MOR agonists increase MMP-9 expression in dissociated neurons of DRG cultures. (A) Triple staining of MMP-9, NeuN (neuronal marker), and DAPI (nuclei marker for all cells). Note that MMP-9 is only expressed in neurons (arrows). Scale, 100 μm. (B) Double staining showing MMP-9 and NeuN expression in dissociated DRG neurons in control cultures and cultures treated with morphine (10 μM, 2 h). Left panels show all neurons (NeuN+) in the field. Right panels show high magnification images of middle panels. Scales, 100 μm (middle) and 25 μm (right). (C) Intensity of MMP-9-positive neurons in DRG cultures after treatment of morphine (1, 10, and 50 μM, 2 h), DAMGO and remifentanil (1 and 10 μM, 2 h), together with the opioid receptor antagonist naloxone and MOR antagonist CTAT (10 and 50 μM). Note that morphine-induced MMP-9 increase is inhibited by naloxone and CTAP. *P < 0.05, compared to control; #
P < 0.05, ANOVA followed by Bonferroni post hoc test. The data were also analyzed by non-parametric Kruskal-Wallis test followed by Mann-Whitney test. n = 4 cultures.