«October 2009 SCIENTIFIC COORDINATOR Pierre Le Neindre, Senior research scientist, INRA (French National Institute for Agricultural Research) ...»
Several experiments in chickens, pigeons and quail have shown that the behavioural responses triggered by nociceptive stimuli are reduced or disappear after an injection of morphine. The impairment of these responses due to morphine treatment shows that the abnormal stance observed in the animals studied is not due to a functional handicap but to a nociceptive phenomenon or even pain. The injection of non-steroidal anti-inflammatory drugs can also improve the stance of chickens which were previously limping. These data demonstrate the existence of receptors for substances like morphine, which is consistent with data from phylogenetic studies showing the presence of opioid-like receptor families in almost the entire vertebrate phylum and even sometimes in very primitive marine invertebrates.
These behavioural responses are not just reflexes because they can also be modulated by endogenous analgesia provoked by other motivated behaviours. Thus, a hen that is about to lay will crouch down on both legs even though she previously avoided standing on a leg in which arthritis had been induced experimentally. Similarly, a strong motivation to feed can diminish or eliminate the expression of joint pain in the chicken.
Cognition Even though birds are quite capable of expressing protective and escape behaviours of nociceptive origin in combination with emotional responses, the level of consciousness, and therefore the characteristics of the sensory and emotional experience of pain, may vary according to the species. It must be borne in mind that the avian class includes animals with a wide range of cognitive abilities. Gallinaceous and web-footed poultry, and quail, which constitute the majority of farmed birds, have very reduced cognitive capacities in comparison to other birds, Expertise scientifique collective "Douleurs animales" 37 especially the corvids and the parrots. Work performed on jays has shown that these birds are capable of forming episodic memories very similar to some of the most complex memory processes in humans. Some parrots seem able to count, to combine shapes and colours from oral instructions, to indicate the location of an object that has been hidden, to such a point that some authors believe they have a high form of consciousness. It is quite possible that the diversities in cognitive abilities and in levels of consciousness (in the sense of alertness and perception of the environment) evolved in parallel.
In conclusion, the neurobiological data confirm the behavioural and physiological results. They suggest that noxious stimuli trigger emotional responses, and in that regard we can suspect the existence of pain in birds, not just nociception, although this is still being debated in the scientific community, especially for farm species.
Existence of pain in reptiles and amphibians There is still very little data in the literature on these two phyla.
Reptiles Phylogenetic review articles on brain functions reveal that the first events associated with a form of consciousness (sleep / wake cycles, primary emotions, positive reinforcement) are seen in the reptiles. Elements concerning the expression of pain (nociception, emotion, phenomenal consciousness) are still too fragmentary to make clear conclusions.
Amphibians In the frog the existence of somatosensory and chemical nociceptors can be demonstrated by the swift and vigorous extension of the hind limbs in response to stinging or to the application of an acidic solution to the skin.
Work on amphibians is mainly focused on the identification of peripheral nociceptors or at the level of the spinal cord. Recently, a chemo-nociceptor that is sensitive to a neuropeptide known for its analgesic effect in the spinal cord in other vertebrates has been characterized in the frog. This receptor is specific to nociceptin and differs in its structure and properties to opioid receptors (mu, delta and kappa) which are found in many animal species. It can be concluded through phylogeny that the analgesic properties of nociceptin found in the frog indicate the presence of elementary nociceptors.
However, the huge differences in the anatomical organization of the nervous system between amphibians and mammals make it very difficult or even impossible, in the current state of our knowledge, to speak about pain in the former. Thus the predominant involvement of the forebrain in the identification of chemical stimuli (smell), in addition to the lack of cortex, makes it difficult to conclude to the existence of elementary sensory emotions associated with a primary form of consciousness.
In conclusion, the escape responses described in amphibians are controlled by brainstem centres which receive information from peripheral nociceptors. These are reflex responses which, if they appear rather elaborate, do not include the participation of emotional awareness in the way it is applicable to mammals. The organization of such protective behavioural responses does not exclude the existence of elementary forms of sensory awareness, often described by the concept of sentiency.
Existence of pain in fish Fish form a very vast and heterogeneous phylogenetic group. Current knowledge on this class is limited to a small number of species and cannot be generalized to all fish.
Anatomy Anatomical and electrophysiological work has recently demonstrated the existence of nociceptors in trout. These nociceptors are located on labial areas of the head and respond to mechanical, thermal and chemical stimuli. They send information to the brain via small trigeminal Aδ and C fibres (a cranial nerve), the number of which is much smaller than in mammals and birds. It should be noted that neither nociceptors nor a system of nociception have 38 Expertise scientifique collective "Douleurs animales" yet been found among cartilaginous fish (elasmobranchs) even though such features are essential for the survival of individuals.
Trout present five different types of nociceptive responses and its nociceptors have similar characteristics to mammals. The receptors do not show, however, the sensitization phenomenon widely described in mammals after invasive chemical or thermal stimulation known to induce inflammatory responses and hyperalgesia. Instead, after such stimulation these receptors display either the same response as that initially observed, or become irreversibly insensitive. Trout and goldfish have opioid receptors which respond to Met-enkephalin and leu-enkephalin, two substances found in the nociceptive system of rodents. A stressful event induces the secretion of met-enkephalin in goldfish.
Behaviour Fear-like behaviour is observed in the trout after introduction of an unfamiliar object into its environment. The same type of behaviour is displayed after a subcutaneous injection of an acidic solution into its mouth; the effect is neutralized by an analgesic treatment of morphine. Other studies in goldfish show the existence of long-term memory, resulting in the avoidance of situations previously associated with a noxious stimulation.
