Autoradiographic experiments to examine uptake, anterograde and retrograde transport of tritiated serotonin in the mammalian brain.

In an attempt to define the potential application of neurotransmitter-specific transport as a method of tracing fiber connections, we have examined the uptake and subsequent ortho- and retrograde transport of tritium-labeled serotonin (3H-5HT) in the cerebellum-raphe pallidus system. Injection of various concentrations of 3H-5HT followed by different post-injection survival times revealed different labeling patterns in the injected sites and different patterns of transport. The most striking feature is that nonseroitonin neurons as well as serotonin cells were able to take up and transport the tritium label in both otho- and retrograde fashion. The non-sertonin-specific nature of this uptake and transport is more obvious at higher concentrations of 3H-5HT (more than 9 X 10(-5) M), with longer survival times and following pretreatment with monoamine oxidase inhibitors. At a concentration of 9 X 10(-6) M 3H-5HT, only specific uptake seems to take place as evidenced by label in known serotinin cells and fiber systems; however, it was impossible to detect by autoradiography any ortho- or retrograde transport at this low concentration. Non-specific uptake and transport were observed following injection into the vestibular nuclei and oculomotor complex. This suggests that non-specific uptake and the transport of 3H-5HT or metabolites may also occur in other regions of the central nervous system.