Pain sensitivity, mood and plasma endocrine levels in man following long-distance running: Effects of naloxone

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Abstract
The effects of intense exercise on pain perception, mood and plasma endocrine levels in man were studied under naloxone and saline conditions. Long-distance runners (12) (mean weekly mileage = 41.5) were evaluated on thermal, ischemic and cold pressor pain tests and on mood visual analog scales (VAS). Blood was drawn for determination of plasma levels of .beta.-endorphin-like immunoreactivity (BEir), growth hormone (GH), ACTH and prolactin (PRL). These procedures were undertaken before and after a 6.3 mile run at 85% of maximal aerobic capacity. Subjects participated on 2 occasions in a double-blind procedure counterbalanced for drug order: on 1 day they received 2 i.v. injections of naloxone (0.8 mg in 2 ml vehicle each) at 20 min intervals following the run; on the other day, 2 equal volume injections of normal saline (2 ml). Sensory decision theory analysis of the responses to thermal stimulation showed that discriminability, P(A), was significantly reduced post-run under the saline condition, a hypoalgesic effect; response bias, B, was unaffected. Ischemic pain reports were significantly reduced post-run on the saline day, also a hypoalgesic effect. Naloxone reversed the post-run ischemic but not thermal hypoalgesic effects. Joy, euphoria, cooperation and conscientiousness VAS ratings were elevated post-run; naloxone attenuated elevation in joy and euphoria ratings only. Plasma levels of BEir, ACTH, GH, and PRL were significantly increased post-run.