Electron microscopic autoradiographic localization of opioid receptors in rat neostriatum.

The regional distribution of opioid receptors in the mammalian central nervous system has been extensively studied by light microscopic autoradiography. Little is known, however, about the fine structural localization of these receptor sites apart from the fact that they are present in both synaptosomal and smooth microsomal subcellular fractions of whole brain homogenates. Previously, using the Met-enkephalin analogue FK 33-824 (Sandoz), we determined that brain opioid binding sites could be visualized by electron microscopic autoradiography. We now report on the ultrastructural distribution of these sites in the caudate-putamen of the rat. The vast majority of specifically labelled opioid binding sites were found to be associated with axo-dendritic (53%), axoaxonic (18%) or axo-somatic (3%) neuronal membrane interfaces. Only 7%, however, were associated with synaptic junctions. These results suggest that opioids act primarily at non-junctional interfaces on the dendrites, axons and soma of neurones in the neostriatum.