Chemoreceptor control of heart rate and behaviour during diving in the tufted duck (Aythya fuligula)

Conceptual foundations of physiological regulation incorporating the free energy principle and self-organized criticality

Bettinger, J. S., K. J. Friston. Conceptual Foundations of Physiological Regulation incorporating the Free Energy Principle & Self-Organized Criticality. NEUROSCI BIOBEHAV REV 23(x) 144-XXX, 2022. Since the late nineteen-nineties, the concept of homeostasis has been contextualized within a broader class of "allostatic" dynamics characterized by a wider-berth of causal factors including social, psychological and environmental entailments; the fundamental nature of integrated brain-body dynamics; plus the role of anticipatory, top-down constraints supplied by intrinsic regulatory models. Many of these evidentiary factors are integral in original descriptions of homeostasis; subsequently integrated; and/or cite more-general operating principles of self-organization. As a result, the concept of allostasis may be generalized to a larger category of variational systems in biology, engineering and physics in terms of advances in complex systems, statistical mechanics and dynamics involving heterogenous (hierarchical/heterarchical, modular) systems like brain-networks and the internal milieu. This paper offers a three-part treatment. 1) interpret "allostasis" to emphasize a variational and relational foundation of physiological stability; 2) adapt the role of allostasis as "stability through change" to include a "return to stability" and 3) reframe the model of homeostasis with a conceptual model of criticality that licenses the upgrade to variational dynamics.

Carotid body stimulation as a potential intervention in sudden death in epilepsy

OBJECTIVE

To investigate carotid body (CB) mechanisms related to sudden death during seizure. Ictal activation of oxygen-conserving reflexes (OCRs) can trigger fatal cardiorespiratory collapse in seizing rats, which presents like human sudden unexpected death in epilepsy (SUDEP). The CB is strongly implicated in OCR pathways; we hypothesize that modulating CB activity will provide insight into these mechanisms of death.

METHODS

Long-Evans rats were anesthetized with urethane. Recordings included: electrocorticography, electrocardiography, respiration via nasal thermocouple, and blood pressure (BP). The mammalian diving reflex (MDR) was activated by cold water delivered through a nasal cannula. Reflex and stimulation trials were repeated up to 16 times (4 pre-intervention, 12 post-intervention) or until death. In some animals, one or both carotid bodies were denervated. In some animals, the CB was electrically stimulated, both with and without MDR. Seizures were induced with kainic acid (KA).

RESULTS

Animals without seizure and with no CB modulation survived all reflexes. Non-seizing animals with CB denervation survived 7.1 ± 5.4 reflexes before death, and only 1 of 7 survived past the 12-trial threshold. Electrical CB stimulation without seizure and without reflex caused significant tachypnea and hypotension. Electrical CB stimulation with seizure and without reflex required higher amplitudes to replicate the physiological responses seen outside seizure. Seizing animals without CB intervention survived 3.2 ± 3.6 trials (per-reflex survival rate 42.0% ± 44.4%), and 0 of 7 survived past the 12-trial threshold. Seizing animals with electrical CB stimulation survived 10.5 ± 4.7 ictal trials (per-reflex survival rate 86.3% ± 35.0%), and 6 of 8 survived past the 12-trial threshold.

SIGNIFICANCE

These results suggest that, during seizure, the ability of the CB to stimulate a restart of respiration is impaired. The CB and its afferents may be relevant to fatal ictal apnea and SUDEP in humans, and CB stimulation may be a relevant intervention technique in these deaths.

