on the right side of the medulla. These observations suggest that a loss of parasympathetic preganglionic neurones supplying the heart with axons in the right vagus nerve, or a remodelling of their cardiac projections, may explain the known impairment of the baroreceptor reflex gain controlling heart rate in hypertension.

Differential control of cardiac functions by the brain

1. The idea is introduced that cardiac rate, contractility or atrioventricular (A-V) conduction spread may be controlled independently by the brain. Limited data from reflex studies are cited to support this view. 2. Evidence is presented that individual autonomic post- and preganglionic neurons have quite specific actions on the heart. Premotor and other central neurons can have preferential actions on heart rate, contractility or A-V conduction. 3. The functional implications of selective cardiac control are discussed.

Central control of atrio-ventricular conduction and left ventricular contractility in the cat heart: Synaptic interactions of vagal preganglionic neurons in the nucleus ambiguus with neuropeptide Y-immunoreactive nerve terminals

In the cat, vagal postganglionic controls of heart rate, atrio-ventricular (AV) conduction and left ventricular contractility are mediated by three separate intrinsic cardiac ganglia, the sinoatrial (SA), AV and cranioventricular (CV) ganglia, respectively. The vagal preganglionic neurons (VPNs) that project to these ganglia are located in the ventrolateral nucleus ambiguus (NA-VL). We have previously shown that the VPNs projecting to the SA, AV and CV ganglia are distinct from one another. We have also demonstrated that neuropeptide Y-immunoreactive (NPY-IR) axon terminals synapse upon VPNs projecting to the SA ganglion. In the present study, we test the hypothesis that those VPNs projecting to the AV ganglion (negative dromotropic VPNs) and those projecting to the CV ganglion (negative inotropic VPNs) are innervated by NPY-IR terminals in NA-VL. A retrograde tracer was injected into the AV or CV ganglion of the cat, and the brains subsequently processed for visualization of tracer and the immunocytochemical visualization of NPY by dual labeling electron-microscopic methods. We observed that 11+/-5% of all axodendritic synapses and 8+/-6% of all axosomatic synapses upon negative inotropic VPNs were NPY-IR. Furthermore, 19+/-14% of all axodendritic synapses upon negative dromotropic VPNs were NPY-IR. A few NPY-IR axosomatic synapses upon negative dromotropic neurons were also observed. NPY-IR terminals in NA-VL occasionally formed axosomatic synapses with NPY-IR neurons and axoaxonic synapses with unlabeled terminals. These results suggest that central NPY afferents to the NA-VL modulate the vagal preganglionic control of AV conduction and left ventricular contractility.

Enkephalins and functionally specific vagal preganglionic neurons to the heart: Ultrastructural studies in the cat

In cat, distinct populations of vagal preganglionic and postganglionic neurons selectively modulate heart rate, atrioventricular conduction and left ventricular contractility, respectively. Vagal preganglionic neurons to the heart originate in the ventrolateral part of nucleus ambiguus and project to postganglionic neurons in intracardiac ganglia, including the sinoatrial (SA), atrioventricular (AV) and cranioventricular (CV) ganglia, which selectively modulate heart rate, AV conduction and left ventricular contractility, respectively. These ganglia receive projections from separate populations of vagal preganglionic neurons. The neurochemical anatomy and synaptic interactions of afferent neurons which mediate central control of these preganglionic neurons is incompletely understood. Enkephalins cause bradycardia when microinjected into nucleus ambiguus. It is not known if this effect is mediated by direct synapses of enkephalinergic terminals upon vagal preganglionic neurons to the heart. The effects of opioids in nucleus ambiguus upon AV conduction and cardiac contractility have also not been studied. We have tested the hypothesis that enkephalinergic nerve terminals synapse upon vagal preganglionic neurons projecting to the SA, AV and CV ganglia. Electron microscopy was used combining retrograde labeling from the SA, AV or CV ganglion with immunocytochemistry for enkephalins in ventrolateral nucleus ambiguus. Eight percent of axodendritic synapses upon negative chronotropic, and 12% of axodendritic synapses upon negative dromotropic vagal preganglionic neurons were enkephalinergic. Enkephalinergic axodendritic synapses were also present upon negative inotropic vagal preganglionic neurons. Thus enkephalinergic terminals in ventrolateral nucleus ambiguus can modulate not only heart rate but also atrioventricular conduction and left ventricular contractility by directly synapsing upon cardioinhibitory vagal preganglionic neurons.

