Time Course and Autonomic Nervous System Regulation of Pain Inhibition

Objectives: The aim of this study was to determine the time course for pain inhibition using a pain-inhibits-pain experimental protocol and test for associations between pain inhibition, pain sensitization, and autonomic nervous system regulation. Methods: Data came from 20 healthy subjects with a mean age of 35. A training session determined custom temperatures that produced a pain rating between 40-50 for heat and cold painful stimuli. The experiment involved two sessions, each consisting of four 120-second contact-heat stimuli applied to the left palm with a 2.5cm square peltier thermode at 20-minute intervals. One of the two sessions was preceded by four 45-second water bath immersions of the foot as a conditioning stimulus. Pain associated with the heat stimulus was measured continuously using an electronic visual analogue scale. Skin temperature as a proxy for autonomic nervous system regulation was monitored continuously using a precision thermocouple system attached to the palmar surface of the left index finger. Results: Overall, subjects sensitized within each trial in both sessions. The greatest sensitization occurred during the first and second trials in the non-water bath session (+15 points on a scale of 0-100). The mean skin temperature dropped from the moment of thermode contact for each trial with little difference between both sessions. A greater change in skin temperature was associated with greater pain inhibition following the cold-water bath immersion. In addition, less skin temperature change was associated with greater pain sensitization across each trial. Conclusion: A robust autonomic response following cold water bath immersion is related to greater inhibition and less sensitization across time, both of which would be considered a healthy response to pain. The protocol will now be tested on patients with orofacial pain. This research was supported by a UFCD Student Summer Research Fellowship, NIH/NIDCR Grant 5T32-DE007200, and a UFCD Seed Grant.