Fig. 1. These confocal images (A–F) illustrate the effect of morphine on noxious stimulus-evoked internalization of the NK-1 receptor in NK-1 receptor-immunoreactive neurons. Eachpanel is from sagittal sections through lamina I of the L4 segment of the spinal cord. The confocal images were taken through the center of neurons that express the NK-1 receptor. In all examples, the noxious stimulus was a 15 sec pinch of the hindpaw.A–C, Saline pretreatment;D–F, morphine pretreatment.A, Contralateral to the noxious stimulus in a rat that received intrathecal saline. The NK-1 receptor–LI is localized to the plasma membrane, indicating that internalization had not occurred.D, Contralateral to the noxious stimulus in a rat that received intrathecal morphine. There is no NK-1 receptor internalization. By contrast, in B, C,E, and F, there is extensive NK-1 receptor–LI in endosomes in the cytoplasm, indicating extensive internalization. B, Ipsilateral in a rat that received intrathecal saline; C, ipsilateral in a rat that received subcutaneous saline; E, ipsilateral in a rat that received intrathecal morphine (30 μg); F, ipsilateral in a rat that received subcutaneous morphine (10 mg/kg). Note that the magnitude of NK-1 receptor internalization in individual cells (number or brightness of endosomes) was not altered by morphine treatment. Scale bar (shown in D): 20 μm.