Non-invasive in-vivo autoradiographic method to measure axonal transport in serotoninergic neurons in the rat brain.

We studied axonal transport of serotoninergic neurons by autoradiography following intravenous administration of alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp). Autoradiograms obtained 24 h after intravenous injection of the tracer demonstrated clearly all raphe nuclei and the major ascending pathway, the medial forebrain bundle (MFB). From these autoradiograms it was clear that radioactivity traveling along the MFB had already reached the substantia nigra and ventrolateral geniculate body nuclei, terminal field. The whole route of the MFB was well visualized from an axial cross-section of a three-dimensional display of data. Autoradiograms obtained at 6 h after injection revealed only the caudal part of the MFB but all raphe nuclei were labelled, indicating that the tracer was in the process of being transported, probably as an alpha-methyl-5-hydroxytryptamine, via the MFB. The axonal transport rate was estimated from the brain autoradiograms of 4 rats killed 6 h after injection of the tracer. The mean distance of the tracer transported via the medial forebrain bundle in 4 rats was 3.8 +/- 0.4 (S.D.) mm, which corresponded to the level of the posterior to mid-hypothalamus. The axonal transport rate calculated from this distance from the medial raphe was 0.63 +/- 0.07 mm/h (14 mm/day). There was no significant difference in the axonal transport rate between the right and left side of the MFB.