Figure 3.

Intrathecal DAMGO and Dlt II reduce heat-induced NK₁ receptor internalization. Internalization of NK₁ receptors was induced by immersing the right hindpaw of male Sprague Dawley rats in a 49°C water bath for 38 s, and lamina I NK₁ receptor-immunoreactive neurons were observed by immunofluorescence. The noxious heat stimulation was applied 10 min after intrathecal injection of saline (A,B), DAMGO 5 μg (C), or Dlt II 10 μg (D). Confocal images of neurons on the contralateral (A) and ipsilateral (B–D) sides of the spinal cord are shown. On the contralateral side of the saline-injected animals, immunolabeling of NK₁ receptors appeared to be at the cell surface (A). However, on the ipsilateral side of the same animals, the noxious heat stimulation induced a significant increase in NK₁ receptor internalization, as evidenced by the intensely labeled intracellular vesicle-like structures (B). When DAMGO (C) or Dlt II (D) was injected, a significant reduction in NK₁ receptor internalization was observed on the ipsilateral side compared with the same side in saline-injected rats. The animals that had received a saline injection but no noxious stimulation had low basal proportions of neurons with internalized NK₁ receptors. This result is illustrated in the graphic representation showing the percentage of neurons with NK₁ receptor internalization induced by heat stimulation for ipsilateral side of the lumbar spinal cord (E). *p < 0.05 (one-way ANOVA with Bonferroni's post hoc test). ***p < 0.001 (one-way ANOVA with Bonferroni's post hoc test). The numbers in parentheses represent the number of animals per group. Error bars indicate the SEM. Scale bar: A, 30 μm.