Synaptic remodeling in the nucleus ambiguus following vagal–hypoglossal nerve anastomosis in the cat

We reported recently the occurrence of a massive and selective elimination of synaptic boutons on motoneurons in the dorsal motor nucleus of the vagus (DMV) in the cat following vagal-hypoglossal nerve anastomosis (VHA) [J. Comp. Neurol. 458 (2003) 195]. This study was aimed to explore the synaptic reorganization in the other major nucleus associated with the vagus, namely, the nucleus ambiguus (NA) following the same treatment. In view of the tremendous difference in function, the NA and DMV are considered to be two ideal nuclei for explanatory studies seeking to elucidate how VHA could induce different plasticity of brainstem neurons influenced by the newly reestablished neural pathway. The present results showed that the vagal efferent neurons in the NA had responded to VHA in a different manner compared with those in the DMV. Firstly, the numbers of axon terminals containing round (R), round with dense-cored (R+D), pleomorphic (P) or flattened (F) synaptic vesicles contacting the NA motoneurons were markedly increased at 500-day postoperation, the longest reinnervation interval. The percent increases in the synapse frequency for R, R+D, P and F boutons were 8.6%, 274.4%, 238.3% and 400.0%, respectively. Secondly, the formation of astroglial ensheathment around the motoneurons in the DMV following VHA was not evident in the NA. Another striking difference was the extensive dendritic sprouting of the NA neurons as opposed to the dendritic retraction of the DMV neurons as shown by a significant increase in distal dendrites of NA motoneurons. The different modes of neural remodeling between NA and DMV may be attributed to the unique nature of the two nuclei to structures they normally supply and their different compatibility with the newly innervated target, viz. tongue skeletal musculature.

Parasympathetic control of the heart. III. Neuropeptide Y-immunoreactive nerve terminals synapse on three populations of negative chronotropic vagal preganglionic neurons

The vagal postganglionic control of cardiac rate is mediated by two intracardiac ganglia, i.e., the sinoatrial (SA) and posterior atrial (PA) ganglia. Nothing is known about the vagal preganglionic neurons (VPNs) that innervate the PA ganglion or about the neurochemical anatomy of central afferents that innervate these VPNs. These issues were examined using light microscopic retrograde labeling methods and dual-labeling electron microscopic histochemical and immunocytochemical methods. VPNs projecting to the PA ganglion are found in a narrow column exclusively in the ventrolateral nucleus ambiguus (NA-VL). These neurons are relatively large (37.6 +/- 2.7 microm by 21.3 +/- 3.4 microm) with abundant cytoplasm and intracellular organelles, rare somatic and dendritic spines, round uninvaginated nuclei, and myelinated axons. Previous physiological data indicated that microinjections of neuropeptide Y (NPY) into the NA-VL cause negative chronotropic effects. The present morphological data demonstrate that NPY-immunoreactive nerve terminals formed 18 +/- 4% of the axodendritic or axosomatic synapses and close appositions on VPNs projecting to the PA ganglion. Three approximately equal populations of VPNs in the NA-VL were retrogradely labeled from the SA and PA ganglia. One population each projects to the SA ganglion, the PA ganglion, or to both the SA and PA ganglia. Therefore, there are both shared and independent pathways involved in the vagal preganglionic controls of cardiac rate. These data are consistent with the hypothesis that the central and peripheral parasympathetic controls of cardiac rate are coordinated by multiple potentially redundant and/or interacting pathways and mechanisms.

