Figure 4.

Sustained exposure to morphine upregulates GFAP expression in the dorsal horn of the spinal cord in female rats and uncouples GFAP expression from OA-like pain behaviour. GFAP protein expression was quantified in the ipsilateral spinal cord dorsal horn at D14 (A) and D21 (B) after intra-articular injection. See supplementary Figure S4, http://links.lww.com/PR9/A224 for full blots. Densitometry of GFAP labelling intensity expressed relative to β-actin in arbitrary units at D14 (C) and D21 (D). Morphine-treated groups had significantly elevated GFAP expression at D14 after intra-articular injection; there was no effect of the MIA model at this time point (n = 5–6/group). Similarly at D21, morphine treatment was also associated with significantly elevated GFAP expression (n = 6/group) irrespective of the presence of OA-like pain. Data are presented as mean ± SEM and analysed through 2-way ANOVA with the Tukey multiple comparison post hoc test. #P < 0.05, #P < 0.01 main effect of drug treatment, *P < 0.05, ***P < 0.001 multiple comparisons between groups. There was no relationship between PWT AUC values and spinal GFAP expression at D14 (E) in either saline-treated or morphine-treated rats. At D21 (F), PWTs were negatively correlated with GFAP expression in the morphine naïve groups but not in the morphine-treated groups (data pooled by drug treatment and correlations determined through Pearson correlation coefficient r). ANOVA, analysis of variance; GFAP, glial fibrillary acidic protein; MIA, monosodium iodoacetate; OA, osteoarthritis; PWT, paw withdrawal threshold.