Heart rate regulation and extreme bradycardia in diving emperor penguins

To investigate the diving heart rate (f(H)) response of the emperor penguin (Aptenodytes forsteri), the consummate avian diver, birds diving at an isolated dive hole in McMurdo Sound, Antarctica were outfitted with digital electrocardiogram recorders, two-axis accelerometers and time depth recorders (TDRs). In contrast to any other freely diving bird, a true bradycardia (f(H) significantly <f(H) at rest) occurred during diving [dive f(H) (total beats/duration)=57+/-2 beats min(-1), f(H) at rest=73+/-2 beats min(-1) (mean +/- s.e.m.)]. For dives less than the aerobic dive limit (ADL; duration beyond which [blood lactate] increases above resting levels), dive f(H)=85+/-3 beats min(-1), whereas f(H) in dives greater than the ADL was significantly lower (41+/-1 beats min(-1)). In dives greater than the ADL, f(H) reached extremely low values: f(H) during the last 5 mins of an 18 min dive was 6 beats min(-1). Dive f(H) and minimum instantaneous f(H) during dives declined significantly with increasing dive duration. Dive f(H) was independent of swim stroke frequency. This suggests that progressive bradycardia and peripheral vasoconstriction (including isolation of muscle) are primary determinants of blood oxygen depletion in diving emperor penguins. Maximum instantaneous surface interval f(H) in this study is the highest ever recorded for emperor penguins (256 beats min(-1)), equivalent to f(H) at V(O(2)) max., presumably facilitating oxygen loading and post-dive metabolism. The classic Scholander-Irving dive response in these emperor penguins contrasts with the absence of true bradycardia in diving ducks, cormorants, and other penguin species.

Control of ventilation in diving birds

Studies on diving ducks indicate that the carotid bodies affect dive duration when the birds are hypoxic before a dive but not when they are hypercapnic. When close to their critical concentrations (beyond which the ducks will not dive), both oxygen and carbon dioxide reduce dive duration but hypercapnia has a much larger influence than hypoxia on surface duration. Also, excessive removal of carbon dioxide before a dive may be as important a factor in preparing for that dive as the replacement of the oxygen used during the previous dive. This observation is compatible with a physiological model of the control of diving behaviour in the Weddell seal which emphasises the significance of the level of carbon dioxide in the blood perfusing the brain.

Optimal diving behaviour and respiratory gas exchange in birds

This review discusses the advancements in our understanding of the physiology and behaviour of avian diving that have been underpinned by optimal foraging theory and the testing of optimal models. To maximise their foraging efficiency during foraging periods, diving birds must balance numerous factors that are directly or indirectly related to the replenishment of the oxygen stores and the removal of excess carbon dioxide. These include (1) the time spent underwater (which diminishes the oxygen supply, increases carbon dioxide levels and may even include a build up of lactate due to anaerobic metabolism), (2) the time spent at the surface recovering from the previous dive and preparing for the next (including reloading their oxygen supply, decreasing their carbon dioxide levels and possibly also metabolising lactate) and (3) the trade-off between maximising oxygen reserves for consumption underwater by taking in more air to the respiratory system, and minimising the energy costs of positive buoyancy caused by this air, to maximise the time available underwater to forage. Due to its importance in avian diving, replenishment of the oxygen stores has become integral to models of optimal diving, which predict the time budgeting of animals foraging underwater. While many of these models have been examined qualitatively, such tests of predictive trends appear fallible and only quantifiable support affords strong evidence of their predictive value. This review describes how the quantification of certain optimal diving models, using tufted ducks, indeed demonstrates some predictive success. This suggests that replenishment of the oxygen stores and removal of excess carbon dioxide have significant influences on the duration of the surface period between dives. Nevertheless, present models are too simplistic to be robust predictors of diving behaviour for individual animals and it is proposed that they require refinement through the incorporation of other variables that also influence diving behaviour such as, perhaps, prey density and predator avoidance.

To What Extent Is the Foraging Behaviour of Aquatic Birds Constrained by Their Physiology?

Aquatic birds have access to limited amounts of usable oxygen when they forage (dive) underwater, so the major physiological constraint to their behaviour is the need to periodically visit the water surface to replenish these stores and remove accumulated carbon dioxide. The size of the oxygen stores and the rate at which they are used (V dot o2) or carbon dioxide accumulates are the ultimate determinants of the duration that aquatic birds can remain feeding underwater. However, the assumption that the decision to terminate a dive is governed solely by the level of the respiratory stores is not always valid. Quantification of an optimal diving model for tufted ducks (Aythya fuligula) shows that while they dive efficiently by spending a minimum amount of time on the surface to replenish the oxygen used during a dive, they dive with nearly full oxygen stores and surface well before these stores are exhausted. The rates of carbon dioxide production during dives and removal during surface intervals are likely to be at least as important a constraint as oxygen; thus, further developments of optimal diving models should account for their effects. In the field, diving birds will adapt to changing environmental conditions and often maximise the time spent submerged during diving bouts. However, other factors influence the diving depths and durations of aquatic birds, and in some circumstances they are unable to forage sufficiently well to provide food for their offspring. The latest developments in telemetry have demonstrated how diving birds can make physiological decisions based on complex environmental factors. Diving penguins can control their inhaled air volume to match the expected depth, likely prey encounter rate, and buoyancy challenges of the following dive.