Parasympathetic control of the heart. II. A novel interganglionic intrinsic cardiac circuit mediates neural control of heart rate

Intracardiac pathways mediating the parasympathetic control of various cardiac functions are incompletely understood. Several intracardiac ganglia have been demonstrated to potently influence cardiac rate [the sinoatrial (SA) ganglion], atrioventricular (AV) conduction (the AV ganglion), or left ventricular contractility (the cranioventricular ganglion). However, there are numerous ganglia found throughout the heart whose functions are poorly characterized. One such ganglion, the posterior atrial (PA) ganglion, is found in a fat pad on the rostral dorsal surface of the right atrium. We have investigated the potential impact of this ganglion on cardiac rate and AV conduction. We report that microinjections of a ganglionic blocker into the PA ganglion significantly attenuates the negative chronotropic effects of vagal stimulation without significantly influencing negative dromotropic effects. Because prior evidence indicates that the PA ganglion does not project to the SA node, we neuroanatomically tested the hypothesis that the PA ganglion mediates its effect on cardiac rate through an interganglionic projection to the SA ganglion. Subsequent to microinjections of the retrograde tracer fast blue into the SA ganglion, >70% of the retrogradely labeled neurons found within five intracardiac ganglia throughout the heart were observed in the PA ganglion. The neuroanatomic data further indicate that intraganglionic neuronal circuits are found within the SA ganglion. The present data support the hypothesis that two interacting cardiac centers, i.e., the SA and PA ganglia, mediate the peripheral parasympathetic control of cardiac rate. These data further support the emerging concept of an intrinsic cardiac nervous system.

Ionotropic glutamate receptor subunit immunoreactivity of vagal preganglionic neurones projecting to the rat heart

The ionotropic glutamate receptor subunits expressed by vagal preganglionic neurones in the rat medulla oblongata were examined by using fluorescence immunolabelling combined with retrograde neuronal tracing. The general population of these neurones in the medulla was identified by intraperitoneal injections of Fluorogold and also with choline acetyltransferase antibodies. Cardiac projecting neurones were specifically identified by applying the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine (DiI) to the heart or by injecting cholera toxin B-subunit into the pericardium. Both tracers labelled populations of neurones lying in the dorsal vagal nucleus, intermediate reticular formation and nucleus ambiguus, and when both tracers were applied simultaneously, approximately 50% of cells were dual-labelled. Control experiments established that the labelling was specific for neurones projecting to the heart. Most vagal preganglionic neurones, including those projecting to the heart, irrespective of their location in the medulla, had a similar profile of glutamate receptor immunoreactivity. Labelling of somata for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) subunit GluR1 was weak or absent, while labelling with antibodies directed to GluR2, a common sequence of GluR2 and GluR3, and GluR4 was moderate or intense. All neurones studied appeared to express the N-methyl-D-aspartate (NMDA) receptor subunit NR1, and while antibodies recognising the NR2A and NR2B splice variants gave strong labelling, immunoreactivity with a NR2B specific antibody was weaker. Weak to moderate labelling was seen in some neurones using antibodies to the kainate receptor subunits KA2 and GluR5-7. These results are consistent with neurophysiological data indicating the presence of AMPA, NMDA and kainate responses in cardiac vagal preganglionic neurones, and suggest that these neurones are similar to other vagal parasympathetic preganglionic neurones in expressing mainly AMPA receptor subunits GluR2-4.

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.

Substance P afferent terminals innervate vagal preganglionic neurons projecting to the trachea of the ferret

Airway disorders, such as asthma and chronic obstructive bronchitis, are, in part, due to abnormalities in the nervous control of the airways. However, the ultrastructural circuitry and neurochemical anatomy of afferents modulating the output of airway-related vagal preganglionic neurons (VPNs) in the nucleus ambiguus are poorly understood. We have examined the potential role of substance P (SP) immunoreactive afferents in the regulation of anatomically identified airway VPNs. Cholera toxin b-subunit conjugated to horseradish peroxidase was used as a retrograde cell body tracer to identify the central VPNs innervating the extra-thoracic trachea. Immunocytochemistry was employed to identify SP afferents. The external formation of the nucleus ambiguus was examined by electron microscopy using a simultaneous double labeling method. Cell bodies of tracheal VPNs were 31.7 +/- 1.18 x 23.0 +/- 1.3 microm (means +/- S.E.M.) in size, contained abundant endoplasmic reticulum, had a round nucleus with a prominent nucleolus, no satellite body and displayed somatic and dendritic spines. Somato-somatic appositions, somato-dendritic appositions without intervening glial processes and dendritic "bundling" commonly seen in esophageal motoneurons were not observed. The ultrastructural morphology of tracheal VPNs were also clearly distinguishable from pharyngeal and laryngeal motoneurons in other divisions of the nucleus ambiguus which lack somatic spines. These data are consistent with the hypothesis that differences in the ultrastructure and synaptology of the different divisions of the nucleus ambiguus may be associated with specific physiological functions. The mean size (+/- S.E.M.) of SP nerve terminals was 1.57 +/- 0.06 x 0.79 +/- 0.03 microm. SP terminals formed 17.5% of the axo-dendritic and 15.9% of the axo-somatic synapses which were observed upon retrogradely labeled tracheal VPNs. Synaptic contacts observed were both symmetric and asymmetric. These synaptic interactions define, in part, the neurochemical anatomy of neuronal circuits modulating vagal preganglionic control of tracheal functions.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Negative chronotropism of the heart is inhibited with lesions of the caudal medulla in the rat

