Pain in amphibians

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Dissection of a frog Frog dissection, 5.JPG
Dissection of a frog

Pain is an aversive sensation and feeling associated with actual, or potential, tissue damage. [1] It is widely accepted by a broad spectrum of scientists and philosophers that non-human animals can perceive pain, including pain in amphibians.

Contents

Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in non-human animals cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions. [2] [3]

Amphibians, particularly anurans, fulfill several physiological and behavioural criteria proposed as indicating that non-human animals may experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements.

Pain in amphibians has societal implications including their exposure to pollutants, (preparation for) cuisine (e.g. frog legs) and amphibians used in scientific research.

Several scientists and scientific groups have expressed the belief that amphibians can feel pain, however, this remains somewhat controversial due to differences in brain structure and the nervous system compared with other vertebrates.

Examples of the three modern orders of amphibians
Red-eyed Tree Frog - Litoria chloris edit1.jpg
Orange-eyed tree frog – (Anura)
Taricha torosa, Napa County, CA.jpg
California newt – (Urodela)
Dermophis mexicanus.jpg
Mexican burrowing caecilian – (Apoda)

Background

The possibility that amphibians and other non-human animals may experience pain has a long history. Initially, pain in non-human animals was based around theoretical and philosophical argument, but more recently has turned to scientific investigation.

Philosophy

Rene Descartes Jan Baptist Weenix - Portrait of Rene Descartes.jpg
René Descartes

The idea that non-human animals might not feel pain goes back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness. [4] [5] [6] In 1789, the British philosopher and social reformist, Jeremy Bentham, addressed in his book An Introduction to the Principles of Morals and Legislation the issue of our treatment of animals with the following often quoted words: "The question is not, Can they reason? nor, can they talk? but, Can they suffer?" [7]

Peter Singer, a bioethicist and author of Animal Liberation published in 1975, suggested that consciousness is not necessarily the key issue: just because animals have smaller brains, or are ‘less conscious’, this does not mean that they are not capable of feeling pain.

Bernard Rollin, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain. [8] In his interactions with scientists and other veterinarians, Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. [8]

Continuing into the 1990s, discussions were further developed on the roles that philosophy and science had in understanding animal cognition and mentality. [9] In subsequent years, it was argued there was strong support for the suggestion that some animals (most likely amniotes) have at least simple conscious thoughts and feelings [10] and that the view animals feel pain differently to higher primates is now a minority view. [4]

Scientific investigation

Cambridge Declaration on Consciousness (2012)

The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates. [11]

In the 20th- and 21st-century, there were many scientific investigations of pain in non-human animals.

Mammals

At the turn of the century, studies were published showing that arthritic rats self-select analgesic opiates. [12] In 2014, the veterinary Journal of Small Animal Practice published an article on the recognition of pain which started "The ability to experience pain is universally shared by all mammals...", [13] and in 2015, it was reported in the science journal Pain that several mammalian species (rat, mouse, rabbit, cat and horse) adopt a facial expression in response to a noxious stimulus that is consistent with the expression of pain. [14]

Birds

At the same time as the investigations using arthritic rats, studies were published showing that birds with gait abnormalities self-select for a diet that contains carprofen, an analgesic. [15] In 2005, it was written "Avian pain is likely analogous to pain experienced by most mammals" [16] and in 2014, "...it is accepted that birds perceive and respond to noxious stimuli and that birds feel pain." [17]

Reptiles

Veterinary articles have been published stating reptiles [18] [19] [20] experience pain in a way analogous to mammals, and that analgesics are effective in this class of vertebrates.

