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Pain in fish: reflections…

Pain in fish: reflections…

Recently a Dutch journal (‘de Volkskrant’; special issue Sir Edmund 25th April 2015, pp 48-50) featured an article on the question of pain in fish. I was one of the interviewed and I suggested that we should think of experiments with more conclusive evidence than we have thus far. Here I reveal a few of my thoughts on the matter.

Pain in fish: reflections…

 

Recently a Dutch journal (‘de Volkskrant’; special issue Sir Edmund 25th April 2015, pp 48-50) featured an article on the question of pain in fish. I was one of the interviewed and I suggested that we should think of experiments with more conclusive evidence than we have thus far. Here I reveal a few of my thoughts on the matter.

 

The other day I was pruning one of my rose trees. The tip of my finger touched one of its thorns. Immediately I withdrew my finger, looked at the little drop of blood, and felt a brief pricking pain. I decided to proceed more carefully with the work, that is, with more attention than before and taking care of not another round of hurting my fingers on the rose tree’s thorns.

 

Now the question is, what would have happened when I would not have felt this brief pricking pain? This is, I think, the crucial question of all research on pain in fish (or animals in general): what is the difference between subjects who do and subjects who do not experience this feeling, we call pain, or to be more precise, the negative affect associated with a damaging (nociceptive) event. Or put in another way: what is the advantage of having this feeling in order to organize on-going or future behaviour, that is, beyond mere nociception - the bodily response of noticing that areas have been damaged? Thus, pain should feature in a formal behavioural model aiding the subject to optimize his/her on-going or future behaviour.

 

We know that how pain feels to people may differ between subjects. We cannot (as yet) access this private sensation beyond what people say. In a model it is therefore crucial to capture that we feel pain, for this is relevant in an evolutionary sense – as this is common to all subjects, i.e. beyond the private sensation and individual differences in ‘the colour of pain’. Hence, how pain feels to animals is of less relevance than that they feel pain. It is that which features in a formal behavioural model and that we should try to capture and measure in our experiments.

 

The immediate reflex is not dependent upon experiencing pain. So, regardless whether I would really feel pain, I will withdraw my finger. This ‘protective’ reflex in itself does not say much on the experience - this is inherent to the nociceptive machinery. Someone who would have seen me, may have noticed that following the event of hurting my finger, my movements were different from than before: (more) cautious, trying to avoid another ‘pricking-event’. Now, one could argue that this feeling increased my awareness of the situation at hand to proceed more carefully and in this sense may also speed learning in this time-frame - say a case of one-trial learning. If so, it would also follow, that if not, learning would be slow - or even impossible.

 

Evidence in humans suggests that awareness-dependent learning is significantly slowed when competing information is present, say, due to distracting events. Our on-line working space, that we experience as awareness, is limited as it, for instance, only allows to attend to one thing at a time. Awareness is serially organized, while information in the central nervous system or the brain is processed in parallel, i.e. at more levels and within more circuits at the same time. Thus, if someone would have distracted me while gardening, it would follow that my behaviour would not have changed to the same extent following the ‘pricking-event’, or it would take more ‘pricking-events’ to adjust my behaviour.

 

An experiment done in mice also suggests that awareness-dependent learning is sensitive to distraction. This is not to say that learning cannot take place without awareness. Indeed, there are many instances, where learning per se occurs without awareness of stimuli - the difference being that certain forms explicitly require awareness. It follows then that we have to organize or search for those instances where it cannot or hardly be done without awareness, showing a difference in the outcome of experiments. For instance, in humans and mice awareness-independent learning is less sensitive (or even insensitive) to distraction.

 

As to the underlying brain circuits, this immediate pricking pain – let’s call it ‘attention-grabbing pain’ - is dependent on fast-transmitting A-delta fibres, and, among others, activity in the anterior cingulate and insular cortex. The insular cortex may be, among others, considered to be the body’s emergency centre, signalling that bodily integrity is at stake – say when you suffocate, are extremely hungry or when you crave for a smoke, addictive substances or for a round of gambling. The anterior cingulate may be seen as an area necessary for awareness and attention to ponder on appropriate action-selection. The experiments in mice as indicated above confirm this notion as this area is critical for awareness-dependent learning, but not for awareness-independent learning.

 

Next to A-delta fibres, we also posses C-fibres, which transmit the more dull or pressing feeling of pain. This is more involved in long-lasting damage and may lead to changes in behaviour to seek rest and allow the body to recover from damage. Following the day of my Friday night soccer match I feel this dull pain in my muscles when I try to move too fast – hence I move more slowly and I am not inclined to do silly things. Again, similar brain areas as indicated above are involved here. Let’s call this pain ‘repair-related pain’. In general land-living animals contain more C-fibres than A-delta fibres, suggesting a functional difference between ‘attention-grabbing pain’ and ‘repair-related pain’, i.e. they sub serve different goals. I’ll return to this below.

