Differential polarization of serotonin transporters in axons versus soma-dendrites: An immunogold electron microscopy study

In spite of the conventional belief that neurotransmitter uptake occurs at the synapses, we demonstrated previously that serotonin transporters and th e high-affinity uptake of serotonin were not confined to the terminals but rather occurred throughout the axons [Zhou F. C. et al. (1998) Brain Res. 8 05, 241-254]. In the present study, the detailed distribution of serotonin transporters over various parts of the neuron was illustrated and analysed morphometrically using a pre-embedding immunogold method with a characteriz ed serotonin transporter antibody at the electron microscopic level. Our fi ndings reveal a highly polarized distribution of serotonin transporters bet ween axons and soma-dendrites in two aspects. (1) On the plasma membrane, s erotonin transporter-immunogold is extremely low on soma-dendrites and syna ptic junctions, but consistently dense along the axons and perisynaptic are a. (2) In contrast, serotonin transporter labeling in the cytoplasm is conc entrated in soma and dendrites, particularly on the membranes of rough endo plasmic reticulum, Golgi complexes and tubulovesicular structures, but low in the axoplasm. The extensive distribution of serotonin transporter along the axolemma sugg ests a broad range of uptake sites beyond synaptic junctions, and is consis tent with the notion that the major mode of transmission for serotonin neur ons is through volume (extrasynaptic) transmission. The highly polarized di stribution also indicates that the major serotonin uptake sites are on axon s and not on soma-dendrites. (C) 1999 IBRO. Published by Elsevier Science L td.