Fish

Several scientists or scientific groups have made statements indicating they believe fish can experience pain. For example, in 2004, Chandroo et al. wrote "Anatomical, pharmacological and behavioural data suggest that affective states of pain, fear and stress are likely to be experienced by fish in similar ways as in tetrapods". [21] In 2009, the European Food Safety Authority published a document stating scientific opinion on the welfare of fish. The document contains many sections indicating that the scientific panel believe fish can experience pain, for example, "Fish that are simply immobilized or paralysed [before euthanasia] would experience pain and suffering..." [22] In 2015, Brown wrote "A review of the evidence for pain perception strongly suggests that fish experience pain in a manner similar to the rest of the vertebrates." [23]

Argument by analogy

In 2012 the American philosopher Gary Varner reviewed the research literature on pain in animals. His findings are summarised in the following table. [24]

Argument by analogy [24]
Property
FishAmphibiansReptilesBirdsMammals
Has nociceptors Green check.svgGreen check.svgGreen check.svgGreen check.svgGreen check.svg
Has brainGreen check.svgGreen check.svgGreen check.svgGreen check.svgGreen check.svg
Nociceptors and brain linkedGreen check.svg ? [lower-alpha 1] / Green check.svg ? [lower-alpha 2] / Green check.svg ? / Green check.svgGreen check.svg
Has endogenous opioids Green check.svgGreen check.svgGreen check.svgGreen check.svgGreen check.svg
Analgesics affect responsesGreen check.svg ? [lower-alpha 3]  ? [lower-alpha 4] Green check.svgGreen check.svg
Response to damaging stimuli similar to humansGreen check.svgGreen check.svgGreen check.svgGreen check.svgGreen check.svg

Notes

  1. ↑ But see [25]
  2. ↑ But see [26]
  3. ↑ But see [27]
  4. ↑ But see [28]

Arguing by analogy, Varner claims that any animal which exhibits the properties listed in the table could be said to experience pain. On that basis, he concludes that all vertebrates, including amphibians, probably experience pain, but invertebrates apart from cephalopods probably do not experience pain. [24] [29]

Experiencing pain

Although there are numerous definitions of pain, almost all involve two key components.

First, nociception is required. [30] This is the ability to detect noxious stimuli which evoke a reflex response that rapidly moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective "feeling" – it is a reflex action. An example would be the rapid withdrawal of a finger that has touched something hot – the withdrawal occurs before any sensation of pain is actually experienced.

The second component is the experience of "pain" itself, or suffering – the internal, emotional interpretation of the nociceptive experience. This is when the withdrawn finger begins to hurt, moments after the withdrawal. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if an animal responds to a stimulus in a similar way, it is likely to have had an analogous experience.

Nociception

Nociception: The reflex arc of a dog with a pin in her paw. Note there is no communication to the brain, but the paw is withdrawn by nervous impulses generated by the spinal cord. There is no conscious interpretation of the stimulus by the dog. Anatomy and physiology of animals A reflex arc.jpg
Nociception: The reflex arc of a dog with a pin in her paw. Note there is no communication to the brain, but the paw is withdrawn by nervous impulses generated by the spinal cord. There is no conscious interpretation of the stimulus by the dog.

Nociception usually involves the transmission of a signal along a chain of nerve fibers from the site of a noxious stimulus at the periphery to the spinal cord and brain. This process evokes a reflex arc response generated at the spinal cord and not involving the brain, such as flinching or withdrawal of a limb. Nociception is found, in one form or another, across all major animal taxa. [30] Nociception can be observed using modern imaging techniques; and a physiological and behavioral response to nociception can be detected.

Emotional pain

Sometimes a distinction is made between "physical pain" and "emotional" or "psychological pain". Emotional pain is the pain experienced in the absence of physical trauma, e.g. the pain experienced after the loss of a loved one, or the break-up of a relationship. It has been argued that only primates can feel "emotional pain", because they are the only animals that have a neocortex – a part of the brain's cortex considered to be the "thinking area". However, research has provided evidence that monkeys, dogs, cats and birds can show signs of emotional pain and display behaviours associated with depression during painful experience, i.e. lack of motivation, lethargy, anorexia, unresponsiveness to other animals. [31]

Physical pain

The nerve impulses of the nociception response may be conducted to the brain thereby registering the location, intensity, quality and unpleasantness of the stimulus. This subjective component of pain involves conscious awareness of both the sensation and the unpleasantness (the aversive, negative affect). The brain processes underlying conscious awareness of the unpleasantness (suffering), are not well understood.

