Respiratory sinus arrhythmia is a major component of heart rate variability in undisturbed, remotely monitored rattlesnakes, Crotalus durissus

Cardiovascular responses and the role of the neurohumoral cardiac regulation during digestion in the herbivorous lizard Iguana iguana

Carnivorous reptiles exhibit an intense metabolic increment during digestion, which is accompanied by several cardiovascular adjustments responsible for meeting the physiological demands of the gastrointestinal system. Postprandial tachycardia, a well-documented phenomenon in these animals, is mediated by the withdrawal of vagal tone associated with the chronotropic effects of non-adrenergic and non-cholinergic (NANC) factors. However, herbivorous reptiles exhibit a modest metabolic increment during digestion and there is no information about postprandial cardiovascular adjustments. Considering the significant impact of feeding characteristics on physiological responses, we investigated cardiovascular and metabolic responses, as well as the neurohumoral mechanisms of cardiac control, in the herbivorous lizard Iguana iguana during digestion. We measured oxygen consumption rate (O2), heart rate (fH), mean arterial blood pressure (MAP), myocardial activity, cardiac autonomic tone, fH/MAP variability and baroreflex efficiency in both fasting and digesting animals before and after parasympathetic blockade with atropine followed by double autonomic blockade with atropine and propranolol. Our results revealed that the peak of O2 in iguanas was reached 24 h after feeding, accompanied by an increase in myocardial activity and a subtle tachycardia mediated exclusively by a reduction in cardiac parasympathetic activity. This represents the first reported case of postprandial tachycardia in digesting reptiles without the involvement of NANC factors. Furthermore, this withdrawal of vagal stimulation during digestion may reduce the regulatory range for short-term fH adjustments, subsequently intensifying the blood pressure variability as a consequence of limiting baroreflex efficiency.

The vagal paradox: A polyvagal solution

Although there is a consistent literature documenting that vagal cardioinhibitory pathways support homeostatic functions, another less frequently cited literature implicates vagal cardioinhibitory pathways in compromises to survival in humans and other mammals. The latter is usually associated with threat reactions, chronic stress, and potentially lethal clinical conditions such as hypoxia. Solving this 'vagal paradox' in studies conducted in the neonatal intensive care unit served as the motivator for the Polyvagal Theory (PVT). The paradox is resolved when the different functions of vagal cardioinhibitory fibers originating in two anatomically distinguishable brainstem areas are recognized. One pathway originates in a dorsal area known as the dorsal motor nucleus of the vagus and the other in a ventral area of the brainstem known as nucleus ambiguus. Unlike mammals, in all ancestral vertebrates from which mammals evolved, cardioinhibitory vagal fibers primarily originate in the dorsal motor nucleus of the vagus. Thus, in mammals the vagus nerve is 'poly' vagal because it contains two distinct efferent pathways. Developmental and evolutionary biology identify a ventral migration of vagal cardioinhibitory fibers that culminate in an integrated circuit that has been labeled the ventral vagal complex. This complex consists of the interneuronal communication of the ventral vagus with the source nuclei involved in regulating the striated muscles of the head and face via special visceral efferent pathways. This integrated system enables the coordination of vagal regulation of the heart with sucking, swallowing, breathing, and vocalizing and forms the basis of a social engagement system that allows sociality to be a potent neuromodulator resulting in calm states that promote homeostatic function. These biobehavioral features, dependent on the maturation of the ventral vagal complex, can be compromised in preterm infants. Developmental biology informs us that in the immature mammal (e.g., fetus, preterm infant) the ventral vagus is not fully functional and myelinization is not complete; this neuroanatomical profile may potentiate the impact of vagal cardioinhibitory pathways originating in the dorsal motor nucleus of the vagus. This vulnerability is confirmed clinically in the life-threatening reactions of apnea and bradycardia in human preterm newborns, which are hypothetically mediated through chronotropic dorsal vagal pathways. Neuroanatomical research documents that the distribution of cardioinhibitory neurons representing these two distinct vagal source nuclei varies among mammals and changes during early development. By explaining the solution of the 'vagal paradox' in the preterm human, the paper highlights the functional cardioinhibitory functions of the two vagal source nuclei and provides the scientific foundation for the testing of hypotheses generated by PVT.

