Stressful and behavioral conditions that affect reversible cardiac arrest in the Nile tilapia, Oreochromis niloticus (Teleostei)

The pancreas does not contribute to the non-adrenergic-non-cholinergic stimulation of heart rate in digesting pythons

Vertebrates elevate heart rate when metabolism increases during digestion. Part of this tachycardia is due to a non-adrenergic-non-cholinergic (NANC) stimulation of the cardiac pacemaker, and it has been suggested these NANC factors are circulating hormones that are released from either gastrointestinal or endocrine glands. The NANC stimulation is particularly pronounced in species with large metabolic responses to digestion, such as reptiles. To investigate the possibility that the pancreas may release hormones that exert positive chronotropic effects on the digesting Burmese python heart, a species with very large postprandial changes in heart rate and oxygen uptake, we evaluate how pancreatectomy affects postprandial heart rate before and after autonomic blockade of the muscarinic and the beta-adrenergic receptors. We also measured the rates of oxygen consumption and evaluated the short-term control of the heart using the spectral analysis of heart rate variability and the baroreflex sequence method. Digestion caused the ubiquitous tachycardia, but the intrinsic heart rate (revealed after the combination of atropine and propranolol) was not affected by pancreatectomy and therefore hormones, such as glucagon and insulin, do not appear to contribute to the regulation of heart rate during digestion in Burmese pythons.

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.

Threatening stimuli elicit a sequential cardiac pattern in arthropods

In order to cope with the challenges of living in dynamic environments, animals rapidly adjust their behaviors in coordination with different physiological responses. Here, we studied whether threatening visual stimuli evoke different heart rate patterns in arthropods and whether these patterns are related with defensive behaviors. We identified two sequential phases of crab's cardiac response that occur with a similar timescale to that of the motor arrest and later escape response. The first phase was modulated by low salience stimuli and persisted throughout spaced stimulus presentation. The second phase was modulated by high-contrast stimuli and reduced by repetitive stimulus presentation. The overall correspondence between cardiac and motor responses suggests that the first cardiac response phase might be related to motor arrest while the second to the escape response. We show that in the face of threat arthropods coordinate their behavior and cardiac activity in a rapid and flexible manner.

The nonpharmacological sequence method provides a reliable evaluation of baroreflex sensitivity in fish

The most commonly used technique to study the barostatic regulation of blood pressure in ectothermic vertebrates consists of determining the heart rate response to pharmacological manipulations of blood pressure, the so-called "Oxford method." Although well established, the Oxford method has some important limitations, such as induction of hypervolemia in small animals and undesired effects of vasoactive drugs on central and peripheral baroreflex components. As an alternative, the sequence method, which consists in the computerized evaluation of naturally-occurring baroreflex adjustments of heart rate without the need for pharmacological administrations, was developed to study baroreflexes. In the present study, we compare this sequence method with the Oxford technique in two teleost species with different life styles, and we assess the optimal software configuration for the employment of the sequence method in fish. Calculation of baroreflex gain through the sequence method was adequate and reliable when the software was configured to search for baroreflex sequences with a minimum length of three cardiac cycles with a delay of one cardiac cycle between fluctuations in mean ventral aortic blood pressure and reflex changes in pulse interval. When properly configured, the sequence and the Oxford methods yielded similar determinations of the baroreflex gain in fish.

GABAA receptors activate fish feeding behaviour via two distinct functional pathways

Benzodiazepines, acting through ionotropic receptors of γ-aminobutyric acid (GABA_A receptors, GABAR), have been shown to modify feeding behaviour and increase appetite in humans and non-human subjects. However, the cellular and molecular mechanisms that underlie connected short-term behavioural fluctuations are still unclear. In the present study, we used Carassius gibelio (Prussian carp) as a model organism to research the impact of scantily explored benzodiazepine phenazepam (PNZ) on feeding behaviour and the related molecular mechanisms of PNZ action at single-cell and single-receptor levels. We found that the feeding activity of C. gibelio is under control of GABARs via two distinct mechanisms: orthosteric (triggered by GABA binding site) and allosteric (triggered by benzodiazepine binding site). PNZ displayed clear stimulatory effects on both mechanisms in a GABA-dependent manner. In addition, orthosteric and allosteric effects were found to be partially competitive, which leads to complex behavioural repercussions of conjoint effects of GABAR ligands.

New evidence on an old question: is the "fight or flight" stage present in the cardiac and respiratory regulation of decapod crustaceans?

The ability to stay alert to subtle changes in the environment and to freeze, fight or flight in the presence of predators requires integrating sensory information as well as triggering motor output to target tissues, both of which are associated with the autonomic nervous system. These reactions, which are commonly related to vertebrates, are the fundamental physiological responses that allow an animal to survive danger. The circulatory activity in vertebrates changes in opposite phases. The stage where circulatory activity is high is termed the "fight or flight stage", while the stage where circulatory activity slows down is termed the "rest and digest stage". It may be assumed that highly evolved invertebrates possess a comparable response system as they also require rapid cardiovascular and respiratory regulation to be primed when necessary. However, in invertebrates, the body plan may have developed such a system very differently. Since this topic is insufficiently studied, it is necessary to extend studies for a comparative analysis. In the present review, we use our own experimental results obtained in the crab Neohelice granulata and both older and newer findings obtained by other authors in decapod crustaceans as well as in other invertebrates, to compare the pattern of change in circulatory activity, especially in the "fight or flight" stage. We conclude that the main features of neuroautonomic regulation of the cardiac function were already present early in evolution, at least in highly evolved invertebrates, although conspicuous differences are also evident.

