Morphine self-administration is inhibited by the antioxidant N‐acetylcysteine and the anti-inflammatory ibudilast; an effect enhanced by their co-administration
María Elena Quintanilla, Paola Morales, Daniela Santapau, Javiera Gallardo, Rocío Rebolledo, Gabriel Riveras, Tirso Acuña, Mario Herrera-Marschitz, Yedy Israel, Fernando Ezquer
Abstract
Background
The treatment of opioid addiction mainly involves the medical administration of methadone or other opioids, aimed at gradually reducing dependence and, consequently, the need for illicit opioid procurement. Thus, initiating opioid maintenance therapy with a lower level of dependence would be advantageous. There is compelling evidence indicating that opioids induce brain oxidative stress and associated glial activation, resulting in the dysregulation of glutamatergic homeostasis, which perpetuates drug intake. The present study aimed to determine whether inhibiting oxidative stress and/or neuroinflammation reduces morphine self-administration in an animal model of opioid dependence.
Introduction
Numerous studies have shown that repeated administration of morphine, the most widely used opioid for treating acute and chronic pain, induces cerebral oxidative stress [1–3] and activates proinflammatory responses in microglia and astrocytes—hallmarks of neuroinflammation—in brain regions associated with morphine dependence, including the hippocampus; ventral tegmental area, nucleus accumbens and prefrontal cortex [4–8]. With respect to the mechanism(s) through which morphine induces oxidative stress, previous studies have shown that morphine-induced activation of μ-opioid receptors inhibits excitatory amino acid transporter type 3 (EAAT3)-mediated cysteine transport into neurons, resulting in decreased levels of glutathione (GSH) and elevated oxidative stress [9].
Materials and method
2.1 Induction of voluntary morphine consumption
Naïve female eight weeks-old rats from the UChB line, selectively bred for ninety generations for their high voluntary ethanol intake [34, 35], were used to induce voluntary morphine consumption as previously described [36]. All experiments were conducted with female rats. The rationale for choosing to work only with females was twofold: (i) we had previously demonstrated that female rats of this strain developed morphine dependence after achieving a stable morphine intake, as observed in the present study (15.6 ± 0.8 mg/kg/day) using the same induction method [36]; and (ii) female rats were selected for translational considerations, such as the urgent need to manage opioid dependence in pregnant women, who are typically treated with long-acting opioids (e.g. methadone).
Results and Discussion
3.1 Morphine self-administration
Fig 1 illustrates that under free-choice concurrent access to a morphine solution (90 mg/L) and water, rats exhibited an average consumption of 14.3 ± 0.2 mg of morphine/kg body weight/day (mean ± SEM) and displayed a high preference for morphine over water (more than 80%). The morphine intake during the baseline period was not significantly different among the four groups of rats (two-way ANOVA). Administration of a loading dose of NAC (70 mg/kg/day; i.p.) for two days (days 50 and 51 of morphine intake) to rats in both the NAC group (green squares) and the NAC + ibudilast group (red squares) resulted in a significant reduction in chronic morphine intake compared to the other two groups treated with vehicle (the control group [black squares] and the group assigned to receive ibudilast [blue squares]) (***p<0.001 for the NAC group and **p<0.01 for the NAC + ibudilast group, respectively, two-way ANOVA followed by Dunnett’s post hoc test).
Acknowledgments
The technical assistance of Robel Vazquez, Juan Santibañez and Carmen Almeyda is greatly appreciated.
Citation: Quintanilla ME, Morales P, Santapau D, Gallardo J, Rebolledo R, Riveras G, et al. (2024) Morphine self-administration is inhibited by the antioxidant N‐acetylcysteine and the anti-inflammatory ibudilast; an effect enhanced by their co-administration. PLoS ONE 19(10): e0312828. https://doi.org/10.1371/journal.pone.0312828
Editor: Shao-Jun Tang, University of Texas Medical Branch at Galveston, UNITED STATES OF AMERICA
Received: May 27, 2024; Accepted: October 11, 2024; Published: October 29, 2024
Copyright: © 2024 Quintanilla et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All data supporting this study are included within the article and supporting material.
Funding: This work was supported by Agencia Nacional de Investigación y Desarrollo (ANID) FONDECYT 1240162 and ACT210012 grants to Fernando Ezquer, and FONDECYT 1231443 to Mario Herrera-Marschitz. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
