Cardiovascular component of the context signal memory in the crab Chasmagnathus

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.

Threat induces cardiac and metabolic changes that negatively impact survival in flies

Adjusting to a dynamic environment involves fast changes in the body’s internal state, characterized by coordinated alterations in brain activity and physiological and motor responses. Threat-induced defensive states are a classic case of coordinated adjustment of bodily responses, cardiac regulation being one of the best characterized examples in vertebrates. A great deal is known regarding the neural basis of invertebrate defensive behaviors, mainly in Drosophila melanogaster. However, whether physiological changes accompany these remains unknown. Here, we set out to describe the internal bodily state of fruit flies upon an inescapable threat and found cardiac acceleration during running and deceleration during freezing. In addition, we found that freezing leads to increased cardiac pumping from the abdomen toward the head-thorax, suggesting mobilization of energy resources. Concordantly, threat-triggered freezing reduces sugar levels in the hemolymph and renders flies less resistant to starvation. The cardiac responses observed during freezing were absent during spontaneous immobility, underscoring the active nature of freezing response. Finally, we show that baseline cardiac activity predicts the amount of freezing upon threat. This work reveals a remarkable similarity with the cardiac responses of vertebrates, suggesting an evolutionarily convergent defensive state in flies. Our findings are at odds with the widespread view that cardiac deceleration while freezing has first evolved in vertebrates and that it is energy sparing. Investigating the physiological changes coupled to defensive behaviors in the fruit fly has revealed that freezing is costly yet accompanied by cardiac deceleration and points to heart activity as a key modulator of defensive behaviors.

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Flies show tight coupling between defensive behaviors and cardiac activity

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Flies bias cardiac pumping toward the head and thorax during defensive behaviors

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After prolonged freezing, sugar levels and resistance to starvation are decreased

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Cardiac reversal rate and rate variability are predictive of freezing intensity

Patterns of heart rate and cardiac pausing in unrestrained resting decapod crustaceans

Measurement of heart rate (HR) has been used as an important physiological indicator in a broad range of taxa. In the present study HR patterns were measured in five species of unrestrained, resting decapod crustaceans. In addition to variation in HR among individuals, it was also very variable within an individual animal. While some of this variation was related to activity, there was also a non-locomotory component. Unstressed, resting crabs exhibited intermittent heart activity, whereas HR in stressed crabs was more stable, suggesting differential control of HR in resting crabs. Once the animals settled in the experimental apparatus they exhibited regular and extended cardiac pauses (acardia) of 15-300-s duration. As with HR, there was a significant variation in the frequency and length of acardic events, which were only observed in inactive crabs. Regaining of HR, following a period of acardia, was characterized by small adjustments in position and movement of the mouthparts. This rhythmic pattern, and the fact that entry into and out of acardia was not instantaneous, suggested that these events were related to release of neurohormones and their subsequent degradation in the system, rather than direct neural control of the heart. Because HR was variable and interrupted by regular periods of acardia, caution is recommended when calculating baseline levels of HR, or using HR alone as an indicator of physiological stress. Incorporating a coefficient of variation for HR and/or measuring the periods of acardia may be a more reliable indicator of physiological stress in decapod crustaceans.

Chemosensitivity and role of swimming legs of mud crab, Scylla paramamosain, in feeding activity as determined by electrocardiographic and behavioural observations

Swimming crabs have a characteristic fifth pair of legs that are flattened into paddles for swimming purposes. The dactyl of these legs bears a thick seta along its edge. The chemoreceptive and feeding properties of the seta are supported with scientific evidence; however, there is no available data on the sensitivity of the setae in portunid crabs. The underlying mechanisms of the chemo- and mechano-sensitivity of appendages and their involvement in feeding activities of the mud crab (Scylla paramamosain) were investigated using electrocardiography and behavioural assay, which focused on the responses of the mud crab to chemical and touch stimulus. Electrocardiography revealed the sensory properties of the appendages. The dactyls of swimming legs and the antennules were chemosensitive, but not mechanosensitive and vice versa for the antennae. However, the mouthparts, claws, and walking legs were chemo- and mechanosensitive. Only the chemosensitive appendages, including the swimming legs, were directly involved in feeding. The flattened dactyls of the swimming legs were more efficient than the pointed dactyls of the walking legs in detecting the food organism crawling on the substrate. The structural features enhanced the capacity of the crab in coming into contact with scattered food items. This study revealed that the swimming legs are important appendages for feeding in the mud crab.

