Pain in invertebrates 1

Pain in invertebrates

A Monarch butterfly, (Danaus plexippus)caterpillar

Pain in invertebrates is a contentious issue. Although there arenumerous definitions of pain, almost all involve two key components.First, nociception is required. This is the ability to detect noxiousstimuli which evokes a reflex response that moves the entire animal, orthe affected part of its body, away from the source of the stimulus. Theconcept of nociception does not 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 thenociceptive experience. Pain is therefore a private, emotionalexperience. Pain cannot be directly measured in other animals,including other humans; responses to putatively painful stimuli can bemeasured, but not the experience itself. To address this problem whenassessing the capacity of other species to experience pain,argument-by-analogy is used. This is based on the principle that if ananimal responds to a stimulus in a similar way to ourselves, it is likelyto have had an analogous experience. Dr Chris Sherwin at theUniversity of Bristol used this line of reasoning to question whetherinvertebrates have the capacity for suffering. He argued that if a pin isstuck in a chimpanzee's finger and she rapidly withdraws her hand,then argument-by-analogy implies that like humans, she felt pain. Whythen, Sherwin questions, does not the inference follow that a cockroach experiences pain when it writhes after beingstuck with a pin?[1] This argument-by-analogy approach has been revisited by Prof. Rob Elwood at the Queen'sUniversity Belfast.[2]

The ability to experience nociception has been subject to natural selection and offers the advantage of reducingfurther harm to the organism. While it might be expected therefore that nociception is widespread and robust,nociception varies across species. For example, the chemical capsaicin is commonly used as a noxious stimulus inexperiments with mammals; however, the African naked mole-rat, Heterocephalus glaber, an unusual rodent speciesthat lacks pain-related neuropeptides (e.g., substance P) in cutaneous sensory fibres, shows a unique and remarkablelack of pain-related behaviours to acid and capsaicin.[3] Similarly, capsaicin triggers nociceptors in someinvertebrates,[4][5] but this substance is not noxious to Drosophila melanogaster.[6] Criteria that may indicate apotential for experiencing pain include:[7]

1.1. Has a suitable nervous system and receptors2.2. Physiological changes to noxious stimuli3.3. Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing,

holding or autotomy4.4. Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local

anaesthetics5.5. Shows trade-offs between stimulus avoidance and other motivational requirements6.6. Shows avoidance learning7.7. High cognitive ability and sentience

Pain in invertebrates 2

Suitable nervous system

Central nervous systemOne suggested reason for rejecting a pain experience in invertebrates is that invertebrate brains are too small.However, brain size does not necessarily equate to complexity of function.[8] Moreover, weight for body-weight, thecephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but asbig or bigger than most fish brains.[9][10]

Charles Darwin wrote of the interaction between size and complexity of invertebrate brains: "It is certain that there may be

extraordinary activity with an extremely small absolute mass of nervous matter; thus the wonderfully diversified instincts, mental

powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin's head. Under

this point of view, the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more so than the brain of

man."[11]

Internal anatomy of a spider, showing the centralnervous system in blue

Invertebrate nervous systems are very unlike those of vertebrates andthis dissimilarity has sometimes been used to reject the possibility of apain experience in invertebrates. In humans, the neocortex of the brainhas a central role in pain and it has been argued that any specieslacking this structure will therefore be incapable of feeling pain.[12]

However, it is possible that different structures may be involved in thepain experience of other animals in the way that, for example,crustacean decapods have vision despite lacking a human visualcortex.[13]

The octopus Amphioctopus marginatus

Two groups of invertebrates have notably complex brains: arthropods(insects, crustaceans, arachnids, and others) and modern cephalopods(octopuses, squid, cuttlefish) and other molluscs.[14] The brains ofarthropods and cephalopods arise from twin parallel nerve cords thatextend through the body of the animal. Arthropods have a central brainwith three divisions and large optical lobes behind each eye for visualprocessing.[14] The brains of the modern cephalopods in particular arehighly developed, comparable in complexity to the brains of somevertebrates (See also: Invertebrate brains). Emerging results suggestthat a convergent evolutionary process has led to the selection ofvertebrate-like neural organization and activity-dependent long-termsynaptic plasticity in these invertebrates.[15] Cephalopods stand out by having a central nervous system that sharesprime electrophysiological and neuroanatomical features with vertebrates like no other invertebrate taxon.[16]

