Brainstem cells of origin of physiologically identified cardiopulmonary nerves in the rhesus monkey (Macaca mulatta)

Molecular cell types as functional units of the efferent vagus nerve

The vagus nerve vitally connects the brain and body to coordinate digestive, cardiorespiratory, and immune functions. Its efferent neurons, which project their axons from the brainstem to the viscera, are thought to comprise "functional units" - neuron populations dedicated to the control of specific vagal reflexes or organ functions. Previous research indicates that these functional units differ from one another anatomically, neurochemically, and physiologically but have yet to define their identity in an experimentally tractable way. However, recent work with genetic technology and single-cell genomics suggests that genetically distinct subtypes of neurons may be the functional units of the efferent vagus. Here we review how these approaches are revealing the organizational principles of the efferent vagus in unprecedented detail.

The Role of Central Oxytocin in Autonomic Regulation

Oxytocin (OXT), a neuropeptide originating from the hypothalamus and traditionally associated with peripheral functions in parturition and lactation, has emerged as a pivotal player in the central regulation of the autonomic nervous system (ANS). This comprehensive ANS, comprising sympathetic, parasympathetic, and enteric components, intricately combines sympathetic and parasympathetic influences to provide unified control. The central oversight of sympathetic and parasympathetic outputs involves a network of interconnected regions spanning the neuroaxis, playing a pivotal role in the real-time regulation of visceral function, homeostasis, and adaptation to challenges. This review unveils the significant involvement of the central OXT system in modulating autonomic functions, shedding light on diverse subpopulations of OXT neurons within the paraventricular nucleus of the hypothalamus and their intricate projections. The narrative progresses from the basics of central ANS regulation to a detailed discussion of the central controls of sympathetic and parasympathetic outflows. The subsequent segment focuses specifically on the central OXT system, providing a foundation for exploring the central role of OXT in ANS regulation. This review synthesizes current knowledge, paving the way for future research endeavors to unravel the full scope of autonomic control and understand multifaceted impact of OXT on physiological outcomes.

Signal processing in the vagus nerve: Hypotheses based on new genetic and anatomical evidence

Each organism must regulate its internal state in a metabolically efficient way as it interacts in space and time with an ever-changing and only partly predictable world. Success in this endeavor is largely determined by the ongoing communication between brain and body, and the vagus nerve is a crucial structure in that dialogue. In this review, we introduce the novel hypothesis that the afferent vagus nerve is engaged in signal processing rather than just signal relay. New genetic and structural evidence of vagal afferent fiber anatomy motivates two hypotheses: (1) that sensory signals informing on the physiological state of the body compute both spatial and temporal viscerosensory features as they ascend the vagus nerve, following patterns found in other sensory architectures, such as the visual and olfactory systems; and (2) that ascending and descending signals modulate one another, calling into question the strict segregation of sensory and motor signals, respectively. Finally, we discuss several implications of our two hypotheses for understanding the role of viscerosensory signal processing in predictive energy regulation (i.e., allostasis) as well as the role of metabolic signals in memory and in disorders of prediction (e.g., mood disorders).

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

Neurocardiology: Structure-Based Function

Cardiac control is mediated via a series of reflex control networks involving somata in the (i) intrinsic cardiac ganglia (heart), (ii) intrathoracic extracardiac ganglia (stellate, middle cervical), (iii) superior cervical ganglia, (iv) spinal cord, (v) brainstem, and (vi) higher centers. Each of these processing centers contains afferent, efferent, and local circuit neurons, which interact locally and in an interdependent fashion with the other levels to coordinate regional cardiac electrical and mechanical indices on a beat-to-beat basis. This control system is optimized to respond to normal physiological stressors (standing, exercise, and temperature); however, it can be catastrophically disrupted by pathological events such as myocardial ischemia. In fact, it is now recognized that autonomic dysregulation is central to the evolution of heart failure and arrhythmias. Autonomic regulation therapy is an emerging modality in the management of acute and chronic cardiac pathologies. Neuromodulation-based approaches that target select nexus points of this hierarchy for cardiac control offer unique opportunities to positively affect therapeutic outcomes via improved efficacy of cardiovascular reflex control. As such, understanding the anatomical and physiological basis for such control is necessary to implement effectively novel neuromodulation therapies. © 2016 American Physiological Society. Compr Physiol 6:1635-1653, 2016.

Parasympathetic preganglionic cardiac motoneurons labeled after voluntary diving

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2 bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known.

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.

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.

Effects of nuclei ambiguus and dorsal motor nuclei of vagus on gastric H+ and HCO3- secretion in rats

AIM: To determine the effects of electrical stimulation of nucleus ambiguus (NA) and dorsal motor nuclei of vagus (DMV) on gastric acid and bicarbonate secretion in rats.