A Theoretical Analysis of Diving Performance in the Weddell Seal (Leptonychotes weddelli)

Marine mammals are constrained in their foraging behaviour because, as obligate air breathers, they must undertake regular trips to the water surface to satisfy the need for respiratory gas exchange. Maximum underwater endurance time is determined by O2 supply and demand, but this does not necessarily imply that O2 is the main factor regulating individual dive and surface times. This study presents a theoretical analysis of diving performance that emphasizes a key role for CO2 in the proximate control of diving behaviour. Computer simulations, based on a mathematical model of the mammalian cardiorespiratory control system, are used to investigate the influence of swimming to depth and other energetic stresses (feeding, thermogenesis, sleep) on predicted diving behaviour in an average adult Weddell seal. The plausibility of the proposed model is supported by the study, which replicated published observations of natural diving behaviour in this species. It is suggested that diving behaviour is tuned to oscillations in respiratory drive and that behavioural and physiological factors can alter the dynamic characteristics of the system to achieve a highly adaptable reciprocal interaction that blurs the boundary between physiology and behaviour.

Physiological control of diving behaviour in the Weddell sealLeptonychotes weddelli: a model based on cardiorespiratory control theory

Despite being obligate air breathers, many species of marine mammal are capable of spending most of their lives submerged in water. How they do this has been a subject of intense interest to physiologists for over a century,yet we still do not have a detailed understanding of the physiological mechanisms underlying this behaviour. What are the proximate mechanisms that trigger the 'decisions' to submerge and return to the surface? The present study proposes a model intended to address this question, based on fundamental concepts of cardiorespiratory control. Two basic hypotheses are examined by computer simulation, using a mathematical model of the mammalian cardiorespiratory control system with parameter values for an adult Weddell seal: (1) that the control of diving can be considered to be a respiratory control problem, and (2) that dives are initiated and maintained by disfacilitation of respiratory drive, not inhibition. Computer simulations confirmed the plausibility of these hypotheses. Simulated diving behaviour and physiological responses (ventilation, cardiac output, blood and tissue gas tensions) were consistent with published data from freely diving Weddell seals. Dives up to the estimated aerobic dive limit (ADL, 18-25 min) could be simulated without the need for active inhibition of breathing in this model. This theoretical analysis suggests that the most important physiological adjustments occur during the surface interval phase of the dive cycle and include hyperventilation accompanied by high cardiac output, appropriate regulation of cerebral blood flow and central chemoreceptor threshold shifts. During dives, cardiac output, distribution of peripheral blood flow, splenic contraction and peripheral chemoreflex drives were found to modulate physiological and behavioural responses, but were not essential for simulated dives to occur. The main conclusion from this study is that the central chemoreceptor may be an important mechanism involved in the regulation of diving behaviour, implying that CO2, not O2, is the key regulatory variable in this model. This model includes and extends the ADL concept and suggests an explicit mechanism by which the respiratory control system may play a central role in the regulation of diving behaviour. It is likely that respiratory mechanisms are an important component of a hierarchical behavioural control system and further studies are required to test the qualitative and quantitative validity of the model.

Breathing Hypoxic Gas Affects the Physiology as Well as the Diving Behaviour of Tufted Ducks

We measured the effects of exposure to hypoxia (15% and 11% oxygen) and hypercapnia (up to 4.5% carbon dioxide) on rates of respiratory gas exchange both between and during dives in tufted ducks, Aythya fuligula, to investigate to what extent these may explain changes in diving behaviour. As found in previous studies, the ducks decreased dive duration (t(d)) and increased surface duration when diving from a hypoxic or hypercapnic gas mix. In the hypercapnic conditions, oxygen consumption during the dive cycle was not affected. Oxygen uptake between dives was reduced by only 17% when breathing a hypoxic gas mix of 11% oxygen. However, estimates of the rate of oxygen metabolism during the foraging periods of dives decreased nearly threefold in 11% oxygen. Given that tufted ducks normally dive well within their aerobic dive limits and that they significantly reduced their t(d) during hypoxia, it is not at all clear why they make this physiological adjustment.

