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

A simple preembedding avidin-biotin-peroxidase complex technique was used t o study the ultrastructural localization of mu-opioid receptor in the rat a rea postrema. By using low concentrations of the first antiserum for incuba tion with a short reaction time to 3,3'-diaminobenzidine, the immunostainin g was faint at the light microscopic level. However, at the electron micros copic level, strong immunoreaction was observed. mu-Opioid receptors were f ound to be localized on the postsynaptic membrane of dendrites, extrasynapt ic plasma membrane, and the surface of the small, clear vesicles in axon te rminals. Of the total 283 immunopositive profiles observed, 68.2% (193 of 2 83) were dendrites, 29.3% (83 of 283) were axon terminals, and 2.5% (7 of 2 83) were myelinated axons. No immunostained neuron bodies were found in the present study; 109 mu-opioid receptor immunoreactive dendrites received sy napses (56.5%, 109 of 193) from nonimmunoreactive (84.4%, 92 of 109) or imm unoreactive (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. (C) 1999 Elsevier Science Inc. All rights reserved.