Opioid involvement in the perception of pain due to endurance exercise in trained man.

Abstract

The purpose of this study was to evaluate the role of endogenous opiates in modulating physical performance during dynamic exercise in conscious man. The plasma concentration of fl-endorphin (BEP) and of adrenocorticotropic hormone (ACTH) along with muscle pain (McGuill Pain Questionnaire) were assessed in 17 trained, male runners before and after running the longest possible distance within 12 min (i.e., the Cooper test). Each runner participated twice in the test (double-blind cross-over design), with a 1-week interval-with or without an injection of the opiate antagonist naloxone (0.8 mg i.v.). The average (SEM) distance reached was 3,198 (45) m in the naloxone test and 3,240 (38) m in the placebo test. The BEP increased significantly during the tests by a factor of 4.1 on naloxone and by 2.8 on placebo (from the normal resting averages of 1.7 and 2.1 pmol/l, respectively). The ACTH also increased significantly by a factor of 2.0 on naloxone and 2.5 on placebo (from the normal resting averages of l 9.3 and 16.8 pmol/l, respectively). There were no significant differences between the naloxone and the placebo test with respect to the increments of BEP or ACTH by exercise. However, the perception of muscle pain was enhanced with naloxone. The increased perception of pain did not decrease the athletes ability to perform in terms of the distance run. We conclude that endogenous opiates are involved in the perception of pain associated with exhaustive exercise and may subserve psychological rather than physiological functions during exercise.

A Meta-Analytic Review of the Hypoalgesic Effects of Exercise

“…Exercise of sufficient intensity and duration results in the release of peripheral and central beta-endorphins which have been associated with changes in pain sensitivity. 15,55,57,64 However, animal research has provided the most consistent support for this hypothesis 23,64 , while the human data has been mixed. 6,18,31,52 Animal data also shows that non-opioid systems exist (e.g., endocannabinoid, neurotransmitters such as serotonin and norephinephrine) and that parameters of the exercise (i.e., duration of session, continuous vs. intermittent, and varying water temperature for swim protocols) may determine which system is activated.…”

Advocating neuroimaging studies of transmitter release in human physical exercise challenges studies

“…Therefore, we conclude that peripheral measurements of endorphins provide only limited information about central opioidergic mechanisms underlying psychophysiological effects. This also seems to apply to receptor blocking studies, which are not summarized here in detail, but which have also revealed equivocal results, ie, either negative32–35 or positive blocking effects in the pain domain 3,36. On the other hand, several studies have reported positive blocking effects in the mood domain,3,37,38 thus supporting the hypothesis of central opioidergic effects mediating mood enhancement.…”

Control and sensation of breathing during cycling exercise in hypoxia under naloxone: a randomised controlled crossover trial

“…Endocannabinoids are also thought to play a role in runner's high [36]. Paulev et al [37] compared naloxone (0.8 mg i.v.) to placebo during a Cooper test (running the longest possible distance within 12 min) in trained subjects.…”