Experimental setup influences the cardiovascular responses of decapod crustaceans to environmental change

The effects of different holding methods on heart rate (HR) changes in the green crab, Carcinus maenas (Linnaeus, 1758), were investigated. Green crabs were held in perforated plastic boxes (with or without a layer of sand) suspended above the bottom of the tank or strapped to a weighted plastic grate. The HR of green crabs classified as unrestrained (plastic box with or without sand) dropped more rapidly compared with restrained (hanging from band, strapped to grate) green crabs. Within 1 h, unrestrained green crabs exhibited periods of cardiac pausing accounting for between 8% and 14% of the hourly time. In contrast, restrained green crabs rarely exhibited cardiac pausing. When the green crabs were subjected to a temperature increase (10–30 °C), the HR of unrestrained green crabs reached higher levels than that of the restrained animals. The four restraining methods were also used to investigate cardiac responses to hypoxia. During progressive hypoxia (100%–20% oxygen), the HR of unrestrained green crabs declined to lower levels than that of the restrained animals. The restraining methods appeared to be more stressful for the green crabs that maintained elevated HRs and were less able to respond to environmental change compared with green crabs that could move freely within a small chamber. This suggests that even subtle changes in experimental design may alter physiological responses.

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.

Does conditioned taste aversion learning in the pond snail Lymnaea stagnalis produce conditioned fear?

In conditioned taste aversion (CTA) training performed on the pond snail Lymnaea stagnalis, a stimulus (the conditional stimulus, CS; e.g., sucrose) that elicits a feeding response is paired with an aversive stimulus (the unconditional stimulus, US) that elicits the whole-body withdrawal response and inhibits feeding. After CTA training and memory formation, the CS no longer elicits feeding. We hypothesize that one reason for this result is that after CTA training the CS now elicits a fear response. Consistent with this hypothesis, we predict the CS will cause (1) the heart to skip a beat and (2) a significant change in the heart rate. Such changes are seen in mammalian preparations exposed to fearful stimuli. We found that in snails exhibiting long-term memory for one-trial CTA (i.e., good learners) the CS significantly increased the probability of a skipped heartbeat, but did not significantly change the heart rate. The probability of a skipped heartbeat was unaltered in control snails given backward conditioning (US followed by CS) or in snails that did not acquire associative learning (i.e., poor learners) after the one-trial CTA training. These results suggest that as a consequence of acquiring CTA, the CS evokes conditioned fear in the conditioned snails, as evidenced by a change in the nervous system control of cardiac activity.

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.

Pavlov's cockroach: classical conditioning of salivation in an insect

Secretion of saliva to aid swallowing and digestion is an important physiological function found in many vertebrates and invertebrates. Pavlov reported classical conditioning of salivation in dogs a century ago. Conditioning of salivation, however, has been so far reported only in dogs and humans, and its underlying neural mechanisms remain elusive because of the complexity of the mammalian brain. We previously reported that, in cockroaches Periplaneta americana, salivary neurons that control salivation exhibited increased responses to an odor after conditioning trials in which the odor was paired with sucrose solution. However, no direct evidence of conditioning of salivation was obtained. In this study, we investigated the effects of conditioning trials on the level of salivation. Untrained cockroaches exhibited salivary responses to sucrose solution applied to the mouth but not to peppermint or vanilla odor applied to an antenna. After differential conditioning trials in which an odor was paired with sucrose solution and another odor was presented without pairing with sucrose solution, sucrose-associated odor induced an increase in the level of salivation, but the odor presented alone did not. The conditioning effect lasted for one day after conditioning trials. This study demonstrates, for the first time, classical conditioning of salivation in species other than dogs and humans, thereby providing the first evidence of sophisticated neural control of autonomic function in insects. The results provide a useful model system for studying cellular basis of conditioning of salivation in the simpler nervous system of insects.