Axoplasmic transport and possible recycling of opiate receptors labelled with 3H-lofentanil.

3H-Lofentanil, an extremely potent opiate drug with a very long duration of action was injected intravenously into rats immediately after a ligature had been tied around the vagus nerve. Radioactivity accumulated on both sides of the ligature 24 hours and, to a larger extent, 48 hours after the injection. In contrast, there was no accumulation in animals pretreated with naloxone, neither in ligated sciatic nerves nor between two ligatures in the vagus nerve. An accumulation of stereospecific 3H-lofentanil binding sites measured in vitro was only detected above the ligature, thus in the proximal part of the nerve. When 3H-lofentanil was injected at different time intervals after ligation, we observed a tremendous drop of labelling in the distal and also but more slowly in the proximal part of the nerve. This could be due to a possible recycling or re-utilization of 3H-lofentanil binding sites. The present data are compatible with an axoplasmic flow and a possible recycling of opiate receptors labelled in vivo after intravenous injection of 3H-lofentanil.