This study tested the hypothesis that older adults would have a stronger response for substance P (facilitatory) but weaker response to β-endorphin (inhibitory), both in magnitude and time-course. Eight younger and 9 older adults underwent 3 experimental sessions using well-validated laboratory pain models: cold pressor task (CPT), contact heat pain (HP), and a non-painful control. Blood was collected through an indwelling catheter at baseline and 3, 15, 30, 45, and 60-minutes post-stimuli administration. Older adults had higher baseline levels of both neuropeptides suggesting increased peripheral activity compared to younger adults. Following CPT, 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, β-endorphin increased between 3 and 15 minutes for both groups with the upsurge substantially higher for older adults. Following HP, younger adults had an immediate surge in circulating substance P and β-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. β-endorphin peaked at 30 minutes for both groups and returned to baseline. No changes were observed during the non-painful control.
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