Immunolabeling for the delta opioid receptor (DOR) is localized primarily to axon terminals in the ventrolateral periaqueductal gray (vlPAG). However, rather than on the plasma membrane, DOR immunoreactivity is usually located within the cytoplasmic compartment, often associated with dense-core vesicles. In this study, the hypothesis that a behavioral stimulus, a cold water swim stress (3 minutes at 4 degrees C; CWSS), could initiate the translocation of the DOR was tested. The subcellular distribution of DOR was examined using a preembedding immunogold-labeling method and ultrastructural analysis in control rats and in rats that had a CWSS. In both cases, dense-core vesicles associated with DOR labeling were often within 100 nm of the plasma membrane. When the dense-core vesicles were near the plasma membrane, sometimes electron-dense "tethers" appeared between the vesicle and the plasma membrane. However, in rats exposed to CWSS, there was a decrease in immunolabeling associated with dense-core vesicles that were near the plasma membrane and a significant increase in DOR immunoreactivity associated with the plasma membrane. In addition, there was a significant increase in the fraction of DOR immunoreactivity associated with large clear-core vesicles; possibly early endosomes. Moreover, after a CWSS, dense-core vesicles containing DOR immunoreactivity could be visualized fusing with the plasma membrane of synaptic boutons. These data suggest the involvement of DOR in the vlPAG in the behavioral response to CWSS. Furthermore, the results support the hypothesis that the cell surface distribution of presynaptic receptors can be regulated in an activity-dependent manner by virtue of transport via dense-core vesicles.
Copyright 2003 Wiley-Liss, Inc.