Agonist-induced activation of phospho-ERK1/2 by the endogenous MOR in wild type striatal neurons was assessed. Neuronal cultures prepared as described under “Experimental Procedures” were used to quantify the expression levels of phospho-ERK1/2. Representative confocal images are shown of pERK, MOR, and arrestin3 immunoreactivity in wild type striatal neurons that were vehicle treated (Basal), or treated for 30 min with fentanyl (100 nM), fentanyl with naloxone (10 μM) pretreatment, or morphine (10 μM). The merged images represent the colocalization of p-ERK and arrestin3 for each treatment described above.