Fig. 2.  Activated p38 mitogen-activated protein kinase (MAPK), but not extracellular signal–regulated kinase (ERK) 1/2, mediate nerve growth factor (NGF)–dependent increases in μ-opioid receptor (MOR) binding sites, immunoreactive cells, and protein concentration in dorsal root ganglia. (A ) MOR-specific [³H]-labeled [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO)–binding experiments with dorsal root ganglion membrane preparations from animals that received intraplantar treatment with vehicle, NGF, NGF plus intrathecal p38-MAPK inhibitor SB203580, or NGF plus intrathecal ERK-1/2 inhibitor PD98059 were performed in the presence or absence of 10 μM unlabeled naloxone to exclude nonspecific binding. After NGF, [³H]DAMGO binding sites significantly increased compared with vehicle. This increase was significantly reversed by intrathecal p38-MAPK inhibitor SB20358, but not by ERK-1/2 inhibitor PD98059. Data are expressed as mean ± SEM. Statistically significant differences versus  control (*) and inhibitor (†) are noted (P < 0.05; one-way ANOVA, Newman-Keuls test). (B ) Quantitative analysis of MOR-specific immunofluorescent cell staining of dorsal root ganglia from animals with intraplantar vehicle (control), intraplantar NGF, intraplantar NGF plus intrathecal p38-MAPK inhibitor SB20358, or NGF plus intrathecal ERK-1/2 inhibitor PD98059 treatment. Data are expressed as mean ± SEM. Statistically significant differences compared with vehicle or p38-MAPK inhibitor treatment are noted (*†P < 0.05; one-way ANOVA, Newman-Keuls test). (C –E ) Western blot analysis of dorsal root ganglia (DRG) immunoblotted with anti-MOR, antiphospho-p38 MAPK, or anti-p38 MAPK showing bands of MOR (50 kDa) (C ), phospho-p38 MAPK (43 kDa) (D ), and p38 MAPK (38 kDa) (E ) in animals with vehicle, NGF, NGF/p38-MAPK inhibitor, or NGF/ERK-1/2 inhibitor treatment. ß-Actin was used as a loading control. The optical density of the protein bands of MOR and p-p38 MAPK, but not p38 MAPK, were significantly increased in NGF-treated groups. This increase was reversed only after treatment with intrathecal p38-MAPK inhibitor SB203580.