Pain in amphibians
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.
[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?
[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.
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.
[14] 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.
"[17] 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.
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".
[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.
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.
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".
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] 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.
The authors claim this study is the first experimental evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries.
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] Early electrophysiological studies in frogs report that noxious mechanical, thermal and chemical stimuli excite primary afferent fibres with slowly conducting axons.
[25][37] All vertebrate species have a common brain archetype divided into the telencephalon and diencephalon (collectively referred to as forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).
[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.
"[43] 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.
[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.
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.
[53] 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).
[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.
[51] 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.
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.
[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.
[75] 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.
[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.
[81] 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).
The 1974 Norwegian Animal Rights Law states it relates to mammals, birds, frogs, salamanders, reptiles, fish, and crustaceans.
