Ultrastructural localization of delta-1 opioid receptor in the dorsal raphe nucleus of the rat.

The ultrastructural localization of delta-1 opioid receptor in the rat dorsal raphe nucleus was studied by the preembedding avidin-biotin-peroxidase complex technique. With application of a low concentration of the first antiserum in incubation and control of short-time reaction to 3,3'-diaminobenzidine, the immunoreaction seemed to be faint at the light microscopic level. At the electron microscopic level, however, delta-1 opioid receptor immunoreaction products were found to be localized specifically on the postsynaptic membrane of dendrites, dense-cored vesicles, and the surface of the small, clear vesicles in axon terminals with strong immunoreactivity. Of the total 659 immunopositive profiles observed, up to 62.4% (411/659) were dendrites, whereas 33.8% (223/659) were axon terminals. The immunostained myelinated axons and perikarya were relatively rare, with the frequencies 0.8% (5/659) and 3.0% (20/659), respectively. Most of the immunopositive dendrites (338/411, 82.2%) were immunostained only at the postsynaptic membranes. Other immunoreactive dendrites showed their immunoreaction products also in some other contents besides the postsynaptic membranes (44/411, 10.7%) or only in those contents but not the postsynaptic membranes (25/411, 6.1%). Only four dendrites showed their immunoreactive results only at the membrane not related to synapse (4/267, 1.0%). No dendrite was found immunostained in all the contents. About half of the immunopositive axon terminals (125/223, 56.1%) were found to make synapse with nonimmunoreactive dendrites (76/223, 34.1%) or immunoreactive dendrites (49/223, 22.0%), while only one was found to make contact with immunoreactive perikarya. The present study showed that delta-1 opioid receptor in the dorsal raphe nucleus is mostly localized on postsynaptic membrane; the main function of the delta-1 receptor in the dorsal raphe nucleus is to receive signals from the opioid-containing axon terminals through synapses.