There have been several published lists of criteria for establishing whether non-human animals experience pain, e.g. [32] [33] Some criteria that may indicate the potential of another species, including amphibians, to feel pain include: [33]

  1. Has a suitable nervous system and sensory receptors
  2. Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics
  3. Physiological changes to noxious stimuli
  4. Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy
  5. Shows avoidance learning
  6. Shows trade-offs between noxious stimulus avoidance and other motivational requirements
  7. High cognitive ability and sentience

Adaptive value

The adaptive value of nociception is obvious; an organism detecting a noxious stimulus immediately withdraws the limb, appendage or entire body from the noxious stimulus and thereby avoids further (potential) injury. However, a characteristic of pain (in mammals at least) is that pain can result in hyperalgesia (a heightened sensitivity to noxious stimuli) and allodynia (a heightened sensitivity to non-noxious stimuli). When this heightened sensitisation occurs, the adaptive value is less clear. First, the pain arising from the heightened sensitisation can be disproportionate to the actual tissue damage caused. Second, the heightened sensitisation may also become chronic, persisting well beyond the tissues healing. This can mean that rather than the actual tissue damage causing pain, it is the pain due to the heightened sensitisation that becomes the concern. This means the sensitisation process is sometimes termed maladaptive. It is often suggested hyperalgesia and allodynia assist organisms to protect themselves during healing, but experimental evidence to support this has been lacking. [34] [35]

In 2014, the adaptive value of sensitisation due to injury was tested using the predatory interactions between longfin inshore squid (Doryteuthis pealeii) and black sea bass (Centropristis striata) which are natural predators of this squid. If injured squid are targeted by a bass, they began their defensive behaviours sooner (indicated by greater alert distances and longer flight initiation distances) than uninjured squid. If anaesthetic (1% ethanol and MgCl2) is administered prior to the injury, this prevents the sensitisation and blocks the behavioural effect. The authors claim this study is the first experimental evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries. [36]

Research findings

Nervous system

Receptors

Frogs have nociceptors in the superficial and deep layers of the skin that transduce mechanical and chemical noxious stimuli. Furthermore, frogs possess neural pathways that support processing and perception of noxious stimuli. Although organization is less well structured compared with mammals, it is now commonly accepted that amphibians possess neuro-anatomical pathways conductive of a complete nociceptive experience. [25]

Nerve fibres

Early electrophysiological studies in frogs report that noxious mechanical, thermal and chemical stimuli excite primary afferent fibres with slowly conducting axons. [37]

There are two types of nerve fibre relevant to pain in amphibians. Group C nerve fibres are a type of sensory nerve fibre which lack a myelin sheath and have a small diameter, meaning they have a low nerve conduction velocity. The suffering associated with burns, toothaches, or crushing injury are caused by C fibre activity. A-delta fibres are another type of sensory nerve fibre, however, these are myelinated and therefore transmit impulses faster than non-myelinated C fibres. A-delta fibres carry cold, pressure and some pain signals, and are associated with acute pain that results in "pulling away" from noxious stimuli. [38]

The skin of frogs contains both Group C fibres and A-delta fibres. [25] [37]

Brain

Brains of vertebrate classes. CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulae. I., olfactory; II., optic nerves. Origin of Vertebrates Fig 019.png
Brains of vertebrate classes. CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulæ. I., olfactory; II., optic nerves.

All vertebrate species have a common brain archetype divided into the telencephalon and diencephalon (collectively referred to as forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). [39] Nervous connections to the telencephalon indicate that frogs may be able to perceive pain. [25]

In 2002, James Rose, from the University of Wyoming, published reviews arguing that fish cannot feel pain because they lack a neocortex in the brain. [40] [41] If the presence of a large, considerably developed neocortex is required for experiencing pain, as Rose suggests, this theory would eliminate birds, amphibians, other non-mammalian animals, and even some mammals from having the capacity to experience pain. [42] Other researchers do not believe that animal consciousness requires a neocortex, but can arise from homologous subcortical brain networks. [11] Animal behaviouralist Temple Grandin argues that fish (and therefore, presumably, amphibians) could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions." [43]