The effect of O2 and CO2 on the dive behavior and heart rate of lesser scaup ducks (Aythya affinis): quantification of the critical that initiates a diving bradycardia

Lesser scaup ducks were trained to dive for short and long durations following exposure to various gas concentrations to determine the influence of oxygen (O2) and carbon dioxide (CO2) on diving behavior and heart rate. Compared with normoxia, hyperoxia (50% O2) significantly increased the duration of long dives, whereas severe hypoxia (9% O2) significantly decreased the duration of both short and long dives. Hypercapnia (5% CO2) had no effect on dive duration. Surface intervals were not significantly altered by the oxygen treatments, but significantly increased following CO2 exposure. Heart rate during diving was unaffected by hyperoxia and hypercapnia, but gradually declined in long dives after severe hypoxia. Thus, our results suggest that during the majority of dives, O2 and CO2 levels in lesser scaup ducks are managed through changes in diving behavior without any major cardiovascular adjustments, but below a threshold PaO2, a bradycardia is evoked to conserve the remaining oxygen for hypoxia sensitive tissues. A model of oxygen store utilization during voluntary diving was developed to estimate the critical PaO2 below which bradycardia is initiated (approximately 26 mmHg) and predicted that this critical PaO2 would be reached 19s into a dive after exposure to severe hypoxia, which corresponded exactly with the time of initiation of bradycardia in the severe hypoxia trials.

Kinematics of hovering hummingbird flight along simulated and natural elevational gradients

Hovering flight is one of the most energetically demanding forms of animal locomotion. Despite the cost, hummingbirds regularly hover at high elevations, where flight is doubly challenging because of reduced air density and oxygen availability. We performed three laboratory experiments to examine how air density and oxygen partial pressure influence wingbeat kinematics. In the first study, we experimentally lowered air density but maintained constant oxygen partial pressure. Under these hypodense but normoxic conditions, hummingbirds increased stroke amplitude substantially and increased wingbeat frequency slightly. In the second experiment, we maintained constant air density but decreased oxygen partial pressure. Under these normodense but hypoxic conditions, hummingbirds did not alter stroke amplitude but instead reduced wingbeat frequency until they could no longer generate enough vertical force to offset body weight. In a final combined experiment, we decreased air density but increased oxygen availability, and found that the wingbeat kinematics were unaffected by supplemental oxygen. We also studied hovering and maximally loaded flight performance for 43 hummingbird species distributed along a natural elevational gradient in Peru. During free hovering flight, hummingbirds showed increased stroke amplitude interspecifically at higher elevations, mirroring the intra-individual responses in our first laboratory experiment. During loaded flight, hummingbirds increased both wingbeat frequency and wing stroke amplitude by 19% relative to free-flight values at any given elevation. We conclude that modulation of wing stroke amplitude is a major compensatory mechanism for flight in hypodense or hypobaric environments. By contrast, increases in wingbeat frequency impose substantial metabolic demands, are only elicited transiently and anaerobically, and cannot be used to generate additional sustained lift at high elevations.

The influence of oxygen and carbon dioxide on diving behaviour of tufted ducks, Aythya fuligula

While optimal diving models focus on the diver's oxygen (O(2)) stores as the predominant factor influencing diving behaviour, many vertebrate species surface from a dive before these stores are exhausted and may commence another dive well after their O(2) stores have been resaturated. This study investigates the influence of hypoxia and also hypercapnia on the dive cycle of tufted ducks, Aythya fuligula, in terms of surface duration and dive duration. The birds were trained to surface into a respirometer box after each dive to a feeding tray so that rates of O(2) uptake (VO2) and carbon dioxide output (VCO2) at the surface could be measured. Although Vco2 initially lagged behind Vo2, both respiratory gas stores were close to full adjustment after the average surface duration, indicating that they probably had a similar degree of influence on surface duration. Chemoreceptors, which are known to influence diving behaviour, detect changes in O(2) and CO(2) partial pressures in the arterial blood. Thus, the need to restore blood gas levels appears to be a strong stimulus to continue ventilation. Mean surface duration coincided with peak instantaneous respiratory exchange ratio due to predive anticipatory hyperventilation causing hypocapnia. For comparison, the relationship between surface duration and O(2) uptake in reanalysed data for two grey seals indicated that one animal tended to dive well after fully restocking its O(2) stores, while the other dived at the point of full restocking. More CO(2) is exchanged than O(2) in tufted ducks during the last few breaths before the first dive of a bout, serving to reduce CO(2) stores and suggesting that hypercapnia rather than hypoxia is more often the limiting factor on asphyxia tolerance during dives. Indeed, according to calculations of O(2) stores and O(2) consumption rates over modal diving durations, a lack of O(2) does not seem to be associated with the termination of a dive in tufted ducks. However, factors other than CO(2) are also likely to be important, and perhaps more so, such as food density and rate of food ingestion. Because some predictive success has been demonstrated for optimal diving models, they should continue to incorporate O(2) stores as a variable, but their validity is likely to be improved by also focusing on CO(2) stores.

