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Endorphins, nociceptors, pain, and fox hunting

Thursday 22nd September 2011

Endorphins, nociceptors, pain, and fox hunting

A friend of mine recently told me about an argument put forward by those who support fox hunting (the kind with horses and hounds) about pain. No worries, say the proponents, the fox is so doped up on endorphins that by the time the hounds get to it, it feels no pain when it gets ripped apart. Therefore, and despite how it might seem to us, the fox dies a painless death.

Well, that surprised me. These kinds of asinine statements delivered with complete confidence have great precedent in the hunters’ lexicon, and are basically meant to delude the unapprised.  But let’s have a bit of a review of pain, endorphins, and nociceptors. Some of this might be a bit technical, but it kills the hunters’ arguments dead.

At the most basic level, the issue relates to animal “consciousness”, a subject that has been endlessly debated by philosophers since the days of Rene Descartes, who said in 1641 that non-humans are nothing but “automata” without souls, minds, or reason. Animals were therefore not conscious, and could not suffer or feel pain. It is quite amazing that such opinions were expressed then (Descartes could not have owned a dog, for example) and even more amazing that vestiges of such sentiments remain 370 years later. Anyone at all familiar with animals knows they are conscious and feel pain just by observing the following:

• Animals have a wide range of behaviour, including playfulness, tenderness, fear, depression, aggression, etc. They can clearly express emotions, and because of their far greater abilities to hear and smell, actually live in a world where perception of the complexities of their environment is greater than ours.

• Animals have a wide range of learning abilities, and not just what they can be “taught” by humans. Wild animals with territories know the landscape intimately, predators learn how to hunt different prey species with different techniques, elephants “remember” distant pastures and waterholes needed in times of drought, etc. Along these lines, social animals have complex individual hierarchies within their groups. Within hyena clans, that could number 60 members or more, every individual knows where they stand in a very complex network of relationships.

• Among a long list, dogs, cats, cows, armadillos, mice, and opossums experience REM sleep (dreaming) – now linked to intelligence and cognitive function.

• Animals clearly experience pain, and this fact has been used for aeons for training and “encouragement”. If animals did not experience pain, whips, spurs, cattle prods, nose rings in bulls and the like would be useless. Wounded animals limp or become immobile to avoid pain, and arthritic animals, for example, improve greatly in mobility when given steroids to relieve pain. It is however telling that animal pain as a rigorously treated subject was not part of the US veterinary school curriculum until 1989 according to Bernard Rollin of Colorado State University.

• Any animal that does not suffer pain is an evolutionary dead end. We rely on the signal of pain to avoid situations that are dangerous and life-threatening.

Besides all that, there is no physiological reason why animals should not feel pain. All mammalian nervous systems consist of the same cells and pathways. All mammals (at least) have nociceptors, sensory neurons found in parts of the body that sense pain internally and externally. They are thus found in the skin and mucosa, and in muscles, joints, bladder, and the digestive tract. They contain two types of axons that allow fast (20 meters/second) signals and slower (2 meters/second) signals to the central nervous system. Pain thus occurs in two phases – an initially sharp pain signal sent by the fast axons, and then a more enduring dull pain signal sent by the slower axons.

So essentially, all the sensory equipment is present to feel pain, and it would seem rather senseless (forgive the pun) to have all systems present and doing nothing in a dog, fox, cat, squirrel, whale, lion, horse, elephant?

The next argument put forward is that the animal being hunted feels no pain as it is put in a great state of fear, stress and anxiety before it is killed. This state would release a high level of endogenous hormones called endorphins (about 20 different types occur in humans) from the pituitary gland into the blood, and from the hypothalamus into the spinal cord and brain. Since the endorphins released into the blood cannot enter the brain in much quantity because of the blood-brain barrier, their physiological importance to pain control is not so obvious. There are other reasons for these endorphins, and I will discuss these below. But what endorphins do (the name comes from a combination of endogenous and morphine) is interact with the opiate receptors in the brain to reduce the perception of pain.

Anyone who has been badly injured knows just how ineffective these endorphins are. Again, it is not that humans make “weaker” endorphins than other mammals, not at all, they are all pretty much identical peptide hormones. It is no wonder that people who are badly injured require high doses of extra morphine to begin to relieve pain.

Endorphins are not only to do with pain. They are also released when you eat spicy food and chocolate, and when you laugh and have sex. It is an important modulator of behaviour, and one of the causes of autism has been linked to an excess of circulating opioids. Endorphins also trigger the release of at least two other hormones – adrenaline and glucagon. Adrenaline, or epinephrine, stimulates the heart rate, contracts blood vessels, and dilates air passages. A good thing to have on board when you are being chased by something. Another good thing to have is glucagon, which allows glycogen stored in the liver to be released as glucose – allowing further sustained activity.

A further consequence of serious injury is often that humans and animals go into a state described as “shock” – a serious medical condition caused by a sudden drop in blood pressure. Prey animals being disembowelled by hyenas and African wild dogs, racehorses with badly broken legs, and humans involved in serious car accidents all seem to be in a stoic and “painless state”, but this is induced by massive trauma that causes such a great overload to sensory systems that the body basically and protectively shuts down. But the injury and pain have to come first.  

To sum up, all mammals have nociceptors to signal pain, and therefore it is more than just plausible that pain in other mammals occurs in the same way it does in humans. There is no difference. Also as in humans, other mammals have endorphin hormones that reduce pain but far from eliminate it when an animal is injured, stressed, and subjected to high levels of fear. So it is entirely credible that animals feel pain exactly the way humans do. The fact that animals do not express pain as we do does not mean that they do not suffer from it. How animals deal with pain is a very different issue from the fact that such pain and trauma are being inflicted, and that is what should cease in “blood sports”.


Posted by Pieter Kat at 15:56

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