Pain in invertebrates 3

Nociceptors

Medicinal leech, Hirudo medicinalis

Nociceptors are sensory receptors that respond to potentially damagingstimuli by sending nerve signals to the brain. Although these neurons ininvertebrates may have different pathways and relationships to the centralnervous system than mammalian nociceptors, nociceptive neurons ininvertebrates often fire in response to similar stimuli as mammals, such ashigh temperature (40 C or more), low pH, capsaicin, and tissue damage. Thefirst invertebrate in which a nociceptive cell was identified was themedicinal leech, Hirudo medicinalis, which has the characteristic segmentedbody of an Annelida, each segment possessing a ganglion containing the T(touch), P (pressure) and N (noxious) cells.[17] Later studies on the responsesof leech neurones to mechanical, chemical and thermal stimulation

motivated researchers to write "These properties are typical of mammalian polymodal nociceptors".[4]

The sea hare, Aplysia

There have been numerous studies of learning and memory usingnociceptors in the sea hare, Aplysia.[18][19][20] Many of these havefocused on mechanosensory neurons innervating the siphon and havingtheir somata (bulbous end) in the abdominal ganglion (LE cells). TheseLE cells display increasing discharge to increasing pressures, withmaximal activation by crushing or tearing stimuli that cause tissueinjury. Therefore, they satisfy accepted definitions of nociceptors.They also show similarities to vertebrate Aδ nociceptors, including aproperty apparently unique (among primary afferents) to nociceptors— sensitization by noxious stimulation. Either pinching or pinning the siphon decreased the threshold of the LE cellsfiring and enhanced soma excitability.[21]

Nociceptors have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodesand arthropods.[22][23]

Physiological changesIn vertebrates, potentially painful stimuli typically produce vegetative modifications such as tachycardia, pupildilation, defecation, arteriole blood gases, fluid and electrolyte imbalance, and changes in blood flow, respiratorypatterns, and endocrine.[24]

The crayfish Procambarus clarkii

At the cellular level, injury or wounding of invertebrates leads to thedirected migration and accumulation of haematocytes (defence cells)and neuronal plasticity, much the same as the responses of humanpatients undergoing surgery or after injury.[25][26] In one study, heartrate in the crayfish, Procambarus clarkii, decreased following clawautotomy during an aggressive encounter.[27]

Recording physiological changes in invertebrates in response tonoxious stimuli will enhance the findings of behavioural observationsand such studies should be encouraged. However, careful control isrequired because physiological changes can occur due to noxious, but

non-pain related events, e.g. cardiac and respiratory activity in crustaceans is highly sensitive and responds tochanges in water level, various chemicals and activity during aggressive encounters.[28]

Pain in invertebrates 4

Protective motor reactionsInvertebrates show a wide range of protective reactions to putatively painful stimuli. However, even unicellularanimals will show protective responses to, for example, extremes of temperature. Many invertebrate protectivereactions appear stereotyped and reflexive in action, perhaps indicating a nociceptive response rather than one ofpain, but other responses are more plastic, especially when competing with other motivational systems (see sectionbelow), indicating a pain response analogous to that of vertebrates.

Mechanical stimulationA selection of invertebrates that show avoidance of noxious mechanical stimulation

The flatworm Pseudoceros dimidiatus

Drosophila larva

The sea hare, Aplysia

Rather than a simple withdrawal reflex, the flatworm, Notoplana aticola, displays a locomotory escape behaviourfollowing pin pricks to the posterior end.[29] Touching the larvae of fruit flies, Drosophila melanogaster, with aprobe causes them to pause and move away from the stimulus, however, stronger mechanical stimulation evokes amore complex corkscrew-like rolling behaviour, i.e. the response is plastic.[30] When a weak tactile stimulus isapplied to the siphon of the sea-hare Aplysia californica, the animal rapidly withdraws the siphon between theparapodia.[21][31][32] It is sometimes claimed this response is an involuntary reflex (e.g. see Aplysia gill and siphonwithdrawal reflex), however, the complex learning associated with this response (see 'Learned Avoidance' below)suggests this view might be overly simplistic.In 2001, Walters and colleagues published a report that described the escape responses of the Tobacco Hornwormcaterpillar Manduca sexta to mechanical stimulation.[33] These responses, particularly their plasticity, wereremarkably similar to vertebrate escape responses."A set of defensive behavior patterns in larval Manduca sexta is described and shown to undergo sensitization following noxious

