The role of spinal inhibitory neuroreceptors in the antihyperalgesic effect of warm water immersion therapy

Objective: Warm water immersion therapy (WWIT) has been widely used in the treatment of various clinical conditions, with analgesic and anti-inflammatory effects. However, its mechanism of action has not been fully investigated. The present study analyzed the role of spinal inhibitory neuroreceptors in the antihyperalgesic effect of WWIT in an experimental model of inflammatory pain. Methods: Mice were injected with complete Freunds adjuvant (CFA; intraplantar [i.pl.]). Paw withdrawal frequency to mechanical stimuli (von Frey test) was used to determine: (1) the effect of intrathecal (i.t.) preadministration of naloxone (a non-selective opioid receptor antagonist; 5 mu g/5 mu l), (2); AM281 (a selective cannabinoid receptor type 1 [CB1] antagonist; 2 mu g/5 mu l), (3); and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; a selective adenosine A(1) receptor antagonist; 10 nmol/5 mu l), on the antihyperalgesic (pain-relieving) effect of WWIT against CFA-induced hyperalgesia. Results: Intrathecal naloxone, AM281, and DPCPX significantly prevented the antihyperalgesic effect of WWIT. This study suggests the involvement of spinal (central) receptors in the antihyperalgesic effect of WWIT in a model of inflammatory pain. Conclusions: Taken together, these results suggest that opioid, CB1, and A(1) spinal receptors might contribute to the pain-relieving effect of WWIT. (C) 2020 Associacao Brasileira de Pesquisa e Pos-Graduacao em Fisioterapia. Published by Elsevier Editora Ltda. All rights reserved.