Neurons in the ventrolateral medulla are essential for cardiorespiratory regulation. It has been suggested that neurons in the caudal ventrolateral medulla are responsible for the negative chronotropic effect of the heart, at least in carnivores, because injection of glutamate into this area decreases heart rate significantly. In the present study, we monitored heart rate both before and after injections of the excitotoxin ibotenic acid into the most caudal part of the ventrolateral medulla in rats. We found that resting heart rate increased significantly by more than 53% (P<0.0001) after the ibotenic acid injections. This result suggests that neurons located in the caudal ventrolateral medulla are responsible for the negative chronotropic effect of the heart in the rat, especially its most caudal part.

Control of negative inotropic vagal preganglionic neurons in the dog: synaptic interactions with substance P afferent terminals in the nucleus ambiguus?

Previous research from this laboratory has shown that substance P-immunoreactive (SP) terminals synapse upon negative chronotropic vagal preganglionic neurons (VPNs), but not upon negative dromotropic VPNs, of the ventrolateral nucleus ambiguus (NA-VL). Moreover, SP agonists injected into NA-VL cause bradycardia without decreasing AV conduction. In the current study, we have: (1) defined the electron microscopic characteristics of the SP neurons of NA-VL in dog; and (2) tested the hypothesis that SP nerve terminals synapse upon negative inotropic VPNs of NA-VL, retrogradely labeled from the cranial medial ventricular (CMV) ganglion. Numerous SP terminals and a few SP neurons were observed in the vicinity of retrogradely labeled neurons. SP terminals were observed forming synapses with unlabeled dendrites and with SP dendrites, but never with the retrogradely labeled neurons. Together, these results and earlier findings suggest that SP agonists may be able to induce bradycardia without decreasing AV conduction or ventricular contractility.

Negative inotropic vagal preganglionic neurons in the nucleus ambiguus of the cat: neuroanatomical comparison with negative chronotropic neurons utilizing dual retrograde tracers

The vagal postganglionic controls of cardiac rate and left ventricular contractility are mediated by separate intracardiac ganglia, the sino-atrial (SA) and cranio-ventricular (CV) ganglia, respectively. We injected a different retrograde tracer into each of these ganglia (in the same animal) and subsequently examined the brain for the presence of single labeled or double labeled vagal preganglionic neurons. Retrogradely labeled cells from either ganglion were found exclusively in the ventrolateral nucleus ambiguus (NA-VL). There was considerable overlap in the distribution of labeled cells from either ganglion, however fewer than 3% of labeled neurons were double labeled. The data are consistent with the hypothesis that the preganglionic controls of cardiac rate and left ventricular contractility are mediated by largely separate but overlapping groups of cardioinhibitory neurons originating from the NA-VL. These neurons have parallel but morphologically independent pathways projecting to the SA and CV ganglia. Physiological experiments are needed to support this hypothesis.