mechanical stimulation. The striking response is a rapid bending that accurately propels the head towards sharply poking or

pinching stimuli applied to most abdominal segments. The strike is accompanied by opening of the mandibles and, sometimes,

regurgitation. The strike may function to dislodge small attackers and startle larger predators. When the same stimuli are applied to

anterior segments, the head is pulled away in a withdrawal response. Noxious stimuli to anterior or posterior segments can evoke a

transient withdrawal (cocking) that precedes a strike towards the source of stimulation and may function to maximize the velocity of

the strike. More intense noxious stimuli evoke faster, larger strikes and may also elicit thrashing, which consists of large, cyclic,

side-to-side movements that are not directed at any target. These are sometimes also associated with low-amplitude quivering

Pain in invertebrates 5

cycles. Striking and thrashing sequences elicited by obvious wounding are sometimes followed by grooming-like behavior."[33]

Tobacco hornworm larva, Manduca sexta

Tobacco Hornworm

Autotomy

Over 200 species of invertebrates are capable of using autotomy (selfamputation) as an avoidance or protective behaviour[34][35] including -

• land slugs (Prophysaon)[36]

• sea snails (Oxynoe panamensis)[37]

• crickets[38]

• spiders[39]

• crabs[40]

• lobsters[35]

• octopuses[35]

These animals can voluntarily shed appendages when necessary for survival. Autotomy can occur in response tochemical, thermal and electrical stimulation, but is perhaps most frequently a response to mechanical stimulationduring capture by a predator. Autotomy serves either to improve the chances of escape or to reduce further damageoccurring to the remainder of the animal such as the spread of a chemical toxin after being stung, but the 'decision' toshed a limb or part of a body and the considerable costs incurred by this, suggests a pain response rather than simplya nociceptive reflex.

Thermal stimulationA heated probe (»42 °C or 108 °F) evokes a complex, corkscrew-like rolling avoidance behaviour in Drosophilalarvae which occurs in as little as 0.4 seconds; a non-heated probe does not cause this avoidance behaviour.[30] Landsnails show an avoidance response to being placed on a hotplate (»40 °C or 104 °F) by lifting the anterior portion ofthe extended foot.[41][42]

Chemical stimulationThe prawn Palaemon elegans shows protective motor reactions when their antennae are treated with the irritantsacetic acid or sodium hydroxide.[43] The prawns specifically groom the treated antennae and rub them against thetank, showing they are aware of the location of the noxious stimulus on their body rather than exhibiting ageneralised response to stimulation.

Wasp stinger, with droplet of venom

Under natural conditions, orb-weaving spiders (Argiope spp.) undergoautotomy (self-amputation) if they are stung in a leg by wasps or bees.Under experimental conditions, when spiders were injected in the leg withbee or wasp venom, they shed this appendage. But if they are injected withonly saline, they rarely autotomize the leg, indicating it is not the physicalinsult or the ingress of fluid per se that causes autotomy. Even moreinterestingly, spiders injected with venom components which cause injectedhumans to report pain (serotonin, histamine, phospholipase A2 and melittin)autotomize the leg, but if the injections contain venom components which donot cause pain to humans, autotomy does not occur.[44]

Electrical stimulation

Pain in invertebrates 6

The sea-slug, Tritonia diomedia, possesses a group of sensory cells, "S-cells", situated in the pleural ganglia, whichinitiate escape swimming if stimulated by electric shock.[23] Similarly, the mantis shrimp Squilla mantis showsavoidance of electric shocks with a strong tail-flick escape response.[45] Both these responses appear to be ratherfixed and reflexive, however, other studies indicate a range of invertebrates exhibit considerably more plasticresponses to electric shocks.Because of their soft bodies, hermit crabs rely on shells for their survival, but, when they are given small electricshocks within their shells, they evacuate these. The response, however, is influenced by the attractivness of the shell;more preferred shells are only evacuated when the crabs are given a higher voltage shock, indicating this is not asimple, reflex behaviour.[13]

