Labeling for keratin protein and the NK1 receptor (NK1R) in the hindpaw skin after intraplantar injection of substance P (SP). Keratin protein (red) and NK1R (green) are shown in control rats (top row of panels), and at 1 h (second row), 3 h (third row), and 6 h (fourth row) after intraplantar injection of 25 μg SP. Increased NK1R staining was observed in keratinocytes at 1 h and 3 h after SP injection, resolving by 6 h post-injection. These results suggest that facilitated SP signaling is not responsible for the prolonged hyperalgesia observed after SP injection (Figure 1). Scale bar = 20 μm.