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Anche i pesci provano dolore
Le trote possiedono recettori polimodali proprio come gli uomini

Scienziati britannici avrebbero trovato le prove che dimostrano che anche i pesci percepiscono il dolore. La scoperta fa da complemento a studi precedenti, secondo i quali sia gli uccelli sia i mammiferi possono provare dolore, mentre era stato ipotizzato che i pesci non ne fossero in grado.
I ricercatori, del Roslin Institute e dell’Università di Edimburgo, hanno osservato regioni nel cervello delle trote arcobaleno che rispondono a stimoli dannosi. Inoltre hanno scoperto che i pesci mostrano reazioni marcate quando vengono esposti a sostanze nocive.
La ricerca è stata pubblicata sulla rivista “Proceedings of the Royal Society London B”. Lynne Sneddon, che ha guidato il gruppo di scienziati, afferma che i profondi mutamenti comportamentali e fisiologici mostrati dalle trote dopo l’esposizione a sostanze nocive sono comparabili con quelli osservati nei mammiferi superiori. I ricercatori hanno anche indagato sulla presenza nei pesci di nocicettori, siti che rispondono a stimoli di danneggiamento dei tessuti.
Applicando stimoli meccanici, termici e chimici alla testa di pesci sotto anestetico, e registrando la loro attività neurale, gli scienziati hanno scoperto 58 recettori che rispondevano ad almeno uno stimolo. “Di questi - spiega Sneddon - ventidue possono essere classificati come nocicettori, in quanto rispondevano alla pressione meccanica ed erano stimolati se riscaldati sopra i 40 gradi Celsius. Diciotto recettori, inoltre, rispondevano a stimolazioni chimiche e possono essere definiti come nocicettori polimodali”. Si tratta dei primi a essere trovati nei pesci, e assomigliano a quelli degli anfibi, degli uccelli e dei mammiferi, compresi quelli degli esseri umani.

Can fish feel pain? This is a question that has troubled anglers and animal rights activists alike in recent years. Research by a team of scientists from the Roslin Institute and the University of Edinburgh appears to provide the answer: "yes". This is the first conclusive evidence indicating pain perception in fish. It has previously been established that birds and mammals are capable of experiencing pain.

This study was undertaken on rainbow trout and demonstrates that not only are there nervous system receptors in the head of the fish that respond to damaging stimuli, but that application of short-acting noxious substances produce profound behavioural and physiological changes in the fish over a prolonged period comparable to those observed in higher mammals. This indicates, the researchers believe, that fish can perceive pain.

Nervous reaction
The presence of nociceptors - receptors that respond preferentially to tissue damaging stimuli - in the rainbow trout was determined using electrophysiological recordings. Neural activity was recorded in prepared, anaesthetised fish whilst mechanical, thermal and chemical stimuli were applied to the head. "We found 58 receptors located on the face and head of the rainbow trout that responded to at least one of the stimuli," says Dr Lynne Sneddon who led the research at the Roslin Institute and is now working at University of Liverpool. "22 of these could be classified as nociceptors in that they responded to mechanical pressure and were stimulated when heated above 40ºC. Of these, 18 receptors also responded to chemical stimulation and can be defined as polymodal nociceptors."

The polymodal nociceptors found in the trout are the first to be found in fish and have similar properties to those found in amphibians, birds and mammals including humans. "Receptor diameter, thermal thresholds and mechanical responses were found to be similar to those measured in higher vertebrate groups," explains Dr Sneddon. "However mechanical thresholds were lower than those found in, for example, human skin. This may be due to the more easily damaged nature of fish skin. The threshold for mechanical stimulation appears to be similar to that for mammalian eye nociceptors."

Nociception not enough
However the demonstration of nociception, the detection of noxious, tissue damaging stimuli, in any animal is not enough to prove pain perception. Nociception may sometimes be accompanied by a reflex reaction such as withdrawal. To demonstrate pain perception it is necessary to show that the animal’s behaviour is adversely affected by a potentially painful experience and these behavioural changes are not simple reflex responses. Therefore the behavioural response to stimuli in fish was also studied.

Bee venom or acetic acid was injected in the lips of some trout, whilst other fish were injected with saline solution or merely handled to form control groups. All fish had previously been conditioned to feed at a feeding ring in their tank where they were collected for handling and/or injection.

"Anomalous behaviours were exhibited by trout subjected to bee venom and acetic acid," says Dr Sneddon. "Fish demonstrated ‘rocking’ motion, strikingly similar to the kind of motion seen in stressed higher vertebrates like mammals, and the trout injected with acetic acid were also observed to rub their lips onto the gravel in their tank and on the tank walls. These do not appear to be reflex responses."

The acid and bee venom injected fish also took almost three times longer to resume feeding activity compared to the saline and handling control groups. The time to resume feeding did not decrease in experiments with fish conditioned to feed on ‘softer’ food.

Hook line and sinker
"Our research demonstrates nociception and suggests that noxious stimulation in the rainbow trout has adverse behavioural and physiological effects. This fulfils the criteria for animal pain," concludes Dr Sneddon. "Previous work on fish has looked at the elasmobranch family, such as stingrays, and primitive vertebrates like the lamprey. These studies did not conclusively show the presence of nociceptors. We believe that our study is the first work with fish of the teleost family and the results may represent an evolutionary divergence between the teleost and elasmobranch lineages."