Opioid system and effects of analgesics

By spinal administration of a range of opioid agonists, it has been demonstrated that frogs have mu (μ)-, delta (δ) and kappa (κ)-opioid binding sites. [44] The kappa sub-types κ1 and κ2 are present in the brains of edible frogs (Rana esculenta). In evolutionary terms, this means the opioid receptor sub-types are already present in amphibians, although the differences between these are less pronounced than in mammals. [45] Sequence comparisons show that the amphibian opioid receptors are highly conserved (70-84% similar to mammals) and are expressed in the central nervous system (CNS) areas apparently involved in pain experience. [32]

When treating amphibians, veterinary practice frequently uses the same analgesics and anesthetics used for mammals. These chemicals act on the nociceptive pathways, blocking signals to the brain where emotional responses to the signals are further processed by certain parts of the brain found in amniotes ("higher vertebrates"). [46] [47]

Effects of morphine and other opioids

The relative analgesic potency of 11 opioid agents (μ-opioid receptor agonists – fentanyl, levorphanol, methadone, morphine, meperidine and codeine; the partial μ agonist – buprenorphine; and the κ-opioid receptor agonists – nalorphine, bremazocine, U50488 and CI-977) in the Northern grass frog produced a dose-dependent and long-lasting analgesia which persists for at least four hours. The relative analgesic potency of μ-opioids in amphibians was correlated with the relative analgesic potency of these same agents recorded in on the mouse writhing and hot plate tests. [48] [49] Other opioid analgesics are effective in amphibians, for example, butorphanol. [50]

Alfaxalone–butorphanol and alfaxalone–morphine combinations are comparable in terms of onset and duration of anaesthesia in Oriental fire-bellied toads (Bombina orientalis). [51]

When an isolated peptide termed "frog's nociception-related peptide" (fNRP) is injected into newts, it increases the latency for newts to flick their tails in response to a hot-beam. The effect is blocked by simultaneous injection of naloxone, thereby indicating evidence for the interaction of fNRP and opioid steps in the analgesia pathways of newts. [52]

Effects of opioid antagonists

Naloxone and naltrexone are both μ-opioid receptor antagonists which, in mammals, negate the analgesic effects of opioids. Morphine analgesia in frogs is blocked by both naloxone and naltrexone, indicating that the effect is mediated at least partially by opioid receptors. [53]

Effects of other analgesics

Direct intraspinal injection of the catecholamines epinephrine and norepinephrine, and the α-adrenergic agents dexmedetomidine and clonidine, produce a dose-dependent elevation of pain thresholds in the Northern leopard frog (Rana pipiens). This analgesia occurs without accompanying motor or sedative effects. [54]

A range of non-opioid drugs administered through the dorsal lymph sac of Northern leopard frogs has demonstrable analgesic effects, established by using the acetic acid test. Chlorpromazine and haloperidol (antipsychotics), chlordiazepoxide (a benzodiazepine) and diphenhydramine (a histamine antagonist) produced moderate to strong analgesic effects, whereas indomethacin and ketorolac (NSAIDs), and pentobarbital (a barbiturate) produced weaker analgesic effects. [55]

Physiological changes

In multiple animal studies, it has been shown that stress causes increases in glucocorticoid levels). [56] Frogs release corticosteroids in response to many environmental factors [57] and this pattern of release is often species-specific within Amphibia [58] More specifically, increased stocking density and hypoxia cause changes in cortisol (one of the glucocorticoids) and white blood cells in American bullfrog tadpoles (Lithobates catesbeianus) indicative of stress. [58]

Analgesia in amphibians can be measured using heart rate and respiratory rate. [51]

Protective motor responses

Amphibians exhibit classic wiping and withdrawal protective motor responses to noxious chemical, heat and mechanical stimuli. [32]

Acetic acid (a strong irritant) applied to the hindlimb of frogs elicits vigorous wiping of the exposed skin; both pH and osmolarity may contribute to the nociception produced. [59] This response is used in a standard test for analgesic effects in frogs, commonly termed the "acetic acid test". In this procedure, dilutions of the acid are placed drop-wise on the dorsum of the frog's thigh until the frog wipes the affected area. [55]

Newts flick their tails in response to it being irradiated by a hot beam, [52] in a very similar manner to that observed in rodents being used in the tail flick test.