FUNDAMENTAL CHALLENGES AND LIKELY REFUTATIONS OF THE FIVE BASIC PREMISES OF THE POLYVAGAL THEORY

The polyvagal collection of hypotheses is based upon five essential premises, as stated by its author (Porges, 2011). Polyvagal conjectures rest on a primary assumption that the brainstem ventral and dorsal regions in mammals each have their own unique mediating effects upon vagal control of heart rate. The polyvagal hypotheses link these putative dorsal- vs. ventral-vagal differences to socioemotional behavior (e.g. defensive immobilization, and social affiliative behaviors, respectively), as well as to trends in the evolution of the vagus nerve (e.g. Porges, 2011 & 2021a). Additionally, it is essential to note that only one measurable phenomenon-as index of vagal processes-serves as the linchpin for virtually every premise. That phenomenon is respiratory sinus arrhythmia (RSA), heart-rate changes coordinated to phase of respiration (i.e. inspiration vs. expiration), often employed as an index of vagally, or parasympathetically, mediated control of heart rate. The polyvagal hypotheses assume that RSA is a mammalian phenomenon, since Porges (2011) states "RSA has not been observed in reptiles." I will here briefly document how each of these basic premises have been shown to be either untenable or highly implausible based on the available scientific literature. I will also argue that the polyvagal reliance upon RSA as equivalent to general vagal tone or even cardiac vagal tone is conceptually a category mistake (Ryle, 1949), confusing an approximate index (i.e. RSA) of a phenomenon (some general vagal process) with the phenomenon, itself.

Adrenergic control of functional characteristics of the cardiovascular system in the South American rattlesnake, Crotalus durissus

In squamate reptiles, extensive innervation of the heart and vascular beds allows for continuous modulation of the cardiovascular system by the autonomic nervous system. The systemic vasculature is the main target of excitatory sympathetic adrenergic fibers, while the pulmonary circulation has been described as less responsive to both nervous and humoral modulators. However, histochemical evidence has demonstrated the presence of adrenergic fibers in pulmonary circulation. Besides, reduced responsiveness is intriguing since the balance of regulation between systemic and pulmonary vascular circuits has critical hemodynamic implications in animals with an undivided ventricle and consequent cardiovascular shunts. The present study investigated the role and functional relevance of α and β-adrenergic stimulation in regulating systemic and mainly the pulmonary circulations in a decerebrate, autonomically responsive rattlesnake preparation. The use of the decerebrate preparation allowed us to observe a new diverse functional modulation of vascular beds and the heart. In resting snakes, the pulmonary vasculature is less reactive to adrenergic agonists at 25 °C. However, the β-adrenergic tone is relevant for modulating resting peripheral pulmonary conductance, while both α- and β-adrenergic tones are relevant for the systemic circuit. Active dynamic modulation of both pulmonary compliance and conductance effectively counterbalances alterations in the systemic circulation to maintain the R-L shunt pattern. Furthermore, we suggest that despite the great attention given to cardiac adjustments, vascular modulation is sufficient to support the hemodynamic adjustments needed to control blood pressure.

Acquirement of the autonomic nervous system modulation evaluated by heart rate variability in medaka (Oryzias latipes)

Small teleosts have recently been established as models of human diseases. However, measuring heart rate by electrocardiography is highly invasive for small fish and not widely used. The physiological nature and function of vertebrate autonomic nervous system (ANS) modulation of the heart has traditionally been investigated in larvae, transparent but with an immature ANS, or in anesthetized adults, whose ANS activity may possibly be disturbed under anesthesia. Here, we defined the frequency characteristics of heart rate variability (HRV) modulated by the ANS from observations of heart movement in high-speed movie images and changes in ANS regulation under environmental stimulation in unanesthetized adult medaka (Oryzias latipes). The HRV was significantly reduced by atropine (1 mM) in the 0.25-0.65 Hz and by propranolol (100 μM) at 0.65-1.25 Hz range, suggesting that HRV in adult medaka is modulated by both the parasympathetic and sympathetic nervous systems within these frequency ranges. Such modulations of HRV by the ANS in adult medaka were remarkably suppressed under anesthesia and continuous exposure to light suppressed HRV only in the 0.25-0.65 Hz range, indicating parasympathetic withdrawal. Furthermore, pre-hatching embryos did not show HRV and the power of HRV developed as fish grew. These results strongly suggest that ANS modulation of the heart in adult medaka is frequency-dependent phenomenon, and that the impact of long-term environmental stimuli on ANS activities, in addition to development of ANS activities, can be precisely evaluated in medaka using the presented method.

