Fig. 6.

Sustained morphine increases noxious stimulation induced NK-1 receptor internalization in deep dorsal horn neurons. Male Sprague–Dawley rats were implanted with two placebo or morphine pellets. Separate groups were exposed to mechanical (2 min pinch) or thermal (2 min at 52 °C) stimulation 2 and 6 days after pellet implantation; the animals were perfused 5 min post-stimulation, and lumbar spinal cord tissues were collected and processed for NK-1 receptor immunoreactivity. The percentage of NK-1 receptor labeled neurons at the L4 segment that contained internalized NK-1 receptors after thermal or mechanical stimulation were counted in laminae I, III, and V of the spinal cord. The thermal and mechanical stimulation induced internalization of the NK-1 receptor in approximately 100% of the NK-1 receptor labeled lamina I cells across all conditions. (A) and (C) show that under 5% of NK-1 receptor labeled neurons in laminae III–V showed NK-1 receptor internalization after the thermal or mechanical stimulation 2 days after placebo or morphine pellet implantation. (B) and (D) show that 6 days after morphine pellet implants, both the thermal and mechanical stimuli induced an increase in the percentage of NK-1 receptor labeled cells that internalized the NK-1 receptor in laminae III and V compared to the placebo-pelleted animals. Graphs represent mean±SEM. Each treatment group consisted of 3–5 rats.