The threshold to Von Frey hairs and response to nociceptive withdrawal can be used to measure the effectiveness of analgesia. [51]

Avoidance learning

Early studies showed that African clawed frogs (Xenopus laevis) learn to avoid electric shocks in an aquatic shuttle-box test [60] and similarly, cane toads (Bufo marinus) learn to avoid electric shocks in a T-maze. [61] Furthermore, American bullfrogs (Rana catesbiana) learn to inhibit their high-priority, biologically adaptive righting reflex to avoid electric shock; after training, they remain passively on their backs rather than exhibiting the normal short-latency, righting response. [62]

Batrachochytrium dendrobatidis is a chytrid fungus that causes the disease chytridiomycosis in amphibians; frogs learn to avoid the fungus after just one exposure. [63]

Trade-offs in motivation

A painful experience may change the motivation for normal behavioural responses. American bullfrogs learn to inhibit their high-priority, biologically adaptive righting reflex to avoid electric shock. After repeated exposure, they remain passively on their backs rather than exhibiting the normal, short-latency, righting response, [62] thereby showing a trade-off in motivation.

Cognitive ability and sentience

Giant salamander Naturalis Biodiversity Center - Andrias japonicus - Japanese giant salamander - Siebold Collection.jpg
Giant salamander

It has been argued that although a high cognitive capacity may indicate a greater likelihood of experiencing pain, it also gives these animals a greater ability to deal with this, leaving animals with a lower cognitive ability a greater problem in coping with pain. [64]

Habituation

Habituation is one of the simplest forms of animal learning. It has been stated there are no qualitative or quantitative differences between vertebrate species in this form of learning [65] indicating there is no difference between mammals and amphibians in this process.

Associative learning

Newts are capable of associative learning. They are able to associate chemical signals from a novel predator with another chemical stimulus when the second stimulus is the skin extract of another newt. [66]

Numeracy

At least some amphibians are capable of numeracy. [67] [68] When offered live fruit flies ( Drosophila virilis), salamanders choose the larger of 1 vs 2 and 2 vs 3. Frogs are able to distinguish between low numbers (1 vs 2, 2 vs 3, but not 3 vs 4) and large numbers (3 vs 6, 4 vs 8, but not 4 vs 6) of prey. This is irrespective of other characteristics, i.e. surface area, volume, weight and movement, although discrimination among large numbers may be based on surface area. [69]

Spatial orientation

The spotted salamander can learn to use visual cues to locate rewards. Ambystoma maculatum by OpenCage.jpg
The spotted salamander can learn to use visual cues to locate rewards.

The Rocky Mountain toad (Bufo woodhousii woodhousii) and Gulf Coast toad (Bufo valliceps) are able to discriminate between left and right positions in a T-maze. [70]

Both the terrestrial toad Rhinella arenarum [71] and the spotted salamander (Ambystoma maculatum) [72] can learn to orient in an open space using visual cues to get to a reward. Furthermore, they prefer using cues close to the reward. This shows a learning phenomenon previously recorded in other taxa including mammals, birds, fish and invertebrates. [71] It has been suggested that male dart frogs of the species Allobates femoralis use spatial learning for way-finding in their local area; they are able to find their way back to their territory when displaced several hundred metres, so long as they are displaced in their local area. [73]

Social learning

Wood frog (Rana sylvatica) tadpoles use social learning to acquire information about predators; the ratio of tutors to observers, but not group size, influences the intensity of learned predator recognition. [74] Wood frog tadpoles also exhibit local enhancement in their social learning, however, spotted salamander larvae do not; this difference in social learning could be largely due to differences in aquatic ecology between tadpoles and salamander larvae. [75]