Baroreflex responses to activity at different temperatures in the South American rattlesnake, Crotalus durissus

In humans, physical exercise imposes narrower limits for the heart rate (f_H) response of the baroreflex, and vascular modulation becomes largely responsible for arterial pressure regulation. In undisturbed reptiles, the baroreflex-related f_H alterations at the operating point (G_op) decreases at elevated body temperatures (T_b) and the vascular regulation changes accordingly. We investigated how the baroreflex of rattlesnakes, Crotalus durissus, is regulated during an activity at different T_b, expecting that activity would reduce the capacity of the cardiac baroreflex neural pathway to buffer arterial pressure fluctuations while being compensated by the vascular neural pathway regulation. Snakes were catheterized for blood pressure assessment at three different T_b: 15, 20 and 30 °C. Data were collected before and after activity at each T_b. Baroreflex gain (G_op) was assessed with the sequence method; the vascular limb, with the time constant of pressure decay (τ), using the two-element Windkessel equation. Both G_op and τ reduced when T_b increased. Activity also reduced G_op and τ in all T_b. The relationship between τ and pulse interval (τ/PI) was unaffected by the temperature at resting snakes, albeit it reduced after activity at 20 °C and 30 °C. The unchanged τ/PI and normalized G_op at different T_b indicated those variables are actively adjusted to work at different f_H and pressure conditions at rest. Our data suggest that during activity, the baroreflex-related f_H response is attenuated and hypertension is buffered by a disproportional increase in the rate which pressure decays during diastole. This compensation seems especially important at higher T_b where G_op is already low.

Quantifying the Autonomic Response to Stressors-One Way to Expand the Definition of "Stress" in Animals

Quantifying how whole organisms respond to challenges in the external and internal environment ("stressors") is difficult. To date, physiological ecologists have mostly used measures of glucocorticoids (GCs) to assess the impact of stressors on animals. This is of course too simplistic as Hans Seyle himself characterized the response of organisms to "noxious stimuli" using multiple physiological responses. Possible solutions include increasing the number of biomarkers to more accurately characterize the "stress state" of animal or just measuring different biomarkers to more accurately characterize the degree of acute or chronic stressors an animal is experiencing. We focus on the latter and discuss how heart rate (HR) and heart rate variability (HRV) may be better predictors of the degree of activation of the sympathetic-adrenal-medullary system and complement or even replace measures of GCs as indicators of animal health, welfare, fitness, or their level of exposure to stressors. The miniaturization of biological sensor technology ("bio-sensors" or "bio-loggers") presents an opportunity to reassess measures of stress state and develop new approaches. We describe some modern approaches to gathering these HR and HRV data in free-living animals with the aim that heart dynamics will be more integrated with measures of GCs as bio-markers of stress state and predictors of fitness in free-living animals.

Postprandial cardiorespiratory responses and the regulation of digestion-associated tachycardia in Nile tilapia (Oreochromis niloticus)

Cardiorespiratory adjustments that occur after feeding are essential to supply the demands of digestion in vertebrates. The well-documented postprandial tachycardia is triggered by an increase in adrenergic activity and by non-adrenergic non-cholinergic (NANC) factors in mammals and crocodilians, while it is linked to a withdrawal of vagal drive and NANC factors in non-crocodilian ectotherms-except for fish, in which the sole investigation available indicated no participation of NANC factors. On the other hand, postprandial ventilatory adjustments vary widely among air-breathing vertebrates, with different species exhibiting hyperventilation, hypoventilation, or even no changes at all. Regarding fish, which live in an environment with low oxygen capacitance that requires great ventilatory effort for oxygen uptake, data on the ventilatory consequences of feeding are also scarce. Thus, the present study sought to investigate the postprandial cardiorespiratory adjustments and the mediation of digestion-associated tachycardia in the unimodal water-breathing teleost Oreochromis niloticus. Heart rate (f_H), cardiac autonomic tones, ventilation rate (f_V), ventilation amplitude, total ventilation and f_H/f_V variability were assessed both in fasting and digesting animals under untreated condition, as well as after muscarinic cholinergic blockade with atropine and double autonomic blockade with atropine and propranolol. The results revealed that digestion was associated with marked tachycardia in O. niloticus, determined by a reduction in cardiac parasympathetic activity and by circulating NANC factors-the first time such positive chronotropes were detected in digesting fish. Unexpectedly, postprandial ventilatory alterations were not observed, although digestion triggered mechanisms that were presumed to increase oxygen uptake, such as cardiorespiratory synchrony.

