Axonal transport of opiate receptor subtypes

The axonal transport of mu and delta receptors in the rat was examined by autoradiography in three fiber systems: vagus nerve, fasciculus retroflexus and corpus callosum. Following ligature or knife cut, sections were incubated with [125I]D-Ala2-MePhe4-Met(O)5-ol-enkephalin to label mu receptors or [125I]D-Ala2-D-Leu5-enkephalin in the presence of 30 nM oxymorphone to selectively label delta receptors. Ligature of the vagus and knife cut of the fasciculus retroflexus produced a time-dependent proximal build-up of mu receptors indicating anterograde axonal flow. In contrast, the corpus callosum has delta but not mu receptors, and these could not be demonstrated to undergo axonal flow. The results suggest that in the fiber tracts examined, mu but not delta opiate receptor subtypes are anterogradely transported toward nerve terminals where they may be inserted in presynaptic membranes.