Age Differences in the Time Course and Magnitude of Changes in Circulating Neuropeptides After Pain Evocation in Humans

This study tested the hypothesis that older adults would have a stronger response for substance P (facilitatory) but weaker response to beta-endorphin (inhibitory), in magnitude as well as time course. Eight younger and 9 older adults underwent 3 experimental sessions using well validated laboratory pain models: cold pressor task, contact heat pain, and a nonpainful control. Blood was collected through an indwelling catheter at baseline and 3, 15, 30, 45, and 60 minutes after stimuli administration. Older adults had higher baseline levels of both neuropeptides suggesting increased peripheral activity compared with younger adults. After the cold pressor task, older adults demonstrated a quick and strong release of substance P with dramatic recovery, whereas young adults maintained a constant low-grade response. Unlike substance P, beta-endorphin increased between 3 and 15 minutes for both groups with the upsurge substantially higher for older adults. After heat pain, younger adults had an immediate surge in circulating substance P and beta-endorphin that was more pronounced than among older adults. However, levels of substance P for younger adults slowly tapered whereas they continued to climb for the older adults through 30 minutes. beta-endorphin peaked at 30 minutes for both groups and returned to baseline. No changes were observed during the nonpainful control session. Perspective: Older adults had higher baseline levels of substance P and beta-endorphin suggesting increased peripheral activity compared with younger adults. After pain evocation, older adults demonstrated a more intense early response for both neuropeptides suggesting peripheral mechanisms involved in the response to pain may change with age. (C) 2017 by the American Pain Society