High-Intensity Swimming Exercise Decreases Glutamate-Induced Nociception by Activation of G-Protein-Coupled Receptors Inhibiting Phosphorylated Protein Kinase A.

Several studies in humans have reported that improved pain control is associated with exercise in a variety of painful conditions, including osteoarthritis, fibromyalgia, and neuropathic pain. Despite the growing amount of experimental data on physical exercise and nociception, the precise mechanisms through which high-intensity exercise reduces pain remain elusive. Since the glutamatergic system plays a major role in pain transmission, we firstly analyzed if physical exercise could be able to decrease glutamate-induced nociception through G-protein-coupled receptor (G-PCR) activation. The second purpose of this study was to examine the effect of exercising upon phosphorylation of protein kinase A (PKA) isoforms induced by intraplantar (i.pl.) glutamate injection in mice. Our results demonstrate that high-intensity swimming exercise decreases nociception induced by glutamate and that i.pl. or intrathecal injections of cannabinoid, opioid, and adenosine receptor antagonists, AM281, naloxone, and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), respectively, prevent this effect. Furthermore, the peripheral A1 and opioid receptors, but not CB1 , are also involved in exercise's effect. We also verified that glutamate injection increases levels of phosphorylated PKA (p-PKA). High-intensity swimming exercise significantly prevented p-PKA increase. The current data show the direct involvement of the glutamatergic system on the hyponociceptive effect of high-intensity swimming exercise as well as demonstrate that physical exercise can activate multiple intracellular pathways through G-PCR activation, which share the same endogenous mechanism, i.e., inhibition of p-PKA.