Figure 3.

Morphine does not prevent the activation of the neural circuit underlying dynamic mechanical allodynia. A, Images of Fos-positive cell nuclei and phospho-ERK immunolabeling in the MDH of control (Vehicle) and morphine injected rats. Fos expression and ERK phosphorylation were induced by light brushing of the ipsilateral lip after intracisternal strychnine; subcutaneous morphine (4 mg · kg⁻¹) or vehicle (isotonic saline) were preemptively administered 25 min before strychnine; scale bar, 50 μm. On the left, the approximate location at which images were taken is indicated on the schematic diagram of a coronal section of the MDH. B, Bar histograms summarizing mean Fos expression and ERK phosphorylation in the different laminae of the ipsilateral MDH after light brushing of the lip after intracisternal injection of strychnine or aCSF in vehicle and morphine injected animals (n = 3–4/group). A, B, I–IIo, Laminae I and outer II; IIi–IIIo, laminae inner II and outer III; IIIi–IV, laminae inner III and IV. **p < 0.01.