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Background: Fibromyalgia (FM) is a chronic painful condition that primarily affects women. The allodynia and hyperalgesia induced by reserpine is a FM model commonly used to study the disease; however, it produces one painful episode, in contrast to FM, which is a chronic painful condition. Reserpine induces hypersensitivity for 10 days, which is resolved by day-21. In this study, we sought to determine whether the resolution of pain was due to the release of endogenous antinociceptive opioids and the activation of spinal opioid receptors. Methods: Female Wistar rats were used in this study. To induce a fibromyalgia-like model, reserpine was subcutaneously administered. Twenty-one days after reserpine administration, subcutaneous or intrathecal opioid blockers were administered to evaluate latent sensitization. Additionally, serum β-endorphin concentration was determined by ELISA. Results: opioid receptors (CTOP and naltrindole) reinstated hypersensitivity. These results suggest that an increased concentration of opioid agonists in reserpinized animals leads to antinociceptive effects. Accordingly, reserpine injection enhanced serum β-endorphin concentration in female rats. Conclusion: Our data suggest that reserpine-induced nociceptive hypersensitivity is a long-lasting condition masked by the compensatory release of β-endorphin and the activation of spinal µ and, to a lesser extent, δ opioid receptors.
Juárez-Núñez et al. (Wed,) studied this question.