Electron microscopic observation of mu-opioid receptor in the rat area postrema.

A simple preembedding avidin-biotin-peroxidase complex technique was used to study the ultrastructural localization of mu-opioid receptor in the rat area postrema. By using low concentrations of the first antiserum for incubation with a short reaction time to 3,3'-diaminobenzidine, the immunostaining was faint at the light microscopic level. However, at the electron microscopic level, strong immunoreaction was observed. Mu-Opioid receptors were found to be localized on the postsynaptic membrane of dendrites, extrasynaptic plasma membrane, and the surface of the small, clear vesicles in axon terminals. Of the total 283 immunopositive profiles observed, 68.2% (193 of 283) were dendrites, 29.3% (83 of 283) were axon terminals, and 2.5% (7 of 283) were myelinated axons. No immunostained neuron bodies were found in the present study; 109 mu-opioid receptor immunoreactive dendrites received synapses (56.5%, 109 of 193) from nonimmunoreactive (84.4%, 92 of 109) or immunoreactive (15.6%, 17 of 109) axon terminals, whereas 84 dendrites (43.5%, 84 of 193) were found without receiving synapses. The present study shows that the mu-opioid receptor in the area postrema plays a role mainly at the synapses.