 

An important aspect of being aware of pain, or in general of emotions, is that they can be recalled when needed and in a situation of conflict be weighed against one another. Say as an animal, I am foraging for food in the expectancy of finding tasty bits. Signals which indicate that predators may be present, may lead to a deliberation between to continue (going for tasty food) or retreat (fear of predation), i.e. expressed as risk-assessment –going back-and-forth or observing a possible threat from a relatively safe place. Thus, emotions may be helpful in dealing with conflicting situations. Conflicting situations recruit also the anterior cingulate areas and are moments where options need to be explicitly deliberated.

 

Three features are critically different in fish.

 

First, brains of fish are different from those of, say, mammals. Gross neuro-anatomy of the forebrain does not reveal the structures, which may be critical in feeling pain - or emotions in general, i.e. an anterior cingulate area or insular cortex are not present at first sight. Yet, functional neuro-anatomy reveals areas, which appear homologues to areas in mammals, such as the hippocampus and amygdala, which play a role in learning processes. Still, areas as the insular cortex and anterior cingulate cortex have not as yet been (unequivocally) found. The critical difference between fish species in general, and, say, mammals is that brain structures are less well differentiated and developed. For instance, some systems only become clearly distinguishable in vertebrates beyond fish species. Thus an important question is whether awareness developed contingent on the outgrowth and further differentiation of brain areas, such as the anterior cingulate area, after other vertebrates, such as mammals, emerged or whether only the domains and its complexity have increased. New technologies for manipulating activity in the brains of fish as well as molecular markers will undoubtedly reveal whether we find the homologues for the anterior cingulate and insular cortex.

 

Second, with 30.000 fish species – more than mammals –living in many different niches, there’s a huge variation in the outlook, organization and structural features of fish brains as well as in the behaviour of fish – far more than in mammals, I suspect. So, rather than treating them all alike, it may be critical to take a precise look at these different species to predict whether and how pain would fit their niche, and hence how this would be represented in brain and behaviour. Maybe we find strong differences between fish species: from fish species where the feeling of pain is not involved at all to fish species where it is clearly critical in survival.

 

Third, in general fish species seem to have more A-delta fibres than C-fibres. The latter may be related to the high level of plasticity that fish have, i.e. high levels of the activity of quick repair mechanisms. Of course, a question then is why this would be the case. This may be related to a trade-off between energy consuming repair processes and body-temperature related processes: ectothermic fish may divert their available energy more to quick repair mechanisms, while endothermic mammals may divert their energy more to maintaining their body temperature. In addition, it may be related to the potential higher levels of damage of land living animals than fish. Thus, in fish-species ‘attention-grabbing pain’ may be more relevant and/or easy to study than ‘repair-related pain’. As indicated above, it may also be studied in a different time-frame: ‘attention-grabbing pain’ is related to the ‘here-and-now’ and hence leads to immediate changes in behaviour as well as may be related to learning processes, i.e. caution, consolidation and retrieval, while ‘repair-related pain’ has a longer time-span and is related to changes in behavioural patterns related to facilitating repair-mechanisms to take place. Next to behaviour following damaging events, conflicting situations may be ideally suited to further dissect differences between the two forms of pain. Data from some studies already lend themselves to look at such differences. Given the differences in niches that fish species are living related to e.g. differences in foraging tactics, this may lead to predictions, which areas are more sensitive for preventing damage. In humans, for instance, some parts of our body are more costly to damage than others and hence are more strongly represented. In addition, it may lead to predictions which species would potentially experience more or less pain – or even as indicated above where it may be absent.

 

So, to advance the discussions and overcome the controversies in this field, it is critical to use paradigms (1) that are tailored to the fish-species, (2) that lead to clear mutually-exclusive predictions following from experiments and observations in relation to ‘attention-grabbing pain’ and/or ‘repair-related pain’, that is, predictions between awareness-dependent and awareness-independent behaviour and learning related to these different forms of pain, and (3) that may be associated with brain-related changes, i.e. first by assessing which areas express changes in immediate-early genes in relation to the event or other molecular markers of relevance, second by changing the activity in these areas to assess causation, and third to relate these areas to their mammalian equivalents. It is clear that the insular cortex may be key next to the anterior cingulate.

 

Finally, the question of pain in fish is not so much different from the question of pain in infants, who cannot report in language their feelings and thoughts. So what applies to evolutionary questions, also applies to ontogenetic questions – in humans and other species alike. Thus the question of pain in infants should be – and is actually - approached along the same lines of reasoning. Quite a few interesting reports have already emerged on this subject as it is obvious that the question of pain-alleviation in infants is of utmost importance for instance in premature born babies. Knowledge obtained in this field is of direct relevance to questions on pain in animals, including fish.

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