In studies on learning and the Aplysia gill and siphon withdrawal reflex, Aplysia received an electric shock on thesiphon each time their gill relaxed below a criterion level.[46] Aplysia learned to keep their gills contracted above thecriterion level - an unlikely outcome if the response was due to a nociceptive experience.Drosophila feature widely in studies of invertebrate nociception and pain. It has been known since 1974[47] thatthese fruit-flies can be trained with sequential presentations of an odour and electric shock (odour-shock training)and will subsequently avoid the odour because it predicts something 'bad'.[48][49] A similar response has been foundin the larvae of this species.[50] In an intruiging study,[51] Drosophila learned two kinds of prediction regarding a'traumatic' experience. If an odour preceded an electric shock during training, it predicted shock and the fliessubsequently avoided it. When the sequence of events during training was reversed, i.e. odour followed shock, theodour predicted relief from shock and flies approached it. The authors termed this latter effect 'relief' learning.Many invertebrate species learn to withdraw from, or alter their behaviour in response to, a conditioned stimuluswhen this has been previously paired with an electric shock - cited by Sherwin[1] - and include snails, leeches,locusts, bees and various marine molluscs.If vertebrate species are used in studies on protective or motor behaviour and they respond in similar ways to thosedescribed above, it is usually assumed that the learning process is based on the animal experiencing a sensation ofpain or discomfort from the stimulus, e.g. an electric shock. Argument-by-analogy suggests an analogous experienceoccurs in invertebrates.

Opioid receptors, effects of local anaesthetics or analgesicsIn vertebrates, opiates modulate nociception and opioid receptor antagonists, e.g. naloxone and CTOP, reverse thiseffect. So, if opiates have similar effects in invertebrates as vertebrates, they should delay or reduce any protectiveresponse and the opioid antagonist should counteract this. It has been found that molluscs and insects have opioidbinding sites or opioid general sensitivity. Certainly there are many examples of neuropeptides involved in vertebratepain responses being found in invertebrates, for example, endorphins have been found in platyhelminthes, molluscs,annelids, crustaceans and insects (see[1][52]). It should be noted, however, that apart from analgesia, there are othereffects of exogenous opiates specifically being involved in feeding behaviour and activation of immunocytes.[53]

These latter functions might explain the presence of opioids and opioid receptors in extremely simple invertebratesand unicellular animals.

Pain in invertebrates 7

Nematodes

Movement of Wild-type C. elegans

Nematodes avoid extremes of temperature.[5] Morphine increases thelatency of this defensive response in the parasitic Ascaris suum.[54] In astudy on the effects of opiates in Caenorhabditis elegans, 76% of anon-treated group exhibited a rapid, reflexive withdrawal to heat,whereas 47%, 36% and 39% of morphine, endomorphin 1 andendomorphin 2 treated worms (respectively) withdrew. These effectswere reversed with the opioid receptor antagonists naloxone andCTOP, leading the authors to conclude that thermonocifensive

behaviour in C. elegans was modulated by opioids.[55]

Molluscs

Helix pomatia, a species of land snail

Slugs and snails posess an opioid receptor system.[56][57] Inexperiments on different terrestrial snails, morphine prolonged thelatency of the snails' raising their foot in response to being placed on ahot (40°C) surface.[42] The analgesic effects of the morphine wereeliminated by naloxone as is seen in humans and other vertebrates.There was also habituation to morphine. Snails administered withmorphine for four days did not differ from the control ones in tests onpain sensitivity and analgesia was achieved only at a higher dose.

