Brain Research, 62 (1973) 431-437 431 © Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands CYTOLOGICAL ASPECTS OF THE AXONAL MIGRATION OF CATECHOLAMINES AND OF THEIR STORAGE MATERIAL J. TAXI AND C. SOTELO Laboratoire de Biologie Animale, Universit~ de Paris VI, 75005 Paris and Laboratoire de Neuro- morphologie, U-106 LN.S.E.R.M., 75014 Paris (France) Ligated sympathetic nerves have been largely used to study axonal transport of catecholamines and their related storage organelles (see references in Dahlstr6ma). In the present communication, different kinds of experiments were carried out on the ligated sciatic nerve of the rat in order to re-study with another approach -- high resolution radioautography -- the problem of axonal transport of catecholamines. Two main questions were analyzed: (A) What is the contribution of axonal transport to the large accumulation of noradrenaline (NA) observed with fluorescence histochemistry 1 in the proximal portion of ligated sympathetic fibers? (B) What are the cytological features in these proximal portions which can be correlated with NA storage? (A) To answer the first question the following experiments were performed (a detailed description of the methods used here are already published12,16): 5 mCi of [3H]NA or of a precursor ([3H]DOPA or [aH]DA) were injected intravenously into a 80-100 g rat. It is already established14,15,17 that noradrenergic neurons are, under these conditions, immediately loaded with the [SH]NA. After a delay of 2 h, necessary to obtain a decrease of the blood concentration in [aH]NA to a negligible level, the sciatic nerves were ligated. From this moment, substances migrating inside the axons must accumulate above the ligature, and among such substances the [3H]NA. The rats were killed and fixed 3 h or 20 h after ligation. No labeling was observed on the proximal portion of the fibers after a period of migration of 20 h, but a moderate labeling was encountered in the 3 h experiments. The disappearance of the labeling between 3 and 20 h in our experiments means that the bulk of fluorescence NA visible after 24 h of ligation t cannot be accounted for by the migration of NA manufactured in the neuronal perikarya, but must be related to local synthesis and storage. When similar experiments were made in animals pretreated with a monoamine- oxidase inhibitor (IMAO), a weak to moderate labeling was observed on proximal fibers not only in rats killed 3 h after ligation, but also in those killed after 20 h. The presence of labeling in these instances is certainly due to the slowing down of NA catabolism. These results, similar to those obtained by Geffen et al. 4 in quite different