Neural control of left ventricular contractility in the dog heart: synaptic interactions of negative inotropic vagal preganglionic neurons in the nucleus ambiguus with tyrosine hydroxylase immunoreactive terminals

Recent physiological evidence indicates that vagal postganglionic control of left ventricular contractility is mediated by neurons found in a ventricular epicardial fat pad ganglion. In the dog this region has been referred to as the cranial medial ventricular (CMV) ganglion [J.L. Ardell, Structure and function of mammalian intrinsic cardiac neurons, in: J.A. Armour, J.L. Ardell (Eds.). Neurocardiology, Oxford Univ. Press, New York, 1994, pp. 95-114; B.X. Yuan, J.L. Ardell, D.A. Hopkins, A.M. Losier, J.A. Armour, Gross and microscopic anatomy of the canine intrinsic cardiac nervous system, Anat. Rec., 239 (1994) 75-87]. Since activation of the vagal neuronal input to the CMV ganglion reduces left ventricular contractility without influencing cardiac rate or AV conduction, this ganglion contains a functionally selective pool of negative inotropic parasympathetic postganglionic neurons. In the present report we have defined the light microscopic distribution of preganglionic negative inotropic neurons in the CNS which are retrogradely labeled from the CMV ganglion. Some tissues were also processed for the simultaneous immunocytochemical visualization of tyrosine hydroxylase (TH: a marker for catecholaminergic neurons) and examined with both light microscopic and electron microscopic methods. Histochemically visualized neurons were observed in a long slender column in the ventrolateral nucleus ambiguus (NA-VL). The greatest number of retrogradely labeled neurons were observed just rostral to the level of the area postrema. TH perikarya and dendrites were commonly observed interspersed with vagal motoneurons in the NA-VL. TH nerve terminals formed axo-dendritic synapses upon negative inotropic vagal motoneurons, however the origin of these terminals remains to be determined. We conclude that synaptic interactions exist which would permit the parasympathetic preganglionic vagal control of left ventricular contractility to be modulated monosynaptically by catecholaminergic afferents to the NA-VL.

Ultrastructural circuitry of cardiorespiratory reflexes: there is a monosynaptic path between the nucleus of the solitary tract and vagal preganglionic motoneurons controlling atrioventricular conduction in the cat

We have tested the hypothesis: (1) that presumptive negative dromotropic vagal preganglionic neurons in the ventrolateral nucleus ambiguus (NA-VL) can be selectively labelled from the heart, by injecting one of two fluorescent tracers into the two intracardiac ganglia which independently control sino-atrial (SA) rate or atrioventricular (AV) conduction; i.e., the SA and AV ganglia, respectively. The NA-VL was examined for the presence of single and/or double labelled cells. Over 91% of vagal preganglionic neurons in the NA-VL projecting to either intracardiac ganglion did not project to the second ganglion. Consequently, we also tested the hypothesis: (2) that there is a monosynaptic connection between neurons of the medial, and/or dorsolateral nucleus of the solitary tract (NTS), rostral to obex, and negative dromotropic neurons in the NA-VL. An anterograde tracer was injected into the NTS, and a retrograde tracer into the AV ganglion. The anterograde marker was found in both myelinated and unmyelinated axons in the NA-VL, as well as in nerve terminals. Axo-somatic and axo-dendritic synapses were detected between terminals labelled from the NTS, and retrogradely labelled negative dromotropic neurons in the NA-VL. This is the first ultrastructural demonstration of a monosynaptic pathway between neurons in the NTS and functionally associated (negative dromotropic) cardioinhibitory neurons. The data are consistent with the hypothesis that the neuroanatomical circuitry mediating the vagal baroreflex control of AV conduction may be composed of as few as four neurons in series, although interneurons may also be interposed within the NTS.

Vagal control of left ventricular contractility is selectively mediated by a cranioventricular intracardiac ganglion in the cat

Activation of the vagus nerve leads to decreases in sinoatrial (SA) rate, atrioventricular (AV) conduction, and myocardial contractility. Previous data are consistent with the hypothesis that vagal control of cardiac rate and AV conduction are mediated by two anatomically separated and physiologically independent parasympathetic intracardiac ganglia located in fat pads on the surface of the right and left atria, respectively. These data suggested that vagal control of ventricular contractility might be mediated through another intracardiac ganglion. We examined the ventricles of cat hearts histologically for the presence of ganglia. Multiple small basophilic ganglia composed of a few neurons, and an occasional larger ganglion were found embedded in the epicardial fat surrounding the cranial margin of the anterior surface of the left ventricle, near the juncture with the right ventricle, which we refer to as the CV ganglion. In anesthetized cats, right cervical vagal stimulation decreased SA rate by 44 +/- 5%, decreased the rate of AV conduction by 68 +/- 14%, and reduced ventricular contractility by 19.5 +/- 5.7%. Vagally induced negative inotropism was almost completely prevented by microinjection of a ganglionic blocking drug into the CV ganglion. However, these injections into the CV ganglion did not significantly effect vagally induced decreases in either SA rate or AV conduction. We conclude: (1) that ganglia are found in a fat pad on the surface of the left ventricle of the cat heart and (2) that the CV ganglion selectively mediates the negative inotropic effect of vagal stimulation on the left ventricle. Greater understanding of the physiological functions of intracardiac neuronal circuits may help in developing new strategies to treat disorders of cardiac contractility such as congestive heart failure.