Crustaceans

Two crustaceans that show responses to analgesics and their agonists

A mantis shrimp, Squilla mantis

The grass prawn, Penaeus monodon

Evidence of the capacity for invertebrates to experience nociception and pain has been widely studied in crustaceans.[28] In the crab Neohelice granulata,[58]</ref> electric shocks delivered via small holes in the carapace elicited a defensive threat display. Injection of morphine reduced the crabs' sensitivity to the shock in a dose-dependent manner, with the effect declining with increasing duration between morphine injection and shock. Naloxone injection inhibited the effects of morphine, as is seen in vertebrates.[59] Morphine also had inhibitory effects on the escape tail-flick response to electric shock in the mantis shrimp, Squilla mantis, that was reversed by

Pain in invertebrates 8

naloxone, indicating that the effect is found in crustacean groups other than decapods.[45] When the irritants aceticacid or sodium hydroxide were applied to the antennae of grass prawns, Penaeus monodon, there was an increase inrubbing and grooming of the treated areas which was not seen if they had previously been treated with a localanaesthetic, benzocaine, however, the benzocaine did not eliminate the level of rubbing seen in response tomechanical stimulation with forceps. There was no effect of benzocaine on the general locomotion of the prawns, sothe reduction in rubbing and grooming was not simply due to inactivity of the animal.[43] Another local anaesthetic,xylocaine, reduced the stress of eyestalk ablation in female Whiteleg shrimps, Litopenaeus vannamei, as indicated bylevels of feeding and swimming.[60]

It has not always been possible to replicate these findings in crustaceans. In one study,[61] three decapod crustaceanspecies, Louisiana red swamp crayfish, white shrimp and grass shrimp, were tested for nociceptive behaviour byapplying sodium hydroxide, hydrochloric acid, or benzocaine to the antennae. This caused no change in behaviour inthese three species compared to controls. Animals did not groom the treated antenna, and there was no difference inmovement of treated individuals and controls. Extracellular recordings of antennal nerves in the Louisiana redswamp crayfish revealed continual spontaneous activity, but no neurons that were reliably excited by the applicationof sodium hydroxide or hydrochloric acid. The authors concluded there was no behavioural or physiologicalevidence that the antennae contained specialized nociceptors that responded to pH. It could be argued thatdifferences in the findings between studies may be due to responses to extreme pH being inconsistently evokedacross species.It has been argued that the analgesic effects of morphine should not be used as a criterion of the ability of animals, atleast crustaceans, to experience pain. In one study, shore crabs, Carcinus maenas received electric shocks in apreferred dark shelter but not if they remained in an unpreferred light area. Analgesia from morphine should haveenhanced movement to the preferred dark area because the crabs would not have experienced 'pain' from the electricshock. However, morphine inhibited rather than enhanced this movement, even when no shock was given. Morphineproduced a general effect of non-responsiveness rather than a specific analgesic effect, which could also explainprevious studies claiming analgesia. However, the researchers argued that other systems such as the enkephalin orsteroid systems might be used in pain modulation by crustaceans and that behavioural responses should beconsidered rather than specific physiological and morphological features.[62]

Insects

The house cricket, Acheta domestica

Morphine extends the period that crickets avoided the heated surface ofa hotplate.[63][64]

Trade-offs between stimulus avoidance andother motivational requirements

This is a particularly important criterion for assessing whether ananimal has the capacity to experience pain rather than only nociception. Nociceptive responses do not requireconsciousness or higher neural processing; this results in relatively fixed, reflexive actions. However, the experienceof pain does involve higher neural centres which also take into account other factors of relevance to the animal, i.e.competing motivations. This means that a response to the experience of pain is likely to be more plastic than anociceptive response when there are competing factors for the animal to consider.

Pain in invertebrates 9

Hermit crabs fighting over a shell

Robert Elwood and Mirjam Appel at the Queen's University of Belfastargue that pain may be inferred when the responses to a noxiousstimulus are not reflexive but are traded off against other motivationalrequirements, the experience is remembered and the situation isavoided in the future. They investigated this by giving hermit crabssmall electric shocks within their shells. Only crabs given shocksevacuated their shells indicating the aversive nature of the stimulus, butfewer crabs evacuated from a preferred species of shell demonstratinga motivational trade-off.[13] Most crabs, however, did not evacuate atthe shock level used, but when these shocked crabs were subsequentlyoffered a new shell, they were more likely to approach and enter thenew shell. They approached the new shell more quickly, investigated itfor a shorter time and used fewer cheliped probes within the aperture prior to moving in. This demonstrates theexperience of the electric shock altered future behaviour in a manner consistent with a marked shift in motivation toget a new shell to replace the one previously occupied.