Criteria for pain perception

Scientists have also proposed that in conjunction with argument-by-analogy, criteria of physiology or behavioural responses can be used to assess the possibility of non-human animals perceiving pain. The following is a table of criteria suggested by Sneddon et al. [32]

Criteria for pain perception in amphibians
Criteria
Anura

Toad 2 (PSF).png

Caudata

Cryptobranchus japonicus.jpg

Gymnophiona

Eocaecilia BW.jpg

Has nociceptors Green check.svg ? ?
Pathways to central nervous systemGreen check.svg ? ?
Central processing in brainGreen check.svg ? ?
Receptors for analgesic drugsGreen check.svg ? ?
Physiological responsesGreen check.svg ? ?
Movement away from noxious stimuliGreen check.svg ? ?
Behavioural changes from normGreen check.svg ? ?
Protective behaviourGreen check.svg ? ?
Responses reduced by analgesic drugsGreen check.svg ? ?
Self-administration of analgesia ? ? ?
Responses with high priority over other stimuliGreen check.svg ? ?
Pay cost to access analgesia ? ? ?
Altered behavioural choices/preferencesGreen check.svg ? ?
Relief learning ? ? ?
Rubbing, limping or guardingGreen check.svg ? ?
Paying a cost to avoid noxious stimulus ? ? ?
Tradeoffs with other requirementsGreen check.svg ? ?

Scientific statements

Several scientists have made statements indicating they believe amphibians can experience pain. For example, -

After examining the morphology of the nervous system of vertebrates, Somme concluded "...most four-legged vertebrates have some state of consciousness..." [76]

Gentz, in a paper on the surgery of amphibians, writes "Postoperative recommendations include ...analgesia" and "Hypothermia is also unacceptable as a sedation technique for painful procedures". [50]

Veterinary articles have been published stating amphibians experience pain in a way analogous to mammals, and that analgesics are effective in control of this class of vertebrates. [77] [78] [79] Shine et al., wrote that most animal ethics committees and the wider community believe that amphibians can feel pain. [80]

Some scientists have been a little more guarded about the experience of amphibians, for example, Michaels et al. wrote that the identification of pain pathways shared between amphibians and other amniotes suggests an ability to experience pain, even if in a different and more restricted sense than in amniote taxa. [81]

Societal implications

Societal implications of pain in amphibians include acute and chronic exposure to pollutants, cuisine and scientific research (e.g. genetic-modification may have detrimental effects on welfare, deliberately-imposed adverse physical, physiological and behavioural states, toe-clipping or other methods of invasive marking and handling procedures which may cause injury).

Culinary

Frogs legs - a culinary dish Frog legs.jpg
Frogs legs – a culinary dish

It has been claimed that frogs killed for eating are "...sliced through the belly while they are still fully conscious and they can take up to an hour to die." [82]

Legislation

In the UK, the legislation protecting animals during scientific research, the Animals (Scientific Procedures) Act 1986, protects amphibians from the moment they become capable of independent feeding. [83] The legislation protecting animals in most other circumstances in the UK is the Animal Welfare Act 2006, which states that in the Act, "'animal means a vertebrate other than man", [84] thereby including amphibians.

The 1974 Norwegian Animal Rights Law states it relates to mammals, birds, frogs, salamanders, reptiles, fish, and crustaceans. [85]

In the US, the legislation protecting animals during scientific research is the Animal Welfare Act. [86] This Act excludes protection of "cold-blooded" animals, thereby excluding amphibians from protection.

See also

Related Research Articles

General anaesthetics are often defined as compounds that induce a loss of consciousness in humans or loss of righting reflex in animals. Clinical definitions are also extended to include an induced coma that causes lack of awareness to painful stimuli, sufficient to facilitate surgical applications in clinical and veterinary practice. General anaesthetics do not act as analgesics and should also not be confused with sedatives. General anaesthetics are a structurally diverse group of compounds whose mechanisms encompass multiple biological targets involved in the control of neuronal pathways. The precise workings are the subject of some debate and ongoing research.