Discrete subregions of the rat midbrain periaqueductal gray project to nucleus ambiguus and the periambigual region

We investigated the organization of projections from the rat midbrain periaqueductal gray to nucleus ambiguus and the periambigual region using retrograde and anterograde tract tracing techniques. Retrograde tracing results revealed that neurons that project to nucleus ambiguus arise from three discrete, longitudinally organized columns of neurons located in the supraoculomotor central gray, lateral and ventrolateral periaqueductal gray. Anterograde tracing studies demonstrated that projections from these three columns of periaqueductal gray neurons terminate with topographic specificity in nucleus ambiguus and the periambigual region. Double-labelling studies demonstrated that periaqueductal gray neurons terminate in close contiguity to cholinergic neurons in the compact, semicompact, loose and external formations of nucleus ambiguus. The present results suggest that projections from periaqueductal gray to nucleus ambiguus may mediate, in part, certain cardiovascular adjustments and vocalizations produced by stimulation of periaqueductal gray.

Vagal afferent innervation of the atria of the rat heart reconstructed with confocal microscopy

We have used confocal microscopy to analyze the vagal afferent innervation of the rat heart. Afferents were labeled by injecting 1,1'-dioleyl-3,3,3',3'-tetramethylindocarbocyanine methanesulfonate (DiI) into the nodose ganglia of animals with prior supranodose de-efferentations, autonomic ganglia were stained with Fluoro-gold, and tissues were examined in whole mounts. Distinctively different fiber specializations were observed in the epi-, myo-, and endocardium: Afferents to the epicardium formed complexes associated with cardiac ganglia. These ganglia consisted of four major ganglionated plexuses, two on each atrium, at junctions of the major vessels with the atria. Ganglionic locations and sizes (left > right) were consistent across animals. In addition to principal neurons (PNs), significant numbers of small intensely fluorescent (SIF) cells were located in each of these plexuses, and vagal afferents provided dense pericellular varicose endings around the SIF cells in each ganglionic plexus, with few if any terminations on PNs. In the myocardium, vagal afferents formed close contacts with cardiac muscles, including conduction fibers. In the endocardium, vagal fibers formed "flower-spray" and "end-net" terminals in connective tissue. With three-dimensional reconstruction of confocal optical sections, a novel polymorphism was seen: Some fibers had one or more collaterals ending as endocardial flower sprays and other collaterals ending as myocardial intramuscular endings. Some unipolar or pseudounipolar neurons within each cardiac ganglionic plexus were retrogradely labeled from the nodose ganglia. In conclusion, vagal afferents form a heterogeneity of differentiated endings in the heart, including structured elements which may mediate chemoreceptor function, stretch reception, and local cardiac reflexes.

What are the roles of substance P and neurokinin-1 receptors in the control of negative chronotropic or negative dromotropic vagal motoneurons? A physiological and ultrastructural analysis

