Scratching activates microglia in the mouse spinal cord.

This study tested the hypothesis that repetitive scratching provoked by two known pruritogens, compound 48/80 and 5'-guanidinonaltrindole (GNTI), is accompanied by activation of microglial cells in the mouse spinal cord. Immunohistochemical studies revealed that the complement receptor 3, also known as cluster determinant 11b (CD11b), a cell surface marker of microglial cells, was upregulated in the spinal cord 10-30 min after a subcutaneous (s.c.) injection of compound 48/80 (50 μg/100 μl) or GNTI (0.3 mg/kg) to the back of the mouse neck. Numerous intensely labeled CD11b-immunoreactive (CD11b-ir) cells, with the appearance of hypertrophic reactive microglia, were distributed throughout the gray and white matter. In contrast, weakly labeled CD11b-ir cells were distributed in the spinal cord from mice injected with saline. Western blots showed that CD11b expression levels were significantly increased in spinal cords of mice injected s.c. with either pruritogen, reached a peak response in about 30 min, and declined to about the basal level in the ensuing 60 min. In addition, phospho-p38 (p-p38) but not p38 levels were upregulated in spinal cords from mice injected with compound 48/80 or GNTI, with a time course parallel to that of CD11b expression. Pretreatment of the mice with nalfurafine (20 µg/kg; s.c.), a κ-opioid receptor agonist that has been shown to suppress scratching, reduced CD11b and p-p38 expression induced by either pruritogen. The results demonstrate, for the first time, that scratch behavior induced by the pruritogens GNTI and compound 48/80 is accompanied by a parallel activation of microglial cells in the spinal cord.