In physiology, nociception, also nocioception; from Latin nocere 'to harm/hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, convert it to a molecular signal, and recognize and characterize the signal to trigger an appropriate defensive response.

<span class="mw-page-title-main">Pain</span> Type of distressing feeling

Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage."

<span class="mw-page-title-main">Sentience</span> Ability to experience feelings and sensations

Sentience is the ability to experience feelings and sensations. It may not necessarily imply higher cognitive functions such as awareness, reasoning, or complex thought processes. Sentience is an important concept in ethics, as the ability to experience happiness or suffering often forms a basis for determining which entities deserve moral consideration, particularly in utilitarianism.

<span class="mw-page-title-main">Nociceptor</span> Sensory neuron that detects pain

A nociceptor is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.

<span class="mw-page-title-main">Periaqueductal gray</span> Nucleus surrounding the cerebral aqueduct

The periaqueductal gray (PAG), also known as the central gray, is a brain region that plays a critical role in autonomic function, motivated behavior and behavioural responses to threatening stimuli. PAG is also the primary control center for descending pain modulation. It has enkephalin-producing cells that suppress pain.

<span class="mw-page-title-main">Nociceptin</span> Chemical compound

Nociceptin/orphanin FQ (N/OFQ), a 17-amino acid neuropeptide, is the endogenous ligand for the nociceptin receptor. Nociceptin acts as a potent anti-analgesic, effectively counteracting the effect of pain-relievers; its activation is associated with brain functions such as pain sensation and fear learning.

Opioid-induced hyperalgesia (OIH) or opioid-induced abnormal pain sensitivity, also called paradoxical hyperalgesia, is an uncommon condition of generalized pain caused by the long-term use of high dosages of opioids such as morphine, oxycodone, and methadone. OIH is not necessarily confined to the original affected site. This means that if the person was originally taking opioids due to lower back pain, when OIH appears, the person may experience pain in the entire body, instead of just in the lower back. Over time, individuals taking opioids can also develop an increasing sensitivity to noxious stimuli, even evolving a painful response to previously non-noxious stimuli (allodynia). This means that if the person originally felt pain from twisting or from sitting too long, the person might now additionally experience pain from a light touch or from raindrops falling on the skin.

Veterinary anesthesia is a specialization in the veterinary medicine field dedicated to the proper administration of anesthetic agents to non-human animals to control their consciousness during procedures. A veterinarian or a Registered Veterinary Technician administers these drugs to minimize stress, destructive behavior, and the threat of injury to both the patient and the doctor. The duration of the anesthesia process goes from the time before an animal leaves for the visit to the time after the animal reaches home after the visit, meaning it includes care from both the owner and the veterinary staff. Generally, anesthesia is used for a wider range of circumstances in animals than in people not only due to their inability to cooperate with certain diagnostic or therapeutic procedures, but also due to their species, breed, size, and corresponding anatomy. Veterinary anesthesia includes anesthesia of the major species: dogs, cats, horses, cattle, sheep, goats, and pigs, as well as all other animals requiring veterinary care such as birds, pocket pets, and wildlife.

<span class="mw-page-title-main">Pain in fish</span>

Fish fulfill several criteria proposed as indicating that non-human animals experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements.

<span class="mw-page-title-main">Pain in animals</span> Pain experienced by non-human animals

Pain negatively affects the health and welfare of animals. "Pain" is defined by the International Association for the Study of Pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." Only the animal experiencing the pain can know the pain's quality and intensity, and the degree of suffering. It is harder, if even possible, for an observer to know whether an emotional experience has occurred, especially if the sufferer cannot communicate. Therefore, this concept is often excluded in definitions of pain in animals, such as that provided by Zimmerman: "an aversive sensory experience caused by actual or potential injury that elicits protective motor and vegetative reactions, results in learned avoidance and may modify species-specific behaviour, including social behaviour." Nonhuman animals cannot report their feelings to language-using humans in the same manner as human communication, but observation of their behaviour provides a reasonable indication as to the extent of their pain. Just as with doctors and medics who sometimes share no common language with their patients, the indicators of pain can still be understood.