Cholinergic regulation along the pulmonary arterial tree of the South American rattlesnake: vascular reactivity, muscarinic receptors, and vagal innervation

Vascular tone in the reptilian pulmonary vasculature is primarily under cholinergic, muscarinic control exerted via the vagus nerve. This control has been ascribed to a sphincter located at the arterial outflow, but we speculated whether the vascular control in the pulmonary artery is more widespread, such that responses to acetylcholine and electrical stimulation, as well as the expression of muscarinic receptors, are prevalent along its length. Working on the South American rattlesnake ( Crotalus durissus), we studied four different portions of the pulmonary artery (truncus, proximal, distal, and branches). Acetylcholine elicited robust vasoconstriction in the proximal, distal, and branch portions, but the truncus vasodilated. Electrical field stimulation (EFS) caused contractions in all segments, an effect partially blocked by atropine. We identified all five subtypes of muscarinic receptors (M1–M5). The expression of the M1 receptor was largest in the distal end and branches of the pulmonary artery, whereas expression of the muscarinic M3 receptor was markedly larger in the truncus of the pulmonary artery. Application of the neural tracer 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindo-carbocyanine perchlorate (DiI) revealed widespread innervation along the whole pulmonary artery, and retrograde transport of the same tracer indicated two separate locations in the brainstem providing vagal innervation of the pulmonary artery, the medial dorsal motor nucleus of the vagus and a ventro-lateral location, possibly constituting a nucleus ambiguus. These results revealed parasympathetic innervation of a large portion of the pulmonary artery, which is responsible for regulation of vascular conductance in C. durissus, and implied its integration with cardiorespiratory control.

Heart rate and cardiac response to exercise during voluntary dives in captive sea turtles (Cheloniidae)

In chelonids, oxygen is primarily stored in the lungs during a dive. Therefore, management of blood oxygen transportation to peripheral tissues by cardiovascular adjustments during submergence is crucial to maximize their dive duration, and consequently, the time spent for ecological activities such as foraging. However, the cardiac response to exercise has rarely been examined in sea turtles. In this study, heart rate and its relationship with exercise during voluntary dives were determined in six captive green turtles (19.4±1.5 kg) by simultaneously recording depth, acceleration and electrocardiogram. Our results demonstrated that the heart rate of green turtles was generally low (11.1±0.4 bpm) during resting dives, but they often exhibited instantaneously extreme tachycardia (up to 78.4 bpm). Green turtles elevated their heart rate up to 39.8±1.5 bpm during ventilation after resting dives, while up to 33.1±1.4 bpm after active dives. The heart rate immediately elevated with onset of exercise, and increased linearly with exercise. This result may indicate that turtles immediately need to transport oxygen from the lungs to peripheral tissues by pulmonary and systemic circulations to meet the metabolic demands of exercise because they mainly store oxygen in their lungs.

Heart rate variability in the tegu lizard, Salvator merianae, its neuroanatomical basis and role in the assessment of recovery from experimental manipulation

Using long-term, remote recordings of heart rate (f_H) on fully recovered, undisturbed lizards, we identified several components of heart rate variability (HRV) associated with respiratory sinus arrhythmia (RSA): 1.) A peak in the spectral representation of HRV at the frequency range of ventilation. 2.) These cardiorespiratory interactions were shown to be dependent on the parasympathetic arm of the autonomic nervous system. 3.) Vagal preganglionic neurons are located in discrete groups located in the dorsal motor nucleus of the vagus and also, in a ventro-lateral group, homologous to the nucleus ambiguus of mammals. 4.) Myelinated nerve fibers in the cardiac vagus enabling rapid communication between the central nervous system and the heart. Furthermore, the study of the progressive recovery of f_H in tegu following anesthesia and instrumentation revealed that 'resting' levels of mean f_H and reestablishment of HRV occurred over different time courses. Accordingly, we suggest that, when an experiment is designed to study a physiological variable reliant on autonomic modulation at its normal, resting level, then postsurgical reestablishment of HRV should be considered as the index of full recovery, rather than mean f_H.