Recent data indicate that there is a cardiotopic organization of negative chronotropic and negative dromotropic neurons in the nucleus ambiguus (NA). Negative dromotropic neurons are found in the rostral ventrolateral NA (rNA-VL), negative chronotropic neurons are found in the caudal ventrolateral NA (cNA-VL), and both types of neurons are found in an intermediate level of the ventrolateral NA (iNA-VL). Substance P (SP) immunoreactive nerve terminals synapse upon negative chronotropic vagal motoneurons in the iNA-VL, and SP microinjections in the NA cause bradycardia. In the present report we have attempted to: (1) define the type of tachykinin receptor which mediates the negative chronotropic effect of SP microinjections into the iNA-VL; (2) define the physiological effect of microinjections of a selective SP agonist into the rNA-VL on atrioventricular (AV) conduction: and (3) find ultrastructural evidence for synaptic interactions of SP-immunoreactive nerve terminals with negative dromotropic vagal motoneurons in the rNA-VL. Microinjections of the excitatory amino acid glutamate (Glu) into the iNA-VL to activate all local vagal preganglionic neurons caused both bradycardia and a decrease in the rate of AV conduction. Injections of the selective neurokinin-1 (NK-1) receptor agonist drug GR-73632 also caused bradycardia, however the rapid onset of agonist induced desensitization prevented an evaluation of potential effects on AV conduction in the iNA-VL. These data suggest that the SP-induced bradycardia which can be elicited from the NA is mediated, at least in part, by NK-1 receptors. Microinjections of Glu into the rNA-VL caused a decrease in AV conduction without an effect on cardiac rate. On the other hand, GR-73632 microinjections into rNA-VL did not affect AV conduction. Following injections of the beta subunit of cholera toxin conjugated to horseradish peroxidase (CTB-HRP) into the left atrial fat pad ganglion which selectively mediates changes in AV conduction, retrogradely labeled neurons were histochemically visualized in the rNA-VL. These tissues were subsequently processed for the simultaneous immunocytochemical visualization of SP, and examined by electron microscopy. Histochemically labeled neurons were large, multipolar, with abundant cytoplasm containing large masses of rough endoplasmic reticulum, and exhibited distinctive dendritic and somatic spines. Unlabeled nerve terminals were noted to form either asymmetric or symmetric synapses with dendrites, dendritic spines, and perikarya of histochemically labeled neurons. SP-immunoreactive nerve terminals were also detected in the rNA-VL. SP terminals typically contained numerous small pleomorphic vesicles, multiple large dense core vesicles, and several mitochondria, and they synapsed upon unlabeled dendritic profiles. A total of 154 SP-immunoreactive nerve terminals were observed on photomicrographs of tissues which also contained histochemically labeled profiles. None made an identifiable synapse with a retrogradely labeled profile on the sections examined. In summary, both physiological and ultrastructural data indicate that SP terminals in the iNA-VL do modify the output of negative chronotropic vagal motoneurons. This effect is mediated by NK-1 receptors. On the other hand both physiological and ultrastructural data indicate that SP terminals in the rNA-VL do not modify the output of negative dromotropic vagal motoneurons. Therefore different mechanisms (neurotransmitters or receptors) mediate the central vagal control of cardiac rate and AV conduction.

Can neurons in the nucleus ambiguus selectively regulate cardiac rate and atrio-ventricular conduction?

Previous anatomic data have described the distribution of presumptive negative chronotropic and negative dromotropic neurons in the ventro-lateral nucleus ambiguus (NA-VL) following injections of retrograde tracers into physiologically selective parasympathetic intracardiac ganglia. Negative dromotropic neurons were preferentially distributed in the rostral NA-VL (rNA-VL). Negative chronotropic neurons were preferentially distributed in the caudal NA-VL (cNA-VL). Significant numbers of both types of cardio-inhibitory neurons were observed to overlap in an intermediate level of the NA-VL (iNA-VL). In the present report, we have examined the effects of microinjections of the excitatory amino-acid glutamate (GLU) into the cNA-VL and iNA-VL on cardiac rate and AV conduction while recording the electrocardiogram in paced and non-paced cat hearts. The data indicate that: (i) excitation of neurons in the cNA-VL causes a 58 +/- 17% reduction in cardiac rate, without influencing AV conduction; and (ii) excitation of neurons in the iNA-VL causes both a reduction in heart rate (68 +/- 12%) and a decrease in the rate of AV conduction (38 +/- 7%). These physiological results support the anatomical inference that neurons in the cNA-VL that are retrogradely labeled from physiologically selective parasympathetic intracardiac ganglia selectively exhibit negative chronotropic properties. Furthermore, the data indicate that there is a longitudinal cardiotopic organization of both negative chronotropic and negative dromotropic neurons in the NA-VL. This CNS organization mirrors the peripheral organization of functionally selective cardiac components of the vagus nerve. Finally, the data are consistent with the hypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA independently control cardiac rate and AV conduction.