Learned avoidanceLearning to avoid a noxious stimulus indicates that prior experience of the stimulus is remembered by the animal andappropriate action taken in the future to avoid or reduce potential damage. This type of response is therefore not thefixed, reflexive action of nociceptive avoidance.

Habituation and sensitizationHabituation and sensitisation are two simple, but widespread, forms of learning. Habituation refers to a type ofnon-associative learning in which repeated exposure to a stimulus leads to decreased responding. Sensitization isanother form of learning in which the progressive amplification of a response follows repeated administrations of astimulus.When a tactile stimulus is applied to the skin of Aplysia californica, the animal withdraws the siphon and gillbetween the parapodia. This defensive withdrawal, known as the Aplysia gill and siphon withdrawal reflex, has beenthe subject of much study on learning behaviour.[32][65][66] Generally, these studies have involved only weak, tactilestimulation and are therefore more relevant to the question of whether invertebrates can experience nociception,however, some studies[46] have used electric shocks to examine this response (See sections on "Electricalstimulation" and "Operant conditioning").Other researchers working with Aplysia were sufficiently impressed about the similarity between invertebrate and mammalianresponses to write - "Persistent nociceptive sensitization of nociceptors in Aplysia displays many functional similarities to

alterations in mammalian nociceptors associated with the clinical problem of chronic pain. Moreover, in Aplysia and mammals the

same cell signaling pathways trigger persistent enhancement of excitability and synaptic transmission following noxious stimulation,

and these highly conserved pathways are also used to induce memory traces in neural circuits of diverse species"[67]

Location avoidanceAvoidance learning was examined in the crab Neohelice granulata by placing the animals in a the dark compartmentof a double-chamber device and allowing them to move towards a light compartment.[68] Experimental crabsreceived a shock in the light compartment, whilst controls did not. After 1 min, both experimental and control crabswere free to return to the dark compartment. The learned outcome was not a faster escape response to the stimulusbut rather refraining from re-entering the light compartment. A single trial was enough to establish an associationbetween light and shock that was detected up to 3 hours later.[69]

Pain in invertebrates 10

Studies on crayfish, Procambarus clarkia, demonstrated that they learned to associate the turning on of a light with ashock that was given 10 seconds later. They learned to respond by walking to a safe area in which the shock was notdelivered.[70] However, this only occurred if the crayfish were facing the area to which they could retreat to avoidthe shock. If they were facing away from the safe area the animal did not walk but responded to the shock by atail-flick escape response. Despite repeated pairings of light and shock the animals did not learn to avoid the shockby tail-flicking in response to light. Curiously, when the animals that had experienced shocks whilst facing awayfrom the safe area were subsequently tested facing towards the safe area they showed a very rapid avoidance of theshock upon the onset of the light. Thus, they seemed to have learned the association although they had not previouslyused it to avoid the shock - much like mammalian latent learning. These studies show an ability in decapods thatfulfils several criteria for pain experience rather than nociception.

Conditioned suppression

A drone bee

Honeybees extend their proboscis when learning about novel odours.In one study on this response, bees learnt to discriminate between twoodours, but then learned to suppress the proboscis extension responsewhen one of the odours was paired with an electric shock.[71] Thisindicates the sensation was aversive to the bee, however, the responsewas plastic rather than simply reflexive, indicating pain rather thannociception.

Operant conditioning

Operant studies using vertebrates have been conducted for many years.In such studies, an animal operates or changes some part of the environment to gain a positive reinforcement oravoid a negative one. In this way, animals learn from the consequence of their own actions, i.e. they use an internalpredictor. Operant responses indicate a voluntary act; the animal exerts control over the frequency or intensity of itsresponses, making these distinct from reflexes and complex fixed action patterns. A number of studies have revealedsurprising similarities between vertebrates and invertebrates in their capacity to use operant responses to gainpositive reinforcements,[72] but also to avoid negative reinforcement that in vertebrates would be described as 'pain'.

