Evaluation of physiologic pain in relation to pain substances in healthy subjects

This study was aimed at finding a relationship between pain modulators in the blood and physiological pain in apparently healthy Nigerians. It also aimed at establishing a pilot study for finding reference values for plasma levels of substance P, serotonin and tryptophan for the first time among Nigerians. Volunteers were made up of 110 residents of Abuja, aged between 21 and 50 years. Cold pressor test was used to induce pain assessing pain intensity, threshold and tolerance. ELISA was used to assay for plasma substance P, serotonin and tryptophan. Pain parameters from cold pressor test were correlated with plasma pain modulators measured. Results from cold pressor test revealed pain intensity to be 5.79 ± 0.25 cm, pain threshold 28.77 ± 2.32 s and pain tolerance 143.62 ± 24.39 s. Blood plasma level of substance P was 116.52 ± 20.53 pg/mL, serotonin 454.18 ± 30.16 ng/mL and tryptophan 12.77 ± 0.67 μg/mL. There was negative correlation between pain threshold and plasma substance P, pain tolerance and plasma substance P and pain threshold and plasma serotonin. There was however a positive correlation between pain intensity and plasma serotonin. In conclusion, the regression formulas may aid in using cold pressor test to predict blood substance levels of the measured pain modulators in a low resource setting like Nigeria where ELISA test is very expensive.