Why Does Pain Tolerance Differ Among People?

            Pain has been a topic of discussion in lecture on many occasions, perhaps because pain is something that most experience, but want to avoid. Though most experience some form of pain, incidents vary in intensity and people’s reactions are of different extremes, as well. Why? What causes these differences in pain tolerance? It was determined in class that pain is the result of certain pattern generators in the nervous system, so it is only natural that one looks to the brain to get to the root of pain tolerance. Research has been done that claims the source is genetic, psychological, or even gender-based. But in fact, this student believes that pain tolerance is the result of a combination of at least these three conditions.

            The first, and most evidentially-supported, argument—the genetic explanation—revolves around the gene that codes for COMT, an enzyme that metabolizes, or breaks down, the neurotransmitter dopamine. Dopamine delivers signals from one brain cell to another, and so can theoretically proliferate pain signals throughout the nervous system. But, COMT depletes the dopamine supply in the brain, freeing receptors in the brain to which the dopamine was bound so that they are available to bind to endorphins, which lead to pain relief (1,3)

The studies done recently on COMT deal with the common val158met polymorphism, in which methionine is substituted for valine at codon 158, causing a “three- to four-fold reduction in the activity of the COMT enzyme” (3). Two alleles code for the expression of this gene: val and met, resulting in an instance of codominance. If an individual receives a val allele from both of his/her parents, the COMT enzyme that is produces is strong, i.e. it readily metabolizes dopamine so that less pain is felt. If an individual receives the met allele from both parents, the COMT enzyme produced is weak; dopamine builds up, suppressing the body’s supply of natural painkillers, or the endogenous opioids (endorphins), so that that individual suffers from low pain tolerance. A heterozygote, receiving the val allele from one parent and the met allele from another parent, has a pain tolerance that lies somewhere in between that of  the homozygotes. In summary, the stronger the COMT, the more receptors that are freed from the grips of dopamine and so the more endorphins that are able to bind to these receptor sites and the more relief from pain that is felt (1, 3, 4).

The lead researcher on the COMT project, Dr. Zubieta, cautions that pain tolerance cannot logically be explained by a single gene, an argument supported by the fact that COMT has other functions in the body; however, COMT must play a very large part in the differences seen in individuals. This statement leads this student to look elsewhere for other explanations of ranges in pain tolerance: psychology.

The psychological research done on this topic operates under the understanding that pain can be manifested in negative emotions, such as anxiety, depression and anger, to name a few. These researchers argue that these negative emotional responses to pain stimuli can be counterbalanced by positive emotional responses; in one very compelling study, the positive emotional responses were produced by sexual fantasies. The subject was told to immerse his/her arm in ice water until he/she could no longer bear the pain. Then, the subjects were separated into three groups: one group was instructed to envision a neutral fantasy (e.g. walking); another group was instructed to envision a sexual fantasy, and the third group was not given any specific instructions. Then, each group underwent the same submersion task as before. Interestingly, those subjects that were asked to think of a sexual fantasy while experiencing pain “handled pain better and experienced less pain [than the subjects under other conditions]. They also were less anxious and depressed, and less angry.” In general, the subjects under the sexual fantasy condition were able to endure the pain for longer. The pleasant emotions produced by the thought of a sexual fantasy counteract the unpleasant thoughts that are a result of pain. The implications of this are that if a person enduring a painful experience imagines something that evokes in them positive emotions, they are able to cope with the pain better, and actually report experiencing less pain. Conversely, if a person experiences negative sensations from sources other than the painful experience in combination with the painful experience, the subject cannot endure as much pain and reports experiencing more pain than other subjects (2).

Psychology accounts for aspects of tolerance that genetics cannot: “mind over matter.” There is a possibility that two people can tolerate the same amount of pain, but that one voices their felt pain more readily than someone else. This could arguably be because one person is mentally stronger than the other. One person might feel an unbearable amount of pain, but stays quiet, does not medicate, goes on with his/her day, etc, while another person, under the same physical stress, manifests his/her pain in moaning, pill-popping, crying, being bed-ridden, etc. Maybe that person that seems stronger has conditioned him-/herself to suppress such manifestations of pain, or maybe he/she has been conditioned by society, so that this idea of “mind over matter” is actually cultural (i.e. feeling pain is a sign of weakness, and in some cultures, weakness is looked down upon more severely than in others).  This conscious suppression of reactions to pain must then involve the I-function, an aspect of the nervous system in which the genetic explanation would not be involved. But, obviously, this cannot be the only explanation for the wide ranges of pain tolerance seen across humanity. The research is quite convincing, but does not take into account the genetic explanation for pain tolerance, just as the genetic explanation does not take into account this theory. So, it leads this student to wonder to what extent pain tolerance is genetic and to what extent is it psychological? Furthermore, is anything else involved in a person’s tolerance level for painful? And in fact, this student has found some research that suggests gender has something to do with it.

Researchers have found that estrogen can act as a natural painkiller. Higher estrogen levels result in a higher pain tolerance, and lower estrogen  levels cause effectively lower pain tolerance in subjects. Granted, the study was done only in women, but it is curious that hormones can effect how one deals with pain. Though this student did not find any research that inspected estrogen levels in males, or levels of male-specific hormones like testosterone, the studies examined here open yet another door through which pain tolerance may pass. Now, one can question whether these significant changes in pain tolerance are due to estrogen levels specifically, or to hormone levels in general. Furthermore, estrogen levels change with menstruation so that a woman’s pain tolerance would, theoretically, also vacillate with changes in the body. So, though genetics and psychology may play a part, a woman’s pain tolerance is not constant, but is subject to manipulations by hormones (3, 5).

It seems that combining the knowledge gained on different theories of pain tolerance—genetic, psychological, and hormonal explanations—leads one to the conclusion that pain tolerance is an intricate neurological output. Not only is one’s basic threshold for pain determined by the presence of certain forms of a gene, but it can be pushed one way or the other by changes in hormone levels and one can conditions     him-/herself to live and work through the pain. In conclusion, this student argues that some aspects of pain tolerance are spontaneous and incontrollable; but a large part of it lies within the I-function.

Works Cited

(1) “Gene Controls Pain Threshold”, http://news.bbc.co.uk/1/hi/health/2784869.stm

(2) “Sexual Fantasies Increase Pain Tolerance”, http://www.hopkinsmedicine.org/press/1999/DEC99/991216.HTM

(3) “The Gene for Pain Tolerance”, http://www.bio.davidson.edu/courses/genomics/2003/talbert/pain.html

(4) “Can’t Stand the Pain? Your Genes Might Be to Blame”, http://www.med.umich.edu/opm/newspage/2003/paingene.htm

(5) “Pain and the Brain: Sex, Hormones & Genetics Affect Brain's Pain Control System, Shaping a Person's Pain Perception”

http://www.med.umich.edu/opm/newspage/2003/painbrain.htm