Research Thesis Assignment

Biol 491 Senior Seminar

Carmen Major

October 16, 2020

Ethnicity is associated with alterations in oxytocin relationships to pain sensitivity in women

It is known that African Americans experience greater pain associated with many different clinical conditions as well as a greater sensitivity to experimental pain tasks when compared to non- Hispanic whites. Not only do African Americans show reduced pain tolerance to experimental pian tests but there is also an absence of the expected endogenous pain inhibitory mechanisms that is documented in Caucasians. Only Caucasians show the expected positive association between higher pain tolerance and greater blood pressure, plasma norepinephrine, and cortisol. All correlations involving these biological measures were very low, which suggests there is inherent differences, alterations or adaptions in one or more biological pathways involves in pain inhibition that could possibly contribute to the ethnic differences in pain. The purpose of this article was to examine the relationship between plasma oxytocin (OT) and pain sensitivity. This study also explores the relation between plasma oxytocin and other factors that are known to influence pain perception.

The association of plasma oxytocin concentration was tested with three types of experimental pain: Ischemic pain tolerance, Hand cold pressor task and Thermal heat pain task. To test this theory, 48 women, 25 African American and 23 non-Hispanic White, were used to examine the effects of ethnicity, gender, and stress on physiological mechanisms involved in endogenous pain regulation. These women were healthy non-smokers, pre-menopausal, medically healthy, normotensive or pre-hypertensive and free of chronic disease or pain disorders. Additionally, none of them took prescription medication of any kind. Each woman underwent an identical experimental laboratory testing once in each of the three phases of the menstrual cycle (early follicular, late follicular, and luteal phases). Only results exclusive to late follicular phase, days 8-12, testing were reported in this article.

During the ischemic pain procedure, the submaximal effort tourniquet was used to occlude blood flow to the forearm in order to induce ischemic pain. Doing this causes a gradually increasing deep, diffuse, dull aching muscle pain. The tourniquet cuff was placed on the subject’s arm above the elbow. The arm was supported and elevated vertically for 30 seconds to promote venous drainage and the cuff was then rapidly inflated to 200 mmHg. The arm was then lowered to rest on a pillow on the arm of a chair and the subject completed 20 handgrip exercises with 30% maximum force for two second durations each, with an inter-squeeze interval of two seconds. When the subjects initially felt a sensation of pain they were instructed to say “pain” and to say “stop” when they were no longer able to tolerate the pain. Immediately after the subjects rated the aspects of pain intensity from 0 to 100 being the most intense sensation imaginable. They also rated unpleasantness from 0 to 100 being the worst pain imaginable. The maximum time limit for this test was 20 minutes to prevent tissue damage and the time to pain threshold and tolerance were recorded.

To perform the Hand cold pressor task, a container was filled with ice and water, which was maintained at 4°C, and a water circulator was placed inside the container to prevent the water from warming near the subject’s hand. The subjects placed their hand in the container to a marked line on their wrist and remined still. The maximum time limit for this test was 5 minutes to prevent tissue damage. The time to pain threshold was recorded and intensity and unpleasantness was accessed.

To perform the Thermal heat pain task, a 1cm diameter contact thromde was created. The threshold and tolerance were determined by increasing the diode temperature in .5°C increments every 5 seconds, stating at 41.5°C until it reached 53°C or until the subjects reach tolerance. The average of the three series were calculated to determine the threshold and tolerance.

African American women demonstrated significantly lower pain tolerance to hand cold pressor pains and a marginally lower tolerance to ischemic pain when compared to non-Hispanic Whites. Based on the ethnic difference in oxytocin concentration, there was no ethnic difference in tolerance to thermal heat pain. African American women had significantly lower baseline plasma OT concentrations that non- Hispanic White women and the levels of OT were also lower in African American compared to Whites. From the relationships of OT to other biological measures, in African Americans higher OT concentrations were significantly correlated with lower systolic blood pressure.

Oxytocin is a powerful hormone that cats as a neurotransmitter in the brain. It is known as the “love drug” as it is associated with feeling good. It is manufactured at many central and peripheral sites, but the paraventricular nucleus of the hypothalamus has been identified as its primary source. It is best known for its milk let down function in lactation, and uterine contraction during parturition. The relationship between oxytocin concentrations and lesser pain sensitivity has been supported. Oxytocin and pain tolerance have an inversely proportional relationship.

This experiment was designed to test the relationship between endogenous OT concentrations and sensitivity to experimental pain in humans. Pre-menopausal women with. Higher circulating OT levels during resting baseline demonstrated greater tolerance ischemic pain and cold pressor pain. There is indeed a link between lower OT levels and reports of greater clinical pain. African American not only differed in OT levels but also in pain- response profile. African Americans demonstrated lower tolerance levels to ischemic, cold pressor and thermal heat pain than their white counterparts. These finding support the theory that differences in OT may be one of various underlying biological mechanisms contributing to the observed ethnic differences in types of clinical and experimental pain. Greater pain sensitivity in African Americans were present regardless of whether pain testing was done after stress or after rest. It can be concluded that reduced oxytocinergic function may be one of many factors underlying pain experiment by African Americans compared to Whites. Lower OT levels and decreased pain tolerance in African American women may reflect the long -term effects of physiological responses to physiosocial stress.

Reference:

Grewen, KM, Light ,KC, Mechlin, B, Girdler, SS. Ethnicity is associated with alterations in oxytocin relationships to pain sensitivity in women. Ethn Health. [Internet]. 2008[October 17, 2020];13(3):219-241. doi: 10.1080/13557850701837310