Selective Accumulation of ³H-Serotonin by Nerve Terminals of Raphe Neurones: An Autoradiographic Study

Abstract

IT is possible to demonstrate an active, saturable uptake process for serotonin (5HT) in vitro in brain slices^1–4, and to show accumulation of 5HT by nerve endings in vivo^5,6. It has been thought that the high affinity portion of this uptake represents a selective accumulation of 5HT by 5HT-containing neurones^4,7. In support of this proposition, we have recently found a selective reduction of the high affinity uptake of ³H-5HT into nuclei-free homogenates prepared from forebrains of rats in which the serotonin-containing cells of the midbrain raphe nuclei were destroyed⁷. Electrolytic destruction of the midbrain raphe nuclei results in degeneration of the serotonin-containing projections to the forebrain as shown by a selective reduction of forebrain 5HT levels and tryptophan hydroxylase activity, and a loss of the histochemical fluorescence of 5HT-ontaining fibres and nerve terminals in the forebrain^8–10. It s presumed that synaptosomes (isolated, intact nerve endings) were responsible for the uptake of ³H-5HT in the nuclei-free homogenates on the following basis: uptake of ³H-5HT was prevented by procedures known to disrupt the synaptosomal membrane, such as osmotic shock and freezing and thawing; after homogenates were centrifuged in discontinuous sucrose density gradients, most of the uptake activity was localized to tissue at the 0.8–1.2 M sucrose interface which contains the bulk of the synaptosomes in the gradient. To investigate further the site(s) of ³H-5HT uptake, we have conducted the following light and electron microscopic investigations.