METHODS: NA and DMV in rats were electrically stimulated. Pylorus ligation or esophagus perfusion was used to collect the gastric secretion. The titratable H⁺ quantum, H⁺ concentration, HCO₃^- secretion quantum were measured.

RESULTS: Electrical stimulation of NA had no effects on the volume of gastric juice, titratable acidity and acid concentration, but elicited a pronounced increase in the total bicarbonate. However, electrical stimulation of DMV significantly increased the titratable acidity, the volume of gastric juice and the acid concentration. Similarly, electrical stimulation of either NA or DMV decreased the respiratory frequency and sinus bradycardia.

CONCLUSION: NA in rats can not control the secretion of gastric acid but the secretion of bicarbonate in gastric juice, while DMV controls the secretion of gastric acid.

Organization of the newborn piglets vagal motor complex: insights into integrated autonomic control mechanisms

Pediatric disorders frequently exhibit dysregulation of sympatho-vagal activity, and impaired control of cardiovascular vagal networks. Factors influencing the maturation of vagal networks are of special interest because they normally protect the heart and circulation, facilitate digestion, and preserve visceral metabolism. At present, scant literature exists regarding the development of vagal innervation of the heart. This study in neonatal swine, Sus scrofa, mapped the normal anatomy of vagal motor cell groups, with special focus on the origins of cardiomotor neurons. Right cardiac nerve branches, or the right thoracic vagal trunk were resected, inserted into capillary glass vials filled with 2% FluoroGold (FG) tracer solutions, and sealed to prevent leakage (false positives). Dorsal and ventral vagal complexes were identified on cross-sectioned tissues incubated in a well-characterized specific FG antibody. Thoracic and abdominal vagal motoneurons were cytologically heterogeneous, and predominantly medium-sized, polygonal cell bodies. Discrete longitudinal cell columns were observed, as well as organized arrays of elongate spindle-shaped cells in formation. Long axes and dendrites appeared to orient toward incoming peripheral sensory and central afferents, and were juxtaposed to cerebral microvasculature. The piglets' dorsal vagal complex is: (i) thick and long, comparable to ruminants, in contrast to much shorter lengths in non-ruminants, and (ii) the chief source of vagal motoneurons, forming discrete, topographically organized parasympathetic cell groups with distinct dendritic arbors. The cardiac motor subnucleus is localized to a highly restricted areal subunit of nucleus ambiguus' external formation in the vicinity of the obex. On the other hand, dorsal cardiac vagal motoneurons were few in number and diffusely distributed. Dorsal vagal motoneurons of neonatal swine likely projected primarily to the enteric nervous system, exerting excitatory influence over gastrointestinal activity.

Attenuation of baroreflex sensitivity after domoic acid lesion of the nucleus ambiguus of rats

The nucleus ambiguus (NA) and the dorsal motor nucleus of the vagus (DmnX) innervate distinct populations of cardiac ganglionic principal neurons. This anatomic evidence suggests that these two nuclei play different roles (Cheng Z and Powley TL, Soc Neurosci Abstr 26: 1189, 2000). However, lesion of the DmnX does not attenuate baroreflex sensitivity (Cheng Z, Guo SZ, Lipton AJ, and Gozal D, J Neurosci 22: 3215-3226, 2002). The present study tested the functional role of the NA in baroreflex control of heart rate (HR). Domoic acid (DA) was injected into the left NA of Sprague-Dawley rats to lesion the NA. The neuronal loss was assessed using retrograde labeling and confocal microscopy. HR changes induced by phenylephrine and sodium nitroprusside administration and after electrical stimulation of the left vagal trunk were measured at 15 days, and HR responses to left NA microinjection of L-glutamate were determined at 180 days postlesion. Compared with vehicle injections, DA lesions significantly reduced the population of NA motor neurons by approximately 68% (P < 0.01) and attenuated baroreflex sensitivity by approximately 83% (P < 0.01) at 15 days. Similarly, electrical stimulation of the vagal trunk of DA-lesioned animals led to attenuated decreases in HR responses. NA neuronal counts were reduced by approximately 81% (P < 0.01) and mean HR responses to l-glutamate injection into the lesioned NA were attenuated by approximately 65% (P < 0.01) at 180 days. Therefore, the NA plays a major role in baroreflex control of HR, and the integrity of the NA is critically important for the normal baroreflex control. In addition, NA lesions produce long-term anatomic and functional dysfunction of the nucleus, and thus it may provide an useful model for functional assessment of respective roles of the NA and DmnX.