Ultrastructure and synaptology of the nucleus ambiguus in the rat: the compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato-somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and, occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 +/- 0.4 S.D. micrograms in diameter). Motoneuronal somatic profiles have 0-5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 x 0.7 microgram in the transverse plane and are up to 3.0 micrograms long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (> 90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injection of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis.

Synaptic interactions of substance P immunoreactive nerve terminals in the baro- and chemoreceptor reflexes of the cat

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo- or baroreceptor afferents, however ultrastructural support of this hypothesis is lacking. In the present report we have traced the central projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary tissues including the commissural NTS (cNTS) were processed for the histochemical visualization of transganglionically labeled CSN afferents and for the immunocytochemical detection of SP by dual labeling light and electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilateral TMB labeling was detected in the dorsal, dorso-lateral, and medial subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were readily identified in single labeled unmyelinated axons, myelinated axons, and nerve terminals in the cNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferents. These ultrastructural data support the hypothesis that SP immunoreactive first order neurons are involved in the origination of the chemo- and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b) other CSN primary afferent terminals; and (c) terminals containing SP. Axo-axonic synapses were also observed between CSN primary afferents which contained SP, and other SP terminals. These observations may mediate the morphological bases for multiple forms of presynaptic inhibition in the cNTS, including those involved in cardiorespiratory integration. In conclusion, our results indicate that SP immunoreactive nerve terminals may be important in both the origination and the modulation of the chemo- and/or baroreceptor reflexes.

Cardiotopic organization of the nucleus ambiguus? An anatomical and physiological analysis of neurons regulating atrioventricular conduction

Previous data indicate that there are anatomically segregated and physiologically independent parasympathetic postganglionic vagal motoneurons on the surface of the heart which are capable of selective control of sinoatrial rate, atrioventricular conduction and atrial contractility. We have injected a retrograde tracer into the cardiac ganglion which selectively regulates atrioventricular conduction (the AV ganglion). Medullary tissues were processed for the histochemical detection of retrogradely labeled neurons by light and electron microscopic methods. Negative dromotropic retrogradely labeled cells were found in a long column in the ventrolateral nucleus ambiguus (NA-VL), which enlarged somewhat at the level of the area postrema, but reached its largest size rostral to the area postrema in an area termed the rostral ventrolateral nucleus ambiguus (rNA-VL). Three times as many cells were observed in the left rNA-VL as compared to the right (P < 0.025). Retrogradely labeled cells were also consistantly observed in the dorsal motor nucleus of the vagus (DMV). The DMV contained one third as many cells as the NA-VL. The right DMV contained twice as many cells as the left (P < 0.05). These data are consistent with physiological evidence that suggests that the left vagus nerve is dominant in the regulation of AV conduction, but that the right vagus nerve is also influential. While recording the electrocardiogram in paced and non-paced hearts, L-glutamate (GLU) was microinjected into the rNA-VL. Microinjections of GLU caused a 76% decrease in the rate of atrioventricular (AV) conduction (P < 0.05) and occasional second degree heart block, without changing heart rate. The effects of GLU were abolished by ipsilateral cervical vagotomy. These physiological data therefore support the anatomical inference that CNS neurons that are retrogradely labeled from the AV ganglion selectively exhibit negative dromotropic properties. Retrogradely labeled negative dromotropic neurons displayed a round nucleus with ample cytoplasm, abundant rough endoplasmic reticulum and the presence of distinctive somatic and dendritic spines. These neurons received synapses from afferent terminals containing small pleomorphic vesicles and large dense core vesicles. These terminals made both asymmetric and symmetric contacts with negative dromotropic dendrites and perikarya, respectively. In conclusion, the data presented indicate that there is a cardiotopic organization of ultrastructurally distinctive negative dromotropic neurons in the NA-VL. This central organization of parasympathetic preganglionic vagal motoneurons mirrors the functional organization of cardioinhibitory postganglionic neurons of the peripheral vagus nerve. These data are further discussed in comparison to a recent report on the light microscopic distribution and ultrastructural characteristics of negative chronotropic neurons in the NA-VL42.(ABSTRACT TRUNCATED AT 400 WORDS)