As summarized in , women in the luteal phase of their menstrual cycle had significantly higher levels of ALLO-ir than men (F(1,82) = 119.12, p < .0001). Men had higher concentrations of β-endorphin than women (F(1, 83) = 23.23, p < .0001). As previously reported in the larger sample (Mechlin et al. 2005
), overall the subset of men also had greater cortisol concentrations than women (F(1,82) = 25.81, p < .0001), greater pain threshold (Fs(1, 82) = 2.51 to 7.63, ps < .05) and tolerance to all three pain tests (Fs(1, 82) = 4.94 to 12.13, ps < .01) collapsing across ethnic groups (i.e. main effect of gender) even after controlling for order of pain tests. Finally, as previously observed in the larger sample, African Americans had lower tolerance values to all three pain tests relative to non-Hispanic Whites, and this was true for both genders and after controlling for order (main effect of ethnicity: Fs(3, 92) = 3.32 to 10.53, ps< .05). There was no main effect of ethnicity on pain threshold for the thermal or tourniquet tasks (Fs(2, 81) = 1.04 and 0.82, respectively), however there was a difference for cold pressor pain threshold (F(2, 81) = 4.17, p < .01), with African Americans exhibiting a lower pain threshold than non-Hispanic Whites.
For the cold pressor task, significantly more non-Hispanic White subjects than African American subjects reached the 5-minute time limit (7 vs. 0; χ2(1) = 11.34, p < .01). There were no ethnic differences in the proportion of subjects that reached the 20-minute time limit for the tourniquet task (3 vs. 6; χ2(1) = 1.75) or the 53°C limit for the thermal heat pain task (1 vs. 0; χ2(1) = 1.19). When comparing subjects who had rest first vs. stress first, there were no significant differences in thermal pain tolerance (48.4°C vs. 48.3°C, respectively), cold pressor tolerance (62 seconds vs. 35 seconds), tourniquet pain tolerance (524 seconds vs. 426 seconds), baseline ALLO-ir (0.91ng/mL vs. 0.85ng/mL), baseline cortisol (7.63pg/mL vs. 8.76pg/mL), or baseline β-endorphin (7.96pg/mL vs. 7.49pg/mL). Therefore, subsequent analyses did not control for the order of stress vs. rest.
Relationships Involving Neuroendocrine Factors and Pain Sensitivity
As summarized in , the only significant partial correlations (partialing out order of pain tests) to emerge involving plasma ALLO-ir, β-endorphin or cortisol concentrations and pain tolerance were seen in the non-Hispanic White subjects. ALLO-ir levels were negatively correlated with thermal heat pain tolerance (r = -.47, p < .01), cold pressor pain tolerance (r = -.35, p < .05), and ischemic pain tolerance (r = -.43, p < .01) in the non-Hispanic Whites. We observed a positive partial correlation between plasma β-endorphin concentrations and cold pressor tolerance levels (r = +.38, p<.05), but only in the non-Hispanic Whites, who also showed the expected positive correlation between plasma cortisol and thermal heat pain tolerance (r = +.39, p<.05) and cold pressor pain tolerance (r = +.32, p < .05). The partial correlation coefficients relating ALLO-ir, β-endorphin, or cortisol concentrations to pain tolerance were uniformly non-significant in the African Americans.
Partial Correlations Relating Pain Tolerance to Neuroendocrine Measures by Ethnicity (Controlling for Order of Pain Tests)
In non-Hispanic Whites, thermal heat pain threshold was significantly correlated with both ALLO-ir (r = -.49, p < .01) and cortisol (r = +.50, p < .01), and ischemic pain threshold was correlated with β-endorphin (r = +.37, p < .05). There were no significant partial correlations involving pain threshold and biological measures in African Americans (rs = -.29 to +.24, ps > .05).
As depicted in , the expected relationship between ALLO-ir concentrations and cortisol concentrations (i.e., negative) was only significant in non-Hispanic Whites (r = -.50, p < .01; see ), though the direction of the relationship was similar in the African Americans (r = -.24; ), but it was not significant. Both groups showed negative correlations involving ALLO-ir and β-endorphin concentrations (rs = -.44 and -.55, ps < .01; see ), while neither group showed a statistically significant relationship involving plasma cortisol and β-endorphin.
Relationship of Neuroendocrine Factors as a Function of Ethnicity
A scatterplot of the relationship between baseline levels of ALLO-ir and Cortisol separated by ethnicity with non-Hispanic Whites displayed in the top panel, and African Americans displayed in the bottom panel.
A scatterplot of the relationship between baseline levels of ALLO-ir and β-endorphin with non-Hispanic Whites displayed in the top panel, and African Americans displayed in the bottom panel.
Since only the non-Hispanic Whites showed significant correlations among ALLO-ir, cortisol, β-endorphin, and pain tolerance, mediational analyses were conducted in that sample only (Baron & Kenny 1986
). Because baseline cortisol and β-endorphin were not correlated with each other, they were not considered together in multiple mediation models but instead each was examined as a simple mediator of the relationship between ALLO-ir and pain tolerance. Despite the numerous intercorrelations between ALLO-ir, cortisol, β-endorphin, and pain tolerance in non-Hispanic Whites, the only measure to meet strict criteria as a mediator in the relationship between ALLO-ir and pain sensitivity was cortisol.
For non-Hispanic Whites, since baseline ALLO-ir was negatively correlated with baseline cortisol (r = -.50, p < .01), and since both
baseline ALLO-ir and baseline cortisol were correlated with thermal heat pain threshold (rs = -.49 and +.50 respectively, ps < .01) ), a series of linear regression analyses were performed (Baron & Kenny 1986
) to test whether cortisol served as a statistical mediator of the relationship between ALLO-ir and thermal heat pain threshold. These analyses indicated that the significant regression coefficient relating baseline ALLO-ir to thermal heat pain threshold (t = -2.85, p < .01; b = -1.72; β = -0.43; R2
= 0.16, p < .01) is reduced (t = -1.34, p > .05; b = -0.85; β = -0.21) and in fact no longer significant, when baseline cortisol (t = 2.80, p < .01; b = 0.34; β = 0.44) is added to the model. Thus, higher baseline concentrations of cortisol met criteria for mediating the relationship between lower baseline ALLO-ir and higher thermal heat pain threshold (z = -2.18, p < .05). Baseline cortisol and baseline ALLO-ir concentrations together accounted for 30% of the variance in thermal heat pain tolerance (F(2, 35) = 8.74, p < .001).
When examining the relationship between ALLO-ir, cortisol, and thermal heat pain tolerance; ALLO-ir, cortisol, and cold pressor pain tolerance; and ALLO-ir, β-endorphin, and cold pressor pain tolerance, analyses indicated that cortisol did not meet statistical criteria as a mediator in these relationships involving cortisol and pain sensitivity.