In conclusion, experimental results in teleost fish confirm the existence of nociceptors and avoidance behaviours which can help memorizing the context where a noxious stimulation was experienced. However, proof for the existence of an emotional component is still lacking, therefore there is no solid evidence to prove that these elementary reactions reflect pain. This issue is still being debated within the scientific community but experimental data is still patchy and limited to a few species.
Existence of pain in cephalopods Very few species of invertebrates are raised for human consumption. Revision of the EU Directive on the use of animals in experiments (EEC 86/609) has extended the scope of application to some invertebrates, including cephalopods. Data on nociception and pain in marine cephalopods should therefore be examined.
The diversity of adaptive niches and species does not exclude the existence of differences in conscious sensory activity (primary consciousness) like alarm, awareness and alertness according to the type of cephalopod.
Neuroanatomy Cognitive and behavioural performances of cephalopods are linked to their considerable brain size (520 million neurons in the octopus). Removal of the cephalic lobes (superior vertical optic lobes), has been performed but only to gain understanding of the neurobiological bases to visual recognition. These lobectomy experiments do not solve issues on homology of brain structures between cephalopods and vertebrates in regards to the processing of nociceptive information.
Behaviour and Cognition The behavioural performances of cephalopods are strongly linked to predation. They reveal important cognitive and adaptive abilities (discrimination between shapes, colour or intensity of stimulations; special memory; learning by visual observation; categorization of shapes) that are very similar to those found of vertebrates.
Data on aversive learning could be relevant for assessing the potential existence of pain in these species, in the sense that any aversive stimulus, whether of nociceptive origin or not, can trigger a minimal withdrawal response or avoidance. Threatening stimuli trigger immediate flight responses in cephalopods, followed by hiding or protective behaviours. This can be caused by a particular element of the environment or the situation, or by the context itself (contextual learning).
Aversive situations are memorised for several days after a single experience. This is typical of the consequences of being exposed to a noxious or potentially dangerous stimulus, such as the reaction to bitterness (quinine) which in many species is associated with the risk of being poisoned.
Expertise scientifique collective "Douleurs animales" 39 In conclusion, cephalopods are clearly sensitive animals, with highly developed memory and cognitive abilities.
While some behavioural expressions described in the literature are characteristic of nociception, the emotional components, associated with pain in higher vertebrates, remain largely unexplored. The level of consciousness so far determined for cephalopods still corresponds to elementary forms of sensory awareness. The debate within the scientific community on emotion and consciousness in cephalopods shows that there is a need to develop further work on this matter.
Conclusion This brief review on the phylogenetic aspect of nociception and pain seems to indicate that elementary solutions have been conserved down through evolution. This is the case for peripheral nociceptors, all of which have free nerve endings that do not have peripheral structures or some spinal neuronal receptors responding to analgesic substances (opioids analgesic neuropeptides) which are found in animal categories as diverse as cephalopods, amphibians, fish, birds or mammals. Protective reflexes are present at all evolutionary levels and are often associated with the ability to memorize aversive sensory experience. However, the organizational diversity of the nervous systems is such that protective behaviours cannot be assimilated to more complex forms of responses to pain and to mental representations of pain as seen in primates (emotions, forms of sensory awareness). The emergence of these components may be dated phylogenetically to the time of transition from the aquatic to the terrestrial environment, including in the embryonic forms (amniotic egg). We still have very little knowledge of this phenomenon and there is a need for an interdisciplinary approach. Thus asserting that basic emotions (primary emotions) exist in lower vertebrates and some aquatic invertebrates is premature.
Reviewing our current knowledge on the neurological mechanisms of nociception and pain reveals the following
The definitions of words and concepts relating to pain, which are accepted worldwide, were originally chosen to characterize pain in humans. Pain and its emotional and cognitive components are well-defined in humans. It is not the case for non-human animals.
Pain is indissociable from an emotional component that is linked to primary emotions. This type of emotion is related to the concept of homeostasis.
Pain is not a single, unequivocal entity. There are different kinds of pain, depending on where it is located in the body tissues (with a special distinction between somatic and visceral tissues), the duration of trauma and the associated neural mechanisms. Different types of pain can be distinguished by their acute or chronic nature or whether or not they are associated with an inflammatory process.
Lack of acute pain management can induce neurobiological changes leading to neural plasticity that may result in changes in sensitivity and for which the interpretation in terms of chronic pain in animals is the subject of scientific debates.
It may be that the various forms of pain described in mammals are not the only ones that exist in the animal kingdom. In this respect, research needs to be undertaken to test the hypothesis of the existence of other forms of pain in infra-mammalian species. That such a hypothesis has never been examined very seriously is probably because the extensive knowledge gained from work on primates, including man, has influenced our concept of pain.
Sensitivity to noxious stimuli, characterized by response thresholds, is modulated by socio-emotional factors such as relationships between conspecifics or the mother-young bond.
Transposing data from one animal species to any other is only relevant from a phylogenetic perspective. There is no consensus in the scientific community (neuroscientists, cognitive philosophers and ethologists) on the abilities of all vertebrates and some invertebrates to feel emotions associated with avoidance of noxious stimuli, to reach 40 Expertise scientifique collective "Douleurs animales" consciousness and experience pain as higher mammals do. A similar question can be posed for nociception: some researchers believe that it participates in the emergence of the most basic forms of consciousness.
Based on the current state of knowledge, we can suppose that pain, with its sensory, cognitive, and emotional components is present in mammals and birds, however it must be borne in mind that there is no consensus for birds.