Effects of Exercise and Peripheral Opioid Receptor Stimulation on Anxiety‐Like Behaviors in a Rat Model of Chronic Pain

Abstract

Patients with fibromyalgia are more likely to suffer from anxiety. In the acidic saline‐induced chronic pain model, rats exhibit long‐lasting, wide‐spread pain along with anxious behaviors. Exercise training has been shown to decrease pain in both patients with fibromyalgia and in this rat model of chronic pain; however the effect of exercise on anxiety and the contribution of peripheral opioid receptors to this effect are unknown. All experiments were performed in female Sprague‐Dawley rats that demonstrated chronic widespread pain due to acidic saline injections and all rats underwent moderate‐intensity treadmill training (4 weeks; 5 days/week; 30 min/day at 15 m/sec). Anxiety‐like behaviors were determined using the open field test. Control rats (n=4) were observed in the open field (90 × 90 cm) for two minutes before (PRE) and after (POST) each exercise bout. Time spent moving was recorded: decreased motility demonstrated anxiety. A separate group of rats (NAL; n=5) received injections of the nonspecific opioid receptor antagonist naloxone (120 μg in 100 μL saline) in both gastrocnemius muscles ten minutes prior to treadmill running in addition to open field testing. Results showed that treadmill training alone had no effect on control rats' average time spent moving in the open field: 32.5 ± 4.2 s (PRE) and 31.2 ± 4.4 s (POST) for week 1; 35.1 ± 4.2 s (PRE) and 31.7 ± 3.8 s (POST) for week 2; 32.7 ± 4.3 s (PRE) and 38.3 ± 4.8 s (POST) for week 3; and 35.6 ± 4.6 s (PRE) and 34.5 ± 3.1 s (POST) for week 4. However, NAL rats demonstrated increased anxiety‐like behaviors indicated by decreased average time spent moving for all weeks tested: 25 ± 3.9 s (PRE) and 14.2 ± 3.2 s (POST) for week 1; 34.6 ± 3.4 s (PRE) and 18.9 ± 2.8 s (POST) for week 2; 35 ± 4.1 s (PRE) and 16.5 ± 3.2 s (POST) for week 3; and 38 ± 4.5 s (PRE) and 17.9 ± 2.9 s (POST) for week 4. In NAL rats, PRE and POST times were significantly different for all four weeks (p<0.05). PRE times were not different between control and NAL groups, however, POST times were significantly different between control and NAL groups for all weeks. Our data suggests that endorphins released during exercise act on peripheral opioid receptors to reduce anxiety‐like behaviors associated with the exercise bout in a rat model of chronic pain. These findings support the use of exercise to reduce anxiety and chronic pain in people with fibromyalgia.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.