The effects of depth on the cardiac and behavioural responses of double-crested cormorants (Phalacrocorax auritus) during voluntary diving

Heart rate and dive behaviour were monitored in double-crested cormorants (Phalacrocorax auritus) during shallow (1 m) and deep diving (12 m), after breathing different gas mixtures, to investigate the role of depth and the accompanying changes in blood gas levels in cardiac and behavioural control during voluntary diving. Pre-dive heart rate in both shallow- and deep-diving birds was approximately three times the resting heart rate (137.9±17.5 beats min–1; mean ± s.d., N=5), falling abruptly upon submersion to around 200–250 beats min–1. During shallow diving, the initial reduction in heart rate was followed by a secondary, more gradual decline, to around the resting level. In contrast, during deep diving, heart rate stabilised at 200–250 beats min–1. In dives of similar duration, mean dive heart rate was significantly lower during shallow diving (163.2±14.0 beats min–1) than during deep diving (216.4±7.7 beats min–1), but in both cases was significantly above the resting value. The difference in cardiac response is probably due to an increase in arterial oxygen tension (PaO2) during the descent phase of deep dives (compression hyperoxia). Exposure to a hyperoxic gas mixture before shallow diving significantly increased mean dive heart rate, while exposure to a hypoxic gas mixture in both the shallow and deep dive tanks significantly reduced mean dive heart rate. In contrast, breathing hypercapnic gas before diving had no significant effect on dive heart rate. We suggest that the cardiac response to voluntary diving in double-crested cormorants is strongly influenced by changes in blood oxygen levels throughout the dive. Dive duration was unaffected by alterations in inspired gas composition, but surface interval duration decreased during hyperoxic gas exposure and increased during hypoxic gas exposure. The most efficient dive pattern (highest dive/pause ratio) was observed after hyperoxic exposure. Our study suggests that blood oxygen level is a powerful stimulus that facilitates the cardiac and behavioural adjustments during foraging that are important components of a strategy allowing double-crested cormorants to maximise the time spent under water and, hence, potential foraging time.

Hovering Performance of Hummingbirds in Hyperoxic Gas Mixtures

Hummingbirds evolved during a period of decline in atmospheric oxygen concentration and currently encounter varying levels of oxygen availability along their elevational distribution. We tested the hypothesis that inspiration of hyperoxic gas increases hummingbird hovering performance when birds are simultaneously challenged aerodynamically. We measured the maximum duration of hovering flight while simultaneously monitoring the rate of oxygen consumption of ruby-throated hummingbirds (Archilochus colubris) in low-density heliox that was either normoxic (21% O2) or hyperoxic (35% O2). As air density decreased below 0.85kgm−3, hummingbirds hovered significantly longer in hyperoxia than in normoxia, but the air density at which the birds could no longer sustain hovering flight was independent of oxygen concentration. At low air densities in hyperoxia flight trials, hummingbirds appeared to increase their rate of oxygen consumption relative to flight sequences at equivalent densities in normoxia trials, but these differences were not significant. We tested the hypothesis that hummingbirds can discriminate between environments that differ in oxygen concentration. In another density-reduction experiment, hummingbirds were allowed to choose between artificial feeders infused with either normoxic or hyperoxic gases. The hypothesis was not supported because birds failed to associate oxygen concentration with a particular feeder independently of air density. Supplemental oxygen thus yields increased hovering duration at intermediate air densities, but the minimum density at which birds can fly is limited exclusively by aerodynamic considerations.