Underside of a snail climbing a bladeof grass, showing the muscular foot

Snail

It has been shown that snails will operate a manipulandum to electricallyself-stimulate areas of their brain. Balaban and Maksimova[73] surgicallyimplanted fine wire electrodes in two regions of the brains of snails (Helix sp.).To receive electrical stimulation of the brain, the snail was required to displacethe end of a rod. When pressing the rod delivered self-stimulation to themesocerebrum (which is involved in sexual activity) the snails increased thefrequency of operating the manipulandum compared to the baseline spontaneousfrequency of operation. However, when stimulation was delivered to the parietalganglion, the snails decreased the frequency of touching the rod compared to thebaseline spontaneous frequency. These increases and decreases in pressing arepositive and negative reinforcement responses typical of those seen withvertebrates.

Pain in invertebrates 11

Aplysia

To examine the gill and siphon withdrawal response to a putatively painful stmulus, Aplysia were tested in pairs.During the initial training period, the experimental animal received a siphon shock each time its gill relaxed below acriterion level, and the yoked control animal received a shock whenever the experimental animal did, regardless ofits own gill position. The experimental animals spent more time with their gills contracted above the criterion levelthan did the control animals during each period, demonstrating operant conditioning.[46]

Drosophila melanogaster

Drosophila

A fly-controlled heat-box has been designed to study operantconditioning in several studies of Drosophila.[74][75][76] Each time a flywalks into the designated half of the tiny dark chamber, the wholespace is heated. As soon as the animal leaves the punished half, thechamber temperature reverts to normal. After a few minutes, theanimals restrict their movements to one-half of the chamber, even ifthe heat is switched off.

A Drosophila flight simulator has been used to examine operantconditioning.[77] The flies are tethered in an apparatus that measuresthe yaw torque of their flight attempts and stabilizes movements of the panorama. The apparatus controls the fly'sorientation based on these attempts. When the apparatus was set up to direct a heat beam on the fly if it "flew" tocertain areas of its panorama, the flies learned to prefer and avoid certain flight orientations in relation to thesurrounding panorama. The flies "avoided" areas that caused them to receive heat.

These experiments show that Drosophila can use operant behaviour and learn to avoid noxious stimuli. However,these responses were plastic, complex behaviours rather than simple reflex actions, consistent more with theexperience of pain rather than simply nociception.

Cognitive abilities

Atta colombica workers transporting leaves

It could be argued that a high cognitive ability is not necessary for theexperience of pain, otherwise, it could be argued that humans with lesscognitive capacity have a lower likelihood of experiencing pain.However, most definitions of pain indicate some degree of cognitiveability. Several of the learned and operant behaviours described aboveindicate that invertebrates have high cognitive abilities. Otherexamples include -• Social transmission of information during the waggle dance of

honeybees.• Idiothetic orientation by spiders, i.e. they memorize information

about their previous movements.[78]

• Detour behaviour in which spiders choose to take an indirect routeto a goal rather than the most direct route, thereby indicatingflexibility in behaviour and route planning, and possibly insightlearning.[1]

• Conceptualisation in the honeybee, Apis mellifera.[79]

• Problem solving in leafcutter ants, Atta colombica.[80]

• Numeracy in the yellow mealworm beetle, Tenebrio molitor,[81] and honeybee.[82]

Pain in invertebrates 12

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mammalian TRPA1/ANKTM1. Current Biology, 16: 1034–1040[7] Elwood, R.W., Barr, S. and Patterson, L., (2009). Pain and stress in crustaceans? Applied Animal Behaviour Science, 118 (3): 128–136[8] Chittka, L. and Niven, J., (2009). Are Bigger Brains Better? Current Biology, 19: R995–R1008, November 17. DOI

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Article Sources and Contributors 15

Article Sources and ContributorsPain in invertebrates  Source: http://en.wikipedia.org/w/index.php?oldid=556990617  Contributors: Anthonyhcole, Bearcat, CarrieVS, DrChrissy, Draeath, DragonflySixtyseven,Electricmuffin11, Epipelagic, Eug, Eumolpo, GB fan, Glacialfox, Jimfbleak, John of Reading, LilHelpa, Logical Cowboy, Mgiganteus1, NotWith, Ontoursecretly, Orenburg1, Ottawahitech,Rjwilmsi, Robert Daoust, SchreiberBike, Squids and Chips, Stemonitis, Steven Walling, Tom.Reding, TotoBaggins, Vanished user 19794758563875, VictorianMutant, Zearin, 7 anonymous edits

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