Characterization of the cardiac ganglion in the crab Neohelice granulata and immunohistochemical evidence of GABA-like extrinsic regulation

The aim of the present work is to provide an anatomical description of the cardiac system in the crab Neohelice granulata and evidence of the presence of GABA by means of immunohistochemistry. The ganglionic trunk was found lying on the inner surface of the heart's dorsal wall. After dissection, this structure appeared as a Y-shaped figure with its major axis perpendicular to the major axis of the heart. Inside the cardiac ganglion, we identified four large neurons of 63.7 μm ± 3.7 in maximum diameter, which were similar to the motor neurons described in other decapods. All the GABA-like immunoreactivity (GABAi) was observed as processes entering mainly the ganglionic trunk and branching in slender varicose fibers, forming a network around the large neurons suggesting that GABAi processes contact them. Our findings strengthen previous results suggesting that the GABAergic system mediates the cardio-inhibitory response upon sensory stimulation.

The effect of hypoxia on ventilation frequency in startled common sole Solea solea

Ventilation frequency (F(V) ) in motionless common sole Solea solea was measured before and after a startling stimulus in normoxia and in hypoxia (15% air saturation). Startling reduced F(V) in normoxia (from mean ±s.e. 41 ± 3·3 beats min⁻¹ to near zero, i.e. 2·0 ± 1·8 beats min⁻¹) and in hypoxia (from mean ±s.e. 80 ± 4·4 to 58·8 ± 12·9 beats min⁻¹). It is suggested that the maintenance of high F(V) in hypoxia may increase the probability of detection by predators compared to normoxia.

Cardiac arrest during gamete release in chum salmon regulated by the parasympathetic nerve system

Cardiac arrest caused by startling stimuli, such as visual and vibration stimuli, has been reported in some animals and could be considered as an extraordinary case of bradycardia and defined as reversible missed heart beats. Variability of the heart rate is established as a balance between an autonomic system, namely cholinergic vagus inhibition, and excitatory adrenergic stimulation of neural and hormonal action in teleost. However, the cardiac arrest and its regulating nervous mechanism remain poorly understood. We show, by using electrocardiogram (ECG) data loggers, that cardiac arrest occurs in chum salmon (Oncorhynchus keta) at the moment of gamete release for 7.39±1.61 s in females and for 5.20±0.97 s in males. The increase in heart rate during spawning behavior relative to the background rate during the resting period suggests that cardiac arrest is a characteristic physiological phenomenon of the extraordinarily high heart rate during spawning behavior. The ECG morphological analysis showed a peaked and tall T-wave adjacent to the cardiac arrest, indicating an increase in potassium permeability in cardiac muscle cells, which would function to retard the cardiac action potential. Pharmacological studies showed that the cardiac arrest was abolished by injection of atropine, a muscarinic receptor antagonist, revealing that the cardiac arrest is a reflex response of the parasympathetic nerve system, although injection of sotalol, a β-adrenergic antagonist, did not affect the cardiac arrest. We conclude that cardiac arrest during gamete release in spawning release in spawning chum salmon is a physiological reflex response controlled by the parasympathetic nervous system. This cardiac arrest represents a response to the gaping behavior that occurs at the moment of gamete release.

The cardiac response of the crab Chasmagnathus granulatus as an index of sensory perception

When an animal's observable behavior remains unaltered, one can be misled in determining whether it is able to sense an environmental cue. By measuring an index of the internal state, additional information about perception may be obtained. We studied the cardiac response of the crab Chasmagnathus to different stimulus modalities: a light pulse, an air puff, virtual looming stimuli and a real visual danger stimulus. The first two did not trigger observable behavior, but the last two elicited a clear escape response. We examined the changes in heart rate upon sensory stimulation. Cardiac response and escape response latencies were also measured and compared during looming stimuli presentation. The cardiac parameters analyzed revealed significant changes (cardio-inhibitory responses) to all the stimuli investigated. We found a clear correlation between escape and cardiac response latencies to different looming stimuli. This study proved useful to examine the perceptual capacity independently of behavior. In addition, the correlation found between escape and cardiac responses support previous results which showed that in the face of impending danger the crab triggers several coordinated defensive reactions. The ability to escape predation or to be alerted to subtle changes in the environment in relation to autonomic control is associated with the complex ability to integrate sensory information as well as motor output to target tissues. This ;fear, fight or flight' response gives support to the idea of an autonomic-like reflexive control in crustaceans.

The ventilatory, cardiac and behavioural responses of resting cuttlefish(Sepia officinalisL.) to sudden visual stimuli

When startled, some animals reduce ventilation rate and heart rate, and become motionless. The function of this response, if any, remains unknown. We used non-invasive ultrasound imaging to monitor the ventilatory, cardiac and postural responses of cuttlefish exposed to sudden visual stimuli. Simultaneously, we recorded cuttlefish behaviour using an overhead video camera. Upon presentation of the sudden visual stimulus (rapidly approaching bird cut-out), cuttlefish rapidly changed the colour and the texture of their skin, taking on characteristics of the Deimatic Display. Cuttlefish also became motionless (behavioural freezing), hyperinflated their mantles, and decreased their ventilation rate and heart rate. We found no evidence of a relationship between the intensity of the Deimatic Display and the intensity of any other measured parameter. Ventilation rate decreased during behavioural freezing. Hyperinflation of the mantle was most intense in preparation for and during behavioural freezing. Heart rate decreases occurred during mantle hyperinflation and were greatest in animals showing the most hyperinflation. Decreased heart rate may not be adaptive per se. Instead, it might be a product of the unusual arrangement of the cuttlefish peripheral vasculature,which could be compressed during mantle hyperinflation. By filling the mantle with water (hyperinflation), this response to sudden stimuli may help cuttlefish prepare for possible flight by jet propulsion, which often follows the Deimatic Display.