Race/Ethnicity Does Not Moderate the Relationship Between Adverse Life Experiences and Temporal Summation of the Nociceptive Flexion Reflex and Pain: Results From the Oklahoma Study of Native American Pain Risk

Abstract

Adverse life experiences (ALEs) are associated with hyperalgesia and chronic pain, but the underlying mechanisms are poorly understood. One potential mechanism is hyperexcitability of spinal neurons (ie, central sensitization). Given that Native Americans (NAs) are more likely to have ALEs and to have a higher prevalence of chronic pain, the relationship between ALEs and spinal hyperexcitability might contribute to their pain risk. The present study assessed temporal summation of the nociceptive flexion reflex (TS-NFR; a correlate of spinal hyperexcitability) and pain (TS-Pain) in 246 healthy, pain-free non-Hispanic whites and NAs. The Life Events Checklist was used to assess the number of ALEs. Multilevel growth models were used to predict TS-NFR and TS-Pain, after controlling for age, perceived stress, psychological problems, negative and positive affect, and painful stimulus intensity. ALEs and negative affect were significantly associated with greater pain, but not enhanced TS-Pain. By contrast, ALEs were associated with enhanced TS-NFR. Race did not moderate these relationships. This finding implies that ALEs promote hyperalgesia as a result of increased spinal neuron excitability. Although relationships between ALEs and the nociceptive flexion reflex/pain were not stronger in NAs, given prior evidence that NAs experience more ALEs, this factor might contribute to the higher prevalence of chronic pain in NAs. PERSPECTIVE: This study found a dose-dependent relationship between ALEs and spinal neuron excitability. Although the relationship was not stronger in NAs than non-Hispanic whites, given prior evidence that NAs experience more ALEs, this could contribute to the higher prevalence of chronic pain in NAs.

Keywords: Temporal summation; adverse life experiences; ethnic differences; negative affect; nociceptive flexion reflex; pain; trauma.

Figure 1.

Testing day timeline and experimental procedures for the Temporal Summation of Pain and Nociceptive Flexion Reflex (TS-Pain; TS-NFR) testing day. The TS-NFR is expanded to show the 5 trains of 3 individually calibrated electrical stimulations, which were delivered at a frequency of 2.0 Hz. The interstimulus interval between each stimulation was randomized and ranged from 8-12 seconds

Figure 2.

Plots depicting individual growth curves for temporal summation of pain (TS-Pain; panel A) and temporal summation of the nociceptive flexion reflex (TS-NFR; panel B). As depicted in the graphs and verified by the multilevel growth curve models, there is significant variance in both the intercepts (mean levels) and slopes (summation across the 3 stimulus trains) for both variables. This indicates that variance is available to be predicted by the growth models.

Figure 3.

The predicted relationships between adverse life experiences and pain (panel A) and negative affect and pain (panel B). The predicted values are derived from the coefficients of the multilevel growth curve model. More adverse life experiences and greater negative affect were associated with higher reported pain during temporal summation of pain testing. This suggests that exposure to adverse life experiences and negative affect have independent effects on pain amplification. variance is available to be predicted by the growth models.

Figure 4.

The predicted relationship between adverse life experiences (ALEs) and temporal summation of the nociceptive flexion reflex (TS-NFR). Individual (simple) regression lines are based on the multilevel growth curve model. NFRs increased over the 3 stimulus train, indicating a temporary hyperexcitability of spinal neurons. However, persons who experienced a higher number of ALEs had greater TS-NFR, suggesting enhanced hyperexcitability (central sensitization) of spinal neurons in these individuals. This could serve as a risk factor for future chronic pain.

Figure 5.

Electromyogram (EMG) traces from the biceps femoris muscle used to measure temporal summation of the nociceptive flexion reflex (TS-NFR). On the left is 6 randomly selected participants that reported 0 adverse life experiences (ALEs) and on the right is 6 randomly selected participants reporting ≥5 ALEs. As illustrated, the summation in NFR across the 3-stimulation train (3 stimulations depicted in the bottom graphs) was greater in the group reporting more ALEs. y-axis range: 0-50 μV, x-axis range: 1500 ms.