Selective accumulation of 3 H-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 slices1–4 and to show accumulation of 5HT by nerve endings in vivo5,6. It has been thought that the high affinity portion of this uptake represents a selective accumulation of 5HT by 5HT-containing neurones4,7. 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 forebrain8–10. 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.