<span class="mw-page-title-main">Pain in crustaceans</span> Ethical debate

There is a scientific debate which questions whether crustaceans experience pain. It is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in an animal, or another human for that matter, cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions.

Pain in babies, and whether babies feel pain, has been a large subject of debate within the medical profession for centuries. Prior to the late nineteenth century it was generally considered that babies hurt more easily than adults. It was only in the last quarter of the 20th century that scientific techniques finally established babies definitely do experience pain – probably more than adults – and developed reliable means of assessing and of treating it. As recently as 1999, it was widely believed by medical professionals that babies could not feel pain until they were a year old, but today it is believed newborns and likely even fetuses beyond a certain age can experience pain.

The hot plate test is a test of the pain response in animals, similar to the tail flick test. Both hot plate and tail-flick methods are used generally for centrally acting analgesic, while peripherally acting drugs are ineffective in these tests but sensitive to acetic acid-induced writhing test.

A nociception assay evaluates the ability of an animal, usually a rodent, to detect a noxious stimulus such as the feeling of pain, caused by stimulation of nociceptors. These assays measure the existence of pain through behaviors such as withdrawal, licking, immobility, and vocalization. The sensation of pain is not a unitary concept; therefore, a researcher must be conscious as to which nociception assay to use.

<span class="mw-page-title-main">Pain in invertebrates</span> Contentious issue

Pain in invertebrates is a contentious issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not necessarily imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or suffering—i.e., the internal, emotional interpretation of the nociceptive experience. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience. It has been argued that if a pin is stuck in a chimpanzee's finger and they rapidly withdraw their hand, then argument-by-analogy implies that like humans, they felt pain. It has been questioned why the inference does not then follow that a cockroach experiences pain when it writhes after being stuck with a pin. This argument-by-analogy approach to the concept of pain in invertebrates has been followed by others.

<span class="mw-page-title-main">Maria Carmela Lico</span> Italian-Brazilian physiologist

Maria Carmela Lico or Licco (1927–1985) spent most of her research life as a physiologist studying the neural mechanisms of pain at the Department of Physiology of the Faculdade de Medicina de Ribeirão Preto (Brazil). Lico produced important insights on the descending control of nociception by limbic structures, specially the septal nuclei.

<span class="mw-page-title-main">Grimace scale</span> Method of assessing pain in non-human animals

The grimace scale (GS), sometimes called the grimace score, is a method of assessing the occurrence or severity of pain experienced by non-human animals according to objective and blinded scoring of facial expressions, as is done routinely for the measurement of pain in non-verbal humans. Observers score the presence or prominence of "facial action units" (FAU), e.g. Orbital Tightening, Nose Bulge, Ear Position and Whisker Change. These are scored by observing the animal directly in real-time, or post hoc from photographs or screen-grabs from videos. The facial expression of the animals is sometimes referred to as the pain face.

<span class="mw-page-title-main">Pain in cephalopods</span> Contentious issue

Pain in cephalopods is a contentious issue. Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in non-human animals, or another human for that matter, cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions.

Placebo analgesia occurs when the administration of placebos leads to pain relief. Because placebos by definition lack active ingredients, the effect of placebo analgesia is considered to result from the patient's belief that they are receiving an analgesic drug or other medical intervention. It has been shown that, in some cases, the endogenous opioid system is critical for mediating placebo analgesia, as evidenced by the ability of such analgesia to be reduced by the opioid antagonist naloxone. However, it is also possible for placebo analgesia to be mediated by non-opioid mechanisms, in which case it would not be affected by naloxone. Other research has indicated that the human spinal cord, prefrontal cortex, and rostral anterior cingulate cortex also play a role in placebo analgesia.

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  83. ↑ "Animals (Scientific Procedures) Act 1986" (PDF). Home Office (UK). Retrieved September 23, 2015.
  84. ↑ "Animal Welfare Act 2006". UK Government. 2006. Retrieved September 25, 2015.
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