Monosynaptic projections from the nucleus retroambiguus region to laryngeal motoneurons in the rhesus monkey

Vocalization and straining-related activities require the activation of laryngeal muscles. The control of laryngeal muscles during these activities is thought to be mediated by a pathway from the periaqueductal gray via premotor neurons in the nucleus retroambiguus to laryngeal motoneurons in the nucleus ambiguus. However, direct contacts between the nucleus retroambiguus and laryngeal motoneurons have never been demonstrated anatomically. Moreover, data in primates about the nucleus retroambiguus-nucleus ambiguus pathway are lacking. Therefore, the present study examines the projection from the nucleus retroambiguus region to laryngeal motoneurons in the rhesus monkey at the light and electron microscopic levels. Injections with wheat germ agglutinin-horseradish peroxidase were made into the nucleus retroambiguus in five rhesus monkeys to anterogradely label fibers in the nucleus ambiguus. In two of these animals, the cricothyroid muscle was injected with cholera toxin subunit b to identify the motoneurons that supply it. The results show that the nucleus retroambiguus region most densely projects to the compact formation of the nucleus ambiguus, whereas cricothyroid motoneurons, which surround the compact formation, receive a moderate projection. The projections are bilateral, with a contralateral predominance. Ultrastructurally, anterogradely labeled terminal profiles from the nucleus retroambiguus contact cholera toxin subunit b-labeled dendrites of cricothyroid motoneurons. The terminal profiles contain primarily spherical vesicles and form asymmetrical contacts with cricothyroid motoneurons. This study demonstrates that the nucleus retroambiguus region projects to the nucleus ambiguus in the primate. Some of these projections include monosynaptic connections to laryngeal motoneurons. This pathway is important for the control of the vocal folds during vocalization and straining-related activities.

Monosynaptic projections from the lateral periaqueductal gray to the nucleus retroambiguus in the rhesus monkey: implications for vocalization and reproductive behavior

The periaqueductal gray (PAG) is known to be essential for vocalization and reproductive behavior. The PAG controls components of these behaviors by means of projections to the nucleus retroambiguus (NRA), a group of premotor neurons in the caudal medulla oblongata. In the accompanying study (VanderHorst et al., 2000 [accompanying study]), the NRA and its lumbosacral projections have been identified in the rhesus monkey. The present light and electron microscopical tracing study describes the PAG-NRA pathway in primates. To locate midbrain neurons projecting to the NRA, wheat germ agglutinin horseradish peroxidase (WGA-HRP) was injected into the NRA in six monkeys. To determine the distribution pattern of PAG axons in the medulla oblongata, WGA-HRP was injected into the PAG and adjacent tegmentum in three additional monkeys. In one of these three monkeys, biotinylated dextran amine and cholera toxin subunit b were injected into the lumbosacral cord to retrogradely identify NRA neurons. The results show that a compact group of neurons in the medial part of the lateral PAG at the intercollicular level sends a dense projection to the NRA. The projection is bilateral with a clear ipsilateral predominance. At the ultrastructural level, there are monosynaptic contacts between PAG fibers and NRA neurons, including NRA neurons that project to the lumbosacral cord. The synaptic contacts were primarily asymmetrical and the labeled terminal profiles contained spherical and dense core vesicles. It is concluded that there exists a strong and direct PAG-NRA pathway in the rhesus monkey. Because NRA neurons projecting to the lower lumbar cord are included, the PAG-NRA projection is likely to be involved not only in vocalization but also in other behaviors, such as receptive posture.

Localization of sympathetic, parasympathetic and sensory neurons innervating the heart of the Beijing duck by means of the retrograde transport of horseradish peroxidase

Sympathetic, parasympathetic and sensory neurons were labeled by injections of horseradish peroxidase into various regions of the heart in 33 Beijing ducks. Sympathetic postganglionic neurons innervating the heart were located in the paravertebral ganglia C15 (C16 is the last cervical segment in the duck) to T3, especially in the ganglion T1. The coronary sulcus and ventricle were more abundantly innervated by sympathetic neurons than the atrium. The left side of the heart was preferentially innervated by sympathetic postganglionic neurons in the left side of paravertebral ganglia but the right side of the heart were equally supplied from the right and left ganglia. Within the medulla oblongata, the number of labeled vagal preganglionic neurons in the nucleus ambiguus was much greater than that in the dorsal motor nucleus of the vagus nerve. Labeled neurons of the nucleus ambiguus were found in many ducks injected into the coronary sulcus. Cardiac sensory neurons were observed in the dorsal root ganglia C15 to T2 (highest in the ganglion T1) and in the nodose and jugular ganglia of the vagus nerve. These labeled neurons probably form the afferent and efferent limbs of cardiac reflexes and control circulation in the Beijing duck.