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. 2024 Mar 12;9(2):e1145. doi: 10.1097/PR9.0000000000001145

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Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of The International Association for the Study of Pain.

This is an open access article distributed under the Creative Commons Attribution License 4.0 (CCBY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 2. — Sustained exposure to morphine is associated with greater MIA-induced pain behaviour in female rats. See Figure 1A for study design and timeline. (A) Weight-bearing asymmetry was observed following unilateral intra-articular injection of MIA in all rats, but to a significantly greater degree in rats treated with morphine. (B) Area under the curve (AUC) analysis of the time course data in (A) confirmed that morphine treatment exacerbated weight-bearing asymmetry when compared with saline treatment in the MIA model (saline/saline = 6, saline/MIA = 6, morphine/saline = 6, and morphine/MIA = 5). (C) Neither systemic administration of saline nor morphine altered PWTs before model induction (morphine/naïve = 12 and saline/naïve = 12). (D) Following intra-articular injection of 2 mg MIA, PWTs were robustly decreased in rats treated with morphine, but not those treated with saline. (E) AUC of the time course data in (D) confirmed no change in PWTs in either the morphine/saline and saline/MIA groups compared with saline/saline group, but a significant decrease in PWTs in the morphine/MIA–treated group (n = 6 for all groups). Data are presented as mean ± SEM. *P = 0.5, **P < 0.001, and ***P < 0.0001 vs morphine/saline; #P < 0.05, ##P < 0.001 vs saline/MIA. RM 2-way ANOVA with the Tukey post hoc testing. ANOVA, analysis of variance; MIA, monosodium iodoacetate; PWT, paw withdrawal threshold.